FLNA regulates neuronal maturation by modulating RAC1-Cofilin activity in the developing cortex.

Periventricular nodular heterotopia (PNH), the most common brain malformation diagnosed in adulthood, is characterized by the presence of neuronal nodules along the ventricular walls. PNH is mainly associated with mutations in the FLNA gene - encoding an actin-binding protein - and patients often develop epilepsy. However, the molecular mechanisms underlying the neuronal failure still remain elusive. It has been hypothesized that dysfunctional cortical circuitry, rather than ectopic neurons, may explain the clinical manifestations. To address this issue, we depleted FLNA from cortical pyramidal neurons of a conditional Flnaflox/flox mice by timed in utero electroporation of Cre recombinase. We found that FLNA regulates dendritogenesis and spinogenesis thus promoting an appropriate excitatory/inhibitory inputs balance. We demonstrated that FLNA modulates RAC1 and cofilin activity through its interaction with the Rho-GTPase Activating Protein 24 (ARHGAP24). Collectively, we disclose an uncharacterized role of FLNA and provide strong support for neural circuit dysfunction being a consequence of FLNA mutations.

An Emerging Role of PRRT2 in Regulating Growth Cone Morphology.

Mutations in the PRRT2 gene are the main cause for a group of paroxysmal neurological diseases including paroxysmal kinesigenic dyskinesia, episodic ataxia, benign familial infantile seizures, and hemiplegic migraine. In the mature central nervous system, the protein has both a functional and a structural role at the synapse. Indeed, PRRT2 participates in the regulation of neurotransmitter release, as well as of actin cytoskeleton dynamics during synaptogenesis. Here, we show a role of the protein also during early stages of neuronal development. We found that PRRT2 accumulates at the growth cone in cultured hippocampal neurons. Overexpression of the protein causes an increase in the size and the morphological complexity of growth cones. In contrast, the growth cones of neurons derived from PRRT2 KO mice are smaller and less elaborated. Finally, we demonstrated that the aberrant shape of PRRT2 KO growth cones is associated with a selective alteration of the growth cone actin cytoskeleton. Our data support a key role of PRRT2 in the regulation of growth cone morphology during neuronal development.

Dysfunction of the serotonergic system in the brain of synapsin triple knockout mice is associated with behavioral abnormalities resembling synapsin-related human pathologies.

Synapsins (Syns) are a family of phosphoproteins associated with synaptic vesicles (SVs). Their main function is to regulate neurotransmitter release by maintaining a reserve pool of SVs at the presynaptic terminal. Previous studies reported that the deletion of one or more Syn genes in mice results in an epileptic phenotype and autism-related behavioral abnormalities. Here we aimed at characterizing the behavioral phenotype and neurobiological correlates of the deletion of Syns in a Syn triple knockout (TKO) mouse model. Wild type (WT) and TKO mice were tested in the open field, novelty suppressed feeding, light-dark box, forced swim, tail suspension and three-chamber sociability tests. Using in vivo electrophysiology, we recorded the spontaneous activity of dorsal raphe nucleus (DRN) serotonin (5-HT) and ventral tegmental area (VTA) dopamine (DA) neurons. Levels of 5-HT and DA in the frontal cortex and hippocampus of WT and TKO mice were also assessed using a High-Performance Liquid Chromatography. TKO mice displayed hyperactivity and impaired social and anxiety-like behavior. Behavioral dysfunctions were accompanied by reduced firing activity of DRN 5-HT, but not VTA DA, neurons. TKO mice also showed increased responsiveness of DRN 5-HT-1A autoreceptors, measured as a reduced dose of the 5-HT-1A agonist 8-OH-DPAT necessary to inhibit DRN 5-HT firing activity by 50%. Finally, hippocampal 5-HT levels were lower in TKO than in WT mice. Overall, Syns deletion in mice leads to a reduction in DRN 5-HT firing activity and hippocampal 5-HT levels along with behavioral alterations reminiscent of human neuropsychiatric conditions associated with Syn dysfunction.

Proline-rich transmembrane protein 2 (PRRT2) regulates the actin cytoskeleton during synaptogenesis.

