Reducing Endogenous α-Synuclein Mitigates the Degeneration of Selective Neuronal Populations in an Alzheimer's Disease Transgenic Mouse Model.

UNLABELLED: Alzheimer's disease (AD) is characterized by the progressive accumulation of amyloid ß (Aß) and microtubule associate protein tau, leading to the selective degeneration of neurons in the neocortex, limbic system, and nucleus basalis, among others. Recent studies have shown that α-synuclein (α-syn) also accumulates in the brains of patients with AD and interacts with Aß and tau, forming toxic hetero-oligomers. Although the involvement of α-syn has been investigated extensively in Lewy body disease, less is known about the role of this synaptic protein in AD. Here, we found that reducing endogenous α-syn in an APP transgenic mouse model of AD prevented the degeneration of cholinergic neurons, ameliorated corresponding deficits, and recovered the levels of Rab3a and Rab5 proteins involved in intracellular transport and sorting of nerve growth factor and brain-derived neurotrophic factor. Together, these results suggest that α-syn might participate in mechanisms of vulnerability of selected neuronal populations in AD and that reducing α-syn might be a potential approach to protecting these populations from the toxic effects of Aß. SIGNIFICANCE STATEMENT: Reducing endogenous α-synuclein (α-syn) in an APP transgenic mouse model of Alzheimer's disease (AD) prevented the degeneration of cholinergic neurons, ameliorated corresponding deficits, and recovered the levels of Rab3a and Rab5 proteins involved in intracellular transport and sorting of nerve growth factor and brain-derived neurotrophic factor. These results suggest that α-syn might participate in mechanisms of vulnerability of selected neuronal populations in AD and that reducing α-syn might be a potential approach to protecting these populations from the toxic effects of amyloid ß.

Forkhead box protein p1 is a transcriptional repressor of immune signaling in the CNS: implications for transcriptional dysregulation in Huntington disease.

Forkhead box protein p1 (Foxp1), a transcription factor showing highly enriched expression in the striatum, has been implicated in central nervous system (CNS) development, but its role in the mature brain is unknown. In order to ascertain functional roles for Foxp1 in the CNS, we have identified gene targets for Foxp1 both in vitro and in vivo using genome-wide expression microarrays and chromatin-immunoprecipitation followed by high-throughput sequencing (ChIP-seq) assays. We found that mouse Foxp1 overexpression in striatal cells elicited expression changes of genes related to immune signaling, transcriptional regulation and a manually curated Huntington's disease (HD)-signaling pathway. Similar results were found when the gene expression data set was integrated with Foxp1-binding data determined from ChIP-seq analysis. In vivo lentiviral-mediated overexpression of human FOXP1 in the context of mutant huntingtin (Htt) protein resulted in a robust downregulation of glial cell-associated, immune genes, including those encoding a variety of cytokines and chemokines. Furthermore, Foxp1-induced expression changes were significantly negatively correlated with those changes elicited by mutant Htt protein in several different HD mouse models, and most significantly in post-mortem caudate from human HD subjects. We finally show that Foxp1 interacts with mutant Htt protein in mouse brain and is present in nuclear Htt aggregates in the striatum of R6/1 transgenic mice. These findings implicate Foxp1 as a key repressor of immune signaling in the CNS and suggest that the loss of Foxp1-mediated gene regulation in HD contributes to the immune dysfunction in this disease. We further suggest that Foxp1-regulated pathways might be important mediators of neuronal-glial cell communication.

In vivo demonstration that alpha-synuclein oligomers are toxic.

The aggregation of proteins into oligomers and amyloid fibrils is characteristic of several neurodegenerative diseases, including Parkinson disease (PD). In PD, the process of aggregation of α-synuclein (α-syn) from monomers, via oligomeric intermediates, into amyloid fibrils is considered the disease-causative toxic mechanism. We developed α-syn mutants that promote oligomer or fibril formation and tested the toxicity of these mutants by using a rat lentivirus system to investigate loss of dopaminergic neurons in the substantia nigra. The most severe dopaminergic loss in the substantia nigra is observed in animals with the α-syn variants that form oligomers (i.e., E57K and E35K), whereas the α-syn variants that form fibrils very quickly are less toxic. We show that α-syn oligomers are toxic in vivo and that α-syn oligomers might interact with and potentially disrupt membranes.

