Modulation of statin-activated shedding of Alzheimer APP ectodomain by ROCK. - AlzForum Alzheimer Research Paper of the Week

Statins are known to increase secretion of APP, but the mechanism by which this occurs is poorly understood [1]. The current manuscript by Pedrini et al. focuses on the effect of statins on Rho and Rho-associated coiled-coil containing kinase 1 (ROCK). The group observes that a constitutively active ROCK prevented the actions of statins on APPsα. This suggests that inhibition of ROCK plays an important role in the mechanism of action of statins. They also performed the converse experiment, and examined how dominant-negative ROCK affects secretion of APPaα. Unfortunately, this is a point where the group's story strays. The dominant-negative ROCK increases APPsα secretion on cells not exposed to statins, but does not increase the actions of statins; thus, the effects of dominant-negative ROCK are not strictly opposite to those of the constitutively active ROCK. These data suggest that ROCK can modulate the effects of statins, but do not explicitly prove that statins act on APPsα through ROCK. Nonetheless, this is a very interesting story which nicely integrates Rho signaling into... Read more

Since the appearance of the first epidemiological and animal studies claiming a connection between cholesterol and Alzheimer disease, at least four different aspects of cholesterol metabolism have been directly linked to AD neuropathology:

(i) clustering of APP and BACE1 into lipid rafts, which facilitates β cleavage of APP (1);
(ii) intracellular cholesterol distribution, which is able to activate the amyloidogenic processing of APP (2);
(iii) ozonolysis of cholesterol, which generates peroxi-derivatives of cholesterol that accelerate the aggregation of Aβ monomers (3), and
(iv) Aβ-mediated oxidation of membrane cholesterol, which liberates H2O2 and aggravates oxidative stress (4).

Therefore, strategies aimed at the modulation of cholesterol metabolism/distribution in the brain have received wide attention for the prevention of AD. Among those, statins seem to be especially welcome, mostly because they are already available, have been widely studied for their role in the prevention of atherosclerosis, and are overall very safe. Statins were introduced as... Read more

Since the appearance of the first epidemiological and animal studies claiming a connection between cholesterol and Alzheimer disease, at least four different aspects of cholesterol metabolism have been directly linked to AD neuropathology:

(i) clustering of APP and BACE1 into lipid rafts, which facilitates β cleavage of APP (1);
(ii) intracellular cholesterol distribution, which is able to activate the amyloidogenic processing of APP (2);
(iii) ozonolysis of cholesterol, which generates peroxi-derivatives of cholesterol that accelerate the aggregation of Aβ monomers (3), and
(iv) Aβ-mediated oxidation of membrane cholesterol, which liberates H2O2 and aggravates oxidative stress (4).

Therefore, strategies aimed at the modulation of cholesterol metabolism/distribution in the brain have received wide attention for the prevention of AD. Among those, statins seem to be especially welcome, mostly because they are already available, have been widely studied for their role in the prevention of atherosclerosis, and are overall very safe. Statins were introduced as pharmacological inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMGCoA) reductase, the rate-limiting enzyme in the biosynthesis of cholesterol, but they were soon shown to do more than just that, including stimulation of bone osteogenesis and inhibition of growth/invasion of certain types of cancers. Some of these effects could be related to their cholesterol-lowering activity, some cannot (at least for now).

This story seems to hold even when we switch to the “molecular” effects of statins: They do more than just inhibit HMGCoA reductase. In this new study by Pedrini et al., a post-translational modification involving isoprenoids—and not cholesterol itself—is shown to affect α cleavage of APP. As the paper points out, isoprenoid (farnesyl and geranylgeranyl) moieties, too, originate from the cholesterol biosynthetic pathway, just a few steps downstream from HMGCoA reductase and a few steps upstream of cholesterol (for review, see 5). This paper is a continuation of previous work from the senior author, Sam Gandy, who has been investigating the mechanisms that regulate α cleavage of APP for a long time. Here, the authors show that inhibition of cholesterol biosynthesis increases α cleavage of APP through a mechanism that is in part independent of cholesterol itself. They used elegant biochemical approaches, including HMGCoA reductase and farnesyl-transferase inhibition in the presence or absence of mevalonate. Since mevalonate is able to bypass HMG-CoA reductase but not farnesyl-transferase inhibition, they managed to identify a novel form of regulation of APP processing that requires isoprenoids. The authors went on to show that such an event seems to involve post-translational modulation of the Rho family of GTPases and Rho-associated coiled-coil containing kinases (ROCKs).

The effect produced by ROCK is completely abolished after deletion of both the pleckstrin homology and the Rho-binding domains, and after inhibition of the kinase activity. The specific roles of the different domains of ROCK or the possible interaction between Rho GTPases and ROCK itself are not explored in detail. However, since a conformational change of ROCK is required for the functional activation of the kinase activity of the protein, it is likely that the Rho-binding domain is necessary for the Rho-mediated activation of ROCK. Therefore, statin-mediated inhibition of the cholesterol biosynthetic pathway may also lead to decreased transfer of isoprenoid moieties to Rho proteins, thereby decreasing their functional activity.

Unfortunately, the paper did not describe what happens to β cleavage of APP or to the production of Aβ peptides. It would be interesting to see whether or not Rho/ROCK proteins can also influence β cleavage, either by diverting APP from the β to the α pathway, or by directly affecting β cleavage of APP. In this regard, it is very tempting to try to find a possible connection with the loe phenotype observed in D. melanogaster (6). Flies are not able to generate cholesterol; the HMG-CoA reductase-dependent pathway stops immediately after the generation of isoprenoids. This pathway is under the inhibitory control of AMP-activated kinase (AMPK), which blocks the biosynthesis of both fatty acids and isoprenoids, and the hydrolysis of diet-derived cholesterol esters. Disruption of AMPK (loe phenotype) in D. melanogaster leads to a marked decrease in the shedding of APPL, the fly homolog of human APP. This event is in part due to the increased levels of isoprenoids, because statin-mediated inhibition of isoprenoid biosynthesis was able to partially recover APPL processing. It seems that both Pedrini et al. and Tschape et al. have found a connection between isoprenoids and APP processing, a connection that has been conserved throughout evolution but that can differ in some aspects, probably because of different molecules situated between isoprenoids and APP. The identification of those molecules will be the next stop…and Sam Gandy and Suzana Petanceska will certainly satisfy our curiosity.

