N-cadherin juxtamembrane domain modulates voltage-gated Ca2+ current via RhoA GTPase and Rho-associated kinase

δ-Catenin controls astrocyte morphogenesis via layer-specific astrocyte-neuron cadherin interactions

Astrocytes control the formation of specific synaptic circuits via cell adhesion and secreted molecules. Astrocyte synaptogenic functions are dependent on the establishment of their complex morphology. However, it is unknown if distinct neuronal cues differentially regulate astrocyte morphogenesis. δ-Catenin was previously thought to be a neuron-specific protein that regulates dendrite morphology. We found δ-catenin is also highly expressed by astrocytes and required both in astrocytes and neurons for astrocyte morphogenesis. δ-Catenin is hypothesized to mediate transcellular interactions through the cadherin family of cell adhesion proteins. We used structural modeling and biochemical analyses to reveal that δ-catenin interacts with the N-cadherin juxtamembrane domain to promote N-cadherin surface expression. An autism-linked δ-catenin point mutation impaired N-cadherin cell surface expression and reduced astrocyte complexity. In the developing mouse cortex, only lower-layer cortical neurons express N-cadherin. Remarkably, when we silenced astrocytic N-cadherin throughout the cortex, only lower-layer astrocyte morphology was disrupted. These findings show that δ-catenin controls astrocyte-neuron cadherin interactions that regulate layer-specific astrocyte morphogenesis.

Injury to Cone Synapses by Retinal Detachment: Differences from Rod Synapses and Protection by ROCK Inhibition

Attachment of a detached retina does not always restore vision to pre-injury levels, even if the attachment is anatomically successful. The problem is due in part to long-term damage to photoreceptor synapses. Previously, we reported on damage to rod synapses and synaptic protection using a Rho kinase (ROCK) inhibitor (AR13503) after retinal detachment (RD). This report documents the effects of detachment, reattachment, and protection by ROCK inhibition on cone synapses. Conventional confocal and stimulated emission depletion (STED) microscopy were used for morphological assessment and electroretinograms for functional analysis of an adult pig model of RD. RDs were examined 2 and 4 h after injury or two days later when spontaneous reattachment had occurred. Cone pedicles respond differently than rod spherules. They lose their synaptic ribbons, reduce invaginations, and change their shape. ROCK inhibition protects against these structural abnormalities whether the inhibitor is applied immediately or 2 h after the RD. Functional restoration of the photopic b-wave, indicating cone-bipolar neurotransmission, is also improved with ROCK inhibition. Successful protection of both rod and cone synapses with AR13503 suggests this drug will (1) be a useful adjunct to subretinal administration of gene or stem cell therapies and (2) improve recovery of the injured retina when treatment is delayed.

β-cells retain a pool of insulin-containing secretory vesicles regulated by adherens junctions and the cadherin binding protein p120 catenin

The β-cells of the islets of Langerhans are the sole producers of insulin in the human body. In response to rising glucose levels, insulin-containing vesicles inside β-cells fuse with the plasma membrane and release their cargo. However, the mechanisms regulating this process are only partly understood. Previous evidence indicated reductions in α-catenin elevate insulin release, while reductions in β-catenin decrease insulin release. α- and β-catenin contribute to cellular regulation in a range of ways but one is as members of the adherens junction complex and these contribute to the development of cell polarity in b-cells. Therefore, we investigated the effects of adherens junctions on insulin release. We show in INS-1E β-cells knockdown of either E- or N-cadherin had only small effects on insulin secretion, but simultaneous knockout of both cadherins resulted in a significant increase in basal insulin release to the same level as glucose-stimulated release. This double knockdown also significantly attenuated levels of p120 catenin, a cadherin binding partner involved in regulating cadherin turnover. Conversely, reducing p120 catenin levels with siRNA destabilized both E- and N-cadherin, and this was also associated with an increase in levels of insulin secreted from INS-1E cells. Furthermore, there were also changes in these cells consistent with higher insulin release, namely reductions in levels of F-actin and increased intracellular free Ca²⁺ levels in response to KCl-induced membrane depolarization. Taken together, these data provide evidence that adherens junctions play important roles in retaining a pool of insulin secretory vesicles within the cell and establish a role for p120 catenin in regulating this process.

