Online

Toker, Nicholas

Neuroscience Graduate Program

2020

M.S.

Notch 1 is an evolutionarily conserved cell signaling receptor with important functions ranging from control of stem cell proliferation and differentiation to regulation of angiogenesis. Accordingly, Notch 1 expression affects a diverse assortment of protein targets. Here, we explore the role that Notch 1 and related signaling molecules play in astrocyte cell biology. Specifically, we investigate exosomes and the transduction of Notch signaling in astrocytes. Exosomes are small (10-150 nm) vesicles secreted by most cell types and have been found to contain cargo ranging from small RNA species to full length transmembrane receptors. We report, for the first time to our knowledge, the identification of the Notch 1 intracellular domain (NICD), the signaling-competent portion of the Notch 1 receptor, within exosomes secreted from astrocytes. Although NICD was detectable in astrocyte exosome samples, we were unable to demonstrate that exosomal NICD was biologically active or signaling-competent in assays measuring Notch target gene expression or neural stem cell proliferation. We also performed experiments interrogating the function of Glypican 4 (a Notch-regulated heperan sulfate proteoglycan) in astrocytes, given our identification of Glypican 4 expression as regulated by Notch 1 by microarray. While Glypican 4 is known to exist in a GPI-tethered and in a secreted form, we detected significant nuclear Glypican 4 expression in astrocytes. Given the nuclear localization and highly punctate nature of Glypican 4 expression in astrocytes, we hypothesized potential involvement in alternative splicing. Nuclear Glypican 4 was found to colocalize with RbFox3/NeuN, a known effector of alternative splicing. Taken together, these experiments underscore the importance of Notch signaling to astrocyte cell biology and provide a basis for further experiments interrogating the mechanistic details underlying these findings.