References:
1. Ehehalt R, Keller P, Haass C, Thiele C, Simons K. Amyloidogenic processing of the Alzheimer beta-amyloid precursor protein depends on lipid rafts. J Cell Biol. 2003 Jan 6;160(1):113-23. Epub 2003 Jan 06. Abstract

2. Puglielli L, Konopka G, Pack-Chung E, Ingano LA, Berezovska O, Hyman BT, Chang TY, Tanzi RE, Kovacs DM. Acyl-coenzyme A: cholesterol acyltransferase modulates the generation of the amyloid beta-peptide. Nat Cell Biol. 2001 Oct;3(10):905-12. Abstract

3. Zhang Q, Powers ET, Nieva J, Huff ME, Dendle MA, Bieschke J, Glabe CG, Eschenmoser A, Wentworth P Jr, Lerner RA, Kelly JW. Metabolite-initiated protein misfolding may trigger Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Apr 6;101(14):4752-7. Epub 2004 Mar 19. Abstract

4. Opazo C, Huang X, Cherny RA, Moir RD, Roher AE, White AR, Cappai R, Masters CL, Tanzi RE, Inestrosa NC, Bush AI. Metalloenzyme-like activity of Alzheimer's disease beta-amyloid. Cu-dependent catalytic conversion of dopamine, cholesterol, and biological reducing agents to neurotoxic H(2)O(2). J Biol Chem. 2002 Oct 25;277(43):40302-8. Epub 2002 Aug 20. Abstract

5. Jackson, S. M. et al. (1997). Signaling molecules derived from the cholesterol biosynthetic pathway. In: Subcellular Biochemistry. Cholesterol: its functions and metabolism in biology and medicine. Bittman, R, ed. (Plenum Press New York, NY), pp. 1-21.

6. Tschape JA, Hammerschmied C, Muhlig-Versen M, Athenstaedt K, Daum G, Kretzschmar D. The neurodegeneration mutant lochrig interferes with cholesterol homeostasis and Appl processing. EMBO J. 2002 Dec 2;21(23):6367-76. Abstract

View all comments by Luigi Puglielli

Gary Landreth's paper in the current issue of The Journal of Neuroscience on statins reducing Aβ-induced microglial inflammatory responses is very elegant work (Cordle and Landreth, 2005). This study shows that statin treatment of microglia and monocytes leads to robust reduction of Aβ-induced Il1β and inducible nitric oxide synthase expression, as well as reduction of nitric oxide production. As isoprenoids and the Rac and Rho-GTPases are implicated as mediators of these effects, this study complements the findings by Pedrini et al.

Furthermore, in 2002, Barbara Cordell's group provided evidence that ApoE secretion from glia requires a prenylated protein entity, and that the reduction of ApoE secretion by statins is due to inhibition of the synthesis of isoprenoids (Naidu et al., 2002).

In 2003, we discussed possible mechanisms by which statins can reduce brain amyloidosis (Petanceska et al., 2003). We hypothesized that... Read more

As Sam Gandy says regarding his research on statin effects in Alzheimer disease: "If it seems like a mess, it is." Hippocrates said, "Every disease has a nature of its own, and each arises from its own natural cause." Why, 2,000 years later, is modern science unable to find a simple "natural cause" for AD?

Are we asking the right questions? Is this a modern disease, with a modern cause? How common are AD lesions in preserved brains from the 19th century? Should we examine the Yerkes and Corsellis collections?

The cholesterol-AD story has confused beginnings, and a messy ending. What government would consider mass-medicating its ageing population with statins to prevent AD, knowing that its best and most dedicated scientists had failed to find a preventable cause of the disease? Those who prefer intervention over prevention will protest that the environmental origins are so murky and multifactorial that treatment and prevention must perforce be piecemeal. It would come as a great shock to such thinking if a simple, preventable cause of the disease were found, which at... Read more

As Sam Gandy says regarding his research on statin effects in Alzheimer disease: "If it seems like a mess, it is." Hippocrates said, "Every disease has a nature of its own, and each arises from its own natural cause." Why, 2,000 years later, is modern science unable to find a simple "natural cause" for AD?

Are we asking the right questions? Is this a modern disease, with a modern cause? How common are AD lesions in preserved brains from the 19th century? Should we examine the Yerkes and Corsellis collections?

The cholesterol-AD story has confused beginnings, and a messy ending. What government would consider mass-medicating its ageing population with statins to prevent AD, knowing that its best and most dedicated scientists had failed to find a preventable cause of the disease? Those who prefer intervention over prevention will protest that the environmental origins are so murky and multifactorial that treatment and prevention must perforce be piecemeal. It would come as a great shock to such thinking if a simple, preventable cause of the disease were found, which at a stroke would wipe out drug development programs and all further research on the disease.

By piecing together the available facts on this disease, it is possible to reach an inductive conclusion, that the simple common cause is Wesson steam-deodorization of polyunsaturated vegetable oils, an industrial process that, since 1900, has been removing some 30 percent of the neuroprotective vitamin E from common frying and salad oils. Reduced antioxidant protection of dietary omega-6 essential fatty acids (linoleic acid in oils) exposes the long-chain EFA of the brain and retina to lipid peroxidation. A major product of arachidonic acid breakdown in neuronal synapses is 4-hydroxynonenal (4-HNE), which is known to inactivate ion-motive ATPases, and glucose and glutamate transporters.

In addition, there is an intriguing possibility that 4-HNE may inactivate α-secretase, by forming adducts with vulnerable amino acids at the catalytic site. Such inactivation would be a key mechanism in a refined oil hypothesis, since it would account for slow β amyloid accumulation.

I propose that my suggested mechanism of HNE-induced inactivation of α-secretase be tested in some laboratory somewhere, by some scientist who retains a native sense of curiosity about causes of disease, unspoilt by commercial temptations. If the prediction is proved correct, government would welcome the breakthrough, which would finally pin down the most critical mechanism in the refined oil hypothesis, paving the way for legislation requiring food oil processors to increase the vitamin E content of refined oils to natural levels (at least 0.6 mg per gm of EFA).

View all comments by Robert Peers

This manuscript confirms and extends a previous study showing that statin treatment can increase the release of sAPPα [1]. The biochemical mechanism by which HMG-CoA reductase inhibition leads to this increase isn’t fully understood. The authors present intriguing data that suggests the small GTPase pathway may be involved. First, a farnesyltransferase inhibitor was shown to increase statin-induced sAPP shedding, implying a farnesylated GTPase may be involved. They then looked at dominant-negative (DN) and constitutively active (CA) forms of ROCK, which is an effector protein kinase of the small GTPase Rho. CA ROCK decreases sAPP release while the DN form increases sAPP release. These results suggest that statin-mediated sAPP shedding could be mediated by isoprenoids, which can regulate the amount of membrane-associated Rho and thus the extent of ROCK activation.

