Connexin36 identified at morphologically mixed chemical/electrical synapses on trigeminal motoneurons and at primary afferent terminals on spinal cord neurons in adult mouse and rat

N-methyl-D-aspartate receptor activation is downstream coupled to pannexin 1 opening by Src kinase in dorsal horn neurons: an essential link for mechanical hyperalgesia in nerve-injured rats

ABSTRACT

A well-recognized molecular entity involved in pain-related neuroplasticity is the N-methyl-D-aspartate receptor (NMDAR), which is crucial for developing chronic pain. Likewise, the pannexin 1 (Panx1) channel has been described as necessary for initiating and maintaining neuropathic pain, driving nociceptive signals dependent on spinal NMDAR through several possible mechanisms. Through behavioral, pharmacological, and molecular approaches, our study in male rats has revealed several key findings: (1) neurons located in spinal cord laminae I and II express functional Panx1 channels in both neuropathic and sham rats. These channels can open (indicated by YOPRO-1 uptake) through the stimulation of NMDARs with intrathecal NMDA; (2) intrathecal NMDA leads to increased expression of pSrc and pPanx1 in dorsal horn neurons. This elevation exacerbates existing mechanical hyperalgesia in nerve-injured rats; (3) inhibition of Src with intrathecal PP2 or blockade of Panx1 with intrathecal 10 Panx effectively mitigates NMDA-induced effects and reduces the spontaneous mechanical hyperalgesia of nerve-injured rats. Notably, while 10 Panx successfully alleviates hyperalgesia, it does not alter pSrc expression; and (4) NMDA-stimulated YOPRO-1 uptake in neurons of laminae I-II of spinal cord slices were prevented by the NMDAR antagonist D-AP5, the Src inhibitor PP2 (but not PP3), as well as with the 10 Panx and carbenoxolone. Therefore, NMDAR activation in dorsal horn neurons triggers an NMDAR-Src-Panx1 signaling pathway, where Panx1 acts as an enhancing effector in neuropathic pain. This implies that disrupting the NMDAR-Panx1 communication (eg, through Src inhibitors and/or Panx1 blockers) may offer a valuable strategy for managing some forms of chronic pain.

The Double-Edged Effect of Connexins and Pannexins of Glial Cells in Central and Peripheral Nervous System After Nerve Injury

Glial cells play pivotal roles in homeostatic regulation and driving reactive pathologic changes after nerve injury. Connexins (Cxs) and pannexins (Panxs) have emerged as seminal proteins implicated in cell-cell communication, exerting a profound impact on the response processes of glial cell activation, proliferation, protein synthesis and secretion, as well as apoptosis following nerve injury. These influences are mediated through various forms, including protein monomers, hemichannel (HC), and gap junction (GJ), mainly by regulating intercellular or intracellular signaling pathways. Multiple Cx and Panx isoforms have been detected in central nervous system (CNS) or peripheral nervous system (PNS). Each isoform exhibits distinct cellular and subcellular localization, and the differential regulation and functional roles of various protein isoforms are observed post-injury. The quantitative and functional alterations of the same protein isoform in different studies remain inconsistent, attributable to factors such as the predominant mode of protein polymerization, the specific injury model, and the injury site. Similarly, the same protein isoforms have different roles in regulating the response processes after nerve injury, thus exerting a double-edged sword effect. This review describes the regulatory mechanisms and bidirectional effects of Cxs and Panxs. Additionally, it surveys the current status of research and application of drugs as therapeutic targets for neuropathic injuries. We summarize comprehensive and up-to-date information on these proteins in the glial cell response to nerve injury, providing new perspectives for future mechanistic exploration and development of targeted therapeutic approaches.

Patterns of connexin36 and eGFP reporter expression among motoneurons in spinal sexually dimorphic motor nuclei in mouse

BACKGROUND

Sexually dimorphic spinal motoneurons (MNs) in the dorsomedial nucleus (DMN) and dorsolateral nucleus (DLN) as well as those in the cremaster nucleus are involved in reproductive behaviours, and the cremaster nucleus additionally contributes to testicular thermoregulation. It has been reported that MNs in DMN and DLN are extensively linked by gap junctions forming electrical synapses composed of connexin36 (Cx36) and there is evidence that subpopulation of MNs in the cremaster nucleus are also electrically coupled by these synapses.

METHODOLOGY

We used immunofluorescence methods to detect enhanced green fluorescent protein (eGFP) reporter for Cx36 expression in these motor nuclei.

RESULTS

We document in male mice that about half the MNs in each of DMN and DLN express eGFP, while the remaining half do not. Further, we found that the eGFP+ vs. eGFP- subsets of MNs in each of these motor nuclei innervate different target muscles; eGFP+ MNs in DMN and DLN project to sexually dimorphic bulbocavernosus and ischiocavernosus muscles, while the eGFP- subsets project to sexually non-dimorphic anal and external urethral sphincter muscles. Similarly, eGFP+ vs. eGFP- cremaster MNs were found to project to anatomically distinct portions of the cremaster muscle. By immunofluorescence, nearly all motoneurons in both DMN and DLN displayed punctate labelling for Cx36, including at eGFP+/eGFP+, eGFP+/eGFP- and eGFP-/eGFP- cell appositions.

CONCLUSIONS

Most if not all motoneurons in DMN and DLN are electrically coupled, including sexually dimorphic and non-dimorphic motoneurons with each other, despite absence of eGFP reporter in the non-dimorphic populations in these nuclei that have selective projections to sexually non-dimorphic target muscles.

Association of connexin36 with adherens junctions at mixed synapses and distinguishing electrophysiological features of those at mossy fiber terminals in rat ventral hippocampus

BACKGROUND

Granule cells in the hippocampus project axons to hippocampal CA3 pyramidal cells where they form large mossy fiber terminals. We have reported that these terminals contain the gap junction protein connexin36 (Cx36) specifically in the stratum lucidum of rat ventral hippocampus, thus creating morphologically mixed synapses that have the potential for dual chemical/electrical transmission.

