Differential Synaptic Vesicle Priming States Determine Synaptic Strength at Hippocampal Glutamatergic Synapses
Mohammad Nour Eddin Mahmoud Aldahabi
János Szentágothai Neurosciences
Dr. Bereczki Dániel
HUN-REN Kísérleti Orvostudományi Kutatóintézet, tanterem
2025-11-24 14:00:00
Neuromorphology and cell biology
Dr. Alpár Alán
Dr. Nusser Zoltán
Dr. Zachar Gergely
Dr. Molnár Gábor
Dr. Dobolyi Árpád
Dr. Tóth Zsuzsanna
Dr. Szabadics János
The variability in neurotransmitter release from a single presynaptic neuron, depending on its postsynaptic target, is a hallmark of cortical network complexity. We found that CA1 PC – FSIN connections exhibit 10 times larger Pv than the PC – mGluR1α-expressing O-LM IN connections. The differential distribution of proteins at presynaptic AZs is crucial for establishing these distinct Pv and STP patterns. O-LM INs express Elfn1, which trans-synaptically recruits mGluR7 to PC axon AZs. We further demonstrate that Elfn1 also selectively recruits Munc13-2, a protein involved in SV priming and docking, to PC AZs targeting mGluR1α+ INs. To determine the roles of Elfn1 and Munc13-2 at the low-Pv PC – O-LM IN connections, we performed knockout experiments. In Elfn1 KO, eEPSCs in O-LM INs showed a 3-fold increase in amplitude and reduced STF, potentially due to the loss of mGluR7, Munc13-2, or both. Conditional genetic deletion of Munc13-2 gene from CA1 PCs resulted in Munc13-2 loss from AZs, but did not affect mGluR7 levels, eEPSC amplitude, or the characteristic STF at PC – O-LM IN connections. These results indicate that Munc13-2 is not essential for the low Pv at PC – O-LM IN synapses and that Munc13-1 alone can mediate both low and high Pv at PC – O-LM and PC – FSIN synapses, respectively.
Freeze-fracture immunolabeling revealed that differences in Ca2+ channel and RS nanotopology or coupling distance do not explain the distinct Pv. Although [Ca2+] transients are 40% larger in FSIN-innervating boutons, matching [Ca2+] entry in both bouton populations still resulted in 7-fold smaller eEPSCs in O-LM cells, suggesting that Pfusion is not the primary factor limiting Pv. However, PDBU application resulted in a ~2.5-fold larger augmentation at PC – O-LM IN synapses compared to PC – FSIN synapses, indicating incomplete vesicle docking or priming. Similar docked vesicle densities ruled out distinct RS occupancies, demonstrating that incompletely primed, yet docked, vesicles limit PC – O-LM IN synapse output.
We used a modelling approach to determine whether the primary difference lies in AP-evoked fusion or upstream vesicle priming processes. We fit a sequential two-step SV priming model to eEPSC peak amplitudes recorded in response to complex presynaptic stimulation. At PC – FSIN connections, Pfusion was 0.6, and 44% of docked SVs were fusion-competent. At PC – O-LM IN synapses, Pfusion was only 40% lower (0.36), while the fraction of well-primed SVs was 6.5-fold smaller. These results demonstrate that the low transmission fidelity at PC – O-LM IN synapses can be explained by a low occupancy of release sites by well-primed SVs.
