Semagacestat for treatment of Alzheimer's disease

Recent advances in Alzheimer's disease: Mechanisms, clinical trials and new drug development strategies

Alzheimer's disease (AD) stands as the predominant form of dementia, presenting significant and escalating global challenges. Its etiology is intricate and diverse, stemming from a combination of factors such as aging, genetics, and environment. Our current understanding of AD pathologies involves various hypotheses, such as the cholinergic, amyloid, tau protein, inflammatory, oxidative stress, metal ion, glutamate excitotoxicity, microbiota-gut-brain axis, and abnormal autophagy. Nonetheless, unraveling the interplay among these pathological aspects and pinpointing the primary initiators of AD require further elucidation and validation. In the past decades, most clinical drugs have been discontinued due to limited effectiveness or adverse effects. Presently, available drugs primarily offer symptomatic relief and often accompanied by undesirable side effects. However, recent approvals of aducanumab (1) and lecanemab (2) by the Food and Drug Administration (FDA) present the potential in disrease-modifying effects. Nevertheless, the long-term efficacy and safety of these drugs need further validation. Consequently, the quest for safer and more effective AD drugs persists as a formidable and pressing task. This review discusses the current understanding of AD pathogenesis, advances in diagnostic biomarkers, the latest updates of clinical trials, and emerging technologies for AD drug development. We highlight recent progress in the discovery of selective inhibitors, dual-target inhibitors, allosteric modulators, covalent inhibitors, proteolysis-targeting chimeras (PROTACs), and protein-protein interaction (PPI) modulators. Our goal is to provide insights into the prospective development and clinical application of novel AD drugs.

PARP16-Mediated Stabilization of Amyloid Precursor Protein mRNA Exacerbates Alzheimer's Disease Pathogenesis

The accumulation and deposition of beta-amyloid (Aβ) are key neuropathological hallmarks of Alzheimer's disease (AD). PARP16, a Poly(ADP-ribose) polymerase, is a known tail-anchored endoplasmic reticulum (ER) transmembrane protein that transduces ER stress during pathological processes. Here, we found that PARP16 was significantly increased in the hippocampi and cortices of APPswe/PS1dE9 (APP/PS1) mice and hippocampal neuronal HT22 cells exposed to Aβ, suggesting a positive correlation between the progression of AD pathology and the overexpression of PARP16. To define the effect of PARP16 on AD progression, adeno-associated virus mediated-PARP16 knockdown was used in APP/PS1 mice to investigate the role of PARP16 in spatial memory, amyloid burden, and neuroinflammation. Knockdown of PARP16 partly attenuated impaired spatial memory, as indicated by the Morris water maze test, and decreased amyloid deposition, neuronal apoptosis, and the production of inflammatory cytokines in the brains of APP/PS1 mice. In vitro experiments demonstrated that the knockdown of PARP16 expression rescued neuronal damage and ER stress triggered by Aβ. Furthermore, we discovered that intracellular PARP16 acts as an RNA-binding protein that regulates the mRNA stability of amyloid precursor protein (APP) and protects targeted APP from degradation, thereby increasing APP levels and AD pathology. Our findings revealed an unanticipated role of PARP16 in the pathogenesis of AD, and at least in part, its association with increased APP mRNA stability.

Mouse model of Alzheimer's disease demonstrates differential effects of early disease pathology on various brain regions

Different parts of the brain are affected distinctively in various stages of the Alzheimer's disease (AD) pathogenesis. Identifying the biochemical changes in specific brain regions is key to comprehend the neuropathological mechanisms in early pre-symptomatic phases of AD. Quantitative proteomics profiling of four distinct areas of the brain of young APP/PS1 mouse model of AD was performed followed by biochemical pathway enrichment analysis. Findings revealed fundamental compositional and functional shifts even in the early stages of the disease. This novel study highlights unique proteome and biochemical pathway alterations in specific regions of the brain that underlie the early stages of AD pathology and will provide a framework for future longitudinal studies. The proteomics data were deposited into the ProteomeXchange Consortium via PRIDE with the identifier PXD019192.