Transcriptomic profiling of sporadic Alzheimer's disease patients.

Alzheimer's disease (AD) manifested before age 65 is commonly referred to as early-onset AD (EOAD) (Reitz et al. Neurol Genet. 2020;6:e512). While the majority (> 90%) of EOAD cases are not caused by autosomal-dominant mutations in PSEN1, PSEN2, and APP, they do have a higher heritability (92-100%) than sporadic late-onset AD (LOAD, 70%) (Wingo et al. Arch Neurol. 2012;69:59-64, Fulton-Howard et al. Neurobiol Aging. 2021;99:101.e1-101.e9). Although the endpoint clinicopathological changes, i.e., Aß plaques, tau tangles, and cognitive decline, are common across EOAD and LOAD, the disease progression is highly heterogeneous (Neff et al. Sci Adv Am Assoc Adv Sci. 2021;7:eabb5398). This heterogeneity, leading to temporally distinct age at onset (AAO) and stages of cognitive decline, may be caused by myriad combinations of distinct disease-associated molecular mechanisms. We and others have used transcriptome profiling in AD patient-derived neuron models of autosomal-dominant EOAD and sporadic LOAD to identify disease endotypes (Caldwell et al. Sci Adv Am Assoc Adv Sci. 2020;6:eaba5933, Mertens et al. Cell Stem Cell. 2021;28:1533-1548.e6, Caldwell et al. Alzheimers Demen. 2022). Further, analyses of large postmortem brain cohorts demonstrate that only one-third of AD patients show hallmark disease endotypes like increased inflammation and decreased synaptic signaling (Neff et al. Sci Adv Am Assoc Adv Sci. 2021;7:eabb5398). Areas of the brain less affected by AD pathology at early disease stages-such as the primary visual cortex-exhibit similar transcriptomic dysregulation as those regions traditionally affected and, therefore, may offer a view into the molecular mechanisms of AD without the associated inflammatory changes and gliosis induced by pathology (Haroutunian et al. Neurobiol Aging. 2009;30:561-73). To this end, we analyzed AD patient samples from the primary visual cortex (19 EOAD, 20 LOAD) using transcriptomic signatures to identify patient clusters and disease endotypes. Interestingly, although the clusters showed distinct combinations and severity of endotypes, each patient cluster contained both EOAD and LOAD cases, suggesting that AAO may not directly correlate with the identity and severity of AD endotypes.

The HDAC inhibitor 4b ameliorates the disease phenotype and transcriptional abnormalities in Huntington's disease transgenic mice.

Transcriptional dysregulation has emerged as a core pathologic feature of Huntington's disease (HD), one of several triplet-repeat disorders characterized by movement deficits and cognitive dysfunction. Although the mechanisms contributing to the gene expression deficits remain unknown, therapeutic strategies have aimed to improve transcriptional output via modulation of chromatin structure. Recent studies have demonstrated therapeutic effects of commercially available histone deacetylase (HDAC) inhibitors in several HD models; however, the therapeutic value of these compounds is limited by their toxic effects. Here, beneficial effects of a novel pimelic diphenylamide HDAC inhibitor, HDACi 4b, in an HD mouse model are reported. Chronic oral administration of HDACi 4b, beginning after the onset of motor deficits, significantly improved motor performance, overall appearance, and body weight of symptomatic R6/2(300Q) transgenic mice. These effects were associated with significant attenuation of gross brain-size decline and striatal atrophy. Microarray studies revealed that HDACi 4b treatment ameliorated, in part, alterations in gene expression caused by the presence of mutant huntingtin protein in the striatum, cortex, and cerebellum of R6/2(300Q) transgenic mice. For selected genes, HDACi 4b treatment reversed histone H3 hypoacetylation observed in the presence of mutant huntingtin, in association with correction of mRNA expression levels. These findings suggest that HDACi 4b, and possibly related HDAC inhibitors, may offer clinical benefit for HD patients and provide a novel set of potential biomarkers for clinical assessment.

