Molecular determinants of astrocyte morpho-functional changes in Alzheimer’s disease

Abstract

As people are living longer, neurodegenerative disorders such as Alzheimer’s disease (AD) are becoming more prevalent but the pursuit for treatments has yet to deliver satisfactory results. By the time AD is diagnosed, the typical hallmarks of tangles and plaques are present and cognitive decline has occurred, pointing future research towards the early stages of the disease. Indeed, recent research has unearthed cell-autonomous atrophy of astrocytes in the early stages of AD, characterised by decreased cell size and loss of processes. This aberrant astrocyte morphology was present in both sporadic and familial AD models and manifests independently of senile plaques. These findings challenge the neuron-centric view of AD, granting glial cells an undeniable role in neurodegeneration. The exact mechanisms underlying aberrant astrocyte morphology are entirely unexplained. This thesis demonstrates that the Cas-family proteins, previously identified by GWAS as genetic risk-factors for late-onset AD, NEDD9 and CASS4 act as regulators of astrocyte morphology.
Primary human cortical astrocytes were transiently transfected in vitro with vectors mediating the overexpression or siRNA-induced knockdown of either NEDD9 or CASS4. Concurrent expression of GFP, which localised throughout the entire cell, permitted the visualisation of complete cellular morphologies, including fine processes. Morphological analysis by visual binning into morphological subtypes or 3D reconstruction followed by morphometric quantifications (e.g. surface area, volume) revealed that overexpression or knockdown of either NEDD9 or CASS4 induced significant changes in astrocyte morphology compared to controls. Moreover, manipulation of Cas-protein levels induced altered expression and sub-cellular distribution of key astrocyte functional markers, including glial fibrillary acid protein (GFAP) and the calcium-binding protein, S100B; mimicking the pathological phenotype reported in human iPSC astrocyte models of AD.
Hence, it appears that both NEDD9 and CASS4 are capable of inducing morphological and functional changes in human astrocytes and may therefore contribute to astrocyte pathology in AD.