Gamma-protocadherins regulate the functional integrity of hypothalamic feeding circuitry in mice

Mouse models for the study of clustered protocadherins

Since their first description, the clustered protocadherins (cPcdhs) have sparked interest for their potential to generate diverse cell-surface recognition cues and their widespread expression in the nervous system. Through the use of mouse models, we have learned a great deal about the functions served by cPcdhs, and how their molecular diversity is regulated. cPcdhs are essential contributors to a host of processes during neural circuit formation, including neuronal survival, dendritic and axonal branching, self-avoidance and targeting, and synapse formation. Their expression is controlled by the interplay of epigenetic marks with proximal and distal elements involving high order DNA looping, regulating transcription factor binding. Here, we will review various mouse models targeting the cPcdh locus and how they have been instructive in uncovering the regulation and function of the cPcdhs.

Clustered Protocadherins Emerge as Novel Susceptibility Loci for Mental Disorders

The clustered protocadherins (cPcdhs) are a subfamily of type I single-pass transmembrane cell adhesion molecules predominantly expressed in the brain. Their stochastic and combinatorial expression patterns encode highly diverse neural identity codes which are central for neuronal self-avoidance and non-self discrimination in brain circuit formation. In this review, we first briefly outline mechanisms for generating a tremendous diversity of cPcdh cell-surface assemblies. We then summarize the biological functions of cPcdhs in a wide variety of neurodevelopmental processes, such as neuronal migration and survival, dendritic arborization and self-avoidance, axonal tiling and even spacing, and synaptogenesis. We focus on genetic, epigenetic, and 3D genomic dysregulations of cPcdhs that are associated with various neuropsychiatric and neurodevelopmental diseases. A deeper understanding of regulatory mechanisms and physiological functions of cPcdhs should provide significant insights into the pathogenesis of mental disorders and facilitate development of novel diagnostic and therapeutic strategies.

The γ-Protocadherins Regulate the Survival of GABAergic Interneurons during Developmental Cell Death

Inhibitory interneurons integrate into developing circuits in specific ratios and distributions. In the neocortex, inhibitory network formation occurs concurrently with the apoptotic elimination of a third of GABAergic interneurons. The cell surface molecules that select interneurons to survive or die are unknown. Here, we report that members of the clustered Protocadherins (cPCDHs) control GABAergic interneuron survival during developmentally-regulated cell death. Conditional deletion of the gene cluster encoding the γ-Protocadherins (Pcdhgs) from developing GABAergic neurons in mice of either sex causes a severe loss of inhibitory populations in multiple brain regions and results in neurologic deficits such as seizures. By focusing on the neocortex and the cerebellar cortex, we demonstrate that reductions of inhibitory interneurons result from elevated apoptosis during the critical postnatal period of programmed cell death (PCD). By contrast, cortical interneuron (cIN) populations are not affected by removal of Pcdhgs from pyramidal neurons or glial cells. Interneuron loss correlates with reduced AKT signaling in Pcdhg mutant interneurons, and is rescued by genetic blockade of the pro-apoptotic factor BAX. Together, these findings identify the PCDHGs as pro-survival transmembrane proteins that select inhibitory interneurons for survival and modulate the extent of PCD. We propose that the PCDHGs contribute to the formation of balanced inhibitory networks by controlling the size of GABAergic interneuron populations in the developing brain.SIGNIFICANCE STATEMENT A pivotal step for establishing appropriate excitatory-inhibitory ratios is adjustment of neuronal populations by cell death. In the mouse neocortex, a third of GABAergic interneurons are eliminated by BAX-dependent apoptosis during the first postnatal week. Interneuron cell death is modulated by neural activity and pro-survival pathways but the cell-surface molecules that select interneurons for survival or death are unknown. We demonstrate that members of the cadherin superfamily, the clustered γ-Protocadherins (PCDHGs), regulate the survival of inhibitory interneurons and the balance of cell death. Deletion of the Pcdhgs in mice causes inhibitory interneuron loss in the cortex and cerebellum, and leads to motor deficits and seizures. Our findings provide a molecular basis for controlling inhibitory interneuron population size during circuit formation.