METHODOLOGY

Here, we used various approaches to characterize molecular and electrophysiological relationships between the Cx36-containing gap junctions at mossy fiber terminals and their postsynaptic elements and to examine molecular relationships at mixed synapses in the brainstem.

RESULTS

In rat and human ventral hippocampus, many of these terminals, identified by their selective expression of vesicular zinc transporter-3 (ZnT3), displayed multiple, immunofluorescent Cx36-puncta representing gap junctions, which were absent at mossy fiber terminals in the dorsal hippocampus. In rat, these were found in close proximity to the protein constituents of adherens junctions (i.e., N-cadherin and nectin-1) that are structural hallmarks of mossy fiber terminals, linking these terminals to the dendritic shafts of CA3 pyramidal cells, thus indicating the loci of gap junctions at these contacts. Cx36-puncta were also associated with adherens junctions at mixed synapses in the brainstem, supporting emerging views of the structural organization of the adherens junction-neuronal gap junction complex. Electrophysiologically induced long-term potentiation (LTP) of field responses evoked by mossy fiber stimulation was greater in the ventral than dorsal hippocampus.

CONCLUSIONS

The electrical component of transmission at mossy fiber terminals may contribute to enhanced LTP responses in the ventral hippocampus.

Connexins Control Glial Inflammation in Various Neurological Diseases

Connexins (Cxs) form gap junctions through homotypic/heterotypic oligomerization. Cxs are initially synthesized in the endoplasmic reticulum, then assembled as hexamers in the Golgi apparatus before being integrated into the cell membrane as hemichannels. These hemichannels remain closed until they combine to create gap junctions, directly connecting neighboring cells. Changes in the intracellular or extracellular environment are believed to trigger the opening of hemichannels, creating a passage between the inside and outside of the cell. The size of the channel pore depends on the Cx isoform and cellular context-specific effects such as posttranslational modifications. Hemichannels allow various bioactive molecules, under ~1 kDa, to move in and out of the host cell in the direction of the electrochemical gradient. In this review, we explore the fundamental roles of Cxs and their clinical implications in various neurological dysfunctions, including hereditary diseases, ischemic brain disorders, degenerative conditions, demyelinating disorders, and psychiatric illnesses. The influence of Cxs on the pathomechanisms of different neurological disorders varies depending on the circumstances. Hemichannels are hypothesized to contribute to proinflammatory effects by releasing ATP, adenosine, glutamate, and other bioactive molecules, leading to neuroglial inflammation. Modulating Cxs' hemichannels has emerged as a promising therapeutic approach.

Characteristics of Electrical Synapses, C-terminals and Small-conductance Ca2+ activated Potassium Channels in the Sexually Dimorphic Cremaster Motor Nucleus in Spinal Cord of Mouse and Rat

Sexually dimorphic motoneurons (MNs) located in lower lumbar spinal cord are involved in mating and reproductive behaviours and are known to be coupled by electrical synapses. The cremaster motor nucleus in upper lumbar spinal cord has also been suggested to support physiological processes associated with sexual behaviours in addition to its thermoregulatory and protective role in maintaining testes integrity. Using immunofluorescence approaches, we investigated whether cremaster MNs also exhibit features reflecting their potential for electrical synaptic communication and examined some of their other synaptic characteristics. Both mice and rats displayed punctate immunolabelling of Cx36 associated with cremaster MNs, indicative of gap junction formation. Transgenic mice with enhanced green fluorescent protein (eGFP) reporter for connexin36 expression showed that subpopulations of cremaster MNs in both male and female mice express eGFP, with greater proportions of those in male mice. The eGFP+ MNs within the cremaster nucleus vs. eGFP- MNs inside and outside this nucleus displayed a 5-fold greater density of serotonergic innervation and exhibited a paucity of innervation by C-terminals arising from cholinergic V0c interneurons. All MNs within the cremaster motor nucleus displayed prominent patches of immunolabelling for SK3 (K+) channels around their periphery, suggestive of their identity as slow MNs, many though not all of which were in apposition to C-terminals. The results provide evidence for electrical coupling of a large proportion of cremaster MNs and suggest the existence of two populations of these MNs with possibly differential innervation of their peripheral target muscles serving different functions.

Interrelationships between spinal sympathetic preganglionic neurons, autonomic systems and electrical synapses formed by connexin36-containing gap junctions

Spinal sympathetic preganglionic neurons (SPNs) are among the many neuronal populations in the mammalian central nervous system (CNS) where there is evidence for electrical coupling between cell pairs linked by gap junctions composed of connexin36 (Cx36). Understanding the organization of this coupling in relation to autonomic functions of spinal sympathetic systems requires knowledge of how these junctions are deployed among SPNs. Here, we document the distribution of immunofluorescence detection of Cx36 among SPNs identified by immunolabelling of their various markers, including choline acetyltransferase, nitric oxide and peripherin in adult and developing mouse and rat. In adult animals, labelling of Cx36 was exclusively punctate and dense concentrations of Cx36-puncta were distributed along the entire length of the spinal thoracic intermediolateral cell column (IML). These puncta were also seen in association with SPN dendritic processes in the lateral funiculus, the intercalated and central autonomic areas and those within and extending medially from the IML. All labelling for Cx36 was absent in spinal cords of Cx36 knockout mice. High densities of Cx36-puncta were already evident among clusters of SPNs in the IML of mouse and rat at postnatal days 10-12. In Cx36^BAC::eGFP mice, eGFP reporter was absent in SPNs, thus representing false negative detection, but was localized to some glutamatergic and GABAergic synaptic terminals. Some eGFP⁺ terminals were found contacting SPN dendrites. These results indicate widespread Cx36 expression in SPNs, further supporting evidence of electrical coupling between these cells, and suggest that SPNs are innervated by neurons that themselves may be electrically coupled.

