The permeability of gap junction channels to probes of different size is dependent on connexin composition and permeant-pore affinities

Molecular permeation through large pore channels: computational approaches and insights

Computational methods such as molecular dynamics (MD) have illuminated how single-atom ions permeate membrane channels and how selectivity among them is achieved. Much less is understood about molecular permeation through eukaryotic channels that mediate the flux of small molecules (e.g. connexins, pannexins, LRRC8s, CALHMs). Here we describe computational methods that have been profitably employed to explore the movements of molecules through wide pores, revealing mechanistic insights, guiding experiments, and suggesting testable hypotheses. This review illustrates MD techniques such as voltage-driven flux, potential of mean force, and mean first-passage-time calculations, as applied to molecular permeation through wide pores. These techniques have enabled detailed and quantitative modeling of molecular interactions and movement of permeants at the atomic level. We highlight novel contributors to the transit of molecules through these wide pathways. In particular, the flexibility and anisotropic nature of permeant molecules, coupled with the dynamics of pore-lining residues, lead to bespoke permeation dynamics. As more eukaryotic large-pore channel structures and functional data become available, these insights and approaches will be important for understanding the physical principles underlying molecular permeation and as guides for experimental design.

Multiple approaches for the evaluation of connexin-43 expression and function in macrophages

Connexins are essential gap junction proteins that play pivotal roles in intercellular communication in various organs of mammals. Connexin-43 (Cx43) is expressed in various components of the immune system, and there is extensive evidence of its participation in inflammation responses. The involvement of Cx43 in macrophage functionality involves the purinergic signaling pathway. Macrophages contribute to defenses against inflammatory reactions such as bacterial sepsis and peritonitis. Several assays can identify the presence and activity of Cx43 in macrophages. Real-time polymerase chain reaction (PCR) can measure the relative mRNA expression of Cx43, whereas western blotting can detect protein expression levels. Using immunofluorescence assays, it is possible to analyze the expression and observe the localization of Cx43 in cells or tissues. Moreover, connexin-mediated gap junction intercellular communication can be evaluated using functional assays such as microinjection of fluorescent dyes or scrape loading-dye transfer. The use of selective inhibitors contributes to this understanding and reinforces the role of connexins in various processes. Here, we discuss these methods to evaluate Cx43 and macrophage gap junctions.

Insights into the role of connexins and specialized intercellular communication pathways in breast cancer: Mechanisms and applications

Gap junctions, membrane-based channels comprised of connexin proteins (Cxs), facilitate direct communication among neighbouring cells and between cells and the extracellular space through their hemichannels. The normal human breast expresses various Cxs family proteins, such as Cx43, Cx30, Cx32, Cx46, and Cx26, crucial for proper tissue development and function. These proteins play a significant role in breast cancer development, progression, and therapy response. In primary tumours, there is often a reduction and cytoplasmic mislocalization of Cx43 and Cx26, while metastatic lesions show an upregulation of these and other Cxs. Although existing research predominantly supports the tumour-suppressing role of Cxs in primary carcinomas through channel-dependent and independent functions, controversies persist regarding their involvement in the metastatic process. This review aims to provide an updated perspective on Cxs in human breast cancer, with a specific focus on intrinsic subtypes due to the heterogeneous nature of this disease. Additionally, the manuscript will explore the role of Cxs in immune interactions and novel forms of intercellular communication, such as tunneling nanotubes and extracellular vesicles, within the breast tumour context and tumour microenvironment. Recent findings suggest that Cxs hold potential as therapeutic targets for mitigating metastasis and drug resistance. Furthermore, they may serve as novel biomarkers for cancer prognosis, offering promising avenues for future research and clinical applications.

Large-pore connexin hemichannels function like molecule transporters independent of ion conduction

Connexin hemichannels were identified as the first members of the eukaryotic large-pore channel family that mediate permeation of both atomic ions and small molecules between the intracellular and extracellular environments. The conventional view is that their pore is a large passive conduit through which both ions and molecules diffuse in a similar manner. In stark contrast to this notion, we demonstrate that the permeation of ions and of molecules in connexin hemichannels can be uncoupled and differentially regulated. We find that human connexin mutations that produce pathologies and were previously thought to be loss-of-function mutations due to the lack of ionic currents are still capable of mediating the passive transport of molecules with kinetics close to those of wild-type channels. This molecular transport displays saturability in the micromolar range, selectivity, and competitive inhibition, properties that are tuned by specific interactions between the permeating molecules and the N-terminal domain that lies within the pore-a general feature of large-pore channels. We propose that connexin hemichannels and, likely, other large-pore channels, are hybrid channel/transporter-like proteins that might switch between these two modes to promote selective ion conduction or autocrine/paracrine molecular signaling in health and disease processes.

Human coronavirus OC43 infection remodels connexin 43-mediated gap junction intercellular communication in vitro

β-coronaviruses cause acute infection in the upper respiratory tract, resulting in various symptoms and clinical manifestations. OC43 is a human β-coronavirus that induces mild clinical symptoms and can be safely studied in the BSL2 laboratory. Due to its low risk, OC43 can be a valuable and accessible model for understanding β-coronavirus pathogenesis. One potential target for limiting virus infectivity could be gap junction-mediated communication. This study aims to unveil the status of cell-to-cell communications through gap junctions in human β-coronavirus infection. Infection with OC43 leads to reduced expression of Cx43 in A549, a lung epithelial carcinoma cell line. Infection with this virus also shows a significant ER and oxidative stress increase. Internal localization of Cx43 is observed post-OC43 infection in the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) region, which impairs the gap junction communication between two adjacent cells, confirmed by Lucifer yellow dye transfer assay. It also affects hemichannel formation, as depicted by the EtBr uptake assay. Impairment of Cx43 trafficking and the ability to form hemichannels and functional GJIC are hampered by virus-induced Golgi apparatus disruption. Altogether, these results suggest that several physiological changes accompany OC43 infection in A549 cells and can be considered an appropriate model system for understanding the differences in gap junction communication post-viral infections. This model system can provide valuable insights for developing therapies against human β-coronavirus infections.IMPORTANCEThe enduring impact of the recent SARS-CoV-2 pandemic underscores the importance of studying human β-coronaviruses, advancing our preparedness for future coronavirus infections. As SARS-CoV-2 is highly infectious, another human β-coronavirus OC43 can be considered an experimental model. One of the crucial pathways that can be considered is gap junction communication, as it is vital for cellular homeostasis. Our study seeks to understand the changes in Cx43-mediated cell-to-cell communication during human β-coronavirus OC43 infection. In vitro studies showed downregulation of the gap junction protein Cx43 and upregulation of the endoplasmic reticulum and oxidative stress markers post-OC43 infection. Furthermore, HCoV-OC43 infection causes reduced Cx43 trafficking, causing impairment of functional hemichannel and GJIC formation by virus-mediated Golgi apparatus disruption. Overall, this study infers that OC43 infection reshapes intercellular communication, suggesting that this pathway may be a promising target for designing highly effective therapeutics against human coronaviruses by regulating Cx43 expression.

Astrocyte regulation of extracellular space parameters across the sleep-wake cycle

Multiple subfields of neuroscience research are beginning to incorporate astrocytes into current frameworks of understanding overall brain physiology, neuronal circuitry, and disease etiology that underlie sleep and sleep-related disorders. Astrocytes have emerged as a dynamic regulator of neuronal activity through control of extracellular space (ECS) volume and composition, both of which can vary dramatically during different levels of sleep and arousal. Astrocytes are also an attractive target of sleep research due to their prominent role in the glymphatic system, a method by which toxic metabolites generated during wakefulness are cleared away. In this review we assess the literature surrounding glial influences on fluctuations in ECS volume and composition across the sleep-wake cycle. We also examine mechanisms of astrocyte volume regulation in glymphatic solute clearance and their role in sleep and wake states. Overall, findings highlight the importance of astrocytes in sleep and sleep research.

