A hypothesis on the aetiology of Alzheimer's disease: description of a model involving a misfolded chaperone

A Scoping Review of Alzheimers Disease Hypotheses: An Array of Uni- and Multi-Factorial Theories

Background

There is a common agreement that Alzheimers disease (AD) is inherently complex; otherwise, a general disagreement remains on its etiological underpinning, with numerous alternative hypotheses having been proposed.

Objective

To perform a scoping review of original manuscripts describing hypotheses and theories of AD published in the past decades.

Results

We reviewed 131 original manuscripts that fulfilled our inclusion criteria out of more than 13,807 references extracted from open databases. Each entry was characterized as having a single or multifactorial focus and assigned to one of 15 theoretical groupings. Impact was tracked using open citation tools.

Results

Three stages can be discerned in terms of hypotheses generation, with three quarter of studies proposing a hypothesis characterized as being single-focus. The most important theoretical groupings were the Amyloid group, followed by Metabolism and Mitochondrial dysfunction, then Infections and Cerebrovascular. Lately, evidence towards Genetics and especially Gut/Brain interactions came to the fore.

Conclusions

When viewed together, these multi-faceted reports reinforce the notion that AD affects multiple sub-cellular, cellular, anatomical, and physiological systems at the same time but at varying degree between individuals. The challenge of providing a comprehensive view of all systems and their interactions remains, alongside ways to manage this inherent complexity.

Stabilization of ubiquitous mitochondrial creatine kinase preprotein by APP family proteins

Amyloid precursor protein (APP) is involved in the pathogenesis of Alzheimer's disease (AD). However, the physiological role of APP and its family members is still unclear. To gain insights into APP function, we used a proteomic approach to identify APP interacting proteins. We report here for the first time a direct interaction between the C-terminal region of APP family proteins and ubiquitous mitochondrial creatine kinase (uMtCK). This interaction was confirmed in vitro as well as in cultured cells and in brain. Interestingly, expression of full-length and C-terminal domain of APP family proteins stabilized uMtCK preprotein in cultured cells. Our data suggest that APP may regulate cellular energy levels and mitochondrial function via a direct interaction and stabilization of uMtCK.

Alpha-fetoprotein structure and function: relevance to isoforms, epitopes, and conformational variants

Mammalian alpha-fetoprotein (AFP) is classified as a member of the albuminoid gene superfamily consisting of albumin, AFP, vitamin D (Gc) protein, and alpha-albumin. Molecular variants of AFP have long been reported in the biomedical literature. Early studies identified isoelectric pH isoforms and lectin-binding variants of AFP, which differed in their physicochemical properties, but not in amino acid composition. Genetic variants of AFP, differing in mRNA kilobase length, were later extensively described in rodent models during fetal/perinatal stages, carcinogenesis, and organ regeneration. With the advent of monoclonal antibodies in the early 1980s, multiple antigenic epitopes on native AFP were detected and categorized, culminating in the identification of six to seven major epitopes. During this period, various AFP-binding proteins and receptors were reported to inhibit certain AFP immunoreactions. Concomittantly, human and rodent AFP were cloned and the amino acid sequences of the translated proteins were divulged. Once the amino acid composition of the AFP molecule was known, enzymatic fragments could be identified and synthetic peptide segments synthesized. Following discovery of the molten globule form in 1981, the existence of transitory, intermediate forms of AFP were acknowledged and their physiological significance was realized. In the present review, the various isoforms and variants of AFP are discussed in light of their potential biological relevance.

Cellular pathogenesis of Alzheimer's disease

The author proposes that paired helical filaments, which contain the protein tau in the fibrillar or beta-pleated sheet conformation, compete with microtubules for binding to nascent, soluble tau. Binding of nascent tau to tau in the beta-pleated sheet conformation autocatalyzes the conformational change into the beta-pleated sheet conformation. As long as sufficient tau is present to stabilize microtubules, neuronal function is normal. However, because paired helical filaments are resistant to proteolysis, they accumulate and eventually bind the bulk of nascent tau. This results in progressive microtubule instability and eventually neuronal death. Senile plaques are involved in Alzheimer's disease pathogenesis in that they contain fibrillar proteins which may function as heteronucleants, catalyzing the fibrillogenesis of other proteins such as tau. In this paradigm, apolipoprotein E4 serves as a heteronucleant for fibrillogenesis of tau.

Antagonistic interactions between yeast chaperones Hsp104 and Hsp70 in prion curing

The maintenance of [PSI], a prion-like form of the yeast release factor Sup35, requires a specific concentration of the chaperone protein Hsp104: either deletion or overexpression of Hsp104 will cure cells of [PSI]. A major puzzle of these studies was that overexpression of Hsp104 alone, from a heterologous promoter, cures cells of [PSI] very efficiently, yet the natural induction of Hsp104 with heat shock, stationary-phase growth, or sporulation does not. These observations pointed to a mechanism for protecting the genetic information carried by the [PSI] element from vicissitudes of the environment. Here, we show that simultaneous overexpression of Ssa1, a protein of the Hsp70 family, protects [PSI] from curing by overexpression of Hsp104. Ssa1 protein belongs to the Ssa subfamily, members of which are normally induced with Hsp104 during heat shock, stationary-phase growth, and sporulation. At the molecular level, excess Ssa1 prevents a shift of Sup35 protein from the insoluble (prion) to the soluble (cellular) state in the presence of excess Hsp104. Overexpression of Ssa1 also increases nonsense suppression by [PSI] when Hsp104 is expressed at its normal level. In contrast, hsp104 deletion strains lose [PSI] even in the presence of overproduced Ssa1. Overproduction of the unrelated chaperone protein Hsp82 (Hsp90) neither cured [PSI] nor antagonized the [PSI]-curing effect of overproduced Hsp104. Our results suggest it is the interplay between Hsp104 and Hsp70 that allows the maintenance of [PSI] under natural growth conditions.

Molecular chaperones and disease

Molecular chaperones are intracellular protein-folding proteins which form part of an ancient cellular response to stress called the heat shock response. They have been the focus for attention during the last decade because of the discovery of their vital role in cell functioning. In very recent years additional roles for these 'topologically-active' proteins in the process of tissue pathology and its treatment have been indicated and are reviewed.