Retinal changes in Alzheimer's disease- integrated prospects of imaging, functional and molecular advances

Alzheimer's Disease (AD) is a devastating neurodegenerative disorder of the brain, clinically characterised by cognitive deficits that gradually worsen over time. There is, at present, no established cure, or disease-modifying treatments for AD. As life expectancy increases globally, the number of individuals suffering from the disease is projected to increase substantially. Cumulative evidence indicates that AD neuropathological process is initiated several years, if not decades, before clinical signs are evident in patients, and diagnosis made. While several imaging, cognitive, CSF and blood-based biomarkers have been proposed for the early detection of AD; their sensitivity and specificity in the symptomatic stages is highly variable and it is difficult to justify their use in even earlier, pre-clinical stages of the disease. Research has identified potentially measurable functional, structural, metabolic and vascular changes in the retina during early stages of AD. Retina offers a distinctively accessible insight into brain pathology and current and developing ophthalmic technologies have provided us with the possibility of detecting and characterising subtle, disease-related changes. Recent human and animal model studies have further provided mechanistic insights into the biochemical pathways that are altered in the retina in disease, including amyloid and tau deposition. This information coupled with advances in molecular imaging has allowed attempts to monitor biochemical changes and protein aggregation pathology in the retina in AD. This review summarises the existing knowledge that informs our understanding of the impact of AD on the retina and highlights some of the gaps that need to be addressed. Future research will integrate molecular imaging innovation with functional and structural changes to enhance our knowledge of the AD pathophysiological mechanisms and establish the utility of monitoring retinal changes as a potential biomarker for AD.

Recent developments of small molecule γ-secretase modulators for Alzheimer's disease

Alzheimer's disease (AD) is the most common form of progressive neurodegenerative disorder, marked by memory loss and a decline in cognitive function. The major hallmarks of AD are the presence of intracellular neurofibrillary tau tangles (NFTs) composed of hyperphosphorylated tau proteins and extracellular plaques composed of amyloid beta peptides (Aβ). The amyloid (Aβ) cascade hypothesis proposes that the AD pathogenesis is initiated by the accumulation of Aβ peptides in the parenchyma of the brain. An aspartyl intramembranal protease called γ-secretase is responsible for the production of Aβ by the cleavage of the amyloid precursor protein (APP). Clinical studies of γ-secretase inhibitors (GSIs) for AD failed due to the lack of substrate specificity. Therefore, γ-secretase modulators (GSMs) have been developed as potential disease modifying agents to modulate the γ-secretase cleavage activity towards the production of toxic Aβ42 peptides. Following the first-generation 'nonsteroidal anti-inflammatory drug' (NSAID) based GSMs, second-generation GSMs (carboxylic acid based NSAID derivatives and non-NSAID derived heterocyclic analogues), as well as natural product-based GSMs, have been developed. In this review, we focus on the recent developments of small molecule-based GSMs that show potential improvements in terms of drug-like properties as well as their current status in human clinical trials and the future perspectives of GSM research.

Upregulation of RIN3 induces endosomal dysfunction in Alzheimer's disease

BACKGROUND

In Alzheimer's Disease (AD), about one-third of the risk genes identified by GWAS encode proteins that function predominantly in the endocytic pathways. Among them, the Ras and Rab Interactor 3(RIN3) is a guanine nucleotide exchange factor (GEF) for the Rab5 small GTPase family and has been implicated to be a risk factor for both late onset AD (LOAD) and sporadic early onset AD (sEOAD). However, how RIN3 is linked to AD pathogenesis is currently undefined.

METHODS

Quantitative PCR and immunoblotting were used to measure the RIN3 expression level in mouse brain tissues and cultured basal forebrain cholinergic neuron (BFCNs). Immunostaining was used to define subcellular localization of RIN3 and to visualize endosomal changes in cultured primary BFCNs and PC12 cells. Recombinant flag-tagged RIN3 protein was purified from HEK293T cells and was used to define RIN3-interactomes by mass spectrometry. RIN3-interacting partners were validated by co-immunoprecipitation, immunofluorescence and yeast two hybrid assays. Live imaging of primary neurons was used to examine axonal transport of amyloid precursor protein (APP) and β-secretase 1 (BACE1). Immunoblotting was used to detect protein expression, processing of APP and phosphorylated forms of Tau.

