https://core.ac.uk/download/pdf/2756387.pdf

Collagens—structure, function, and biosynthesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Basement membranes: structure, assembly and role in tumour angiogenesis

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Role of Extracellular Matrix in Adaptation of Tendon and Skeletal Muscle to Mechanical Loading

Apoptosis plays an important role in development and maintenance of tissue homeostasis. Intensive efforts have been made to explore the molecular mechanisms of the apoptotic signaling pathways including the initiation, mediation, execution and regulation of apoptosis. Caspases are central effectors of apoptosis. Cells undergo apoptosis through two major pathways, namely the extrinsic pathway (death receptor pathway) or the intrinsic pathway (the mitochondrial pathway). Finally, the contents of dead cells are packaged into apoptotic bodies, which are recognized by neighboring cells or macrophages and cleared by phagocytosis. Cellular apoptosis is tightly controlled by a complex regulatory networks including balancing pro-survival signals. De-regulation of apoptosis may lead to pathological disorders such as developmental defects, autoimmune diseases, neurodegeneration or cancer. Increasing attention is being focused on alternative signaling pathways leading to cell death including necrosis, autophagy, and mitotic catastrophe. Understanding of cell death signaling pathways is relevant to understanding cancer and to developing more effective therapeutics.

Overview of cell death signaling pathways

Bone-marrow–derived stromal cells are known to protect against acute lung injury. Jahar Bhattacharya and colleagues now show that one way these cells offer such protection is to transfer their mitochondria to the injured lung epithelia to improve the bioenergetics of the recipient cells, thus probably allowing them to recover from injury more efficiently.

/pdf/mitochondrial-transfer-from-bone-marrow-derived-stromal-4dyhxy9vw0.pdf

Mitochondrial transfer from bone-marrow-derived stromal cells to pulmonary alveoli protects against acute lung injury.

Mice lacking the membrane tyrosine kinase c-mer have been shown to have altered macro-phage cytokine production and defective phagocytosis of apoptotic cells despite normal phagocytosis of other particles. We show here that c-mer–deficient mice have impaired clearance of infused apoptotic cells and that they develop progressive lupus-like autoimmunity, with antibodies to chromatin, DNA, and IgG. The autoimmunity appears to be driven by endogenous antigens, with little polyclonal B cell activation. These mice should be an excellent model for studying the role of apoptotic debris as an immunogenic stimulus for systemic autoimmunity.

/pdf/delayed-apoptotic-cell-clearance-and-lupus-like-autoimmunity-4q4fmrvrhy.pdf

Delayed apoptotic cell clearance and lupus-like autoimmunity in mice lacking the c-mer membrane tyrosine kinase.

The collagen family of proteins consists of 19 types encoded by 33 genes. One of the more recently discovered collagens is the α1 chain of type XV. Type XV collagen is comprised of a 577-amino-acid, highly interrupted, triple-helical region that is flanked by amino and carboxy noncollagenous domains of 555 and 256 residues, respectively. To address questions of where this collagen is localized and what its function may entail, we produced a bacteria-expressed recombinant protein representing the first half of the type XV collagen carboxy-terminal domain in order to generate highly specific polyclonal antisera. Immunoscreening of an expression library with the affinity-purified antibody revealed three clones coding for part of the type XV triple-helical region and the entire noncollagenous carboxy-terminus. Western blot analysis of human tissue homogenates identified a 116-kDa collagenase-sensitive protein and a 27-kDa collagenase-resistant fragment, whose electrophoretic mobilities were unchanged in the presence and absence of reductant. Northern blot hybridization to human tissue RNAs indicated that type XV has a prevalent and widespread distribution. To determine the precise localization of type XV collagen, immunohistochemical analyses at the light- and electron-microscopic levels were performed. Type XV exhibited a surprisingly restricted and uniform presence in many human tissues as evidenced by a strong association with vascular, neuronal, mesenchymal, and some epithelial basement membrane zones. These data suggest that type XV collagen may function in some manner to adhere basement membrane to the underlying connective tissue stroma.

