Project Abstract (30 lines) Neurodegenerative diseases (NDDs) are devastating conditions that rob individuals of their cognitive function, mobility, and ability to function in the world. Ultimately, many of these diseases are fatal. Today, a combined 6.5 million Americans suffer from NDDs, encompassing Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), and Huntington’s disease (HD). However, by 2030, 1 in 5 Americans will be over the age of 65, and because NDDs strike primarily in mid- to late-life, the incidence is expected to soar as our population ages. These circumstances highlight the increasing urgency for the development of effective treatments and cures for NDDs, which are currently lacking. While NDDs differ in their inciting mechanisms, research has now clearly demonstrated the presence of shared features of downstream pathophysiology, notably, the role of a dysregulated neuroinflammatory system. Recent studies in humans and animal models have uncovered a population of microglia (Disease-associated microglia, DAMs), that are defined by a distinct transcriptional signature, and are conserved across several different NDDs. Intriguingly, the DAM signature includes upregulation of a number of different members of the MS4A gene family, for which polymorphisms have been linked to AD by numerous genome-wide association studies (GWAS). The upregulation of AD-associated MS4A genes in DAMs begs the questions of whether MS4A genes might play a common role in NDDs, which share a DAM signature, and furthermore, whether MS4A genes impact the functional properties of microglia in the context of NDDs. Excitingly, our lab has found that across three animal NDD models (5XFAD, MAPT, SOD1G93A), mice deficient for either of two individual Ms4a family members exhibit improved disease phenotypes, extension of lifespan, and amelioration of histopathological disease hallmarks. Thus, this proposal will test the hypothesis that multiple MS4As act in concert to drive pathology in a mouse model of ALS and regulate microglial transcriptional as well as functional responses to the NDD milieu. To test this hypothesis, aim 1 will investigate the impact of simultaneous deletion of the entire MS4A gene family on ALS. To this end, we have generated a novel mouse genetic reagent in which the entire MS4A gene cluster is deleted. These mice will be crossed to the SOD1G93A ALS mouse model and pathological features of ALS, including motor defects, lifespan, microgliosis, neuronal loss, and synapse elimination will be assessed. Aim 2 will examine the impact of MS4A deficiency on the transcriptional and functional properties of spinal cord microglia isolated from end-stage SOD1G93A mice. Specifically, I will utilize single-cell RNA sequencing (scRNA-seq) in tandem with spatial transcriptomics (MERFISH) to evaluate the impact of MS4A deletion on the DAM population. In parallel, I will examine how MS4A deficiency in vi...