The b1 isoform of protocadherin-gamma (Pcdhγ) interacts with the microtubule-destabilizing protein SCG10

Expression of protocadherin-γC4 protein in the rat brain

It has been proposed that the combinatorial expression of γ-protocadherins (Pcdh-γs) and other clustered protocadherins (Pcdhs) provides a code of molecular identity and individuality to neurons, which plays a major role in the establishment of specific synaptic connectivity and formation of neuronal circuits. Particular attention has been directed to the Pcdh-γ family, for which experimental evidence derived from Pcdh-γ-deficient mice shows that they are involved in dendrite self-avoidance, synapse development, dendritic arborization, spine maturation, and prevention of apoptosis of some neurons. Moreover, a triple-mutant mouse deficient in the three C-type members of the Pcdh-γ family (Pcdh-γC3, Pcdh-γC4, and Pcdh-γC5) shows a phenotype similar to the mouse deficient in whole Pcdh-γ family, indicating that the latter is largely due to the absence of C-type Pcdh-γs. The role of each individual C-type Pcdh-γ is not known. We have developed a specific antibody to Pcdh-γC4 to reveal the expression of this protein in the rat brain. The results show that although Pcdh-γC4 is expressed at higher levels in the embryo and earlier postnatal weeks, it is also expressed in the adult rat brain. Pcdh-γC4 is expressed in both neurons and astrocytes. In the adult brain, the regional distribution of Pcdh-γC4 immunoreactivity is similar to that of Pcdh-γC4 mRNA, being highest in the olfactory bulb, dentate gyrus, and cerebellum. Pcdh-γC4 forms puncta that are frequently apposed to glutamatergic and GABAergic synapses. They are also frequently associated with neuron-astrocyte contacts. The results provide new insights into the cell recognition function of Pcdh-γC4 in neurons and astrocytes.

The Role of Protocadherin 19 (PCDH19) in Neurodevelopment and in the Pathophysiology of Early Infantile Epileptic Encephalopathy-9 (EIEE9)

PCDH19 is considered one of the most clinically relevant genes in epilepsy, second only to SCN1A. To date about 150 mutations have been identified as causative for PCDH19-female epilepsy (also known as early infantile epileptic encephalopathy-9, EIEE9), which is characterized by early onset epilepsy, intellectual disabilities, and behavioral disturbances. Although little is known about the physiological role of PCDH19 and the pathogenic mechanisms that lead to EIEE9, in this review, we will present latest researches focused on these aspects, underlining protein expression, its known functions and the mechanisms by which the protein acts, with particular interest in PCDH19 extracellular and intracellular roles in neurons.

Astroprincin (FAM171A1, C10orf38): A Regulator of Human Cell Shape and Invasive Growth

Our group originally found and cloned cDNA for a 98-kDa type 1 transmembrane glycoprotein of unknown function. Because of its abundant expression in astrocytes, it was called the protein astroprincin (APCN). Two thirds of the evolutionarily conserved protein is intracytoplasmic, whereas the extracellular domain carries two N-glycosidic side chains. APCN is physiologically expressed in placental trophoblasts, skeletal and hearth muscle, and kidney and pancreas. Overexpression of APCN (cDNA) in various cell lines induced sprouting of slender projections, whereas knockdown of APCN expression by siRNA caused disappearance of actin stress fibers. Immunohistochemical staining of human cancers for endogenous APCN showed elevated expression in invasive tumor cells compared with intratumoral cells. Human melanoma cells (SK-MEL-28) transfected with APCN cDNA acquired the ability of invasive growth in semisolid medium (Matrigel) not seen with control cells. A conserved carboxyterminal stretch of 21 amino acids was found to be essential for APCN to induce cell sprouting and invasive growth. Yeast two-hybrid screening revealed several interactive partners, of which ornithine decarboxylase antizyme-1, NEEP21 (NSG1), and ADAM10 were validated by coimmunoprecipitation. This is the first functional description of APCN. These data show that APCN regulates the dynamics of the actin cytoskeletal and, thereby, the cell shape and invasive growth potential of tumor cells.

