This is a very interesting manuscript that is noteworthy for its proposed mechanism linking ApoE and cerebrovascular dysfunction. The authors use a combination of genetic and pharmacological manipulations to propose a link between ApoE, cyclophilin A, and MMP9 activation. Although promising, substantially more work will be necessary to determine if the pathways/targets proposed in this manuscript are suitable for the treatment of Alzheimer's and other diseases associated with ApoE4. Below are several points worthy of further consideration:

1. The work offers yet another model by which ApoE may be mediating a general neurotoxic outcome in the brain (to be added to the many others already in the literature). As ApoE is linked to several degenerative diseases, and recovery after stroke, a general mechanism as proposed by Zlokovic and colleagues is reasonable; however, there is a wide range of previous work claiming different "general" mechanisms. My fear is that ApoE4 is pleiotropic, affecting a number of cell biological mechanisms; thus, pinpointing a specific cellular...  Read more

This is a very interesting manuscript that is noteworthy for its proposed mechanism linking ApoE and cerebrovascular dysfunction. The authors use a combination of genetic and pharmacological manipulations to propose a link between ApoE, cyclophilin A, and MMP9 activation. Although promising, substantially more work will be necessary to determine if the pathways/targets proposed in this manuscript are suitable for the treatment of Alzheimer's and other diseases associated with ApoE4. Below are several points worthy of further consideration:

1. The work offers yet another model by which ApoE may be mediating a general neurotoxic outcome in the brain (to be added to the many others already in the literature). As ApoE is linked to several degenerative diseases, and recovery after stroke, a general mechanism as proposed by Zlokovic and colleagues is reasonable; however, there is a wide range of previous work claiming different "general" mechanisms. My fear is that ApoE4 is pleiotropic, affecting a number of cell biological mechanisms; thus, pinpointing a specific cellular mechanism may prove elusive. Nevertheless, the authors make a concerted effort to establish a molecular mechanism driving blood-brain barrier disruption, namely, the activation of MMP9 via cyclophilin A, dismantling of the basement membrane, and downregulation of proteins regulating endothelial tight junctions.

2. Although the genetic manipulations look compelling, caution is necessary when interpreting results from pharmacological manipulations. There is no extensive pharmacokinetics/pharmacodynamics to fully assure the reader that these molecules (cyclosporine, PDTC, and SB-3CT) are specifically acting via the mechanisms proposed.

3. The model proposed by Zlokovic and colleagues does not account for the most validated observation related to Alzheimer's and ApoE4, namely, that ApoE4 increases the risk of developing amyloid plaques. Furthermore, it has been proposed that ApoE4 carriers show a reduction in Aβ efflux. This being said, it is not unreasonable to assume that ApoE is modulating multiple areas of biology as discussed above.

View all comments by Ryan Watts

This paper represents a natural progression for the group that has clarified the effect of different isoforms of ApoE on the transporters that clear Aβ from the brain. Here, the group has made significant contributions in demonstrating how ApoE exerts its effect on the blood-brain barrier. The authors demonstrated first that the absence of ApoE or the presence of human ApoE4 in mice results in a leaky blood-brain barrier, associated with decreased levels of proteins expressed at the tight junctions, and decreased levels of collagen IV. Collagen IV is a glycoprotein present at the basement membranes, and it prevents the formation of Aβ fibrils. Apart from their clearance across the endothelium into the blood, solutes and Aβ are eliminated by perivascular drainage along cerebrovascular basement membranes (Hawkes et al., 2011) .

The authors then demonstrate that ApoE4 is associated with high expression of cyclophilin A (CypA) in pericytes and increased expression of matrix metalloproteinase 9 (MMP9). A series of very elegant in-vivo experiments, coupled with pharmacological and...  Read more

This paper represents a natural progression for the group that has clarified the effect of different isoforms of ApoE on the transporters that clear Aβ from the brain. Here, the group has made significant contributions in demonstrating how ApoE exerts its effect on the blood-brain barrier. The authors demonstrated first that the absence of ApoE or the presence of human ApoE4 in mice results in a leaky blood-brain barrier, associated with decreased levels of proteins expressed at the tight junctions, and decreased levels of collagen IV. Collagen IV is a glycoprotein present at the basement membranes, and it prevents the formation of Aβ fibrils. Apart from their clearance across the endothelium into the blood, solutes and Aβ are eliminated by perivascular drainage along cerebrovascular basement membranes (Hawkes et al., 2011) .