Type XV collagen exhibits a widespread distribution in human tissues but a distinct localization in basement membrane zones.

Nineteen types, the product of 33 genes, comprise the collagen family of proteins. Types I, II, III, V, and XI constitute the fibrillar collagens, whereas types IV, VI to X, and XII to XIX represent the structurally diverse, nonfibrillar members. Type XIX collagen was discovered from the sequence of rhabdomyosarcoma cDNA clones. The type XIX chain consists of 1142 amino acids that contribute primarily to a unique five subdomain triple-helical region. To characterize the protein, to determine the tissue distribution, and to provide some insight into its function, we generated two type XIX-specific polyclonal antibodies. One was directed against a recombinant molecule containing amino-terminal sequences, and the second was derived from a synthetic peptide corresponding to most of the short carboxy terminus. These antibodies were used in immunoblot assays of rhabdomyosarcoma cell/matrix homogenates to identify a 165-kd disulfide-bonded and bacterial collagenase-sensitive protein. Immunohistochemical analysis of type XIX collagen was performed for human skeletal muscle, spleen, prostate, kidney, liver, placenta, colon, and skin. In contrast to Northern blot hybridizations, which showed very low levels of the 12-kb transcript in few tissues, the protein was found in all tissues examined. The type XIX collagen distribution was restricted to vascular, neuronal, mesenchymal, and some epithelial basement membrane zones, which is similar to the profile recently established (Ref. 8) and further extended here for type XV collagen. Nevertheless, localization of type XIX exhibited significant differences from type XV collagen that were particularly evident in the kidney, liver, and spleen. This report, in conjunction with the type XV results and other studies of type XVIII collagen, indicates the existence of a new collagen subgroup founded on their widespread presence in basement membrane zones regardless of chain homology. In addition to their role in basement membrane-stromal interactions, the pronounced vascular association suggests involvement of these related collagen types with angiogenic and pathological processes.

Biochemical and immunohistochemical characterization of human type XIX defines a novel class of basement membrane zone collagens.

We examined phenotype-specific changes in gap junction protein [connexin (Cx)] expression and function by cultured rat alveolar type II cells. Type II cells cultured on extracellular matrix in medi...

Phenotypic control of gap junctional communication by cultured alveolar epithelial cells

We analyzed the pattern of gap junction protein (connexin) expression in vivo by indirect immunofluorescence. In normal rat lung sections, connexin (Cx)32 was expressed by type II cells, whereas Cx43 was more ubiquitously expressed and Cx46 was expressed by occasional alveolar epithelial cells. In response to bleomycin-induced lung injury, Cx46 was upregulated by alveolar epithelial cells, whereas Cx32 and Cx43 expression were largely unchanged. Given that Cx46 may form gap junction channels with either Cx43 or Cx32, we examined the ability of primary alveolar epithelial cells cultured for 6 days, which express Cx43 and Cx46, to form heterocellular gap junctions with cells expressing other connexins. Day 6 alveolar epithelial cells formed functional gap junctions with other day 6 cells or with HeLa cells transfected with Cx43 (HeLa/Cx43), but they did not communicate with HeLa/Cx32 cells. Furthermore, day 6 alveolar epithelial cells formed functional gap junction channels with freshly isolated type II cells. Taken together, these data are consistent with the notion that type I and type II alveolar epithelial cells communicate through gap junctions compatible with Cx43.

https://www.physiology.org/doi/pdf/10.1152/ajplung.2001.280.6.L1085

Valsamma Abraham

Papers

Delayed apoptotic cell clearance and lupus-like autoimmunity in mice lacking the c-mer membrane tyrosine kinase.

Type XV collagen exhibits a widespread distribution in human tissues but a distinct localization in basement membrane zones.

Biochemical and immunohistochemical characterization of human type XIX defines a novel class of basement membrane zone collagens.

Phenotypic control of gap junctional communication by cultured alveolar epithelial cells

Heterocellular gap junctional communication between alveolar epithelial cells