The Amyloid Cascade Hypothesis 2.0 for Alzheimer's Disease and Aging-Associated Cognitive Decline: From Molecular Basis to Effective Therapy

Abstract

With the long-standing amyloid cascade hypothesis (ACH) largely discredited, there is an acute need for a new all-encompassing interpretation of Alzheimer's disease (AD). Whereas such a recently proposed theory of AD is designated ACH2.0, its commonality with the ACH is limited to the recognition of the centrality of amyloid-β (Aβ) in the disease, necessitated by the observation that all AD-causing mutations affect, in one way or another, Aβ. Yet, even this narrow commonality is superficial since AD-causing Aβ of the ACH differs distinctly from that specified in the ACH2.0: Whereas in the former, the disease is caused by secreted extracellular Aβ, in the latter, it is triggered by Aβ-protein-precursor (AβPP)-derived intraneuronal Aβ (iAβ) and driven by iAβ generated independently of AβPP. The ACH2.0 envisions AD as a two-stage disorder. The first, asymptomatic stage is a decades-long accumulation of AβPP-derived iAβ, which occurs via internalization of secreted Aβ and through intracellular retention of a fraction of Aβ produced by AβPP proteolysis. When AβPP-derived iAβ reaches critical levels, it activates a self-perpetuating AβPP-independent production of iAβ that drives the second, devastating AD stage, a cascade that includes tau pathology and culminates in neuronal loss. The present study analyzes the dynamics of iAβ accumulation in health and disease and concludes that it is the prime factor driving both AD and aging-associated cognitive decline (AACD). It discusses mechanisms potentially involved in AβPP-independent generation of iAβ, provides mechanistic interpretations for all principal aspects of AD and AACD including the protective effect of the Icelandic AβPP mutation, the early onset of FAD and the sequential manifestation of AD pathology in defined regions of the affected brain, and explains why current mouse AD models are neither adequate nor suitable. It posits that while drugs affecting the accumulation of AβPP-derived iAβ can be effective only protectively for AD, the targeted degradation of iAβ is the best therapeutic strategy for both prevention and effective treatment of AD and AACD. It also proposes potential iAβ-degrading drugs.

Keywords: Aβ protein precursor (AβPP)-independent generation of iAβ; BACE1 and BACE2 activators as AD and AACD drugs; aging-related cognitive dysfunction (AACD); iAβ depletion therapy for AD and AACD; intraneuronal Aβ (iAβ); the amyloid cascade hypothesis 2.0 (ACH2.0).

Figure 1

Mechanistic aspects of iAβ dynamics in the ACH2.0 perspective: The AD Engine. Left box: The life-long accumulation of intraneuronal Aβ (iAβ) produced in the AβPP proteolytic pathway. Two distinct processes contribute to this accumulation: importation of secreted extracellular Aβ inside the cell and retention within the neuron of a fraction of Aβ generated by gamma-cleavage of the C99 fragment of AβPP on intracellular rather than on plasma membranes. Such accumulation of iAβ is a normal physiological process common to healthy individuals and future AD patients. It becomes detrimental if and when it reaches the critical threshold and activates the second, symptomatic stage of AD. In the majority of population this threshold is not reached within the lifespan of an individual and no AD occurs. Middle box: When iAβ, accumulated in a life-long process, reaches the critical threshold invoked above, it mediates the elicitation of the integrated stress response, ISR (or of a yet undefined pathway marked XXX). This occurs via the documented activation of two eIF2α kinases, PKR and HRI (other eIF2α kinases, or yet unidentified mediators denoted “???” could be also involved). Activated PKR and/or HRI phosphorylate eIF2α and thus trigger the ISR. Top box: The ISR manifests itself as an acute decline in the protein synthesis output. The reduction in the global cellular protein synthesis occurs via the suppression of the cap-dependent initiation of translation. Concurrently, the ISR promotes cap-independent translation of selected mRNA species; among those are mRNAs encoding specific transcription factors. The ISR-induced transcriptions factors, or translation products of the genes activated by these factors, plausibly include components critical for the activation of the AβPP-independent iAβ generation pathway. The bulk, if not the whole iAβ output is retained within affected neurons. Right box: The increased influx of iAβ generated in the AβPP-independent manner substantially elevates its steady-state levels. Arched arrows: As the result of a drastic augmentation of iAβ levels, pathways leading to the elicitation of the integrated stress response are sustained, and the activity of the AβPP-independent iAβ generation pathway and uninterrupted influx of iAβ are perpetuated. These continuous cycles of iAβ-stimulated propagation of its own production constitute an engine that drives AD, the AD Engine. Only when the AD Engine is activated does the disease commence. A possibility that the agent driving the second AD stage is not iAβ cannot be excluded; this would not, however, change the logic of the thesis and is discussed in detail in [2].

