Physiology and biophysics of hemi-gap-junctional channels expressed in Xenopus oocytes

High extracellular magnesium inhibits mineralized matrix deposition and modulates intracellular calcium signaling in human bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) have the potential to differentiate into several cell types and provide an attractive source of autologous cells for regenerative medicine. However, their cellular biology is not fully understood. Similar to Ca(2+), extracellular Mg(2+) plays an important role in the functions of the skeletal system. Here, we examined the effects of extracellular Mg(2+) on the deposition of calcium phosphate matrix and Ca(2+) signaling with or without ATP stimulation in human bone marrow-derived mesenchymal stem cells (hBMSCs). We found that high extracellular Mg(2+) concentration ([Mg(2+)]e) inhibited extracellular matrix mineralization in hBMSCs in vitro. hBMSCs also produced a dose-dependent decrease in the frequency of calcium oscillations during [Mg(2+)]e elevation with a slight suppression on oscillation amplitude. In addition, spontaneous ATP release was inhibited under high [Mg(2+)]e levels and exogenous ATP addition stimulated oscillation reappear. Taken together, our results indicate that high [Mg(2+)]e modulates calcium oscillations via suppression of spontaneous ATP release and inactivates purinergic receptors, resulting in decreased extracellular mineralized matrix deposition in hBMSCs. Therefore, the high magnesium environment created by the rapid corrosion of Mg alloys may result in the dysfunction of calcium-dependent physiology processes and be disadvantageous to hBMSCs physiology.

Roles of gap junctions, connexins, and pannexins in epilepsy

Enhanced gap junctional communication (GJC) between neurons is considered a major factor underlying the neuronal synchrony driving seizure activity. In addition, the hippocampal sharp wave ripple complexes, associated with learning and seizures, are diminished by GJC blocking agents. Although gap junctional blocking drugs inhibit experimental seizures, they all have other non-specific actions. Besides interneuronal GJC between dendrites, inter-axonal and inter-glial GJC is also considered important for seizure generation. Interestingly, in most studies of cerebral tissue from animal seizure models and from human patients with epilepsy, there is up-regulation of glial, but not neuronal gap junctional mRNA and protein. Significant changes in the expression and post-translational modification of the astrocytic connexin Cx43, and Panx1 were observed in an in vitro Co⁺⁺ seizure model, further supporting a role for glia in seizure-genesis, although the reasons for this remain unclear. Further suggesting an involvement of astrocytic GJC in epilepsy, is the fact that the expression of astrocytic Cx mRNAs (Cxs 30 and 43) is several fold higher than that of neuronal Cx mRNAs (Cxs 36 and 45), and the number of glial cells outnumber neuronal cells in mammalian hippocampal and cortical tissue. Pannexin expression is also increased in both animal and human epileptic tissues. Specific Cx43 mimetic peptides, Gap 27 and SLS, inhibit the docking of astrocytic connexin Cx43 proteins from forming intercellular gap junctions (GJs), diminishing spontaneous seizures. Besides GJs, Cx membrane hemichannels in glia and Panx membrane channels in neurons and glia are also inhibited by traditional gap junctional pharmacological blockers. Although there is no doubt that connexin-based GJs and hemichannels, and pannexin-based membrane channels are related to epilepsy, the specific details of how they are involved and how we can modulate their function for therapeutic purposes remain to be elucidated.

Connexin and Pannexin hemichannels are regulated by redox potential

Connexins (Cxs) and Pannexins (Panxs) are two non-related protein families, having both the property to form hemichannels at the plasma membrane. There are 21 genes coding for different Cx based proteins and only 3 for Panx. Under physiological conditions, these hemichannels (Cxs and Panxs) present a low open probability, but when open, they allow the release of signaling molecules to the extracellular space. However, under pathological conditions, these hemichannels increase their open probability, inducing important lysis of metabolites, and ionic imbalance, which in turn induce the massive entry of Ca⁺² to the cell. Actually, it is well recognized that Cxs and Panxs based channels play an important role in several diseases and -in many cases- this is associated with an aberrant hemichannel opening. Hemichannel opening and closing are controlled by a plethora of signaling including changes of the voltage plasma membrane, protein-protein interactions, and several posttranslational modifications, including protein cleavage, phosphorylation, glycosylation, hydroxylation and S-nitrosylation, among others. In particular, it has been recently shown that the cellular redox status modulates the opening/closing and permeability of at least Cx43, Cx46, and Panx1 hemichannels. Thus, for example, the gaseous transmitter nitric oxide (NO) can induce the S-nitrosylation of these proteins modulating in turn several of their properties. The reason is that the redox status of a cell is fundamental to set their response to the environment and also plays an important role in several pathologies. In this review, I will discuss how NO and other molecules associated with redox signaling modulate Cxs and Panx hemichannels properties.

