Project Summary/Abstract This is an application for a diversity supplement according to FOA: Research Supplements to Promote Diversity in Health-Related Research (PA-21-071) for Ms. Tatiana M. Gonzalez, a PhD student in my lab. Ms. Gonzalez will be working on experiments for her PhD studies on the awarded and currently active R01 grant (AA027079, entitled: “Mechanisms of synaptic loss by the classical complement pathway in motor circuit development and disease”, PI: G.Z. Mentis). The essence of this grant is to unravel the molecular and cellular mechanisms responsible for the disruption of neuronal networks through synaptic elimination which leads to compromised motor output, both during normal development and in the motor neuron disease, spinal muscular atrophy (SMA). These mechanisms underlie the refinement of immature sensory-motor circuits during early normal development and are the main cause for the severe phenotypic deficits in SMA. The Specific Aims of our currently active grant are as follows: In Aim 1, we will determine whether aberrant activation of the classical complement pathway induces loss of vulnerable synapses in SMA. In Aim 2, we will investigate the requirement of the classical complement pathway for normal development of sensory-motor circuits. In Aim 3, we will determine the role of MEGF10 and MERTK in mediating C1q and C3 (proteins of the classical complement) in tagging and eliminating sensory synapses. The studies that will be investigated by Ms. Tatiana Gonzalez, will involve aspects of Aim 2 and Aim 3. Specifically, she will investigate to which extent supernumerary and inappropriate proprioceptive sensory synapses are formed through C1q and C3 involvement. To address this, Ms. Gonzalez will utilize a novel retrograde viral-mediated neuronal circuit tracing assay to selectively label and quantify hindlimb muscle-specific sensory proprioceptive synapses on muscle-identified spinal motor neurons in mice at different developmental ages. This assay relies on the expression of parvalbumin (PV) by proprioceptors; to prime fluorescent reporter expression in the proprioceptors, a parvalbumin-directed FlpO recombinase (PV::FlpO) mouse line is crossed with Cre- and Flp-dependent tdTomato reporter mice (Ai65). PV::FlpO;Ai65 neonatal mice will be injected intramuscularly at birth with CAV2-CRE virus into the tibialis anterior (TA) to activate tdTomato expression in proprioceptive sensory neurons innervating the TA. To label motor neurons innervating the antagonistic Gastrocnemius (Gs) muscle, these mice will concomitantly be injected with scAAV6-GFP virus to express GFP in Gs motor neurons. In this manner, proprioceptive TA fibers will be labelled with TdTomato (in red) and Gs motor neurons with GFP (in green). Using immunohistochemistry and confocal microscopy, excitatory glutamatergic proprioceptive sensory synapses will be labeled using an anti-VGluT1 antibody and the presence of inappropriate synapses will be detected through...