Photobiomodulation augments the effects of mitochondrial transplantation in the treatment of spinal cord injury in rats by facilitating mitochondrial transfer to neurons via Connexin 36

Mitochondrial transplantation is a promising treatment for spinal cord injury (SCI), but it has the disadvantage of low efficiency of mitochondrial transfer to targeted cells. Here, we demonstrated that Photobiomodulation (PBM) could promote the transfer process, thus augmenting the therapeutic effect of mitochondrial transplantation. In vivo experiments, motor function recovery, tissue repair, and neuronal apoptosis were evaluated in different treatment groups. Under the premise of mitochondrial transplantation, the expression of Connex36 (Cx36), the trend of mitochondria transferred to neurons, and its downstream effects, such as ATP production and antioxidant capacity, were evaluated after PBM intervention. In in vitro experiments, dorsal root ganglia (DRG) were cotreated with PBM and 18β-GA (a Cx36 inhibitor). In vivo experiments showed that PBM combined with mitochondrial transplantation could increase ATP production and reduce oxidative stress and neuronal apoptosis levels, thereby promoting tissue repair and motor function recovery. In vitro experiments further verified that Cx36 mediated the transfer of mitochondria into neurons. PBM could facilitate this progress via Cx36 both in vivo and in vitro. The present study reports a potential method of using PBM to facilitate the transfer of mitochondria to neurons for the treatment of SCI.

Detecting Early Cognitive Decline in Alzheimer's Disease with Brain Synaptic Structural and Functional Evaluation

Early cognitive decline in patients with Alzheimer's (AD) is associated with quantifiable structural and functional connectivity changes in the brain. AD dysregulation of Aβ and tau metabolism progressively disrupt normal synaptic function, leading to loss of synapses, decreased hippocampal synaptic density and early hippocampal atrophy. Advances in brain imaging techniques in living patients have enabled the transition from clinical signs and symptoms-based AD diagnosis to biomarkers-based diagnosis, with functional brain imaging techniques, quantitative EEG, and body fluids sampling. The hippocampus has a central role in semantic and episodic memory processing. This cognitive function is critically dependent on normal intrahippocampal connections and normal hippocampal functional connectivity with many cortical regions, including the perirhinal and the entorhinal cortex, parahippocampal cortex, association regions in the temporal and parietal lobes, and prefrontal cortex. Therefore, decreased hippocampal synaptic density is reflected in the altered functional connectivity of intrinsic brain networks (aka large-scale networks), including the parietal memory, default mode, and salience networks. This narrative review discusses recent critical issues related to detecting AD-associated early cognitive decline with brain synaptic structural and functional markers in high-risk or neuropsychologically diagnosed patients with subjective cognitive impairment or mild cognitive impairment.

On the Organization of Connexin36 Expression in Electrically Coupled Cholinergic V0c Neurons (Partition Cells) in the Spinal Cord and Their C-terminal Innervation of Motoneurons

Large cholinergic neurons (V0c neurons; aka, partition cells) in the spinal cord project profusely to motoneurons on which they form C-terminal contacts distinguished by their specialized postsynaptic subsurface cisterns (SSCs). The V0c neurons are known to be rhythmically active during locomotion and release of acetylcholine (ACh) from their terminals is known to modulate the excitability of motoneurons in what appears to be a task-dependent manner. Here, we present evidence that a subpopulation of V0c neurons express the gap junction forming protein connexin36 (Cx36), indicating that they are coupled by electrical synapses. Based on immunofluorescence imaging and the use of Cx36BAC-enhanced green fluorescent protein (eGFP) mice in which C-terminals immunolabelled for their marker vesicular acetylcholine transporter (vAChT) are also labelled for eGFP, we found a heterogeneous distribution of eGFP+ C-terminals on motoneurons at cervical, thoracic and lumber spinal levels. The density of C-terminals on motoneurons varied as did the proportion of those that were eGFP+ vs. eGFP-. We present evidence that fast vs. slow motoneurons have a greater abundance of these terminals and fast motoneurons also have the highest density that were eGFP+. Thus, our results indicate that a subpopulation of V0c neurons projects preferentially to fast motoneurons, suggesting that the capacity for synchronous activity conferred by electrical synapses among networks of coupled V0c neurons enhances their dynamic capabilities for synchronous regulation of motoneuron excitability during high muscle force generation. The eGFP+ vs. eGFP- V0c neurons were more richly innervated by serotonergic terminals, suggesting their greater propensity for regulation by descending serotonergic systems.

The Roles of Calmodulin and CaMKII in Cx36 Plasticity

Anatomical and electrophysiological evidence that gap junctions and electrical coupling occur between neurons was initially confined to invertebrates and nonmammals and was thought to be a primitive form of synaptic transmission. More recent studies revealed that electrical communication is common in the mammalian central nervous system (CNS), often coexisting with chemical synaptic transmission. The subsequent progress indicated that electrical synapses formed by the gap junction protein connexin-36 (Cx36) and its paralogs in nonmammals constitute vital elements in mammalian and fish synaptic circuitry. They govern the collective activity of ensembles of coupled neurons, and Cx36 gap junctions endow them with enormous adaptive plasticity, like that seen at chemical synapses. Moreover, they orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie the fundamental integrative processes, such as memory and learning. Here, we review the available mechanistic evidence and models that argue for the essential roles of calcium, calmodulin, and the Ca2+/calmodulin-dependent protein kinase II in integrating calcium signals to modulate the strength of electrical synapses through interactions with the gap junction protein Cx36.