RESULTS

We have shown that RIN3 mRNA level was significantly increased in the hippocampus and cortex of APP/PS1 mouse brain. Basal forebrain cholinergic neurons (BFCNs) cultured from E18 APP/PS1 mouse embryos also showed increased RIN3 expression accompanied by early endosome enlargement. In addition, via its proline rich domain, RIN3 recruited BIN1(bridging integrator 1) and CD2AP (CD2 associated protein), two other AD risk factors, to early endosomes. Interestingly, overexpression of RIN3 or CD2AP promoted APP cleavage to increase its carboxyl terminal fragments (CTFs) in PC12 cells. Upregulation of RIN3 or the neuronal isoform of BIN1 increased phosphorylated Tau level. Therefore, upregulation of RIN3 expression promoted accumulation of APP CTFs and increased phosphorylated Tau. These effects by RIN3 was rescued by the expression of a dominant negative Rab5 (Rab5^S34N) construct. Our study has thus pointed to that RIN3 acts through Rab5 to impact endosomal trafficking and signaling.

CONCLUSION

RIN3 is significantly upregulated and correlated with endosomal dysfunction in APP/PS1 mouse. Through interacting with BIN1 and CD2AP, increased RIN3 expression alters axonal trafficking and procession of APP. Together with our previous studies, our current work has thus provided important insights into the role of RIN3 in regulating endosomal signaling and trafficking.

The Beta Amyloid Dysfunction (BAD) Hypothesis for Alzheimer's Disease

Beta amyloid, Aβ 1–42, originally named as Amyloid A4 protein, is one of the most investigated peptides in neuroscience and has attracted substantial interest since its discovery as the main insoluble fibril-type protein in cerebrovascular amyloid angiopathy (Glenner and Wong, 1984; Masters et al., 1985) of Alzheimer’s disease (AD). From the very beginning, Aβ was regarded per se as a “bad molecule,” triggering the so-called “beta amyloid cascade hypothesis” (Hardy and Higgins, 1992). This hypothesis ignored any physiological function for in situ generated Aβ monomer with normal production and turnover rate (Bateman et al., 2006). Accordingly, pan-Aβ-related therapeutic approaches were designed to eliminate or lower the three structural isoforms in parallel: (1) the pre-amyloid monomer, (2) the misfolded oligomer, and (3) the final fibril. While we already knew about poor correlations between plaques and cognitive decline quite early (Terry et al., 1991), data for an essential benign physiological role for Aβ monomer at low concentrations were also not considered to be relevant. Here, a different Beta Amyloid hypothesis is described, the so-called “Beta Amyloid Dysfunction hypothesis,” which, in contrast to the “Beta Amyloid Cascade hypothesis,” builds on the homeostasis of essential Aβ monomer in the synaptic vesicle cycle (SVC). Disease-relevant early pathology emerges through disturbance of the Aβ homeostasis by so far unknown factors leading to the formation of misfolded Aβ oligomers. These early species interfere with the synaptic physiological Aβ monomer regulation and exert their neurotoxicity via various receptors for sticky oligomer-type Aβ aggregates. The Beta Amyloid Dysfunction (BAD) hypothesis is introduced and shown to explain negative clinical results of Gamma-secretase and Beta-secretase (BACE) inhibitors as well as pan-Aβ isotype unselective immunotherapies. This hypothesis gives guidance to what needs to be done therapeutically to revive successful clinical testing in AD for this highly validated target. The BAD hypothesis will need further refinement in particular through more detailed exploration for the role of physiological Aβ monomer.

