Alzheimer's Disease is Driven by Intraneuronally Retained Beta-Amyloid Produced in the AD-Specific, βAPP-Independent Pathway: Current Perspective and Experimental Models for Tomorrow

On the Inadequacy of the Current Transgenic Animal Models of Alzheimer's Disease: The Path Forward

For at least two reasons, the current transgenic animal models of Alzheimer's disease (AD) appear to be patently inadequate. They may be useful in many respects, the AD models; however, they are not. First, they are incapable of developing the full spectrum of the AD pathology. Second, they respond spectacularly well to drugs that are completely ineffective in the treatment of symptomatic AD. These observations indicate that both the transgenic animal models and the drugs faithfully reflect the theory that guided the design and development of both, the amyloid cascade hypothesis (ACH), and that both are inadequate because their underlying theory is. This conclusion necessitated the formulation of a new, all-encompassing theory of conventional AD-the ACH2.0. The two principal attributes of the ACH2.0 are the following. One, in conventional AD, the agent that causes the disease and drives its pathology is the intraneuronal amyloid-β (iAβ) produced in two distinctly different pathways. Two, following the commencement of AD, the bulk of Aβ is generated independently of Aβ protein precursor (AβPP) and is retained inside the neuron as iAβ. Within the framework of the ACH2.0, AβPP-derived iAβ accumulates physiologically in a lifelong process. It cannot reach levels required to support the progression of AD; it does, however, cause the disease. Indeed, conventional AD occurs if and when the levels of AβPP-derived iAβ cross the critical threshold, elicit the neuronal integrated stress response (ISR), and trigger the activation of the AβPP-independent iAβ generation pathway; the disease commences only when this pathway is operational. The iAβ produced in this pathway reaches levels sufficient to drive the AD pathology; it also propagates its own production and thus sustains the activity of the pathway and perpetuates its operation. The present study analyzes the reason underlying the evident inadequacy of the current transgenic animal models of AD. It concludes that they model, in fact, not Alzheimer's disease but rather the effects of the neuronal ISR sustained by AβPP-derived iAβ, that this is due to the lack of the operational AβPP-independent iAβ production pathway, and that this mechanism must be incorporated into any successful AD model faithfully emulating the disease. The study dissects the plausible molecular mechanisms of the AβPP-independent iAβ production and the pathways leading to their activation, and introduces the concept of conventional versus unconventional Alzheimer's disease. It also proposes the path forward, posits the principles of design of productive transgenic animal models of the disease, and describes the molecular details of their construction.

The Amyloid Cascade Hypothesis 2.0: Generalization of the Concept

Recently, we proposed the Amyloid Cascade Hypothesis 2.0 (ACH2.0), a reformulation of the ACH. In the former, in contrast to the latter, Alzheimer's disease (AD) is driven by intraneuronal amyloid-β (iAβ) and occurs in two stages. In the first, relatively benign stage, Aβ protein precursor (AβPP)-derived iAβ activates, upon reaching a critical threshold, the AβPP-independent iAβ-generating pathway, triggering a devastating second stage resulting in neuronal death. While the ACH2.0 remains aligned with the ACH premise that Aβ is toxic, the toxicity is exerted because of intra- rather than extracellular Aβ. In this framework, a once-in-a-lifetime-only iAβ depletion treatment via transient activation of BACE1 and/or BACE2 (exploiting their Aβ-cleaving activities) or by any means appears to be the best therapeutic strategy for AD. Whereas the notion of differentially derived iAβ being the principal moving force at both AD stages is both plausible and elegant, a possibility remains that the second AD stage is enabled by an AβPP-derived iAβ-activated self-sustaining mechanism producing a yet undefined deleterious "substance X" (sX) which anchors the second AD stage. The present study generalizes the ACH2.0 by incorporating this possibility and shows that, in this scenario, the iAβ depletion therapy may be ineffective at symptomatic AD stages but fully retains its preventive potential for both AD and the aging-associated cognitive decline, which is defined in the ACH2.0 framework as the extended first stage of AD.

