Elucidating the Role of Altered Heterogeneous Nuclear Ribonucleoprotein A1 Expression in the Pathogenesis of Neurodegeneration in an In Vitro Model of Multiple Sclerosis

Abstract

Neuronal and axonal damage, collectively known as neurodegeneration, are salient pathogenic features of multiple sclerosis (MS) and are thought to underlie permanent disability in MS, particularly in progressive forms of the disease. Despite decades of research, the etiology of neurodegeneration in MS remains relatively unknown and there are no treatments available that target its pathogenesis. Research in MS as well as other neurologic diseases has established that dysfunctional RNA binding proteins (RBPs) are a prominent pathogenic feature and may contribute to the pathogenesis of neurodegeneration. Neurons from MS cortex have been shown to exhibit dysfunctional features of the RBP heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1), whereby hnRNP A1 is mislocalized from its homeostatic nuclear location to the cytoplasm, resulting in loss of hnRNP A1 function within the neuron. We hypothesized that loss-of-function of hnRNP A1 modelled using siRNA in differentiated Neuro-2a cells would have detrimental effects on neuronal health and viability. Through RNA sequencing (RNAseq) followed by gene ontology (GO) analyses, we found that hnRNP A1 is involved in important biological processes, including RNA metabolism, neuronal function, neuronal morphology, neuronal viability, and stress granule (SG) formation. We confirmed several of these roles by showing that hnRNP A1 knockdown caused a reduction of neurite outgrowth (p<0.001), which correlated with decreased hnRNP A1 expression (p<0.05), increased cell cytotoxicity (p<0.05), and increased punctate staining of a necroptotic cell death marker, phospho-mixed lineage kinase domain like pseudokinase (pMLKL). Additionally, we demonstrated that hnRNP A1 knockdown disrupts the formation of cytoplasmic SGs (p<0.0001) after stress induction. These findings present novel insights into how hnRNP A1 loss-of-function in neurons may contribute to neuronal dysfunction and death. Further, it implicates hnRNP A1 dysfunction, particularly decreased hnRNP A1 expression, in the pathogenesis of neurodegeneration in MS and other neurodegenerative diseases.