Epithelial cell–cell junctions and plasma membrane domains

AbstractEpithelial cells form a barrier against the environment, but are also required for the regulated exchange of molecules between an organism and its surroundings. Epithelial cells are characterised by a remarkable polarization of their plasma membrane, evidenced by the appearance of structurally, compositionally, and functionally distinct surface domains. Here we consider the (in)dependence of epithelial cell polarisation and the function of smaller plasma membrane domains (e.g. adherens junctions, gap junctions, tight junctions, apical lipid rafts, caveolae, and clathrin-coated pits) in the development and maintenance of cell surface polarity. Recent evidence of cross-talk and/or overlap between the different cell–cell junction components and alternate functions of junction components, including gene expression regulation, are discussed in the context of cell surface polarity

Lens connexins α3Cx46 and α8Cx50 interact with zonula occludens protein-1 (ZO-1)

Connexin α1Cx43 has previously been shown to bind to the PDZ domain–containing protein ZO-1. The similarity of the carboxyl termini of this connexin and the lens fiber connexins α3Cx46 and α8Cx50 suggested that these connexins may also interact with ZO-1. ZO-1 was shown to be highly expressed in mouse lenses. Colocalization of ZO-1 with α3Cx46 and α8Cx50 connexins in fiber cells was demonstrated by immunofluorescence and by fracture-labeling electron microscopy but showed regional variations throughout the lens. ZO-1 was found to coimmunoprecipitate with α3Cx46 and α8Cx50, and pull-down experiments showed that the second PDZ domain of ZO-1 was involved in this interaction. Transiently expressed α3Cx46 and α8Cx50 connexins lacking the COOH-terminal residues did not bind to the second PDZ domain but still formed structures resembling gap junctions by immunofluorescence. These results indicate that ZO-1 interacts with lens fiber connexins α3Cx46 and α8Cx50 in a manner similar to that previously described for α1Cx43. The spatial variation in the interaction of ZO-1 with lens gap junctions is intriguing and is suggestive of multiple dynamic roles for this association

Regulation of connexin43 gap junctional communication by phosphatidylinositol 4,5-bisphosphate

Cell–cell communication through connexin43 (Cx43)-based gap junction channels is rapidly inhibited upon activation of various G protein–coupled receptors; however, the mechanism is unknown. We show that Cx43-based cell–cell communication is inhibited by depletion of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P2) from the plasma membrane. Knockdown of phospholipase Cβ3 (PLCβ3) inhibits PtdIns(4,5)P2 hydrolysis and keeps Cx43 channels open after receptor activation. Using a translocatable 5-phosphatase, we show that PtdIns(4,5)P2 depletion is sufficient to close Cx43 channels. When PtdIns(4,5)P2 is overproduced by PtdIns(4)P 5-kinase, Cx43 channel closure is impaired. We find that the Cx43 binding partner zona occludens 1 (ZO-1) interacts with PLCβ3 via its third PDZ domain. ZO-1 is essential for PtdIns(4,5)P2-hydrolyzing receptors to inhibit cell–cell communication, but not for receptor–PLC coupling. Our results show that PtdIns(4,5)P2 is a key regulator of Cx43 channel function, with no role for other second messengers, and suggest that ZO-1 assembles PLCβ3 and Cx43 into a signaling complex to allow regulation of cell–cell communication by localized changes in PtdIns(4,5)P2

A Kinase Interacting Protein 1 regulates mitochondrial protein levels in energy metabolism and promotes mitochondrial turnover after exercise

