Abstract
Huntington’s disease (HD) is a devastating monogenic, dominant, hereditary, neurodegenerative disease. HD is caused by the expansion of CAG repeats in exon 1 of the huntingtin (HTT) gene, IT15, resulting in an expanded polyglutamine (polyQ) residue in the N-terminus of the HTT protein. HD is characterized by the accumulation of mutant HTT (mHTT) in neural and somatic cells. Progressive brain atrophy occurs initially in the striatum and extends to different brain regions with progressive decline in cognitive, behavioral and motor functions. Astrocytes are the most abundant cell type in the brain and play an essential role in neural development and maintaining homeostasis in the central nervous system (CNS). There is increasing evidence supporting the involvement of astrocytes in the development of neurodegenerative diseases such as Parkinson’s disease (PD), Huntington’s disease (HD), Alzheimer’s disease (AD), and amyotrophic lateral sclerosis (ALS). We have generated neural progenitor cells (NPCs) from induced pluripotent stem cells (iPSCs) of transgenic HD monkeys as a model for studying HD pathogenesis. We have reported that NPCs can be differentiated in vitro into mature neural cells, such as neurons and glial cells, and are an excellent tool to study the pathogenesis of HD. To better understand the role of astrocytes in HD pathogenesis and discover new therapies to treat HD, we have developed an astrocyte differentiation protocol and evaluated the efficacy of RNAi to ameliorate HD phenotypes in astrocytes. The resultant astrocytes expressed canonical astrocyte-specific markers examined by immunostaining and real-time PCR. Flow cytometry (FACS) analysis showed that the majority of the differentiated NPCs (95.7%) were positive for an astrocyte specific marker, glial fibrillary acidic protein (GFAP). Functionalities of astrocytes were evaluated by glutamate uptake assay and electrophysiology. Expression of mHTT in differentiated astrocytes induced cytosolic mHTT aggregates and nuclear inclusions, suppressed the expression of SOD2 and PGC1, reduced ability to uptake glutamate, decreased 4-aminopyridine (4-AP) response, and shifted I/V plot measured by electrophysiology, which are consistent with previous reports on HD astrocytes and patient brain samples. However, expression of small-hairpin RNA against HTT (shHD) ameliorated and reversed aforementioned HD phenotypes in astrocytes. This represents a demonstration of a novel non-human primate (NHP) astrocyte model for studying HD pathogenesis and a platform for discovering novel HD treatments.
Highlights
Huntington’s disease (HD) is a devastating monogenic, hereditary, neurodegenerative disease characterized by progressive brain atrophy in striatum, cortex and other brain areas [1]
The homogeneity of astrocytes was confirmed by fluorescence-activated cell sorting (FACS) analysis (Fig 1D) with over 96% positive for glial fibrillary acidic protein (GFAP) (96.7%, Fig 1D)
HD and shHD astrocytes expressed canonical astrocyte-specific markers, ALDH1L1, GFAP, and S100β, while the expression of Neural progenitor cells (NPCs) specific marker (PAX6) and neural-specific marker (MAP2) were not observed, which correspond with real-time quantitative PCR data shown in section (Fig 2A and 2B)
Summary
Huntington’s disease (HD) is a devastating monogenic, hereditary, neurodegenerative disease characterized by progressive brain atrophy in striatum, cortex and other brain areas [1]. The psychophysiological phenotypes include cognitive, behavioral, and motor function deficits and psychiatric abnormalities [2,3]. HD affects about 3–10 people in every 100,000 people in Western Europe and North America, and juvenile cases account for 4.92% of cases, with an early age of onset at 20 [4,5]. The juvenile form of HD is associated with more severe chorea, dystonia, and neurodegeneration in the frontal and temporal lobes [5]. The primary etiology of HD is the neurodegeneration of basal ganglia, which partially explains the pronounced motor and cognitive symptoms observed in HD patients [6]. Following the onset of the disease, the atrophy spreads to other cerebral areas, exacerbating HD symptoms
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