The connexin gene family in mammals

In Search of a Unifying Concept in Human Diseases

Throughout the history of biological/medicine sciences, there has been opposing strategies to find solutions to complex human disease problems. Both empirical and deductive approaches have led to major insights and concepts that have led to practical preventive and therapeutic benefits for the human population. The classic definitions of "science" (to know) has been paired with the classic definition of technology (to do). One knew more as the technology developed, and that development was often based on science. In other words, one could do more if science could improve the technology. In turn, this made possible to know more science with improved technology. However, with the development of new technologies of today in biology and medicine, major advances have been made, such as the information from the Human Genome Project, genetic engineering techniques and the use of bioinformatic uses of sophisticated computer analyses. This has led to the renewed idea that Precision Medicine, while raising some serious ethical concerns, also raises the expectation of improved potential of risk predictions for prevention and treatment of various genetically and environmentally influenced human diseases. This new field Artificial Intelligence, as a major handmaiden to Precision Medicine, is significantly altering the fundamental means of biological discovery. However, can today's fundamental premise of "Artificial Intelligence", based on identifying DNA, as the primary nexus of human health and disease, provide the practical solutions to complex human diseases that involve the interaction of those genes with the broad spectrum of "environmental factors"? Will it be "precise" enough to provide practical solutions for prevention and treatments of diseases? In this "Commentary", with the example of human carcinogenesis, it will be challenged that, without the integration of mechanistic and hypothesis-driven approaches with the "unbiased" empirical analyses of large numbers of data, the Artificial Intelligence approach with fall short.

Effects of Alpha-Connexin Carboxyl-Terminal Peptide (aCT1) and Bowman-Birk Protease Inhibitor (BBI) on Canine Oral Mucosal Melanoma (OMM) Cells

Oral mucosal melanomas (OMM) are aggressive cancers in dogs, and are good models for human OMM. Gap junctions are composed of connexin units, which may have altered expression patterns and/or subcellular localization in cancer cells. Cell-to-cell communication by gap junctions is often impaired in cancer cells, including in melanomas. Meanwhile, the upregulated expression of the gap junction protein connexin 43 (Cx43) inhibits melanoma progression. The α-connexin carboxyl-terminal (aCT1) peptide reportedly maintains Cx43 expression and cell-cell communication in human mammary cells and increases the communication activity through gap junctions in functional assays, therefore causing decreased cell proliferation. The Bowman-Birk protease inhibitor (BBI), a component of soybeans, induces Cx43 expression in several tumor cells as a trypsin–chymotrypsin inhibition function, with antineoplastic effects. This study investigated the effect of aCT1 peptide and BBI treatment, alone or in combination, on TLM1 canine melanoma cell viability. Cell viability after treatment with aCT1, the reverse sequence peptide (R-pep), and/or BBI for 5 days was analyzed by PrestoBlue assay. Immunofluorescence was used to observe Cx43 localization and expression. aCT1 (200 μM) alone did not significantly decrease cell viability in TLM1 cells, whereas BBI (400 μg/ml) alone significantly decreased the TLM1 viability. Combined treatment with both aCT1 (200 μM) and BBI (400 μg/ml) significantly decreased cell viability in TLM1 cells. Cx43 expression, as identified by immunostainings in TLM1 cells, was increased in the cell membrane after the combination treatment with BBI and aCT1. This dual treatment can be combined to achieve the anticancer activity, possibly by increasing Cx 43 expression and affecting Cx43 migration to the cell membrane. In conclusion, a treatment strategy targeting Cx43 with BBI and aCT1 may possibly lead to new effective therapies for canine OMM.

Connexins during 500 Million Years-From Cyclostomes to Mammals

It was previously shown that the connexin gene family had relatively similar subfamily structures in several vertebrate groups. Still, many details were left unclear. There are essentially no data between tunicates, which have connexins that cannot be divided into the classic subfamilies, and teleosts, where the subfamilies are easily recognized. There are also relatively few data for the groups that diverged between the teleosts and mammals. As many of the previously analyzed genomes have been improved, and many more genomes are available, we reanalyzed the connexin gene family and included species from all major vertebrate groups. The major results can be summarized as follows: (i) The same connexin subfamily structures are found in all Gnathostomata (jawed vertebrates), with some variations due to genome duplications, gene duplications and gene losses. (ii) In contrast to previous findings, birds do not have a lower number of connexins than other tetrapods. (iii) The cyclostomes (lampreys and hagfishes) possess genes in the alpha, beta, gamma and delta subfamilies, but only some of the genes show a phylogenetic affinity to specific genes in jawed vertebrates. Thus, two major evolutionary transformations have occurred in this gene family, from tunicates to cyclostomes and from cyclostomes to jawed vertebrates.