Mutations in proline-rich transmembrane protein 2 (PRRT2) have been recently identified as the leading cause of a clinically heterogeneous group of neurological disorders sharing a paroxysmal nature, including paroxysmal kinesigenic dyskinesia and benign familial infantile seizures. To date, studies aimed at understanding its physiological functions in neurons have mainly focused on its ability to regulate neurotransmitter release and neuronal excitability. Here, we show that PRRT2 expression in non-neuronal cell lines inhibits cell motility and focal adhesion turnover, increases cell aggregation propensity, and promotes the protrusion of filopodia, all processes impinging on the actin cytoskeleton. In primary hippocampal neurons, PRRT2 silencing affects the synaptic content of filamentous actin and perturbs actin dynamics. This is accompanied by defects in the density and maturation of dendritic spines. We identified cofilin, an actin-binding protein abundantly expressed at the synaptic level, as the ultimate effector of PRRT2. Indeed, PRRT2 silencing unbalances cofilin activity leading to the formation of cofilin-actin rods along neurites. The expression of a cofilin phospho-mimetic mutant (cof-S3E) is able to rescue PRRT2-dependent defects in synapse density, spine number and morphology, but not the alterations observed in neurotransmitter release. Our data support a novel function of PRRT2 in the regulation of the synaptic actin cytoskeleton and in the formation of synaptic contacts.

Dysfunctional Autophagy and Endolysosomal System in Neurodegenerative Diseases: Relevance and Therapeutic Options.

Autophagy and endolysosomal trafficking are crucial in neuronal development, function and survival. These processes ensure efficient removal of misfolded aggregation-prone proteins and damaged organelles, such as dysfunctional mitochondria, thus allowing the maintenance of proper cellular homeostasis. Beside this, emerging evidence has pointed to their involvement in the regulation of the synaptic proteome needed to guarantee an efficient neurotransmitter release and synaptic plasticity. Along this line, an intimate interplay between the molecular machinery regulating synaptic vesicle endocytosis and synaptic autophagy is emerging, suggesting that synaptic quality control mechanisms need to be tightly coupled to neurosecretion to secure release accuracy. Defects in autophagy and endolysosomal pathway have been associated with neuronal dysfunction and extensively reported in Alzheimer's, Parkinson's, Huntington's and amyotrophic lateral sclerosis among other neurodegenerative diseases, with common features and emerging genetic bases. In this review, we focus on the multiple roles of autophagy and endolysosomal system in neuronal homeostasis and highlight how their defects probably contribute to synaptic default and neurodegeneration in the above-mentioned diseases, discussing the most recent options explored for therapeutic interventions.

Melatonin MT1 receptor as a novel target in neuropsychopharmacology: MT1 ligands, pathophysiological and therapeutic implications, and perspectives.

Melatonin (MLT), a neuromodulator mainly acting through two G-protein coupled receptors MT1 and MT2, regulates many brain functions, including circadian rhythms, mood, pain and sleep. MLT and non-selective MT1/MT2 receptor agonists are clinically used in neuropsychiatric and/or sleep disorders. However, the selective roles of the MT1 and MT2 receptors need to be clarified. Here, we review the role of the MT1 receptor in neuropsychopharmacology, describe the anatomical localization of MT1 receptors in the brain, discuss the medicinal chemistry, biochemistry and molecular aspects of the receptor, and explore the findings linking MT1 receptors to psychiatric and neurological disorders. MT1 receptors are localized in brain regions which regulate circadian rhythms, sleep, and mood, such as the suprachiasmatic nucleus, cortex, hippocampus, dorsal raphe nucleus and lateral hypothalamus. Their activation modulates intracellular signaling pathways also targeted by psychoactive drugs, including antidepressants and mood stabilizers. MT1 receptor knockout mice display increased anxiety, a depressive-like phenotype, increased propensity to reward and addiction, and reduced Rapid-Eye-Movement sleep. These behavioral dysfunctions are associated with altered serotonergic and noradrenergic neurotransmissions. Several studies indicate that the MT1, rather than MT2, receptor is implicated in circadian rhythm regulation. The involvement of MT1 receptors in Alzheimer's and Huntington diseases has also been proposed. Postmortem studies in depressed patients have further confirmed the possible involvement of MT1 receptors in depression. Overall, there is substantial evidence indicating a role for MT1 receptor in modulating brain function and mood. Consequently, this MLT receptor subtype deserves to be further examined as a novel target for neuropsychopharmacological drug development.

