Reactive Astrogliosis and the Tripartite Synapse in Alzheimer's Disease

Massachusetts General Hospital, Boston MA

Investigators

Abstract

PROJECT SUMMARY/ABSTRACT Synaptic loss is considered the best correlate of cognitive decline in Alzheimer’s disease (AD) and has been largely attributed to the toxicity of amyloid-β (Aβ) and phospho-tau (pTau) species, with microglia engulfing and eliminating Aβ- and pTau-tagged vulnerable synapses. While astrocytes participate in the so-called “tripartite” excitatory synapses, and reactive astrocytes surrounding Aβ plaques and pTau neurofibrillary tangles in the AD brain are known to suffer dramatic morphological, functional, and molecular changes (collectively termed reactive astrogliosis), the net effect of AD reactive astrogliosis on synaptic health remains a crucial gap of knowledge. On one hand, reactive astrocytes may contribute to synaptic loss by secreting synaptotoxic factors (e.g., C3, lipocalin-2) and suppressing the release of synaptogenic factors (e.g., thrombospondins). On the other hand, a key feature of reactive astrocytes is the upregulation of the cytoskeletal intermediate filament glial fibrillary acidic protein (GFAP), which has been implicated in the maintenance of the normal structure and function of tripartite excitatory synapses (e.g., by trafficking glutamate transporters and potassium-buffering channels to the astrocyte peripheral leaflets). Here we propose to investigate the role of GFAP upregulation by reactive astrocytes in AD-related synaptic dysfunction and loss. Our central hypothesis is that the GFAP upregulation associated with AD reactive astrogliosis is an adaptive compensatory response of astrocytes to protect synapses in the setting of pTau-induced synaptic toxicity. Our overall goal in the present proposal is to decipher whether and how tripartite synapses are altered by the GFAP upregulation characteristic of AD reactive astrocytes. The rationale for this project is that, based on our preliminary data, GFAP upregulation by reactive astrocytes may protect synapses from pTau toxicity; therefore, understanding this potentially compensatory mechanism could hold therapeutic promise against AD-related cognitive decline. Our central hypothesis will be tested by investigating whether and how astrocyte GFAP upregulation impacts tripartite excitatory synapses in a tauopathy mouse model of AD, and whether and how blunting this astrocyte response impacts the proteome of tripartite excitatory synapses in a tauopathy mouse model of AD. Specifically, we will overexpress GFAP in astrocytes via viral gene delivery (aim 1) and blunt Gfap upregulation in reactive astrocytes with a mouse-specific Gfap anti-sense oligonucleotide (ASO, aim 2) in AD tau transgenic mice and wild-type littermates. We will then investigate changes in the protein composition and structure of tripartite synapses by performing tandem tag mass (TMT)-based quantitative proteomics on synaptoneurosome preparations and array tomography on ultrathin sections, respectively. Upon successful completion of these aims, our expected outcome is to have delineated the changes in protein composition and structure of tripartite excitatory synapses downstream GFAP upregulation and to have elucidated whether this feature of AD reactive astrogliosis is toxic, protective, or neutral with respect to synapses.