CRISPR/Cas9 interrogation of the mouse Pcdhg gene cluster reveals a crucial isoform-specific role for Pcdhgc4

The mammalian Pcdhg gene cluster encodes a family of 22 cell adhesion molecules, the gamma-Protocadherins (γ-Pcdhs), critical for neuronal survival and neural circuit formation. The extent to which isoform diversity–a γ-Pcdh hallmark–is required for their functions remains unclear. We used a CRISPR/Cas9 approach to reduce isoform diversity, targeting each Pcdhg variable exon with pooled sgRNAs to generate an allelic series of 26 mouse lines with 1 to 21 isoforms disrupted via discrete indels at guide sites and/or larger deletions/rearrangements. Analysis of 5 mutant lines indicates that postnatal viability and neuronal survival do not require isoform diversity. Surprisingly, given reports that it might not independently engage in trans-interactions, we find that γC4, encoded by Pcdhgc4, is the only critical isoform. Because the human orthologue is the only PCDHG gene constrained in humans, our results indicate a conserved γC4 function that likely involves distinct molecular mechanisms.

The γ-Protocadherins (γ-Pcdhs) are a family of 22 molecules that serve many crucial functions during neural development. They can combine to form multimers at the cell surface, such that each combination specifically recognizes the same combination at the surface of other cells. In this way, 22 molecules can generate thousands of distinct recognition complexes. To test the extent to which molecular diversity is required for the γ-Pcdhs to serve their many functions, we used CRISPR/Cas9 gene editing to make a series of mouse mutants in which different combinations of the γ-Pcdhs are disrupted. We report 25 new mouse lines with between 1 and 21 intact members of the γ-Pcdh family. Further, we found that for the critical function of neuronal survival–and consequently the survival of the animal–the molecular diversity was not essential. Rather, a single member of the family called γC4 was the only one necessary or sufficient for this function; databases of human genome sequences suggest that this important role is conserved. These new strains will be invaluable for disentangling the role of molecular diversity in the γ-Pcdhs’ functions, and as we have already found, will help identify specific functions for specific γ-Pcdh family members.

Combinatorial Effects of Alpha- and Gamma-Protocadherins on Neuronal Survival and Dendritic Self-Avoidance

The clustered protocadherins (Pcdhs) comprise 58 cadherin-related proteins encoded by three tandemly arrayed gene clusters, Pcdh-α, Pcdh-β, and Pcdh-γ (Pcdha, Pcdhb, and Pcdhg, respectively). Pcdh isoforms from different clusters are combinatorially expressed in neurons. They form multimers that interact homophilically and mediate a variety of developmental processes, including neuronal survival, synaptic maintenance, axonal tiling, and dendritic self-avoidance. Most studies have analyzed clusters individually. Here, we assessed functional interactions between Pcdha and Pcdhg clusters. To circumvent neonatal lethality associated with deletion of Pcdhgs, we used Crispr-Cas9 genome editing in mice to combine a constitutive Pcdha mutant allele with a conditional Pcdhg allele. We analyzed roles of Pcdhas and Pcdhgs in the retina and cerebellum from mice (both sexes) lacking one or both clusters. In retina, Pcdhgs are essential for survival of inner retinal neurons and dendritic self-avoidance of starburst amacrine cells, whereas Pcdhas are dispensable for both processes. Deletion of both Pcdha and Pcdhg clusters led to far more dramatic defects in survival and self-avoidance than Pcdhg deletion alone. Comparisons of an allelic series of mutants support the conclusion that Pcdhas and Pcdhgs function together in a dose-dependent and cell-type-specific manner to provide a critical threshold of Pcdh activity. In the cerebellum, Pcdhas and Pcdhgs also cooperate to mediate self-avoidance of Purkinje cell dendrites, with modest but significant defects in either single mutant and dramatic defects in the double mutant. Together, our results demonstrate complex patterns of redundancy between Pcdh clusters and the importance of Pcdh cluster diversity in postnatal CNS development.SIGNIFICANCE STATEMENT The formation of neural circuits requires diversification and combinatorial actions of cell surface proteins. Prominent among them are the clustered protocadherins (Pcdhs), a family of ∼60 neuronal recognition molecules. Pcdhs are encoded by three closely linked gene clusters called Pcdh-α, Pcdh-β, and Pcdh-γ. The Pcdhs mediate a variety of developmental processes, including neuronal survival, synaptic maintenance, and spatial patterning of axons and dendrites. Most studies to date have been limited to single clusters. Here, we used genome editing to assess interactions between Pcdh-α and Pcdh-γ gene clusters. We examined two regions of the CNS, the retina and cerebellum and show that the 14 α-Pcdhs and 22 γ-Pcdhs act synergistically to mediate neuronal survival and dendrite patterning.

