Boosting Synapse Formation in Human iPSC Motor Neurons for Better Disease Modeling

The intricate workings of human synapses, the tiny junctions where neurons communicate, are fundamental to brain function, yet notoriously difficult to study at high resolution. Traditional methods using brain tissue or animal models have limitations, prompting a push towards human-specific models. Human induced pluripotent stem cells (iPSCs) offer a promising alternative, providing patient-specific neurons and a renewable resource, but historically, achieving robust and reproducible synapse formation in iPSC-derived neurons, especially motor neurons, has been a significant hurdle due to issues like variable differentiation and low synaptic density.

To overcome this, a new study focused on optimizing motor neuron differentiation protocols and introducing specific glutamatergic modulators, CX516 and CDPPB, during the differentiation process. Within just 28 days, these modified human iPSC-derived motor neurons developed into morphologically and functionally mature neuronal networks, characterized by phase-bright somata, elongated axons, and dense axodendritic branching. The effectiveness of this combined approach was rigorously validated through multiple techniques, including immunolabeling of synaptic markers, electrophysiology, calcium imaging, live-cell synaptic vesicle recycling assays, and high-resolution ultrastructural imaging like cryo-electron tomography (cryo-ET).

The result, as lead author Rostami notes in the paper, is that "This combined approach of optimized differentiation and targeted neuromodulation yielded reproducible MN networks with enhanced synaptic density and function, providing a robust in vitro platform for investigating human MN physiology, synaptic mechanisms, and disease-relevant synaptopathies." This advancement offers a powerful tool for deeper insights into neurological disorders and the development of new therapeutic strategies within a human-specific context.