Figure 2

Dynamics of Aβ accumulation and the disease in AD-affected population: ACH and ACH2.0 perspectives. Blue lines: Levels of Aβ (panels A,B) or iAβ (panels C–E). Red lines: Degree of neuronal damage. Black lines: Indicator lines; no noticeable neuronal damage. Red blocks: Apoptotic zone. Threshold T: The level of Aβ and the consequent level of neurodegeneration causing symptomatic manifestation of AD. Threshold T1: The level of AβPP-derived iAβ triggering cellular processes leading to the activation of the AβPP-independent generation of iAβ. Threshold T2: The level of iAβ and the consequent degree of neurodegeneration causing cellular commitment to apoptosis and acute AD symptoms. Panels A,B (SAD, FAD respectively): Dynamics of AD in the ACH perspective. Extracellular Aβ accumulates and the degree of neuronal damage increases proportionally. When the T threshold is crossed, the symptomatic AD stage commences. Panels C,D (SAD, FAD respectively): Dynamics of AD in the ACH2.0 perspective. Following crossing of the T1 threshold by iAβ produced in the AβPP proteolytic pathway, its generation in the AβPP-independent pathway commences. Since the entire Aβ output of the AβPP-independent pathway is retained intraneuronally, the rate of iAβ accumulation greatly accelerates and its levels substantially and rapidly increase, which causes, via the cascade involving tau pathology, significant neuronal damage and triggers initial AD symptoms. When iAβ, and the consequent degree of neuronal damage reach and cross the T2 threshold, the cell apoptotic pathway is triggered and acute AD symptoms manifest. (Panel E): iAβ dynamics in subjects (including non-human mammals) with an inoperative second stage. AβPP-derived iAβ reaches and crosses the T1 threshold but the AβPP-independent iAβ generation pathway remains inoperative. Neither levels of iAβ causing AD-related damage nor the T2 threshold are reached, no AD occurs.

Figure 3

iAβ dynamics in the affected neuronal population of an individual patient in the ACH2.0 perspective. Blue lines: Levels of iAβ in individual AD-affected neurons. Threshold T1: The level of AβPP-derived iAβ triggering cellular processes leading to the activation of the AβPP-independent generation of iAβ. Threshold T2: The level of iAβ and the consequent degree of neurodegeneration causing cellular commitment to apoptosis and acute AD symptoms. Red blocks: Apoptotic zone. Vertical blue arrows: Commencement of the occurrence of AD symptoms. Panel A: Individual neurons reach and cross the T1 threshold with a stochastic distribution within a broad time interval, which primarily determines the duration of the disease. Subsequent to the T1 threshold crossing by AβPP-derived iAβ, the AβPP-independent iAβ generation pathway is activated, the rate of iAβ accumulation and its cellular levels are sharply elevated, and neuronal damage rapidly increases. Following crossing of the T2 threshold, neurons enter the apoptotic pathway and are ultimately lost. When sufficient fraction of neurons lose their functionality or die, AD symptoms manifest while a substantial proportion of affected neurons have not yet crossed the T1 threshold. Panel B: With the progression of the disease, additional neurons cross first the T1 and then the T2 thresholds and the disease reaches its end stage. Panel A’: The neuronal crossing of the T1 threshold occurs within relatively short time interval. Subsequent to the crossing of the T1 threshold, the affected neurons advance toward and cross the T2 threshold in a broad stochastic distribution; the temporal duration of this distribution determines the duration of the disease. When the neuronal damage or loss occurred to a degree sufficient for symptomatic manifestation of the disease, the majority, if not the entire population, of the affected neurons already crossed the T1 threshold. Panel B’: As the disease progresses, more neurons reach the T2 threshold and enter the apoptotic pathway; eventually, the end stage is reached.