JMJD3 Is Crucial for the Female AVPV RIP-Cre Neuron-Controlled Kisspeptin-Estrogen Feedback Loop and Reproductive Function

The hypothalamic-pituitary-gonadal axis controls development, reproduction, and metabolism. Although most studies have focused on the hierarchy from the brain to the gonad, many questions remain unresolved concerning the feedback from the gonad to the central nervous system, especially regarding the potential epigenetic modifications in hypothalamic neurons. In the present report, we generated genetically modified mice lacking histone H3 lysine 27 (H3K27) demethylase Jumonji domain-containing 3 (JMJD3) in hypothalamic rat-insulin-promoter-expressing neurons (RIP-Cre neurons). The female mutant mice displayed late-onset obesity owing to reduced locomotor activity and decreased energy expenditure. JMJD3 deficiency in RIP-Cre neurons also results in delayed pubertal onset, an irregular estrous cycle, impaired fertility, and accelerated ovarian failure in female mice owing to the dysregulation of the hypothalamic-ovarian axis. We found that JMJD3 directly regulates Kiss1 gene expression by binding to the Kiss1 promoter and triggering H3K27me3 demethylation in the anteroventral periventricular (AVPV) nucleus. Further study confirmed that the aberrations arose from impaired kisspeptin signaling in the hypothalamic AVPV nucleus and subsequent estrogen deficiency. Estrogen replacement therapy can reverse obesity in mutant mice. Moreover, we demonstrated that Jmjd3 is an estrogen target gene in the hypothalamus. These results provide direct genetic and molecular evidence that JMJD3 is a key mediator for the kisspeptin-estrogen feedback loop.

Clustered Protocadherins Are Required for Building Functional Neural Circuits

Neuronal identity is generated by the cell-surface expression of clustered protocadherin (Pcdh) isoforms. In mice, 58 isoforms from three gene clusters, Pcdhα, Pcdhβ, and Pcdhγ, are differentially expressed in neurons. Since cis-heteromeric Pcdh oligomers on the cell surface interact homophilically with that in other neurons in trans, it has been thought that the Pcdh isoform repertoire determines the binding specificity of synapses. We previously described the cooperative functions of isoforms from all three Pcdh gene clusters in neuronal survival and synapse formation in the spinal cord. However, the neuronal loss and the following neonatal lethality prevented an analysis of the postnatal development and characteristics of the clustered-Pcdh-null (Δαβγ) neural circuits. Here, we used two methods, one to generate the chimeric mice that have transplanted Δαβγ neurons into mouse embryos, and the other to generate double mutant mice harboring null alleles of both the Pcdh gene and the proapoptotic gene Bax to prevent neuronal loss. First, our results showed that the surviving chimeric mice that had a high contribution of Δαβγ cells exhibited paralysis and died in the postnatal period. An analysis of neuronal survival in postnatally developing brain regions of chimeric mice clarified that many Δαβγ neurons in the forebrain were spared from apoptosis, unlike those in the reticular formation of the brainstem. Second, in Δαβγ/Bax null double mutants, the central pattern generator (CPG) for locomotion failed to create a left-right alternating pattern even in the absence of neurodegeneraton. Third, calcium imaging of cultured hippocampal neurons showed that the network activity of Δαβγ neurons tended to be more synchronized and lost the variability in the number of simultaneously active neurons observed in the control network. Lastly, a comparative analysis for trans-homophilic interactions of the exogenously introduced single Pcdh-γA3 isoforms between the control and the Δαβγ neurons suggested that the isoform-specific trans-homophilic interactions require a complete match of the expressed isoform repertoire at the contacting sites between interactive neurons. These results suggested that combinations of clustered Pcdh isoforms are required for building appropriate neural circuits.