Connexin36 localization along axon initial segments in the mammalian CNS

Electrical synapses formed by gap junctions occur at a variety of neuronal subcellular sites in the mammalian central nervous system (CNS), including at somatic, dendritic and axon terminal compartments. Numerous electrophysiological studies using mice and rats, as well as computer modelling approaches, have predicted the additional occurrence of electrical synapses between axons near their emergence from neuronal somata. Here, we used immunofluorescence methods to search for localization of the neuronal gap junction-forming protein connexin36 (Cx36) along axon initial segments (AISs) labelled for the AIS marker ankyrinG. Immunofluorescent Cx36-puncta were found to be associated with AISs in several CNS regions of mice, including the spinal cord, inferior olive and cerebral cortex. Localization of Cx36-puncta at AISs was confirmed by confocal single scan and 3D imaging, immunofluorescence intensity profiling and high resolution structured illumination microscopy (SIM). AISs measuring up to 30 µm in length displayed typically a single Cx36-punctum and the incidence of these long AISs displaying Cx36-puncta ranged from 3% to 7% in the inferior olive and in various layers of the cerebral cortex. In the inferior olive, the gap junction associated protein zonula occludens-1 (ZO-1) was found to be co-localized with Cx36-puncta on AISs, indicating that these puncta have some of the molecular constituents of gap junctions. Our results add to the neuronal subcellular locations at which Cx36 is deployed, and raise possibilities for its involvement in novel functions in the AIS compartment.

Altered Expression of Glial Gap Junction Proteins Cx43, Cx30, and Cx47 in the 5XFAD Model of Alzheimer's Disease

Glial gap junction proteins, called connexins (Cxs), form gap junctions in the central nervous system (CNS) to allow the bidirectional cytosolic exchange of molecules between adjacent cells. Their involvement in inheritable diseases and the use of experimental animal models that closely mimic such diseases revealed the critical role of glial GJs in myelination and homeostasis. Cxs are also implicated in acquired demyelinating disorders, such as Multiple Sclerosis (MS) and Alzheimer's disease (AD). Animal and human studies have revealed a role of the astrocytic Cx43 in the progression of AD but the role of Cx47, which is the main partner of Cx43 in the astrocyte-oligodendrocyte GJs is still unknown. The aim of this study was to investigate the astrocytic connexins, Cx43 and Cx30 in relation to oligodendrocytic Cx47 in the cortex and thalamus of the 5XFAD mouse model of AD. The model was characterized by increased Aβ deposition, gliosis, neuronal loss, and memory impairment. Compared to wild-type mice, Cx43 and Cx30 showed increased immunoreactivity in older 5XFAD mice, reflecting astrogliosis, while Cx47 immunoreactivity was reduced. Moreover, Cx47 GJ plaques showed reduced colocalization with Cx43 plaques. Oligodendrocyte precursor cells (OPCs) and mature oligodendrocyte populations were also depleted, and myelin deficits were observed. Our findings indicate reduced astrocyte-oligodendrocyte gap junction connectivity and possibly a shift in Cx43 expression toward astrocyte-astrocyte gap junctions and/or hemichannels, that could impair oligodendrocyte homeostasis and myelination. However, other factors, such as Aβ toxicity, could directly affect oligodendrocyte survival in AD. Our study provides evidence that Cxs might have implications in the progression of AD, although the role of oligodendrocyte Cxs in AD requires further investigation.

Fictive Scratching Patterns in Brain Cortex-Ablated, Midcollicular Decerebrate, and Spinal Cats

Background: The spinal cord’s central pattern generators (CPGs) have been explained by the symmetrical half-center hypothesis, the bursts generator, computational models, and more recently by connectome circuits. Asymmetrical models, at odds with the half-center paradigm, are composed of extensor and flexor CPG modules. Other models include not only flexor and extensor motoneurons but also motoneuron pools controlling biarticular muscles. It is unknown whether a preferred model can explain some particularities that fictive scratching (FS) in the cat presents. The first aim of this study was to investigate FS patterns considering the aiming and the rhythmic periods, and second, to examine the effects of serotonin (5HT) on and segmental inputs to FS.

Methods: The experiments were carried out first in brain cortex-ablated cats (BCAC), then spinalized (SC), and for the midcollicular (MCC) preparation. Subjects were immobilized and the peripheral nerves were used to elicit the Monosynaptic reflex (MR), to modify the scratching patterns and for electroneurogram recordings.

Results: In BCAC, FS was produced by pinna stimulation and, in some cases, by serotonin. The scratching aiming phase (AP) initiates with the activation of either flexor or extensor motoneurons. Serotonin application during the AP produced simultaneous extensor and flexor bursts. Furthermore, WAY 100635 (5HT1A antagonist) produced a brief burst in the tibialis anterior (TA) nerve, followed by a reduction in its electroneurogram (ENG), while the soleus ENG remained silent. In SC, rhythmic phase (RP) activity was recorded in the soleus motoneurons. Serotonin or WAY produced FS bouts. The electrical stimulation of Ia afferent fibers produced heteronymous MRes waxing and waning during the scratch cycle. In MCC, FS began with flexor activity. Electrical stimulation of either deep peroneus (DP) or superficial peroneus (SP) nerves increased the duration of the TA electroneurogram. Medial gastrocnemius (MG) stretching or MG nerve electrical stimulation produced a reduction in the TA electroneurogram and an initial MG extensor burst. MRes waxed and waned during the scratch cycle.

Conclusion: Descending pathways and segmental afferent fibers, as well as 5-HT and WAY, can change the FS pattern. To our understanding, the half-center hypothesis is the most suitable for explaining the AP in MCC.

Could electrical coupling contribute to the formation of cell assemblies?

Cell assemblies and central pattern generators (CPGs) are related types of neuronal networks: both consist of interacting groups of neurons whose collective activities lead to defined functional outputs. In the case of a cell assembly, the functional output may be interpreted as a representation of something in the world, external or internal; for a CPG, the output ‘drives’ an observable (i.e. motor) behavior. Electrical coupling, via gap junctions, is critical for the development of CPGs, as well as for their actual operation in the adult animal. Electrical coupling is also known to be important in the development of hippocampal and neocortical principal cell networks. We here argue that electrical coupling – in addition to chemical synapses – may therefore contribute to the formation of at least some cell assemblies in adult animals.

