Investigators find mutant ATXN1 gene impacts directly on VEGF production.
Treating the inherited neurodegenerative disease spinocerebellar ataxia type 1 (SCA1) using VEGF improves motor function and restores cerebellar pathology in a mouse model of the disease, suggesting it could represent a therapeutic approach for human patients, scientists claim.
A team led by Puneet Opal, Ph.D., at the Northwestern University Feinberg School of Medicine, and colleagues in the U.S. and Germany, found that VEGF expression is depressed in the affected brain regions of a mouse model of SCA1, due to the activity of the mutated ataxin-1 (ATXN1) gene that causes SCA1. The team subsequently found that SCA1 pathology could be reduced and motor function improved in mice engineered to overexpress VEGF, or those that were treated using intracerebellar administration of VEGF. Dr. Opal et al. report their findings in Nature Medicine in a paper titled “Vascular endothelial growth factor ameliorates the ataxic phenotype in a mouse model of spinocerebellar ataxia type 1.”
Transcriptional changes are one of the earliest pathogenic signatures in SCA1 mouse models, indicating that dysregulated gene expression caused by mutant ATXN1 is central to the pathogenesis of SCA1, the researchers explain. However, the direct targets of ATXN1-induced repression, particularly in the most vulnerable Purkinje cell population in the cerebellum, haven’t been identified. The team used laser-capture microdissection (LCM) to isolate Purkinje neurons from SCA1 knock-in mice, and undertake PCR to further study gene expression dysregulation in the disease by examining the expression of candidate genes involved in key neurodegenerative pathways. They found that one of the genes downregulated in SC1A encodes VEGFA, an angiogenic and neurotrophic factor implicated in motor neuron disorders. The animals exhibited decreased VEGFA mRNA levels as early as postnatal day 30, which is before any pathological signs of the disease become manifest.
In fact, the level of decrease in VEGFA mRNA was greater than that of three other genes—Gsbs, Homer3, and Slc1a6—which previous research had shown were downregulated in SCA1. Further analysis showed that levels of VEGF protein were 30% less in SCA1 cerebella than in wild-type cerebella, and the decrease was most prominent in Purkinje neurons. The team went on to use a VEGFA luciferase reporter assay to see whether mutant ATXN1 directly affects VEGFA mRNA expression by modulating promoter activity. Interestingly, both the mutant ATXN1 (ATXN1-84Q) and wild-type ATXN1 (ATXN1-2Q) repressed reported activity.
“The ability of even wild-type ATXN1 to cause repression is in keeping with the observation that overexpression of wild-type ATXN1 can induce pathology in animal models and the notion that SCA1 can in part be caused by a gain of ATXN1 normal function,” the authors state. Conversely, mutating a phosphorylation site critical for ATXN1 toxicity disrupted VEGFA promotor repression. Chromatin immunoprecipitation (ChIP) studies confirmed that ATXN1 interacted directly with the VEGFA promoter, but not the promotor for the closely related VEGFC.
The fact that VEGF is an angiogenic factor pointed to the probability that reduced levels could contribute to cerebellar dysfunction by limiting angiogenesis. Indeed, the researchers found that cerebellar microvessel density and vessel length were both depressed in SCA1 mice, and there was additional evidence of hypoxia. Separate studies on normal mixed cerebellar neuronal cultures that express VEGF and its receptor VEGFR2 demonstrated that antibody- or inhibitor-mediated blockage of VEGF or VEGFR2, respectively, resulted in decreased neurite length and increased cell death.
The team finally evaluated the effects of restoring VEGF expression in SCA1 mice either by increasing VEGF gene expression, or administering exogenous VEGF directly to the brain. Trasngenic SCA1 mice that also overexpressed human VEGF in neurons demonstrated significantly enhanced motor performance at 13 weeks and 6 months of age, and exhibited improved cerebellar pathology including cerebellar microvessel density. Similarly, continuously delivering mouse VEGF to SCA1 animals using an intracerebroventricular osmotic pump also improved motor performance and restored cerebellar pathology.
“Our findings suggest a role for VEGF in SCA1 pathogenesis and indicate that restoring VEGF levels may be a potentially useful treatment in patients with SCA1,” the authors conclude. Moreover, they suggest, “alterations in VEGF itself or sequelae of VEGF signaling in blood or cerebrospinal fluid of affected individuals could prove to be a biomarker of disease progression.”