Transthyretin is an abundant blood protein that binds and transports thyroid hormones. It has been known for a number of years that transthyretin can also bind the β amyloid peptide (Aβ) associated with Alzheimer disease. Both in vitro studies and in vivo studies using the nematode worm C. elegans have shown that transthyretin can inhibit the aggregation of Aβ into insoluble amyloid fibers. This study by Buxbaum et al. uses transgenic mouse models to demonstrate that increased expression of transthyretin can protect transgenic mice from behavioral deficits caused by Aβ expression, and loss of transthyretin expression exacerbates these behavioral deficits. These studies support the idea that transthyretin might have a natural role as a chaperone protein for Aβ, serving to combat the aggregation of Aβ into amyloid or some other toxic form.

Could manipulation of transthyretin expression in people help protect them from Alzheimer disease? This is a tricky question, because paradoxically transthyretin itself is associated with amyloid disease. Familial amyloid...  Read more

Transthyretin is an abundant blood protein that binds and transports thyroid hormones. It has been known for a number of years that transthyretin can also bind the β amyloid peptide (Aβ) associated with Alzheimer disease. Both in vitro studies and in vivo studies using the nematode worm C. elegans have shown that transthyretin can inhibit the aggregation of Aβ into insoluble amyloid fibers. This study by Buxbaum et al. uses transgenic mouse models to demonstrate that increased expression of transthyretin can protect transgenic mice from behavioral deficits caused by Aβ expression, and loss of transthyretin expression exacerbates these behavioral deficits. These studies support the idea that transthyretin might have a natural role as a chaperone protein for Aβ, serving to combat the aggregation of Aβ into amyloid or some other toxic form.

Could manipulation of transthyretin expression in people help protect them from Alzheimer disease? This is a tricky question, because paradoxically transthyretin itself is associated with amyloid disease. Familial amyloid polyneuropathy, a fatal disease, is caused by mutations in transthyretin that cause the transthyretin protein itself to form amyloid. Normal (not mutated) transthyretin can also form amyloid deposits in the heart and brain, as is observed in cases of systemic senile amyloidosis. Interestingly, small heat shock proteins, classic chaperone proteins that can inhibit Aβ from forming amyloid, also form insoluble deposits by themselves under appropriate conditions. Perhaps proteins evolved to interact with aggregation-prone proteins become predisposed to aggregate themselves. These considerations suggest that manipulation of the expression transthyretin (or other putative Aβ chaperone proteins) might be therapeutic, but might require careful titration of the expression of these proteins. This study also raises the possibility that reduced expression of transthyretin might be a risk factor for developing Alzheimer disease.

View all comments by Chris Link

Transthyretin (TTR) is a blood and cerebrospinal fluid (CSF) carrier protein for thyroxine and retinol (in association with the retinol-binding protein). In the last few years an increasing number of reports have linked TTR to Alzheimer disease (AD). Specifically, TTR has been suggested as a neuroprotective factor for disease progression, given its ability to sequester and clear the amyloid-β peptide (Aβ) out of the brain.

This article generally confirms the previous reports for a role of TTR in AD. The study shows that 1) in the absence of TTR there is increased amyloid load in the brain of APP transgenic mice; 2) overexpression of 90 copies of the human TTR gene in APP transgenic mice decreases amyloid load; 3) TTR overexpression in APP transgenic mice reverts the cognitive impairment normally observed in this animal model of AD. Of note, this study confirms a previous one (1) in which the absence of TTR was shown to accelerate the memory decline normally associated with age. This may be related to a TTR function that is ”independent of its interaction with Aβ,” as...  Read more

Transthyretin (TTR) is a blood and cerebrospinal fluid (CSF) carrier protein for thyroxine and retinol (in association with the retinol-binding protein). In the last few years an increasing number of reports have linked TTR to Alzheimer disease (AD). Specifically, TTR has been suggested as a neuroprotective factor for disease progression, given its ability to sequester and clear the amyloid-β peptide (Aβ) out of the brain.

