Heterozygous mutations of the NRXN1 gene, which encodes the presynaptic cell-adhesion molecule neurexin-1, were repeatedly associated with autism and schizophrenia. However, diverse clinical presentations of NRXN1 mutations in patients raise the question whether heterozygous NRXN1 mutations alone directly impair synaptic function. To address this question under conditions that precisely control for genetic background, we generated human ES cells with different heterozygous conditional NRXN1 mutations, and analyzed two different types of isogenic control and NRXN1-mutant neurons derived from these ES cells. Both heterozygous NRXN1 mutations selectively impaired neurotransmitter release in human neurons without changing neuronal differentiation or synapse formation. Moreover, NRXN1-mutant human neurons exhibited increased levels of CASK, a critical synaptic scaffolding protein that binds to neurexin-1. Our results show that, unexpectedly, heterozygous inactivation of NRXN1 directly impairs synaptic function in human neurons, and illustrate the value of this conditional deletion approach for studying the functional effects of disease-associated mutations.

Mutations in the contactin-associated protein 2 (CNTNAP2) gene encoding CASPR2, a neurexin-related cell-adhesion molecule, predispose to autism, but the function of CASPR2 in neural circuit assembly remains largely unknown. In a knockdown survey of autism candidate genes, we found that CASPR2 is required for normal development of neural networks. RNAi-mediated knockdown of CASPR2 produced a cell-autonomous decrease in dendritic arborization and spine development in pyramidal neurons, leading to a global decline in excitatory and inhibitory synapse numbers and a decrease in synaptic transmission without a detectable change in the properties of these synapses. Our data suggest that in addition to the previously described role of CASPR2 in mature neurons, where CASPR2 organizes nodal microdomains of myelinated axons, CASPR2 performs an earlier organizational function in developing neurons that is essential for neural circuit assembly and operates coincident with the time of autism spectrum disorder (ASD) pathogenesis.dendrite | synaptogenesis A utism spectrum disorders (ASDs) comprise a heterogeneous group of early developmental diseases characterized by repetitive and stereotypic behaviors and impairments in social interactions and language development. ASDs are highly heritable, with recent studies linking mutations at hundreds of genes to ASDs (1-3). These findings raised the fundamental question of how these genes might act in neural circuits without being essential for all brain function. Here, we have attempted to address this question by using a Ca 2+ -imaging screening platform to visualize changes in excitatory network activity following shRNA-mediated knockdown (KD) of prominent cell-adhesion molecules that have been repeatedly implicated in the development of ASDs. We found that molecular manipulation of several ASD candidate genes profoundly influences network activity as monitored by this assay. We observed the biggest effects with the neuronal cell-adhesion molecule contactin-associated protein 2 (CASPR2) that is encoded by the CNTNAP2 gene, leading us to specifically focus on the mechanisms by which this cell-adhesion molecule influences neural circuit development.Mutations in the CNTNAP2 gene have been repeatedly identified in ASD patients (for review, see ref. 4). In addition, mutations in CNTNAP2 also have been linked to epilepsy (5-7), Tourette syndrome (8, 9), schizophrenia (5, 7, 10), attention deficit hyperactivity disorder (ADHD) (11), learning disability (12, 13), and language impairment (14-16). Thus, CNTNAP2 is of central importance for human brain function, as additionally shown by recent in vivo MRI studies in which variations in the CNTNAP2 gene were associated with reduced frontal gray matter and altered functional connectivity (17,18).CASPR2 is a member of the contactin-associated protein family (19). CASPRs are referred to as neurexin IV in Drosophila and are highly homologous to neurexins, which are presynaptic celladhesion molecules (20-23). CASPR2 is best known for its role in mye...

α- and β-neurexins are presynaptic cell-adhesion molecules implicated in autism and schizophrenia. We find that although β-neurexins are expressed at much lower levels than α-neurexins, conditional knockout of β-neurexins with continued expression of α-neurexins dramatically decreased neurotransmitter release at excitatory synapses in cultured cortical neurons. The β-neurexin knockout phenotype was attenuated by CB1-receptor inhibition which blocks presynaptic endocannabinoid signaling or by 2-arachidonoylglycerol synthesis inhibition which impairs postsynaptic endocannabinoid release. In synapses formed by CA1-region pyramidal neurons onto burst-firing subiculum neurons, presynaptic in vivo knockout of β-neurexins aggravated endocannabinoid-mediated inhibition of synaptic transmission and blocked LTP; presynaptic CB1-receptor antagonists or postsynaptic 2-arachidonoylglycerol synthesis inhibition again reversed this block. Moreover, conditional knockout of β-neurexins in CA1-region neurons impaired contextual fear memories. Thus, our data suggest that presynaptic β-neurexins control synaptic strength in excitatory synapses by regulating postsynaptic 2-arachidonoylglycerol synthesis, revealing an unexpected role for β-neurexins in the endocannabinoid-dependent regulation of neural circuits.