Germline mutations directions are different between introns of the same gene: case study of the gene coding for amyloid-beta precursor protein

Amyloid-beta precursor protein (APP) is highly conserved in mammals. This feature allowed us to compare nucleotide usage biases in fourfold degenerated sites along the length of its coding region for 146 species of mammals and birds in search of fragments with significant deviations. Even though cytosine usage has the highest value in fourfold degenerated sites in APP coding region from all tested placental mammals, in contrast to marsupial mammals with the bias toward thymine usage, the most frequent germline and somatic mutations in human APP coding region are C to T and G to A transitions. The same mutational AT-pressure is characteristic for germline mutations in introns of human APP gene. However, surprisingly, there are several exceptional introns with deviations in germline mutations rates. The most of those introns surround exons with exceptional biases in nucleotide usage in fourfold degenerated sites. Existence of such fragments in exons 4 and 5, as well as in exon 14, can be connected with the presence of lncRNA genes in complementary strand of DNA. Exceptional nucleotide usage bias in exons 16 and 17 that contain a sequence encoding amyloid-beta peptides can be explained either by the presence of yet unmapped lncRNA(s), or by the autonomous expression of a short mRNA that encodes just C-terminal part of the APP providing an alternative source of amyloid-beta peptides. This hypothesis is supported by the increased rate of T to C transitions in introns 16-17 and 17-18 of Human APP gene relatively to other introns.

Anti-Amyloidogenic Effects of Metasequoia glyptostroboides Fruits and Its Active Constituents

Alzheimer's disease (AD) is a serious neurodegenerative brain disease that interferes with daily life. The accumulation of beta-amyloid (Aβ), along with oxidative stress-inducing neurocellular apoptosis, has been considered one of the causes of AD. Thus, the purpose of this study is to find natural products that can reduce Aβ accumulation. The ethanol extract of Metasequoia glyptostroboides Hu & Cheng fruits (Cupressaceae) significantly reduced the aggregation of Aβ into oligomers and fibrils determined by Thioflavin T (ThT) assay. The solvent-partitioned ethyl acetate layer was further separated based on the bioassay-guided isolation method combined with the ThT assay. As a result, five compounds were isolated and elucidated as taxoquinone (1), sugiol (2), suginal (3), sandaracopimarinol (4), and sandaracopimaradien-19-ol (5) by comparing NMR data with references. All the compounds significantly reduced the aggregation of Aβ and enhanced the disaggregation of pre-formed Aβ aggregates in a dose-dependent manner. Furthermore, the inhibition of Aβ aggregation by the compounds protected PC12 cells from Aβ aggregate-induced toxicity. Among the five compounds, sandaracopimarinol (4) and sandaracopimaradien-19-ol (5) were the most effective. These results suggest that M. glyptostroboides and isolated five compounds have a potential for further study to be developed as anti-AD agents.

Correlation between microRNA-320 and postoperative delirium in patients undergoing tibial fracture internal fixation surgery

Background

Although the incidence of postoperative delirium (POD) in the elderly after surgery are rising as individuals are living longer, the pathogenesis of POD remains poorly understood. It has been suggested that miRNA-320 may play a role in POD based on animal study and human study.

Methods

We first carried out an animal study, and designed and conducted a human study based on the result of animal study. The aged rats were randomly assigned to five groups: the control (C), anesthesia and surgery (AS), saline (NS), agomir-320 (AG), and antagomir-320 (AT) groups. Postoperative spatial learning and memory in rats were analyzed by the Morris water maze and the open field tests. The plasma levels of insulin-like growth factor-1 (IGF-1), amyloid precursor protein (APP) proteins, miRNA320 and IGF-1mRNA were measured by ELISA and qRT-PCR, respectively. A total of 240 Chinese Han patients over 65 years who underwent tibial fracture internal fixation were included in the PNDABLE study. POD cases and non-POD controls (1:1 matched) were selected by an anesthesiologist using Confusion Assessment Method.

Results

For Group AS, the escape latency was significantly longer and the ratio of time spent in the target quadrant was significantly reduced, APP and miR-320 were upregulated and IGF-1mRNA was downregulated compared with Group C. For Group AG, the escape latency was significantly longer and the ratio of time spent in the target quadrant was significantly reduced, APP and miR-320 were upregulated and IGF-1mRNA was downregulated compared with Group AS. For Group AT, the escape latency was significantly reduced and the ratio of time spent in the target quadrant was significantly longer, APP and miR-320 were downregulated and IGF-1mRNAwas upregulated compared with Group AS. Compared with NPOD patients, the expressions of plasma miR-320 and APP protein were increased and the expression of plasma IGF-1 mRNA was decreased in POD patients after surgery.

