Overexpression of connexin43 alters the mutant phenotype of midgestational wnt-1 null mice resulting in recovery of the midbrain and cerebellum

Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Gap junctions (GJs) are aqueous channels that allow cells to communicate via physiological signals directly. The role of gap junctional connectivity in determining single-cell functions has long been recognized. However, GJs have another important role: the regulation of large-scale anatomical pattern. GJs are not only versatile computational elements that allow cells to control which small molecule signals they receive and emit, but also establish connectivity patterns within large groups of cells. By dynamically regulating the topology of bioelectric networks in vivo, GJs underlie the ability of many tissues to implement complex morphogenesis. Here, a review of recent data on patterning roles of GJs in growth of the zebrafish fin, the establishment of left-right patterning, the developmental dysregulation known as cancer, and the control of large-scale head-tail polarity, and head shape in planarian regeneration has been reported. A perspective in which GJs are not only molecular features functioning in single cells, but also enable global neural-like dynamics in non-neural somatic tissues has been proposed. This view suggests a rich program of future work which capitalizes on the rapid advances in the biophysics of GJs to exploit GJ-mediated global dynamics for applications in birth defects, regenerative medicine, and morphogenetic bioengineering. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 643-673, 2017.

The connexin43-dependent transcriptome during brain development: importance of genetic background

Use of null mutant mice is a powerful way to evaluate the role of specific proteins in brain function. Studies performed on knockout mice have revealed some unexpected roles of the gap junction proteins (connexins). Thus, analyses of gene expression in connexin43 (Cx43) null brains indicated that deletion of a single gene (Gja1) induced expression level change of numerous other genes located on all chromosomes and involved in a wide diversity of functional pathways. The significant overlap between alterations in gene expression level, control and coordination in Cx43 knockout and knockdown astrocytes raised the possibility that Gja1 represents a transcriptomic node of gene regulatory networks. However, conditional deletion of Gja1 in astrocytes of two mouse strains resulted in remarkably different phenotypes. In order to evaluate the influence of the genetic background on the transcriptome, we performed microarray studies on brains of GFAP-Cre:Cx43(f/f) C57Bl/6 and 129/SvEv mice. The surprisingly low number of Cx43 core genes (regulated in all Cx43 nulls regardless of strain) and the high number of differently regulated genes in the two Cx43 conditional knockouts indicate high influence of mouse strain on brain transcriptome. This article is part of a Special Issue entitled Electrical Synapses.

Precerebellar cell groups in the hindbrain of the mouse defined by retrograde tracing and correlated with cumulative Wnt1-cre genetic labeling

The precerebellar nuclei are hindbrain and spinal cord centers that send fibers to the cerebellum. The neurons of the major hindbrain precerebellar nuclei are derived from the rhombic lip. Wnt1, a developmentally important gene involved in intercellular signaling, is expressed in the developing rhombic lip. We sought to investigate the relationship between the cell clusters expressing Wnt1 and the precerebellar nuclei in the hindbrain. We therefore defined the hindbrain precerebellar nuclei by retrograde tracing, following cerebellar injections of HRP, and compared these results with the cell clusters expressing Wnt1 in newborn mice. We found that 39 distinct hindbrain nuclei project to the cerebellum. Of these nuclei, all but three (namely the oral pontine reticular nucleus, the caudal pontine reticular nucleus, and the subcoeruleus nucleus) contain neurons expressing Wnt1. This shows a high degree of overlap between the precerebellar nuclei and the nuclei that express Wnt1. However, it should be noted that neurons expressing Wnt1 are also found in the superior olivary complex, which is a basal plate derivative lacking cerebellar projections.

Gap Junctions and Biophysical Regulation of Bone Cells

Communication between osteoblasts, osteoclasts, and osteocytes is integral to their ability to build and maintain the skeletal system and respond to physical signals. Various physiological mechanisms, including nerve communication, hormones, and cytokines, play an important role in this process. More recently, the important role of direct, cell-cell communication via gap junctions has been established. In this review, we demonstrate the integral role of gap junctional intercellular communication (GJIC) in skeletal physiology and bone cell mechanosensing.