Epigenomic and metabolic responses of hypothalamic POMC neurons to gestational nicotine exposure in adult offspring

Background

Epidemiological and animal studies have reported that prenatal nicotine exposure (PNE) leads to obesity and type-2 diabetes in offspring. Central leptin-melanocortin signaling via hypothalamic arcuate proopiomelanocortin (POMC) neurons is crucial for the regulation of energy and glucose balance. Furthermore, hypothalamic POMC neurons were recently found to mediate the anorectic effects of nicotine through activation of acetylcholine receptors. Here, we hypothesized that PNE impairs leptin-melanocortinergic regulation of energy balance in first-generation offspring by altering expression of long non-coding RNAs (lncRNAs) putatively regulating development and/or function of hypothalamic POMC neurons.

Methods

C57BL/6J females were exposed ad libitum to nicotine through drinking water and crossed with C57BL/6J males. Nicotine exposure was sustained during pregnancy and discontinued at parturition. Offspring development was monitored from birth into adulthood. From the age of 8 weeks, central leptin-melanocortin signaling, diabetes, and obesity susceptibility were assessed in male offspring fed a low-fat or high-fat diet for 16 weeks. Nicotine-exposed and non-exposed C57BL/6J females were also crossed with C57BL/6J males expressing the enhanced green fluorescent protein specifically in POMC neurons. Transgenic male offspring were subjected to laser microdissections and RNA sequencing (RNA-seq) analysis of POMC neurons for determination of nicotine-induced gene expression changes and regulatory lncRNA/protein-coding gene interactions.

Results

Contrary to expectation based on previous studies, PNE did not impair but rather enhanced leptin-melanocortinergic regulation of energy and glucose balance via POMC neurons in offspring. RNA-seq of laser microdissected POMC neurons revealed only one consistent change, upregulation of Gm15851, a lncRNA of yet unidentified function, in nicotine-exposed offspring. RNA-seq further suggested 82 cis-regulatory lncRNA/protein-coding gene interactions, 19 of which involved coding genes regulating neural development and/or function, and revealed expression of several previously unidentified metabolic, neuroendocrine, and neurodevelopment pathways in POMC neurons.

Conclusions

PNE does not result in obesity and type 2 diabetes but instead enhances leptin-melanocortinergic feeding and body weight regulation via POMC neurons in adult offspring. PNE leads to selective upregulation of Gm15851, a lncRNA, in adult offspring POMC neurons. POMC neurons express several lncRNAs and pathways possibly regulating POMC neuronal development and/or function.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0348-2) contains supplementary material, which is available to authorized users.

The γ-Protocadherin-C3 isoform inhibits canonical Wnt signalling by binding to and stabilizing Axin1 at the membrane

The 22 γ-Protocadherin (γ-Pcdh) adhesion molecules encoded by the Pcdhg gene cluster play critical roles in nervous system development, including regulation of dendrite arborisation, neuronal survival, and synaptogenesis. Recently, they have been implicated in suppression of tumour cell growth by inhibition of canonical Wnt signalling, though the mechanisms through which this occurs remain unknown. Here, we show differential regulation of Wnt signalling by individual γ-Pcdhs: The C3 isoform uniquely inhibits the pathway, whilst 13 other isoforms upregulate signalling. Focusing on the C3 isoform, we show that its unique variable cytoplasmic domain (VCD) is the critical one for Wnt pathway inhibition. γ-Pcdh-C3, but not other isoforms, physically interacts with Axin1, a key component of the canonical Wnt pathway. The C3 VCD competes with Dishevelled for binding to the DIX domain of Axin1, which stabilizes Axin1 at the membrane and leads to reduced phosphorylation of Wnt co-receptor Lrp6. Finally, we present evidence that Wnt pathway activity can be modulated up (by γ-Pcdh-A1) or down (by γ-Pcdh-C3) in the cerebral cortex in vivo, using conditional transgenic alleles. Together, these data delineate opposing roles for γ-Pcdh isoforms in regulating Wnt signalling and identify Axin1 as a novel protein interactor of the widely-expressed γ-Pcdh-C3 isoform.

