Alzheimer's Disease Therapeutics: Alternatives to Amyloid 2019

There is a lot of evidence that amyloid-β (Aβ) is a key trigger in Alzheimer’s disease (AD) pathogenesis. It begins to accumulate in the brain beginning ~ 20 years prior to the onset of cognitive decline. It is an accompanied by an innate immune response which appears to protect against some of the downstream consequences of Aβ. Just before cognitive decline, tau accumulation spreads from the medial temporal lobe to neocortical regions. This spread is accompanied by an additional innate immune response which together appears to drive neurodegeneration in the regions in which this process occurs as well as corresponding cognitive changes. These data suggest that targeting Aβ may be beneficial in delaying the onset of AD when done before it builds up (primary prevention) or possibly when it is building up (secondary prevention) but prior to cognitive decline. Activating the innate immune response when Aβ is accumulating may also be beneficial. However, inhibiting the brain’s innate immune response once significant tau accumulation begins to occur or with symptoms onset may be the best approach as the disease progresses. Preventing the spreading of tau pathology, decreasing tau levels, and clearing abnormally phosphorylated, aggregated tau may prove beneficial both as a secondary prevention as well as a treatment during the symptomatic phase of AD. As apoE contributes to both amyloid deposition as well as to tau-mediated neurodegeneration, decreasing apoE levels in the brain may prove useful as a primary prevention, secondary prevention, or during the early symptomatic phase of AD.

Individuals with Alzheimer disease have substantially different rates of progression – for largely uncertain reasons. We tested the hypothesis that different properties in the likelihood that tau propagation efficiency might underlie part of this variation. Tau isolated from postmortem frontal cortex showed different biochemical properties from different individuals, including a spectrum of protease resistance, behavior on size exclusion columns, behavior on an in vitro seeding assay, and differences in specific phosphorylation patterns as assessed by ELISA or Mass Spec. We postulate that these differences in tau cross cases may impact the rate of tau propagation across neural systems, and thus clinical rate of progression.

Increasing evidence indicates that neurodegenerative disease, such as Alzheimer’s disease (AD), is the product of an individual’s genetic risk in the context of their environmental exposures. The immune system plays a major role in arbitrating the gene-environment interactions that contribute to risk for AD. Peripheral immune cell signaling plays an important role in AD through regulating immune cell interactions within the microvasculature and meninges of the CNS, as well as the gut and BBB. A diet high in fat and sugar dysregulates several signaling pathways that trigger immune and metabolic responses that can result in metabolic syndrome and increased risk for AD. Among these, cytokines regulate peripheral immune cell trafficking to inflamed tissues, including the gut and brain. The cytokine tumor necrosis factor (TNF) is elevated in AD patients, regulates brain and gut barrier permeability, and mediates conditions involved in metabolic syndrome. Soluble TNF is a key mediator of peripheral immune cell contributions to AD-like pathology and metabolic dysfunction. We investigated the effects of chronic high-fat-high-carbohydrate (HFHC) diet-induced peripheral inflammation on neuroinflammation and AD-like pathology in young 5xFAD mice. They were HFHC or a control diet (CD) for 8 weeks. After 4 weeks, brain-permeant XPro1595 was given systemically to selectively inhibit solTNF signaling. XPro1595 reversed several HFHC diet-induced phenotypes in 5xFAD mice, including abnormal immune cell traffic to the brain, alterations in the gut microbiome, “leaky gut” syndrome, and brain inflammation. Our findings suggest timely targeting of solTNF with XPro1595 may reduce risk for neuroinflammation and neurodegeneration without immunosuppression.

Coauthors: Kathryn MacPherson, Lori N. Eidson, Mary K. Herrick, Maria Elizabeth Rodrigues, Lindsey Sniffen, Sean D. Kelly, Adam Hamilton, Danielle Oliver, Yuan Yang, and Jianjun Chang, Emory University School of Medicine, Atlanta.

Alzheimer’s disease and related dementias are pervasive disorders characterized by changes in brain activity, whose complex etiologies involves multiple brain cell types, but remain poorly characterized. To address this challenge, we analyze 80,660 single-nucleus transcriptomes from prefrontal cortex of disease vs. control individuals, and integrate these data with providing the single-cell cortical views of brain pathologies. We identify transcriptionally-distinct cell subpopulations capturing six major brain cell-types, and analyzed their association with pathology. In the case of Alzheimer’s disease (AD), we uncover coherent neuronal and glial subpopulations associated with AD, and identified regulators of myelination, inflammation, and neuron survival as top markers characterizing them. We find that the strongest AD-associated changes appear early in disease progression, and are highly cell-type specific. By contrast, genes up-regulated in late-stage disease progression are common across cell types, and primarily involved in global stress response. Surprisingly, we found that cells isolated from female individuals are overrepresented in the AD-associated cell subpopulations, revealing substantially different transcriptional responses between sexes. We also combined single-cell profiles, tissue-level variation, and genetic variation across healthy and diseased individuals to deconvolve bulk profiles into single-cell profiles, to recognize changes in cell type proportion associated with disease and aging, and to partition genetic effects into the individual cell types where they act. These revealed a change in cell type proportion associated with aging and with AD, with decreased excitatory and inhibitory neurons, and increased astrocytes, which can become cytotoxic. They also revealed genetic variants underlying cell-type-proportion, including in the TMEM106B locus, which has been previously implicated in Fronto-temporal lobal degeneration (FTLD), and was associated with decreased fraction of inhibitory neurons, but not associated with AD directly. These results provide a roadmap for translating genetic findings into mechanistic insights and ultimately new therapeutic avenues for complex brain disorders.

