Background Neuronal activity at gamma frequency is impaired in schizophrenia (SZ) and is considered critical for cognitive performance. Such impairments are thought to be due to reduced N-Methyl-D-Aspartate Receptor (NMDAR)-mediated inhibition from parvalbumin (PV) interneurons, rather than a direct role of impaired NMDAR signaling on pyramidal neurons. However, recent studies suggest a direct role of pyramidal neurons in regulating gamma oscillations. In particular, a computational model has been proposed in which phasic currents from pyramidal cells could drive synchronized feedback inhibition from interneurons. As such, impairments in pyramidal neuron activity could lead to abnormal gamma oscillations. However, this computational model has not been tested experimentally and the molecular mechanisms underlying pyramidal neuron dysfunction in SZ remain unclear. Methods In the present study, we tested the hypothesis that SZ-related phenotypes could arise from reduced NMDAR signaling in pyramidal neurons using forebrain pyramidal neurons specific NMDA-R1 knocked-out mice. Results The mice displayed increased baseline gamma power as well as socio-cognitive impairments. These phenotypes were associated with increased pyramidal cell excitability due to changes in inherent membrane properties. Interestingly, mutant mice showed decreased expression of GIRK2 channels, which has been linked to increase neuronal excitability. Conclusions Our data demonstrate for the first time that NMDAR hypofunction in pyramidal cells is sufficient to cause electrophysiological, molecular, neuropathological and behavioral changes related to SZ.

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SUMMARYLigand-specific negative regulation of cytokine-induced signaling relies on down regulation of the cytokine receptors. Down regulation of the IFNAR1 sub-unit of the Type I interferon (IFN) receptor proceeds via lysosomal receptor proteolysis, which is triggered by ubiquitination that depends on IFNAR1 serine phosphorylation. While IFN-inducible phosphorylation, ubiquitination and degradation requires the catalytic activity of the Tyk2 Janus kinase, here we found the ligand-and Tyk2-independent pathway that promotes IFNAR1 phosphorylation, ubiquitination, and degradation when IFNAR1 is expressed at high levels. A major cellular kinase activity that is responsible for IFNAR1 phosphorylation in vitro does not depend on either ligand or Tyk2 activity. Inhibition of ligand-independent IFNAR1 degradation suppresses cell proliferation. We discuss the signaling events that might lead to ubiquitination and degradation of IFNAR1 via ligand-dependent and independent pathways and their potential physiologic significance.

The ubiquitination of the receptor that mediates signaling induced by the polypeptide pituitary hormone prolactin (PRL) has been shown to lead to the degradation of this receptor and to the ensuing negative regulation of cellular responses to PRL. However, the mechanisms of PRL receptor (PRLr) proteolysis remain largely to be determined. Here we provide evidence that PRLr is internalized and primarily degraded via the lysosomal pathway. Ubiquitination of PRLr is essential for the rapid internalization of PRLr, which proceeds through a pathway dependent on clathrin and the assembly polypeptide 2 (AP2) adaptor complexes. Recruitment of AP2 to PRLr is stimulated by PRLr ubiquitination, which also is required for the targeting of already internalized PRLr to the lysosomal compartment. While mass spectrometry analysis revealed that both monoubiquitination and polyubiquitination (via both K48-and K63-linked chains) occur on PRLr, the results of experiments using forced expression of ubiquitin mutants indicate that PRLr polyubiquitination via K63-linked chains is important for efficient interaction of PRLr with AP2 as well as for efficient internalization, postinternalization sorting, and proteolytic turnover of PRLr. We discuss how specific ubiquitination may regulate early and late stages of endocytosis of PRLr and of related receptors to contribute to the negative regulation of the magnitude and duration of downstream signaling.Endocytosis of signaling receptors is a major mechanism used by cells to restrict the magnitude and duration of signal transduction induced by extracellular ligands. Ligand-induced endocytosis of cell surface receptors may occur through clathrin-dependent or -independent pathways. The clathrin-dependent pathway links receptors with clathrin-coated vesicles, which are specialized invaginations of the plasma membrane that concentrate receptors that become internalized. This pathway relies on the interaction of the assembly polypeptide 2 (AP2) clathrin adaptor complexes with specific endocytic signals located within the cytoplasmic domain of the receptors (5).AP2 complexes, which are involved in the assembly of clathrin triskelions at the plasma membrane, are composed of four components, including two adaptin subunits (␣ and ␤2) and two smaller subunits ( 2 and 2); among these subunits, each has different biological functions (34). There are specific endocytic motifs that are essential for receptor clustering on the membrane and clathrin-dependent internalization of receptors. For example, both tyrosine-and leucine-based motifs can be recognized by the AP2 complex through the interaction with its 2 subunit and with the ␤2 or ␣/ 2 hemicomplexes, respectively (5,8,13).The process of receptor endocytosis is not exclusively regulated by the linear motifs located on the cytoplasmic tail of receptors. Posttranslational modification by ubiquitination has also emerged as an important factor in the endocytosis and sorting of surface receptors. Ubiquitin is a 76-amino-acid protein that forms an isopeptide...