As the authors acknowledge in the discussion, there are a couple of inconsistencies in the data that are confusing. Their data suggests that the effects of statins are mediated at the plasma membrane. They also... Read more

This manuscript confirms and extends a previous study showing that statin treatment can increase the release of sAPPα [1]. The biochemical mechanism by which HMG-CoA reductase inhibition leads to this increase isn’t fully understood. The authors present intriguing data that suggests the small GTPase pathway may be involved. First, a farnesyltransferase inhibitor was shown to increase statin-induced sAPP shedding, implying a farnesylated GTPase may be involved. They then looked at dominant-negative (DN) and constitutively active (CA) forms of ROCK, which is an effector protein kinase of the small GTPase Rho. CA ROCK decreases sAPP release while the DN form increases sAPP release. These results suggest that statin-mediated sAPP shedding could be mediated by isoprenoids, which can regulate the amount of membrane-associated Rho and thus the extent of ROCK activation.

As the authors acknowledge in the discussion, there are a couple of inconsistencies in the data that are confusing. Their data suggests that the effects of statins are mediated at the plasma membrane. They also conclude that this effect may be mediated through a farnesylated form of Rho via ROCK. There are three isoforms of Rho (A,B,C) [2]. RhoA and C are only geranylgeranylated and located mainly at the plasma membrane. RhoB can be farnesylated or geranylgeranylated and is found primarily in the endosomes, suggesting a spatial disconnect. One critical issue is the specificity of the farnesyl transferase inhibitor that was used. If this effect is specific, treatment with FPP and not GGPP should block the increase in sAPP.

Since the small GTPase pathway is so complex, DN and CA forms of these proteins can often have unexpected effects. It would be informative to look directly at the isoforms of Rho, as well as other GTPases that are theoretically not involved in sAPP processing.

Finally, the ROCK inhibitor Y-27632 had no effect on sAPP release. This unexpected result could be a result of multiple activities since it is known that this compound can affect multiple kinases [3]. A variety of more specific and potent ROCK inhibitors have now been developed that can be screened to more thoroughly probe this effect [4].

Despite these issues, this manuscript provides an intriguing association between the alpha secretase processing of APP and the isoprenoid pathway, which has also been recently implicated in γ-secretase processing. A paper by Zhou et al. suggests that NSAIDs mediate their Aβ42 lowering effect through inhibition Rho [5]. We presented data at the 2004 Society for Neuroscience meeting suggesting that NSAIDs do not act through Rho. Instead, our data suggests that NSAIDs, as well as isoprenoids, directly target the γ-secretase complex to modulate Aβ production.

How these effects and the isoprenoid pathway interact with all APP processing pathways remains to be determined. Clearly, the isoprenoid pathway and the numerous GTPases that are influenced by these metabolites are complex and incompletely understood. Moreover, almost nothing is known about isoprenoid metabolism in the brain (besides the fact that the enzymes that regulate it are abundant). Further research into the role of isoprenoids and small GTPase in APP metabolism and Alzheimer’s disease is required and may provide important insight into disease mechanism and novel therapeutic strategies.

References:
1. Kojro E, Gimpl G, Lammich S, Marz W, Fahrenholz F. Low cholesterol stimulates the nonamyloidogenic pathway by its effect on the alpha-secretase ADAM 10. Proc Natl Acad Sci U S A 2001, 98:5815-20. Abstract

2. Wheeler AP, Ridley AJ. Why three Rho proteins? RhoA, RhoB, RhoC, and cell motility. Exp Cell Res 2004, 301:43-9. Abstract

3. Breitenlechner C, Gassel M, Hidaka H, Kinzel V, Huber R, Engh RA, Bossemeyer D. Protein kinase A in complex with Rho-kinase inhibitors Y-27632, Fasudil, and H-1152P: structural basis of selectivity. Structure (Camb) 2003, 11:1595-607. Abstract

4. Sasaki Y, Suzuki M, Hidaka H. The novel and specific Rho-kinase inhibitor (S)-(+)-2-methyl-1-[(4-methyl-5-isoquinoline)sulfonyl]-homopiperazine as a probing molecule for Rho-kinase-involved pathway. Pharmacol Ther 2002, 93:225-32. Abstract

5. Zhou Y, Su Y, Li B, Liu F, Ryder JW, Wu X, Gonzalez-DeWhitt PA, Gelfanova V, Hale JE, May PC, Paul SM, Ni B. Nonsteroidal anti-inflammatory drugs can lower amyloidogenic Abeta42 by inhibiting Rho. Science 2003, 302:1215-7. Abstract

View all comments by Thomas Kukar

Pedrini et al. identified two connected pathways with ROCK1 as the central player. Their findings indicate that ROCK1 inhibits α-secretase activity; two different statins inhibit ROCK1 via reducing isoprenylation of the Rho GTPases. Thus, statins could activate α-secretase, at least in part, via inhibition of ROCK1.

Regulation of α-secretase and γ-secretase (Zhou et al. 2003) activities by the Rho/ROCK1 phosphorylation pathway may provide interesting clues to the neuronal function of the secretases. The role of the Rho GTPases in cell motility and axon guidance is well established. In neuronal cell lines, RhoA/ROCK are activated in response to repulsive cues and lead to growth cone collapse. In contrast, attractive cues activate Cdc42 and Rac GTPases, which, in turn, promote extension of axons to appropriate targets. The growth cone integrates multiple signals to produce coordinated changes in cytoskeletal dynamics. These changes are mediated by signaling via the C-terminal tails of axon guidance molecules, such as DCC, N-cadherin, NCAM, LAR, ephrinA/B, by activating either... Read more

Pedrini et al. identified two connected pathways with ROCK1 as the central player. Their findings indicate that ROCK1 inhibits α-secretase activity; two different statins inhibit ROCK1 via reducing isoprenylation of the Rho GTPases. Thus, statins could activate α-secretase, at least in part, via inhibition of ROCK1.

Regulation of α-secretase and γ-secretase (Zhou et al. 2003) activities by the Rho/ROCK1 phosphorylation pathway may provide interesting clues to the neuronal function of the secretases. The role of the Rho GTPases in cell motility and axon guidance is well established. In neuronal cell lines, RhoA/ROCK are activated in response to repulsive cues and lead to growth cone collapse. In contrast, attractive cues activate Cdc42 and Rac GTPases, which, in turn, promote extension of axons to appropriate targets. The growth cone integrates multiple signals to produce coordinated changes in cytoskeletal dynamics. These changes are mediated by signaling via the C-terminal tails of axon guidance molecules, such as DCC, N-cadherin, NCAM, LAR, ephrinA/B, by activating either the Rho/ROCK (repulsion) or the Cdc42 and Rac (attraction) pathways. Interestingly, many of the signaling proteins are substrates for α-secretase-like and γ-secretase cleavages. The studies by Pedrini and Zhou suggest that the RhoA/ROCK pathway may regulate α- and γ-secretase activities to produce specific coordinated changes in growth cone collapse.