A Kinase Interacting Protein 1 (AKIP1) is a signalling adaptor that promotes mitochondrial respiration and attenuates mitochondrial oxidative stress in cultured cardiomyocytes. We sought to determine whether AKIP1 influences mitochondrial function and the mitochondrial adaptation in response to exercise in vivo. We assessed mitochondrial respiratory capacity, as well as electron microscopy and mitochondrial targeted-proteomics in hearts from mice with cardiomyocyte-specific overexpression of AKIP1 (AKIP1-TG) and their wild type (WT) littermates. These parameters were also assessed after four weeks of voluntary wheel running. In contrast to our previous in vitro study, respiratory capacity measured as state 3 respiration on palmitoyl carnitine was significantly lower in AKIP1-TG compared to WT mice, whereas state 3 respiration on pyruvate remained unaltered. Similar findings were observed for maximal respiration, after addition of FCCP. Mitochondrial DNA damage and oxidative stress markers were not elevated in AKIP1-TG mice and gross mitochondrial morphology was similar. Mitochondrial targeted-proteomics did reveal reductions in mitochondrial proteins involved in energy metabolism. Exercise performance was comparable between genotypes, whereas exercise-induced cardiac hypertrophy was significantly increased in AKIP1-TG mice. After exercise, mitochondrial state 3 respiration on pyruvate substrates was significantly lower in AKIP1-TG compared with WT mice, while respiration on palmitoyl carnitine was not further decreased. This was associated with increased mitochondrial fission on electron microscopy, and the activation of pathways associated with mitochondrial fission and mitophagy. This study suggests that AKIP1 regulates the mitochondrial proteome involved in energy metabolism and promotes mitochondrial turnover after exercise. Future studies are required to unravel the mechanistic underpinnings and whether the mitochondrial changes are required for the AKIP1-induced physiological cardiac growth.</p

DWORF Extends Life Span in a PLN-R14del Cardiomyopathy Mouse Model by Reducing Abnormal Sarcoplasmic Reticulum Clusters

BACKGROUND: The p.Arg14del variant of the PLN (phospholamban) gene causes cardiomyopathy, leading to severe heart failure. Calcium handling defects and perinuclear PLN aggregation have both been suggested as pathological drivers of this disease. Dwarf open reading frame (DWORF) has been shown to counteract PLN regulatory calcium handling function in the sarco/endoplasmic reticulum (S/ER). Here, we investigated the potential disease-modulating action of DWORF in this cardiomyopathy and its effects on calcium handling and PLN aggregation. METHODS: We studied a PLN-R14del mouse model, which develops cardiomyopathy with similar characteristics as human patients, and explored whether cardiac DWORF overexpression could delay cardiac deterioration. To this end, R14Δ/Δ (homozygous PLN-R14del) mice carrying the DWORF transgene (R14Δ/ΔDWORFTg [R14Δ/Δ mice carrying the DWORF transgene]) were used. RESULTS: DWORF expression was suppressed in hearts of R14Δ/Δ mice with severe heart failure. Restoration of DWORF expression in R14Δ/Δ mice delayed cardiac fibrosis and heart failure and increased life span &gt;2-fold (from 8 to 18 weeks). DWORF accelerated sarcoplasmic reticulum calcium reuptake and relaxation in isolated cardiomyocytes with wild-type PLN, but in R14Δ/Δ cardiomyocytes, sarcoplasmic reticulum calcium reuptake and relaxation were already enhanced, and no differences were detected between R14Δ/Δ and R14Δ/ΔDWORFTg. Rather, DWORF overexpression delayed the appearance and formation of large pathogenic perinuclear PLN clusters. Careful examination revealed colocalization of sarcoplasmic reticulum markers with these PLN clusters in both R14Δ/Δ mice and human p.Arg14del PLN heart tissue, and hence these previously termed aggregates are comprised of abnormal organized S/ER. This abnormal S/ER organization in PLN-R14del cardiomyopathy contributes to cardiomyocyte cell loss and replacement fibrosis, consequently resulting in cardiac dysfunction. CONCLUSIONS: Disorganized S/ER is a major characteristic of PLN-R14del cardiomyopathy in humans and mice and results in cardiomyocyte death. DWORF overexpression delayed PLN-R14del cardiomyopathy progression and extended life span in R14Δ/Δ mice, by reducing abnormal S/ER clusters.</p