Cerebellar lipid differences between R6/1 transgenic mice and humans with Huntington's disease.

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by motor, psychiatric, and cognitive abnormalities. In this present study, we tested whether abnormal motor behavior in a mouse model of HD, the R6/1 transgenic (Tg) mice, was associated with changes in cerebellar lipid composition and gene expression. We report altered motor behavior, which was associated with abnormal expression of glycosyltransferase genes in the cerebellum of R6/1 Tg mice. Cerebellar wet weight and total ganglioside concentration was significantly lower in R6/1 Tg mice than in wild-type (Wt) mice. Furthermore, the Purkinje cell-enriched ganglioside LD1 and the granule cell-enriched ganglioside GD1a were significantly lower in R6/1 Tg mice than in Wt mice. The myelin-enriched lipid sulfatides was also reduced in the cerebellum of R6/1 Tg mice. In contrast to the R6/1 Tg mice, total cerebellar ganglioside concentration did not differ between HD and control subjects. However, expression of several cerebellar glycosyltransferases genes was significantly less in HD subjects than in control subjects. Our findings indicate that the R6/1 Tg mice have severe cerebellar glycosphingolipid (GSL) abnormalities that may account, in part, for their abnormal motor behavior. Although the cerebellar lipid abnormalities found in the R6/1 Tg mice were not found in these HD subjects, the R6/1 Tg mice may be useful for evaluating the role of GSLs in cerebellar development.

Functional roles for the striatal-enriched transcription factor, Bcl11b, in the control of striatal gene expression and transcriptional dysregulation in Huntington's disease.

Transcriptional dysregulation has emerged as a central pathogenic mechanism in Huntington's disease (HD), which is associated with neuropathological changes predominantly in the striatum. Here we demonstrate that expression of Bcl11b (a.k.a. CTIP2), a transcription factor exhibiting highly-enriched localization in adult striatum, is significantly decreased in HD cells, mouse models and human subjects and that overexpression of Bcl11b attenuates toxic effects of mutant huntingtin in cultured striatal neurons. We show that Bcl11b directly activates the proximal promoter regions of striatal-enriched genes and can increase mRNA levels of striatal-expressing genes. We further demonstrate an interaction between Bcl11b and huntingtin protein in cultured cells and brain homogenates from HD R6/1 and YAC72 transgenic mice. We propose that sequestration and/or decreased expression of Bcl11b in HD is responsible, at least in part, for the dysregulation of striatal gene expression observed in HD and may contribute to the specificity of pathology observed in this disease.

Perinatal programming of neurodevelopment: epigenetic mechanisms and the prenatal shaping of the brain.

The recent years have witnessed an exponential growth in the knowledge of epigenetic mechanisms, and piling evidence now links DNA methylation and histone modifications with a wide range of physiological processes from embryonic development to memory formation and behavior. Not surprisingly, deregulation of epigenetic modifications is associated with human diseases as well.An important feature of epigenetics is the ability of transducing environmental input into biological signaling, mainly by modulation of the transcriptome in response to a particular scenario. This characteristic generates developmental plasticity and allows the manifestation of a variety of phenotypes from the same genome.The early-life years represent a period of particular susceptibility to epigenetic alteration, as active changes in DNA methylation and histone marks are occurring as part of developmental programs and in response to environmental cues, which notably include psychosocial stimulation and maternal behavior. Memory formation and storage, response to stress in adult life, behavior, and manifestation of neurodegenerative conditions can all be imprinted in the organism by epigenetic modifications that contribute to shape the brain during prenatal or early postnatal life. Moreover, if these epigenetic alterations are preserved in the germ line, changes induced in one generation are likely inherited by future offspring. Programming by transgenerational inheritance thus represents a central mechanism by which environmental conditions may influence disease risk across multiple generations.As novel techniques emerge and as genome-wide profiling of disease-associated methylomes is achieved, epigenetic marks open a new source for biomarker discovery.