Figure 4

Dynamics of AD: Effect of the rate of accumulation of AβPP-derived iAβ. iAβ: Intraneuronal Aβ levels; T1: The level of iAβ that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ, and activation of the AD Engine. T2: The level of iAβ that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. The T1 threshold is constant and is chosen deliberately low, so that the extent of AβPP-derived iAβ-accumulation-related neuronal damage prior to the T1 threshold’s crossing is insignificant and inconsequential. The Figure does not consider the effect of the rate of iAβ (mostly iAβ generated independently of AβPP) accumulation in the second AD stage, which is assumed constant for purposes of this analysis. The lifespan is assumed to end at 100 years of age. In panel A, the rate of AβPP-derived iAβ accumulation is such that AD symptoms manifest at about 65 years of age (statistical age of the commencement of AD). As the rate of AβPP-derived iAβ accumulation decreases, the timing of its reaching and crossing the T1 threshold, and consequently of the commencement of stage two of AD, increases. In panel B, this timing is such that AD symptoms manifest at about 85 years of age. In panel C, AβPP-derived iAβ crosses the T1 threshold and initiates the AβPP-independent iAβ production pathway so late that, while the manifestation of AD symptoms commences, the disease does not run its complete course within the lifespan of an individual. In panel D, the rate of AβPP-derived iAβ accumulation is sufficiently low for it not to reach the T1 threshold within the lifespan of an individual. Therefore, the depicted process is, in contrast to the analogous process in panels A through C, not “the first stage of AD”. Note that given a sufficient lifespan, AβPP-derived iAβ would eventually cross the T1 threshold and AD would inevitably occur.

Figure 5

Dynamics of AD and AACD: Effect of the extent of the T1 threshold. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ (more precisely, between the T⁰ and the maximum level reached by AβPP-derived iAβ short of the T1 threshold). The rate of accumulation of AβPP-derived iAβ and the extent of the T⁰ threshold are constant, and the lifespan is assumed to terminate at 100 years of age; the only variable is the extent of the T1 threshold. In panel A, the T1 threshold is chosen deliberately low, so that the accumulation of AβPP-derived iAβ results in no significant neuronal damage. With the increase of the extent of the T1 threshold, such damage would inevitably occur at the sub-T1 levels of AβPP-derived iAβ; to indicate the extent of iAβ accumulation where such damage commences, another threshold, the T⁰ is introduced in panel B and it is posited that it is this AβPP-derived iAβ-inflicted neuronal damage, occurring between the thresholds T⁰ and T1 which causes AACD (on the more precise definition of the upper AACD boundary, see text). In panel C, the extent of the T1 threshold increases. With the rate of AβPP-derived iAβ accumulation and the extent of the T⁰ threshold remaining constant, the AACD Zone increases accordingly, as does the duration and the severity of the dysfunction. While the timing of the commencement of AACD does not change with the increasing extent of the T1 threshold, the timing of the commencement of the second AD stage increases in a direct proportion, and the probability of developing AD within the remaining lifespan decreases in an inverse proportion to the increase in the extent of the T1 threshold. In panel D, the extent of the T1 threshold is such that the level of AβPP-derived iAβ does not reach the T1 threshold within the lifespan of an individual. With the extent of the T⁰ threshold and the rate of AβPP-derived iAβ accumulation fixed, the timing of the commencement of AACD remains constant, but the AACD Zone further increases. On the other hand, since the T1 threshold is not crossed, there is no activation of the AβPP-independent iAβ production pathway, no stage two of AD ensues, no AD occurs. Note, however, that the T1 threshold would be crossed and AD would certainly occur provided the lifespan is long enough.