The vexing complexity of the amyloidogenic pathway

The role of the amyloidogenic pathway in the etiology of Alzheimer's disease (AD), particularly the common sporadic late onset forms of the disease, is controversial. To some degree, this is a consequence of the failure of drug and therapeutic antibody trials based either on targeting the proteases in this pathway or its amyloid end products. Here, we explore the formidable complexity of the biochemistry and cell biology associated with this pathway. For example, we review evidence that the immediate precursor of amyloid-β, the C99 domain of the amyloid precursor protein (APP), may itself be toxic. We also review important new results that appear to finally establish a direct genetic link between mutations in APP and the sporadic forms of AD. Based on the complexity of amyloidogenesis, it seems possible that a major contributor to the failure of related drug trials is that we have an incomplete understanding of this pathway and how it is linked to Alzheimer's pathogenesis. If so, this highlights a need for further characterization of this pathway, not its abandonment.

Aberrant Neuronal Cell Cycle Re-Entry: The Pathological Confluence of Alzheimer's Disease and Brain Insulin Resistance, and Its Relation to Cancer

Aberrant neuronal cell cycle re-entry (CCR) is a phenomenon that precedes and may mechanistically lead to a majority of the neuronal loss observed in Alzheimer's disease (AD). Recent developments concerning the regulation of aberrant neuronal CCR in AD suggest that there are potential intracellular signaling "hotspots" in AD, cancer, and brain insulin resistance, the latter of which is characteristically associated with AD. Critically, these common signaling nodes across different human diseases may represent currently untapped therapeutic opportunities for AD. Specifically, repurposing of existing US Food and Drug Administration-approved pharmacological agents, including experimental therapeutics that target the cell cycle in cancer, may be an innovative avenue for future AD-directed drug discovery and development. In this review we discuss overlapping aspects of AD, cancer, and brain insulin resistance from the perspective of neuronal CCR, and consider strategies to exploit them for prevention or therapeutic intervention of AD.

Advances in developing novel therapeutic strategies for Alzheimer's disease

Alzheimer's Disease (AD), the most prevalent neurodegenerative disease of aging, affects one in eight older Americans. Nearly all drug treatments tested for AD today have failed to show any efficacy. There is a great need for therapies to prevent and/or slow the progression of AD. The major challenge in AD drug development is lack of clarity about the mechanisms underlying AD pathogenesis and pathophysiology. Several studies support the notion that AD is a multifactorial disease. While there is abundant evidence that amyloid plays a role in AD pathogenesis, other mechanisms have been implicated in AD such as tangle formation and spread, dysregulated protein degradation pathways, neuroinflammation, and loss of support by neurotrophic factors. Therefore, current paradigms of AD drug design have been shifted from single target approach (primarily amyloid-centric) to developing drugs targeted at multiple disease aspects, and from treating AD at later stages of disease progression to focusing on preventive strategies at early stages of disease development. Here, we summarize current strategies and new trends of AD drug development, including pre-clinical and clinical trials that target different aspects of disease (mechanism-based versus non-mechanism based, e.g. symptomatic treatments, lifestyle modifications and risk factor management).

Dysregulation of Rab5-mediated endocytic pathways in Alzheimer's disease

Increasing evidence has pointed to that dysregulation of the endo-lysosomal system is an early cellular phenotype of pathogenesis for Alzheimer's disease (AD). Rab5, a small GTPase, plays a critical role in mediating these processes. Abnormal overactivation of Rab5 has been observed in post-mortem brain samples of Alzheimer's patients as well as brain samples of mouse models of AD. Recent genome-wide association studies of AD have identified RIN3 (Ras and Rab interactor 3) as a novel risk factor for the disease. RIN3 that functions as a guanine nucleotide exchange factor for Rab5 may serve as an important activator for Rab5 in AD pathogenesis. In this review, we present recent research highlights on the possible roles of dysregulation of Rab5-mediated endocytic pathways in contributing to early pathogenesis of AD.