Role of α-synuclein penetration into the membrane in the mechanisms of oligomer pore formation.

Parkinson's disease (PD) and dementia with Lewy bodies are common disorders of the aging population and characterized by the progressive accumulation of α-synuclein (α-syn) in the central nervous system. Aggregation of α-syn into oligomers with a ring-like appearance has been proposed to play a role in toxicity. However, the molecular mechanisms and the potential sequence of events involved in the formation of pore-like structures are unclear. We utilized computer modeling and cell-based studies to investigate the process of oligomerization of wild-type and A53T mutant α-syn in membranes. The studies suggest that α-syn penetrates the membrane rapidly, changing its conformation from α-helical towards a coiled structure. This penetration facilitates the incorporation of additional α-syn monomers in the complex, and the subsequent displacement of phospholipids and the formation of oligomers in the membrane. This process occurred more rapidly, and with a more favorable energy of interaction, for mutant A53T compared with wild-type α-syn. After 4 ns of simulation of the protein-membrane model, α-syn had penetrated through two-thirds of the membrane. By 9 ns, the penetration of the annular α-syn oligomers can result in the formation of pore-like structures that fully perforate the lipid bilayer. Experimental incubation of recombinant α-syn in synthetic membranes resulted in the formation of similar pore-like complexes. Moreover, mutant (A53T) α-syn had a greater tendency to accumulate in neuronal membrane fractions in cell cultures, resulting in greater neuronal permeability, as demonstrated with the calcein efflux assay. These studies provide a sequential molecular explanation for the process of α-syn oligomerization in the membrane, and support the role of formation of pore-like structures in the pathogenesis of the neurodegenerative process in PD.

Glycolipid and ganglioside metabolism imbalances in Huntington's disease.

We have explored genome-wide expression of genes related to glycobiology in exon 1 transgenic Huntington's disease (HD) mice using a custom-designed GLYCOv2 chip and Affymetrix microarray analyses. We validated, using quantitative real-time PCR, abnormal expression levels of genes encoding glycosyltransferases in the striatum of R6/1 transgenic mice, as well as in postmortem caudate from human HD subjects. Many of these genes show differential regional expression within the CNS, as indicated by in situ hybridization analysis, suggesting region-specific regulation of this system in the brain. We further show disrupted patterns of glycolipids (acidic and neutral lipids) and/or ganglioside levels in both the forebrain of the R6/1 transgenic mice and caudate samples from human HD subjects. These findings reveal novel disruptions in glycolipid/ganglioside metabolic pathways in the pathology of HD and suggest that the development of new targets to restore glycosphingolipid balance may act to ameliorate some symptoms of HD.

Selective deficits in the expression of striatal-enriched mRNAs in Huntington's disease.

We have identified and cataloged 54 genes that exhibit predominant expression in the striatum. Our hypothesis is that such mRNA molecules are likely to encode proteins that are preferentially associated with particular physiological processes intrinsic to striatal neurons, and therefore might contribute to the regional specificity of neurodegeneration observed in striatal disorders such as Huntington's disease (HD). Expression of these genes was measured simultaneously in the striatum of HD R6/1 transgenic mice using Affymetrix oligonucleotide arrays. We found a decrease in expression of 81% of striatum-enriched genes in HD transgenic mice. Changes in expression of genes associated with G-protein signaling and calcium homeostasis were highlighted. The most striking decrement was observed for a newly identified subunit of the sodium channel, beta 4, with dramatic decreases in expression beginning at 8 weeks of age. A subset of striatal genes was tested by real-time PCR in caudate samples from human HD patients. Similar alterations in expression were observed in human HD and the R6/1 model for the striatal genes tested. Expression of 15 of the striatum-enriched genes was measured in 6-hydroxydopamine-lesioned rats to determine their dependence on dopamine innervation. No changes in expression were observed for any of these genes. These findings demonstrate that mutant huntingtin protein causes selective deficits in the expression of mRNAs responsible for striatum-specific physiology and these may contribute to the regional specificity of degeneration observed in HD.