The potential antiepileptogenic effect of neuronal Cx36 gap junction channel blockage

Epilepsy is one of the most prevalent neurological disorders and can result in neuronal injury and degeneration. Consequently, research into new antiepileptic drugs capable of providing protection against neuronal injury and degeneration is extremely important. Neuronal Cx36 gap junction channels have been found to play an important role in epilepsy; thus, pharmacological interference using Cx36 gap junction channel blockers may be a promising strategy for disrupting the synchronization of neurons during seizure activity and protecting neurons. Based on these promising findings, several in vivo and in vitro studies are ongoing and the first encouraging results have been published. The results bring hope that neurons can be protected from injury and degeneration in patients with epilepsy, which is currently impossible.

Phylogeny of teleost connexins reveals highly inconsistent intra- and interspecies use of nomenclature and misassemblies in recent teleost chromosome assemblies

BACKGROUND

Based on an initial collecting of database sequences from the gap junction protein gene family (also called connexin genes) in a few teleosts, the naming of these sequences appeared variable. The reasons could be (i) that the structure in this family is variable across teleosts, or (ii) unfortunate naming. Rather clear rules for the naming of genes in fish and mammals have been outlined by nomenclature committees, including the naming of orthologous and ohnologous genes. We therefore analyzed the connexin gene family in teleosts in more detail. We covered the range of divergence times in teleosts (eel, Atlantic herring, zebrafish, Atlantic cod, three-spined stickleback, Japanese pufferfish and spotted pufferfish; listed from early divergence to late divergence).

RESULTS

The gene family pattern of connexin genes is similar across the analyzed teleosts. However, (i) several nomenclature systems are used, (ii) specific orthologous groups contain genes that are named differently in different species, (iii) several distinct genes have the same name in a species, and (iv) some genes have incorrect names. The latter includes a human connexin pseudogene, claimed as GJA4P, but which in reality is Cx39.2P (a delta subfamily gene often called GJD2like). We point out the ohnologous pairs of genes in teleosts, and we suggest a more consistent nomenclature following the outlined rules from the nomenclature committees. We further show that connexin sequences can indicate some errors in two high-quality chromosome assemblies that became available very recently.

CONCLUSIONS

Minimal consistency exists in the present practice of naming teleost connexin genes. A consistent and unified nomenclature would be an advantage for future automatic annotations and would make various types of subsequent genetic analyses easier. Additionally, roughly 5% of the connexin sequences point out misassemblies in the new high-quality chromosome assemblies from herring and cod.

Human adult stem cells as the target cells for the initiation of carcinogenesis and for the generation of "cancer stem cells"

The inference to stem cells has been found in ancient myths and the concept of stem cells has existed in the fields of plant biology, developmental biology and embryology for decades. In the field of cancer research, the stem cell theory was one of the earliest hypotheses on the origin of a cancer from a single cell. However, an opposing hypothesis had it that an adult differentiated somatic cell could "de-differentiate" to become a cancer cell. Only within the last decade, via the "cloning" of Dolly, the sheep, did the field of stem cell biology really trigger an exciting revolution in biological research. The isolation of human embryonic stem cells has created a true revolution in the life sciences that has led to the hope that these human stem cells could lead to (a) basic science understanding of gene regulation during differentiation and development; (b) stem cell therapy; (c) gene therapy via stem cells; (d) the use of stem cells for drug discovery; (e) screening for toxic effects of chemicals; and (f) understand the aging and diseases of aging processes.

Evolution of energy metabolism, stem cells and cancer stem cells: how the warburg and barker hypotheses might be linked

The evolutionary transition from single cells to the metazoan forced the appearance of adult stem cells and a hypoxic niche, when oxygenation of the environment forced the appearance of oxidative phosphorylation from that of glycolysis. The prevailing paradigm in the cancer field is that cancers start from the "immortalization" or "re-programming" of a normal, differentiated cell with many mitochondria, that metabolize via oxidative phosphorylation. This paradigm has been challenged with one that assumes that the target cell for carcinogenesis is the normal, immortal adult stem cell, with few mitochondria. This adult organ-specific stem cell is blocked from "mortalizing" or from "programming" to be terminally differentiated. Two hypotheses have been offered to explain cancers, namely, the "stem cell theory" and the "de-differentiation" or "re-programming" theory. This Commentary postulates that the paleochemistry of the oceans, which, initially, provided conditions for life' s energy to arise via glycolysis, changed to oxidative phosphorylation for life' s processes. In doing so, stem cells evolved, within hypoxic niches, to protect the species germinal and somatic genomes. This Commentary provides support for the "stem cell theory", in that cancer cells, which, unlike differentiated cells, have few mitochondria and metabolize via glycolysis. The major argument against the "de-differentiation theory" is that, if re-programming of a differentiated cell to an "induced pluri-potent stem cell" happened in an adult, teratomas, rather than carcinomas, should be the result.

