SUMMARY Cortical circuit dysfunction is a primary pathophysiology that underlies prominent neurological symptoms of Fragile X Syndrome (FXS). Yet the precise way in which circuit development in the cortex is altered in FXS has not been fully elucidated. Recent work by us, and others, has established that local circuit interneurons (INs) may be a key to understanding cortical circuits in FXS. We demonstrated that the density, maturity and activity of parvalbumin (PV) cortical INs are all reduced in the Fmr1 knockout (KO) mouse model of FXS. Here we propose to address outstanding questions in the field by determining how the birth, migration and connectivity of PV INs are disrupted in Fmr1 KO mice, and how this leads to sensory hypersensitivity and tactile defensiveness. We will incorporate a detailed analysis of PV INs using birth dating, neuroanatomical and functional studies to define how the abnormal integration of PV INs into feedforward circuits in the primary somatosensory cortex (S1) contributes to atypical sensory processing. In preliminary studies, we demonstrate that, in response to repetitive whisker stimulation, Fmr1 KO mice display maladaptive avoidance behaviors that correlate with a lack of neuronal adaptation of layer (L) 2/3 pyramidal neurons in S1. We also show that PV INs and their precursors from the medial ganglionic eminence (MGE) are hypoactive in S1 of Fmr1 KO mice by postnatal day (P) 6, and that increasing their activity for a few days using excitatory DREADDs significantly increases their density by P15. We will now determine whether similar early activity manipulations of MGE-derived INs, or later on in more mature PV INs, can restore the loss of sensory adaptation of L2/3 neurons and ameliorate tactile defensiveness in Fmr1 KO mice. We will address the following important questions: 1. What are the contributions of neurogenesis, migration, connectivity and developmental apoptosis to the reduced density of PV INs in FXS? 2. How do MGE-derived INs and pyramidal neurons interact during the early postnatal critical period and how is their ‘handshake’ different in FXS? 3. Is the hypoactivity of PV INs or their precursors causal to the circuit and behavior deficits of Fmr1 KO mice? The mechanistic experimental design employs cell type-specific intersectional genetics, in vivo calcium imaging, chemogenetics, and ex vivo circuit channel-rhodopsin connectivity mapping, among others. An important goal of this grant is to identify whether targeting INs is a viable path for therapeutics in FXS. As such a novel class of allosteric modulating drugs of Kv3.1 channels (responsible for fast-spiking characteristics of PV INs) will be tested in Fmr1 KO mice. Overall, the collaboration between the laboratories of Dr. Carlos Portera-Cailliau (co-PI, PL) at UCLA and Dr. Anis Contractor (co-PI) at Northwestern University will enable a comprehensive approach to understanding the developmental and functional contributions of INs to the pathoph...