The authors then demonstrate that ApoE4 is associated with high expression of cyclophilin A (CypA) in pericytes and increased expression of matrix metalloproteinase 9 (MMP9). A series of very elegant in-vivo experiments, coupled with pharmacological and genetic manipulation, demonstrates that CypA, nuclear factor κB, and MMP9 are responsible for the breakdown in the BBB observed in the presence of ApoE4. Furthermore, the study demonstrates that the endothelium lipoprotein receptor responsible for clearance of Aβ is also present on pericytes. Isoforms of ApoE regulate the expression and function of lipoprotein receptor on pericytes. ApoE3 binds with high affinity to LRP1, whereas ApoE4 does not bind to pericyte LRP1, a result very similar to that observed in previous studies of the interactions of ApoE and LRP1 at the vascular level.

Recently, this group led by Berislav Zlokovic has clarified many of the physiological roles of pericytes in maintaining the integrity of the neurovascular unit (Winkler et al., 2011). The present study, through a series of important findings about how pericytes interact with ApoE and influence the integrity of the blood-brain barrier, is a major step in clarifying the factors behind the pathogenesis of neurodegenerative disorders. Pericytes may provide the motive force for the drainage of solutes from the extracellular spaces along vascular basement membranes. Defects in the function of pericytes may be associated with a failure of elimination of Aβ by lipoprotein receptors as well as by perivascular drainage. It is possible that targeting the activity of pericytes may become a therapeutic strategy in the treatment of neurodegenerative diseases.

References:
Hawkes, C. A., W. Hartig, et al. (2011). Perivascular drainage of solutes is impaired in the ageing mouse brain and in the presence of cerebral amyloid angiopathy. Acta Neuropathol 121(4): 431-443. Abstract

Winkler, E. A., R. D. Bell, et al. (2011). Central nervous system pericytes in health and disease. Nat Neurosci 14(11): 1398-1405. Abstract

View all comments by Roxana Carare
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View all comments by Roy O. Weller

Fascinating paper! I am somewhat hesitant to extrapolate its relevance directly to humans; intuitively the effects seem too large for this. But mechanistically, the findings are concordant with Nishitsuji et al. This will require confirmation, of course, but the idea and the plausible mechanism definitely warrant detailed scrutiny and extension of these studies by other labs. For instance, Boucher et al. showed that loss of LRP1 in vascular smooth muscle cells results in increased activation of MMP2 and MMP9, which fits well with the results reported here by Bell et al.

What I find further tantalizing is the link it offers to cerebral amyloid angiopathy, which occurs so frequently in ApoE4 carriers. I wonder how exactly these mechanisms might be connected. On the other hand, if human ApoE4 carriers were suffering from such a large degree of blood-brain barrier (BBB) leakage, would one not expect this to manifest itself clinically in a more prominent manner? Perhaps the effect in humans is smaller than in the mouse? On the other hand, an increased incidence of glomerular...  Read more

Fascinating paper! I am somewhat hesitant to extrapolate its relevance directly to humans; intuitively the effects seem too large for this. But mechanistically, the findings are concordant with Nishitsuji et al. This will require confirmation, of course, but the idea and the plausible mechanism definitely warrant detailed scrutiny and extension of these studies by other labs. For instance, Boucher et al. showed that loss of LRP1 in vascular smooth muscle cells results in increased activation of MMP2 and MMP9, which fits well with the results reported here by Bell et al.

What I find further tantalizing is the link it offers to cerebral amyloid angiopathy, which occurs so frequently in ApoE4 carriers. I wonder how exactly these mechanisms might be connected. On the other hand, if human ApoE4 carriers were suffering from such a large degree of blood-brain barrier (BBB) leakage, would one not expect this to manifest itself clinically in a more prominent manner? Perhaps the effect in humans is smaller than in the mouse? On the other hand, an increased incidence of glomerular nephropathy has also been reported to be associated with ApoE4, raising the possibility that the ApoE4 effect at the BBB may extend to the related mesangial cells in the kidney glomerulus.