Homophilic Protocadherin Cell-Cell Interactions Promote Dendrite Complexity

Growth of a properly complex dendrite arbor is a key step in neuronal differentiation and a prerequisite for neural circuit formation. Diverse cell surface molecules, such as the clustered protocadherins (Pcdhs), have long been proposed to regulate circuit formation through specific cell-cell interactions. Here, using transgenic and conditional knockout mice to manipulate γ-Pcdh repertoire in the cerebral cortex, we show that the complexity of a neuron's dendritic arbor is determined by homophilic interactions with other cells. Neurons expressing only one of the 22 γ-Pcdhs can exhibit either exuberant or minimal dendrite complexity, depending only on whether surrounding cells express the same isoform. Furthermore, loss of astrocytic γ-Pcdhs, or disruption of astrocyte-neuron homophilic matching, reduces dendrite complexity cell non-autonomously. Our data indicate that γ-Pcdhs act locally to promote dendrite arborization via homophilic matching, and they confirm that connectivity in vivo depends on molecular interactions between neurons and between neurons and astrocytes.

Structure and Sequence Analyses of Clustered Protocadherins Reveal Antiparallel Interactions that Mediate Homophilic Specificity

Clustered protocadherin (Pcdh) proteins mediate dendritic self-avoidance in neurons via specific homophilic interactions in their extracellular cadherin (EC) domains. We determined crystal structures of EC1-EC3, containing the homophilic specificity-determining region, of two mouse clustered Pcdh isoforms (PcdhγA1 and PcdhγC3) to investigate the nature of the homophilic interaction. Within the crystal lattices, we observe antiparallel interfaces consistent with a role in trans cell-cell contact. Antiparallel dimerization is supported by evolutionary correlations. Two interfaces, located primarily on EC2-EC3, involve distinctive clustered Pcdh structure and sequence motifs, lack predicted glycosylation sites, and contain residues highly conserved in orthologs but not paralogs, pointing toward their biological significance as homophilic interaction interfaces. These two interfaces are similar yet distinct, reflecting a possible difference in interaction architecture between clustered Pcdh subfamilies. These structures initiate a molecular understanding of clustered Pcdh assemblies that are required to produce functional neuronal networks.

Protocadherins and hypothalamic development: do they play an unappreciated role?

Normal brain development requires coordinated cell movements at precise times. It has long been established that cell-cell adhesion proteins of the cadherin superfamily are involved in the adhesion and sorting of cells during tissue morphogenesis. In the present review, we focus on protocadherins, which form the largest subfamily of the cadherin superfamily and mediate homophilic cell-cell adhesion in the developing brain. These molecules are highly expressed during neural development and the exact roles that they play are still emerging. Although, historically, protocadherins were considered to provide mechanical and chemical connections between adjacent cells, recent research suggests that they may also serve as molecular identity markers of neurones to help guide cell recognition and sorting, cell migration, outgrowth of neuronal processes, and synapse formation. This phenomenon of single cell diversity stems, in part, from the vast variation in protein structure, genomic organisation and molecular function of the protocadherins. Although expression profiles and genetic manipulations have provided evidence for the role of protocadherins in the developing brain, we have only begun to construct a complete understanding of protocadherin function. We examine our current understanding of how protocadherins influence brain development and discuss the possible roles for this large superfamily within the hypothalamus. We conclude that further research into these underappreciated but vitally important genes will shed insight into hypothalamic development and perhaps the underlying aetiology of neuroendocrine disorders.