Conclusions

MiRNA-320 might play a role in up-regulating the levels of IGF-1mRNA and APP protein, which offered a new target for POD treatment.

Trial registration

Correlation of perioperative neurocognitive disorders with lifestyle and biomarkers. ChiCTR2000033439. Registered 1 June 2020.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12871-022-01612-w.

Neuropathology of Aging in Cats and its Similarities to Human Alzheimer’s Disease

Elderly cats develop age-related behavioral and neuropathological changes that ultimately lead to cognitive dysfunction syndrome (CDS). These neuropathologies share similarities to those seen in the brains of humans with Alzheimer’s disease (AD), including the extracellular accumulation of ß-amyloid (Aβ) and intraneuronal deposits of hyperphosphorylated tau, which are considered to be the two major hallmarks of AD. The present study assessed the presence and distribution of Aβ and tau hyperphosphorylation within the cat brain (n = 55 cats), and how the distribution of these proteins changes with age and the presence of CDS. For this, immunohistochemistry was performed on seven brain regions from cats of various ages, with and without CDS (n = 10 with CDS). Cats accumulate both intracytoplasmic and extracellular deposits of Aβ, as well as intranuclear and intracytoplasmic hyperphosphorylated tau deposits. Large extracellular aggregates of Aβ were found in elderly cats, mainly in the cortical brain areas, with occasional hippocampal aggregates. This may suggest that these aggregates start in cortical areas and later progress to the hippocampus. While Aβ senile plaques in people with AD have a dense core, extracellular Aβ deposits in cats exhibited a diffuse pattern, similar to the early stages of plaque pathogenesis. Intraneuronal Aβ deposits were also observed, occurring predominantly in cortical brain regions of younger cats, while older cats had few to no intraneuronal Aβ deposits, especially when extracellular aggregates were abundant. Intracytoplasmic hyperphosphorylated tau was found within neurons in the brains of elderly cats, particularly in those with CDS. Due to their ultrastructural features, these deposits are considered to be pre-tangles, which are an early stage of the neurofibrillary tangles seen in AD. The largest numbers of pre-tangles are found mainly in the cerebral cortex of elderly cats, whereas lower numbers were found in other regions (i.e., entorhinal cortex and hippocampus). For the first time, intranuclear tau was found in both phosphorylated and non-phosphorylated states within neurons in the cat brain. The highest numbers of intranuclear deposits were found in the cortex of younger cats, and this tended to decrease with age. In contrast, elderly cats with pre-tangles had only occasional or no nuclear labelling.

The effects of antimony on Alzheimer's disease-like pathological changes in mice brain

We have previously identified antimony (Sb) as a newly nerve poison which leads to neuronal apoptosis. However, the relationship between Sb exposure and Alzheimer's disease (AD) process lacks direct evidence. HE staining and Nissl staining showed significant nerve damage after Sb exposure. Therefore, we further evaluated Sb-associated AD risk by detecting accumulation of β-amyloid protein (Aβ) and neurofibrillary tangles (NFTs) in the brains of mice exposed to Sb for 4 and 8 weeks, and even 1 year. The results showed that dose of 20 mg/kg induced Aβ accumulation, but not tau hyperphosphorylation after exposure for 4 week. Eight weeks later, both 10 and 20 mg/kg dramatically triggered Aβ accumulation and increased tau phosphorylation at ser199. At the same time, 20 mg/kg could also increase tau phosphorylation at ser396 and number of NFTs. One years later, we found all of AD hallmarks detected in present study showed positive results in the brains of mice exposed to Sb at 10 and 20 mg/kg. In summary, our data provided direct evidence of Sb-associated AD risk, drawing more attention to Sb-triggered neurotoxicity.