On the occurrence and enigmatic functions of mixed (chemical plus electrical) synapses in the mammalian CNS

Electrical synapses with diverse configurations and functions occur at a variety of interneuronal appositions, thereby significantly expanding the physiological complexity of neuronal circuitry over that provided solely by chemical synapses. Gap junctions between apposed dendritic and somatic plasma membranes form "purely electrical" synapses that allow for electrical communication between coupled neurons. In addition, gap junctions at axon terminals synapsing on dendrites and somata allow for "mixed" (dual chemical+electrical) synaptic transmission. "Dual transmission" was first documented in the autonomic nervous system of birds, followed by its detection in the central nervous systems of fish, amphibia, and reptiles. Subsequently, mixed synapses have been detected in several locations in the mammalian CNS, where their properties and functional roles remain undetermined. Here, we review available evidence for the presence, complex structural composition, and emerging functional properties of mixed synapses in the mammalian CNS.

Dye-coupling between neonatal spinal motoneurons and interneurons revealed by prolonged back-filling of a ventral root with a low molecular weight tracer in the mouse

We investigated dye-coupling between motoneurons in the L6 segment of the neonatal mouse spinal cord that contains limb-innervating motoneurons and sexually dimorphic motor nuclei. Using an isolated spinal cord preparation, we back-filled the cut, L6 ventral root with the small molecule Neurobiotin, or the much larger dextran-conjugated fluorophores for 16–24 hours. Motoneurons and parasympathetic preganglionic neurons were filled with both markers, but dye-coupling was only seen with Neurobiotin fills. Following a neurobiotin fill, fluorescence was observed in contralateral motoneurons, in motoneurons innervating adjacent ventral roots, and in ChAT-negative, putative interneurons outside of the motoneuron pools in addition to the directly back-labeled L6 motoneurons. It is known that the gap junction protein connexin-36 is widely expressed in the sexually dimorphic motoneurons of the L6 segment, suggesting that the dye-coupling is mediated by gap junctions. The technique has revealed previously unknown connections of motoneurons in the neonatal mouse cord that are likely to play important roles in the development and function of spinal circuits.

Downregulation of Neuronal and Dendritic Connexin36-Made Electrical Synapses Without Glutamatergic Axon Terminals in Spinal Anterior Horn Cells From the Early Stage of Amyotrophic Lateral Sclerosis

Connexin36 (Cx36) forms gap junctions between neurons, which are called electrical synapses, enabling adjacent neurons to communicate directly. The participation of chemical synapses in neurodegeneration in amyotrophic lateral sclerosis (ALS) has long been indicated, but it remains unclear whether electrical synapses are involved in the pathogenesis of ALS. We performed extensive immunopathological analyses using mutant superoxide dismutase 1 (SOD1^G93A) transgenic mice and their littermates to investigate whether Cx36-made electrical synapses are affected in motor neuron diseases. We found that in the lamina IX of the lumbar spinal cord from wild type mice, about half of the Cx36 puncta existed independently of chemical synapse markers, while the rest coexisted with chemical synapse markers, such as vesicular glutamate transporter 1 (VGLUT1), which is a glutamatergic axon terminal marker, and/or glutamate decarboxylase 65 (GAD65), which is a GABAergic axon terminal marker. Cx36 single or Cx36/GAD65 double positive puncta, but not VGLUT1-containing puncta, were preferentially decreased on neuronal and dendritic surfaces of the anterior horn cells in the early stage of SOD1^G93A ALS mice. Moreover, in five human autopsied sporadic ALS cases with bulbar or upper limb onset, Cx36 immunoreactivity was diminished in the proximal dendrites and neuropils of well-preserved large motor neurons in the lumbar anterior horns. These findings suggest that downregulation of neuronal and dendritic Cx36 in the spinal anterior horns commonly occurs from the early stage of hereditary and sporadic ALS. Cx36-made electrical synapses without glutamatergic signaling appear to be more vulnerable than other chemical synapses and electrical synapses with glutamatergic signaling in the early stage of motor neuron degeneration, suggesting involvement of Cx36-made electrical synapses in the pathogenesis of human ALS.

E3 ubiquitin ligases LNX1 and LNX2 localize at neuronal gap junctions formed by connexin36 in rodent brain and molecularly interact with connexin36

Electrical synapses in the mammalian central nervous system (CNS) are increasingly recognized as highly complex structures for mediation of neuronal communication, both with respect to their capacity for dynamic short- and long-term modification in efficacy of synaptic transmission and their multimolecular regulatory and structural components. These two characteristics are inextricably linked, such that understanding of mechanisms that contribute to electrical synaptic plasticity requires knowledge of the molecular composition of electrical synapses and the functions of proteins associated with these synapses. Here, we provide evidence that the key component of gap junctions that form the majority of electrical synapses in the mammalian CNS, namely connexin36 (Cx36), directly interacts with the related E3 ubiquitin ligase proteins Ligand of NUMB protein X1 (LNX1) and Ligand of NUMB protein X2 (LNX2). This is based on immunofluorescence colocalization of LNX1 and LNX2 with Cx36-containing gap junctions in adult mouse brain versus lack of such coassociation in LNX null mice, coimmunoprecipitation of LNX proteins with Cx36, and pull-down of Cx36 with the second PDZ domain of LNX1 and LNX2. Furthermore, cotransfection of cultured cells with Cx36 and E3 ubiquitin ligase-competent LNX1 and LNX2 isoforms led to loss of Cx36-containing gap junctions between cells, whereas these junctions persisted following transfection with isoforms of these proteins that lack ligase activity. Our results suggest that a LNX protein mediates ubiquitination of Cx36 at neuronal gap junctions, with consequent Cx36 internalization, and may thereby contribute to intracellular mechanisms that govern the recently identified modifiability of synaptic transmission at electrical synapses.