G protein-coupled receptor (GPCR) signaling pathways mediate the transmission of signals from the extracellular environment to the generation of cellular responses, a process that is critically important for neurons and neurotransmitter action. The ability to promptly respond to rapidly changing stimulation requires timely inactivation of G proteins, a process controlled by a family of specialized proteins known as regulators of G protein signaling (RGS). The R7 group of RGS proteins (R7 RGS) has received special attention due to their pivotal roles in the regulation of a range of crucial neuronal processes such as vision, motor control, reward behavior and nociception in mammals. Four proteins in this group: RGS6, RGS7, RGS9 and RGS11 share a common molecular organization of three modules: (i) the catalytic RGS domain, (ii) a GGL domain that recruits Gβ5, an outlying member of the G protein beta subunit family, and (iii) a DEP/DHEX domain that mediates interactions with the membrane anchor proteins R7BP and R9AP. As heterotrimeric complexes, R7 RGS proteins not only associate with and regulate a number of G protein signaling pathway components, but have also been found to form complexes with proteins that are not traditionally associated with G protein signaling. This review summarizes our current understanding of the biology of the R7 RGS complexes including their structure/functional organization, protein-protein interactions and physiological roles.

Hippocampal CA1 region neurons specifically target latrophilin-2 (Lphn2), an adhesion-type GPCR, to dendritic spines in the stratum lacunosum-moleculare. In this study, Lphn2 controls assembly of excitatory synapses formed by presynaptic entorhinal cortex afferents but not by Schaffer-collateral afferents, suggesting a synaptic recognition function.

A member of regulator of G-protein signaling family, RGS9-2, is an essential modulator of signaling through neuronal dopamine and opioid G-protein-coupled receptors. Recent findings indicate that the abundance of RGS9-2 determines sensitivity of signaling in the locomotor and reward systems in the striatum. In this study we report the mechanism that sets the concentration of RGS9-2 in vivo, thus controlling G-protein signaling sensitivity in the region. We found that RGS9-2 possesses specific degradation determinants which target it for constitutive destruction by lysosomal cysteine proteases. Shielding of these determinants by the binding partner R7 binding-protein (R7BP) controls RGS9-2 expression at the posttranslational level. In addition, binding to R7BP in neurons targets RGS9-2 to the specific intracellular compartment, the postsynaptic density. Implementation of this mechanism throughout ontogenetic development ensures expression of RGS9-2/type 5 G-protein ␤ subunit/R7BP complexes at postsynaptic sites in unison with increased signaling demands at mature synapses.

