Mesenchymal stromal cell secretome-induced synaptogenesis is mediated by thrombospondin-1

Mesenchymal stromal cells (MSCs) have been proposed as a promising therapeutic tool for traumatic disorders of the nervous system, due to their neuro-regenerative properties. These outcomes have been extensively connected to paracrine mechanisms. Because synaptic reintegration of injured axons into existing circuitry is required for functional recovery, we investigated the synaptogenic potential of MSC secretome. Using neuronal primary cultures, hippocampal organotypic slices, and human cortical brain organoids, we found that MSC secretome induces presynaptic differentiation. Moreover, the number of axodendritic synapses also increases after treatment with MSC secretome. This increase in synapses correlates with an enhancement in synaptic activity, revealing that newly formed synapses are functional. Finally, we unraveled the mechanism underlying this effect and identified thrombospondin-1 (TSP1) as the major synaptogenic factor in MSC secretome. Together, our findings demonstrate that MSC secretome displays synaptogenic properties, pointing to a potential role as a cell-free therapy to restore lost synaptic connectivity.