The work of Pedrini et al. adds to our understanding of the mechanisms by which intracellular lipid metabolism regulates secretase activities. Isoprenoids line up with membrane cholesterol, cholesteryl-esters, phospholipids, and ceramide in regulating APP processing. The identification of the downstream effector, ROCK1, for isoprenoid-mediated regulation of α-secretase sets this pathway apart from the others. This pathway is likely to account, at least in part, for the Aβ-lowering effects of statins by activating α-secretase. Cholesterol-lowering effects of statins have recently come under scrutiny by Abad-Rodriguez et al., (J. Cell Biol, 2004). This paper shows that slightly reduced membrane cholesterol leads to elevated Aβ production, instead of a decrease. More than a 35 percent reduction in membrane cholesterol is required to achieve inhibition of Aβ generation. These findings already suggest the existence of at least two different pathways by which statins may regulate APP processing. Meanwhile, reduction of cholesteryl-esters is accompanied by an increase in membrane cholesterol, and yet Aβ generation is decreased (Puglielli et al, 2001; Hutter-Paier et al., 2004). Clearly, APP processing is not simply modulated by levels of membrane cholesterol, but is influenced by the complex interplay of a number of lipid and protein components of the cell. How exactly isoprenoids fit into this interplay will likely be the subject of further studies from the laboratory of Sam Gandy and of others investigating the role of lipids in regulating Aβ production.

View all comments by Dora M. Kovacs

Clincial evidence suggests that long- term use of statins is associated with a decreased risk of Alzheimer disease (AD). As these drugs block the synthesis of cholesterol, much research has been focused on the importance of cholesterol metabolism in the pathogenesis of AD. Recently, it has been appreciated that statins can also exert biological effects independently of cholesterol. HMGCoA inhibition also blocks the production of isoprenyl precursors, and these isoprenyl groups are required for the proper function of Rho family GTPases. For example, it has been shown that inhibition of Rho contributes to the in vitro antiinflammatory effects of statins (Cordle et al., 2005).

In their recent paper, Pedrini et al. address an important issue by looking at cholesterol-independent effects of statins on APP metabolism. This group has previously shown that, in vitro, treatment of neuroblastoma cells with statins leads to an increase in shedding of sAPPα (Parvathy et al., 2004). In the present work, they expand on this theme by showing that the effects of statins on APP metabolism... Read more

Clincial evidence suggests that long- term use of statins is associated with a decreased risk of Alzheimer disease (AD). As these drugs block the synthesis of cholesterol, much research has been focused on the importance of cholesterol metabolism in the pathogenesis of AD. Recently, it has been appreciated that statins can also exert biological effects independently of cholesterol. HMGCoA inhibition also blocks the production of isoprenyl precursors, and these isoprenyl groups are required for the proper function of Rho family GTPases. For example, it has been shown that inhibition of Rho contributes to the in vitro antiinflammatory effects of statins (Cordle et al., 2005).

In their recent paper, Pedrini et al. address an important issue by looking at cholesterol-independent effects of statins on APP metabolism. This group has previously shown that, in vitro, treatment of neuroblastoma cells with statins leads to an increase in shedding of sAPPα (Parvathy et al., 2004). In the present work, they expand on this theme by showing that the effects of statins on APP metabolism are independent of cholesterol, and by identifying Rho-associated coiled-coil containing kinase (ROCK) as a possible downstream signaling target that may be disrupted by statin treatment.

The authors show that statins increase levels of holo-APP about twofold, yet increase sAPPα shedding three- to fourfold. These data suggest that inhibition of Rho family proteins preferentially drives the α-secretase pathway, though the mechanism remains undetermined. The most interesting data in the paper suggest that ROCK could be the key regulator of APP metabolism in this paradigm. ROCK is a kinase that is activated upon Rho activation. Thus, inhibition of Rho by statins could block ROCK activation and thus relieve a constitutive inhibitory influence exerted by this pathway. By using dominant-negative (DN) and dominant-active (DA) ROCK constructs, Pedrini et al. show that a DN ROCK increases shedding of sAPPα and that DA ROCK decreases sAPPα shedding. While not conclusive, these data suggest that ROCK regulates APP metabolism, and that statins may increase sAPPα shedding via inhibition of ROCK activity. These findings are consistent with our finding that broadly acting inhibitors of Rho proteins, such as Toxin A of C. difficile and isoprenyltransferase inhibitors (unpublished data), elevate sAPPα levels. Paradoxically, Pedrini et al. found that the well-documented ROCK inhibitor Y27632 blocked statin-induced sAPP generation, a finding which remains unexplained.

At face value, it seems as though the increased shedding of sAPPα upon statin treatment would ameliorate the disease process, as an increase in non-amyloidogenic APP processing is usually associated with a decrease in amyloidogenic processing. However, Pedrini et al. demonstrated that treatment with statins results in a twofold increase in holo-APP. We and Bob Vassar’s lab have shown that this results in a corresponding increase in Aβ peptide levels. Pedrini et al. do not show the effect of statins on Aβ levels. Thus, the statin-mediated elevation of cellular APP levels results in an increase in steady-state holo-APP levels, with a commensurate increase in both sAPPα and Aβ production. The data by Pedrini et al. suggest that sAPPα may be preferentially increased, but it is unclear if this phenomenon is separable from increased Aβ production. Thus, it is unclear whether the Rho-ROCK pathway will become an appropriate therapeutic target.

References:
Cordle A, Landreth G. 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors attenuate beta-amyloid-induced microglial inflammatory responses. J Neurosci. 2005 Jan 12;25(2):299-307. Abstract

Parvathy S, Ehrlich M, Pedrini S, Diaz N, Refolo L, Buxbaum JD, Bogush A, Petanceska S, Gandy S. Atorvastatin-induced activation of Alzheimer's alpha secretase is resistant to standard inhibitors of protein phosphorylation-regulated ectodomain shedding. J Neurochem. 2004 Aug;90(4):1005-10. Abstract

View all comments by Gary Landreth

Sam Gandy’s group’s study underscores an emerging role for isoprenoid-mediated regulation of APP processing and its possible relationship to Alzheimer disease pathogenesis. Over a year ago, we reported that GGPP, one of the isoprenoids synthesized in the mevalonate biosynthetic pathway, preferentially increases the generation of the more amyloidogenic Aβ species, Aβ42 (Zhou et al., Science 2003). Based on our experiments using dominant-negative and constitutively active Rho, as well as the ROCK inhibitor Y27632, we concluded that GGPP mediates an increase of Aβ42 through activation of the Rho/ROCK pathway, possibly by modulating γ-secretase.