Transfer of cardiomyocyte-derived extracellular vesicles to neighboring cardiac cells requires tunneling nanotubes during heart development

Rationale: Extracellular vesicles (EVs) are thought to mediate intercellular communication during development and disease. Yet, biological insight to intercellular EV transfer remains elusive, also in the heart, and is technically challenging to demonstrate. Here, we aimed to investigate biological transfer of cardiomyocyte-derived EVs in the neonatal heart. Methods: We exploited CD9 as a marker of EVs, and generated two lines of cardiomyocyte specific EV reporter mice: Tnnt2-Cre; double-floxed inverted CD9/EGFP and αMHC-MerCreMer; double-floxed inverted CD9/EGFP. The two mouse lines were utilized to determine whether developing cardiomyocytes transfer EVs to other cardiac cells (non-myocytes and cardiomyocytes) in vitro and in vivo and investigate the intercellular transport pathway of cardiomyocyte-derived EVs. Results: Genetic tagging of cardiomyocytes was confirmed in both reporter mouse lines and proof of concept in the postnatal heart showed that, a fraction of EGFP+/MYH1- non-myocytes exist firmly demonstrating in vivo cardiomyocyte-derived EV transfer. However, two sets of direct and indirect EGFP +/- cardiac cell co-cultures showed that cardiomyocyte-derived EGFP+ EV transfer requires cell-cell contact and that uptake of EGFP+ EVs from the medium is limited. The same was observed when co-cultiring with mouse macrophages. Further mechanistic insight showed that cardiomyocyte EV transfer occurs through type I tunneling nanotubes. Conclusion: While the current notion assumes that EVs are transferred through secretion to the surroundings, our data show that cardiomyocyte-derived EV transfer in the developing heart occurs through nanotubes between neighboring cells. Whether these data are fundamental and relate to adult hearts and other organs remains to be determined, but they imply that the normal developmental process of EV transfer goes through cell-cell contact rather than through the extracellular compartment.

Gap junctions in Turing-type periodic feather pattern formation

Periodic patterning requires coordinated cell-cell interactions at the tissue level. Turing showed, using mathematical modeling, how spatial patterns could arise from the reactions of a diffusive activator-inhibitor pair in an initially homogeneous 2D field. Most activators and inhibitors studied in biological systems are proteins, and the roles of cell-cell interaction, ions, bioelectricity, etc. are only now being identified. Gap junctions (GJs) mediate direct exchanges of ions or small molecules between cells, enabling rapid long-distance communications in a cell collective. They are therefore good candidates for propagating nonprotein-based patterning signals that may act according to the Turing principles. Here, we explore the possible roles of GJs in Turing-type patterning using feather pattern formation as a model. We found 7 of the 12 investigated GJ isoforms are highly dynamically expressed in the developing chicken skin. In ovo functional perturbations of the GJ isoform, connexin 30, by siRNA and the dominant-negative mutant applied before placode development led to disrupted primary feather bud formation. Interestingly, inhibition of gap junctional intercellular communication (GJIC) in the ex vivo skin explant culture allowed the sequential emergence of new feather buds at specific spatial locations relative to the existing primary buds. The results suggest that GJIC may facilitate the propagation of long-distance inhibitory signals. Thus, inhibition of GJs may stimulate Turing-type periodic feather pattern formation during chick skin development, and the removal of GJ activity would enable the emergence of new feather buds if the local environment were competent and the threshold to form buds was reached. We further propose Turing-based computational simulations that can predict the sequential appearance of these ectopic buds. Our models demonstrate how a Turing activator-inhibitor system can continue to generate patterns in the competent morphogenetic field when the level of intercellular communication at the tissue scale is modulated.

Assembly mechanisms of the neuronal gap junction channel connexin 36 elucidated by Cryo-EM

Electrical synapses are essential components of neural circuits. Neuronal signal transduction across electrical synapses is primarily mediated by gap junction channels composed of Connexin36 (Cx36), the lack of which causes impaired electrical coupling between certain neurons including cortical interneurons and thalamic reticular nucleus (TRN) neurons. However, the structural basis underlying Cx36 function and assembly remains elusive. Recently, Lee et al. reported cryo-EM structures of Cx36, thus provided first insights of its gating mechanism. Here, we report a consistent cryo-EM structure of Cx36 determined in parallel, and describe unique interactions underpinning its assembly mechanism in complementary to the competing work. In particular, we found non-canonical electrostatic interactions between protomers from opposing hemichannels and a steric complementary site between adjacent protomers within a hemichannel, which together provide a structural explanation for the assembly specificity in homomeric and heteromeric gap junction channels.

Simulation of gap junction formation reveals critical role of Cys disulfide redox state in connexin hemichannel docking

Video Abstract.

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

Interaction of human keratinocytes and nerve fiber terminals at the neuro-cutaneous unit

Traditionally, peripheral sensory neurons are assumed as the exclusive transducers of external stimuli. Current research moves epidermal keratinocytes into focus as sensors and transmitters of nociceptive and non-nociceptive sensations, tightly interacting with intraepidermal nerve fibers at the neuro-cutaneous unit. In animal models, epidermal cells establish close contacts and ensheath sensory neurites. However, ultrastructural morphological and mechanistic data examining the human keratinocyte-nerve fiber interface are sparse. We investigated this exact interface in human skin applying super-resolution array tomography, expansion microscopy, and structured illumination microscopy. We show keratinocyte ensheathment of afferents and adjacent connexin 43 contacts in native skin and have applied a pipeline based on expansion microscopy to quantify these parameter in skin sections of healthy participants versus patients with small fiber neuropathy. We further derived a fully human co-culture system, visualizing ensheathment and connexin 43 plaques in vitro. Unraveling human intraepidermal nerve fiber ensheathment and potential interaction sites advances research at the neuro-cutaneous unit. These findings are crucial on the way to decipher the mechanisms of cutaneous nociception.

Biotin-cGMP and -cAMP are able to permeate through the gap junctions of some amacrine cells in the mouse retina despite their large size

Gap junctions transmit electrical signals in neurons and serve metabolic coupling and chemical communication. Gap junctions are made of intercellular channels with large pores, allowing ions and small molecules to permeate. In the mammalian retina, intercellular coupling fulfills many vital functions in visual signal processing but is also implicated in promoting cell death after insults, such as excitotoxicity or hypoxia. Conversely, some studies also suggested a role for retinal gap junctions in neuroprotection. Recently, gap junctions were also advocated as conduits for therapeutic drug delivery in neurodegenerative disorders. This requires the permeation of rather large molecules through retinal gap junctions. However, the permeability of retinal networks for molecules >0.6 kDa has not been tested systematically. Here, we used the cut-loading method and probed gap junctional networks in the mouse retina for their permeability to cGMP and cAMP coupled to Biotin, using the well-characterized tracer Neurobiotin as control. Biotin-cGMP and -cAMP have a molecular weight of >0.8 kDa. We show that they cannot pass the gap junctions of horizontal cells but can permeate through the gap junctions of specific amacrine cells in the inner retina. These amacrine cells do not comprise AII amacrine cells and nitric oxide-releasing amacrine cells but some unknown type. In summary, we show that some retinal gap junctions are large enough to let molecules >0.8 kDa pass, making the intercellular delivery of therapeutic agents - already successfully exploited, for example, in cancer - also feasible in neurodegenerative diseases.