Molecular and functional interaction between protocadherin-γC5 and GABAA receptors

We have found that the γ2 subunit of the GABA(A) receptor (γ2-GABA(A)R) specifically interacts with protocadherin-γC5 (Pcdh-γC5) in the rat brain. The interaction occurs between the large intracellular loop of the γ2-GABA(A)R and the cytoplasmic domain of Pcdh-γC5. In brain extracts, Pcdh-γC5 coimmunoprecipitates with GABA(A)Rs. In cotransfected HEK293 cells, Pcdh-γC5 promotes the transfer of γ2-GABA(A)R to the cell surface. We have previously shown that, in cultured hippocampal neurons, endogenous Pcdh-γC5 forms clusters, some of which associate with GABAergic synapses. Overexpression of Pcdh-γC5 in hippocampal neurons increases the density of γ2-GABA(A)R clusters but has no significant effect on the number of GABAergic contacts that these neurons receive, indicating that Pcdh-γC5 is not synaptogenic. Deletion of the cytoplasmic domain of Pcdh-γC5 enhanced its surface expression but decreased the association with both γ2-GABA(A)R clusters and presynaptic GABAergic contacts. Cultured hippocampal neurons from the Pcdh-γ triple C-type isoform knock-out (TCKO) mouse (Pcdhg(tcko/tcko)) showed plenty of GABAergic synaptic contacts, although their density was reduced compared with sister cultures from wild-type and heterozygous mice. Knocking down Pcdh-γC5 expression with shRNA decreased γ2-GABA(A)R cluster density and GABAergic innervation. The results indicate that, although Pcdh-γC5 is not essential for GABAergic synapse formation or GABA(A)R clustering, (1) Pcdh-γC5 regulates the surface expression of GABA(A)Rs via cis-cytoplasmic interaction with γ2-GABA(A)R, and (2) Pcdh-γC5 plays a role in the stabilization and maintenance of some GABAergic synapses.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Cell-type-specific localization of protocadherin β16 at AMPA and AMPA/Kainate receptor-containing synapses in the primate retina

Protocadherins (Pcdhs) are thought to be key features of cell-type-specific synapse formation. Here we analyzed the expression pattern of Pcdh subunit β16 (β16) in the primate retina by applying antibodies against β16, different subunits of ionotropic glutamate receptors (GluRs), and cell-type-specific markers as well as by coimmunoprecipitation and Western blots. Immunocytochemical localization was analyzed by confocal microscopy and preembedding electron microscopy. In the outer plexiform layer (OPL) H1, but not H2, horizontal cells expressed β16 as revealed by the strong reduction of β16 at short-wavelength-sensitive cones. β16 colocalized with the GluR subunits GluR2-4 at horizontal cell dendritic tips and with GluR2-4 and GluR6/7 at the desmosome-like junctions. At the latter, these AMPA and kainate receptor subunits were found to be clustered within single synaptic hot spots. Additionally, β16-labeled dendritic tips of OFF cone bipolar cells appeared in triad-associated positions at the cone pedicle base, pointing to β16 expression by OFF midget or DB3 bipolar cells. In the inner plexiform layer, β16 was localized also postsynaptically at most of the glutamatergic synapses. Overall, we provide evidence for a cell-type-specific localization of β16 together with GluRs at defined postsynaptic sites and a coexistence of AMPA and kainate receptors within single synaptic hot spots. This study supports the hypothesis that β16 plays an important role in the formation and/or stabilization of specific glutamatergic synapses, whereas our in vivo protein biochemical results argue against the existence of protein complexes formed by β16 and GluRs.