References:
Nishitsuji K, Hosono T, Nakamura T, Bu G, Michikawa M. Apolipoprotein E regulates the integrity of tight junctions in an isoform-dependent manner in an in vitro blood-brain barrier model. J Biol Chem. 2011 May 20;286(20):17536-42. Abstract

Boucher P, Gotthardt M, Li WP, Anderson RG, Herz J. LRP: role in vascular wall integrity and protection from atherosclerosis. Science. 2003 Apr 11;300(5617):329-32. Abstract

View all comments by Joachim Herz

ApoE, Microvascular Injury, and Blood-Brain Barrier Compromise in Sporadic (Late-Onset) Alzheimer’s Disease: A Shining New Light for Therapeutic Intervention
Alzheimer’s disease (AD) is a genetically diverse spectrum of disorders that includes both familial and sporadic forms (1). The familial forms of the disease are seen in less than 10 percent of cases, and are associated with mutations on chromosomes 21 (amyloid precursor protein) (2-4), 14 (presenilin I) (5-7), and 1 (presenilin II) (8-9). Patients generally present with symptoms of cognitive impairment at an early age, have a rapidly progressive course, and exhibit severe pathologic alterations in their brains. Patients with the more common late-onset sporadic form of the disease (90 percent) are likely to be homozygous for the ApoE4 gene on chromosome 19, which codes for the high-density lipoprotein ApoE4 (10). Such patients typically exhibit symptoms of cognitive impairment later in life, have a more slowly progressive clinical course, and a variable degree of brain AD pathology. Despite the unequivocal...  Read more

ApoE, Microvascular Injury, and Blood-Brain Barrier Compromise in Sporadic (Late-Onset) Alzheimer’s Disease: A Shining New Light for Therapeutic Intervention
Alzheimer’s disease (AD) is a genetically diverse spectrum of disorders that includes both familial and sporadic forms (1). The familial forms of the disease are seen in less than 10 percent of cases, and are associated with mutations on chromosomes 21 (amyloid precursor protein) (2-4), 14 (presenilin I) (5-7), and 1 (presenilin II) (8-9). Patients generally present with symptoms of cognitive impairment at an early age, have a rapidly progressive course, and exhibit severe pathologic alterations in their brains. Patients with the more common late-onset sporadic form of the disease (90 percent) are likely to be homozygous for the ApoE4 gene on chromosome 19, which codes for the high-density lipoprotein ApoE4 (10). Such patients typically exhibit symptoms of cognitive impairment later in life, have a more slowly progressive clinical course, and a variable degree of brain AD pathology. Despite the unequivocal association between ApoE4 and late-onset sporadic AD, the mechanism(s) through which ApoE4 contributes to the pathogenesis of sporadic AD remain(s) elusive.

Numerous brain imaging studies by SPECT, CT, PET, and MRI have documented a preferential decrease in cerebral blood flow to brain areas affected by AD, as well as an increase in small vessel disease in Alzheimer's patients (11-18). Microvascular disease is a common finding at autopsy in the brains of elderly patients, and significant microvascular pathology has been extensively described in AD (19-25). Various components of the fragmented vascular basement membrane are found within senile (neuritic) plaques, raising the question of whether plaque formation and microvascular pathology are somehow closely linked (26-30). Previous studies by our group and others have documented that agrin, the major heparan-sulfate proteoglycan component of the cerebral capillary basement membrane, becomes fragmented in sporadic AD, compromising microvascular structural integrity (31-34). We have also demonstrated that this structural damage is greater in AD patients with the ApoE4 genotype, and correlates with the appearance of serum-derived proteins in the brain, presumably due to a defective blood-brain barrier (35-36).

Thus far, evidence supporting a derangement in blood-brain barrier integrity in AD has been derived from clinical studies using the CSF/serum protein ratios of albumin, haptoglobin, and IgG. These studies can be roughly divided into two groups: those finding no evidence of a blood-brain barrier defect in AD (37-40), and those concluding that there is a significant compromise in blood-brain barrier integrity (41-45). Problems with experimental design may account for some of these discrepancies. Sample sizes were often limited to a very small number of patients. Most of the earlier studies failed to consider the severity of AD as a significant variable in their analyses, combining patients with both early and advanced disease into the same AD cohort. Clinical criteria for the diagnosis of AD were often vaguely defined. A trend for improvement in study design is evident in the three most recent studies, which have all concluded that blood-brain barrier integrity is clearly compromised in AD patients (43-45).