Connexins in Cancer, the Possible Role of Connexin46 as a Cancer Stem Cell-Determining Protein

Cancer is a widespread and incurable disease caused by genetic mutations, leading to uncontrolled cell proliferation and metastasis. Connexins (Cx) are transmembrane proteins that facilitate intercellular communication via hemichannels and gap junction channels. Among them, Cx46 is found mostly in the eye lens. However, in pathological conditions, Cx46 has been observed in various types of cancers, such as glioblastoma, melanoma, and breast cancer. It has been demonstrated that elevated Cx46 levels in breast cancer contribute to cellular resistance to hypoxia, and it is an enhancer of cancer aggressiveness supporting a pro-tumoral role. Accordingly, Cx46 is associated with an increase in cancer stem cell phenotype. These cells display radio- and chemoresistance, high proliferative abilities, self-renewal, and differentiation capacities. This review aims to consolidate the knowledge of the relationship between Cx46, its role in forming hemichannels and gap junctions, and its connection with cancer and cancer stem cells.

Mitochondrial Connexins and Mitochondrial Contact Sites with Gap Junction Structure

Mitochondria contain connexins, a family of proteins that is known to form gap junction channels. Connexins are synthesized in the endoplasmic reticulum and oligomerized in the Golgi to form hemichannels. Hemichannels from adjacent cells dock with one another to form gap junction channels that aggregate into plaques and allow cell-cell communication. Cell-cell communication was once thought to be the only function of connexins and their gap junction channels. In the mitochondria, however, connexins have been identified as monomers and assembled into hemichannels, thus questioning their role solely as cell-cell communication channels. Accordingly, mitochondrial connexins have been suggested to play critical roles in the regulation of mitochondrial functions, including potassium fluxes and respiration. However, while much is known about plasma membrane gap junction channel connexins, the presence and function of mitochondrial connexins remain poorly understood. In this review, the presence and role of mitochondrial connexins and mitochondrial/connexin-containing structure contact sites will be discussed. An understanding of the significance of mitochondrial connexins and their connexin contact sites is essential to our knowledge of connexins' functions in normal and pathological conditions, and this information may aid in the development of therapeutic interventions in diseases linked to mitochondria.

Gap junctions in Turing-type periodic feather pattern formation

Periodic patterning requires coordinated cell-cell interactions at the tissue level. Turing showed, using mathematical modeling, how spatial patterns could arise from the reactions of a diffusive activator-inhibitor pair in an initially homogenous two-dimensional field. Most activators and inhibitors studied in biological systems are proteins, and the roles of cell-cell interaction, ions, bioelectricity, etc. are only now being identified. Gap junctions (GJs) mediate direct exchanges of ions or small molecules between cells, enabling rapid long-distance communications in a cell collective. They are therefore good candidates for propagating non-protein-based patterning signals that may act according to the Turing principles. Here, we explore the possible roles of GJs in Turing-type patterning using feather pattern formation as a model. We found seven of the twelve investigated GJ isoforms are highly dynamically expressed in the developing chicken skin. In ovo functional perturbations of the GJ isoform, connexin 30, by siRNA and the dominant-negative mutant applied before placode development led to disrupted primary feather bud formation, including patches of smooth skin and buds of irregular sizes. Later, after the primary feather arrays were laid out, inhibition of gap junctional intercellular communication in the ex vivo skin explant culture allowed the emergence of new feather buds in temporal waves at specific spatial locations relative to the existing primary buds. The results suggest that gap junctional communication may facilitate the propagation of long-distance inhibitory signals. Thus, the removal of GJ activity would enable the emergence of new feather buds if the local environment is competent and the threshold to form buds is reached. We propose Turing-based computational simulations that can predict the appearance of these ectopic bud waves. Our models demonstrate how a Turing activator-inhibitor system can continue to generate patterns in the competent morphogenetic field when the level of intercellular communication at the tissue scale is modulated.

Divergence between Hemichannel and Gap Junction Permeabilities of Connexin 30 and 26

Cx30 has been proposed to play physiological functions in the kidney and cochlea, and this has often been associated with its hemichannel role (deafness mutants frequently affecting hemichannels more than gap junctions), implicated in ATP release. Here, we used heterologous expression systems (Xenopus oocytes and N2A cells) to describe the properties of Cx30 hemichannels, with the objective of better understanding their physiological functions. As previously observed, Cx30 hemichannels gated in response to transmembrane voltage (V0) and extracellular [Ca2+] (pK[Ca2+] of 1.9 μM in the absence of Mg++). They show minimal charge selectivity with respect to small ions (ratio of Na+: K+: Cl- of 1: 0.4: 0.6) and an MW cut-off for Alexa Dyes between 643 (Alex 488) and 820 Da (Alexa 594). However, while cations follow the expected drop in conductance with size (Na+ to TEA+ is 1: 0.3), anions showed an increase, with a ratio of Cl- to gluconate conductance of 1:1.4, suggesting favorable interactions between larger anions and the pore. This was further explored by comparing the permeabilities of both hemichannels and gap junctions to the natural anion (ATP), the release of which has been implicated in Ca++ signaling through hemichannels. We extended this analysis to two closely related connexins co-expressed in the cochlear, Cx26 and Cx30. Cx30 and 26 hemichannels displayed similar permeabilities to ATP, but surprisingly Cx26 gap junctions were six times more permeable than their hemichannels and four times more permeable than Cx30 gap junctions. This suggests a significant physiological difference in the functions of Cx26 and Cx30 gap junctions in organs where they are co-expressed, at least with regard to the distribution of energy resources of the cells. It also demonstrates that the permeability characteristics of hemichannels can significantly diverge from that of their gap junctions for some connexins but not others.

The Multifaceted Role of Connexins in Tumor Microenvironment Initiation and Maintenance

Today's research on the processes of carcinogenesis and the vital activity of tumor tissues implies more attention be paid to constituents of the tumor microenvironment and their interactions. These interactions between cells in the tumor microenvironment can be mediated via different types of protein junctions. Connexins are one of the major contributors to intercellular communication. They form the gap junctions responsible for the transfer of ions, metabolites, peptides, miRNA, etc., between neighboring tumor cells as well as between tumor and stromal cells. Connexin hemichannels mediate purinergic signaling and bidirectional molecular transport with the extracellular environment. Additionally, connexins have been reported to localize in tumor-derived exosomes and facilitate the release of their cargo. A large body of evidence implies that the role of connexins in cancer is multifaceted. The pro- or anti-tumorigenic properties of connexins are determined by their abundance, localization, and functionality as well as their channel assembly and non-channel functions. In this review, we have summarized the data on the contribution of connexins to the formation of the tumor microenvironment and to cancer initiation and progression.