Figure 6

Symptoms of AACD-associated cognitive impairment may overlap with and could be indistinguishable from those of AD-associated mild cognitive impairment. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ, and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to apoptosis. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ. The rate of accumulation of AβPP-derived iAβ and the extent of the T⁰ threshold are constant, and the lifespan is assumed to terminate at 100 years of age; the only variable is the extent of the T1 threshold. In panel A the T1 is high and is not reached within the lifespan of an individual. AACD commences with the crossing of the T⁰ threshold and continues for the remaining portion of the lifespan (gradient-pink box). In this case, iAβ-caused cognitive impairment is clearly attributable to AACD. In panel B, the T1 threshold is lowered. The same range of iAβ within the gradient-pink box as shown in panel A is divided in panel B into two portions: pre-T1 crossing and post-T1 crossing. Because the range of iAβ within the gradient-pink boxes in panels A and B is the same, the symptoms are also the same. But pre-T1 crossing, they comprise AACD-associated cognitive impairment, whereas post-T1 crossing, they constitute AD-associated mild cognitive impairment. In panel C, the same iAβ range within the gradient-pink box as in panels A and B occurs entirely post-T1 crossing. Since the iAβ range within the box is the same as in other panels, the symptoms also are, but now they constitute, in their entirety, AD-associated mild cognitive impairment. Note that since the rate of iAβ accumulation is greater post-T1 than pre-T1 crossing, the duration of symptoms decreases in successive panels.

Figure 7

Protective effect of the Icelandic mutation for AD and AACD: Mechanistic interpretation in the ACH2.0 perspective. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ, and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ (more precisely, between the T⁰ and the maximum level reached by AβPP-derived iAβ short of the T1 threshold). Panels A, B, and C depict three principal variants of the iAβ-caused disease occurring in wild-type AβPP carriers. The rate of AβPP-derived iAβ accumulation is assumed constant and so is the extent of the T⁰ threshold; the lifespan in each case is limited to 100 years of age. On the other hand, the extent of the T1 threshold is variable and dictates whether AACD and AD do or do not occur. In panel A, the T1 threshold is below the AβPP-derived iAβ level (T⁰ threshold) required for the initiation of AACD. When the T1 threshold is reached, the AβPP-independent iAβ generation pathway is activated and AD commences. In panel B, the T⁰ threshold level is below that of the T1 threshold. When the levels of AβPP-derived iAβ reach the former, AACD commences and persists until AβPP-derived iAβ crosses the latter, i.e., for the duration of the AACD Zone (gradient-pink box), whereupon it evolves into AD. In panel C, the extent of the T1 threshold is such that at a given rate of accumulation of AβPP-derived iAβ, the T1 threshold cannot be reached, the AβPP-independent iAβ generation pathway cannot be activated, and AD cannot occur within the lifetime of an individual. When AβPP-derived iAβ levels cross the T⁰ threshold, AACD commences and continues for the remaining part of the lifespan. Panels A’, B’, and C’ depict mechanistic interpretation of the protective effect of the Icelandic AβPP mutation within the framework of the ACH2.0. In all three variants of potential AD/AACD, the rate of accumulation of AβPP-derived iAβ is lowered. In panel A’, it is such that levels of AβPP-derived iAβ do not reach the T1 threshold within the lifespan of an individual. In panels B’ and C’, the rate of accumulation of AβPP-derived iAβ is rendered such that its levels do not reach the T⁰ (and T1) threshold within the individual’s lifetime. Accordingly, in all three variants, neither AACD nor AD occurs within the lifespan of the Icelandic mutation carriers (or occurs substantially later than in wild-type AβPP carriers).

Figure 8

Early onset of AD in carriers of category One FAD mutations: Mechanistic interpretation in the ACH2.0 perspective. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ, and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ (more precisely, between the T⁰ and the maximum level reached by AβPP-derived iAβ short of the T1 threshold). Assumed lifespan: 100 years. Panels A, B, C, D: Kinetics of iAβ accumulation and progression of disease in wild-type AβPP carriers. Panels A and B: The extent of the T⁰ threshold exceed that of the T1; no AACD occurs. Panel A: The T1 threshold is not crossed; no AD occurs. Panel B: The T1 threshold is reached and crossed; the late onset AD ensues. Panels C and D: The T⁰ threshold is lower than the T1. Panel C: The T1 threshold is not crossed; AACD commences upon crossing of the T⁰ threshold and continues for the remaining lifespan. Panel D: Both the T⁰ and T1 thresholds are crossed; AACD commences upon the crossing of the former and evolves into late onset AD when the latter is reached. Panels A’, B’, C’, D’: Dynamics of iAβ accumulation and progression of disease in carriers of category One FAD mutations. The steady-state influx of AβPP-derived iAβ is increased and its rate of accumulation augmented. Consequently, the T1 threshold is reached earlier, AβPP-independent production of iAβ is activated sooner, and the early-onset AD ensues. Panels A’ and B’: The T⁰ levels exceed those of T1; no AACD occurs. Panels C’ and D’: The extent of the T⁰ threshold is lower than that of the T1 threshold, and the early-onset AD is preceded by the AACD phase. Due to the steepness of AβPP-derived iAβ accumulation, the duration of the AACD phase is much shorter in mutants than in wild-type AβPP carriers; it rapidly evolves (upon crossing of the T1 threshold by AβPP-derived iAβ) into early onset AD and therefore could be hard to diagnose as a separate condition. Note that the only dynamic alteration caused by category One FAD mutations is the augmentation of the rate of accumulation of AβPP-derived iAβ.

