Deciphering the role of the actin cytoskeleton in central nervous system synapses

Actin is the predominant cytoskeletal structure in both the pre- and the post-synaptic compartment of excitatory synapses, which are formed between the distal part of the axon and the distal site of dendritic spines. Tropomyosin (Tpm) has been regarded as a master regulator of actin dynamics in mammalian cells. Tpm isoforms found in neurons are encoded by the Tpm1, Tpm3 and Tpm4 genes, and have a distinct temporal and spatial distribution. Tpm3 and Tpm4 gene products have been found to segregate to the postsynaptic region of central nervous system synapses. Here, the detailed morphometric analysis of mouse primary hippocampal neurons, isolated from wild type, Tpm3 knock-out (KO) and Tpm4.2 KO mice has shown a significant difference of the axonal and dendritic length and complexity between the investigated groups. The dynamics of Tpm3.1 and Tpm4.2 at the postsynaptic compartment was further investigated via Fluorescent Recovery After Photobleaching (FRAP) experiments of the dendritic spines, where it has been discovered that Tpm3.1 and Tpm4.2 have distinct kinetic features. Moreover, the analysis of the Tpm4.2 interactome using the biotin ligase proximity-dependent labelling (BioID2 assay), confirmed the involvement of Tpm4.2 in synaptic metabolic pathways, as well as in synaptic transmission and connectivity. Overall, the effect of Tpm3.1 and Tpm4.2 isoforms on neuron morphology and function of dendritic spines provides the evidence for their potential regulation of different actin filament populations at the synaptic compartment. These findings have the potential to further elucidate the specific role of the actin cytoskeleton in neurodegenerative diseases, as well as to help to target specific molecular pathways for the prevention of these diseases.