Semaphorin signalling is required for early differentiation of spinal dorsal horn neurons
The birth of postmitotic neurons from neuroprogenitor cells is a unique process happen shortly after the neurulation. Spinal cord neurons migrate away from the ventricular zone after leaving their cell cycle and populating a highly organized layered structure. Semaphorins are important players for the further neuronal differentiation particularly in axon pathfinding. However, little is known about these roles in the neuronal cell cycle, even though the expression of semaphorin receptors in the ventricular zone of developing spinal cord. This study gains insights into the possible role of this early expression of secreted semaphorins and their receptors. We applied a dominant negative approach to the main receptors of semaphorins neuropilin 1 and 2 to disrupt the signalling pathway of secreted semaphorins. We electroporated plasmids into the spinal cords of mice and chicken embryos coding the dominant negative neuropilin receptors and the GFP for labelling. Those of cells expressing either the dominant negative neuropilin 1 or 2 stuck in the ventricular zone compared to the control GFP expressing spinal cords. Following BrdU and immunohistochemistry against cell cycle markers showed, that in case of neuropilin 2, most of the labelled cells were postmitotic but failed to develop (or withdraw) migratory (leading and trailing) processes in the presumable dorsal horn. Downregulating of neuropilin 1 gave similar phenotype, however we found that many of the cells were co-labelled with BrdU indicating their prolonged stay in the mitotic cycle. These results suggest that semaphorin signalling is required for early differentiation of the spinal dorsal horn neurons.