Marija Cvetanovic

Ataxin-1 ( ATXN1 ) was originally identified as a gene in which abnormal expansion of glutamine encoding CAG repeats causes inherited neurodegenerative disease spinocerebellar ataxia type 1. Spinocerebellar ataxia type 1 is characterized by alterations in movement, cognition, and mood, with severe pathology in the cerebellum and brain stem. Studies in spinocerebellar ataxia type 1 mouse models indicated that CAG expansion predominantly causes pathogenic ATXN1 gain-of-function in the cerebellum. Conversely, mice lacking ATXN1 are severely impaired in cognitive tests and exhibit abnormalities in cortical functions. Subsequent genetic and functional studies have associated ATXN1 with intelligence and several neurological conditions, including Alzheimer’s disease, amyotrophic lateral sclerosis, schizophrenia, and multiple sclerosis. In some conditions, association is found with CAG expansions in ATXN1 and in others with the loss of ATXN1 expression. Additionally, recent studies indicate the role of ATXN1 in glial cells. Here, we review current genetic and functional evidence of how ATXN1 CAG expansion and loss of ATXN1 function contribute to brain dysfunction in spinocerebellar ataxia type 1, Alzheimer’s disease, and multiple sclerosis, with an emphasis on glial cells.

Microglia, resident immune cells of the brain, are important players in neurodegeneration. While microglial activation is a hallmark of many neurodegenerative diseases, the specific role of microglia intrinsic factors in microglial activation and disease pathogenesis remains unknown. Spinocerebellar ataxia type-1 (SCA1) is an inherited autosomal dominant neurodegenerative disease characterized by severe neuronal loss and early microglial activation in the cerebellum. SCA1 is caused by CAG repeat expansion in the ubiquitously expressed ATAXIN1 ( ATXN1) gene. Using human microglia differentiated from SCA1 patient derived iPSCs, we found that mutant ATXN1 is sufficient to alter morphology, gene and protein expression in human microglia in a cell-autonomous manner. Moreover, compared to controls, human SCA1 microglia exhibited increased phagocytosis and pro-inflammatory cytokine production, indicating an immune priming. To determine the extent to which mutant ATXN1 in microglia contributes to SCA1 pathogenesis and behavioral symptoms, we removed mutant ATXN1 from microglia and macrophages in a novel conditional SCA1 mouse model, f-ATXN1 ^146Q/2Q mice. Microglial mutant ATXN1 reduction led to a marked correction in microglia phenotype, in particular in the transcriptomic signature of interferon type 1 mediated immune response, reduced microglial density and resulted in smaller microglia with reduced branching in the cerebellum. Pathology of Purkinje neurons and cerebellar astrogliosis were also ameliorated. Utilizing a battery of behavioral tests, we found that microglia and macrophage mutant ATXN1 reduction ameliorated cognitive, mood, and motor deficits in SCA1 mice. Together, these results indicate that mutant ATXN1 directly impacts microglial phenotype in SCA1, contributing to SCA1 pathology and behavioral deficits.

Ataxin-1 (ATXN1) is a protein in which expansion of its polyglutamine tract causes the neurodegenerative disorder spinocerebellar ataxia type 1 (SCA1) via a gain-of-function. Wild type ATXN1 was recently shown to have a protective role in regulating severity of experimental autoimmune encephalomyelitis (EAE), a well-established mouse model for Multiple sclerosis (MS). This study further investigates the role of ATXN1 with an expanded polyglutamine tract in the context of MS using an EAE mouse model. Hemizygous Atxn1 (Atxn12Q/−) mice or f-ATXN1146Q/2Q, heterozygous mice that have one copy of the endogenous mouse gene replaced with a polyQ expanded pathogenic human ATXN1 gene, were injected with myelin oligodendrocytes glycoprotein (MOG35 − 55) peptide to induce EAE. Immunohistochemical and biochemical approaches were used to analyze the degree of demyelination, cell loss, axonal degeneration as well as detecting the activated immune cells and inflammatory cytokines upon EAE induction in Atxn12Q/− and f-ATXN1146Q/2Q mice. Our findings reveal that a loss-of-function of wild type Atxn1 in Atxn12Q/− and f-ATXN1146Q/2Q mice significantly exacerbates the EAE symptoms, leading to increased demyelination, oligodendrocytes loss, heightened axon degeneration, and greater clinical disability in affected mice. Importantly, the data reveals that neurotoxic astrocytes are activated at acute stage of disease (PID-14) and at the chronic stage of disease (PID-30) neurotoxic astrocytes no longer show signs of activation. The data also demonstrated enhanced infiltration of immune cells into the lesions of mutant mice. These results indicate that ATXN1 plays a protective role in modulating immune responses and maintaining neural integrity during MS. Importantly, expansion of the polyQ tract in ATXN1 results in a loss-of-function in ATXN1’s ability to dampen the immune response. Understanding the functional role of ATXN1 in MS pathogenesis may open new avenues for therapeutic strategies aimed at mitigating disease progression.