Regulated intramembrane proteolysis of the AXL receptor kinase generates an intracellular domain that localizes in the nucleus of cancer cells

Deregulation of the TAM (TYRO3, AXL, and MERTK) family of receptor tyrosine kinases (RTKs) has recently been demonstrated to predominately promote survival and chemoresistance of cancer cells. Intramembrane proteolysis mediated by presenilin/γ-secretase is known to regulate the homeostasis of some RTKs. In the present study, we demonstrate that AXL, but not TYRO3 or MERTK, is efficiently and sequentially cleaved by α- and γ-secretases in various types of cancer cell lines. Proteolytic processing of AXL redirected signaling toward a secretase-mediated pathway, away from the classic, well-known, ligand-dependent canonical RTK signaling pathway. The AXL intracellular domain cleavage product, but not full-length AXL, was further shown to translocate into the nucleus via a nuclear localization sequence that harbored a basic HRRKK motif. Of interest, we found that the γ-secretase-uncleavable AXL mutant caused an elevated chemoresistance in non-small-cell lung cancer cells. Altogether, our findings suggest that AXL can undergo sequential processing mediated by various proteases kept in a homeostatic balance. This newly discovered post-translational processing of AXL may provide an explanation for the diverse functions of AXL, especially in the context of drug resistance in cancer cells.-Lu, Y., Wan, J., Yang, Z., Lei, X., Niu, Q., Jiang, L., Passtoors, W. M., Zang, A., Fraering, P. C., Wu, F. Regulated intramembrane proteolysis of the AXL receptor kinase generates an intracellular domain that localizes in the nucleus of cancer cells.

One protein, multiple pathologies: multifaceted involvement of amyloid β in neurodegenerative disorders of the brain and retina

Accumulation of amyloid β (Aβ) and its aggregates in the ageing central nervous system is regarded synonymous to Alzheimer's disease (AD) pathology. Despite unquestionable advances in mechanistic and diagnostic aspects of the disease understanding, the primary cause of Aβ accumulation as well as its in vivo roles remains elusive; nonetheless, the majority of the efforts to address pathological mechanisms for therapeutic development are focused towards moderating Aβ accumulation in the brain. More recently, Aβ deposition has been identified in the eye and is linked with distinct age-related diseases including age-related macular degeneration, glaucoma as well as AD. Awareness of the Aβ accumulation in these markedly different degenerative disorders has led to an increasing body of work exploring overlapping mechanisms, a prospective biomarker role for Aβ and the potential to use retina as a model for brain related neurodegenerative disorders. Here, we present an integrated view of current understanding of the retinal Aβ deposition discussing the accumulation mechanisms, anticipated impacts and outlining ameliorative approaches that can be extrapolated to the retina for potential therapeutic benefits. Further longitudinal investigations in humans and animal models will determine retinal Aβ association as a potential pathognomonic, diagnostic or prognostic biomarker.

Amyloid precursor protein-mediated endocytic pathway disruption induces axonal dysfunction and neurodegeneration

The endosome/lysosome pathway is disrupted early in the course of both Alzheimer's disease (AD) and Down syndrome (DS); however, it is not clear how dysfunction in this pathway influences the development of these diseases. Herein, we explored the cellular and molecular mechanisms by which endosomal dysfunction contributes to the pathogenesis of AD and DS. We determined that full-length amyloid precursor protein (APP) and its β-C-terminal fragment (β-CTF) act though increased activation of Rab5 to cause enlargement of early endosomes and to disrupt retrograde axonal trafficking of nerve growth factor (NGF) signals. The functional impacts of APP and its various products were investigated in PC12 cells, cultured rat basal forebrain cholinergic neurons (BFCNs), and BFCNs from a mouse model of DS. We found that the full-length wild-type APP (APPWT) and β-CTF both induced endosomal enlargement and disrupted NGF signaling and axonal trafficking. β-CTF alone induced atrophy of BFCNs that was rescued by the dominant-negative Rab5 mutant, Rab5S34N. Moreover, expression of a dominant-negative Rab5 construct markedly reduced APP-induced axonal blockage in Drosophila. Therefore, increased APP and/or β-CTF impact the endocytic pathway to disrupt NGF trafficking and signaling, resulting in trophic deficits in BFCNs. Our data strongly support the emerging concept that dysregulation of Rab5 activity contributes importantly to early pathogenesis of AD and DS.