This landmark paper by Bell et al. demonstrates, through an elegant series of experiments in genetically altered mice, that expression of human ApoE4 and lack of murine ApoE leads to BBB breakdown by activating a proinflammatory CypA-nuclear factor-κB-matrix-metalloproteinase-9 pathway in pericytes (46). This then leads to neuronal uptake of multiple blood-derived neurotoxic proteins, and microvascular and cerebral blood flow reductions. The potential therapeutic relevance of these animal model investigations is strongly supported by prior studies using postmortem brain tissue from Alzheimer's patients, generously provided by their families.

Aging and brain trauma in human patients may both impair the BBB (perhaps synergistically) through the exact mechanisms described in this exciting report, setting into motion a cascade of pathologic processes that destabilize brain fluid homeostasis and lead to cognitive decline. Information gained from these experiments may lead to earlier identification and therapeutic intervention. Pharmacologic and epigenetic manipulations, related to preserving the neurovascular unit and BBB, clearly represent an exciting new approach for reducing the onset and progression of dementia in sporadic AD patients.

References:
1. Davies P. The genetics of Alzheimer's disease: a review and a discussion of the implications. Neurobiol Aging 1986; 7: 459-66. Abstract

2. Tanzi RE, Gusella JF, Watkins PC, Bruns GA, St George-Hyslop P, Van Keuren ML, et al. Amyloid beta protein gene: cDNA, mRNA distribution, and genetic linkage near the Alzheimer locus. Science 1987;235:880-4. Abstract

3. Kang J, Lemaire HG, Unterbeck A, Salbaum JM, Masters CL, Grzeschik KH, et al. The precursor of Alzheimer's disease amyloid A4 protein resembles a cell-surface receptor. Nature 1987;325:733-6. Abstract

4. Robakis NK, Wisniewski HM, Jenkins EC, Devine-Gage EA, Houck GE, Yao XL, et al. Chromosome 21q21 sublocalisation of gene encoding beta-amyloid peptide in cerebral vessels and neuritic (senile) plaques of people with Alzheimer disease and Down syndrome. Lancet 1987;1:384-5. Abstract

5. Campion D, Flaman JM, Brice A, Hannequin D, Dubois B, Martin C, et al. Mutations of the presenilin I gene in families with early-onset Alzheimer's disease. Hum Mol Genet 1995;4:2373-7. Abstract

6. Cruts M, Backhovens H, Wang SY, Van Gassen G, Theuns J, De Jonghe CD, et al. Molecular genetic analysis of familial early-onset Alzheimer's disease linked to chromosome 14q24.3. Hum Mol Genet 1995;4:2363-71. Abstract

7. Sherrington R, Rogaev EI, Liang Y, Rogaeva EA, Levesque G, Ikeda M, Chi, H., et al. Cloning of a gene bearing missense mutations in early-onset familial Alzheimer's disease. Nature 1995;375:754-60. Abstract

8. Cruts M, Hendriks L, Van Broeckhoven C. The presenilin genes: a new gene family involved in Alzheimer disease pathology. Hum Mol Genet 1996;5 Spec No:1449-55. Abstract

9. Kovacs DM, Fausett HJ, Page KJ, Kim TW, Moir RD, Merriam DE, et al. Alzheimer-associated presenilins 1 and 2: neuronal expression in brain and localization to intracellular membranes in mammalian cells. Nat Med 1996;2:224-9. Abstract

10. Corder EH, Saunders AM, Strittmatter WJ, Schmechel DE, Gaskell PC, Small GW, et al. Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer's disease in late onset families. Science 1993;261:921-3. Abstract

11. Fujii K, Sadoshima S, Okada Y, Yao H, Kuwabara Y, Ichiya Y, et al. Cerebral blood flow and metabolism in normotensive and hypertensive patients with transient neurologic deficits. Stroke 1990;21:283-90. Abstract

12. Nakane H, Ibayashi S, Fujii K, Irie K, Sadoshima S, Fujishima M. Cerebral blood flow and metabolism in hypertensive patients with cerebral infarction. Angiology 1995;46:801-10. Abstract