This article generally confirms the previous reports for a role of TTR in AD. The study shows that 1) in the absence of TTR there is increased amyloid load in the brain of APP transgenic mice; 2) overexpression of 90 copies of the human TTR gene in APP transgenic mice decreases amyloid load; 3) TTR overexpression in APP transgenic mice reverts the cognitive impairment normally observed in this animal model of AD. Of note, this study confirms a previous one (1) in which the absence of TTR was shown to accelerate the memory decline normally associated with age. This may be related to a TTR function that is ”independent of its interaction with Aβ,” as recognized by Buxbaum et al.

This last observation points to a function of TTR in behavior that may be unrelated to its ability to sequester Aβ and prevent Aβ deposition. Therefore, the role of TTR in preventing Aβ deposition may not be connected to the cognitive performance improvement observed in APP transgenic mice overexpressing TTR.

As for the role of TTR in preventing amyloid deposition, shown in at least two studies (this one and [2]), it is of relevance to discuss the origin of TTR within the brain. Within the brain, TTR expression is restricted to the choroid plexus (from where it is secreted towards the CSF) (2) and the meninges (4). It is therefore important to clarify whether the overexpression of TTR (90 copies of the gene) in mice originates the synthesis of the protein in other, “non-natural” sites of the brain parenchyma, which may be misleading in interpreting the role of TTR in AD.

TTR, among other CSF proteins (cystatin C, apolipoprotein J, and insulin growth factor 1, [5-7]) is reported to be protective in AD, not only by sequestering Aβ from reaching concentrations that may promote deposition as amyloid, but also by facilitating Aβ clearance out of the brain through receptors located both in the choroid plexus (7) and in the endothelial cells of the blood-brain barrier (8). It is therefore reasonable to suggest that increasing the levels of these proteins might be a therapeutic approach in AD. However, this possibility raises main concerns, of which two should certainly be investigated carefully. First, all these proteins have well-described physiological functions, some of which relate to behavior. Increasing their concentrations may pose health risks higher than the potential benefit for AD. Second, it is necessary to further study whether and how these CSF proteins can successfully reach the major brain sites of amyloid deposition in AD.

References:
1. Sousa JC, Marques F, Dias-Ferreira E, Cerqueira JJ, Sousa N, Palha JA. Transthyretin influences spatial reference memory. Neurobiol Learn Mem. 2007 Oct;88(3):381-5. Abstract

2. Choi SH, Leight SN, Lee VM, Li T, Wong PC, Johnson JA, Saraiva MJ, Sisodia SS. Accelerated Abeta deposition in APPswe/PS1deltaE9 mice with hemizygous deletions of TTR (transthyretin). J Neurosci. 2007 Jun 27;27(26):7006-10. Abstract

3. Sousa JC, Cardoso I, Marques F, Saraiva MJ, Palha JA. Transthyretin and Alzheimer's disease: where in the brain? Neurobiol Aging. 2007 May;28(5):713-8. Abstract

4. Blay P, Nilsson C, Owman C, Aldred A, Schreiber G. Transthyretin expression in the rat brain: effect of thyroid functional state and role in thyroxine transport. Brain Res. 1993 Dec 31;632(1-2):114-20. Abstract

5. Mi W, Pawlik M, Sastre M, Jung SS, Radvinsky DS, Klein AM, Sommer J, Schmidt SD, Nixon RA, Mathews PM, Levy E. Cystatin C inhibits amyloid-beta deposition in Alzheimer's disease mouse models. Nat Genet. 2007 Dec;39(12):1440-2. Abstract

6. Bell RD, Sagare AP, Friedman AE, Bedi GS, Holtzman DM, Deane R, Zlokovic BV. Transport pathways for clearance of human Alzheimer's amyloid beta-peptide and apolipoproteins E and J in the mouse central nervous system. J Cereb Blood Flow Metab. 2007 May;27(5):909-18. Abstract

7. Carro E, Spuch C, Trejo JL, Antequera D, Torres-Aleman I. Choroid plexus megalin is involved in neuroprotection by serum insulin-like growth factor I. J Neurosci. 2005 Nov 23;25(47):10884-93. Abstract

8. Deane R, Zlokovic BV. Role of the blood-brain barrier in the pathogenesis of Alzheimer's disease. Curr Alzheimer Res. 2007 Apr;4(2):191-7. Abstract