Lysophosphatidic Acid and Several Neurotransmitters Converge on Rho-Kinase 2 Signaling to Manage Motoneuron Excitability

Intrinsic membrane excitability (IME) sets up neuronal responsiveness to synaptic drive. Several neurotransmitters and neuromodulators, acting through G-protein-coupled receptors (GPCRs), fine-tune motoneuron (MN) IME by modulating background K⁺ channels TASK1. However, intracellular partners linking GPCRs to TASK1 modulation are not yet well-known. We hypothesized that isoform 2 of rho-kinase (ROCK2), acting as downstream GPCRs, mediates adjustment of MN IME via TASK1. Electrophysiological recordings were performed in hypoglossal MNs (HMNs) obtained from adult and neonatal rats, neonatal knockout mice for TASK1 (task1^–/–) and TASK3 (task3^–/–, the another highly expressed TASK subunit in MNs), and primary cultures of embryonic spinal cord MNs (SMNs). Small-interfering RNA (siRNA) technology was also used to knockdown either ROCK1 or ROCK2. Furthermore, ROCK activity assays were performed to evaluate the ability of various physiological GPCR ligands to stimulate ROCK. Microiontophoretically applied H1152, a ROCK inhibitor, and siRNA-induced ROCK2 knockdown both depressed AMPAergic, inspiratory-related discharge activity of adult HMNs in vivo, which mainly express the ROCK2 isoform. In brainstem slices, intracellular constitutively active ROCK2 (aROCK2) led to H1152-sensitive HMN hyper-excitability. The aROCK2 inhibited pH-sensitive and TASK1-mediated currents in SMNs. Conclusively, aROCK2 increased IME in task3^–/–, but not in task1^–/– HMNs. MN IME was also augmented by the physiological neuromodulator lysophosphatidic acid (LPA) through a mechanism entailing G_αi/o-protein stimulation, ROCK2, but not ROCK1, activity and TASK1 inhibition. Finally, two neurotransmitters, TRH, and 5-HT, which are both known to increase MN IME by TASK1 inhibition, stimulated ROCK2, and depressed background resting currents via G_αq/ROCK2 signaling. These outcomes suggest that LPA and several neurotransmitters impact MN IME via G_αi/o/G_αq-protein-coupled receptors, downstream ROCK2 activation, and subsequent inhibition of TASK1 channels.

Generation and transplantation of reprogrammed human neurons in the brain using 3D microtopographic scaffolds

Cell replacement therapy with human pluripotent stem cell-derived neurons has the potential to ameliorate neurodegenerative dysfunction and central nervous system injuries, but reprogrammed neurons are dissociated and spatially disorganized during transplantation, rendering poor cell survival, functionality and engraftment in vivo. Here, we present the design of three-dimensional (3D) microtopographic scaffolds, using tunable electrospun microfibrous polymeric substrates that promote in situ stem cell neuronal reprogramming, neural network establishment and support neuronal engraftment into the brain. Scaffold-supported, reprogrammed neuronal networks were successfully grafted into organotypic hippocampal brain slices, showing an ∼3.5-fold improvement in neurite outgrowth and increased action potential firing relative to injected isolated cells. Transplantation of scaffold-supported neuronal networks into mouse brain striatum improved survival ∼38-fold at the injection site relative to injected isolated cells, and allowed delivery of multiple neuronal subtypes. Thus, 3D microscale biomaterials represent a promising platform for the transplantation of therapeutic human neurons with broad neuro-regenerative relevance.

Human pluripotent stem cell derived neurons have the potential for cell replacement therapy for brain injury and disease but problems on transplantation need to be overcome. Here, the authors use a microtopographic scaffold to graft neurons into both hippocampal organoids and the mouse brain striatum.

