Characterizing the Role of MuSK Serine Phosphorylation

The neuromuscular junction (NMJ) represents the central node of communication between motor neurons, skeletal muscle and Schwann cells. Every voluntary movement such as breathing, walking, talking and eating relies on the appropriate function of the NMJ, and as such, much of our behavior, wellbeing and productivity are governed by this essential structure. Disorders affecting the neuromuscular system are a large group of diverse diseases including motor neuron diseases, muscular dystrophies, autoimmune diseases and congenital myasthenic syndromes. NMJs form when a motor axon reaches a muscle fiber. Acetylcholine receptors become concentrated at the site of innervation and processes at the molecular and cellular level lead to the development of a mature and functional NMJ. The receptor tyrosine kinase MuSK is the key signaling molecule at the NMJ whose activity is required for the formation of a mature and functional NMJ. It is well established that MuSK kinase activity is tightly regulated to ensure proper NMJ development and that mutations that affect MuSK kinase activity are causally involved in congenital myasthenic syndromes. Signal transduction downstream of MuSK regulates acetylcholine receptor clustering but to understand the exact mechanism of action further studies are required. MuSK activity and function is regulated by posttranslational modifications such as phosphorylation. We have recently identified a novel serine phosphorylation in MuSK and within this project I propose to functionally characterize the role of serine phosphorylation for MuSK activation and ultimately for NMJ development. The proposed study tackles so far poorly understood aspects of MuSK function and will therefor provide valuable novel insights into the molecular mechanisms, which control the formation of a functional neuromuscular junction. We also expect to increase our knowledge about the molecular basis of pathology in patients carrying MuSK mutations thereby aiding to the diagnosis and treatment of congenital myasthenic syndromes.

The neuromuscular junction (NMJ) represents the central node of communication between motor neurons, skeletal muscle and Schwann cells. Every voluntary movement such as breathing, walking, talking and eating relies on the appropriate function of the NMJ, and as such, much of our behavior, wellbeing and productivity are governed by this essential structure. Disorders affecting the neuromuscular system are a large group of diverse diseases including motor neuron diseases, muscular dystrophies, autoimmune diseases and congenital myasthenic syndromes. NMJs form when a motor axon reaches a muscle fiber. Acetylcholine receptors become concentrated at the site of innervation and processes at the molecular and cellular level lead to the development of a mature and functional NMJ. The receptor tyrosine kinase MuSK is the key signaling molecule at the NMJ whose activity is required for the formation of a mature and functional NMJ. It is well established that MuSK kinase activity is tightly regulated to ensure proper NMJ development and that mutations that affect MuSK kinase activity are causally involved in congenital myasthenic syndromes. Signal transduction downstream of MuSK regulates acetylcholine receptor clustering but to understand the exact mechanism of action further studies are required. MuSK activity and function is regulated by posttranslational modifications such as phosphorylation. We have recently identified a novel serine phosphorylation on MuSK S751. Within this project we have identified Calcium/Calmodulin-dependent kinase 2b (CamK2b) as kinase that is able to phosphorylate S751. But CamK2b activity is compensated by other kinases as deletion of CamK2b in muscle cells or mice results in normal MuSK phosphorylation and NMJ development. We further studied the role of serine phosphorylation in a mouse model where MuSK carries a phosphomimetic substitution (D) in the place of S751. Adult mice expressing MuSK S751D form bigger NMJs but show normal motor behavior. Ongoing experiments focus on the role of MuSK S751D during aging. With these studies we will gain novel insights into the molecular mechanisms, which control the development of a functional neuromuscular junction as well as enhanced knowledge about the molecular basis of pathology in patients carrying MuSK mutations thereby aiding to the diagnosis and treatment of congenital myasthenic syndromes.