Figure 9

Early onset of AD in carriers of category Two FAD mutations: Mechanistic interpretation in the ACH2.0 perspective. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ (more precisely, between the T⁰ and the maximum level reached by AβPP-derived iAβ short of the T1 threshold). Assumed lifespan: 100 years. Panels A, B, C, D: Kinetics of iAβ accumulation and progression of disease in wild-type AβPP carriers. Panels A and B: The T⁰ levels exceed those of T1; no AACD occurs. Panel A: The T1 threshold is not crossed; no AD occurs. Panel B: The T1 threshold is reached and crossed; the late onset AD ensues. Panels C and D: The T⁰ threshold is lower than T1. Panel C: The T1 threshold is not crossed; AACD commences upon crossing of the T⁰ threshold and continues for the remaining lifespan. Panel D: Both the T⁰ and T1 thresholds are crossed; AACD commences upon crossing of the former and evolves into late onset AD when the latter is reached. Panels A’, B’, C’, D’: Dynamics of iAβ accumulation and progression of disease in carriers of category two FAD mutations, which cause not only the augmentation of steady-state influx of AβPP-derived iAβ and the increase in its rate of accumulation but also the reduction in the extent of the T1 threshold. The T1 threshold is reached earlier, AβPP-independent production of iAβ is activated sooner, and the early-onset AD ensues. Panels A’ and B’: The T⁰ levels exceed those of T1; no AACD occurs. Panels C’ and D’: The extent of the T⁰ threshold is lower than that of the T1 threshold, and the early-onset AD is preceded by the AACD phase. Due to the steepness of AβPP-derived iAβ accumulation, the duration of the AACD phase is much shorter than in wild-type AβPP carriers; it rapidly evolves (upon crossing of the T1 threshold by AβPP-derived iAβ) into early onset AD and therefore could be hard to diagnose as a separate condition. Note that dynamic changes caused by category two FAD mutations are not only the increase in the rate of accumulation of AβPP-derived iAβ but also the reduction in the extent of the T1 and, probably, T⁰ thresholds.

Figure 10

The Icelandic AβPP mutation as the ultimate guide for AD and AACD therapy: Effect of the imitation of the mode of mutation’s operation. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink Boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ (more precisely, between the T⁰ and the maximum level reached by AβPP-derived iAβ short of the T1 threshold). Orange boxes: Duration of treatment’s administration. The lifespan is assumed to terminate at 100 years of age. Panels A, B, C: Dynamics of iAβ accumulation in AD-affected neurons and the progression of the disease in the wild-type AβPP carriers in the absence of treatment. In panel A, the T⁰ levels exceed those of T1 and no AACD occurs. In panels B and C, the T⁰ threshold is lower than T1. AACD commences upon crossing of the T⁰ threshold and evolves into AD when the T1 threshold is crossed (panel B) or continues for the remaining lifespan if the T1 threshold is not crossed (panel C). Panels A’, B’: Dynamics of AβPP-derived iAβ accumulation under a drug that suppresses its steady-state influx and precludes its further accumulation. The crossing of the T1 and T⁰ thresholds is prevented and no disease ensues for the duration of the treatment. Panel C’: The same drug is administered after the T⁰ crossing. It precludes further accumulation of AβPP-derived iAβ and stops or slows the progression of AACD for the duration of the treatment. Thus, a drug, which suppresses the accumulation of AβPP-derived iAβ, can be only preventive for AD but may constitute a valid treatment for AACD.