In our opinion, the most important finding reported in our paper is the one showing that physiological lipids, such as GGPP, can regulate the generation of the amyloidogenic species Aβ42. Interestingly, isoprenoids are generated not only endogenously but also can be taken up through the diet. Thus, dietary isoprenoids could also regulate APP processing and Aβ synthesis and contribute to AD... Read more

Sam Gandy’s group’s study underscores an emerging role for isoprenoid-mediated regulation of APP processing and its possible relationship to Alzheimer disease pathogenesis. Over a year ago, we reported that GGPP, one of the isoprenoids synthesized in the mevalonate biosynthetic pathway, preferentially increases the generation of the more amyloidogenic Aβ species, Aβ42 (Zhou et al., Science 2003). Based on our experiments using dominant-negative and constitutively active Rho, as well as the ROCK inhibitor Y27632, we concluded that GGPP mediates an increase of Aβ42 through activation of the Rho/ROCK pathway, possibly by modulating γ-secretase.

In our opinion, the most important finding reported in our paper is the one showing that physiological lipids, such as GGPP, can regulate the generation of the amyloidogenic species Aβ42. Interestingly, isoprenoids are generated not only endogenously but also can be taken up through the diet. Thus, dietary isoprenoids could also regulate APP processing and Aβ synthesis and contribute to AD pathogenesis.

At last year’s International Conference on Alzheimer’s and Related Diseases in Philadelphia, Todd Golde’s group reported that they had confirmed the effect of GGPP on Aβ generation. However, based on their finding that the generation of Aβ can also be increased by GGPP in the isolated lipid rafts, they suggested that isoprenoids may act directly on the γ-secretase complex instead of through a Rho/ROCK signaling pathway.

In the present paper, Steve Pedrini and colleagues performed a series of elegant experiments demonstrating that isoprenoids regulate APPα shedding through modulating ROCK activity. However, the consequence of modulating APPα shedding by ROCK on Aβ generation is still under investigation by this group. As Dr. Gandy said in the Q&A, the effect of small G-proteins and their effectors on certain cellular functions, such as APP processing, is complicated because of “some moment-to-moment balance of which pathways prevail.” Add to that interwoven and feedback signal transduction pathways controlled by these small G-proteins, and the studies are truly complex to both perform and interpret.

Regardless of the exact mechanism, the fundamental question of whether long-term, high-dose consumption of dietary isoprenoids could impact central APP processing, Aβ synthesis, and AD neuropathology should be addressed. Experiments designed to feed APP transgenic or wild-type mice isoprenoid-supplemented food daily for many months, and then looking for effects on APP processing/brain neuropathology should prove informative. If dietary isoprenoids indeed aggravate the progress of brain amyloid deposition in APP transgenic mice, one might reasonably speculate on their possible role in contributing to the pathogenesis of AD.

View all comments by Steven Paul
View all comments by Yan Zhou

I sincerely thank Alzforum for publishing my provocative comment on AD and cholesterol, albeit somewhat sanitized of its original pungency! If my theory about refined oils causing sporadic AD is correct, then "stripped" oil (containing little or no vitamin E, after prolonged heating) would be a good means of inducing neuronal lipid peroxidation in culture, which should generate both measurable 4-hydroxynonenal and reduced formation of secreted APP (sAPP), along with a mysterious rise in Aβ. My best wishes go to anybody who may care to do this experiment! Let us fortify ourselves with three observations that should encourage us:

1. Safflower oil, given as 20 percent of the diet, caused learning impairment in weaned rat pups (Harman et al., 1976). When the experiment was repeated with vitamin E supplementation, no harmful effects were seen on learning. Harman's safflower oil may have been typical steam-refined oil, which has about 0.45 mg of vitamin E per gm of essential fatty acids, compared with 0.65 mg in cottonseed oil, 0.36... Read more

I sincerely thank Alzforum for publishing my provocative comment on AD and cholesterol, albeit somewhat sanitized of its original pungency! If my theory about refined oils causing sporadic AD is correct, then "stripped" oil (containing little or no vitamin E, after prolonged heating) would be a good means of inducing neuronal lipid peroxidation in culture, which should generate both measurable 4-hydroxynonenal and reduced formation of secreted APP (sAPP), along with a mysterious rise in Aβ. My best wishes go to anybody who may care to do this experiment! Let us fortify ourselves with three observations that should encourage us:

1. Safflower oil, given as 20 percent of the diet, caused learning impairment in weaned rat pups (Harman et al., 1976). When the experiment was repeated with vitamin E supplementation, no harmful effects were seen on learning. Harman's safflower oil may have been typical steam-refined oil, which has about 0.45 mg of vitamin E per gm of essential fatty acids, compared with 0.65 mg in cottonseed oil, 0.36 mg in corn oil, and a miserly 0.28 mg in soya oil (Herting and Drury, 1963). Lipid peroxidation is seen in animal experiments when the level drops below 0.6 mg. Even if Harman had used cold pressed safflower oil, an initially adequate vitamin E level would have been reduced by the deep-freeze cold-storage he mentions in his paper. More recently, Greg Cole at UCLA has found that safflower oil (source unstated) aggravates transgenically induced AD pathology in mice.

2. M K Horwitt, in the only human vitamin E deprivation trial ever done (at the Elgin mental hospital in Illinois, during the 1960s) observed increased H2O2-induced red blood cell haemolysis after giving stripped corn oil, in one phase of the trial. Such haemolysis is considered to reflect a membrane weakened by lipid peroxidation, so this test might be a good clinical test for current brain peroxidation, due to early Alzheimer's, or to current refined oil consumption at any age. Other markers of brain peroxidation include F2 isoprostanes in blood and urine, and expired air pentane or ethane, as seen in children with attention deficit hyperactivity disorder (Nutritional Neuroscience, Sept 2003)—another refined oil syndrome, arising in pregnancy and aggravated postnatally by refined oils in the child's diet.

3. In hundreds of my patients exposed to refined frying and salad oils, or oily cakes and dips, I have observed and described a typical "refined oil syndrome," consisting of short-term memory impairment, night blindness, and characteristic glare sensitivity (easily provoked with a clinical pen-torch). Vitamin E rapidly corrects the memory deficit, but fish oil is required to improve the visual symptoms.

