Differentiating connexin hemichannels and pannexin channels in cellular ATP release.

Adenosine triphosphate (ATP) plays a fundamental role in cellular communication, with its extracellular accumulation triggering purinergic signaling cascades in a diversity of cell types. While the roles for purinergic signaling in health and disease have been well established, identification and differentiation of the specific mechanisms controlling cellular ATP release is less well understood. Multiple mechanisms have been proposed to regulate ATP release with connexin (Cx) hemichannels and pannexin (Panx) channels receiving major focus. However, segregating the specific roles of Panxs and Cxs in ATP release in a plethora of physiological and pathological contexts has remained enigmatic. This multifaceted problem has arisen from the selectivity of pharmacological inhibitors for Panxs and Cxs, methodological differences in assessing Panx and Cx function and the potential compensation by other isoforms in gene silencing and genetic knockout models. Consequently, there remains a void in the current understanding of specific contributions of Panxs and Cxs in releasing ATP during homeostasis and disease. Differentiating the distinct signaling pathways that regulate these two channels will advance our current knowledge of cellular communication and aid in the development of novel rationally-designed drugs for modulation of Panx and Cx activity, respectively.

Specializations of intercellular junctions are associated with the presence and absence of hair cell regeneration in ears from six vertebrate classes.

Sensory hair cell losses lead to hearing and balance deficits that are permanent for mammals, but temporary for nonmammals because supporting cells in their ears give rise to replacement hair cells. In mice and humans, vestibular supporting cells grow exceptionally large circumferential F-actin belts and their junctions express E-cadherin in patterns that strongly correlate with postnatal declines in regeneration capacity. In contrast, chicken supporting cells retain thin F-actin belts throughout life and express little E-cadherin. To determine whether the junctions in chicken ears might be representative of other ears that also regenerate hair cells, we investigated inner ears from dogfish sharks, zebrafish, bullfrogs, Xenopus, turtles, and the lizard, Anolis. As in chickens, the supporting cells in adult zebrafish, Xenopus, and turtle ears retained thin circumferential F-actin belts and expressed little E-cadherin. Supporting cells in adult sharks and bullfrogs also retained thin belts, but were not tested for E-cadherin. Supporting cells in adult Anolis exhibited wide, but porous webs of F-actin and strong E-cadherin expression. Anolis supporting cells also showed some cell cycle reentry when cultured. The results reveal that the association between thin F-actin belts and low E-cadherin is shared by supporting cells in anamniotes, turtles, and birds, which all can regenerate hair cells. Divergent junctional specializations in supporting cells appear to have arisen independently in Anolis and mammals. The presence of webs of F-actin at the junctions in Anolis appears compatible with supporting cell proliferation, but the solid reinforcement of the F-actin belts in mammals is associated with its absence.

The junctions that don't fit the scheme: special symmetrical cell-cell junctions of their own kind.

Immunocytochemical, electron-, and immunoelectron-microscopical studies have revealed that, in addition to the four major "textbook categories" of cell-cell junctions (gap junctions, tight junctions, adherens junctions, and desmosomes), a broad range of other junctions exists, such as the tiny puncta adhaerentia minima, the taproot junctions (manubria adhaerentia), the plakophilin-2-containing adherens junctions of mesenchymal or mesenchymally derived cell types including malignantly transformed cells, the composite junctions (areae compositae) of the mature mammalian myocardium, the cortex adhaerens of the eye lens, the interdesmosomal "sandwich" or "stud" junctions in the subapical layers of stratified epithelia and the tumors derived therefrom, and the complexus adhaerentes of the endothelial and virgultar cells of the lymph node sinus. On the basis of their sizes and shapes, other morphological criteria, and their specific molecular ensembles, these junctions and the genes that encode them cannot be subsumed under one of the major categories mentioned above but represent special structures in their own right, appear to serve special functions, and can give rise to specific pathological disorders.

The arm-repeat protein NPRAP (neurojungin) is a constituent of the plaques of the outer limiting zone in the retina, defining a novel type of adhering junction.

In the retina, special plaque-bearing adhering junctions are aligned to form a planar system (the "outer limiting zone," OLZ) of heterotypic connections between the photoreceptor cells and the surrounding glial cells ("Müller cells"), together with homotypic junctions. In the plaques of these junctions, which contain N-cadherin-and possibly also related cadherins-we have identified, by immunolocalization techniques, a recently discovered neural tissue-specific protein, neurojungin, a member of the plakoglobin/armadillo protein family. In these plaques we have also detected other adherens plaque proteins, such as alpha- and beta-catenin, protein p120, and vinculin, as well as proteins known as constituents of tight junction plaques, such as symplekin and protein ZO-1, and the desmosomal plaque protein plakophilin 2. This unusual combination of proteins and the demonstrated absence of plakoglobin define the OLZ junctions as a new and distinct category of adhering junction, which probably has special architectural functions.

Structure of intercellular junctions in the endothelium.

Endothelial cell junctions are complex structures formed by transmembrane adhesive molecules linked to a network of cytoplasmic/cytoskeletal proteins. At least three different types of endothelial junctions have been described: tight junctions, gap junctions and adherens junctions. These structures have some features and components in common with epithelium but also some which are specific for endothelium. We still know very little about the pathologic consequences of alterations in the functional behaviour or synthesis of endothelial cell junction proteins. It is possible that pathologies linked to altered endothelial permeability and vascular organization (e.g. hemangiomas, scleroderma, and other types of vasculitis) are associated with structural alterations in endothelial junction organization. In addition, changes in endothelial permeability properties are associated with the early stages of atherosclerosis and many inflammatory diseases.

Adhesive proteins at endothelial cell-to-cell junctions and leukocyte extravasation.

Endothelial cell junctions are complex structures formed by transmembrane adhesive molecules linked to a network of cytoplasmic/cytoskeletal proteins. At least four different types of endothelial junctions have been described (tight junctions, gap junctions, adherence junctions and syndesmos or complexus adhaerentes). Leukocytes adhesion to endothelial cells is frequently followed by their extravasation. The mechanisms which regulate the passage of leukocytes through endothelial clefts remain to be clarified. Many indirect data suggest that leukocytes might transfer signals to endothelial cells both through the release of active agents and adhesion to the endothelial cell surface. These signals could induce the disorganization of interendothelial junctions and facilitate leukocyte transmigration.

Cell junctions during the early development of the sea urchin embryo (Paracentrotus lividus).

Thin sections, lanthanum tracer and the freeze-fracture technique revealed the presence of different types of cell junctions in early sea urchin (Paracentrotus lividus) embryos. During the first four cleavage cycles, which are characterized by synchrony of cell division, sister blastomeres were connected only by intercellular bridges, formed as a result of incomplete cytokinesis; no trace of other junctions was found at these stages. From the 16-cell stage onwards, septate junctions and gap junctions began to appear between blastomeres. It is postulated that cell-cell interactions may provide a mechanism for the propagation of signals necessary for the coordination of cell proliferation and differentiation.