Regulation of neuronal high-voltage activated Ca(V)2 Ca(2+) channels by the small GTPase RhoA

High-Voltage-Activated (HVA) Ca(2+) channels are known regulators of synapse formation and transmission and play fundamental roles in neuronal pathophysiology. Small GTPases of Rho and RGK families, via their action on both cytoskeleton and Ca(2+) channels are key molecules for these processes. While the effects of RGK GTPases on neuronal HVA Ca(2+) channels have been widely studied, the effects of RhoA on the HVA channels remains however elusive. Using heterologous expression in Xenopus laevis oocytes, we show that RhoA activity reduces Ba(2+) currents through CaV2.1, CaV2.2 and CaV2.3 Ca(2+) channels independently of CaVβ subunit. This inhibition occurs independently of RGKs activity and without modification of biophysical properties and global level of expression of the channel subunit. Instead, we observed a marked decrease in the number of active channels at the plasma membrane. Pharmacological and expression studies suggest that channel expression at the plasma membrane is impaired via a ROCK-sensitive pathway. Expression of constitutively active RhoA in primary culture of spinal motoneurons also drastically reduced HVA Ca(2+) current amplitude. Altogether our data revealed that HVA Ca(2+) channels regulation by RhoA might govern synaptic transmission during development and potentially contribute to pathophysiological processes when axon regeneration and growth cone kinetics are impaired.

Cell adhesion and intracellular calcium signaling in neurons

Cell adhesion molecules (CAMs) play indispensable roles in the developing and mature brain by regulating neuronal migration and differentiation, neurite outgrowth, axonal fasciculation, synapse formation and synaptic plasticity. CAM-mediated changes in neuronal behavior depend on a number of intracellular signaling cascades including changes in various second messengers, among which CAM-dependent changes in intracellular Ca²⁺ levels play a prominent role. Ca²⁺ is an essential secondary intracellular signaling molecule that regulates fundamental cellular functions in various cell types, including neurons. We present a systematic review of the studies reporting changes in intracellular Ca²⁺ levels in response to activation of the immunoglobulin superfamily CAMs, cadherins and integrins in neurons. We also analyze current experimental evidence on the Ca²⁺ sources and channels involved in intracellular Ca²⁺ increases mediated by CAMs of these families, and systematically review the role of the voltage-dependent Ca²⁺ channels (VDCCs) in neurite outgrowth induced by activation of these CAMs. Molecular mechanisms linking CAMs to VDCCs and intracellular Ca²⁺ stores in neurons are discussed.

Development of cranial placodes: insights from studies in chick

This review focuses on how research, using chick as a model system, has contributed to our knowledge regarding the development of cranial placodes. This review highlights when and how molecular signaling events regulate early specification of placodal progenitor cells, as well as the development of individual placodes including morphological movements. In addition, we briefly describe various techniques used in chick that are important for studies in cell and developmental biology.

N-cadherin induces partial differentiation of cholinergic presynaptic terminals in heterologous cultures of brainstem neurons and CHO cells

N-cadherin is a calcium-sensitive cell adhesion molecule commonly expressed at synaptic junctions and contributes to formation and maturation of synaptic contacts. This study used heterologous cell cultures of brainstem cholinergic neurons and transfected Chinese Hamster Ovary (CHO) cells to examine whether N-cadherin is sufficient to induce differentiation of cholinergic presynaptic terminals. Brainstem nuclei isolated from transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of choline acetyltransferase (ChAT) transcriptional regulatory elements (ChAT^BACEGFP) were cultured as tissue explants for 5 days and cocultured with transfected CHO cells for an additional 2 days. Immunostaining for synaptic vesicle proteins SV2 and synapsin I revealed a ~3-fold increase in the area of SV2 immunolabeling over N-cadherin expressing CHO cells, and this effect was enhanced by coexpression of p120-catenin. Synapsin I immunolabeling per axon length was also increased on N-cadherin expressing CHO cells but required coexpression of p120-catenin. To determine whether N-cadherin induces formation of neurotransmitter release sites, whole-cell voltage-clamp recordings of CHO cells expressing α3 and β4 nicotinic acetylcholine receptor (nAChR) subunits in contact with cholinergic axons were used to monitor excitatory postsynaptic potentials (EPSPs) and miniature EPSPs (mEPSPs). EPSPs and mEPSPs were not detected in both, control and in N-cadherin expressing CHO cells in the absence or presence of tetrodotoxin (TTX). These results indicate that expression of N-cadherin in non-neuronal cells is sufficient to initiate differentiation of presynaptic cholinergic terminals by inducing accumulation of synaptic vesicles; however, development of readily detectable mature cholinergic release sites and/or clustering of postsynaptic nAChR may require expression of additional synaptogenic proteins.