Stem cells in toxicology: fundamental biology and practical considerations

This "Commentary" has examined the use of human stem cells for detection of toxicities of physical, chemical, and biological toxins/toxicants in response to the challenge posed by the NRC Report, "Toxicity Testing in the 21st Century: A vision and Strategy." Before widespread application of the use of human embryonic, pluripotent, "iPS," or adult stem cells be considered, the basic characterization of stem cell biology should be undertaken. Because no in vitro system can mimic all factors that influence cells in vivo (individual genetic, gender, developmental, immunological and diurnal states; niche conditions; complex intercellular interactions between stem, progenitor, terminal differentiated cells, and the signaling from extracellular matrices, oxygen tensions, etc.), attempts should be made to use both embryonic and adult stem cells, grown in three dimension under "niche-like" conditions. Because many toxins and toxicants work by "epigenetic" mechanisms and that epigenetic mechanisms play important roles in regulating gene expression and in the pathogenesis of many human diseases, epigenetic toxicity must be incorporated in toxicity testing. Because modulation of gap junctional intercellular communication by epigenetic agents plays a major role in homeostatic regulation of both stem and progenitor cells in normal tissues, the modulation of this biological process by both endogenous and endogenous chemicals should be incorporated as an end point to monitor for potential toxicities or chemo-preventive attributes. In addition, modulation of quantity, as well as the quality, of stem cells should be considered as potential source of a chemical's toxic potential in affecting any stem cell-based pathology, such as cancer.

The gap junction as a "Biological Rosetta Stone": implications of evolution, stem cells to homeostatic regulation of health and disease in the Barker hypothesis

The discovery of the gap junction structure, its functions and the family of the "connexin" genes, has been basically ignored by the major biological disciplines. These connexin genes code for proteins that organize to form membrane-associated hemi-channels, "connexons", co-join with the connexons of neighboring cells to form gap junctions. Gap junctions appeared in the early evolution of the metazoan. Their fundamental functions, (e.g., to synchronize electrotonic and metabolic functions of societies of cells, and to regulate cell proliferation, cell differentiation, and apoptosis), were accomplished via integrating the extra-cellular triggering of intra-cellular signaling, and therefore, regulating gene expression. These functions have been documented by genetic mutations of the connexin genes and by chemical modulation of gap junctions. Via genetic alteration of connexins in knock-out and transgenic mice, as well as inherited connexin mutations in various human syndromes, the gap junction has been shown to be directly linked to many normal cell functions and multiple diseases, such as birth defects, reproductive, neurological disorders, immune dysfunction and cancer. Specifically, the modulation of gap junctional intercellular communication (GJIC), either by increasing or decreasing its functions by non-mutagenic chemicals or by oncogenes or tumor suppressor genes in normal or "initiated" stem cells and their progenitor cells, can have a major impact on tumor promotion or cancer chemoprevention and chemotherapy. The overview of the roles of the gap junction in the evolution of the metazoan and its potential in understanding a "systems" view of human health and aging and the diseases of aging will be attempted.

Hypoxia in high glucose followed by reoxygenation in normal glucose reduces the viability of cortical astrocytes through increased permeability of connexin 43 hemichannels

Brain ischemia causes more extensive injury in hyperglycemic than normoglycemic subjects, and the increased damage is to astroglia as well as neurons. In the present work, we found that in cortical astrocytes from rat or mouse, reoxygenation after hypoxia in a medium mimicking interstitial fluid during ischemia increases hemichannel activity and decreases cell-cell communication via gap junctions as indicated by dye uptake and dye coupling, respectively. These effects were potentiated by high glucose during the hypoxia in a concentration-dependent manner (and by zero glucose) and were not observed in connexin 43(-/-) astrocytes. The responses were transient and persistent after short and long periods of hypoxia, respectively. The persistent responses were associated with a progressive reduction in cell viability that was prevented by La(3+) or peptides that block connexin 43 (Cx43) hemichannels or by inhibition of p38 MAP kinase prior to hypoxia-reoxygenation but not by treatments that block pannexin hemichannels. Block of Cx43 hemichannels did not affect the reduction in gap junction mediated dye coupling observed during reoxygenation. Cx43 hemichannels may be a novel therapeutic target to reduce cell death following stroke, particularly in hyperglycemic conditions.

