Doc2 Proteins Are Not Required for the Increased Spontaneous Release Rate in Synaptotagmin-1-Deficient Neurons

Regulated secretion is controlled by Ca 2+ sensors with different affinities and subcellular distributions. Inactivation of Syt1 (synaptotagmin-1), the main Ca 2+ sensor for synchronous neurotransmission in many neurons, enhances asynchronous and spontaneous release rates, suggesting that Syt1 inhibits other sensors with higher Ca 2+ affinities and/or lower cooperativities. Such sensors could include Doc2a and Doc2b, which have been implicated in spontaneous and asynchronous neurotransmitter release and compete with Syt1 for binding SNARE complexes. Here, we tested this hypothesis using triple-knock-out mice. Inactivation of Doc2a and Doc2b in Syt1-deficient neurons did not reduce the high spontaneous release rate. Overexpression of Doc2b variants in triple-knock-out neurons reduced spontaneous release but did not rescue synchronous release. A chimeric construct in which the C2AB domain of Syt1 was substituted by that of Doc2b did not support synchronous release either. Conversely, the soluble C2AB domain of Syt1 did not affect spontaneous release. We conclude that the high spontaneous release rate in synaptotagmin-deficient neurons does not involve the binding of Doc2 proteins to Syt1 binding sites in the SNARE complex. Instead, our results suggest that the C2AB domains of Syt1 and Doc2b specifically support synchronous and spontaneous release by separate mechanisms. (Both male and female neurons were studied without sex determination)

Neuronal F-Box protein FBXO41 regulates synaptic transmission and hippocampal network maturation

FBXO41 is a neuron-specific E3 ligase subunit implicated in epileptic encephalopathies. Fbxo41 null mutant (KO) mice show behavioral deficits and early lethality. Here, we report that loss of FBXO41 causes defects in synaptic transmission and brain development. Cultured Fbxo41 KO neurons had normal morphology and showed no signs of degeneration. Single-cell electrophysiology showed a lower synaptic vesicle release probability in excitatory neurons. Inhibitory neurons exhibited reduced synaptophysin expression, a smaller readily releasable pool, and decreased charge transfer during repetitive stimulation. In Fbxo41 KO hippocampal slices at postnatal day 6, the dentate gyrus was smaller with fewer radial-glial-like cells and immature neurons. In addition, neuronal migration was delayed. Two-photon calcium imaging showed a delayed increase in network activity and synchronicity. Together, our findings point toward a role for FBXO41 in synaptic transmission and postnatal brain development, before behavioral deficits are detected in Fbxo41 KO mice

Neuronal F-Box protein FBXO41 regulates synaptic transmission and hippocampal network maturation

FBXO41 is a neuron-specific E3 ligase subunit implicated in epileptic encephalopathies. Fbxo41 null mutant (KO) mice show behavioral deficits and early lethality. Here, we report that loss of FBXO41 causes defects in synaptic transmission and brain development. Cultured Fbxo41 KO neurons had normal morphology and showed no signs of degeneration. Single-cell electrophysiology showed a lower synaptic vesicle release probability in excitatory neurons. Inhibitory neurons exhibited reduced synaptophysin expression, a smaller readily releasable pool, and decreased charge transfer during repetitive stimulation. In Fbxo41 KO hippocampal slices at postnatal day 6, the dentate gyrus was smaller with fewer radial-glial-like cells and immature neurons. In addition, neuronal migration was delayed. Two-photon calcium imaging showed a delayed increase in network activity and synchronicity. Together, our findings point toward a role for FBXO41 in synaptic transmission and postnatal brain development, before behavioral deficits are detected in Fbxo41 KO mice