Project Summary/Abstract Pathological disturbances caused by neuromuscular disorders can disrupt the motor circuits that drive muscle stimulation. For example, muscle paralysis induces compensatory changes in motor circuit excitability. Although much is known about neuronal mechanisms that alter intrinsic excitability, far less is known about the intercellular mechanisms that monitor and control synaptic strength at the circuit level, perhaps because the complexity of mammalian circuits has precluded intensive efforts to characterize circuit-level coordination of network excitability. This proposal harnesses the simple genetics and known neuroanatomy of C. elegans to investigate a novel inter-tissue feedback pathway that couples neuromuscular junction (NMJ) or muscle activity with synaptic strength of synapses on interneurons that reside two synaptic layers upstream of the NMJ. This feedback pathway was discovered while characterizing the Vascular Endothelial Growth Factor Receptors ver-1 and ver-4, and their ligand pvf-1, which are hits from an RNAi screen for genes that affect a mechanosensory reflex circuit. When excitatory signaling at the NMJ is lost or when muscles cannot contract, a sensor generates a signal to the command interneuron to increase its synaptic strength to compensate for decreased behavioral output (reflex response). Literature from the muscle hypertrophy field points to a sensor candidate, unc-22/titin, which has spring-like properties and a stretch-activated kinase domain. It is well established that titin kinase initiates hypertrophy signaling, an active response to muscle overload that includes muscle growth and sarcomere reorganization. Does titin initiate a feedback signal that adjusts network strength to control drive onto the muscle? This proposal tests the hypothesis that a sensor in muscle, perhaps unc-22/titin, detects changes in muscle contraction and initiates an inter-tissue signal, mediated by pvf-1 and ver signaling, that modulates the synaptic strength of a command interneuron two synaptic layers upstream from muscle. Preliminary results supporting this hypothesis show that animals with defective NMJ signaling or muscle contraction have increased surface glutamate receptor (GluR) levels (correlate of increased synaptic strength) in interneurons that drive the NMJ. Loss of either unc-22/titin or pvf-1 block this increased GluR abundance, implicating both in the feedback pathway. Proposed experiments will characterize feedback pathway triggers, test if unc-22/titin is a muscle stretch sensor that initiates the feedback signal, and identify the inter-tissue signal by examining the role of pvf-1 and ver signaling in this pathway. Completion of this exciting project will result in a novel description of a mechanism that neuronal circuits use to monitor circuit output and adjust synaptic strength accordingly. Further, this project identifies a retrograde signal that operates more than one synaptic layer upstrea...