View all comments by Joao Sousa

This paper shows that overexpression of wild-type human transthyretin (TTR) in APP transgenic mice ameliorates Aβ amyloid deposition and improves cognitive function. Targeted silencing of the mouse endogenous TTR gene accelerated the development of the neuropathologic phenotype, confirming recent reports of enhanced TTR expression in the brain of APP transgenic mice and enhanced Aβ amyloid deposition in these mice lacking TTR. Using in vitro techniques, a direct binding between TTR and Aβ is shown, extending previous in vitro studies by Alexander L. Scharzman and Dmitry Goldgaber that showed that binding of TTR to Aβ results in decreased amyloid formation.

While the precise molecular nature of the transthyretin-binding species of Aβ was not defined, the data show that tetrameric TTR binds aggregated Aβ. The findings suggest that a physical interaction between TTR and Aβ prevents the toxicity and plaque formation by interfering with aggregation of Aβ species larger than monomers. While the endogenous protein most likely has an ongoing role in prevention of amyloid formation,...  Read more

This paper shows that overexpression of wild-type human transthyretin (TTR) in APP transgenic mice ameliorates Aβ amyloid deposition and improves cognitive function. Targeted silencing of the mouse endogenous TTR gene accelerated the development of the neuropathologic phenotype, confirming recent reports of enhanced TTR expression in the brain of APP transgenic mice and enhanced Aβ amyloid deposition in these mice lacking TTR. Using in vitro techniques, a direct binding between TTR and Aβ is shown, extending previous in vitro studies by Alexander L. Scharzman and Dmitry Goldgaber that showed that binding of TTR to Aβ results in decreased amyloid formation.

While the precise molecular nature of the transthyretin-binding species of Aβ was not defined, the data show that tetrameric TTR binds aggregated Aβ. The findings suggest that a physical interaction between TTR and Aβ prevents the toxicity and plaque formation by interfering with aggregation of Aβ species larger than monomers. While the endogenous protein most likely has an ongoing role in prevention of amyloid formation, its concentration may not be sufficient under pathological conditions that favor amyloid formation. It is suggested that increasing cerebral TTR synthesis is a potential therapeutic/prophylactic approach to human Alzheimer disease. However, induction of expression of the full-length protein may prove to have negative effects, especially because wild-type TTR can form amyloid fibrils. It is more likely that for therapeutic purposes, a biologically active peptidomimetic compound with the Aβ-binding properties of TTR can be designed.

It is of special interest that potentially amyloidogenic proteins can bind to each other and inhibit amyloid fibril formation. Aβ has a high tendency to form amyloidogenic aggregations, and the formation of amyloid fibrils is inhibited by binding to the tetrameric form of wild-type TTR. Unlike TTR, only a Leu68Gln variant of cystatin C can form amyloid fibrils. However, both wild-type and variant cystatin C bind monomeric soluble Aβ and inhibit Aβ oligomerization and fibril formation. Future studies will show whether cerebral or systemic amyloidoses can be halted or prevented by modulation of expression of another amyloidogenic protein, or more likely by a drug that will be developed to mimic the function of such a protein.

View all comments by Efrat Levy

Transthyretin (TTR) interaction with Aβ in the CSF has been known at least since 1994 when Schwarzman and colleagues (Schwarzman et al., 1994) concluded that TTR was the major Aβ binding protein in the CSF, observing a decrease in the aggregation state of the peptide. Two years later, the same group confirmed the inhibitory effect of TTR on Aβ formation and consequent reduction in its toxicity (Schwarzman et al., 1996). Later on, the same group of researchers performed in vitro studies using different TTR mutations and concluded on the differential binding (i.e., physical interaction) and inhibition of Aβ aggregation by those variants to (Schwarzman et al., 2004). At this point, the characterization of the interaction between the two molecules was missing.

The work by Buxbaum and coworkers further explores the protective role of TTR using animal models, but does not unravel mechanisms behind the observed protection; details on the physical interaction between the two molecules are still missing.