I did a small pilot study in 1993, finding that 12 patients diagnosed with AD had all used refined oils for decades, compared with 20 controls with excellent memories, none of whom had any regular exposure to refined oils (Peers, 1993). It is time we found out what these oils can do in the laboratory!

View all comments by Robert Peers

I recommend this paper

Please see our commentary on this important study at PLoS Medicine eLetters page

View all comments by Alexei R. Koudinov

BRG1 and BRM are subunits of the SWI/SNF chromatin remodeling complex which have been implicated in the regulation of gene expression, cell cycle control, and oncogenesis.

The Liu group [1] reports that the BAF (BRG1 associated factor) complex results in promoter activation of CSF-1 and promotes Z-DNA formation. A conformational change from B-DNA to Z-DNA in the hippocampus in AD is reported by Suram et al. [2], as is increased serum CSF-1 [3]. This might lead us to expect increased BRG1 in AD, and consequently increased ROCK1.

The Emerson group [4] reports that BRG1 binds to zinc finger proteins through a unique N-terminal domain that is not present in BRM. BRM interacts with two ankyrin repeat proteins that are critical components of Notch signal transduction. SWI/SNF BRG1 complexes, but not BRM, bind to the CREB transcription factor only when CREB is phosphorylated. DYRK1A, a gene in the Down syndrome critical region, has been found to phosphorylate CREB.

The findings by the Emerson lab would seem to provide a targeted therapy in AD as well as DS. They state... Read more

BRG1 and BRM are subunits of the SWI/SNF chromatin remodeling complex which have been implicated in the regulation of gene expression, cell cycle control, and oncogenesis.

The Liu group [1] reports that the BAF (BRG1 associated factor) complex results in promoter activation of CSF-1 and promotes Z-DNA formation. A conformational change from B-DNA to Z-DNA in the hippocampus in AD is reported by Suram et al. [2], as is increased serum CSF-1 [3]. This might lead us to expect increased BRG1 in AD, and consequently increased ROCK1.

The Emerson group [4] reports that BRG1 binds to zinc finger proteins through a unique N-terminal domain that is not present in BRM. BRM interacts with two ankyrin repeat proteins that are critical components of Notch signal transduction. SWI/SNF BRG1 complexes, but not BRM, bind to the CREB transcription factor only when CREB is phosphorylated. DYRK1A, a gene in the Down syndrome critical region, has been found to phosphorylate CREB.

The findings by the Emerson lab would seem to provide a targeted therapy in AD as well as DS. They state that they can screen for molecules that block the association between chromatin remodeling complexes and the specific transcription factors with which they interact. Might this be more beneficial than statins alone, which have inhibited ROCK1?

It's interesting that amyloid-β precursor protein forms a transcriptionally active complex with the chromatin remodeling enzyme,Tip60.

References:
1. Liu R, Liu H, Chen X, Kirby M, Brown PO, Zhao K. Regulation of CSF1 promoter by the SWI/SNF-like BAF complex. Cell. 2001 Aug 10;106(3):309-18. Abstract

2. Suram A, Rao KS, Latha KS, Viswamitra MA. First evidence to show the topological change of DNA from B-dNA to Z-DNA conformation in the hippocampus of Alzheimer's brain. Neuromolecular Med. 2002;2(3):289-97. Abstract

3. Kong QL, Zhang JM, Zhang ZX, Ge PJ, Xu YJ, Mi RS, Zhao YH, Sui YP, He W. [Serum levels of macrophage colony stimulating factor in the patients with Alzheimer's disease] Zhongguo Yi Xue Ke Xue Yuan Xue Bao. 2002 Jun;24(3):298-301. Chinese. Abstract

4. http://www.salk.edu/otm/alltech.html

View all comments by Mary Reid

The role of statins in modifying both cholesterol- and isoprenoid-related Abeta production is of consierable interest, as reported here. Alternatively, however, the effects of statins on endothelial integrity and function (via increase of eNOS and decrease of Endothelin-1, e.g.) may be especially important in sporadic Alzheimer's disease. There is extensive evidence for the key role of vascular risk factors in sporadic AD; and endothelial-secreted cytokines have been shown (for example) to be important for development and division of neural stem cells. The pleiotropic effects of statins raise many possibilities regarding which of their effects on cholesterol, Abeta, or other signalling pathways may account for their effectiveness in vascular disorders, and their potential efficacy in AD may well involve more than Abeta.

References:
Breteler, M. Vascular risk factors for Alzheimer's disease: an epidemiologic perspective. Neurobiol Aging. 2000, 21:153-60. Seshadri, S. et al. Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. New Engl. J Med,2000; 346:476-483 Shen, Q et al. Endothelial cells stimulate self-renewal and expand neurogenesis of neural stem cells. Science, 2004; 304:1338-1340 Laufs, U and Liao, JK. Post-transcriptional regulation of endothelial nitric oxide synthase mRNA stability by Rho GTPase. J Biol Chem, 1998; 273: 24266-71

View all comments by David Drachman

This paper is most remarkable. The authors show that statin treatment, which has long been thought to be beneficial for Alzheimer disease patients, has two independent and diverging effects on APP processing. In a novel in-vitro system, the authors have been able to decipher the cholesterol-dependent and isoprenoid-dependent role of statins. The effects are surprisingly different. While low cholesterol reduced APP processing and Aβ generation, as expected, low isoprenoid levels enhanced intracellular accumulation of APP and its proteolytic products, including Aβ. Several recent studies have implicated a potential role of intraneuronal Aβ as an early pathological hallmark in AD patients. Together with recent reports that intracellular accumulation of Aβ is observed prior to neuronal death in APP/PS1 mouse models, one wonders whether statin treatment is indeed beneficial for Alzheimer disease patients.

View all comments by Thomas Bayer

Have you considered the possibility that a mechanism of statin action in AD may be related to its stimulatory effect on cerebral blood flow?

View all comments by James Crawford

The paper by Cole and colleagues is a very elegant manuscript because it provides important new insights into how statins might affect APP processing. The observation that inhibition of isoprenoid metabolism increases intracellular Aβ accumulation is surprising and important for the field to realize. However, the enzymes that drive isoprenoid synthesis have a very high affinity for their substrates, which means that isoprenoid synthesis remains intact even when cholesterol synthesis is partially blocked. Whether statins would actually cause this [Aβ accumulation] to occur in vivo remains an open question because statin treatment does not necessarily fully reduce cholesterol synthesis under the conditions used clinically (depending on the particular statin and dose utilized). This manuscript is also important because it elegantly defines careful methods for dissecting out the effects of cholesterol metabolism on the cell. By defining four treatment paradigms, the authors provide a roadmap for future studies into cholesterol biology.