Vitamin K2 suppresses malignancy of HuH7 hepatoma cells via inhibition of connexin 43

The anti-cancer potential of vitamin K(2) (VK(2)) in hepatoma has gained considerable attention but the underlying mechanisms are unclear. Treatment of HuH7 hepatoma cells with VK(2) produced a normal liver phenotype. Following treatment of cells with VK(2), there was an increase in gap junctional intercellular communication activity, accompanied by up-regulation of connexin 32 (Cx32), dominantly expressed in normal hepatocyte. In contrast, Cx43 expression was inhibited. Moreover, the effect of VK(2) on Cx32 was abolished by over-expression of Cx43. Taken together, we propose that the anti-tumor effect of VK(2) is at least partly due to a decrease in Cx43 promoter activity.

Expression of connexin 43 in normal canine testes and canine testicular tumors

In human testis, gap junctions containing connexin(Cx)43 are located within the seminiferous epithelium between Sertoli cells and between Sertoli and germ cells. Cx43 is known to play a role in the differentiation and proliferation of these cell types. It can further be associated with human seminoma development. The dog has been proposed as a model for studies of the male reproductive system, because of the frequent occurrence of testicular neoplasms. Thus, we investigated Cx43-mRNA and -protein expression in testes of normal prepubertal dogs, adult dogs, and in canine testicular tumors. Sertoli cells in prepubertal cords express Cx43 mRNA, but do synthesize only less Cx43 protein. Within the seminiferous tubules, Cx43 mRNA was detected in Sertoli cells, spermatogonia, and spermatocytes. Cx43 protein was mainly present in the basal compartment. In canine testicular tumors Cx43 mRNA was detectable in both seminoma and neoplastic Sertoli cells, whereas Cx43 protein was only found in neoplastic Sertoli cells. Our data indicate that Cx43 is regulated differentially in testicular tumors and that alterations of Cx43 expression may be involved in the pathogenesis of canine testicular malignancies. This study represents the first morphological work on the spatiotemporal expression pattern of Cx43 in normal and neoplastic canine testis.

Evolutionary selection pressure and family relationships among connexin genes

We suggest an extension ofconnexinorthology relationships across the major vertebrate lineages. We first show that the conserved domains of mammalianconnexins(encoding the N-terminus, four transmembrane domains and two extracellular loops) are subjected to a considerably more strict selection pressure than the full-length sequences or the variable domains (the intracellular loop and C-terminal tail). Therefore, the conserved domains are more useful for the study of family relationships over larger evolutionary distances. The conserved domains ofconnexinswere collected from chicken,Xenopus tropicalis, zebrafish, pufferfish, green spotted pufferfish,Ciona intestinalisandHalocynthia pyriformis(two tunicates). A total of 305connexinsequences were included in this analysis. Phylogenetic trees were constructed, from which the orthologies and the presumed evolutionary relationships between the sequences were deduced. The tunicateconnexinsstudied had the closest, but still distant, relationships to vertebrateconnexin36,39.2,43.4,45and47. The main structure in theconnexinfamily known from mammals pre-dates the divergence of bony fishes, but some additional losses and gains ofconnexinsequences have occurred in the evolutionary lineages of subsequent vertebrates. Thus, theconnexingene family probably originated in the early evolution of chordates, and underwent major restructuring with regard to gene and subfamily structures (including the number of genes in each subfamily) during early vertebrate evolution.

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.

Status and applications of genomic resources for the gray, short-tailed opossum, Monodelphis domestica, an American marsupial model for comparative biology

Owing to its small size, favourable reproductive characteristics, and simple husbandry, the gray, short-tailed opossum, Monodelphis domestica, has become the most widely distributed and intensively utilised laboratory-bred research marsupial in the world today. This article provides an overview of the current state and future projections of genomic resources for this species and discusses the potential impact of this growing resource base on active research areas that use M. domestica as a model system. The resources discussed include: fully arrayed, bacterial artificial chromosome (BAC) libraries; an expanding linkage map; developing full-genome BAC-contig and chromosomal fluorescence in situ hybridisation maps; public websites providing access to the M. domestica whole-genome-shotgun sequence trace database and the whole-genome sequence assembly; and a new project underway to create an expressed-sequence database and microchip expression arrays for functional genomics applications. Major research areas discussed span a variety of genetic, evolutionary, physiologic, reproductive, developmental, and behavioural topics, including: comparative immunogenetics; genomic imprinting; reproductive biology; neurobiology; photobiology and carcinogenesis; genetics of lipoprotein metabolism; developmental and behavioural endocrinology; sexual differentiation and development; embryonic and fetal development; meiotic recombination; genome evolution; molecular evolution and phylogenetics; and more.