Disorders of neurogenesis and cortical development.

The development of the cerebral cortex requires complex sequential processes that have to be precisely orchestrated. The localization and timing of neuronal progenitor proliferation and of neuronal migration define the identity, laminar positioning, and specific connectivity of each single cortical neuron. Alterations at any step of this organized series of events-due to genetic mutations or environmental factors-lead to defined brain pathologies collectively known as malformations of cortical development (MCDs), which are now recognized as a leading cause of drug-resistant epilepsy and intellectual disability. In this heterogeneous group of disorders, macroscopic alterations of brain structure (eg, heterotopic nodules, small or absent gyri, double cortex) can be recognized and probably subtend a general reorganization of neuronal circuits. In this review, we provide an overview of the molecular mechanisms that are implicated in the generation of genetic MCDs associated with aberrations at various steps of neurogenesis and cortical development.

El desarrollo de la corteza cerebral requiere de una secuencia de complejos procesos que tienen que estar coordinados con precisión. La localización y la cronología de la proliferación de las neuronas precursoras y de la migración neuronal definen la identidad, el posicionamiento laminar y la conectividad específica de cada una de las neuronas corticales. Las alteraciones en cualquier etapa de esta serie organizada de acontecimientos- debidas a mutaciones genéticas o a factores ambientales- llevan a patologías cerebrales definidas que en conjunto se denominan malformaciones del desarrollo cortical (MDC), las cuales son reconocidas actualmente como causa de epilepsia resistente a fármacos e incapacidad intelectual. En este grupo heterogéneo de trastornos, las alteraciones macroscópicas de la estructura cerebral (por ej. nódulos heterotópicos, giros pequeños o ausentes, doble corteza) pueden ser reconocidas y es probable que subtiendan a una reorganización general de los circuitos neuronales. En esta revisión se entrega una panorámica de los mecanismos moleculares que se han involucrado en la generación de las MDC asociadas con aberraciones en varias etapas de la neurogénesis y del desarrollo cortical.

Le développement du cortex cérébral fait appel à des processus séquentiels complexes qui doivent être orchestrés précisément. La localisation et la chronologie de la prolifération de neurones précurseurs et celles de la migration neuronale définissent l'identité, le positionnement laminaire et la connectivité spécifique de chaque neurone cortical unique. Toute modification, quel que soit le stade de ces séries organisées d'événements (en raison de mutations génétiques ou de facteurs environnementaux), entraîne des pathologies cérébrales définies, globalement connues sous le terme de malformations du développement cortical (MDC). Ces malformations sont maintenant reconnues comme principalement responsables de la résistance aux médicaments contre l'épilepsie et du déficit intellectuel. Dans ce groupe hétérogène de maladies, les modifications macroscopiques de la structure cérébrale (par exemple, nodules hétérotopiques, gyrus petit ou absent, double cortex) peuvent être identifiées et probablement sous-tendre une réorganisation générale des circuits neuronaux. Cet article présente une vue d'ensemble des mécanismes moléculaires impliqués dans l'apparition de MDC génétiques associées à des aberrations à des stades différents de la neurogenèse et du développement cortical.

eIF4B phosphorylation at Ser504 links synaptic activity with protein translation in physiology and pathology.

Neuronal physiology requires activity-driven protein translation, a process in which translation initiation factors are key players. We focus on eukaryotic initiation factor 4B (eIF4B), a regulator of protein translation, whose function in neurons is undetermined. We show that neuronal activity affects eIF4B phosphorylation and identify Ser504 as a phosphorylation site regulated by casein kinases and sensitive to the activation of metabotropic glutamate receptors. Ser504 phosphorylation increases eIF4B recruitment to the pre-initiation complex and influences eIF4B localization at synapses. Moreover, Ser504 phosphorylation modulates the translation of protein kinase Mζ. Therefore, by sensing synaptic activity, eIF4B could adjust translation to neuronal needs, promoting adaptive changes in synaptic plasticity. We also show that Ser504 phosphorylation is increased in vivo in a rat model of epilepsy during epileptogenesis i.e. when translation drives maladaptive synaptic changes. We propose eIF4B as a mediator between neuronal activity and translation, with relevance in the control of synaptic plasticity.