Abstract Dysfunctions of motor neuron (MN) axons that form both descending motor tracts and peripheral nerve tracts are widely observed in amyotrophic lateral sclerosis (ALS), which often precede MN soma (and axon) degeneration and significantly contribute to disease pathology. Although astroglial adhesion signals are essential for proper axon growth and guidance, whether and how glial adhesion molecules play a role in axon dysfunction/degeneration in ALS remains largely unknown. Although astroglia conditioned medium (ACM) derived from the mouse SOD1 mutant model or from human ALS patient brain astroglia is able to substantially modulate health and survival of MNs, astroglial factors in ACM that modulate MN survival in ALS conditions remain essentially unidentified. Exosomes (50-150 nm in diameter), a major type of secreted extracellular vesicles (EVs), are released from multivesicular bodies (MVBs, an intermediate endosome structure) during endosome maturation. EVs and exosomes secreted from central nervous system (CNS) cell types have emerged as an important intercellular pathway that is implicated in the pathogenesis of neurodegenerative diseases including ALS. How astroglia-derived exosomes affect (motor) neuron survival especially axon properties in ALS remains largely unexplored. In this project, we intend to investigate novel stimulatory and protective roles of astroglial exosomes, especially exosomal adhesion molecule HepaCAM signaling to neurons, in promoting axon growth, functions, and (motor) neuron survival. We will also determine whether dysfunctional HepaCAM signaling from SOD1G93A astroglial exosomes to (motor) neurons contributes to MN axon dysfunction and degeneration in ALS. Based on our preliminary results, we propose the following aims in this project: Aim 1: Determine the effect of astroglial exosomes on (motor) neuron axon growth and functions; Aim 2: Investigate loss-of-function of astroglial exosome signaling to (motor) neurons in ALS models; Aim 3: Elucidate HepaCAM-mediated astroglial exosome signaling to (motor) neurons in ALS models; We have generated a large amount of preliminary data to support our rationales and to demonstrate feasibility for proposed aims. We will employ mouse genetics, primary neuron and astroglial cultures, molecular biology, virus injections, various imaging, and biochemical approaches to complete these aims. Outcomes from this project will reveal a new astroglial exosomal HepaCAM-mediated mechanism in modulating (motor) neuron axon growth and survival. It will also provide important knowledge about the loss-of-functional effect of astroglial exosomes in ALS. These studies will significantly advance our understanding of the astroglial dysfunction in ALS and help develop new astroglia- based neuroprotective strategies.