Structural and Intermolecular Associations Between Connexin36 and Protein Components of the Adherens Junction-Neuronal Gap Junction Complex

Intimate structural and functional relationships between gap junctions and adherens junctions have been demonstrated in peripheral tissues, but have not been thoroughly examined in the central nervous system, where adherens junctions are often found in close proximity to neuronal gap junctions. Here, we used immunofluorescence approaches to document the localization of various protein components of adherens junctions in relation to those that we have previously reported to occur at electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36). The adherens junction constituents N-cadherin and nectin-1 were frequently found to localize near or overlap with Cx36-containing gap junctions in several brain regions examined. This was also true of the adherens junction-associated proteins α-catenin and β-catenin, as well as the proteins zonula occludens-1 and AF6 (aka, afadin) that were reported constituents of both adherens junctions and gap junctions. The deployment of the protein constituents of these junctions was especially striking at somatic contacts between primary afferent neurons in the mesencephalic trigeminal nucleus (MesV), where the structural components of adherens junctions appeared to be maintained in connexin36 null mice. These results support emerging views concerning the multi-molecular composition of electrical synapses and raise possibilities for various structural and functional protein-protein interactions at what now can be considered the adherens junction-neuronal gap junction complex. Further, the results point to intracellular signaling pathways that could potentially contribute to the assembly, maintenance and turnover of this complex, as well as to the dynamic nature of neuronal communication at electrical synapses.

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Neurons and satellite glial cells in adult rat lumbar dorsal root ganglia express connexin 36

Previous studies have shown that following peripheral nerve injury there was a downregulation of the gap junction protein connexin 36 (Cx36) in the spinal cord; however, it is not known whether Cx36 protein is expressed in the dorsal root ganglia (DRGs), nor if its levels are altered following peripheral nerve injuries. Here we address these aspects in the adult rat lumbar DRG. Cx36 mRNA was detected using qRT-PCR, and Cx36 protein was identified in DRG sections using immunohistochemistry (IHC) and immunofluorescence (IF). Double staining revealed that Cx36 co-localizes with both anti-β-III tubulin, a neuronal marker, and anti-glutamine synthetase, a satellite glial cell (SGC) marker. In neurons, Cx36 staining was mostly uniform in somata and fibers of all sizes and its intensity increased at the cell membranes. This labeling pattern was in contrast with Cx36 IF dots mainly found at junctional membranes in islet beta cells used as a control tissue. Co-staining with anti-Cx43 and anti-Cx36 showed that whereas mostly uniform staining of Cx36 was found throughout neurons and SGCs, Cx43 IF puncta were localized to SGCs. Cx36 mRNA was expressed in normal lumbar DRG, and it was significantly down-regulated in L4 DRG of rats that underwent sciatic nerve injury resulting in persistent hypersensitivity. Collectively, these findings demonstrated that neurons and SGCs express Cx36 protein in normal DRG, and suggested that perturbation of Cx36 levels may contribute to chronic neuropathic pain resulting from a peripheral nerve injury.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Motoneurons regulate the central pattern generator during drug-induced locomotor-like activity in the neonatal mouse

Motoneurons are traditionally viewed as the output of the spinal cord that do not influence locomotor rhythmogenesis. We assessed the role of motoneuron firing during ongoing locomotor-like activity in neonatal mice expressing archaerhopsin-3 (Arch), halorhodopsin (eNpHR), or channelrhodopsin-2 (ChR2) in Choline acetyltransferase neurons (ChAT⁺) or Arch in LIM-homeodomain transcription factor Isl1⁺ neurons. Illumination of the lumbar cord in mice expressing eNpHR or Arch in ChAT⁺ or Isl1⁺ neurons, depressed motoneuron discharge, transiently decreased the frequency, and perturbed the phasing of the locomotor-like rhythm. When the light was turned off motoneuron firing and locomotor frequency both transiently increased. These effects were not due to cholinergic neurotransmission, persisted during partial blockade of gap junctions and were mediated, in part, by AMPAergic transmission. In spinal cords expressing ChR2, illumination increased motoneuron discharge and transiently accelerated the rhythm. We conclude that motoneurons provide feedback to the central pattern generator (CPG) during drug-induced locomotor-like activity.

DOI:http://dx.doi.org/10.7554/eLife.26622.001

Spinal control of motor outputs by intrinsic and externally induced electric field potentials

Despite numerous studies on spinal neuronal systems, several issues regarding their role in motor behavior remain unresolved. One of these issues is how electric fields associated with the activity of spinal neurons influence the operation of spinal neuronal networks and how effects of these field potentials are combined with other means of modulating neuronal activity. Another closely related issue is how external electric field potentials affect spinal neurons and how they can be used for therapeutic purposes such as pain relief or recovery of motor functions by transspinal direct current stimulation. Nevertheless, progress in our understanding of the spinal effects of electric fields and their mechanisms has been made over the last years, and the aim of the present review is to summarize the recent findings in this field.

Antidromic-rectifying gap junctions amplify chemical transmission at functionally mixed electrical-chemical synapses

Neurons communicate through chemical synapses and electrical synapses (gap junctions). Although these two types of synapses often coexist between neurons, little is known about whether they interact, and whether any interactions between them are important to controlling synaptic strength and circuit functions. By studying chemical and electrical synapses between premotor interneurons (AVA) and downstream motor neurons (A-MNs) in the Caenorhabditis elegans escape circuit, we found that disrupting either the chemical or electrical synapses causes defective escape response. Gap junctions between AVA and A-MNs only allow antidromic current, but, curiously, disrupting them inhibits chemical transmission. In contrast, disrupting chemical synapses has no effect on the electrical coupling. These results demonstrate that gap junctions may serve as an amplifier of chemical transmission between neurons with both electrical and chemical synapses. The use of antidromic-rectifying gap junctions to amplify chemical transmission is potentially a conserved mechanism in circuit functions.

