Dissecting the dynamics and bioenergetics of local mRNA translation during synaptic plasticity

Long-term synaptic plasticity, which is thought to be at the basis of memory formation, is often dependent on rapid and precisely regulated protein synthesis to alter the composition of the post-synaptic compartment. Proteins can either be transported to stimulated excitatory synapses or be synthetized in dendrites. The latter process, local translation at the spine, is fundamental to maintain the correct synaptic proteome and it is tightly regulated by specific and still largely unknown mechanisms. RNA is transported to dendrites in mRNP granules but its translation is stalled until these organelles receive the correct signals, stopping at appropriate sites in dendrites and initiating protein synthesis. Newly produced proteins can then reach the synapse and participate in structural and functional plasticity as well as in the maturation of spines during neuronal development. mRNA of many proteins of the post-synaptic density such as CaMKII, PSD-95, Shank proteins, BDNF can be found in dendrites and the translation of each species is regulated by specific and sometimes independent mechanisms. The relevance of this process is highlighted by the fact that the absence or dysfunction of mRNA binding proteins regulating translation is associated with neurodevelopmental disorders such as the Fragile X syndrome. The aim of this thesis is to analyze the processes underlying protein synthesis at the synaptic spine in normal conditions and during plasticity. β-actin mRNA, which is fundamental for structural changes of dendritic spines, will be used as a representative example of how transport, docking and translation are influenced or induced by glutamate stimulation of post-synaptic compartments. Then, considering that the massive protein synthesis required for spines enlargement or shrinkage is an extremely energy consuming process, the role of precisely located mitochondrial compartments in providing high levels of ATP in this phase of plasticity will be uncovered. In the last years, RNA dynamics in dendrites have mainly been studied at a single synapse or single RNA species level, to define how the expression of specific proteins was regulated. Nonetheless, since supercomplexes underlying synaptic function are composed of hundreds of different proteins, the adoption of a systematic approach could help discover more refined mechanisms determining synaptic differences. To this end, the use of in situ transcriptomics data obtained by expansion sequencing will allow spatially resolved mapping of mRNA species in dendrites to precisely define how mRNA populations associate and vary in different conditions. ​