News from Mars: Two-Tier Paradox, Intracellular PCR, Chimeric Junction Shift, Dark Matter mRNA and Other Remarkable Features of Mammalian RNA-Dependent mRNA Amplification. Implications for Alzheimer's Disease, RNA-Based Vaccines and mRNA Therapeutics

Molecular Biology, a branch of science established to examine the flow of information from "letters" encrypted into DNA structure to functional proteins, was initially defined by a concept of DNA-to-RNA-to-Protein information movement, a notion termed the Central Dogma of Molecular Biology. RNA-dependent mRNA amplification, a novel mode of eukaryotic protein-encoding RNA-to-RNA-to-Protein genomic information transfer, constitutes the extension of the Central Dogma in the context of mammalian cells. It was shown to occur in cellular circumstances requiring exceptionally high levels of production of specific polypeptides, e.g. globin chains during erythroid differentiation or defined secreted proteins in the context of extracellular matrix deposition. Its potency is reflected in the observed cellular levels of the resulting amplified mRNA product: At the peak of the erythroid differentiation, for example, the amount of globin mRNA produced in the amplification pathway is about 1500-fold higher than the amount of its conventionally generated counterpart in the same cells. The cellular enzymatic machinery at the core of this process, RNA-dependent RNA polymerase activity (RdRp), albeit in a non-conventional form, was shown to be constitutively and ubiquitously present, and RNA-dependent RNA synthesis (RdRs) appeared to regularly occur, in mammalian cells. Under most circumstances, the mammalian RdRp activity produces only short antisense RNA transcripts. Generation of complete antisense RNA transcripts and amplification of mRNA molecules require the activation of inducible components of the mammalian RdRp complex. The mechanism of such activation is not clear. The present article suggests that it is triggered by a variety of cellular stresses and occurs in the context of stress responses in general and within the framework of the integrated stress response (ISR) in particular. In this process, various cellular stresses activate, in a stress type-specific manner, defined members of the mammalian translation initiation factor 2α, eIF2α, kinase family: PKR, GCN2, PERK and HRI. Any of these kinases, in an activated form, phosphorylates eIF2α. This results in suppression of global cellular protein synthesis but also in activation of expression of select group of transcription factors including ATF4, ATF5 and CHOP. These transcription factors either function as inducible components of the RdRp complex or enable their expression. The assembly of the competent RdRp complex activates mammalian RNA-dependent mRNA amplification, which appears to be a two-tier process. Tier One is a "chimeric" pathway, named so because it results in an amplified chimeric mRNA molecule containing a fragment of the antisense RNA strand at its 5' terminus. Tier Two further amplifies one of the two RNA end products of the chimeric pathway and constitutes the physiologically occurring intracellular polymerase chain reaction, iPCR. Depending on the structure of the initial mRNA amplification progenitor, the chimeric pathway, Tier One, may result in multiple outcomes including chimeric mRNA that produces either a polypeptide identical to the original, conventional mRNA progenitor-encoded protein or only its C-terminal fragment, CTF. The chimeric RNA end product of Tier One may also produce a polypeptide that is non-contiguously encoded in the genome, activate translation from an open reading frame, which is "silent" in a conventionally transcribed mRNA, or initiate an abortive translation. In sharp contrast, regardless of the outcome of Tier One, the mRNA end product of Tier Two of mammalian mRNA amplification, the iPCR pathway, always produces a polypeptide identical to a conventional mRNA progenitor-encoded protein. This discordance is referred to as the Two-Tier Paradox and discussed in detail in the present article. On the other hand, both Tiers are similar in that they result in heavily modified mRNA molecules resistant to reverse transcription, undetectable by reverse transcription-based methods of sequencing and therefore constituting a proverbial "Dark Matter" mRNA, despite being highly ubiquitous. It appears that in addition to their other functions, the modifications of the amplified mRNA render it compatible, unlike the bulk of cellular mRNA, with phosphorylated eIF2α in translation, implying that in addition to being extraordinarily abundant due to the method of its generation, amplified mRNA is also preferentially translated under the ISR conditions, thus augmenting the efficiency of the amplification process. The vital importance of powerful mechanisms of amplification of protein-encoding genomic information in normal physiology is self-evident. Their malfunctions or misuse appear to be associated with two types of abnormalities, the deficiency of a protein normally produced by these mechanisms and the mRNA amplification-mediated overproduction of a protein normally not generated by such a process. Certain classes of beta-thalassemia exemplify the first type, whereas the second type is represented by overproduction of beta-amyloid in Alzheimer's disease. Moreover, the proposed mechanism of Alzheimer's disease allows a crucial and verifiable prediction, namely that the disease-causing intraneuronally retained variant of beta-amyloid differs from that produced conventionally by βAPP proteolysis in that it contains the additional methionine or acetylated methionine at its N-terminus. Because of its extraordinary evidential value as a natural reporter of the mRNA amplification pathway, this feature, if proven, would, arguably, constitute the proverbial Holy Grail not only for Alzheimer's disease but also for the mammalian RNA-dependent mRNA amplification field in general. Both examples are discussed in detail in the present article, which summarizes and systematizes our current understanding of the field and describes two categories of reporter constructs, one for the chimeric Tier of mRNA amplification, another for the iPCR pathway; both reporter types are essential for elucidating underlying molecular mechanisms. It also suggests, in light of the recently demonstrated feasibility of RNA-based vaccines, that the targeted intracellular amplification of exogenously introduced amplification-eligible antigen-encoding mRNAs via the induced or naturally occurring RNA-dependent mRNA amplification pathway could be of substantial benefit in triggering a fast and potent immune response and instrumental in the development of future vaccines. Similar approaches can also be effective in achieving efficient and sustained expression of exogenous mRNA in mRNA therapeutics.