Organizational principles of the connexin-related brain transcriptome

We have found that deletion of genes encoding the gap junction proteins Cx43, Cx32 and Cx36 alter the expression levels of large numbers of genes in mouse brain located on all chromosomes and encoding proteins from all major functional categories. Gene regulation in Cx32 and Cx43 null brains was more similar than that in the Cx36 null brain, suggesting the possibility of transcriptomic controls exerted by both genes on both astrocytes and oligodendrocytes. In order to explore the nature of expression linkage among the genes, we examined coordinated expression patterns in wild-type and connexin null brains. Coordination with Cx43 in wild-type brain predicted regulation in Cx43 nulls with considerable accuracy. Moreover, interlinkage within gene networks was greatly perturbed in the Cx43 null brain. These findings suggest several principles regarding regulatory transcriptomic networks involving gap junction genes and raise the issue of the underlying cause of connexin null phenotypes as well as mechanisms of regulation.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

Inhibition of gap junction communication at ectopic Eph/ephrin boundaries underlies craniofrontonasal syndrome

Mutations in X-linked ephrin-B1 in humans cause craniofrontonasal syndrome (CFNS), a disease that affects female patients more severely than males. Sorting of ephrin-B1–positive and –negative cells following X-inactivation has been observed in ephrin-B1^+/− mice; however, the mechanisms by which mosaic ephrin-B1 expression leads to cell sorting and phenotypic defects remain unknown. Here we show that ephrin-B1^+/− mice exhibit calvarial defects, a phenotype autonomous to neural crest cells that correlates with cell sorting. We have traced the causes of calvarial defects to impaired differentiation of osteogenic precursors. We show that gap junction communication (GJC) is inhibited at ectopic ephrin boundaries and that ephrin-B1 interacts with connexin43 and regulates its distribution. Moreover, we provide genetic evidence that GJC is implicated in the calvarial defects observed in ephrin-B1^+/− embryos. Our results uncover a novel role for Eph/ephrins in regulating GJC in vivo and suggest that the pleiotropic defects seen in CFNS patients are due to improper regulation of GJC in affected tissues.

Mutations inephrin-B1 associated with X-linked CFNS result in aberrant formation of facial bone structures.Ephrin-B1 interacts with gap junctions to influence the cell sorting events that underlie the CFNS deformations.

Connexin43 and the brain transcriptome of newborn mice

Our previously reported cDNA array datasets from neonatal wild-type and Cx43-/- (approved gene symbol Gja1) mouse brains were further analyzed to identify underlying interlinkages in the brain transcriptome. The analysis revealed that no gene cohort sharing either primary function or chromosomal location was significantly altered (up-and down-regulation were roughly balanced) in Cx43-/- brains, but each cohort exhibited significant perturbation of transcript abundance proportions and reduced expression variability and coordination. By comparing pairwise expression correlations of all genes with one another in wild-type brains, we found genes exhibiting remarkable similarity or opposition to the coordination profile (set of synergistically, antagonistically, and independently expressed partners) of Cx43, one of the most similar being pannexin1, a vertebrate homolog of invertebrate gap junction proteins. This study indicates striking redundancy of expression controls over functional pathways and suggests that certain genes may play roles similar to or opposite that of Cx43 in organizing the brain transcriptome.

Pannexin expression in the cerebellum

Pannexin1 and pannexin2 are members of the pannexin gene family which are widely expressed in the central nervous system. Here we present an overview of pannexin expression and distribution in the mouse cerebellum. Pannexin1 and pannexin2 are expressed in the Purkinje cells and in some cells of the granule cell layer. Pannexin2 is also expressed in the stellate cells of the molecular layer. A differential expression of pannexin1 and pannexin2 mRNA is observed during cerebellar development. These findings constitute the first indication of the involvement of pannexin molecules in the developing cerebellum. Although the functional relevance of these molecules remains currently unknown, the abundance of pannexins in the Purkinje cells suggests that they may contribute to the generation of cerebellar rhythms.