A recent report by Costa et al., FEBS Letters, provides a Kd for the WT TTR...  Read more

Transthyretin (TTR) interaction with Aβ in the CSF has been known at least since 1994 when Schwarzman and colleagues (Schwarzman et al., 1994) concluded that TTR was the major Aβ binding protein in the CSF, observing a decrease in the aggregation state of the peptide. Two years later, the same group confirmed the inhibitory effect of TTR on Aβ formation and consequent reduction in its toxicity (Schwarzman et al., 1996). Later on, the same group of researchers performed in vitro studies using different TTR mutations and concluded on the differential binding (i.e., physical interaction) and inhibition of Aβ aggregation by those variants to (Schwarzman et al., 2004). At this point, the characterization of the interaction between the two molecules was missing.

The work by Buxbaum and coworkers further explores the protective role of TTR using animal models, but does not unravel mechanisms behind the observed protection; details on the physical interaction between the two molecules are still missing.

A recent report by Costa et al., FEBS Letters, provides a Kd for the WT TTR and soluble Aβ interaction, and goes further, showing that TTR also binds to oligomeric and fibrillar Aβ with similar affinities; most interestingly, it is demonstrated that besides inhibiting Aβ aggregation, TTR is able to disaggregate the peptide fibrils in vitro, opening new perspectives on the role of TTR in Aβ deposition. Regarding TTR variants, the intensity of binding inversely correlates with the amyloidogenic potential (TTR T119M > WT > V30M > Y78F > L55P), immediately drawing attention to TTR protein stability and Aβ binding. The mechanism underlying TTR protection in Aβ toxicity is largely unknown.

References:
Schwarzman AL, Gregori L, Vitek MP, Lyubski S, Strittmatter WJ, Enghilde JJ, Bhasin R, Silverman J, Weisgraber KH, Coyle PK. Transthyretin sequesters amyloid beta protein and prevents amyloid formation. Proc Natl Acad Sci U S A. 1994 Aug 30;91(18):8368-72. Abstract

Schwarzman AL, Goldgaber D. Interaction of transthyretin with amyloid beta-protein: binding and inhibition of amyloid formation. Ciba Found Symp. 1996;199:146-60; discussion 160-4. Abstract

Schwarzman AL, Tsiper M, Wente H, Wang A, Vitek MP, Vasiliev V, Goldgaber D. Amyloidogenic and anti-amyloidogenic properties of recombinant transthyretin variants. Amyloid. 2004 Mar;11(1):1-9. Abstract

Costa R, Gonçalves A, Saraiva MJ, Cardoso I. Transthyretin binding to A-Beta peptide - Impact on A-Beta fibrillogenesis and toxicity. FEBS Lett. 2008 Mar 19;582(6):936-42. Abstract

View all comments by Isabel Cardoso

This paper shows the generation of a novel model of cerebral (non-Aβ) amyloid deposition. The authors generated transgenic mice expressing a mutant form of the BRI gene, found in patients affected by familial Danish dementia (FDD). FDD is a rare inherited disease that causes progressive dementia that, like AD, is neuropathologically characterized by amyloid deposition (ADan), neurofibrillary tangle formation (identical to that seen in AD), and neuronal cell loss. This model provides an exciting new tool in which to study the abnormal changes in the brain that lead to dementia. Comparing the similarities and differences of these two related neurological diseases may provide important clues to how AD develops.

View all comments by Nikolaos K. Robakis

This is a beautiful paper from Dr. Golde's lab showing for the first time that a peptide derived from the BRI2 gene is able to reduce cerebral Aβ deposition in vivo in an AD mouse model and that the same peptide inhibits Aβ aggregation in vitro. The peptide is a 23 amino acid long (Bri2-23) C-terminal fragment generated by the pro-protein convertases processing (Kim et al., 1999) of BRI2, a 266-amino-acid, type-II single transmembrane domain protein (Vidal et al., 1999). Using recombinant adeno-associated virus 1 (rAAV1)-mediated gene transfer in TgCRND8 mice, the investigators show a dramatic suppressive effect of the BRI2 transgene—and a BRI2-Aβ1–40 fusion protein (Kim et al., 2007)—on parenchymal Aβ accumulation. Importantly, the investigators found no evidence for alterations in the steady-state levels of APP or APP CTFβ in TgCRND8 mice expressing the virally delivered BRI2-Aβ1–40 or BRI2 transgenes, but rather that the Bri2–23 peptide could directly inhibit Aβ1–42 fibrillogenesis in vitro.