Cell-to-cell contact dependence and junctional protein content are correlated with in vivo maturation of pancreatic beta cells

In this study, we investigated the cellular distribution of junctional proteins and the dependence on cell–cell contacts of pancreatic beta cells during animal development. Fetus and newborn rat islets, which display a relatively poor insulin secretory response to glucose, present an immature morphology and cytoarchitecture when compared with young and adult islets that are responsive to glucose. At the perinatal stage, beta cells display a low junctional content of neural cell adhesion molecule (N-CAM), α- and β-catenins, ZO-1, and F-actin, while a differential distribution of N-CAM and Pan-cadherin was seen in beta cells and nonbeta cells only from young and adult islets. In the absence of intercellular contacts, the glucose-stimulated insulin secretion was completely blocked in adult beta cells, but after reaggregation they partially reestablished the secretory response to glucose. By contrast, neonatal beta cells were poorly responsive to sugar, regardless of whether they were arranged as intact islets or as isolated cells. Interestingly, after 10 days of culturing, neonatal beta cells, known to display increased junctional protein content in vitro, became responsive to glucose and concomitantly dependent on cell–cell contacts. Therefore, our data suggest that the developmental acquisition of an adult-like insulin secretory pattern is paralleled by a dependence on direct cell–cell interactions.

Endogenous Rho-kinase signaling maintains synaptic strength by stabilizing the size of the readily releasable pool of synaptic vesicles

Rho-associated kinase (ROCK) regulates neural cell migration, proliferation and survival, dendritic spine morphology, and axon guidance and regeneration. There is, however, little information about whether ROCK modulates the electrical activity and information processing of neuronal circuits. At neonatal stage, ROCKα is expressed in hypoglossal motoneurons (HMNs) and in their afferent inputs, whereas ROCKβ is found in synaptic terminals on HMNs, but not in their somata. Inhibition of endogenous ROCK activity in neonatal rat brainstem slices failed to modulate intrinsic excitability of HMNs, but strongly attenuated the strength of their glutamatergic and GABAergic synaptic inputs. The mechanism acts presynaptically to reduce evoked neurotransmitter release. ROCK inhibition increased myosin light chain (MLC) phosphorylation, which is known to trigger actomyosin contraction, and reduced the number of synaptic vesicles docked to active zones in excitatory boutons. Functional and ultrastructural changes induced by ROCK inhibition were fully prevented/reverted by MLC kinase (MLCK) inhibition. Furthermore, ROCK inhibition drastically reduced the phosphorylated form of p21-associated kinase (PAK), which directly inhibits MLCK. We conclude that endogenous ROCK activity is necessary for the normal performance of motor output commands, because it maintains afferent synaptic strength, by stabilizing the size of the readily releasable pool of synaptic vesicles. The mechanism of action involves a tonic inhibition of MLCK, presumably through PAK phosphorylation. This mechanism might be present in adults since unilateral microinjection of ROCK or MLCK inhibitors into the hypoglossal nucleus reduced or increased, respectively, whole XIIth nerve activity.

The many faces of SMN: deciphering the function critical to spinal muscular atrophy pathogenesis

Spinal muscular atrophy (SMA) is the leading genetic cause of infant death, affecting 1 in 6000–10,000 live births. SMA is an autosomal recessive disorder characterized by the degeneration of α-motor neurons, and lower limb and proximal muscle weakness and wasting. SMA is the result of the deletion of or mutations in the survival motor neuron (SMN)1 gene. Currently, our understanding of how loss of the widely expressed SMN leads to the selective pathogenesis observed in SMA is limited. Here, we discuss the known nuclear and cytoplasmic functions of the SMN protein and how they relate to the SMA pathology reported in motor neurons, striated muscle and at neuromuscular junctions. While a vast amount of work in various cell and animal models has increased our knowledge of the many functions of the SMN protein, we have yet to come to a full understanding of which role(s) are central to SMA pathogenesis.

