Communication through GJ channels is regulated at different

Communication through GJ channels is regulated at different levels and includes GJ plaque internalization into a single cell. Those vesicles contain the membrane of both cells and th assembled Cx43. The C-terminal domain of Cx43 interacts with different peptides and proteins. However, the internalization of these vesicles containing the GJ plaque and Cx43-interactome is related to degradation and downregulation of GJIC. However, new studies would be needed to investigate if adjacent cells are also able to transfer materials and proteins through the internalization of GJ plaques formed by Cx43, as they do through disconnected released EVs and exosomes. In this study, we have found that contacting CHs, BCs and SCs are able to transfer different proteins including several chaperones and cell-surface proteins, such as calnexin, endoplasmin, Cy7.5 maleimide(non-sulfonated) or CD44 (Table 1, Table 2, Table 3). Because of the short time frame of the co-culture system performed in this study (4 h), these proteins are likely to be transferred through membrane internalization or nanotube connections. Additionally, we cannot discount that other mechanisms could be involved in the transfer of proteins that do not require cellular contacts (such as EVS). The findings reported here will have significant implications on the mechanism involved in the transfer of signalling molecules and nutrients, providing a path toward new studies in order to fully understand the molecular and metabolic communication that regulates the behaviour of synovial cells, chondrocytes and cells in subchondral bone in both physiological and pathological conditions such as OA.
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Introduction Intercellular communication is an indispensable event for multicellular organisms, from invertebrates to chordates. The main machinery mediating this function is the gap junction. Gap junctions, an assembly of communicating channels that penetrate two adjoining cell membranes — referred to as gap junction channels, facilitate the transfer of small molecules such as nucleotides, second messengers, ions, and peptides between cells [1]. This function is associated with many biological processes, such as cardiac development, immune system function, fertility [2], and electrically mediated neuronal synchronization [3]. Malfunction of gap junction communication leads to many human diseases, such as hearing loss, skin disorders, neuropathies, cataracts, and cardiovascular disease [4]. Early evidence of gap junctions in association with electrical synapses was provided by the giant fiber motor neuron of cray fish [5], marking the dawn of research on intercellular channels. The machinery of the intercellular channels is formed by subunit families with four transmembrane domains, connexin in chordates and innexin in pre-chordates, which have little similarity in terms of their amino acid sequences. It remains unclear whether these two protein families are in a relationship of evolutionary convergence or divergence [6, 7, 8]. Pannexin and leucine-rich repeat-containing 8 (LRRC8) protein families also have weak homology with innexin [9,10]; these are not thought to form gap junction channels, but rather to function as single membrane channels [10,11]. Until a few years ago, the only high-resolution structure of gap junction channels was for a channel composed of connexin 26 (Cx26) [,]. It has been by no means easy to obtain the crystals of gap junction proteins diffracting to high-resolution possibly due to flexibility in the cytoplasmic domains. Given the 3D crystals of Cx26 composed of fully docked junction channels [,], it is unavoidable to have crystal-packing contacts mediated by the cytoplasmic domains that have not yet been resolved for connexin channels. Recent advances in cryo-EM have promoted structural studies without crystallization, which led to atomic resolution of the structures of Caenorhabditis elegans innexin-6 (Ce-INX6) and LRRC8A [,15, 16, 17]. All these proteins have four transmembrane helices, TM1–TM4, and two extracellular loops, E1 and E2, and the cytoplasmic domains contain the N-terminus, the C-terminus, and the cytoplasmic loop. This review highlights structural studies focusing on connexins and innexins as the main components forming gap junction channels, and specifically discusses the N-terminal conformation, which has been implicated as essential to channel function.