Background Behavioral symptoms in individuals with autism spectrum disorder (ASD) have been attributed to abnormal neuronal connectivity, but the molecular bases of these behavioral and brain phenotypes are largely unknown. Human genetic studies have implicated Protocadherin 10 (PCDH10), a member of the δ2 subfamily of non-clustered protocadherin genes, in ASD. PCDH10 expression is enriched in the basolateral amygdala, a brain region implicated in the social deficits of ASD. Previous reports indicate that Pcdh10 plays a role in axon outgrowth and glutamatergic synapse elimination, but its roles in social behaviors and amygdala neuronal connectivity are unknown. We hypothesized that haploinsufficiency of Pcdh10 would reduce social approach behavior and alter the structure and function of amygdala circuits. Methods Mice lacking one copy of Pcdh10 (Pcdh10+/−) and wildtype littermates (WT) were assessed for social approach and other behaviors. The lateral/basolateral amygdala was assessed for dendritic spine number and morphology, and amygdala circuit function was studied using voltage sensitive dye imaging. Expression of Pcdh10 and N-methyl-D-aspartate receptor (NMDAR) subunits was assessed in post-synaptic density fractions of amygdala. Results Male Pcdh10+/− mice have reduced social approach behavior, as well as impaired gamma synchronization, abnormal spine morphology, and reduced levels of NMDAR subunits in amygdala. Social approach deficits in Pcdh10+/− males were rescued with acute treatment with the NMDAR partial agonist d-cycloserine. Conclusions Our studies reveal that male Pcdh10+/− mice have synaptic and behavioral deficits, and establish Pcdh10+/− mice as a novel genetic model for investigating neural circuitry and behavioral changes relevant to ASD.

Linear endocytic motifs of signaling receptors as well as their ubiquitination determine the rate of ligand-induced endocytosis that mediates down-regulation of these receptors and restricts the magnitude and duration of their respective signal transduction pathways. We previously hypothesized that, in the absence of its cognate ligand, type I interferon (IFN), the IFN␣ receptor chain 1 (IFNAR1) receptor chain is protected from basal endocytosis by a hypothetical masking complex that prevents the Tyr-based endocytic motif within IFNAR1 from interacting with components of the adaptin protein complex 2 (AP2). Here we identify a member of the Janus kinase (Jak) family, Tyk2, as a component of such a masking complex. In the absence of ligand or of receptor chain ubiquitination, binding of Janus kinase Tyk2 within the proximity of the Tyr-based linear motif of IFNAR1 is required to prevent IFNAR1 internalization and to maintain its cell surface expression. Furthermore, interaction experiments revealed that Tyk2 physically shields this Tyrbased motif from the recognition by the AP50 subunit of AP2. These data delineate a long-sought ligand-and ubiquitin-independent mechanism by which Tyk2 contributes to both the regulation of total IFNAR1 levels as well as the regulation of the cell surface density of this receptor chain.Cells react to diverse environmental stimuli by expressing specific receptors that recognize these stimuli and initiate specific signaling pathways that enable a cell or a tissue to cope with an altered environment. Down-regulation of these signaling receptors represents the most specific mode of limiting the magnitude and duration of given signal transduction pathways. For transmembrane receptors displayed at the cell surface, ligand-stimulated endocytosis is a major mechanism by which the ability of a cell to react to a ligand is restricted. In addition, basal ligand-independent internalization may determine how responsive a naive cell could be to a subsequent encounter with a particular ligand.Mechanisms mediating internalization of signaling receptors involve a dynamic plasma membrane exchange resulting in bulk endocytosis and a cargo-specific clathrin-dependent endocytosis. Interaction of clathrin lattices formed on the plasma membrane with cargo receptors relies on adaptor complexes such as the adaptin protein-2 complex (AP2), 4 which recognizes specific linear endocytic motifs present within the cytoplasmic domains of the target receptor. For example, the 2 subunit of AP2 (also termed AP50) is known to recognize Tyr-based linear endocytic motifs and to enable the AP2-dependent tethering of cargo to clathrin molecules (reviewed in Refs. 1-4).Receptor ubiquitination has also emerged as a key endocytic signal for numerous eukaryotic cell surface receptors (reviewed in Refs. 5-8). For several receptors (such as epidermal growth factor receptor, growth hormone receptor, interferon ␣ receptor chain 1 (IFNAR1)), ubiquitination is stimulated by the ligand. This stimulation is mediated by the ligand-ind...

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Recent molecular genetics studies have suggested various trans-synaptic processes for pathophysiologic mechanisms of neuropsychiatric illnesses. Examination of pre- and post-synaptic scaffolds in the brains of patients would greatly aid further investigation, yet such an approach in human postmortem tissue has yet to be tested. We have examined three methods using density gradient based purification of synaptosomes followed by detergent extraction (Method 1) and the pH based differential extraction of synaptic membranes (Methods 2 and 3). All three methods separated fractions from human postmortem brains that were highly enriched in typical PSD proteins, almost to the exclusion of pre-synaptic proteins. We examined these fractions using electron microscopy (EM) and verified the integrity of the synaptic membrane and PSD fractions derived from human postmortem brain tissues. We analyzed protein composition of the PSD fractions using two dimensional liquid chromatography tandem mass spectrometry (2D LC-MS/MS) and observed known PSD proteins by mass spectrometry. Immunoprecipitation and immunoblot studies revealed that expected protein-protein interactions and certain posttranscriptional modulations were maintained in PSD fractions. Our results demonstrate that PSD fractions can be isolated from human postmortem brain tissues with a reasonable degree of integrity. This approach may foster novel postmortem brain research paradigms in which the stoichiometry and protein composition of specific microdomains are examined.