13. Nobili F, Rodriguez G, Marenco S, De Carli F, Gambaro M, Castello C, et al. Regional cerebral blood flow in chronic hypertension. A correlative study. Stroke 1993;24:1148-53. Abstract

14. Eberling JL, Jagust WJ, Reed BR, Baker MG. Reduced temporal lobe blood flow in Alzheimer's disease. Neurobiol Aging 2000 1992;13:483-91. Abstract

15. DeKosky ST, Shih WJ, Schmitt FA, Coupal J, Kirkpatrick C. Assessing utility of single photon emission computed tomography (SPECT) scan in Alzheimer disease: correlation with cognitive severity. Alzheimer Dis Assoc Disord 1990;4:14-23. Abstract

16. Scheltens P, Barkhof F, Valk J, Algra PR, van der Hoop RG, Nauta J, et al. White matter lesions on magnetic resonance imaging in clinically diagnosed Alzheimer's disease. Evidence for heterogeneity. Brain Res 1992;115:735-48. Abstract

17. Doddy RS, Massman PJ, Mawad M, Nance M. Cognitive consequences of subcortical magnetic resonance imaging changes in Alzheimer's disease: comparison to small vessel ischemic vascular dementia. Neuropsychiatry Neuropsychol Behav Neurol 1998;11:191-9. Abstract

18. Skoog I, Lernfelt B, Landahl S, Palmertz B, Andreasson LA, Nilsson L, et al. 15-year longitudinal study of blood pressure and dementia. Lancet 1996:1141-5. Abstract

19. Buee L, Hof PR, Delacourte A. Brain microvascular changes in Alzheimer's disease and other dementias. Ann N Y Acad Sci 1997;826::7-24. Abstract

20. Perry G, Smith MA, McCann CE, Siedlak SL, Jones PK, Friedland RP. Cerebrovascular muscle atrophy is a feature of Alzheimer's disease. Brain Res 1998;791:63-6. Abstract

21. Mancardi GL, Perdelli F, Rivano C, Leonardi A, Bugiani O. Thickening of the basement membrane of cortical capillaries in Alzheimer's disease. Acta Neuropathol (Berl) 1980;49:79-83. Abstract

22. Thomas T, Thomas G, McLendon C, Sutton T, Mullan M. beta-Amyloid-mediated vasoactivity and vascular endothelial damage. Nature 1996;380:168-71. Abstract

23. Claudio L. Ultrastructural features of the blood-brain barrier in biopsy tissue from Alzheimer's disease patients. Acta Neuropathol (Berl) 1996;91:6-14. Abstract

24. Mancardi GL, Tabaton M, Liwnicz BH. Endothelial mitochondrial content of cerebral cortical capillaries in Alzheimer's disease. An ultrastructural quantitative study. Eur Neurol 1985;24:49-52. Abstract

25. Perlmutter LS, Myers MA, Barron E. Vascular basement membrane components and the lesions of Alzheimer's disease: light and electron microscopic analyses. Microsc Res Tech 1994;28:204-15. Abstract

26. Perry G, Siedlak SL, Richey P, Kawai M, Cras P, Kalaria RN, et al. Association of heparan sulfate proteoglycan with the neurofibrillary tangles of Alzheimer's disease. J Neurosci 1991;11:3679-83. Abstract

27. Snow AD, Mar H, Nochlin D, Kimata K, Kato M, Suzuki S, et al. The presence of heparan sulfate proteoglycans in the neuritic plaques and congophilic angiopathy in Alzheimer's disease. Am J Pathol 1988;133:456-63. Abstract

28. Roll FJ, Madri JA, Albert J, Furthmayr H. Codistribution of collagen types IV and AB2 in basement membranes and mesangium of the kidney. an immunoferritin study of ultrathin frozen sections. J Cell Biol 1980;85:597-616. Abstract

29. Murtomaki S, Risteli J, Risteli L, Koivisto UM, Johansson S, Liesi P. Laminin and its neurite outgrowth-promoting domain in the brain in Alzheimer's disease and Down's syndrome patients. J Neurosci Res 1992;32:261-73. Abstract

