Balanced development of excitatory and inhibitory synapses is required for normal brain function, and their imbalance may underlie pathogenesis of neuropsychiatric disorders. Compared with many identified trans-synaptic adhesion complexes that organize excitatory synapses, little is known about organizers specific for inhibitory synapses. Here we report Slit and NTRK-like family member 3 (Slitrk3) as a postsynaptic adhesion molecule that selectively regulates inhibitory synapse development via trans-interaction with axonal tyrosine phosphatase receptor PTPδ. Slitrk3 expressed in fibroblasts triggers only inhibitory presynaptic differentiation in contacting axons of cocultured rat hippocampal neurons. Recombinant Slitrk3 preferentially localizes to inhibitory postsynaptic sites. Slitrk3-deficient mice exhibit decreases in inhibitory but not excitatory synapse number and function in hippocampal CA1 neurons and exhibit increased seizure susceptibility and spontaneous epileptiform activity. Slitrk3 requires trans-interaction with axonal PTPδ to induce inhibitory presynaptic differentiation. These results identify Slitrk3-PTPδ as an inhibitory-specific trans-synaptic organizing complex required for normal functional GABAergic synapse development.

The organization of neural progenitors in the developing mammalian neuroepithelium is marked by cadherin-based adherens junctions. Whereas RhoA, a founding member of the small Rho GTPase family, has been shown to play important roles in epithelial adherens junctions, its physiological roles in neural development remain uncertain due to the lack of specific loss-of-function studies. Here, we show that RhoA protein accumulates at adherens junctions in the developing mouse brain and colocalizes to the cadherin–catenin complex. Conditional deletion of RhoA in midbrain and forebrain neural progenitors using Wnt1-Cre and Foxg1-Cre mice, respectively, disrupts apical adherens junctions and causes massive dysplasia of the brain. Furthermore, RhoA -deficient neural progenitor cells exhibit accelerated proliferation, reduction of cell- cycle exit, and increased expression of downstream target genes of the hedgehog pathway. Consequently, both lines of conditional RhoA -deficient embryos exhibit expansion of neural progenitor cells and exencephaly-like protrusions. These results demonstrate a critical role of RhoA in the maintenance of apical adherens junctions and the regulation of neural progenitor proliferation in the developing mammalian brain.

SUMMARY In the vertebrate retina, neurites from distinct neuronal cell types are constrained within the plexiform layers, allowing for establishment of retinal lamination. However, the mechanisms by which retinal neurites are segregated within the inner or outer plexiform layers are not known. We find that the transmembrane semaphorins Sema5A and Sema5B constrain neurites from multiple retinal neuron subtypes within the inner plexiform layer (IPL). In Sema5A−/−; Sema5B−/− mice, retinal ganglion cells (RGCs), amacrine and bipolar cells exhibit severe defects leading to neurite mistargeting into the outer portions of the retina. These targeting abnormalities are more prominent in the outer (OFF) layers of the IPL and result in functional defects in select RGC response properties. Sema5A and Sema5B inhibit retinal neurite outgrowth through PlexinA1 and PlexinA3 receptors both in vitro and in vivo. These findings define a set of ligands and receptors required for the establishment of inner retinal lamination and function.

Mutations in SLITRK1 are found in patients with Tourette's syndrome and trichotillomania. SLITRK1 encodes a transmembrane protein containing leucine-rich repeats that is produced predominantly in the nervous system. However, the role of this protein is largely unknown, except that it can modulate neurite outgrowth in vitro. To clarify the role of Slitrk1 in vivo, we developed Slitrk1-knockout mice and analyzed their behavioral and neurochemical phenotypes. Slitrk1-deficient mice exhibited elevated anxiety-like behavior in the elevated plus-maze test as well as increased immobility time in forced swimming and tail suspension tests. Neurochemical analysis revealed that Slitrk1-knockout mice had increased levels of norepinephrine and its metabolite 3-methoxy-4-hydroxyphenylglycol. Administration of clonidine, an a2-adrenergic agonist that is frequently used to treat patients with Tourette's syndrome, attenuated the anxiety-like behavior of Slitrk1-deficient mice in the elevated plusmaze test. These results lead us to conclude that noradrenergic mechanisms are involved in the behavioral abnormalities of Slitrk1-deficient mice. Elevated anxiety due to Slitrk1 dysfunction may contribute to the pathogenesis of neuropsychiatric diseases such as Tourette's syndrome and trichotillomania.

No abstract

Slitrks are type I transmembrane proteins that share conserved leucine-rich repeat domains similar to those in the secreted axonal guidance molecule Slit. They also show similarities to Ntrk neurotrophin receptors in their carboxy-termini, sharing a conserved tyrosine residue. Among 6 Slitrk family genes in mammals, Slitrk6 has a unique expression pattern, with strong expression in the sensory epithelia of the inner ear. We generated Slitrk6-knockout mice and investigated the development of their auditory and vestibular sensory organs. Slitrk6-deficient mice showed pronounced reduction in the cochlear innervation. In the vestibule, the innervation to the posterior crista was often lost, reduced, or sometimes misguided. These defects were accompanied by the loss of neurons in the spiral and vestibular ganglia. Cochlear sensory epithelia from Slitrk6-knockout mice have reduced ability in promoting neurite outgrowth of spiral ganglion neurons. Indeed the Slitrk6-deficient inner ear showed a mild but significant decrease in the expression of Bdnf and Ntf3, both of which are essential for the innervation and survival of sensory neurons. In addition, the expression of Ntrk receptors, including their phosphorylated forms was decreased in Slitrk6-knockout cochlea. These results suggest that Slitrk6 promotes innervation and survival of inner ear sensory neurons by regulating the expression of trophic and/or tropic factors including neurotrophins from sensory epithelia.

Summary In mammalian spinal cord, group Ia proprioceptive afferents form selective monosynaptic connections with a select group of motor pool targets. The extent to which sensory recognition of motor neurons contributes to the selectivity of sensory-motor connections remains unclear. We show here that proprioceptive sensory afferents that express PlexinD1 avoid forming monosynaptic connections with neurons in Sema3E+ motor pools, yet are able to form direct connections with neurons in Sema3Eoff motor pools. Anatomical and electrophysiological analysis of mice in which Sema3E-PlexinD1 signaling has been deregulated or inactivated genetically reveals that repellent signaling underlies aspects of the specificity of monosynaptic sensory-motor connectivity in these reflex arcs. A semaphorin-based system of motor neuron recognition and repulsion therefore contributes to the formation of specific sensory-motor connections in mammalian spinal cord.

Lrfn2/SALM1 is a PSD-95-interacting synapse adhesion molecule, and human LRFN2 is associated with learning disabilities. However its role in higher brain function and underlying mechanisms remain unknown. Here, we show that Lrfn2 knockout mice exhibit autism-like behavioural abnormalities, including social withdrawal, decreased vocal communications, increased stereotyped activities and prepulse inhibition deficits, together with enhanced learning and memory. In the hippocampus, the levels of synaptic PSD-95 and GluA1 are decreased. The synapses are structurally and functionally immature with spindle shaped spines, smaller postsynaptic densities, reduced AMPA/NMDA ratio, and enhanced LTP. In vitro experiments reveal that synaptic surface expression of AMPAR depends on the direct interaction between Lrfn2 and PSD-95. Furthermore, we detect functionally defective LRFN2 missense mutations in autism and schizophrenia patients. Together, these findings indicate that Lrfn2/LRFN2 serve as core components of excitatory synapse maturation and maintenance, and their dysfunction causes immature/silent synapses with pathophysiological state.