Function of Cap2 in Motor Neurons and Skeletal Muscles – SONA.CAP 2

The ability of the human body to move relies on the cooperation between the nervous and muscular systems. A signal generated in the brain is transmitted through motor neurons to muscle fibers, triggering muscle contraction. This transmission is made possible by a specialized type of synapse known as the neuromuscular junction (NMJ).

In this synapse, the neurotransmitter acetylcholine is released from synaptic vesicles at the terminal of the motor neuron’s axon. It then diffuses across the synaptic cleft and binds to acetylcholine receptors (AChRs) located on the muscle fiber membrane. These receptors are highly clustered, and their organization depends on numerous scaffolding proteins that interact with the cellular cytoskeleton, particularly with actin filaments. The actin cytoskeleton undergoes dynamic remodeling in response to internal and external signals. One of the proteins involved in this remodeling process is Cap2.

Previous studies have shown that Cap2 is essential for the development and proper function of cardiac muscle, and its absence causes severe cardiac abnormalities in both mice and humans. In skeletal muscles, Cap2 dysfunction is associated with muscle weakness and the formation of abnormal protein aggregates known as nemaline rods, which impair muscle function. Our collaborative research with a German group has demonstrated that Cap2 knockout (Cap2 KO) mice develop ring fibers in fast-twitch muscles. These are myofibrils oriented transversely relative to other fibers in the muscle, leading to defective muscle contraction. Interestingly, similar ring fibers have been observed in humans with myopathies of unknown origin. These findings indicate that Cap2 plays a crucial role in skeletal muscle physiology, though its specific function at the neuromuscular junction remains unexplored.

Our preliminary, unpublished results show that loss of Cap2 in Cap2 KO mice leads to abnormal motor neuron axon morphology, defective AChR clustering, and altered synaptic junction size. These findings suggest that Cap2 is important for both sides of the synapse — the motor neuron and the muscle fiber. To study these roles in more detail, I have generated tissue-specific knockout mutants, lacking Cap2 only in motor neurons (Cap2 nKO) or only in skeletal muscles (Cap2 mKO).

Within this project, I will conduct a comprehensive characterization of these mutants, focusing on the morphology and ultrastructure of neuromuscular junctions using various microscopy techniques. Additionally, I will perform functional analyses of synaptic performance, including locomotor behavior tests, muscle strength measurements, and electrophysiological recordings.

The results of this project will provide new insights into the role of Cap2 at the neuromuscular junction and may serve as a foundation for developing potential therapies for neuromuscular junction disorders.