30. McGeer PL, Zhu SG, Dedhar S. Immunostaining of human brain capillaries by antibodies to very late antigens. J Neuroimmunol 1990.;26:213-8. Abstract

31. Berzin TM, Zipser BD, Rafii MS, Kuo-Leblanc V, Yancopoulos GD, Glass DJ, et al. Agrin and microvascular damage in Alzheimer's disease. Neurobiol Aging 2000 2002;21(Mar-Apr):349-55. Abstract

32. Donahue JE, Berzin TM, Rafii MS, Glass DJ, Yancopoulos GD, Fallon JR, et al. Agrin in Alzheimer's disease: altered solubility and abnormal distribution within microvasculature and brain parenchyma. Proc Natl Acad Sci U S A 1999;96:6468-72. Abstract

33. Cotman SL, Halfter W, Cole GJ. Agrin binds to beta-amyloid (Abeta), accelerates abeta fibril formation, and is localized to Abeta deposits in Alzheimer's disease brain. Mol Cell Neurosci 2002; Feb;15((2)):183-98. Abstract

34. Verbeek MM, Otte-Holler I, van den Born J, van den Heuvel LP, David G, Wesseling P, et al. Agrin is a major heparan sulfate proteoglycan accumulating in Alzheimer's disease brain. Am J Pathol 1999;155:2115-25. Abstract

35. Salloway S, Gur T, Berzin T, Tavares R, Zipser B, Correia S, Hovanesian V, Fallon J, Kuo-Leblanc V, Glass D, Hulette C, Rosenberg C, Vitek M, Stopa E. Effect of ApoE genotype on microvascular basement membrane in Alzheimer's disease. J Neurol Sci 2002; 203-204: 183-7. Abstract

36. Zipser BD, Johanson , CE, Gonzalez L, Berzin TM, Tavares R, Hulette CM, Vitek MP, Hovanesian V, Stopa EG. Microvascular injury and blood-brain barrier leakage in Alzheimer’s disease. Neurobiol Aging 2006; 7; 977-986. Abstract

37. Leonardi A, Gandolfo C, Caponnetto C, Arata L, Vecchia R. The integrity of the blood-brain barrier in Alzheimer's type and multi-infarct dementia evaluated by the study of albumin and IgG in serum and cerebrospinal fluid. J Neurol Sci 1985;67:253-61. Abstract

38. Kay AD, May C, Papadopoulos NM, Costello R, Atack JR, Luxenberg JS, et al. CSF and serum concentrations of albumin and IgG in Alzheimer's disease. Neurobiol Aging 2000 1987;8:21-5. Abstract

39. Frolich L, Kornhuber J, Ihl R, Fritze J, Maurer K, Riederer P. Integrity of the blood-CSF barrier in dementia of Alzheimer type: CSF/serum ratios of albumin and IgG. Eur Arch Psychiatry Clin Neurosci 1991;240::363-6. Abstract

40. Mecocci P, Parnetti L, Reboldi GP, Santucci C, Gaiti A, Ferri C, et al. Blood-brain-barrier in a geriatric population: barrier function in degenerative and vascular dementias. Acta Neurol Scand 1991;84:210-3. Abstract

41. Alafuzoff I, Adolfsson R, Bucht G, Winblad B. Albumin and immunoglobulin in plasma and cerebrospinal fluid, and blood-cerebrospinal fluid barrier function in patients with dementia of Alzheimer type and multi-infarct dementia. J Neurol Sci 1983;60:465-72. Abstract

42. Blennow K, Wallin A, Fredman P, Karlsson I, Gottfries CG, Svennerholm L. Blood-brain barrier disturbance in patients with Alzheimer's disease is related to vascular factors. Acta Neurol Scand 1990;81:323-6. Abstract

43. Hampel H, Muller-Spahn F, Berger C, Haberl A, Ackenheil M, Hock C. Evidence of blood-cerebrospinal fluid-barrier impairment in a subgroup of patients with dementia of the Alzheimer type and major depression: a possible indicator for immunoactivation. Dementia 1995;6:348-54. Abstract

44. Skoog I, Wallin A, Fredman P, Hesse C, Aevarsson O, Karlsson I, et al. A population study on blood-brain barrier function in 85-year-olds: relation to Alzheimer's disease and vascular dementia. Neurology 1998;50:966-71. Abstract

45. Wada H. Blood-brain barrier permeability of the demented elderly as studied by cerebrospinal fluid-serum albumin ratio. Intern Med 1998;37:509-13. Abstract

46. Bell RD, Winkler EA, Singh I, Sagare AP, Deane R, Wu Z, Holtzman DM, Betsholtz C, Armulik A, Sallstrom J, Berk BC, Zlokovic BV. Apolipoprotein E controls cerebrovascular integrity via cyclophilin A. Nature. 2012 May 17.

