Beyond its well-documented role in vesicle endocytosis, clathrin has also been implicated in the internalization of large particles such as viruses, pathogenic bacteria, and even latex beads. We have discovered an additional clathrin-dependent endocytic process that results in the internalization of large, double-membrane vesicles at lateral membranes of cells that are coupled by gap junctions (GJs). GJ channels bridge apposing cell membranes to mediate the direct transfer of electrical currents and signaling molecules from cell to cell. Here, we report that entire GJ plaques, clusters of GJ channels, can be internalized to form large, double-membrane vesicles previously termed annular gap junctions (AGJs). These internalized AGJ vesicles subdivide into smaller vesicles that are degraded by endo/lysosomal pathways. Mechanistic analyses revealed that clathrin-dependent endocytosis machinery-components, including clathrin itself, the alternative clathrin-adaptor Dab2, dynamin, myosin-VI, and actin are involved in the internalization, inward movement, and degradation of these large, intercellular double-membrane vesicles. These findings contribute to the understanding of clathrin's numerous emerging functions. INTRODUCTIONThe role of clathrin in endocytosis is well documented. This protein forms a typical curved lattice around endocytic vesicles that are internalized at the plasma membrane (PM). In addition, the involvement of clathrin in several uncharacteristic endocytic processes has been reported, including the internalization of viruses, pathogenic bacteria, and large latex beads (Aggeler and Werb, 1982;Ehrlich et al., 2004;Rust et al., 2004;Veiga and Cossart, 2005). Here, we describe another function of the clathrin-dependent endocytic machinery that results in the internalization of large, doublemembrane vesicles at lateral PMs of cells that are coupled by gap junctions (GJs).GJs are ubiquitously distributed channels that connect the cytoplasms of two apposing cells each participating in this connection via a half channel termed a connexon to provide direct cell-to-cell communication. Connexons are hexamers of four-pass membrane proteins called connexins (Cxs;Bruzzone et al., 1996;Kumar and Gilula, 1996). Once transported to the PM, GJ channels cluster into two-dimensional arrays termed plaques that can be composed of a few to many thousands of individual channels and vary from a few square nanometers to many square micrometers (Bruzzone et al., 1996; Falk, 2000a;Severs et al., 2001). GJ channels can open and close (gate) and physiological parameters, including intracellular pH, Ca 2ϩ concentration, and Cx phosphorylation, are known to modulate GJ channel gating and the extent of GJ-mediated intercellular coupling (Delmar et al., 2004;Lampe and Lau, 2004;Moreno, 2005). However, the extent of intercellular coupling could also be regulated through altering the number of GJ channels in the PM.Cxs have a surprisingly short half-life of only 1-5 h, leading to a rapid GJ and Cx protein turnover (Fallon and Goodeno...

Background Alterations in cardiac energy metabolism downstream of neurohormonal stimulation play a crucial role in the pathogenesis of heart failure (HF). The chronic adrenergic stimulation that accompanies HF is a signaling abnormality that leads to the up-regulation of G protein-coupled receptor kinase 2 (GRK2), which is pathological in the myocyte during disease progression in part due to uncoupling of the β-adrenergic receptor (βAR) system. In this study we explored the possibility that enhanced GRK2 expression and activity, as seen during HF, can negatively affect cardiac metabolism as part of its pathogenic profile. Methods and Results Positron Emission Tomography (PET) studies revealed that transgenic mice with cardiac-specific overexpression of GRK2 negatively impacted cardiac metabolism by inhibiting glucose uptake and desensitization of insulin signaling, which increases after ischemic injury and precedes HF development. Mechanistically, GRK2 interacts with and directly phosphorylates insulin receptor substrate-1 (IRS1) in cardiomyocytes causing insulin-dependent negative signaling feedback including inhibition of membrane translocation of the glucose transporter, GLUT4. This identifies IRS1 as a novel non-receptor target for GRK2 and represents a new pathological mechanism for this kinase in the failing heart. Importantly, inhibition of GRK2 activity prevents post-ischemic defects in myocardial insulin signaling and improves cardiac metabolism via normalized glucose uptake, which appears to participate in GRK2-targeted prevention of HF. Conclusions Our data provide novel insight into how GRK2 is pathological in the injured heart. Moreover, it appears to be a critical mechanistic link within neurohormonal crosstalk governing cardiac contractile signaling/function through βARs and metabolism through the insulin receptor.

Direct intercellular communication mediated by gap junctions (GJs) is a hallmark of normal cell and tissue physiology. In addition, GJs significantly contribute to physical cell-cell adhesion. Clearly, these cellular functions require precise modulation. Typically, GJs represent arrays of hundreds to thousands of densely packed channels, each one assembled from two half-channels (connexons), that dock head-on in the extracellular space to form the channel arrays that link neighboring cells together. Interestingly, docked GJ channels cannot be separated into connexons under physiological conditions, posing potential challenges to GJ channel renewal and physical cell-cell separation. We described previously that cells continuously—and effectively after treatment with natural inflammatory mediators—internalize their GJs in an endo-/exocytosis process that utilizes clathrin-mediated endocytosis components, thus enabling these critical cellular functions. GJ internalization generates characteristic cytoplasmic double-membrane vesicles, described and termed earlier annular GJs (AGJs) or connexosomes. Here, using expression of the major fluorescent-tagged GJ protein, connexin 43 (Cx43-GFP/YFP/mApple) in HeLa cells, analysis of endogenously expressed Cx43, ultrastructural analyses, confocal colocalization microscopy, pharmacological and molecular biological RNAi approaches depleting cells of key-autophagic proteins, we provide compelling evidence that GJs, following internalization, are degraded by autophagy. The ubiquitin-binding protein p62/sequestosome 1 was identified in targeting internalized GJs to autophagic degradation. While previous studies identified proteasomal and endo-/lysosomal pathways in Cx43 and GJ degradation, our study provides novel molecular and mechanistic insights into an alternative GJ degradation pathway. Its recent link to health and disease lends additional importance to this GJ degradation mechanism and to autophagy in general.