Epileptogenic Q555X SYN1 mutant triggers imbalances in release dynamics and short-term plasticity

Abstract

Synapsin I (SynI) is a synaptic vesicle (SV) phosphoprotein playing multiple roles in synaptic transmission and plasticity by differentially affecting crucial steps of SV trafficking in excitatory and inhibitory synapses. SynI knockout (KO) mice are epileptic, and nonsense and missense mutations in the human SYN1 gene have a causal role in idiopathic epilepsy and autism. To get insights into the mechanisms of epileptogenesis linked to SYN1 mutations, we analyzed the effects of the recently identified Q555X mutation on neurotransmitter release dynamics and short-term plasticity (STP) in excitatory and inhibitory synapses. We used patch-clamp electrophysiology coupled to electron microscopy and multi-electrode arrays to dissect synaptic transmission of primary SynI KO hippocampal neurons in which the human wild-type and mutant SynI were expressed by lentiviral transduction. A parallel decrease in the SV readily releasable pool in inhibitory synapses and in the release probability in excitatory synapses caused a marked reduction in the evoked synchronous release. This effect was accompanied by an increase in asynchronous release that was much more intense in excitatory synapses and associated with an increased total charge transfer. Q555X-hSynI induced larger facilitation and post-tetanic potentiation in excitatory synapses and stronger depression after long trains in inhibitory synapses. These changes were associated with higher network excitability and firing/bursting activity. Our data indicate that imbalances in STP and release dynamics of inhibitory and excitatory synapses trigger network hyperexcitability potentially leading to epilepsy/autism manifestations.

Subconvulsant doses of pentylenetetrazol uncover the epileptic phenotype of cultured synapsin-deficient Helix serotonergic neurons in the absence of excitatory and inhibitory inputs

“…The importance of characterizing the excitable phenotype developed by Syn deficiencies is highlighted by the fact that Syn deletion and mutations have been associated with the development of epileptic phenotypes in animals (Li et al, 1995;Rosahl et al, 1995;Gitler et al, 2004;Etholm and Heggelund, 2009;Ketzef et al, 2011;Etholm et al, 2013;Ketzef and Gitler, 2014). In humans, epilepsy and other disorders have been linked to mutations in SynI (Garcia et al, 2004;Fassio et al, 2011b;Lignani et al, 2013;Giannandrea et al, 2013) and SynII genes (Cavalleri et al, 2007;Lakhan et al, 2010;Corradi et al, 2014). In addition, the seizure susceptibility of mice TKO brain slices has been reported to anticipate epileptic phenotypes (Boido et al, 2010;Feliciano et al, 2013).…”

“…It is generally accepted that Syn deficiency reduces GABAergic transmission and not affects or increases glutamatergic transmission, thus leading to a positive imbalance towards hyperexcitability (Terada et al, 1999;Gitler et al, 2004;Baldelli et al, 2007;Chiappalone et al, 2009;Ketzef et al, 2011;Farisello et al, 2012;Lignani et al, 2013;Feliciano et al, 2013;Medrihan et al, 2013;Medrihan et al, 2014 Klee, 1976 andAltrup, 2004). Finally, a key advantage of this monosynaptic cell model is that the presynaptic and postsynaptic compartments can be selectively targeted by injections and specific networks can be constructed by directly plating individual neurons on MEA electrodes.…”

Knock-down of synapsin alters cell excitability and action potential waveform by potentiating BK and voltage-gated Ca2+ currents in Helix serotonergic neurons

“…In addition, epidemiological studies reported several Syn mutations associated with several disorders, such as epilepsy in humans (García et al, 2004;Cavalleri et al, 2007;Lakhan et al, 2010;Fassio et al, 2011b;Lignani et al, 2013;Corradi et al, 2014).…”

“…In mammals it has been shown that almost all single, double or triple Syn knock-out (KO) mice develop a severe epileptic phenotype induced by sensory stimulation (for review see Cesca et al, 2010 andFassio et al, 2011a). In humans, several nonsense and missense Syn mutations associated with epilepsy, among other disorders, have been reported and characterized to some degree, for both SYN1 (García et al, 2004;Fassio et al, 2011b;Lignani et al, 2013, Giannandrea et al, 2013 and SYN2 genes (Cavalleri et al, 2007;Lakhan et al, 2010;Corradi et al, 2014).…”

“…Epileptic disorders have been related with deficiencies in Syn-regulated processes such as neurite outgrowth, synapse formation or neurotransmitter release. In fact, the development of epilepsy in Syn mutants has been associated with i) alterations of neurite outgrowth and synaptogenesis leading to abnormal circuitry during maturation (Cesca et al, 2010), and ii) modification of the synaptic vesicle pool mobility and release with consequent differential changes of the rate of neurotransmitter release in excitatory and inhibitory synapses, thus causing an excitation/inhibition imbalance in network excitability (Gitler et al, 2004;Baldelli et al, 2007;Chiappalone et al, 2009;Noebels et al, 2010;Pitkänen and Lukasiuk, 2011;Farisello et al, 2012;Lignani et al, 2013).…”