Figure 11

Protective action of the Icelandic AβPP mutation can be improved upon: effect of the transient depletion of iAβ. iAβ: Intraneuronal Aβ levels. T⁰: iAβ levels that trigger neuronal damage manifesting as AACD. T1: iAβ level that triggers elicitation of the integrated stress response, initiation of AβPP-independent generation of iAβ and the activation of the AD Engine. T2: iAβ level that triggers cell’s commitment to the apoptotic pathway. Red blocks: Fraction of affected neurons either committed to apoptosis or dead. Gradient-pink boxes: “AACD Zone”, the differential between the T⁰ and T1 threshold levels of AβPP-derived iAβ (more precisely, between the T⁰ and the maximum level reached by AβPP-derived iAβ short of the T1 threshold). Orange boxes: Duration of treatment’s administration. The lifespan is assumed to terminate at 100 years of age. Panels A, B, C: Dynamics of iAβ accumulation in AD-affected neurons and the progression of the disease in the wild-type AβPP carriers in the absence of treatment. In panel A, the T⁰ levels exceed those of T1 and no AACD occurs. In panels B and C, the T⁰ threshold is lower than T1. AACD commences upon crossing of the T⁰ threshold and evolves into AD when the T1 threshold is crossed (panel B) or continues for the remaining lifespan if the T1 threshold is not crossed (panel C). Panels A’, B’: Dynamics of AβPP-derived iAβ accumulation following a transient iAβ depletion treatment administered prior to the crossing of the T1 and T⁰ thresholds. The iAβ population is collapsed and its accumulation is resumed from a low baseline. The duration of the treatment is defined by the desired extent of iAβ depletion and could be as short as few days, akin to an antibiotic treatment’s regimen. As shown, iAβ is completely (or nearly completely) depleted and its build-up to the T1 and T⁰ levels would exceed an individual’s lifespan; no disease would occur. Panel C’: A transient iAβ depletion treatment is applied to AACD patient after the T⁰ crossing. Following the depletion, iAβ levels are well below the T⁰ threshold and the patient is technically cured of AACD (subject to complete recovery of the affected neurons following the iAβ depletion treatment). As shown, de novo accumulating AβPP-derived iAβ does not reach the T⁰ threshold and AACD does not recur within the remaining lifetime of the treated patient. Note that whereas complete or nearly complete iAβ depletion is shown in panels A’–C’, any reduction in its baseline would be therapeutically beneficial in proportion to the extent of the depletion.

Figure 12

Effect of transient iAβ depletion therapy via its targeted degradation at various symptomatic stages of AD. Blue lines: affected neurons. iAβ: Level of intraneuronal Aβ. T1 threshold: Levels of iAβ triggering elicitation of the ISR, initiation of AβPP-independent production of iAβ, activation of the AD Engine, and the commencement of the second stage of AD. T2 threshold: Levels of iAβ triggering neurons’ entrance into the apoptotic pathway. Red blocks: Apoptotic zone. Orange boxes: Active transient iAβ depletion via its targeted degradation by Aβ-cleaving activities of BACE1 and/or BACE2 or by any other suitable agent; levels of iAβ are reset and the accumulation of AβPP-derived iAβ resumes from a low baseline. Panel A: The transient iAβ depletion therapy is implemented at the early symptomatic stage of AD, when the bulk of the affected neurons are still viable. Following the reset of iAβ levels, its build-up starts de novo, supported only by the AβPP proteolytic pathway. It is anticipated that iAβ levels will not reach the T1 threshold and AD will not recur within the remaining lifetime of an SAD patient. Panels B, C, and D: The transient iAβ depletion treatment is implemented at progressively advanced stages of AD. The results are analogous to those depicted in panel A. However, at this AD stages increasing number of affected neurons cross the T2 threshold and commit apoptosis. This leaves a progressively smaller number of affected neurons that retained their viability and can be redeemed.

