Light and Electron Microscopy to Study Structural Aspects of Synapse Evolution

Having insights on how the early steps of brain development have changed during evolution is fundamental in understanding the cell biological bases that provide humans’ higher integration and computation ability. Compared to other mammals, primates’ brain contains a greater number of neural stem and progenitor cells, and, among primate’s species, some types of neurons differ structurally, especially in terms of length and branching of dendrites. Previous studies (Schörnig et al.) have shown that iPSCs derived neurons (iNeurons) from non-human primates and humans differ in their maturation in vitro from both the biochemical and functional point of view and that these differences are paralleled by a different expression of synaptic markers, as highlighted by immunofluorescence experiments. We are presently investigating in vitro synapse maturation in human and chimpanzee iNeurons, searching for species-specific differences in synapse architecture and ultrastructure exploiting electron microscopy. Thanks to transmission electron microscopy, we will be able to correlate all information on synapse ultrastructure modifications in time and between the two species with the previously reported differences. We then plan to transfect iNeurons from the two species with PSD95-FingRs constructs able to bind endogenous PSD95 without affecting the characteristics of dendritic spines, with a transcriptional feedback system. This new cell model allows us to follow the expression level and the localization of the endogenous protein in living cells during neuronal maturation and to further characterize the formation of the postsynaptic compartment and its maturation with Correlative Light and Electron Microscopy.