View all comments by Edward G. Stopa

This paper by Bell and colleagues reports exciting findings that suggest novel mechanisms underlying the role of ApoE genotype in neurodegeneration. They implicate ApoE4 in the breakdown of blood-brain barrier (BBB) integrity, an effect that is mediated by cyclophilin A. The compromised BBB appears to facilitate accumulation of blood-derived neurotoxic proteins, including fibrin, hemosiderin, and thrombin in ApoE4 mice. The authors delineate the temporal course of these changes and provide evidence that vascular dysfunction as reflected in disruption of the BBB precedes neuronal dysfunction in ApoE-negative and ApoE4 mice. These findings provide novel insights into the role of ApoE genotype in provoking neuronal dysfunction/synaptic failure. While extrapolating findings from animal models to humans is fraught with many a broken promise, it is tempting to speculate on the potential implications for Alzheimer’s disease.

These results may offer a mechanistic explanation for the observations that cognitively normal individuals who are ApoE4 carriers show evidence for early...  Read more

This paper by Bell and colleagues reports exciting findings that suggest novel mechanisms underlying the role of ApoE genotype in neurodegeneration. They implicate ApoE4 in the breakdown of blood-brain barrier (BBB) integrity, an effect that is mediated by cyclophilin A. The compromised BBB appears to facilitate accumulation of blood-derived neurotoxic proteins, including fibrin, hemosiderin, and thrombin in ApoE4 mice. The authors delineate the temporal course of these changes and provide evidence that vascular dysfunction as reflected in disruption of the BBB precedes neuronal dysfunction in ApoE-negative and ApoE4 mice. These findings provide novel insights into the role of ApoE genotype in provoking neuronal dysfunction/synaptic failure. While extrapolating findings from animal models to humans is fraught with many a broken promise, it is tempting to speculate on the potential implications for Alzheimer’s disease.

These results may offer a mechanistic explanation for the observations that cognitively normal individuals who are ApoE4 carriers show evidence for early neuronal dysfunction/synaptic failure (1,2). More recently, ApoE4 carriers were found to be especially susceptible to neurotoxic adverse effects observed in patients in a clinical trial of a humanized monoclonal antibody against amyloid-β. The spectrum of imaging abnormalities in these individuals includes vasogenic edema, sulcal effusions, microhemorrhages, and hemosiderin deposits (3). Whether or not the findings reported by Bell and colleagues will eventually lead to the identification of therapeutic targets against neuronal dysfunction or neurotoxicity in at-risk individuals remains to be seen.

References:
1. Thambisetty M, Beason-Held L, An Y, Kraut MA, Resnick SM. APOE epsilon4 genotype and longitudinal changes in cerebral blood flow in normal aging. Arch Neurol. 2010 Jan;67(1):93-8. Abstract

2. Reiman EM, Chen K, Alexander GE, Caselli RJ, Bandy D, Osborne D, Saunders AM, Hardy J. Functional brain abnormalities in young adults at genetic risk for late-onset Alzheimer's dementia. Proc Natl Acad Sci U S A. 2004 Jan 6;101(1):284-9. Abstract

3. Sperling R, Salloway S, Brooks DJ, Tampieri D, Barakos J, Fox NC, Raskind M, Sabbagh M, Honig LS, Porsteinsson AP, Lieberburg I, Arrighi HM, Morris KA, Lu Y, Liu E, Gregg KM, Brashear HR, Kinney GG, Black R, Grundman M. Amyloid-related imaging abnormalities in patients with Alzheimer's disease treated with bapineuzumab: a retrospective analysis. Lancet Neurol. 2012 Mar;11(3):241-9. Abstract

View all comments by Madhav Thambisetty