Protocadherins branch out: Multiple roles in dendrite development

The proper formation of dendritic arbors is a critical step in neural circuit formation, and as such defects in arborization are associated with a variety of neurodevelopmental disorders. Among the best gene candidates are those encoding cell adhesion molecules, including members of the diverse cadherin superfamily characterized by distinctive, repeated adhesive domains in their extracellular regions. Protocadherins (Pcdhs) make up the largest group within this superfamily, encompassing over 80 genes, including the ∼60 genes of the α-, β-, and γ-Pcdh gene clusters and the non-clustered δ-Pcdh genes. An additional group includes the atypical cadherin genes encoding the giant Fat and Dachsous proteins and the 7-transmembrane cadherins. In this review we highlight the many roles that Pcdhs and atypical cadherins have been demonstrated to play in dendritogenesis, dendrite arborization, and dendritic spine regulation. Together, the published studies we discuss implicate these members of the cadherin superfamily as key regulators of dendrite development and function, and as potential therapeutic targets for future interventions in neurodevelopmental disorders.

Human IAPP-induced pancreatic β cell toxicity and its regulation by autophagy

Pancreatic islets in patients with type 2 diabetes mellitus (T2DM) are characterized by loss of β cells and formation of amyloid deposits derived from islet amyloid polypeptide (IAPP). Here we demonstrated that treatment of INS-1 cells with human IAPP (hIAPP) enhances cell death, inhibits cytoproliferation, and increases autophagosome formation. Furthermore, inhibition of autophagy increased the vulnerability of β cells to the cytotoxic effects of hIAPP. Based on these in vitro findings, we examined the pathogenic role of hIAPP and its relation to autophagy in hIAPP-knockin mice. In animals fed a standard diet, hIAPP had no toxic effects on β cell function; however, hIAPP-knockin mice did not exhibit a high-fat-diet-induced compensatory increase in β cell mass, which was due to limited β cell proliferation and enhanced β cell apoptosis. Importantly, expression of hIAPP in mice with a β cell-specific autophagy defect resulted in substantial deterioration of glucose tolerance and dispersed cytoplasmic expression of p62-associated toxic oligomers, which were otherwise sequestrated within p62-positive inclusions. Together, our results indicate that increased insulin resistance in combination with reduced autophagy may enhance the toxic potential of hIAPP and enhance β cell dysfunction and progression of T2DM.

High frequency of rare variants with a moderate-to-high predicted biological effect in protocadherin genes of extremely obese

Relatively rare variants with a moderate-to-high biological effect may contribute to the genetic predisposition of common disorders. To investigate this for obesity, we performed exome sequencing for 30 young (mean age: 29.7 years) extremely obese Caucasian subjects (mean body mass index: 51.1 kg/m(2); m/f = 11/29). Rare variants with a moderate-to-high predicted biological effect were assembled and subjected to functional clustering analysis. It showed that the 55 clustered protocadherin genes on chromosome 5q31 have a significantly (P = 0.002) higher frequency of rare variants than a set of 325 reference genes. Since the protocadherin genes are expressed in the hypothalamus, we tested another 167 genes related to the function of the hypothalamus, but in those genes, the frequency of rare variants was not different from that of the reference genes. To verify the relation of variation in the protocadherin genes with extreme obesity, we analyzed data from more than 4,000 European Americans present on the Exome Variant Server, representing a sample of the general population. The significant enrichment of rare variants in the protocadherin genes was only observed with the group of extremely obese individuals but not in the "general population", indicating an association between rare variants in the protocadherin cluster genes and extreme obesity.

γ-protocadherins control cortical dendrite arborization by regulating the activity of a FAK/PKC/MARCKS signaling pathway

The 22 γ-protocadherins (γ-Pcdhs) potentially specify thousands of distinct homophilic adhesive interactions in the brain. Neonatal lethality of mice lacking the Pcdh-γ gene cluster has, however, precluded analysis of many brain regions. Here, we use a conditional Pcdh-γ allele to restrict mutation to the cerebral cortex and find that, in contrast to other central nervous system phenotypes, loss of γ-Pcdhs in cortical neurons does not affect their survival or result in reduced synaptic density. Instead, mutant cortical neurons exhibit severely reduced dendritic arborization. Mutant cortices have aberrantly high levels of protein kinase C (PKC) activity and of phosphorylated (inactive) myristoylated alanine-rich C-kinase substrate, a PKC target that promotes arborization. Dendrite complexity can be rescued in Pcdh-γ mutant neurons by inhibiting PKC, its upstream activator phospholipase C, or the γ-Pcdh binding partner focal adhesion kinase. Our results reveal a distinct role for the γ-Pcdhs in cortical development and identify a signaling pathway through which they play this role.