The role of Cx36 and Cx43 in 4-aminopyridine-induced rhythmic activity in the spinal nociceptive dorsal horn: an electrophysiological study in vitro

Connexin (Cx) proteins and gap junctions support the formation of neuronal and glial syncytia that are linked to different forms of rhythmic firing and oscillatory activity in the CNS. In this study, quantitative reverse transcription polymerase chain reaction (RT-qPCR) was used to profile developmental expression of two specific Cx proteins, namely glial Cx43 and neuronal Cx36, in postnatal lumbar spinal cord aged 4, 7, and 14 days. Extracellular electrophysiology was used to determine the contribution of Cx36 and Cx43 to a previously described form of 4-aminopyridine (4-AP)-induced 4-12 Hz rhythmic activity within substantia gelatinosa (SG) of rat neonatal dorsal horn (DH) in vitro. The involvement of Cx36 and Cx43 was probed pharmacologically using quinine, a specific uncoupler of Cx36 and the mimetic peptide blocker Gap 26 which targets Cx43. After establishment of 4-12 Hz rhythmic activity by 4-AP (25 μmol/L), coapplication of quinine (250 μmol/L) reduced 4-AP-induced 4-12 Hz rhythmic activity (P < 0.05). Preincubation of spinal cord slices with Gap 26 (100 μmol/L), compromised the level of 4-AP-induced 4-12 Hz rhythmic activity in comparison with control slices preincubated in ACSF alone (P < 0.05). Conversely, the nonselective gap junction "opener" trimethylamine (TMA) enhanced 4-12 Hz rhythmic behavior (P < 0.05), further supporting a role for Cx proteins and gap junctions. These data have defined a physiological role for Cx36 and Cx43 in rhythmic firing in SG, a key nociceptive processing area of DH. The significance of these data in the context of pain and Cx proteins as a future analgesic drug target requires further study.

Connexin 50 modulates Sox2 expression in spinal-cord-derived ependymal stem/progenitor cells

Ion channels included in the family of Connexins (Cx) have been reported to influence the secondary expansion of traumatic spinal cord injury (SCI) and neuropathic pain following SCI. However, Cxs also contribute to spinal cord neurogenesis during the remyelinating process and functional recovery after SCI. Certain Cxs have been recently related to the control of cell proliferation and the differentiation of neuronal progenitors. Adult spinal-cord-derived ependymal stem progenitor cells (epSPC) show high expression levels of Cx50 in non-pathological conditions and lower expression when they actively proliferate after injury (epSPCi). We explore the role of Cx50 in the ependymal population in the modulation of Sox2, a crucial factor of neural progenitor self-renewal and a promising target for promoting neuronal-cell-fate induction for neuronal tissue repair. Short-interfering-RNA ablation or over-expression of Cx50 regulates the expression of Sox2 in both epSPC and epSPCi. Interestingly, Cx50 and Sox2 co-localize at the nucleus indicating a potential role for this ion channel beyond cell-to-cell communication in the spinal cord. In vivo and in vitro experiments with Clotrimazole, a specific pharmacological modulator of Cx50, show the convergent higher expression of Cx50 and Sox2 in the isolated epSPC/epSPCi and in spinal cord tissue. Therefore, the pharmacological modulation of Cx50 might constitute an interesting mechanism for Sox2 induction to modulate the endogenous regenerative potential of neuronal tissue with a potential application in regenerative therapies.

Connexin 50 Expression in Ependymal Stem Progenitor Cells after Spinal Cord Injury Activation

Ion channels included in the family of Connexins (Cx) help to control cell proliferation and differentiation of neuronal progenitors. Here we explored the role of Connexin 50 (Cx50) in cell fate modulation of adult spinal cord derived neural precursors located in the ependymal canal (epSPC). epSPC from non-injured animals showed high expression levels of Cx50 compared to epSPC from animals with spinal cord injury (SCI) (epSPCi). When epSPC or epSPCi were induced to spontaneously differentiate in vitro we found that Cx50 favors glial cell fate, since higher expression levels, endogenous or by over-expression of Cx50, augmented the expression of the astrocyte marker GFAP and impaired the neuronal marker Tuj1. Cx50 was found in both the cytoplasm and nucleus of glial cells, astrocytes and oligodendrocyte-derived cells. Similar expression patterns were found in primary cultures of mature astrocytes. In addition, opposite expression profile for nuclear Cx50 was observed when epSPC and activated epSPCi were conducted to differentiate into mature oligodendrocytes, suggesting a different role for this ion channel in spinal cord beyond cell-to-cell communication. In vivo detection of Cx50 by immunohistochemistry showed a defined location in gray matter in non-injured tissues and at the epicenter of the injury after SCI. epSPCi transplantation, which accelerates locomotion regeneration by a neuroprotective effect after acute SCI is associated with a lower signal of Cx50 within the injured area, suggesting a minor or detrimental contribution of this ion channel in spinal cord regeneration by activated epSPCi.

