IMAGING THE DYNAMICS OF PRESYNAPTIC CAMP AT HIPPOCAMPAL MOSSY FIBER SYNAPSES IN CONTROL CONDITIONS AND IN A MODEL OF FRAGILE X

Cyclic adenosine monophosphate (cAMP) is a second messenger that, together with its downstream effectors such as protein kinase A (PKA), plays a pivotal role in presynaptic mechanisms. At the hippocampal mossy fiber (MF) synapse onto CA3 pyramidal cells, a rise in presynaptic cAMP levels produces short (PTP, post-tetanic potentiation) and long-term (LTP) changes in vesicle release probability. Synaptic facilitation arises from an increase in the number of release sites and in the clustering of calcium channels at the presynaptic terminal (Shahoha et al., 2022). However, the dynamics of presynaptic cAMP in the context of synaptic transmission and neuromodulation, and of consequent PKA activation, have remained elusive. To address this question, we assessed presynaptic cAMP levels by combining the fluorescent cAMP biosensor cAMPFIRE with two-photon microscopy in hippocampal organotypic slices. Using this approach, we can quantify changes in presynaptic cAMP levels at MF synapses in response to neuromodulators and electrical stimulation. Additionally, the loss of the RNA-binding protein FMRP in fragile X syndrome (FXS), the most common inherited form of intellectual disability arising from silencing of the FMR1 gene, leads to elevated levels of phosphodiesterase 2A (PDE2A), the enzyme responsible for cAMP degradation. We propose that the impairment of Mf-CA3 synapses observed in a mouse model of FXS arises from a deregulation of PKA. We currently evaluate cAMP dynamics in a model of FMRP deletion in MF synapses in order to elucidate how the absence of FMRP can affect presynaptic mechanisms.