View all comments by Benjamin Wolozin

Downregulation of clathrin-mediated intracellular transport; desensitization of receptor-mediated ester endocytosis, and RNAi antisense against cell synthesis of cholesterol could prove a powerful synergy of therapeutic treatment in this area. Decreased hydrolytic activity in lysosmes would further ensure less risk of bursting a cell (although targeting specific lysis may prove useful in overly active glial that cannot be suppressed or reverted back to inactive state).

Isoprenoids that show a detrimental role to Alzheimers onset and progression might possibly show also show neuroprotective roles in future treatment modalities. Statins, although promising, are not the miracle some people belived they were.

View all comments by Jacob Mack

I find this paper encouraging to research in the area of statins and effects on various esters, their constituents and other biochmeical markers in Alzheimers. I am curious, though, how we may be able to maximize isoprenoid activity, lower cholesterol, (possibly through further clathrin downregulation), and block signal transduction cell receptors themselves. Maybe desensitize some and sensitize others in order to further find the efficacy of statins and new emerging delivery systems of them.

Would it be fair to say that optimum lysosomal activity coupled with repressed cell uptake of cholesterol; and combined with cannabinoid-mediated lipid interference (arachidonic acid and others) of endocytotoxicity might in fact deal with many of the extra- and intracellular amyloid deposits. Then by using CB-2 mediated immune response we would partially suppress microglial activation. Then follow that up with a regiment of antioxidants, for we know that amyloid and immune cells oxidize (either immune system dependent/coupled with) so much cortical/subcortical matter, and, of course... Read more

This excellent paper very elegantly untangled the differential and independent mechanisms by which Ab production is affected by isoprenoids and cholesterol. Unfortunately, the above discussion whether statin treatment in humans could increase intracellular Ab takes us away from the main and very important finding that the isoprenoid pathway is involved in Ab generation.

As it has been pointed out in the paper and in the Q&A section above, it is experimentally possible to use statins in vitro at a concentration that shuts off HMG-CoA reductase activity. Only under these specific circumstances the isoprenoid pathway is shut down too. For a number of reasons such an approach would be incompatible with life. Animals need cholesterol to maintain functional membranes, cells continuously shed cholesterol from the plasma membrane and this cholesterol must be replenished. Contrary to popular belief, cells produce most of their cholesterol needs themselves by de-novo synthesis, only a minor part is hepatocyte- or diet-derived.

Notwithstanding the perilous consequences of... Read more

This excellent paper very elegantly untangled the differential and independent mechanisms by which Ab production is affected by isoprenoids and cholesterol. Unfortunately, the above discussion whether statin treatment in humans could increase intracellular Ab takes us away from the main and very important finding that the isoprenoid pathway is involved in Ab generation.

As it has been pointed out in the paper and in the Q&A section above, it is experimentally possible to use statins in vitro at a concentration that shuts off HMG-CoA reductase activity. Only under these specific circumstances the isoprenoid pathway is shut down too. For a number of reasons such an approach would be incompatible with life. Animals need cholesterol to maintain functional membranes, cells continuously shed cholesterol from the plasma membrane and this cholesterol must be replenished. Contrary to popular belief, cells produce most of their cholesterol needs themselves by de-novo synthesis, only a minor part is hepatocyte- or diet-derived.

Notwithstanding the perilous consequences of isoprenoid depletion, without HMG-CoA reductase activity the animal would sooner or later run out of cholesterol stores and die. Similar statin brain concentrations (0.25µM) as the minimal concentration used in the elegant in-vitro studies by Vassar had been reported in mice by Gibson Wood. These high levels were achieved by feeding 50 times the maximum clinical dose, could be maintained only for brief periods of time and steady state levels were considerably lower. Cell-culture studies define mechanisms, not therapeutic strategies. In light of the existing data, this part of the discussion is difficult to comprehend. The necessary statin dosage would have to be enormously above clinical standards before harmful accumulation of intracellular Ab occurs. That the patient would be dead by that time for other reasons shows only how unrealistic this discussion is. Like Robert Vassar, I don’t see any evidence that clinical statin dosages could possibly cause relevant intracellular Ab accumulation. In a way, millions of patients on statins give living confirmation for this year by year.

View all comments by Tobias Hartmann

The new paper raises legitimate questions regarding the potential for artifactual associations emerging from epidemiological studies. My position remains cautiously optimistic because of the faint but positive signal emerging from the Sparks et al. trial (see ARF related news story). Randomized, double-blind placebo-controlled clinical trial data trump epidemiological data every time. The size of the Sparks et al. study (<50 subjects) tempers my enthusiasm, and, like others, I await the results of the large simvastatin clinical trial that is headed by Mary Sano and the ADCS.

View all comments by Samuel Gandy

The cholesterol and statin story in AD has been a never-ending battle since its inception in the late 1980s, and the current paper sends a mixed message. It seems that if the authors exclude the final year of medications from consideration, there is no reduced hazard risk (HR), but if the final year of current statin use is included in the analysis, there is a near significant or significant (for AD with or without vascular factors) reduction in the hazard ratio. One must also consider that an individual who may have taken a statin for, say, 1 month would be included in the "ever statin use." I would suggest the take-home message may be that longer exposure to statins produces a reduced risk of AD later in life.

I am sure that the statin story with regard to treatment of AD will be sorted out by the results of LEADe and CLASP: the two large multicenter trials testing atorvastatin and simvastatin, respectively. The way to determine the effect of statins on prevention of AD (reduced risk) is to directly test for benefit in a double-blind, placebo-controlled prevention trial of... Read more

The recent epidemiological study by Rea and colleagues adds yet more complexity (and confusion) to the issue of statin use and AD risk. It’s difficult to draw any firm conclusions from the study, since the reported outcomes vary so distinctly as a function of analysis parameters. The gold standard will always be double-blind, case-controlled studies, and for good reason. The results from the statin clinical trial(s) in the pipeline will hopefully shed more light on this important issue.