Mutations in the BRI2 gene cause neurodegenerative diseases characterized by...  Read more

This is a beautiful paper from Dr. Golde's lab showing for the first time that a peptide derived from the BRI2 gene is able to reduce cerebral Aβ deposition in vivo in an AD mouse model and that the same peptide inhibits Aβ aggregation in vitro. The peptide is a 23 amino acid long (Bri2-23) C-terminal fragment generated by the pro-protein convertases processing (Kim et al., 1999) of BRI2, a 266-amino-acid, type-II single transmembrane domain protein (Vidal et al., 1999). Using recombinant adeno-associated virus 1 (rAAV1)-mediated gene transfer in TgCRND8 mice, the investigators show a dramatic suppressive effect of the BRI2 transgene—and a BRI2-Aβ1–40 fusion protein (Kim et al., 2007)—on parenchymal Aβ accumulation. Importantly, the investigators found no evidence for alterations in the steady-state levels of APP or APP CTFβ in TgCRND8 mice expressing the virally delivered BRI2-Aβ1–40 or BRI2 transgenes, but rather that the Bri2–23 peptide could directly inhibit Aβ1–42 fibrillogenesis in vitro.

Mutations in the BRI2 gene cause neurodegenerative diseases characterized by cerebral amyloid deposition (Vidal et al., 1999, 2000), and transgenic mice overexpressing a mutant form of BRI2 show cerebral amyloid (ADan) deposition (Vidal et al., 2008). Interestingly, the amino-termini of the amyloid peptides (ABri and ADan) contain the amino acid sequence of the anti-amyloidogenic peptide Bri2-23. The unexpected findings of Kim et al. generate even more questions regarding the normal role of the still poorly characterized BRI2 gene and how mutations in BRI2 lead to neurodegeneration. More work is needed to determine whether the Bri2-23 peptide is able to depolymerize mature Aβ fibrils and if the anti-amyloidogenic properties of Bri2-23 are also shared by the C-terminal peptides generated by other members of the BRI gene family (Vidal et al., 2001). The use of increasing levels of BRI2 in the brain for the treatment of AD as proposed by Kim and collaborators (Kim et al., 2008) is an interesting idea; however, we believe that since the normal function of BRI2 (and the Bri2-23 peptide) is not known, caution should be taken in attempting therapies based on the overexpression of BRI2 alone.

References:
Kim SH, Wang R, Gordon DJ, Bass J, Steiner DF, Lynn DG, Thinakaran G, Meredith SC, Sisodia SS. Furin mediates enhanced production of fibrillogenic ABri peptides in familial British dementia. Nat Neurosci. 1999 Nov;2(11):984-8. Abstract

Kim J, Onstead L, Randle S, Price R, Smithson L, Zwizinski C, Dickson DW, Golde T, McGowan E. Abeta40 inhibits amyloid deposition in vivo. J Neurosci. 2007 Jan 17;27(3):627-33. Abstract

Vidal R, Frangione B, Rostagno A, Mead S, Révész T, Plant G, Ghiso J. A stop-codon mutation in the BRI gene associated with familial British dementia. Nature. 1999 Jun 24;399(6738):776-81. Abstract

Vidal R, Revesz T, Rostagno A, Kim E, Holton JL, Bek T, Bojsen-Møller M, Braendgaard H, Plant G, Ghiso J, Frangione B. A decamer duplication in the 3' region of the BRI gene originates an amyloid peptide that is associated with dementia in a Danish kindred. Proc Natl Acad Sci U S A. 2000 Apr 25;97(9):4920-5. Abstract

Vidal R, Calero M, Révész T, Plant G, Ghiso J, Frangione B. Sequence, genomic structure and tissue expression of Human BRI3, a member of the BRI gene family. Gene. 2001 Mar 21;266(1-2):95-102. Abstract

Vidal R, Barbeito AG, Miravalle L, Ghetti B. Cerebral Amyloid Angiopathy and Parenchymal Amyloid Deposition in Transgenic Mice Expressing the Danish Mutant Form of Human BRI(2). Brain Pathol. 2008 Apr 10; Abstract