Connexin36 expression in major centers of the auditory system in the CNS of mouse and rat: Evidence for neurons forming purely electrical synapses and morphologically mixed synapses

Electrical synapses formed by gap junctions composed of connexin36 (Cx36) are widely distributed in the mammalian central nervous system (CNS). Here, we used immunofluorescence methods to document the expression of Cx36 in the cochlear nucleus and in various structures of the auditory pathway of rat and mouse. Labeling of Cx36 visualized exclusively as Cx36-puncta was densely distributed primarily on the somata and initial dendrites of neuronal populations in the ventral cochlear nucleus, and was abundant in superficial layers of the dorsal cochlear nucleus. Other auditory centers displaying Cx36-puncta included the medial nucleus of the trapezoid body (MNTB), regions surrounding the lateral superior olivary nucleus, the dorsal nucleus of the medial lemniscus, the nucleus sagulum, all subnuclei of the inferior colliculus, and the auditory cerebral cortex. In EGFP-Cx36 transgenic mice, EGFP reporter was detected in neurons located in each of auditory centers that harbored Cx36-puncta. In the ventral cochlear nuclei and the MNTB, many neuronal somata were heavily innervated by nerve terminals containing vesicular glutamate transporter-1 (vglut1) and Cx36 was frequently localized at these terminals. Cochlear ablation caused a near total depletion of vglut1-positive terminals in the ventral cochlear nuclei, with a commensurate loss of labeling for Cx36 around most neuronal somata, but preserved Cx36-puncta at somatic neuronal appositions. The results suggest that electrical synapses formed by Cx36-containing gap junctions occur in most of the widely distributed centers of the auditory system. Further, it appears that morphologically mixed chemical/electrical synapses formed by nerve terminals are abundant in the ventral cochlear nucleus, including those at endbulbs of Held formed by cochlear primary afferent fibers, and those at calyx of Held synapses on MNTB neurons.

Gap junction proteins and their role in spinal cord injury

Gap junctions are specialized intercellular communication channels that are formed by two hexameric connexin hemichannels, one provided by each of the two adjacent cells. Gap junctions and hemichannels play an important role in regulating cellular metabolism, signaling, and functions in both normal and pathological conditions. Following spinal cord injury (SCI), there is damage and disturbance to the neuronal elements of the spinal cord including severing of axon tracts and rapid cell death. The initial mechanical disruption is followed by multiple secondary cascades that cause further tissue loss and dysfunction. Recent studies have implicated connexin proteins as playing a critical role in the secondary phase of SCI by propagating death signals through extensive glial networks. In this review, we bring together past and current studies to outline the distribution, changes and roles of various connexins found in neurons and glial cells, before and in response to SCI. We discuss the contribution of pathologically activated connexin proteins, in particular connexin 43, to functional recovery and neuropathic pain, as well as providing an update on potential connexin specific pharmacological agents to treat SCI.

Heterotypic gap junctions at glutamatergic mixed synapses are abundant in goldfish brain

Gap junctions provide for direct intercellular electrical and metabolic coupling. The abundance of gap junctions at "large myelinated club ending (LMCE)" synapses on Mauthner cells (M-cells) of the teleost brain provided a convenient model to correlate anatomical and physiological properties of electrical synapses. There, presynaptic action potentials were found to evoke short-latency electrical "pre-potentials" immediately preceding their accompanying glutamate-induced depolarizations, making these the first unambiguously identified "mixed" (i.e., chemical plus electrical) synapses in the vertebrate CNS. We recently showed that gap junctions at these synapses exhibit asymmetric electrical resistance (i.e., electrical rectification), which we correlated with total molecular asymmetry of connexin composition in their apposing gap junction hemiplaques, with connexin35 (Cx35) restricted to axon terminal hemiplaques and connexin34.7 (Cx34.7) restricted to apposing M-cell plasma membranes. We now show that similarly heterotypic neuronal gap junctions are abundant throughout goldfish brain, with labeling exclusively for Cx35 in presynaptic hemiplaques and exclusively for Cx34.7 in postsynaptic hemiplaques. Moreover, the vast majority of these asymmetric gap junctions occur at glutamatergic axon terminals. The widespread distribution of heterotypic gap junctions at glutamatergic mixed synapses throughout goldfish brain and spinal cord implies that pre- vs. postsynaptic asymmetry at electrical synapses evolved early in the chordate lineage. We propose that the advantages of the molecular and functional asymmetry of connexins at electrical synapses that are so prominently expressed in the teleost CNS are unlikely to have been abandoned in higher vertebrates. However, to create asymmetric coupling in mammals, where most gap junctions are composed of connexin36 (Cx36) on both sides, would require some other mechanism, such as differential phosphorylation of connexins on opposite sides of the same gap junction or on asymmetric differences in the complement of their scaffolding and regulatory proteins.

Connexin36 in gap junctions forming electrical synapses between motoneurons in sexually dimorphic motor nuclei in spinal cord of rat and mouse

Pools of motoneurons in the lumbar spinal cord innervate the sexually dimorphic perineal musculature, and are themselves sexually dimorphic, showing differences in number and size between male and female rodents. In two of these pools, the dorsomedial nucleus (DMN) and the dorsolateral nucleus (DLN), dimorphic motoneurons are intermixed with non-dimorphic neurons innervating anal and external urethral sphincter muscles. As motoneurons in these nuclei are reportedly linked by gap junctions, we examined immunofluorescence labeling for the gap junction-forming protein connexin36 (Cx36) in male and female mice and rats. Fluorescent Cx36-labeled puncta occurred in distinctly greater amounts in the DMN and DLN of male rodents than in other spinal cord regions. These puncta were localized to motoneuron somata, proximal dendrites, and neuronal appositions, and were distributed either as isolated or large patches of puncta. In both rats and mice, Cx36-labeled puncta were associated with nearly all (> 94%) DMN and DLN motoneurons. The density of Cx36-labeled puncta increased dramatically from postnatal days 9 to 15, unlike the developmental decreases in these puncta observed in other central nervous system regions. In females, Cx36 labeling of puncta in the DLN was similar to that in males, but was sparse in the DMN. In enhanced green fluorescent protein (EGFP)-Cx36 transgenic mice, motoneurons in the DMN and DLN were intensely labeled for the EGFP reporter in males, but less so in females. The results indicate the presence of Cx36-containing gap junctions in the sexually dimorphic DMN and DLN of both male and female rodents, suggesting coupling of not only sexually dimorphic but also non-dimorphic motoneurons in these nuclei.