The Rea study does, however, bring to light a couple of general issues (some of which have been discussed previously on Alzforum) that may or may not be resolved in the upcoming prospective clinical trials (e.g., CLASP). If statin use indeed influences AD risk, what duration of use is needed to achieve the effect? I don’t think the “ever use” versus “never use” in the Rea paper is useful in sorting this out. And perhaps more importantly, when do statins need to be taken in order to achieve proposed protection? AD pathology is known to begin years, perhaps decades, prior to cognitive symptoms.... Read more

Recently there has been much debate as to whether statin therapy offers a benefit for Alzheimer disease (AD), and whether statins reduce AD incidence and/or progression remains an open question (Jick et al., 2000; Wolozin et al., 2000; Shepherd et al., 2002; Zandi et al., 2005; Sparks et al., 2005). The prospective cohort study by Rea and colleagues is certainly interesting, and several important factors are brought into consideration, including analysis of the effects of statin use duration, the type of statin used (lipophilicity profile) and patient characteristics. Most importantly, however, this study demonstrates how analysis of the same data set in two different ways can lead to diverging conclusions. Their analysis indicates that antecedent statin use in the population of elderly patients examined was not associated with a lower risk of dementia when primary analysis incorporated a 1-year lag. However, if the data is analyzed in a way similar to that of case-controlled studies, whereby analysis was based on current statin use compared to non-use, without a lag period,... Read more

Recently there has been much debate as to whether statin therapy offers a benefit for Alzheimer disease (AD), and whether statins reduce AD incidence and/or progression remains an open question (Jick et al., 2000; Wolozin et al., 2000; Shepherd et al., 2002; Zandi et al., 2005; Sparks et al., 2005). The prospective cohort study by Rea and colleagues is certainly interesting, and several important factors are brought into consideration, including analysis of the effects of statin use duration, the type of statin used (lipophilicity profile) and patient characteristics. Most importantly, however, this study demonstrates how analysis of the same data set in two different ways can lead to diverging conclusions. Their analysis indicates that antecedent statin use in the population of elderly patients examined was not associated with a lower risk of dementia when primary analysis incorporated a 1-year lag. However, if the data is analyzed in a way similar to that of case-controlled studies, whereby analysis was based on current statin use compared to non-use, without a lag period, statin use appears somewhat beneficial in protecting against dementia. Thus, this study not only indicates that different results are obtained when a lag period is incorporated into the study design, but also ultimately questions whether statins affect the development of dementia at all.

However, it should be considered that AD is a slow and insidious disease, with abnormal physiological alterations likely preceding the clinical manifestations of the disease by several decades. Indeed, familial AD (FAD) mutations are known to elevate the levels of Aβ42 and thus, while patients with FAD likely overproduce Aβ42 from an early age, clinical symptoms are not typically observed until the third to fifth decades. In the current study, participants had a mean age of 75 years and, as discussed, it may well be the case that statin exposure much earlier in life may be required in order for these drugs to beneficially affect dementia risk. In addition to examining a younger population, it may also be of benefit to extend the lag period; given the insidious nature of AD, a year may not be sufficient for statins to beneficially affect the underlying pathophysiology.

In summary, while there is no doubt that this study provides important insights for the analysis of cohort studies, future prospective studies involving the analysis of a younger population and incorporating longer lag times in the study design would be informative. Ultimately, to provide the definitive answer to the question, are statins protective in AD?, it may be necessary to perform primary prevention studies to determine whether continuous long-term statin use in mid-life (long before the onset of dementia) could prevent or significantly delay AD.

View all comments by Sarah L. Cole
View all comments by Robert Vassar

I recommend the Primary Papers

We would like to respond to Dr. Wolozin on his disagreement with the interpretations of our results. His views focus mainly on cholesterol synthesis, when, in fact, our work suggests that changes in cholesterol synthesis are not responsible for the “cholesterol sequestration” phenotype observed in neurons challenged with Aβ during the experimental window. Although the finding that Aβ inhibited cholesterol synthesis seemed paradoxical to the intensive filipin staining, it is not unprecedented since the drug U18666A is a potent inhibitor of cholesterol synthesis and induces a similar pattern of cholesterol sequestration. Our rationale for examining SREBP-2 as the target for Aβ came from the observations that, although both Aβ and pravastatin significantly reduced cholesterol synthesis, pravastatin (at the concentration used in our study) did not cause cholesterol sequestration, nor did it cause apoptosis.

Moreover, in agreement with Dr. Wolozin’s concepts on HMGCoA and prenylation, we did not observe any significant change in protein prenylation in neurons treated with... Read more

We would like to respond to Dr. Wolozin on his disagreement with the interpretations of our results. His views focus mainly on cholesterol synthesis, when, in fact, our work suggests that changes in cholesterol synthesis are not responsible for the “cholesterol sequestration” phenotype observed in neurons challenged with Aβ during the experimental window. Although the finding that Aβ inhibited cholesterol synthesis seemed paradoxical to the intensive filipin staining, it is not unprecedented since the drug U18666A is a potent inhibitor of cholesterol synthesis and induces a similar pattern of cholesterol sequestration. Our rationale for examining SREBP-2 as the target for Aβ came from the observations that, although both Aβ and pravastatin significantly reduced cholesterol synthesis, pravastatin (at the concentration used in our study) did not cause cholesterol sequestration, nor did it cause apoptosis.

Moreover, in agreement with Dr. Wolozin’s concepts on HMGCoA and prenylation, we did not observe any significant change in protein prenylation in neurons treated with pravastatin. This suggests that the conditions of pravastatin treatment were insufficient to achieve enough HMGCoA inhibition, even though we did not measure HMGCoA reductase activity or levels. At no point in our work have we claimed that HMGCoA reductase was involved in the effects of Aβ, and we would not be confident to directly extrapolate levels of HMGCoA reductase from SREBP-2 levels; thus, we do not understand the remark about accuracy that has been made. Finally, as Dr. Wolozin suspected, SREBP-2 regulates many other enzymes of the mevalonate pathway including farnesyl diphosphate synthase, which catalyzes formation of farnesyl-PP; therefore, inhibition of SREBP-2 can be expected to have a higher impact on prenylation. We believe that our work presents strong evidence that SREBP-2 is a target of Aβ. We have identified one pathway affected by the decrease of nuclear SREBP-2, but we expect that other targets of SREBP-2 could also play important roles.

References:
Horton JD, Shah NA, Warrington JA, Anderson NN, Park SW, Brown MS, Goldstein JL. Combined analysis of oligonucleotide microarray data from transgenic and knockout mice identifies direct SREBP target genes. Proc Natl Acad Sci U S A. 2003 Oct 14;100(21):12027-32. Abstract

Horton JD, Goldstein JL, Brown MS. SREBPs: activators of the complete program of cholesterol and fatty acid synthesis in the liver. J Clin Invest. 2002 May;109(9):1125-31. Abstract

View all comments by Amany Mohamed
View all comments by Elena Posse de Chaves