Figure 13

Dynamics of iAβ accumulation and of the disease at the second, symptomatic stage of AD. Blue lines: Affected neurons. iAβ: Levels of intraneuronal Aβ. T1 threshold: Levels of iAβ triggering elicitation of the ISR, initiation of AβPP-independent production of iAβ, activation of the AD Engine and the commencement of the second stage of AD. T2 threshold: Levels of iAβ triggering neurons’ entrance into the apoptotic pathway. Red blocks: Apoptotic zone. All kinetic parameters up to and including the crossing of the T1 threshold are identical in all panels whereas the kinetic parameters following the T1 crossing and the commencement of the second AD stage are different. In panels A and A’, the extent of the T2 threshold is the same but the rates of accumulation of iAβ produced in the AβPP-independent iAβ production pathway are different. It is much greater in panel A than in panel A’. Accordingly, the rate of progression of the disease is much slower, the timing of its symptomatic manifestation is significantly greater, and its duration is substantially longer in panel A’ than in panel A. In panels B and B’, both the extent of the T2 threshold and the initial (fastest) rate of accumulation of iAβ produced independently of AβPP are identical but the stochastic distribution of the latter in the affected neurons is much wider in panel B’ than in panel B. Accordingly, the duration of the disease is significantly longer in panel B’ than in panel B. In panels C and C’, the rate of accumulation of iAβ produced in the AβPP-independent iAβ production pathway and it’s stochastic distribution in the affected neurons are the same but the extents of the T2 threshold differ. In panel C’, it is substantially higher than in panel C. Consequently, the timing of the symptomatic manifestation of the disease is greater and the duration of the disease is significantly longer in panel C’ than in panel C.

Figure 14

Sequential manifestation of the AD pathology in defined brain compartments: The rate of accumulation of iAβ produced independently of AβPP differs in diverse regions of the affected brain. iAβ: Level of intraneuronal Aβ. T1 threshold: Level of iAβ level triggering elicitation of the ISR, initiation of AβPP-independent production of iAβ, activation of the AD Engine and the commencement of the second stage of AD. T2 threshold: Levels of iAβ triggering neurons’ entrance into the apoptotic pathway. Red blocks: Apoptotic zone. Each line represents individual affected neurons. Lines of different colors above the T1 threshold: The affected neurons in various defined parts of the AD-afflicted brain. The rate of iAβ accumulation differs in different parts of the brain, due to either diverse, brain compartment-specific, efficiencies of the AβPP-independent iAβ generation pathway or varied rates of iAβ clearing. Panel A: The early symptomatic stage of AD. Only the entorhinal cortex and possibly the hippocampus are affected; neither significant accumulation of iAβ produced in the AβPP-independent pathway, nor AD neuropathology yet occurred in other brain compartments. Panels B, C, D: With the progression of AD toward the end stage (panel D), iAβ produced in the AβPP-independent pathway accumulates and the AD pathology commences and expends in temporally sequential manner in other defined compartments of the affected brain. Note that if the therapeutic intervention, via transient administration of BACE1 and/or BACE2 activators or of other iAβ-depleting agents, were implemented at an early symptomatic stage of AD (panel A), the progression of the disease in the brain compartment already affected at this stage would cease, and the AD pathology would not commence, due to iAβ depletion, in other brain compartments, which would remain largely intact. The progression of AD in the affected brain compartment would not resume and other brain compartments would stay pathology-free for the remaining lifespan of a patient.

Figure 15

Sequential manifestation of the AD pathology in defined brain compartments: The extent of the T2 threshold is variable, either separately or simultaneously with the rate of AβPP-independent iAβ accumulation, in distinct defined regions of the affected brain. iAβ: Level of intraneuronal Aβ. T1 threshold: Level of iAβ level triggering elicitation of the ISR, initiation of AβPP-independent production of iAβ, activation of the AD Engine, and the commencement of the second stage of AD. T2 threshold: Levels of iAβ triggering neurons’ entrance into the apoptotic pathway. Red blocks: Apoptotic zone. Each line represents individual affected neurons. T2’ through T2’’’’: Extents of the T2 threshold in separate define brain compartments. Lines of different colors above the T1 threshold: The affected neurons in various defined parts (signified by different colors) of the AD-afflicted brain. Dynamics of iAβ accumulation and the disease in separate defined brain regions are superimposed. Levels of AβPP-derived iAβ reach and cross the T1 threshold in a narrow temporal window in all affected neurons throughout the entire brain. Following the T1 crossing, the bulk of iAβ is produced in the AβPP-independent pathway. Panel A: The rate of AβPP-independent iAβ accumulation and its stochastic distribution are identical throughout the entire AD-affected brain, but extents of the T2 threshold are different in diverse defined brain compartments. The T2 threshold is reached and the affected neurons commit to apoptosis and die at different times in different brain regions; consequently, the AD pathology manifests in a sequential temporal order. Panel B: Both the rate of AβPP-independent iAβ accumulation and the extent of the T2 threshold are variable in separate defined regions of the affected brain and both contribute to sequential temporal manifestation of the AD pathology by determining the timing of its occurrence. Note that the extents of temporal shifts (e.g., in the T2 threshold crossings) could be significantly greater when both parameters are variable in defined regions of the brain. The depicted inverse proportionality between rates of AβPP-independent iAβ accumulation and extents of the T2 threshold (panel B) is shown for purposes of comparison and graphic convenience only; it is just one of multiple possible combinations of these two parameters in various defined regions of the AD-affected brain.