A variable cytoplasmic domain segment is necessary for γ-protocadherin trafficking and tubulation in the endosome/lysosome pathway

The variable portion of the γ-protocadherin (Pcdh-γ) cytoplasmic domain (VCD) controls Pcdh-γ trafficking and organelle tubulation in the endolysosome system. Active VCD segments are conserved in Pcdh-γA and Pcdh-γB subfamilies.

Clustered protocadherins (Pcdhs) are arranged in gene clusters (α, β, and γ) with variable and constant exons. Variable exons encode cadherin and transmembrane domains and ∼90 cytoplasmic residues. The 14 Pcdh-αs and 22 Pcdh-γs are spliced to constant exons, which, for Pcdh-γs, encode ∼120 residues of an identical cytoplasmic moiety. Pcdh-γs participate in cell–cell interactions but are prominently intracellular in vivo, and mice with disrupted Pcdh-γ genes exhibit increased neuronal cell death, suggesting nonconventional roles. Most attention in terms of Pcdh-γ intracellular interactions has focused on the constant domain. We show that the variable cytoplasmic domain (VCD) is required for trafficking and organelle tubulation in the endolysosome system. Deletion of the constant cytoplasmic domain preserved the late endosomal/lysosomal trafficking and organelle tubulation observed for the intact molecule, whereas deletion or excision of the VCD or replacement of the Pcdh-γA3 cytoplasmic domain with that from Pcdh-α1 or N-cadherin dramatically altered trafficking. Truncations or internal deletions within the VCD defined a 26–amino acid segment required for trafficking and tubulation in the endolysosomal pathway. This active VCD segment contains residues that are conserved in Pcdh-γA and Pcdh-γB subfamilies. Thus the VCDs of Pcdh-γs mediate interactions critical for Pcdh-γ trafficking.

Chemoaffinity revisited: dscams, protocadherins, and neural circuit assembly

The chemoaffinity hypothesis for neural circuit assembly posits that axons and their targets bear matching molecular labels that endow neurons with unique identities and specify synapses between appropriate partners. Here, we focus on two intriguing candidates for fulfilling this role, Drosophila Dscams and vertebrate clustered protocadherins (Pcdhs). In each, a complex genomic locus encodes large numbers of neuronal transmembrane proteins with homophilic binding specificity, individual members of which are expressed combinatorially. Although these properties suggest that Dscams and Pcdhs could act as specificity molecules, they may do so in ways that challenge traditional views of how neural circuits assemble.

PDCD10/CCM3 acts downstream of {gamma}-protocadherins to regulate neuronal survival

γ-Protocadherins (PCDH-γ) regulate neuronal survival in the vertebrate central nervous system. The molecular mechanisms of how PCDH-γ mediates this function are still not understood. In this study, we show that through their common cytoplasmic domain, different PCDH-γ isoforms interact with an intracellular adaptor protein named PDCD10 (programmed cell death 10). PDCD10 is also known as CCM3, a causative genetic defect for cerebral cavernous malformations in humans. Using RNAi-mediated knockdown, we demonstrate that PDCD10 is required for the occurrence of apoptosis upon PCDH-γ depletion in developing chicken spinal neurons. Moreover, overexpression of PDCD10 is sufficient to induce neuronal apoptosis. Taken together, our data reveal a novel function for PDCD10/CCM3, acting as a critical regulator of neuronal survival during development.