Connexins and Pannexins: Important Players in Neurodevelopment, Neurological Diseases, and Potential Therapeutics

Cell-to-cell communication is essential for proper embryonic development and its dysfunction may lead to disease. Recent research has drawn attention to a new group of molecules called connexins (Cxs) and pannexins (Panxs). Cxs have been described for more than forty years as pivotal regulators of embryogenesis; however, the exact mechanism by which they provide this regulation has not been clearly elucidated. Consequently, Cxs and Panxs have been linked to congenital neurodegenerative diseases such as Charcot-Marie-Tooth disease and, more recently, chronic hemichannel opening has been associated with adult neurodegenerative diseases (e.g., Alzheimer's disease). Cell-to-cell communication via gap junctions formed by hexameric assemblies of Cxs, known as connexons, is believed to be a crucial component in developmental regulation. As for Panxs, despite being topologically similar to Cxs, they predominantly seem to form channels connecting the cytoplasm to the extracellular space and, despite recent research into Panx1 (Pannexin 1) expression in different regions of the brain during the embryonic phase, it has been studied to a lesser degree. When it comes to the nervous system, Cxs and Panxs play an important role in early stages of neuronal development with a wide span of action ranging from cellular migration during early stages to neuronal differentiation and system circuitry formation. In this review, we describe the most recent available evidence regarding the molecular and structural aspects of Cx and Panx channels, their role in neurodevelopment, congenital and adult neurological diseases, and finally propose how pharmacological modulation of these channels could modify the pathogenesis of some diseases.

Connexin37 Regulates Cell Cycle in the Vasculature

Control of vascular cell growth responses is critical for development and maintenance of a healthy vasculature. Connexins - the proteins comprising gap junction channels - are key regulators of cell growth in diseases such as cancer, but their involvement in controlling cell growth in the vasculature is less well appreciated. Connexin37 (Cx37) is one of four connexin isotypes expressed in the vessel wall. Its primary role in blood vessels relies on its unique ability to transduce flow-sensitive signals into changes in cell cycle status of endothelial (and perhaps, mural) cells. Here, we review available evidence for Cx37's role in the regulation of vascular growth, vessel organization, and vascular tone in healthy and diseased vasculature. We propose a novel mechanism whereby Cx37 accomplishes this with a phosphorylation-dependent transition between closed (growth-suppressive) and multiple open (growth-permissive) channel conformations that result from interactions of the C-terminus with cell-cycle regulators to limit or support cell cycle progression. Lastly, we discuss Cx37 and its downstream signaling as a novel potential target in the treatment of cardiovascular disease, and we address outstanding research questions that still challenge the development of such therapies.

Antigen transfer and its effect on vaccine-induced immune amplification and tolerance

Antigen transfer refers to the process of intercellular information exchange, where antigenic components including nucleic acids, antigen proteins/peptides and peptide-major histocompatibility complexes (p-MHCs) are transmitted from donor cells to recipient cells at the thymus, secondary lymphoid organs (SLOs), intestine, allergic sites, allografts, pathological lesions and vaccine injection sites via trogocytosis, gap junctions, tunnel nanotubes (TNTs), or extracellular vesicles (EVs). In the context of vaccine inoculation, antigen transfer is manipulated by the vaccine type and administration route, which consequently influences, even alters the immunological outcome, i.e., immune amplification and tolerance. Mainly focused on dendritic cells (DCs)-based antigen receptors, this review systematically introduces the biological process, molecular basis and clinical manifestation of antigen transfer.

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Importance of the Microenvironment and Mechanosensing in Adipose Tissue Biology

The expansion of adipose tissue is an adaptive mechanism that increases nutrient buffering capacity in response to an overall positive energy balance. Over the course of expansion, the adipose microenvironment undergoes continual remodeling to maintain its structural and functional integrity. However, in the long run, adipose tissue remodeling, typically characterized by adipocyte hypertrophy, immune cells infiltration, fibrosis and changes in vascular architecture, generates mechanical stress on adipose cells. This mechanical stimulus is then transduced into a biochemical signal that alters adipose function through mechanotransduction. In this review, we describe the physical changes occurring during adipose tissue remodeling, and how they regulate adipose cell physiology and promote obesity-associated dysfunction in adipose tissue.

Active role of glia-like supporting cells in the organ of Corti: Membrane proteins and their roles in hearing

The organ of Corti, located in the cochlea in the inner ear, is one of the major sensory organs involved in hearing. The organ of Corti consists of hair cells, glia-like supporting cells, and the cochlear nerve, which work in harmony to receive sound from the outer ear and transmit auditory signals to the cochlear nucleus in the auditory ascending pathway. In this process, maintenance of the endocochlear potential, with a high potassium gradient and clearance of electrolytes and biochemicals in the inner ear, is critical for normal sound transduction. There is an emerging need for a thorough understanding of each cell type involved in this process to understand the sophisticated mechanisms of the organ of Corti. Hair cells have long been thought to be active, playing a primary role in the cochlea in actively detecting and transmitting signals. In contrast, supporting cells are thought to be silent and function to support hair cells. However, growing lines of evidence regarding the membrane proteins that mediate ionic movement in supporting cells have demonstrated that supporting cells are not silent, but actively play important roles in normal signal transduction. In this review, we summarize studies that characterize diverse membrane proteins according to the supporting cell subtypes involved in cochlear physiology and hearing. This review contributes to a better understanding of supporting cell functions and facilitates the development of potential therapeutic tools for hearing loss.

Connexinplexity: the spatial and temporal expression of connexin genes during vertebrate organogenesis

Animal development requires coordinated communication between cells. The Connexin family of proteins is a major contributor to intercellular communication in vertebrates by forming gap junction channels that facilitate the movement of ions, small molecules, and metabolites between cells. Additionally, individual hemichannels can provide a conduit to the extracellular space for paracrine and autocrine signaling. Connexin-mediated communication is widely used in epithelial, neural, and vascular development and homeostasis, and most tissues likely use this form of communication. In fact, Connexin disruptions are of major clinical significance contributing to disorders developing from all major germ layers. Despite the fact that Connexins serve as an essential mode of cellular communication, the temporal and cell-type-specific expression patterns of connexin genes remain unknown in vertebrates. A major challenge is the large and complex connexin gene family. To overcome this barrier, we determined the expression of all connexins in zebrafish using single-cell RNA-sequencing of entire animals across several stages of organogenesis. Our analysis of expression patterns has revealed that few connexins are broadly expressed, but rather, most are expressed in tissue- or cell-type-specific patterns. Additionally, most tissues possess a unique combinatorial signature of connexin expression with dynamic temporal changes across the organism, tissue, and cell. Our analysis has identified new patterns for well-known connexins and assigned spatial and temporal expression to genes with no-existing information. We provide a field guide relating zebrafish and human connexin genes as a critical step toward understanding how Connexins contribute to cellular communication and development throughout vertebrate organogenesis.

Gap Junction-Dependent and -Independent Functions of Connexin43 in Biology

For the first time in animal evolution, the emergence of gap junctions allowed direct exchanges of cellular substances for communication between two cells. Innexin proteins constituted primordial gap junctions until the connexin protein emerged in deuterostomes and took over the gap junction function. After hundreds of millions of years of gene duplication, the connexin gene family now comprises 21 members in the human genome. Notably, GJA1, which encodes the Connexin43 protein, is one of the most widely expressed and commonly studied connexin genes. The loss of Gja1 in mice leads to swelling and a blockage of the right ventricular outflow tract and death of the embryos at birth, suggesting a vital role of Connexin43 gap junction in heart development. Since then, the importance of Connexin43-mediated gap junction function has been constantly expanded to other types of cells. Other than forming gap junctions, Connexin43 can also form hemichannels to release or uptake small molecules from the environment or even mediate many physiological processes in a gap junction-independent manner on plasma membranes. Surprisingly, Connexin43 also localizes to mitochondria in the cell, playing important roles in mitochondrial potassium import and respiration. At the molecular level, Connexin43 mRNA and protein are processed with very distinct mechanisms to yield carboxyl-terminal fragments with different sizes, which have their unique subcellular localization and distinct biological activities. Due to many exciting advancements in Connexin43 research, this review aims to start with a brief introduction of Connexin43 and then focuses on updating our knowledge of its gap junction-independent functions.