Synchronous and asynchronous transmitter release at nicotinic synapses are differentially regulated by postsynaptic PSD-95 proteins

The rate and timing of information transfer at neuronal synapses are critical for determining synaptic efficacy and higher network function. Both synchronous and asynchronous neurotransmitter release shape the pattern of synaptic influences on a neuron. The PSD-95 family of postsynaptic scaffolding proteins, in addition to organizing postsynaptic components at glutamate synapses, acts transcellularly to regulate synchronous glutamate release. Here we show that PSD-95 family members at nicotinic synapses on chick ciliary ganglion neurons in culture execute multiple functions to enhance transmission. Together, endogenous PSD-95 and SAP102 in the postsynaptic cell appear to regulate transcellularly the synchronous release of transmitter from presynaptic terminals onto the neuron while stabilizing postsynaptic nicotinic receptor clusters under the release sites. Endogenous SAP97, in contrast, has no effect on receptor clusters but acts transcellularly from the postsynaptic cell through N-cadherin to enhance asynchronous release. These separate and parallel regulatory pathways allow postsynaptic scaffold proteins to dictate the pattern of cholinergic input a neuron receives; they also require balancing of PSD-95 protein levels to avoid disruptive competition that can occur through common binding domains.

RGK GTPase-dependent CaV2.1 Ca2+ channel inhibition is independent of CaVbeta-subunit-induced current potentiation

RGK (Rad-Gem-Rem) GTPases have been described as potent negative regulators of the Ca(2+) influx via high-threshold voltage-activated Ca(2+) channels. Recent work, mostly performed on Ca(V)1.2 Ca(2+) channels, has highlighted the crucial role played by the channel auxiliary Ca(V)beta subunits and identified several GTPase and beta-subunit protein domains involved in this regulation. We now extend these conclusions by producing the first complete characterization of the effects of Gem, Rem, and Rem2 on the neuronal Ca(V)2.1 Ca(2+) channels expressed with Ca(V)beta(1) or Ca(V)beta(2) subunits. Current inhibition is limited to a decrease in amplitude with no modification in the voltage dependence or kinetics of the current. We demonstrate that this inhibition can occur for Ca(V)beta constructs with impaired capacity to induce current potentiation, but that it is lost for Ca(V)beta constructs deleted for their beta-interaction domain. The RGK C-terminal last approximately 80 amino acids are sufficient to allow potent current inhibition and in vivo beta-subunit/Gem interaction. Interestingly, although Gem and Gem carboxy-terminus induce a completely different pattern of beta-subunit cellular localization, they both potently inhibit Ca(V)2.1 channels. These data therefore set the status of neuronal Ca(V)2.1 Ca(2+) channel inhibition by RGK GTPases, emphasizing the role of short amino acid sequences of both proteins in beta-subunit binding and channel inhibition and revealing a new mechanism for channel inhibition.

N-cadherin modulates voltage activated calcium influx via RhoA, p120-catenin, and myosin-actin interaction

N-cadherin is a transmembrane adhesion receptor that contributes to neuronal development and synapse formation through homophilic interactions that provide structural-adhesive support to contacts between cell membranes. In addition, N-cadherin homotypic binding may initiate cell signaling that regulates neuronal physiology. In this study, we investigated signaling capabilities of N-cadherin that control voltage activated calcium influx. Using whole-cell voltage clamp recording of isolated inward calcium currents in freshly isolated chick ciliary ganglion neurons we show that the juxtamembrane region of N-cadherin cytoplasmic domain regulates high-threshold voltage activated calcium currents by interacting with p120-catenin and activating RhoA. This regulatory mechanism requires myosin interaction with actin. Furthermore, N-cadherin homophilic binding enhanced voltage activated calcium current amplitude in dissociated neurons that have already developed mature synaptic contacts in vivo. The increase in calcium current amplitude was not affected by brefeldin A suggesting that the effect is caused via direct channel modulation and not by increasing channel expression. In contrast, homotypic N-cadherin interaction failed to regulate calcium influx in freshly isolated immature neurons. However, RhoA inhibitors enhanced calcium current amplitude in these immature neurons, suggesting that the inhibitory effect of RhoA on calcium entry is regulated during neuronal development and synapse maturation. These results indicate that N-cadherin modulates voltage activated calcium entry by a mechanism that involves RhoA activity and its downstream effects on the cytoskeleton, and suggest that N-cadherin provides support for synaptic maturation and sustained synaptic activity by facilitating voltage activated calcium influx.