Figure 16

Principal stages of mammalian RNA-dependent mRNA amplification. Boxed line: Sense RNA. Single line: Antisense RNA. “AUG”: Codon for translation-initiating methionine. “TCE”: 3′-terminal complementary element of the antisense RNA; “ICE”: Internal complementary element of the antisense RNA. Filled yellow circle: Helicase/nucleotide-modifying activity complex. Blue lines (single and boxed): RNA strands following their separation by a helicase/modifying activity. Red arrows: Site of cleavage of the chimeric intermediate. Top panel: The progenitor of the mRNA amplification pathway: conventional, genome-transcribed mRNA. Middle panel: Projected stages of the chimeric pathway of mammalian mRNA amplification. The internal complementary element (ICE) is situated within a portion of antisense RNA corresponding to the 5′UTR of conventional mRNA progenitor; consequently, the chimeric RNA end product contains the entire coding region of conventional mRNA. Stage 1: RdRp-mediated transcription of the antisense RNA from the gene-transcribed sense RNA progenitor. Stage 2: Strand separation; helicase activity mounts 3′ poly(A) of the sense RNA and moves along it. Stage 3: TCE/ICE-facilitated folding of antisense RNA into self-priming conformation. Stage 4: 3’ terminus of the antisense RNA is extended into the sense RNA. Stage 5: Double-stranded portion of the hairpin structure is separated into sense and antisense RNA by helicase activity. Stage 6: When helicase reaches single-stranded portion of hairpin structure, it (or associated activity) cleaves the chimeric intermediate. Stage 7: 3′-trucated antisense RNA and chimeric RNA end products of the chimeric mRNA amplification pathway. Bottom panel: The ICE element is situated within a segment of antisense RNA corresponding to the coding region of conventional mRNA. Consequently, the amplified chimeric RNA end-product contains a 5′-truncated coding region of conventional mRNA. The translational outcome is decided by the location of the 5′-most translation initiation codon; if it is in-frame, translation would yield the C-terminal fragment of conventionally encoded polypeptide. Stages 3′ through 7′ correspond to stages 3 through 7.

Figure 17

Chimeric pathway of human AβPP mRNA amplification resulting in mRNA encoding the C100 fragment of AβPP: Projected principal stages. Lowercase letters: Nucleotide sequence of the antisense RNA. Uppercase letters: Nucleotide sequence of the sense RNA. Highlighted in yellow: The 3′ terminal (top) and the internal (bottom) elements of the human antisense AβPP RNA. “2011–2013”: Nucleotide positions (from the 3′ terminus of the antisense AβPP RNA) of the “uac” (highlighted in blue) corresponding to the “AUG” (highlighted in green) encoding Met671 in the human AβPP mRNA. Panel a: TCE/ICE-facilitated folding of the human AβPP antisense RNA into self-priming configuration. Panel b: Extension of self-primed AβPP antisense RNA into sense RNA (highlighted in gray). Red arrow: Cleavage of chimeric RNA intermediate following separation of sense and antisense RNA (not shown). The cleavage is shown at the 5′ end of the TCE element; it may also occur at one of the TCE/ICE mismatches. Panel c: Chimeric RNA end product of RNA-dependent amplification of human AβPP mRNA (highlighted in gray). It consists of antisense portion (the TCE or part thereof) extended into 5′ truncated coding region of human AβPP mRNA. Its first, 5′-most translation initiation codon is the in-frame AUG (highlighted in green) that encodes Met671 of human AβPP; when translated, it would produce the C100 fragment of AβPP.