γ-protocadherins are enriched and transported in specialized vesicles associated with the secretory pathway in neurons

Gamma protocadherins (Pcdh-γs) resemble classical cadherins and have the potential to engage in cell-cell interactions with homophilic properties. Emerging evidence suggests non-conventional roles for some protocadherins in neural development. We sought to determine whether Pcdh-γ trafficking in neurons is consistent with an intracellular role for these molecules. Here we show that, in contrast to the largely surface localization of classical cadherins, endogenous Pcdh-γs are primarily intracellular in rat neurons in vivo and are equally distributed within organelles of subsynaptic dendritic and axonal compartments. A strikingly higher proportion of Pcdh-γ-containing organelles in synaptic compartments was observed at postnatal day 16. To determine the origin of Pcdh-γ-trafficking organelles, we isolated organelles with Pcdh-γ antibody-coupled magnetic beads from brain organelle suspensions. Vesicles with high levels of COPII and endoplasmic reticulum-Golgi intermediate compartment (ERGIC) components were isolated with the Pcdh-γ antibody but not with the classical cadherin antibody. In cultured hippocampal neurons, Pcdh-γ immunolabeling partially overlapped with calnexin- and COPII-positive puncta in dendrites. Mobile Pcdh-γ-GFP profiles dynamically codistributed with a DsRed construct coupled to ER retention signals by live imaging. Pcdh-γ expression correlated with accumulations of tubulovesicular and ER-like organelles in dendrites. Our results are consistent with the possibility that Pcdh-γs could have a unique function within the secretory pathway in addition to their documented surface roles.

Combinatorial homophilic interaction between gamma-protocadherin multimers greatly expands the molecular diversity of cell adhesion

The specificity of interactions between neurons is believed to be mediated by diverse cell adhesion molecules, including members of the cadherin superfamily. Whereas mechanisms of classical cadherin adhesion have been studied extensively, much less is known about the related protocadherins (Pcdhs), which together make up the majority of the superfamily. Here we use quantitative cell aggregation assays and biochemical analyses to characterize cis and trans interactions among the 22-member gamma-Pcdh family, which have been shown to be critical for the control of synaptogenesis and neuronal survival. We show that gamma-Pcdh isoforms engage in trans interactions that are strictly homophilic. In contrast to classical cadherins, gamma-Pcdh interactions are only partially Ca(2+)-dependent, and their specificity is mediated through the second and third extracellular cadherin (EC) domains (EC2 and EC3), rather than through EC1. The gamma-Pcdhs also interact both covalently and noncovalently in the cis-orientation to form multimers both in vitro and in vivo. In contrast to gamma-Pcdh trans interactions, cis interactions are highly promiscuous, with no isoform specificity. We present data supporting a model in which gamma-Pcdh cis-tetramers represent the unit of their adhesive trans interactions. Unrestricted tetramerization in cis, coupled with strictly homophilic interactions in trans, predicts that the 22 gamma-Pcdhs could form 234,256 distinct adhesive interfaces. Given the demonstrated role of the gamma-Pcdhs in synaptogenesis, our data have important implications for the molecular control of neuronal specificity.

LC3-dependent intracellular membrane tubules induced by gamma-protocadherins A3 and B2: a role for intraluminal interactions

Clustered protocadherins (Pcdhs) are a family of cadherin-like molecules arranged in gene clusters (alpha, beta, and gamma). gamma-Protocadherins (Pcdh-gammas) are involved in cell-cell interactions, but their prominent intracellular distribution in vivo and different knock-out phenotypes suggest that these molecules participate in still unidentified processes. We found using correlative light and electron microscopy that Pcdh-gammaA3 and -gammaB2, but not -gammaC4, -alpha1, or N-cadherin, generate intracellular juxtanuclear membrane tubules when expressed in cells. These tubules recruit the autophagy marker MAP1A/1B LC3 (LC3) but are not associated with autophagic vesicles. Lipidation of LC3 is required for its coclustering with Pcdh-gamma tubules, suggesting the involvement of an autophagic-like molecular cascade. Expression of wild-type LC3 with Pcdh-gammaA3 increased tubule length whereas expression of lipidation-defective LC3 decreased tubule length relative to Pcdh-gammaA3 expressed alone. The tubules were found to emanate from lysosomes. Deletion of the luminal/extracellular domain of Pcdh-gammaA3 preserved lysosomal targeting but eliminated tubule formation whereas cytoplasmic deletion eliminated both lysosomal targeting and tubule formation. Deletion of the membrane-proximal three cadherin repeats resulted in tubes that were narrower than those produced by full-length molecules. These results suggest that Pcdh-gammaA and -gammaB families can influence the shape of intracellular membranes by mediating intraluminal interactions within organelles.