Altered Cl- homeostasis hinders forebrain GABAergic interneuron migration in a mouse model of intellectual disability.

Impairments of inhibitory circuits are at the basis of most, if not all, cognitive deficits. The impact of OPHN1, a gene associate with intellectual disability (ID), on inhibitory neurons remains elusive. We addressed this issue by analyzing the postnatal migration of inhibitory interneurons derived from the subventricular zone in a validated mouse model of ID (OPHN1-/y mice). We found that the speed and directionality of migrating neuroblasts were deeply perturbed in OPHN1-/y mice. The significant reduction in speed was due to altered chloride (Cl-) homeostasis, while the overactivation of the OPHN1 downstream signaling pathway, RhoA kinase (ROCK), caused abnormalities in the directionality of the neuroblast progression in mutants. Blocking the cation-Cl- cotransporter KCC2 almost completely rescued the migration speed while proper directionality was restored upon ROCK inhibition. Our data unveil a strong impact of OPHN1 on GABAergic inhibitory interneurons and identify putative targets for successful therapeutic approaches.

Oligophrenin-1 regulates number, morphology and synaptic properties of adult-born inhibitory interneurons in the olfactory bulb.

Among the X-linked genes associated with intellectual disability, Oligophrenin-1 (OPHN1) encodes for a Rho GTPase-activating protein, a key regulator of several developmental processes, such as dendrite and spine formation and synaptic activity. Inhibitory interneurons play a key role in the development and function of neuronal circuits. Whether a mutation of OPHN1 can affect morphology and synaptic properties of inhibitory interneurons remains poorly understood. To address these open questions, we studied in a well-established mouse model of X-linked intellectual disability, i.e. a line of mice carrying a null mutation of OPHN1, the development and function of adult generated inhibitory interneurons in the olfactory bulb. Combining quantitative morphological analysis and electrophysiological recordings we found that the adult generated inhibitory interneurons were dramatically reduced in number and exhibited a higher proportion of filopodia-like spines, with the consequences on their synaptic function, in OPHN1 ko mice. Furthermore, we found that olfactory behaviour was perturbed in OPHN1 ko mice. Chronic treatment with a Rho kinase inhibitor rescued most of the defects of the newly generated neurons. Altogether, our data indicated that OPHN1 plays a key role in regulating the number, morphology and function of adult-born inhibitory interneurons and contributed to identify potential therapeutic targets.

Aberrant Patterns of Sensory-Evoked Activity in the Olfactory Bulb of LRRK2 Knockout Mice.

The LRRK2 gene is the major genetic determinant of familiar Parkinson's disease (PD). Leucine-rich repeat kinase 2 (LRRK2) is a multidomain protein involved in several intracellular signaling pathways. A wealth of evidence indicates that LRRK2 is enriched at the presynaptic compartment where it regulates vesicle trafficking and neurotransmitter release. However, whether the role of LRRK2 affects neuronal networks dynamic at systems level remains unknown. Addressing this question is critical to unravel the impact of LRRK2 on brain function. Here, combining behavioral tests, electrophysiological recordings, and functional imaging, we investigated neuronal network dynamics, in vivo, in the olfactory bulb of mice carrying a null mutation in LRRK2 gene (LRRK2 knockout, LRRK2 KO, mice). We found that LRRK2 KO mice exhibit olfactory behavioral deficits. At the circuit level, the lack of LRRK2 expression results in altered gamma rhythms and odorant-evoked activity with significant impairments, while the spontaneous activity exhibited limited alterations. Overall, our data in the olfactory bulb suggest that the multifaced role of LRRK2 has a strong impact at system level when the network is engaged in active sensory processing.

Modelling genetic mosaicism of neurodevelopmental disorders in vivo by a Cre-amplifying fluorescent reporter.

Genetic mosaicism, a condition in which an organ includes cells with different genotypes, is frequently present in monogenic diseases of the central nervous system caused by the random inactivation of the X-chromosome, in the case of X-linked pathologies, or by somatic mutations affecting a subset of neurons. The comprehension of the mechanisms of these diseases and of the cell-autonomous effects of specific mutations requires the generation of sparse mosaic models, in which the genotype of each neuron is univocally identified by the expression of a fluorescent protein in vivo. Here, we show a dual-color reporter system that, when expressed in a floxed mouse line for a target gene, leads to the creation of mosaics with tunable degree. We demonstrate the generation of a knockout mosaic of the autism/epilepsy related gene PTEN in which the genotype of each neuron is reliably identified, and the neuronal phenotype is accurately characterized by two-photon microscopy.