View all comments by Bernardino Ghetti
View all comments by Ruben Vidal

There are between 50 and 100 experimental manipulations that have been shown to alter the pathologic and/or behavioral phenotypes of various transgenic models of human Alzheimer disease. The description in this paper of the effect of the Bri protein, the agent of familial British dementia, by Todd Golde and his colleagues, is the latest example in which overexpressing a transgene encoding a wild-type protein in TgCRND8 model AD mice has an ameliorative effect on the AD phenotype. These observations are quite striking in the context of three other instances in which the expressed protein suppressing the AD phenotype is a precursor of a protein in which the wild-type or a mutant form is the proximal cause of human CNS or systemic amyloidosis. Similar effects have been found for cystatin C in Aβ Tg2576 (Mi et al., 2007) or APP23 (Kaeser et al., 2007) double transgenics; animals in which gelsolin, the precursor in the Finnish form of familial amyloidotic polyneuropathy (  Read more

There are between 50 and 100 experimental manipulations that have been shown to alter the pathologic and/or behavioral phenotypes of various transgenic models of human Alzheimer disease. The description in this paper of the effect of the Bri protein, the agent of familial British dementia, by Todd Golde and his colleagues, is the latest example in which overexpressing a transgene encoding a wild-type protein in TgCRND8 model AD mice has an ameliorative effect on the AD phenotype. These observations are quite striking in the context of three other instances in which the expressed protein suppressing the AD phenotype is a precursor of a protein in which the wild-type or a mutant form is the proximal cause of human CNS or systemic amyloidosis. Similar effects have been found for cystatin C in Aβ Tg2576 (Mi et al., 2007) or APP23 (Kaeser et al., 2007) double transgenics; animals in which gelsolin, the precursor in the Finnish form of familial amyloidotic polyneuropathy (Hirko et al., 2007), has been expressed in Tg2576 and APP695/mutantPS1 mice transgenic for Aβ, and our own work describing the profound effect of overexpressing a transgene encoding wild-type human transthyretin in the APP23 model of AD (Buxbaum et al., 2008).

Why should these proteins in particular have such an effect? If we assume that the excessive generation of Aβ1-42, its misfolding and subsequent aggregation into toxic oligomers and fibrils, is intrinsic to AD (as represented by these models), there are a variety of possible mechanisms that could explain the results. The overexpressed amyloid precursors may have a direct interaction with the Aβ fragment or its oligomers in the brain to either disaggregate them or accelerate their aggregation into larger non-toxic multimers that can be more rapidly engulfed and degraded by glia. They may bind to some factor that is critical for the generation of Aβ or its aggregation, reducing the concentration of fibrillogenic precursor. They may interfere with a downstream process responsible for neurotoxicity, having no impact on aggregation per se but a strong effect on the behavioral phenotype.

In the gelsolin instance, the gene was introduced by hydrodynamic gene delivery and appeared to only be expressed in the periphery, not in the brain. Hence, its effect is hypothesized to be based on its action as a “plasma sink” for Aβ, increasing its transport from the brain to the systemic circulation, thereby decreasing the effective intracerebral Aβ concentration. A similar notion involving the CSF compartment has previously been proposed for the transthyretin effect. We think this unlikely (see below).

The observations could be trivial since it is also possible that the effects may be mouse specific and have no relationship to human disease. Equally unlikely is the possibility that the apparent proclivity of this set of proteins to have the observed effect may represent a strong ascertainment bias in which the proteins in question are only a small sample of the universe of proteins that can do this, and the molecules that have been assayed for this property have been chosen precisely because they are amyloid precursors. For the purposes of the rest of my discussion I will ignore the last two possibilities and assume that the observations in the double transgenics and the gelsolin animals have some biologic relevance.

