Syntaphilin loss enhances mitochondrial axonal transport and neuromuscular junction formation in a human stem cell derived neuromuscular assembloid model.

BACKGROUND: The neuromuscular junction (NMJ) is the synapse between motor neurons and skeletal muscle and controlls movement. Impaired synaptic transmission and NMJ degeneration has been observed during healthy ageing and is also implicated in several neuromuscular diseases. On account of the high energy demands of being distally located and large sized, NMJs are enriched with mitochondria. This enrichment is dependent on transport of mitochondria across the axon to the NMJ. METHODS: We first established a human 3D neuromuscular assembloid model to study in-vitro NMJs, by fusing human stem cell derived spinal cord organoids and primary skeletal muscle spheroids. To determine whether enhancing axonal mitochondrial transport modulates NMJ formation and maintenance, we generated a CRISPR-Cas9 meditated knockout of syntaphilin in human stem cells. RESULTS: Firstly, we characterised the neuromuscular assembloid model which showed functional innervated NMJs as measured by juxtaposed neurofilament+ axons and α-bungarotoxin+ acetylcholine receptors. Secondly, we showed that spinal cord selective genetic ablation of syntaphilin – an axonally localised mitochondrial anchor protein – resulted in increased mitochondrial motility in motor neurons, and consequently increased axonal density and NMJ formation. CONCLUSION: This proof-of-concept study demonstrated that enhancing mitochondrial mobility could provide a therapeutic target to prevent NMJ degeneration.