1 Scientific RepoRts | 6:34877 | DOI: 10.1038/srep34877 www.nature.com/scientificreports Disruption of ArhGAP15 results in hyperactive Rac1, afects the architecture and function of hippocampal inhibitory neurons and causes cognitive defcits Valentina Zamboni 1 , Maria Armentano 1 , Gabriella sarò 1 , Elisa Ciraolo 1 , Alessandra Ghigo 1 , Giulia Germena 1 , Alessandro Umbach 1 , Pamela Valnegri 2 , Maria Passafaro 2 , Valentina Carabelli 3 , Daniela Gavello 3 , Veronica Bianchi 4 , Patrizia D’Adamo 4 , Ivan de Curtis 4 , Nadia El-Assawi 5 , Alessandro Mauro 5 , Lorenzo Priano 5 , Nicola Ferri 6 , Emilio Hirsch 1,* & Giorgio R. Merlo 1,* During brain development, the small GTPases Rac1/Rac3 play key roles in neuronal migration, neuritogenesis, synaptic formation and plasticity, via control of actin cytoskeleton dynamic. Their activity is positively and negatively regulated by GEFs and GAPs molecules, respectively. However their in vivo roles are poorly known. The ArhGAP15 gene, coding for a Rac-specifc GAP protein, is expressed in both excitatory and inhibitory neurons of the adult hippocampus, and its loss results in the hyperactivation of Rac1/Rac3. In the CA3 and dentate gyrus (DG) regions of the ArhGAP15 mutant hippocampus the CR+, PV+ and SST+ inhibitory neurons are reduced in number, due to reduced efciency and directionality of their migration, while pyramidal neurons are unafected. Loss of ArhGAP15 alters neuritogenesis and the balance between excitatory and inhibitory synapses, with a net functional result consisting in increased spike frequency and bursts, accompanied by poor synchronization. Thus, the loss of ArhGAP15 mainly impacts on interneuron-dependent inhibition. Adult ArhGAP15 -/- mice showed defective hippocampus- dependent functions such as working and associative memories. These fndings indicate that a normal architecture and function of hippocampal inhibitory neurons is essential for higher hippocampal functions, and is exquisitely sensitive to ArhGAP15-dependent modulation of Rac1/Rac3. Te small GTPases of the Rho family, comprising Rho1, Rac1/3 and cdc42, tightly control the dynamic and re-organization of the actin cytoskeleton, an activity at the basis of neuronal migration, neuritogenesis and spine formation 1,2 . Rac GTPases govern lamellipodium and membrane rufe formation, coordinate actin polymeriza- tion and microtubule stability and contribute to determine cell polarity in many cell types, including cortical and hippocampal neurons 1,3,4 . Rac1 is highly and widely expressed in the embryonic and adult brain, while Rac3 is expressed at low levels in the developing nervous system 3 . Rac1/3 have been shown to participate in nearly all steps of neuronal genesis, maturation and circuit formation, including migration, neuritogenesis and spine formation, in vitro and in vivo 3–7 . While the full disruption of Rac1 causes early embryonic lethality, conditional deletion of Rac1 the brain results in axon guidance and radial migration defects 8 , while synapsin1-cre-mediated conditional deletion 1 Department of Molecular Biotechnologies and Health Sciences, University of torino, italy. 2 institute for neuroscience, cnR Milano, italy. 3 Department of Drug Science, University of torino, italy. 4 IRCSS San Rafaele Scientific institute and San Raffaele University, Division of neuroscience, Milano, italy. 5 Department of neurosciences, University of turin & Div. of neurology and neurorehabilitation, S.Giuseppe Hospital, istituto Auxologico italiano iRccS, Piancavallo (VB), italy. 6 Department of Pharmaceutical and Pharmacological Science, University of Padova, italy. * these authors jointly supervised this work. correspondence and requests for materials should be addressed to e.H. (email: emilio.hirsch@unito.it) or G.R.M. (email: giorgioroberto.merlo@unito.it) Received: 26 April 2016 Accepted: 19 September 2016 Published: 07 October 2016 OPEN