Cellular and molecular mechanisms underlying plasma membrane functionality and integrity.

The plasma membrane not only protects the cell from the extracellular environment, acting as a selective barrier, but also regulates cellular events that originate at the cell surface, playing a key role in various biological processes that are essential for the preservation of cell homeostasis. Therefore, elucidation of the mechanisms involved in the maintenance of plasma membrane integrity and functionality is of utmost importance. Cells have developed mechanisms to ensure the quality of proteins that inhabit the cell surface, as well as strategies to cope with injuries inflicted to the plasma membrane. Defects in these mechanisms can lead to the development or onset of several diseases. Despite the importance of these processes, a comprehensive and holistic perspective of plasma membrane quality control is still lacking. To tackle this gap, in this Review, we provide a thorough overview of the mechanisms underlying the identification and targeting of membrane proteins that are to be removed from the cell surface, as well as the membrane repair mechanisms triggered in both physiological and pathological conditions. A better understanding of the mechanisms underlying protein quality control at the plasma membrane can reveal promising and unanticipated targets for the development of innovative therapeutic approaches.

The Role of Proteostasis in the Regulation of Cardiac Intercellular Communication.

Given the low mitotic activity of cardiomyocytes, the contractile unit of the heart, these cells strongly rely on efficient and highly regulated mechanisms of protein degradation to eliminate unwanted potentially toxic proteins. This is particularly important in the context of disease, where an impairment of protein quality control mechanisms underlies the onset and development of diverse cardiovascular maladies. One of the biological processes which is tightly regulated by proteolysis mechanisms is intercellular communication. The different types of cells that form the heart, including cardiomyocytes, endothelial cells, fibroblasts, and macrophages, can communicate directly, through gap junctions (GJ) or tunneling nanotubes (TNT), or at long distances, via extracellular vesicles (EV) or soluble factors.The direct communication between cardiomyocytes is vital to ensure the anisotropic propagation of the electrical impulse, which allows the heart to beat in a coordinated and synchronized manner, as a functional syncytium. The rapid and efficient propagation of the depolarization wave is mainly conducted by low resistance channels called GJ, formed by six subunits of a family of proteins named Cxs. Dysfunctional GJ intercellular communication, due to increased degradation and/or redistribution of connexin43 (Cx43), the main Cx present in the heart, has been associated with several cardiac disorders, such as myocardial ischemia, hypertrophy, arrhythmia, and heart failure. Besides electrical coupling, a fine-tuned exchange of information, namely proteins and microRNAs, conveyed by EV is important to ensure organ function and homeostasis. Disease-induced deregulation of EV-mediated communication between cardiac cells has been implicated in diverse processes such as inflammation, angiogenesis, and fibrosis. Therefore, a better understanding of the mechanisms whereby proteolysis modulates the cross talk between cardiac cells is of utmost importance to develop new strategies to tackle diseases caused by defects in intercellular communication.

Canonical and Non-Canonical Roles of Connexin43 in Cardioprotection.

Since the mid-20th century, ischemic heart disease has been the world's leading cause of death. Developing effective clinical cardioprotection strategies would make a significant impact in improving both quality of life and longevity in the worldwide population. Both ex vivo and in vivo animal models of cardiac ischemia/reperfusion (I/R) injury are robustly used in research. Connexin43 (Cx43), the predominant gap junction channel-forming protein in cardiomyocytes, has emerged as a cardioprotective target. Cx43 posttranslational modifications as well as cellular distribution are altered during cardiac reperfusion injury, inducing phosphorylation states and localization detrimental to maintaining intercellular communication and cardiac conduction. Pre- (before ischemia) and post- (after ischemia but before reperfusion) conditioning can abrogate this injury process, preserving Cx43 and reducing cell death. Pre-/post-conditioning has been shown to largely rely on the presence of Cx43, including mitochondrial Cx43, which is implicated to play a major role in pre-conditioning. Posttranslational modifications of Cx43 after injury alter the protein interactome, inducing negative protein cascades and altering protein trafficking, which then causes further damage post-I/R injury. Recently, several peptides based on the Cx43 sequence have been found to successfully diminish cardiac injury in pre-clinical studies.

Biological Functions of Connexin43 Beyond Intercellular Communication.

Connexin43 (Cx43) is commonly associated with direct cell-cell communication through gap junctions (GJs). However, recent groundbreaking studies have challenged this dogma, implicating Cx43 in other biological processes, such as transcription, metabolism, autophagy, and ion channel trafficking. How Cx43 participates in these processes remains largely unknown, although its high turnover rate, capacity to bind to myriad proteins, and the discovery of truncated isoforms of Cx43, ascribe to this protein unanticipated roles in chief processes that require fine-tuned regulation. Accordingly, Cx43 can be regarded as a central integrative hub to which diverse cues converge to be processed in a concerted manner. In this review, we examine the noncanonical roles of Cx43 and discuss the implications of these functions in human diseases and future therapeutic strategies.

Exosomes secreted by cardiomyocytes subjected to ischaemia promote cardiac angiogenesis.

AIMS: Myocardial infarction (MI) is the leading cause of morbidity and mortality worldwide and results from an obstruction in the blood supply to a region of the heart. In an attempt to replenish oxygen and nutrients to the deprived area, affected cells release signals to promote the development of new vessels and confer protection against MI. However, the mechanisms underlying the growth of new vessels in an ischaemic scenario remain poorly understood. Here, we show that cardiomyocytes subjected to ischaemia release exosomes that elicit an angiogenic response of endothelial cells (ECs). METHODS AND RESULTS: Exosomes secreted by H9c2 myocardial cells and primary cardiomyocytes, cultured either in control or ischaemic conditions were isolated and added to ECs. We show that ischaemic exosomes, in comparison with control exosomes, confer protection against oxidative-induced lesion, promote proliferation, and sprouting of ECs, stimulate the formation of capillary-like structures and strengthen adhesion complexes and barrier properties. Moreover, ischaemic exosomes display higher levels of metalloproteases (MMP) and promote the secretion of MMP by ECs. We demonstrate that miR-222 and miR-143, the relatively most abundant miRs in ischaemic exosomes, partially recapitulate the angiogenic effect of exosomes. Additionally, we show that ischaemic exosomes stimulate the formation of new functional vessels in vivo using in ovo and Matrigel plug assays. Finally, we demonstrate that intramyocardial delivery of ischaemic exosomes improves neovascularization following MI. CONCLUSIONS: This study establishes that exosomes secreted by cardiomyocytes under ischaemic conditions promote heart angiogenesis, which may pave the way towards the development of add-on therapies to enhance myocardial blood supply.