Swedish mutant APP-based BACE1 binding site peptide reduces APP β-cleavage and cerebral Aβ levels in Alzheimer's mice

BACE1 initiates amyloid-β (Aβ) generation and the resultant cerebral amyloidosis, as a characteristic of Alzheimer's disease (AD). Thus, inhibition of BACE1 has been the focus of a large body of research. The most recent clinical trials highlight the difficulty involved in this type of anti-AD therapy as evidenced by side effects likely due to the ubiquitous nature of BACE1, which cleaves multiple substrates. The human Swedish mutant form of amyloid protein precursor (APPswe) has been shown to possess a higher affinity for BACE1 compared to wild-type APP (APPwt). We pursued a new approach wherein harnessing this greater affinity to modulate BACE1 APP processing activity. We found that one peptide derived from APPswe, containing the β-cleavage site, strongly inhibits BACE1 activity and thereby reduces Aβ production. This peptide, termed APPswe BACE1 binding site peptide (APPsweBBP), was further conjugated to the fusion domain of the HIV-1 Tat protein (TAT) at the C-terminus to facilitate its biomembrane-penetrating activity. APPwt and APPswe over-expressing CHO cells treated with this TAT-conjugated peptide resulted in a marked reduction of Aβ and a significant increase of soluble APPα. Intraperitoneal administration of this peptide to 5XFAD mice markedly reduced β-amyloid deposits as well as improved hippocampal-dependent learning and memory.

Function, therapeutic potential and cell biology of BACE proteases: current status and future prospects

The β-site APP cleaving enzymes 1 and 2 (BACE1 and BACE2) were initially identified as transmembrane aspartyl proteases cleaving the amyloid precursor protein (APP). BACE1 is a major drug target for Alzheimer's disease because BACE1-mediated cleavage of APP is the first step in the generation of the pathogenic amyloid-β peptides. BACE1, which is highly expressed in the nervous system, is also required for myelination by cleaving neuregulin 1. Several recent proteomic and in vivo studies using BACE1- and BACE2-deficient mice demonstrate a much wider range of physiological substrates and functions for both proteases within and outside of the nervous system. For BACE1 this includes axon guidance, neurogenesis, muscle spindle formation, and neuronal network functions, whereas BACE2 was shown to be involved in pigmentation and pancreatic β-cell function. This review highlights the recent progress in understanding cell biology, substrates, and functions of BACE proteases and discusses the therapeutic options and potential mechanism-based liabilities, in particular for BACE inhibitors in Alzheimer's disease. The protease BACE1 is a major drug target in Alzheimer disease. Together with its homolog BACE2, both proteases have an increasing number of functions within and outside of the nervous system. This review highlights recent progress in understanding cell biology, substrates, and functions of BACE proteases and discusses the therapeutic options and potential mechanism-based liabilities, in particular for BACE inhibitors in Alzheimer disease.

Roles of matrix metalloproteinases and their natural inhibitors in prostate cancer progression

Matrix metalloproteinases (MMPs), a group of zinc-dependent endopeptidases involved in the degradation of the extracellular matrix, play an important role in tissue remodeling associated with various physiological processes such as morphogenesis, angiogenesis, and tissue repair, as well as pathological processes including cirrhosis, arthritis and cancer. The MMPs are well established as mediators of tumor invasion and metastasis by breaking down connective tissue barriers. Although there has been a vast amount of literature on the role of MMPs in invasion, metastasis and angiogenesis of various cancers, the role of these endopeptidases in prostate cancer progression has not been systematically reviewed. This overview summarizes findings on the tissue and blood expression of MMPs, their function, regulation and prognostic implication in human prostate cancer, with a focus on MMP-2, -7, -9, MT1-MMP and tissue inhibitor of metalloproteinase 1 (TIMP-1). This review also summarizes the efficacy and failure of early-generation matrix metalloproteinase inhibitors (MMPIs) in the treatment of metastatic prostate cancer and highlights the lessons and challenges for next generation MMPIs.