Glioblastoma-Astrocyte Connexin 43 Gap Junctions Promote Tumor Invasion

Glioblastoma multiforme (GBM), classified as World Health Organization grade IV astrocytoma, is the deadliest adult cancer of the central nervous system. An important contributing factor to poor survival rates in GBM is extensive invasion, which decreases the efficacy of resection and subsequent adjuvant therapies. These treatments could be markedly improved with increased resolution of the genetic and molecular initiators and effectors of invasion. Connexin 43 (Cx43) is the principal astrocytic gap junction (GJ) protein. Despite the heterogeneity of GBM, a subpopulation of cells in almost all GBM tumors express Cx43. Functional GJs between GBM cells and astrocytes at the tumor edge are of critical interest for understanding invasion. In this study, we find that both in vitro and in ex vivo slice cultures, GBM is substantially less invasive when placed in a Cx43-deficient astrocyte environment. Furthermore, when Cx43 is deleted in GBM, the invasive phenotype is recovered. These data strongly suggest that there are opposing roles for Cx43 in GBM migration. We find that Cx43 is localized to the tumor edge in our ex vivo model, suggesting that GBM-astrocyte GJ communication at the tumor border is a driving force for invasion. Finally, we find that by a Cx43-dependent mechanism, but likely not direct channel-mediated diffusion, miRNAs associated with cell-matrix adhesion are transferred from GBM to astrocytes and miR-19b promotes invasion, revealing a role for post-transcriptional manipulation of astrocytes in fostering an invasion-permissive peritumoral niche. IMPLICATIONS: Cx43-mediated communication, specifically miRNA transfer, profoundly impacts glioblastoma invasion and may enable further therapeutic insight.

Limited Impact of Murine Placental MDR1 on Fetal Exposure of Certain Drugs Explained by Bypass Transfer Between Adjacent Syncytiotrophoblast Layers

Purpose

Multidrug resistance protein 1 (MDR1) is located at the interface between two syncytiotrophoblast layers in rodent placenta, and may influence fetal drug distribution. Here, we quantitatively compare the functional impact per single MDR1 molecule of MDR1 at the placental barrier and blood-brain barrier in mice.

Methods

MDR1A and MDR1B proteins were quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Paclitaxel or digoxin was continuously administered to pregnant Mdr1a^−/−/Mdr1b^−/− or wild-type mice, and the drug concentrations in the maternal and fetal plasma and maternal brain were quantified by LC-MS/MS.

Results

MDR1A and MDR1B proteins are expressed in the membrane of mouse placental labyrinth, and total MDR1 at the placental barrier amounts to about 30% of that at the blood-brain barrier. The fetal-to-maternal plasma concentration ratio of digoxin was only marginally affected in Mdr1a^−/−/Mdr1b^−/− mice, while that of paclitaxel showed a several-fold increase. No such difference between the two drugs was found in the maternal brain distribution. The impact per single MDR1 molecule on the fetal distribution of digoxin was calculated to be much lower than that on the brain distribution, but this was not the case for paclitaxel. Our pharmacokinetic model indicates that the impact of placental MDR1 is inversely correlated to the ratio of permeability through gap junctions connecting the two syncytiotrophoblast layers to passive diffusion permeability.

Conclusion

Our findings indicate that murine placental MDR1 has a minimal influence on the fetal concentration of certain substrates, such as digoxin, due to bypass transfer, probably via connexin26 gap junctions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11095-022-03165-6.

The role of connexins in breast cancer: from misregulated cell communication to aberrant intracellular signaling

In spite of clinical advancements and improved diagnostic techniques, breast cancers are the leading cause of cancer-associated deaths in women worldwide. Although 70% of early breast cancers can be cured, there are no efficient therapies against metastatic breast cancers. Several factors including connexins and gap junctions play roles in breast tumorigenesis. Connexins are critical for cellular processes as a linkage between connexin mutations and hereditary disorders demonstrated their importance for tissue homeostasis. Further, alterations in their expression, localization and channel activities were observed in many cancers including breast cancer. Both channel-dependent and independent functions of connexins were reported in initiation and progression of cancers. Unlike initial reports suggesting tumor suppressor functions, connexins and gap junctions have stage, context and isoform dependent effects in breast cancers similar to other cancers. In this review, we tried to describe the current understanding of connexins in tumorigenesis specifically in breast cancers.

Connexin 43 Gene Ablation Does Not Alter Human Pluripotent Stem Cell Germ Lineage Specification

During embryonic germ layer development, cells communicate with each other and their environment to ensure proper lineage specification and tissue development. Connexin (Cx) proteins facilitate direct cell–cell communication through gap junction channels. While previous reports suggest that gap junctional intercellular communication may contribute to germ layer formation, there have been limited comprehensive expression analyses or genetic ablation studies on Cxs during human pluripotent stem cell (PSC) germ lineage specification. We screened the mRNA profile and protein expression patterns of select human Cx isoforms in undifferentiated human induced pluripotent stem cells (iPSCs), and after directed differentiation into the three embryonic germ lineages: ectoderm, definitive endoderm, and mesoderm. Transcript analyses by qPCR revealed upregulation of Cx45 and Cx62 in iPSC-derived ectoderm; Cx45 in mesoderm; and Cx30.3, Cx31, Cx32, Cx36, Cx37, and Cx40 in endoderm relative to control human iPSCs. Generated Cx43 (GJA1) CRISPR-Cas9 knockout iPSCs successfully differentiated into cells of all three germ layers, suggesting that Cx43 is dispensable during directed iPSC lineage specification. Furthermore, qPCR screening of select Cx transcripts in our GJA1-/- iPSCs showed no significant Cx upregulation in response to the loss of Cx43 protein. Future studies will reveal possible compensation by additional Cxs, suggesting targets for future CRISPR-Cas9 ablation studies in human iPSC lineage specification.

Engaging biological oscillators through second messenger pathways permits emergence of a robust gastric slow-wave during peristalsis

Peristalsis, the coordinated contraction—relaxation of the muscles of the stomach is important for normal gastric motility and is impaired in motility disorders. Coordinated electrical depolarizations that originate and propagate within a network of interconnected layers of interstitial cells of Cajal (ICC) and smooth muscle (SM) cells of the stomach wall as a slow-wave, underly peristalsis. Normally, the gastric slow-wave oscillates with a single period and uniform rostrocaudal lag, exhibiting network entrainment. Understanding of the integrative role of neurotransmission and intercellular coupling in the propagation of an entrained gastric slow-wave, important for understanding motility disorders, however, remains incomplete. Using a computational framework constituted of a novel gastric motility network (GMN) model we address the hypothesis that engaging biological oscillators (i.e., ICCs) by constitutive gap junction coupling mechanisms and enteric neural innervation activated signals can confer a robust entrained gastric slow-wave. We demonstrate that while a decreasing enteric neural innervation gradient that modulates the intracellular IP₃ concentration in the ICCs can guide the aboral slow-wave propagation essential for peristalsis, engaging ICCs by recruiting the exchange of second messengers (inositol trisphosphate (IP₃) and Ca²⁺) ensures a robust entrained longitudinal slow-wave, even in the presence of biological variability in electrical coupling strengths. Our GMN with the distinct intercellular coupling in conjunction with the intracellular feedback pathways and a rostrocaudal enteric neural innervation gradient allows gastric slow waves to oscillate with a moderate range of frequencies and to propagate with a broad range of velocities, thus preventing decoupling observed in motility disorders. Overall, the findings provide a mechanistic explanation for the emergence of decoupled slow waves associated with motility impairments of the stomach, offer directions for future experiments and theoretical work, and can potentially aid in the design of new interventional pharmacological and neuromodulation device treatments for addressing gastric motility disorders.