Targeting cerebrovascular Rho-kinase in stroke

BACKGROUND

Rho and Rho-associated kinase (ROCK) play pivotal roles in pathogenesis of vascular diseases including stroke. ROCK is expressed in all cell types relevant to stroke, and regulates a range of physiological processes.

OBJECTIVE

To provide an overview of ROCK as an experimental therapeutic target in cerebral ischemia, and the translational opportunities and obstacles in the prophylaxis and treatment of stroke.

METHODS

Relevant literature was reviewed.

RESULTS

ROCK activity is upregulated in chronic vascular risk factors such as diabetes, hyperlipidemia and hypertension, and more acutely by cerebral ischemia. ROCK activation is predicted to increase the risk of cerebral ischemia, and worsen the ischemic tissue outcome and functional recovery. Evidence suggests that ROCK inhibition is protective in models of cerebral ischemia. The benefit is mediated through multiple mechanisms.

CONCLUSION

ROCK is a promising therapeutic target in all stages of stroke.

RhoA GTPase and F-actin dynamically regulate the permeability of Cx43-made channels in rat cardiac myocytes

Gap junctions are clusters of transmembrane channels allowing a passive diffusion of ions and small molecules between adjacent cells. Connexin43, the main channel-forming protein expressed in ventricular myocytes, can associate with zonula occludens-1, a scaffolding protein linked to the actin cytoskeleton and to signal transduction molecules. The possible influence of Rho GTPases, major regulators of cellular junctions and of the actin cytoskeleton, in the modulation of gap junctional intercellular communication (GJIC) was examined. The activation of RhoA by cytoxic necrotizing factor 1 markedly enhanced GJIC, whereas its specific inhibition by the Clostridium botulinum C3 exoenzyme significantly reduced it. RhoA activity affects GJIC without major cellular redistribution of junctional plaques or changes in the Cx43 phosphorylation pattern. As these GTPases frequently act via the cortical cytoskeleton, the importance of F-actin in the modulation of GJIC was investigated by means of agents interfering with actin polymerization. Cytoskeleton stabilization by phalloidin slowed down the kinetics of channel rundown in the absence of ATP, whereas its disruption by cytochalasin D rapidly and markedly reduced GJIC despite ATP presence. Cytoskeleton stabilization by phalloidin markedly reduced the consequences of RhoA activation or inactivation. This mechanism appears to be the first described capable to both up- or down-regulate GJIC through RhoA activation or, conversely, inhibition. The inhibition of Rho downstream kinase effectors had no effect on GJIC. The present results provide further insight into the gating and regulation of junctional channels and identify a new downstream target for the small G-protein RhoA.

Ultrastructural evidence for pre- and postsynaptic localization of Cav1.2 L-type Ca2+ channels in the rat hippocampus

In the hippocampal formation, Ca(v)1.2 (L-type) voltage-gated Ca(2+) channels mediate Ca(2+) signals that can trigger long-term alterations in synaptic efficacy underlying learning and memory. Immunocytochemical studies indicate that Ca(v)1.2 channels are localized mainly in the soma and proximal dendrites of hippocampal pyramidal neurons, but electrophysiological data suggest a broader distribution of these channels. To define the subcellular substrates underlying Ca(v)1.2 Ca(2+) signals, we analyzed the localization of Ca(v)1.2 in the hippocampal formation by using antibodies against the pore-forming alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2). By light microscopy, alpha(1)1.2-like immunoreactivity (alpha(1)1.2-IR) was detected in pyramidal cell soma and dendritic fields of areas CA1-CA3 and in granule cell soma and fibers in the dentate gyrus. At the electron microscopic level, alpha(1)1.2-IR was localized in dendrites, but also in axons, axon terminals, and glial processes in all hippocampal subfields. Plasmalemmal immunogold particles representing alpha(1)1.2-IR were more significant for small- than large-caliber dendrites and were largely associated with extrasynaptic regions in dendritic spines and axon terminals. These findings provide the first detailed ultrastructural analysis of Ca(v)1.2 localization in the brain and support functionally diverse roles of these channels in the hippocampal formation.