Regenerative therapeutics hold the promise of self-renewal and repair. Ageing and age-associated neurodegenerative diseases are marked by a decline in these functions but a capacity for regeneration is retained well into the 7th decade of life and can endure under disease conditions. The challenge in developing therapeutics to promote self-renewal in the nervous system is to activate regenerative and repair pathways in the context of progressive degeneration.

Neurosteroids regulate both regeneration and repair systems in the brain, and among this class of molecules, allopregnanolone has been extensively investigated for its role to promote regeneration in both the central and peripheral nervous systems. In brain, allopregnanolone induced neurogenesis and survival of new neurons in both aging and Alzheimer’s disease discovery models. Regeneration of neural stem cells and subsequent neuronal differentiation were accompanied by restoration of learning and memory function. In addition to promoting regeneration, allopregnanolone increased expression of mechanisms that promote mitochondrial function, myelin and white matter regeneration, clearance of β-amyloidand reduced nflammation and hyperphosphorylated tau.

Primary outcomes from a Phase 1b/2a double-blind placebo controlled clinical trial in persons with Stage 2-3 Alzheimer’s disease treated with allopregnanolone demonstrated safety, predictable pharmacokinetics and maximally tolerated dose. Secondary outcomes were predictive of regeneration and consistent with translational analyses of efficacy.

Challenges for regenerative therapeutics span discovery and translational models, non-linear dose profiles, optimization of treatment regimen and activation of multiple targets. Therapeutics with pleiotropic mechanisms of action that promote regeneration and repair while also reducing burden of pathology are likely to exert greater therapeutic efficacy. In that regard, Allopregnanolone serves as promising therapeutic for Alzheimer’s disease to promote regeneration, repair and recovery of cognitive function.

There is evidence of increased neuronal excitability in subfields of the medial temporal lobe during the early symptomatic phase of Alzheimer’s disease (i.e., the Mild Cognitive Impairment / Prodromal phase of disease). It was initially hypothesized that this increased neuronal excitability was compensatory, and therefore reflected a beneficial response to the presence of increasing pathology. However, molecular and imaging data from both animal models and humans strongly suggest that this neuronal hyperactivity reflects increased vulnerability to accumulating pathology.  A small randomized clinical trial has been completed showing that treatment with low dose Levetiracetam decreases hippocampal hyperactivity and improves memory performance among individuals with mild cognitive impairment. A large clinical trial is now underway to further examine the efficacy of this treatment approach. Additionally, there are ongoing studies to clarify the underlying mechanisms by which neuronal hyperactivity may accelerate the progression of disease.

Alzheimer’s disease (AD) is defined by amyloid beta (Aβ) plaques and neurofibrillary tangles and characterized by neurodegeneration and memory loss. Apolipoprotein E (apoE) is the major genetic risk factor for AD with multiple roles in AD pathogenesis, primarily assumed to occur within the brain. Importantly, as many AD patients also have vascular co-morbidities including Aβ deposition in cerebral vessels known as cerebral amyloid angiopathy (CAA) and microhemorrhages, promoting cerebrovascular resilience may therefore be a promising therapeutic or preventative strategy for AD. Plasma high-density lipoproteins (HDL) have several vasoprotective functions and are associated with reduced AD risk in epidemiological studies. In mice, deficiency of apoA-I, the primary protein component of HDL, increases CAA and cognitive dysfunction, whereas overexpression of apoA-I from its native promoter in liver and intestine has the opposite effect and lessens neuroinflammation. Similarly, acute peripheral administration of HDL reduces soluble Aβ pools in the brain. New animal model data support a role for apoA-I, the major apolipoprotein in HDL, in reducing astrocyte reactivity to parenchymal and vascular amyloid in the cortex and attenuating parenchymal and vascular ICAM-1 in the hippocampus. Studies using novel 3-dimensional engineered human cerebral vessels show that HDL, especially the fraction of HDL enriched in apoE, reduces Aβ deposition and Aβ-induced endothelial activation and is providing new insights into multiple mechanisms by which HDL can protect the cerebrovasculature. Taken together, HDL may be an attractive non-amyloid approach to prevent or treat cerebrovascular dysfunction for AD.

Objectives: Animal models of Alzheimer’s disease (AD) recapitulate the severe amyloidosis and neuroinflammation that is evident in the human disease. Neuroinflammation is associated with activation of astrocytes and microglia in response to injury, but the role of peripheral tissues and more importantly, the microbiota in regulating innate immunity that in turn leads to CNS dysfunction has not been defined. We have tested the hypothesis that the composition of the intestinal microbiome plays a role in modulating neuro-inflammation that will ultimately influence amyloid deposition in two established mouse models of Aβ-amyloidosis.

Methods: We orally administered a combination of antibiotics to induce rapid and sustained alterations in gut microbial populations. The antibiotic cocktail was administered either postnatally or throughout the lifetime of the animal prior to cull and we employed IHC, biochemical and transcriptomic assays to evaluate amyloid deposition and neuroinflammation in the mouse models.

Results: Our studies indicate that alterations in the microbiome parallel changes in plasma cytokines and chemokines, reductions in amyloid deposition and modulation of morphological and transcriptional landscapes of microglia that only occurs in male, but not female animals.

Conclusions: Our studies reveal an unexpected, but significant alteration in amyloid deposition and microglial phenotypes in the brains of transgenic mice upon treatment with orally administered antibiotics.

Acknowledgments: This work was supported by Cure Alzheimer’s Fund (CAF), Open Philanthropy Fund and Good Ventures Foundation.