The coordinated contraction and relaxation of the muscles of the stomach, known as peristalsis is important for normal gastric motility and primarily governed by electrical depolarizations that originate and propagate within a network of interconnected layers of interstitial cells of Cajal (ICCs) and smooth muscle cells of the stomach wall as a slow-wave. Under normal conditions, a gastric slow-wave oscillates with a single period and uniform rostrocaudal lag, exhibiting network entrainment. However, the understanding of intrinsic and extrinsic mechanisms that ensure propagation of a robust entrained slow-wave remains incomplete. Here, using a computational framework, we show that in conjunction with an enteric neural innervation gradient along the rostrocaudal ICC chain, and intercellular electrical coupling, the intercellular exchange of inositol trisphosphate between ICCs prevents decoupling by extending the longitudinal entrainment range along the stomach wall, even when variability in intercellular coupling exists. The findings from our study indicate ways that ensure the rostrocaudal spread of a robust gastric slow-wave and provide a mechanistic explanation for the emergence of decoupled slow waves associated with motility impairments of the stomach.

A new side-effect of sufentanil: increased monocyte-endothelial adhesion

BACKGROUND

Opioids have been identified by the World Health Organization to be 'indispensable for the relief of pain and suffering'. Side-effects, such as nausea, vomiting, postoperative delirium, and effects on breathing, of opioids have been well investigated; however, the influence of opioids on monocyte-endothelial adherence has never been reported. Therefore, we explored the effects of representative opioids, fentanyl, sufentanil, and remifentanil, on monocyte-endothelial adherence and the underlying mechanisms.

METHODS

We built a cell adhesion model with U937 monocytes and human umbilical vein endothelial cells (HUVECs). Two kinds of connexin43 (Cx43) channel inhibitors, 18-α-GA and Gap 27, were used to alter Cx43 channel function in U937 monocytes and HUVECs, respectively, to determine the effects of Cx43 channels on U937-HUVEC adhesion. Subsequently, the effects of fentanyl, sufentanil and remifentanil on Cx43 channel function and U937-HUVEC adhesion were explored.

RESULTS

When fentanyl, sufentanil and remifentanil acted on monocytes or endothelial cells, their effects on monocyte-endothelial adherence differed. When acting on U937 monocytes, sufentanil significantly increased U937-HUVEC adhesion which was associated with reduced release of ATP from Cx43 channels, while fentanyl and remifentanil did not have these influences. Although sufentanil could also inhibit Cx43 channel function in HUVECs, it had no effect on ATP release from HUVECs or U937-HUVECs adhesion.

CONCLUSIONS

We demonstrated that sufentanil application increases monocyte-endothelial adherence which was associated with reduced release of ATP from Cx43 channels in monocytes. This side-effect of sufentanil should be considered seriously by clinicians.

Atrial Fibrillation in Heart Failure Is Associated with High Levels of Circulating microRNA-199a-5p and 22-5p and a Defective Regulation of Intracellular Calcium and Cell-to-Cell Communication

MicroRNAs (miRNAs) participate in atrial remodeling and atrial fibrillation (AF) promotion. We determined the circulating miRNA profile in patients with AF and heart failure with reduced ejection fraction (HFrEF), and its potential role in promoting the arrhythmia. In plasma of 98 patients with HFrEF (49 with AF and 49 in sinus rhythm, SR), differential miRNA expression was determined by high-throughput microarray analysis followed by replication of selected candidates. Validated miRNAs were determined in human atrial samples, and potential arrhythmogenic mechanisms studied in HL-1 cells. Circulating miR-199a-5p and miR-22-5p were significantly increased in HFrEF patients with AF versus those with HFrEF in SR. Both miRNAs, but particularly miR-199a-5p, were increased in atrial samples of patients with AF. Overexpression of both miRNAs in HL-1 cells resulted in decreased protein levels of L-type Ca²⁺ channel, NCX and connexin-40, leading to lower basal intracellular Ca²⁺ levels, fewer inward currents, a moderate reduction in Ca²⁺ buffering post-caffeine exposure, and a deficient cell-to-cell communication. In conclusion, circulating miR-199a-5p and miR-22-5p are higher in HFrEF patients with AF, with similar findings in human atrial samples of AF patients. Cells exposed to both miRNAs exhibited altered Ca²⁺ handling and defective cell-to-cell communication, both findings being potential arrhythmogenic mechanisms.

Mechanisms of Connexin Regulating Peptides

Gap junctions (GJ) and connexins play integral roles in cellular physiology and have been found to be involved in multiple pathophysiological states from cancer to cardiovascular disease. Studies over the last 60 years have demonstrated the utility of altering GJ signaling pathways in experimental models, which has led to them being attractive targets for therapeutic intervention. A number of different mechanisms have been proposed to regulate GJ signaling, including channel blocking, enhancing channel open state, and disrupting protein-protein interactions. The primary mechanism for this has been through the design of numerous peptides as therapeutics, that are either currently in early development or are in various stages of clinical trials. Despite over 25 years of research into connexin targeting peptides, the overall mechanisms of action are still poorly understood. In this overview, we discuss published connexin targeting peptides, their reported mechanisms of action, and the potential for these molecules in the treatment of disease.

Centrosome, the Newly Identified Passenger through Tunneling Nanotubes, Increases Binucleation and Proliferation Marker in Receiving Cells

Type 1 tunneling nanotubes (TNTs-1) are long, cytoplasmic protrusions containing actin, microtubules and intermediate filaments that provide a bi-directional road for the transport of various components between distant cells. TNT-1 formation is accompanied by dramatic cytoskeletal reorganization offering mechanical support for intercellular communication. Although the centrosome is the major microtubule nucleating center and also a signaling hub, the relationship between the centrosome and TNTs-1 is still unexplored. We provide here the first evidence of centrosome localization and orientation towards the TNTs-1 protrusion site, which is implicated in TNT-1 formation. We also envision a model whereby synchronized reorientation of the Golgi apparatus along with the centrosome towards TNTs-1 ensures effective polarized trafficking through TNTs-1. Furthermore, using immunohistochemistry and live imaging, we observed for the first time the movement of an extra centrosome within TNTs-1. In this regard, we hypothesize a novel role for TNTs-1 as a critical pathway serving to displace extra centrosomes and potentially to either protect malignant cells against aberrant centrosome amplification or contribute to altering cells in the tumor environment. Indeed, we have observed the increase in binucleation and proliferation markers in receiving cells. The fact that the centrosome can be both as the base and the user of TNTs-1 offers new perspectives and new opportunities to follow in order to improve our knowledge of the pathophysiological mechanisms under TNT control.