E-cadherin and cell adhesion: a role in architecture and function in the pancreatic islet

BACKGROUND/AIMS

The efficient secretion of insulin from beta-cells requires extensive intra-islet communication. The cell surface adhesion protein epithelial (E)-cadherin (ECAD) establishes and maintains epithelial tissues such as the islets of Langerhans. In this study, the role of ECAD in regulating insulin secretion from pseudoislets was investigated.

METHODS

The effect of an immuno-neutralising ECAD on gross morphology, cytosolic calcium signalling, direct cell-to-cell communication and insulin secretion was assessed by fura-2 microfluorimetry, Lucifer Yellow dye injection and insulin ELISA in an insulin-secreting model system.

RESULTS

Antibody blockade of ECAD reduces glucose-evoked changes in [Ca(2+)](i) and insulin secretion. Neutralisation of ECAD causes a breakdown in the glucose-stimulated synchronicity of calcium oscillations between discrete regions within the pseudoislet, and the transfer of dye from an individual cell within a cell cluster is attenuated in the absence of ECAD ligation, demonstrating that gap junction communication is disrupted. The functional consequence of neutralising ECAD is a significant reduction in insulin secretion.

CONCLUSION

Cell adhesion via ECAD has distinct roles in the regulation of intercellular communication between beta-cells within islets, with potential repercussions for insulin secretion.

Pharmacology of cell adhesion molecules of the nervous system

Cell adhesion molecules (CAMs) play a pivotal role in the development and maintenance of the nervous system under normal conditions. They also are involved in numerous pathological processes such as inflammation, degenerative disorders, and cancer, making them attractive targets for drug development. The majority of CAMs are signal transducing receptors. CAM-induced intracellular signalling is triggered via homophilic (CAM-CAM) and heterophilic (CAM - other counter-receptors) interactions, which both can be targeted pharmacologically. We here describe the progress in the CAM pharmacology focusing on cadherins and CAMs of the immunoglobulin (Ig) superfamily, such as NCAM and L1. Structural basis of CAM-mediated cell adhesion and CAM-induced signalling are outlined. Different pharmacological approaches to study functions of CAMs are presented including the use of specific antibodies, recombinant proteins, and synthetic peptides. We also discuss how unravelling of the 3D structure of CAMs provides novel pharmacological tools for dissection of CAM-induced signalling pathways and offers therapeutic opportunities for a range of neurological disorders.

Modulation of human Kv1.5 channel kinetics by N-cadherin

Kv1.5 is expressed in multiple tissues including heart, brain, macrophages, as well as vascular, airway, and intestinal smooth muscle cells. Kv1.5 currents contribute to cardiac repolarization. In cardiac myocytes Kv1.5 colocalizes with N-cadherin. As Kv1.5 expression increases following establishment of cell-cell contacts and N-cadherin influences the activity of other ion channels, we explored whether N-cadherin participates in the regulation of Kv1.5 activity. To this end, we expressed Kv1.5 in Xenopus oocytes with or without additional expression of N-cadherin. Coexpression of N-cadherin was followed by a approximately 2- to 3-fold increase of Kv1.5 induced current. The effect of N-cadherin was not paralleled by significant alterations of Kv1.5 channel abundance within the oocyte cell membrane but resulted primarily from accelerated recovery from inactivation. In conclusion, N-cadherin modifies Kv1.5 channel activity and is thus a novel candidate signaling molecule participating in the regulation of a variety of functions including cardiac action potential and vascular tone.