The cerebellum’s role in cognition and its functional bi-directional connectivity with prefrontal cortex (PFC) is now recognized. However, how chronic cerebellar dysfunction affects PFC function and cognition remains less understood. Spinocerebellar ataxia type 1 (SCA1), is an inherited, fatal neurodegenerative disease caused by an abnormal expansion of glutamine (Q) encoding CAG repeats in the gene Ataxin-1 (ATXN1) and characterized by severe loss of Purkinje cells (PCs) in the cerebellum. Patients with SCA1 suffer from movement and balance deficits, cognitive decline and premature lethality. Cognitive deficits significantly impact patient’s quality of life, yet how exactly cerebellar degeneration contributes to cognitive deficits and PFC dysfunction in SCA1 is unknown. We have previously demonstrated that expression of mutant ATXN1 only in cerebellar Purkinje cells (PCs) is sufficient to cause cognitive deficits in a transgenic ATXN1[82Q] mouse line. To understand how cerebellar dysfunction impacts the PFC, we examined neuronal activity, synaptic density, and gene expression changes in the PFC of ATXN1[82Q] mice. Remarkably, we found decreased neuronal activity, reduced synaptic density, and altered expression of immediate early genes and pathways involved in glucose metabolism, inflammation and amphetamine in the PFC of ATXN1[82Q] mice. Furthermore, we characterized cellular and molecular PFC dysfunction in a novel conditional knock-in SCA1 line, f-ATXN1^146Q mice, expressing floxed human expanded ATXN1 throughout the brain. Intriguingly, we found an increased number of neurons, increased synaptic density and large gene expression alterations in the PFC of f-ATXN1^146Q mice. Finally, to precisely determine the role of cerebellar dysfunction in cognitive deficits and PFC dysfunction in SCA1, we crossed f-ATXN1^146Q mice with Pcp2-Cre mice expressing Cre recombinase in PCs to delete expanded ATXN1 only in PCs. Surprisingly, we have found that deleting expanded ATXN1 in PCs exacerbated cognitive deficits and PFC dysfunction in these mice. Our findings demonstrate that circumscribed cerebellar dysfunction is sufficient to impact PFC activity and synaptic connectivity impairing cognition. However, when multiple brain regions are impacted in disease, cerebellar dysfunction may ameliorate PFC pathology and cognitive performance.

Spinocerebellar ataxia type 1 (SCA1) is a fatal neurodegenerative disease caused by an expanded polyglutamine tract in the widely expressed ataxin-1 (ATXN1) protein. To elucidate anatomical regions and cell types that underlie mutant ATXN1-induced disease phenotypes, we developed a floxed conditional knockin mouse ( f-ATXN1 ^146Q/2Q ) with mouse Atxn1 coding exons replaced by human ATXN1 exons encoding 146 glutamines. f-ATXN1 ^146Q/2Q mice manifested SCA1-like phenotypes including motor and cognitive deficits, wasting, and decreased survival. Central nervous system (CNS) contributions to disease were revealed using f-ATXN1 ^146Q/2Q ; Nestin-Cre mice, which showed improved rotarod, open field, and Barnes maze performance by 6–12 weeks of age. In contrast, striatal contributions to motor deficits using f-ATXN1 ^146Q/2Q ; Rgs9-Cre mice revealed that mice lacking ATXN1 ^146Q/2Q in striatal medium-spiny neurons showed a trending improvement in rotarod performance at 30 weeks of age. Surprisingly, a prominent role for muscle contributions to disease was revealed in f-ATXN1 ^146Q/2Q ; ACTA1-Cre mice based on their recovery from kyphosis and absence of muscle pathology. Collectively, data from the targeted conditional deletion of the expanded allele demonstrated CNS and peripheral contributions to disease and highlighted the need to consider muscle in addition to the brain for optimal SCA1 therapeutics.

The extracellular matrix (ECM) surrounds cells in the brain, providing structural and functional support. Emerging studies demonstrate that the ECM plays important roles during development, in the healthy adult brain, and in brain diseases. The aim of this review is to briefly discuss the physiological roles of the ECM and its contribution to the pathogenesis of brain disease, highlighting the gene expression changes, transcriptional factors involved, and a role for microglia in ECM regulation. Much of the research conducted thus far on disease states has focused on “omic” approaches that reveal differences in gene expression related to the ECM. Here, we review recent findings on alterations in the expression of ECM-associated genes in seizure, neuropathic pain, cerebellar ataxia, and age-related neurodegenerative disorders. Next, we discuss evidence implicating the transcription factor hypoxia-inducible factor 1 (HIF-1) in regulating the expression of ECM genes. HIF-1 is induced in response to hypoxia, and also targets genes involved in ECM remodeling, suggesting that hypoxia could contribute to ECM remodeling in disease conditions. We conclude by discussing the role microglia play in the regulation of the perineuronal nets (PNNs), a specialized form of ECM in the central nervous system. We show evidence that microglia can modulate PNNs in healthy and diseased brain states. Altogether, these findings suggest that ECM regulation is altered in brain disease, and highlight the role of HIF-1 and microglia in ECM remodeling.