Role of folate-conjugated glycol-chitosan nanoparticles in modulating the activated macrophages to ameliorate inflammatory arthritis: in vitro and in vivo activities

Activated macrophages are the primary targets in rheumatoid arthritis (RA) management. So, we report efficacious, dual-functional Methotrexate (MTX) loaded folate-conjugated pH-responsive glycol-chitosan nanoparticles (MFGCN) prepared by nano-precipitation and zero-order cross-linking reaction for targeting inflamed arthritic tissue. Physical characterization by DLS, SEM and TEM indicated a spherical, smooth morphology with a diameter ~ 300 nm. ¹H NMR and FTIR indicated folic acid conjugation to GC by zero-order cross-linkers. In vitro release kinetics in PBS showed pH-responsive and sustained release behaviour of MFGCN. Enhanced cellular uptake and cytotoxicity of MFGCN in LPS(+)RAW and activated peritoneal macrophages (Mϕ) were observed when compared to LPS(-)RAW cells. MFGCN-induced mitochondrial membrane potential (MMP) perturbations indicated apoptosis. Oxidative stress was evident by significant increase in ROS and RNS, 4 h post incubation with MFGCN. Negligible hemolysis by FGCN and MFGCN on rat RBC's indicated biocompatibility. In vivo biodistribution of MFGCN in adjuvant-induced arthritis (AIA) rats indicated RA targetability. Prolonged blood circulation coupled with higher concentrations of ^99mTc-MFGCN at the arthritic site was observed post 24 h of injection. The gamma scintigraphic image confirmed accumulation of radiolabelled MFGCN in arthritic paw when compared to the non-inflamed paw, confirming the selective uptake of ^99mTc-MFGCN by folate-overexpressing macrophages in the arthritic synovium thereby proving its targeted efficacy and theranostic potential. In AIA rats, MFGCN lowers arthritic signs, improves antioxidant response and decreases pro-inflammatory cytokines, suggesting its potential in targeting activated macrophages of synovium. Graphical abstract.

Approaches to Study Gap Junctional Coupling

Astrocytes and oligodendrocytes are main players in the brain to ensure ion and neurotransmitter homeostasis, metabolic supply, and fast action potential propagation in axons. These functions are fostered by the formation of large syncytia in which mainly astrocytes and oligodendrocytes are directly coupled. Panglial networks constitute on connexin-based gap junctions in the membranes of neighboring cells that allow the passage of ions, metabolites, and currents. However, these networks are not uniform but exhibit a brain region-dependent heterogeneous connectivity influencing electrical communication and intercellular ion spread. Here, we describe different approaches to analyze gap junctional communication in acute tissue slices that can be implemented easily in most electrophysiology and imaging laboratories. These approaches include paired recordings, determination of syncytial isopotentiality, tracer coupling followed by analysis of network topography, and wide field imaging of ion sensitive dyes. These approaches are capable to reveal cellular heterogeneity causing electrical isolation of functional circuits, reduced ion-transfer between different cell types, and anisotropy of tracer coupling. With a selective or combinatory use of these methods, the results will shed light on cellular properties of glial cells and their contribution to neuronal function.

The Role of Connexin 43 in Lung Disease

The term lung disease describes a broad category of disorders that impair lung function. More than 35 million Americans have a preventable chronic lung disease with high mortality rates due to limited treatment efficacy. The recent increase in patients with lung disease highlights the need to increase our understanding of mechanisms driving lung inflammation. Connexins, gap junction proteins, and more specifically connexin 43 (Cx43), are abundantly expressed in the lung and are known to play a role in lung diseases. This review focuses on the role of Cx43 in pathology associated with acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD) and asthma. Additionally, we discuss the role of Cx43 in preventing disease through the transfer of mitochondria between cells. We aim to highlight the need to better understand what cell types are expressing Cx43 and how this expression influences lung disease.

Antigen-loaded monocyte administration induces potent therapeutic antitumor T cell responses

Efficacy of dendritic cell (DC) cancer vaccines is classically thought to depend on their antigen-presenting cell (APC) activity. Studies show, however, that DC vaccine priming of cytotoxic T lymphocytes (CTLs) requires the activity of endogenous DCs, suggesting that exogenous DCs stimulate antitumor immunity by transferring antigens (Ags) to endogenous DCs. Such Ag transfer functions are most commonly ascribed to monocytes, implying that undifferentiated monocytes would function equally well as a vaccine modality and need not be differentiated to DCs to be effective. Here, we used several murine cancer models to test the antitumor efficacy of undifferentiated monocytes loaded with protein or peptide Ag. Intravenously injected monocytes displayed antitumor activity superior to DC vaccines in several cancer models, including aggressive intracranial glioblastoma. Ag-loaded monocytes induced robust CTL responses via Ag transfer to splenic CD8+ DCs in a manner independent of monocyte APC activity. Ag transfer required cell-cell contact and the formation of connexin 43-containing gap junctions between monocytes and DCs. These findings demonstrate the existence of an efficient gap junction-mediated Ag transfer pathway between monocytes and CD8+ DCs and suggest that administration of tumor Ag-loaded undifferentiated monocytes may serve as a simple and efficacious immunotherapy for the treatment of human cancers.

Impact of Four Common Hydrogels on Amyloid-β (Aβ) Aggregation and Cytotoxicity: Implications for 3D Models of Alzheimer's Disease

The physiochemical properties of hydrogels utilized in 3D culture can be used to modulate cell phenotype and morphology with a striking resemblance to cellular processes that occur in vivo. Indeed, research areas including regenerative medicine, tissue engineering, in vitro cancer models, and stem cell differentiation have readily utilized 3D biomaterials to investigate cell biological questions. However, cells are only one component of this biomimetic milieu. In many models of disease such as Alzheimer's disease (AD) that could benefit from the in vivo-like cell morphology associated with 3D culture, other aspects of the disease such as protein aggregation have yet to be methodically considered in this 3D context. A hallmark of AD is the accumulation of the peptide amyloid-β (Aβ), whose aggregation is associated with neurotoxicity. We have previously demonstrated the attenuation of Aβ cytotoxicity when cells were cultured within type I collagen hydrogels versus on 2D substrates. In this work, we investigated the extent to which this phenomenon is conserved when Aβ is confined within hydrogels of varying physiochemical properties, notably mesh size and bioactivity. We investigated the Aβ structure and aggregation kinetics in solution and hydrogels composed of type I collagen, agarose, hyaluronic acid, and polyethylene glycol using fluorescence correlation spectroscopy and thioflavin T assays. Our results reveal that all hydrogels tested were associated with enhanced Aβ aggregation and Aβ cytotoxicity attenuation. We suggest that confinement itself imparts a profound effect, possibly by stabilizing Aβ structures and shifting the aggregate equilibrium toward larger species. If this phenomenon of altered protein aggregation in 3D hydrogels can be generalized to other contexts including the in vivo environment, it may be necessary to reevaluate aspects of protein aggregation disease models used for drug discovery.

Collagen hydrogel confinement of Amyloid-β (Aβ) accelerates aggregation and reduces cytotoxic effects

Alzheimer's disease (AD) is the most common form of dementia and is associated with the accumulation of amyloid-β (Aβ), a peptide whose aggregation has been associated with neurotoxicity. Drugs targeting Aβ have shown great promise in 2D in vitro models and mouse models, yet preclinical and clinical trials for AD have been highly disappointing. We propose that current in vitro culture systems for discovering and developing AD drugs have significant limitations; specifically, that Aβ aggregation is vastly different in these 2D cultures carried out on flat plastic or glass substrates vs. in a 3D environment, such as brain tissue, where Aβ confinement alters aggregation kinetics and thermodynamics. In this work, we identified attenuation of Aβ cytotoxicity in 3D hydrogel culture compared to 2D cell culture. We investigated Aβ structure and aggregation in solution vs. hydrogel using Transmission Electron Microscopy (TEM), Fluorescence Correlation Spectroscopy (FCS), and Thioflavin T (ThT) assays. Our results reveal that the equilibrium is shifted to stable extended β-sheet (ThT positive) aggregates in hydrogels and away from the relatively unstable/unstructured presumed toxic oligomeric Aβ species in solution. Volume exclusion imparted by hydrogel confinement stabilizes unfolded, presumably toxic species, promoting stable extended β-sheet fibrils. STATEMENT OF SIGNIFICANCE: Alzheimer's disease (AD) is a devastating disease and has been studied for over 100 years. Yet, no cure exists and only 5 prescription drugs are FDA-approved to temporarily treat the AD symptoms of declining brain functions related to thinking and memory. Why don't we have more effective treatments to cure AD or relieve AD symptoms? We propose that current culture methods based upon cells cultured on flat, stiff substrates have significant limitations for discovering and developing AD drugs. This study provides strong evidence that AD drugs should be tested in 3D culture systems as a step along the development pathway towards new, more effective drugs to treat AD.