Transthyretin, cystatin C, and gelsolin have been found in Aβ deposits in human AD brains. It has also been shown that in vitro the proteins directly interact with some form of Aβ, in the case of transthyretin most likely a subfibrillar aggregate. These proteins are apparently protective. We believe that their intrinsic amyloidogenicity indicates that they are predisposed to transiently expose their internal hydrophobic sequences to the external (with respect to the protein’s structure) aqueous milieu. If this occurs for a prolonged period or in a substantial portion of their conformational ensemble—conditions more likely for mutant forms of the proteins—the molecules will self-aggregate. However, if the molecule interacts with the hydrophobic portion of another similarly predisposed protein, the interaction can create a hydrophobic micro-environment for that protein domain. If the time frame is short enough, the remaining portions of the two interacting molecules re-fold to re-submerge the hydrophobic region into the internal portion of the native folded molecule. This process most resembles domain swapping but involves regions smaller than full domains and is temporally much more transient. Thus, there could be a series of proteins that are capable of protectively interacting with Aβ or its pre-toxic aggregates serving as “amateur” or “non-professional” chaperones for this particular cargo molecule.

Why should such a mechanism be necessary? The relative frequency of neurodegenerative disorders related to gain of toxic function by misfolded proteins suggests that the usual proteostatic mechanisms operating in neurons are limited. The relative hypersensitivity of neurons to hyperthermia is consistent with this view. It is apparent that during the evolution of the central nervous system, selection has favored the production of limited amounts of functional small peptides. These, because of their size, are less likely to misfold, and are secreted in vesicles that are at neuronal termini, thus not exposing the rest of the cellular milieu to high concentrations of potentially misfolded molecules.

These mechanisms serve the neuron well under most circumstances, unless there are destabilizing mutations in intrinsic neuronal proteins (e.g., α-synuclein, Huntingtin, SOD1). They may also fail when there is an interaction with an infectious agent capable of re-templating the folding of an endogenous protein. The system itself may become less effective (as in aging) for as yet unknown reasons. Under such circumstances, other mechanisms, such as those employing the “amateurs,” are recruited to cope. It is noteworthy that the transcription of transthyretin in the brain has been seen to increase in transgenic AD models. Interestingly, the AD models all require some degree of overexpression of the mutant Aβ construct, suggesting that the intrinsic murine neuronal proteostatic system functions well until it is overloaded. Old mice do not have an AD equivalent in the absence of overexpression of a human AD gene.

It is also possible that the amyloidogenic proteins are not truly “non-professionals” but represent previously unrecognized elements of the neuronal chaperome. Richard Morimoto’s work in C. elegans is consistent with such a hypothesis in that mutations in known elements of the proteostatic machinery reduce the number of glutamines required to produce a neuropathologic phenotype in a poly-Q model of Huntington’s disease, but the effects of such mutations are not seen until the system is stressed, for example, by a misfolded protein challenge [see Bar Harbor Report 2007]. More broadly, cellular proteostasis networks and their role in health and disease are elegantly reviewed in Balch et al., 2008.

Can these notions be experimentally tested for the proteins discussed here? Each observation should be validated by silencing the gene in question. Thus far, only deletion of the transthyretin gene has been tested for its effect on the development of a model of human Aβ transgene-induced murine AD. It accelerated the development of Aβ deposits in two different transgenic models, displaying a gene dose effect strongly supporting the notion that the observations were biologically relevant. If homozygous silencing of the gene in question is lethal, the effect of hemizygous silencing or siRNA knockdown of the gene on amplifying the Aβ phenotype should be reproduced as independent validation of the effect of the particular protein in question.

The protein should be tested for its ability to bind to Aβ in vitro by some standard assay of protein interaction, and the nature of the molecular species of both the “chaperone” protein and Aβ involved in the binding should be defined. The protein should quantitatively inhibit the cytotoxicity of Aβ to neuronally derived targets at concentrations consistent with those attainable in vivo. Most difficult, but certainly most definitive, would be the demonstration of complexes between the protein and Aβ isolated from the target tissue of animals expressing both transgenes and controls.

It would be desirable to determine whether introduction of the gene encoding the protein of interest somewhere in the course of the disease, rather than from conception, would have an impact on the development of the AD phenotype, suggesting that there might be some elements of these interactions that could be therapeutically exploitable. While it is conceivable that the observations made with respect to these four amyloid precursors are the result of ascertainment bias, until such bias is demonstrated the limits of the phenomena should be precisely defined and the underlying chemistry and biology thoroughly explored to determine if there is any “there” there.

View all comments by Joel Buxbaum