Alzheimer's Disease Prevention and Treatment: Case for Optimism

A paradigm shift is under way in the Alzheimer's field. A view of Alzheimer's disease, AD, prevailing until now, the old paradigm, maintains that it is initiated and driven by the overproduction and extracellular accumulation of beta-amyloid, Aβ; a peptide assumed to be derived, both in health and disease, solely by proteolysis of its large precursor, βAPP. In AD, according to this view, Aβ overproduction-associated neurodegeneration begins early, accumulates throughout the lifespan, and manifests symptomatically late in life. A number of drugs, designed within the framework of exceptionality of the βAPP proteolytic/secretory pathway in Aβ production in Alzheimer's disease, achieved spectacular successes in treatment, even the reversal, of AD symptoms in animal models. Without exception, they all exhibited equally spectacular failures in human clinical trials. This paradigm provides few causes for optimism with regard to prevention and treatment of AD. In its context, the disease is considered untreatable in the symptomatic phase; even prodromal cases are assumed too advanced for treatment because Aβ-triggered damages have been accumulating for preceding decades, presumably starting in the early twenties and, to be effective, this is when therapeutic intervention should commence and continue for life. The new paradigm does not dispute the seminal role of Aβ in AD but posits that beta-amyloid produced in the βAPP proteolytic/secretory pathway causes AD in humans no more than it does in non-human mammals that share this pathway with humans, accumulate Aβ as they age, but do not develop the disease. Alzheimer's disease, according to this outlook, is driven by the AD-specific pathway of Aβ production, independent of βAPP and absent in animals. Its activation, late in life, occurs through accumulation, via both cellular uptake of secreted Aβ and neuronal retention of a fraction of beta-amyloid produced in the βAPP proteolytic pathway, of intraneuronal Aβ, which triggers mitochondrial dysfunction. Cellular stresses associated with mitochondrial dysfunction, or, probably, the integrated stress response, ISR, elicited by it, activate an AD-specific Aβ production pathway. In it, every conventionally produced βAPP mRNA molecule potentially serves repeatedly as a template for production of severely 5'-truncated mRNA encoding C99 fragment of βAPP, the immediate precursor of Aβ that is processed in a non-secretory pathway, apparently in a neuron-specific manner. The resulting intraneuronally retained Aβ augments mitochondrial dysfunction, which, in turn, sustains the activity of the βAPP mRNA amplification pathway. These self-propagating Aβ overproduction/mitochondrial dysfunction mutual feedback cycles constitute the engine that drives AD and ultimately triggers neuronal death. In this paradigm, preventive treatment can be initiated any time prior to commencement of βAPP mRNA amplification. Moreover, there are good reasons to believe that with a drug blocking the amplification pathway, it would be possible not only to preempt the disease but also stop and reverse it even when early AD symptoms are already manifested. Thus, the new paradigm introduces a novel theory of Alzheimer's disease. It explains the observed discordances, determines defined therapeutic targets, provides blueprints for a new generation of conceptually distinct AD models and specifies design of a reporter for the mRNA amplification pathway. Most importantly, it offers detailed guidance and tangible hope for prevention of the disease and its treatment at the early symptomatic stages.