Evolutionary origins of chemical synapses

Synaptic transmission is a fundamental neurobiological process by which neurons interact with each other and non-neuronal cells. It involves release of active substances from the presynaptic neuron onto receptive elements of postsynaptic cells, inducing waves of spreading electrochemical response. While much has been learned about the cellular and molecular mechanisms driving and governing transmitter release and sensing, the evolutionary origin of synaptic connections remains obscure. Herein, we review emerging evidence and concepts suggesting that key components of chemical synapse arose independently from neurons, in different functional and biological contexts, before the rise of multicellular living forms. We argue that throughout evolution, distinct synaptic constituents have been co-opted from ancestral forms for a new role in early metazoan, leading to the rise of chemical synapses and neurotransmission. Such a mosaic model of the origin of chemical synapses agrees with and supports the pluralistic hypothesis of evolutionary change.

Permeation of Molecules through Astroglial Connexin 43 Hemichannels Is Modulated by Cytokines with Parameters Depending on the Permeant Species

Recent studies indicate that connexin hemichannels do not act as freely permeable non-selective pores, but they select permeants in an isoform-specific manner with cooperative, competitive and saturable kinetics. The aim of this study was to investigate whether the treatment with a mixture of IL-1β plus TNF-α, a well-known pro-inflammatory condition that activates astroglial connexin 43 (Cx43) hemichannels, could alter their permeability to molecules. We found that IL-1β plus TNF-α left-shifted the dye uptake rate vs. dye concentration relationship for Etd and 2-NBDG, but the opposite took place for DAPI or YO-PRO-1, whereas no alterations were observed for Prd. The latter modifications were accompanied of changes in K_d (Etd, DAPI, YO-PRO-1 or 2-NBDG) and Hill coefficients (Etd and YO-PRO-1), but not in alterations of V_max. We speculate that IL-1β plus TNF-α may distinctively affect the binding sites to permeants in astroglial Cx43 hemichannels rather than their number in the cell surface. Alternatively, IL-1β plus TNF-α could induce the production of endogenous permeants that may favor or compete for in the pore-lining residues of Cx43 hemichannels. Future studies shall elucidate whether the differential ionic/molecule permeation of Cx43 hemichannels in astrocytes could impact their communication with neurons in the normal and inflamed nervous system.

Cisplatin-induced ototoxicity in organotypic cochlear cultures occurs independent of gap junctional intercellular communication

Cisplatin is a very effective chemotherapeutic, but severe and permanent hearing loss remains a prevalent side effect. The processes underpinning cisplatin-induced ototoxicity are not well understood. Gap junction channels composed of connexin (Cx) subunits allow for the passage of small molecules and ions between contacting neighboring cells. These specialized channels have been postulated to enhance cisplatin-induced cell death by spreading “death signals” throughout the supporting cells of the organ of Corti. This study sought to investigate the role of Cx43 in cisplatin-induced ototoxicity using organotypic cochlear cultures from control and two Cx43-mutant mouse strains harboring either a moderate (Cx43^I130T/+) or severe (Cx43^G60S/+) reduction of Cx43 function. Cochlear cultures from Cx43-mutant mice with a severe reduction in Cx43-based gap junctional intercellular communication (GJIC) had an enhanced number of hair cells that were positive for cleaved caspase 3, a marker of active apoptosis, after cisplatin treatment. In cisplatin-treated organotypic cochlear cultures, there was a decrease in the co-localization of Cx26 and Cx30 compared with untreated cultures, suggesting that cisplatin causes reorganization of connexin composition in supporting cells. Both Cx26 and Cx30 protein expression as well as GJIC were decreased in organotypic cochlear cultures treated with the gap-junction blocker carbenoxolone. When cisplatin and carbenoxolone were co-administered, there were no differences in hair cell loss compared with cisplatin treatment alone. Using cisplatin-treated control and Cx43-ablated organ of Corti derived HEI-OC1 mouse cells, we found that greatly reducing GJIC led to preferential induction of an ER stress pathway. Taken together, this study strongly suggests that inhibition of GJIC in organ of Corti cells does not lead to differential susceptibility to cisplatin-induced ototoxicity. Although cisplatin causes the same degree of cell death in gap junction competent and incompetent cochlear cells, the engagement of the mitochondrial dysregulation and ER stress differs.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

The diffusive injection micropipette (DIMP)

The microinjection of fluorescent probes into live cells is an essential component in the toolbox of modern cell biology. Microinjection techniques include the penetration of the plasma membrane and, if present, the cell wall with micropipettes, and the application of pressure or electrical currents to drive the micropipette contents into the cell. These procedures interfere with cellular functions and therefore may induce artifacts. We designed the diffusive injection micropipette (DIMP) that avoids most of the possible artifacts due to the drastically reduced volume of its fluid contents and the utilization of diffusion for cargo delivery into the target cell. DIMPs were successfully tested in plant, fungal, and animal cells. Using the continuity of cytoplasmic dynamics over ten minutes after impalement of Nicotiana trichome cells as a criterion for non-invasiveness, we found DIMPs significantly less disruptive than conventional pressure microinjection. The design of DIMPs abolishes major sources of artifacts that cannot be avoided by other microinjection techniques. Moreover, DIMPs are inexpensive, easy to produce, and can be applied without specific equipment other than a micromanipulator. With these features, DIMPs may become the tool of choice for studies that require the least invasive delivery possible of materials into live cells.

Structural determinants underlying permeant discrimination of the Cx43 hemichannel

Connexin (Cx) gap junction channels comprise two hemichannels in neighboring cells, and their permeability is well-described, but permeabilities of the single Cx hemichannel remain largely unresolved. Moreover, determination of isoform-specific Cx hemichannel permeability is challenging because of concurrent expression of other channels with similar permeability profiles and inhibitor sensitivities. The mammalian Cx hemichannels Cx30 and Cx43 are gated by extracellular divalent cations, removal of which promotes fluorescent dye uptake in both channels but atomic ion conductance only through Cx30. To determine the molecular determinants of this difference, here we employed chimeras and mutagenesis of predicted pore-lining residues in Cx43. We expressed the mutated channels in Xenopus laevis oocytes to avoid background activity of alternative channels. Oocytes expressing a Cx43 hemichannel chimera containing the N terminus or the first extracellular loop from Cx30 displayed ethidium uptake and, unlike WT Cx43, ion conduction, an observation further supported by molecular dynamics simulations. Additional C-terminal truncation of the chimeric Cx43 hemichannel elicited an even greater ion conductance with a magnitude closer to that of Cx30. The inhibitory profile for the connexin hemichannels depended on the permeant, with conventional connexin hemichannel inhibitors having a higher potency toward the ion conductance pathway than toward fluorescent dye uptake. Our results demonstrate a permeant-dependent, isoform-specific inhibition of connexin hemichannels. They further reveal that the outer segments of the pore-lining region, including the N terminus and the first extracellular loop, together with the C terminus preclude ion conductance of the open Cx43 hemichannel.