N-cadherin signaling in synapse formation and neuronal physiology

Neural cadherin (N-cadherin) is an adhesion receptor that is localized in abundance at neuronto- neuron synapses. N-cadherin contains an extracellular domain that binds to other cadherins on juxtaposed cell membranes, a single-pass transmembrane region, and a cytoplasmic tail that interacts with various proteins, including catenins, kinases, phosphatases, and presenilin 1. N-cadherin contributes to the structural and functional organization of the synaptic complex by ensuring the adhesion between synaptic membranes and organizing the underlying actin cytoskeleton. Additionally, recent findings have shown that N-cadherin may participate in synaptic physiology by regulating calcium influx through voltage-activated calcium currents. The diverse activities of N-cadherin stem from its ability to operate as both an adhesion molecule that links cytoskeletons across cell membranes and a ligand-activated homophilic receptor capable of initiating intracellular signaling. An important mechanism of cadherin signaling is the regulation of small Rho guanosine triphosphatase activity that affects cytoskeleton dynamics and calcium influx. Because both the regulation of cadherin adhesive activity and cadherin-mediated signaling are affected by the binding of molecules to the intracellular domain, changes in the composition of the N-cadherin complex are central to the regulation of cadherin-mediated functions. This article focuses on the roles that N-cadherin might play at the level of the synapse through its effect on adhesion and signaling in the proximity of the synaptic junction.

Cadherins in development: cell adhesion, sorting, and tissue morphogenesis

Tissue morphogenesis during development is dependent on activities of the cadherin family of cell-cell adhesion proteins that includes classical cadherins, protocadherins, and atypical cadherins (Fat, Dachsous, and Flamingo). The extracellular domain of cadherins contains characteristic repeats that regulate homophilic and heterophilic interactions during adhesion and cell sorting. Although cadherins may have originated to facilitate mechanical cell-cell adhesion, they have evolved to function in many other aspects of morphogenesis. These additional roles rely on cadherin interactions with a wide range of binding partners that modify their expression and adhesion activity by local regulation of the actin cytoskeleton and diverse signaling pathways. Here we examine how different members of the cadherin family act in different developmental contexts, and discuss the mechanisms involved.

p120-ctn: A nexus for contextual signaling via Rho GTPases

p120 catenin (p120) is the prototypic member of a subfamily of armadillo repeat domain proteins involved in intercellular adhesion. Recent evidence indicates that p120 associates with classical cadherins and regulates their stability. Ectopic p120 expression results in a variety of morphological effects, and promotes cell migration. There is now strong evidence that p120 acts, at least in part, through regulation of Rho GTPases. The data suggest that p120 may act as a signaling nexus, conveying messages from the cellular micro- and macro-environment to the cell's interior. By regulating Rho GTPases in a context-dependent manner p120 can exert profound effects on cellular responses from synaptic plasticity to vesicle trafficking, as well as regulate the motile vs. sessile, and possibly the proliferative vs. quiescent phenotype of epithelial cells. Here, we review the new evidence on the relationship of p120 to Rho GTPases, and discuss potential roles for the p120-Rho connection in normal and malignant cells.

The molecular basis for calcium-dependent axon pathfinding

Ca(2+) signals have profound and varied effects on growth cone motility and guidance. Modulation of Ca(2+) influx and release from stores by guidance cues shapes Ca(2+) signals, which determine the activation of downstream targets. Although the precise molecular mechanisms that underlie distinct Ca(2+)-mediated effects on growth cone behaviours remain unclear, recent studies have identified important players in both the regulation and targets of Ca(2+) signals in growth cones.

Assembly of the N-cadherin complex during synapse formation involves uncoupling of p120-catenin and association with presenilin 1

N-cadherin is an adhesion receptor that participates in both interaction between immature pre- and postsynaptic neurons and in the stabilization and function of matured neuron-neuron synapses. To better understand how the N-cadherin complex contributes to synapse formation, we examined its distribution and composition during synapse formation in the chick ciliary neurons. It was found that at early phases of synaptogenesis, N-cadherin is distributed in small clusters on the cell surface and primarily associates with p120-catenin and beta-catenin. In contrast, as synaptic contacts matured, larger N-cadherin clusters were found localized adjacent to the active zone and associated with PS1 and gamma-catenin, while p120- and beta-catenin were dispersed among other cell regions, including axons. As it is known that PS1 binds gamma-catenin and that uncoupled p120-catenin can alter the cytoskeleton via its effect on Rho GTPases, these changes in the molecular composition of the N-cadherin complex (represented by the uncoupling of p120-catenin and association with PS1) may correspond to distinct functional states of the complex involved in synaptic maturation.