SummaryHeterozygous mutations in proline-rich transmembrane protein 2 (PRRT2) underlie a group of paroxysmal disorders, including epilepsy, kinesigenic dyskinesia, and migraine. Most of the mutations lead to impaired PRRT2 expression, suggesting that loss of PRRT2 function may contribute to pathogenesis. We show that PRRT2 is enriched in presynaptic terminals and that its silencing decreases the number of synapses and increases the number of docked synaptic vesicles at rest. PRRT2-silenced neurons exhibit a severe impairment of synchronous release, attributable to a sharp decrease in release probability and Ca2+ sensitivity and associated with a marked increase of the asynchronous/synchronous release ratio. PRRT2 interacts with the synaptic proteins SNAP-25 and synaptotagmin 1/2. The results indicate that PRRT2 is intimately connected with the Ca2+-sensing machinery and that it plays an important role in the final steps of neurotransmitter release.

Several genes predisposing to autism spectrum disorders (ASDs) with or without epilepsy have been identified, many of which are implicated in synaptic function. Here we report a Q555X mutation in synapsin 1 (SYN1), an X-linked gene encoding for a neuron-specific phosphoprotein implicated in the regulation of neurotransmitter release and synaptogenesis. This nonsense mutation was found in all affected individuals from a large French-Canadian family segregating epilepsy and ASDs. Additional mutations in SYN1 (A51G, A550T and T567A) were found in 1.0 and 3.5% of French-Canadian individuals with autism and epilepsy, respectively. The majority of these SYN1 mutations were clustered in the proline-rich D-domain which is substrate of multiple protein kinases. When expressed in synapsin I (SynI) knockout (KO) neurons, all the D-domain mutants failed in rescuing the impairment in the size and trafficking of synaptic vesicle pools, whereas the wild-type human SynI fully reverted the KO phenotype. Moreover, the nonsense Q555X mutation had a dramatic impact on phosphorylation by MAPK/Erk and neurite outgrowth, whereas the missense A550T and T567A mutants displayed impaired targeting to nerve terminals. These results demonstrate that SYN1 is a novel predisposing gene to ASDs, in addition to epilepsy, and strengthen the hypothesis that a disturbance of synaptic homeostasis underlies the pathogenesis of both diseases.

See Lerche (doi:) for a scientific commentary on this article. PRRT2 mutations cause heterogeneous paroxysmal neurological disorders. Using iPSC-derived neurons from patients homozygous for a nonsense PRRT2 mutation and cortical neurons from PRRT2-knockout mice, Fruscione et al. show that PRRT2 is a negative modulator of voltage-dependent NaV1.2/1.6 channels. Increased neuronal excitability may contribute to the paroxysmal nature of PRRT2-linked diseases.

Alterations in the formation of brain networks are associated with several neurodevelopmental disorders. Mutations in TBC1 domain family member 24 (TBC1D24) are responsible for syndromes that combine cortical malformations, intellectual disability, and epilepsy, but the function of TBC1D24 in the brain remains unknown. We report here that in utero TBC1D24 knockdown in the rat developing neocortex affects the multipolar-bipolar transition of neurons leading to delayed radial migration. Furthermore, we find that TBC1D24-knockdown neurons display an abnormal maturation and retain immature morphofunctional properties. TBC1D24 interacts with ADP ribosylation factor (ARF)6, a small GTPase crucial for membrane trafficking. We show that in vivo, overexpression of the dominant-negative form of ARF6 rescues the neuronal migration and dendritic outgrowth defects induced by TBC1D24 knockdown, suggesting that TBC1D24 prevents ARF6 activation. Overall, our findings demonstrate an essential role of TBC1D24 in neuronal migration and maturation and highlight the physiological relevance of the ARF6-dependent membrane-trafficking pathway in brain development.dendritogenesis | synaptogenesis | epileptic encephalopathies | gene | RNA interferenceT he mammalian cerebral cortex is a multilayered structure derived from cells of the neural tube (1). Cortical projection neurons are generated from proliferating progenitor cells located in the ventricular zone (VZ) adjacent to the lateral ventricle (2). After their final mitotic division, the majority of neurons migrate radially, along radial glia fibers, from the VZ toward the pial surface, where each successive generation passes one another and settles in an inside-out pattern within the cortical plate (CP) (3, 4). When neurons reach their final destination, they stop migrating and order themselves into specific "architectonic" patterns, according to a complex signaling pathway, guiding cells to the correct location in the brain (5). Overall, these steps lead to the formation of a six-layered cortex that plays a key role in cognitive processes. It is therefore not surprising that disruption of early cortical development causes severe neurological conditions usually featuring intellectual disabilities (IDs) and epilepsy, which may be associated with brain malformations (6).Over the past decades, advances in our understanding of the genetics of ID and epilepsy have revolutionized the diagnostic and the clinical approach to these disorders, and an increasing number of pathogenic gene mutations have been identified. TBC1D24 is a novel epilepsy-related gene mutated in different autosomal recessive forms of early-onset epilepsy, which can be accompanied by variable degrees of ID (7-11). The TBC1D24 gene encodes a protein of 553 aa that contains a Tre2/Bub2/Cdc16 (TBC) domain, shared by Rab GTPase-activating proteins (Rab-GAPs) and a TLDc domain of unknown function (8). The TBC1D24 protein is expressed in the brain and interacts with the ADP ribosylation factor (ARF) 6, a small GTP-binding prot...