A member of the RGS (regulators of G protein signaling) family, RGS9-2 is a critical regulator of G protein signaling pathways that control locomotion and reward signaling in the brain. RGS9-2 is specifically expressed in striatal neurons where it forms complexes with its newly discovered partner, R7BP (R7 family binding protein). Interaction with R7BP is important for the subcellular targeting of RGS9-2, which in native neurons is found in plasma membrane and its specializations, postsynaptic densities. Here we report that R7BP plays an additional important role in determining proteolytic stability of RGS9-2. We have found that co-expression with R7BP dramatically elevates the levels of RGS9-2 and its constitutive subunit, G␤5. Measurement of the RGS9-2 degradation kinetics in cells indicates that R7BP markedly reduces the rate of RGS9-2⅐G␤5 proteolysis. Lentivirus-mediated RNA interference knockdown of the R7BP expression in native striatal neurons results in the corresponding decrease in RGS9-2 protein levels. Analysis of the molecular determinants that mediate R7BP/RGS9-2 binding to result in proteolytic protection have identified that the binding site for R7BP in RGS proteins is formed by pairing of the DEP (Disheveled, EGL-10, Pleckstrin) domain with the R7H (R7 homology), a domain of previously unknown function that interacts with four putative ␣-helices of the R7BP core. These findings provide a mechanism for the regulation of the RGS9 protein stability in the striatal neurons.RGS (regulators of G protein signaling) proteins comprise a large family of proteins that control the duration of signal transduction through G protein-coupled receptors (GPCR) 2 (1, 2). RGS proteins act to accelerate the rate of GTP hydrolysis on G protein ␣ subunits, thus facilitating G protein inactivation and the subsequent termination of signaling through GPCRs (reviewed in Ref.3). A mounting body of evidence from clinical studies and genetic animal models indicate that the action of RGS proteins is essential for the normal functioning of almost all systems in the organism (4 -6). In the nervous system, many critical neuronal processes appear to be regulated by the R7 RGS proteins, a subfamily conserved in a variety of animals from Caenorhabditis elegans to humans (2, 7). In mammals, the R7 subfamily consists of four highly homologous proteins: RGS6, RGS7, RGS9, and RGS11, all of which are expressed predominantly in the nervous system (8).Arguably the best studied member of this group is RGS9. It exists in two splice isoforms, RGS9-1 and RGS9-2, which regulate vision and reward behavior, respectively (9). Although the role of RGS9-1 in vertebrate phototransduction has been well established (reviewed in Ref. 10), much remains to be learned about the molecular mechanisms that regulate RGS9-2 function. Previous studies have found that RGS9-2 in the striatum is involved in the modulation of -opoid (11, 12) and D2 dopamine (13-15) receptor responses. Studies of RGS9-deficient mice revealed increased locomotor responses, elevated re...

Neurotransmitter signaling via G protein coupled receptors is crucially controlled by regulators of G protein signaling (RGS) proteins that shape the duration and extent of the cellular response. In the striatum, members of the R7 family of RGS proteins modulate signaling via D2 dopamine and -opioid receptors controlling reward processing and locomotor coordination. Recent findings have established that R7 RGS proteins function as macromolecular complexes with two subunits: type 5 G protein ␤ (G␤5) and R7 binding protein (R7BP). In this study, we report that the subunit compositions of these complexes in striatum undergo remodeling upon changes in neuronal activity. We found that under normal conditions two equally abundant striatal R7 RGS proteins, RGS9-2 and RGS7, are unequally coupled to the R7BP subunit, which is present in complex predominantly with RGS9-2 rather than with RGS7. Changes in the neuronal excitability or oxygenation status resulting in extracellular calcium entry, uncouples RGS9-2 from R7BP, triggering its selective degradation. Concurrently, released R7BP binds to mainly intracellular RGS7 and recruits it to the plasma membrane and the postsynaptic density. These observations introduce activity-dependent remodeling of R7 RGS complexes as a new molecular plasticity mechanism in striatal neurons and suggest a general model for achieving rapid posttranslational subunit rearrangement in multisubunit complexes.Members of the regulator of G protein signaling (RGS) family are ubiquitous negative regulators of signal transmission via G protein-coupled receptors. RGS proteins act to limit the extent and duration of G protein-coupled receptor signaling by accelerating the GTP hydrolysis rate on the ␣ subunits of heterotrimeric G proteins, thus promoting their inactivation (see references 25 and 46 for reviews). The action of RGS proteins is essential for normal functioning of a wide range of fundamental processes including cell division (24), neuronal excitability (47), photoreception (22), angiogenesis (20), vasoconstriction (55), and many others.R7 RGS subfamily is one of six distinct groups of more than 30 diverse RGS proteins (46,64). This subfamily is comprised of four proteins: RGS6, RGS7, RGS9, and RGS11 with similar multidomain organizations (46, 64) and predominant neuronal expression patterns (17). Studies in mice indicate that R7 RGS proteins crucially regulate several critical aspects of nervous system function, such as vision (12, 45), motor control (4, 30), and nociception (15, 48, 62), placing a significant emphasis on the elucidation of their mechanisms.A unique property of R7 RGS proteins is their constitutive association with the type 5 G protein beta (G␤5) subunit (6, 35). Binding to a G protein gamma-like domain in the core of R7 RGS proteins (28), G␤5 is tightly integrated into the structure of the RGS molecule (8). The ability to form complexes with G␤5 was shown to be essential for the folding and stability of R7 RGS proteins (23,60), and knockout of G␤5 in mice results in comple...