Proteomics analysis reveals overlapping functions of clustered protocadherins

The three tandem-arrayed protocadherin (Pcdh) gene clusters, namely Pcdh-alpha, Pcdh-beta, and Pcdh-gamma, play important roles in the development of the vertebrate central nervous system. To gain insight into the molecular action of PCDHs, we performed a systematic proteomics analysis of PCDH-gamma-associated protein complexes. We identified a list of 154 non-redundant proteins in the PCDH-gamma complexes. This list includes nearly 30 members of clustered Pcdh-alpha, -beta, and -gamma families as core components of the complexes and additionally over 120 putative PCDH-associated proteins. We validated a selected subset of PCDH-gamma-associated proteins using specific antibodies. Analysis of the identities of PCDH-associated proteins showed that the majority of them overlap with the proteomic profile of postsynaptic density preparations. Further analysis of membrane protein complexes revealed that several validated PCDH-gamma-associated proteins exhibit reduced levels in Pcdh-gamma-deficient brain tissues. Therefore, PCDH-gamma s are required for the integrity of the complexes. However, the size of the overall complexes and the abundance of many other proteins remained unchanged, raising a possibility that PCDH-alphas and PCDH-betas might compensate for PCDH-gamma function in complex formation. As a test of this idea, RNA interference knockdown of both PCDH-alphas and PCDH-gamma s showed that PCDHs have redundant functions in regulating neuronal survival in the chicken spinal cord. Taken together, our data provide evidence that clustered PCDHs coexist in large protein complexes and have overlapping functions during vertebrate neural development.

Regulatory roles of the cadherin superfamily

Charged with the task of providing a molecular link between adjacent cells, the cadherin superfamily consists of over 100 members and populates the genomes of organisms ranging from vertebrates to cniderians. This breadth hints at what decades of research has confirmed: that cadherin-based adhesion and signaling events regulate diverse cellular processes including cell-sorting, differentiation, cell survival, proliferation, cell polarity, and cytoskeletal organization.

Cytoplasmic domain of protocadherin-α enhances homophilic interactions and recognizes cytoskeletal elements

Cell adhesion molecules of the protocadherin-alpha (pcdh-alpha), -beta, and -gamma families have been proposed to be synaptic specifiers. Pcdh-alpha and -gamma family members localize in part to synapses, and deletion of all pcdh-gammas in mouse affects synaptogenesis. Little is known, however, about the binding specificities and intracellular signaling of protocadherins. Using heterologous expression of tagged constructs, immunostaining, and biotinylation of surface components followed by Western blots we demonstrate that pcdh-alphas undergo homophilic interactions that are significantly enhanced by the cytoplasmic domain. Pcdh-alphas cloned from chick ciliary ganglion have one of two cytoplasmic constant regions (A- and B-types). Screening a yeast two-hybrid library of ciliary ganglion cDNA with the A-type domain yielded a fragment of neurofilament M (NFM); screening with B-type domain yielded a fragment of the actin-bundling protein fascin. Cotransfection of HEK cells with the constructs indicated that the NFM and A-type fragments codistributed as did the fascin and B-type fragments, and the latter could be coimmunoprecipitated. Antibody-induced clustering of full-length pcdh-alphas on the surface of transfected HEK cells induced coclustering of the interacting NFM fragment. Native full-length NFM in tissue extracts bound specifically to the A-type domain on beads, while native full-length fascin in tissue extracts specifically coimmunoprecipitated with pcdh-alpha. Immunostaining neurons demonstrated codistribution of full-length pcdh-alpha with both NFM and actin filaments. These findings suggest cytoskeletal links for pcdh-alphas and identify candidate targets. They also demonstrate homophilic interactions for pcdh-alphas as described for classical cadherins.

Mammalian cadherins and protocadherins: about cell death, synapses and processing

Cadherins have been known for a long time to be key elements in many important biological processes. In particular, the role of classical cadherins in mediating adhesion has been examined in great detail. Over recent years, the accumulation of experimental tools and mice mutants has allowed more refined analysis of cadherin functions, and new aspects such as signaling and synapse dynamics have become the center of interest. In addition, the study of mice lacking the entire protocadherin-gamma cluster shed the first light on a possible novel function of members of this cadherin family in synapse formation and cell survival during development.