Regulation of connexin gene expression during skeletal muscle regeneration in the adult rat

In the adult skeletal muscle, various kinds of trauma promote proliferation of satellite cells that differentiate into myoblasts forming new myofibers or to repair the damaged one. The aim of present work was to perform a comparative spatial and temporal analysis of connexin (Cx) 37, Cx39, Cx40, Cx43, and Cx45 expression in the adult regenerating skeletal muscle in response to crush injury. Within 24 h from injury, Cx37 expression was upregulated in the endothelial cells of blood vessels, and, 5 days after injury, Cx37-expressing cells were found inside the area of lesion and formed clusters generating new blood vessels with endothelial cells expressing Cx37. Three days after injury, Cx39 mRNA was selectively expressed in myogenin-positive cells, forming rows of closely apposed cell nuclei fusing in myotubes. Cx40 mRNA-labeled cells were observed within 24 h from injury in the endothelium of blood vessels, and, 5 days after lesion, Cx40-labeled cells were found inside the area of lesion-forming rows of myogenin-positive, closely apposed cells coexpressing Cx39. Within 24 h from lesion, both Cx43 and Cx45 mRNAs were upregulated in individual cells, and some of them were positive for M-cadherin. Three days after injury, a large number of both Cx43 and Cx45 mRNA-labeled and myogenin-positive cells were found inside the area of lesion. Taken together, these results show that at least four Cxs, out of five expressed in regenerating skeletal muscle, can be differentially involved in communication of myogenic cells during the process of cell proliferation, aggregation, and fusion to form new myotubes or to repair damaged myofibers.

The Lens Circulation

The lens is the largest organ in the body that lacks a vasculature. The reason is simple: blood vessels scatter and absorb light while the physiological role of the lens is to be transparent so it can assist the cornea in focusing light on the retina. We hypothesize this lack of blood supply has led the lens to evolve an internal circulation of ions that is coupled to fluid movement, thus creating an internal micro-circulatory system, which makes up for the lack of vasculature. This review covers the membrane transport systems that are believed to generate and direct this internal circulatory system.

Cell-cell communication beyond connexins: the pannexin channels

Direct cell-to-cell communication through specialized intercellular channels is a characteristic feature of virtually all multi-cellular organisms. The remarkable functional conservation of cell-to-cell coupling throughout the animal kingdom, however, is not matched at the molecular level of the structural protein components. Thus protostomes (including nematodes and flies) and deuterostomes (including all vertebrates) utilize two unrelated families of gap-junction genes, innexins and connexins, respectively. The recent discovery that pannexins, a novel group of proteins expressed by several organisms, are able to form intercellular channels has started a quest to understand their evolutionary relationship and functional contribution to cell communication in vivo. There are three pannexin genes in mammals, two of which are co-expressed in the developing and adult brain. Of note, pannexin1 can also form Ca2+-activated hemichannels that open at physiological extracellular Ca2+ concentrations and exhibit distinct pharmacological properties.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

Estrogen and bisphenol A disrupt spontaneous [Ca(2+)](i) oscillations in mouse oocytes

The present work aims to study the effects of estrogen or endocrine disrupters (EDs) on the dynamic changes in intracellular Ca(2+) concentration of mouse immature oocytes (IOs) loaded with Ca(2+)-sensitive dye Fura-2 using an image analyzer. The majority of IOs isolated from the ovary exhibited spontaneous Ca(2+) oscillations at regular intervals. Entry of external Ca(2+), probably through gap junctions, contributes to Ca(2+) oscillations since they were reversibly inhibited by removing Ca(2+) from the bathing medium or by the application of a gap-junction inhibitor carbenoxolone (CBX, 30 microM). Both 17beta-estradiol (E2) and E2-BSA, a membrane impermeable estrogen, shortened the duration of Ca(2+) oscillations in a dose-dependent manner (1-1000 nM), and produced an irregular pattern of the oscillations, strongly suggesting that E2 acts on the plasma membrane of the oocyte. For bisphenol A (BPA), one of the estrogen-mimicking EDs, a 10,000-fold higher concentration (100 microM) was necessary to exert similar inhibitory action to that of E2.

The novel mouse connexin39 gene is expressed in developing striated muscle fibers

The recently identified mouse connexin39 (mCx39) gene encodes a peptide of 364 amino acids that shows only 61% sequence similarity to its putative human orthologue connexin40.1 (hCx40.1). The coding regions of mCx39 and hCx40.1 are located on two different exons as described for murine and human connexin36. Northern blot and RT-PCR analyses revealed that mCx39 is expressed after embryonic day (ED) 13.5 up to birth and is absent from the adult stage. Polyclonal antibodies raised to a peptide corresponding to the 16 C-terminal amino acid residues detected a protein band of about 40 kDa apparent molecular mass in lysates of several embryonic tissues. In sections of ED14.5, ED16.5 and neonatal (P0) tissues, immunofluorescent signals were prominent between myotubes in the developing diaphragm, within the intercostal muscle, in the region around the occipital bone, as well as in muscles of the limb, tongue and connective tissue around the eye. These antibodies yielded punctate signals on apposed plasma membranes of HeLa cells transfected with Cx39 cDNA but did not react with wild-type cells. Furthermore, no intercellular permeation of microinjected neurobiotin and other tracers could be detected in Cx39 transfected HeLa cells. However, after microinjection of Alexa488 into myotubes of dissected neonatal diaphragm, we found spreading of this dye into neighbouring cells. As expression of no other known connexin could be verified in these cells, intercellular dye transfer might result from functional expression of Cx39 in developing striated muscle fibers.