Endocytosis of alpha 2-macroglobulin is developmentally regulated during myogenesis

Trypanosoma cruzi: acute infection affects expression of alpha-2-macroglobulin and A2MR/LRP receptor differently in C3H and C57BL/6 mice

Although a complete cellular and humoral immune response is elicited in Chagas' disease, recent data suggest that other natural elements of innate immunity may also contribute to the initial host primary defense. alpha-Macroglobulins are a family of plasma proteinase inhibitors that are acute-phase reactants in Trypanosoma cruzi-infected mice and humans. Mice contain a tetrameric alpha-2-macroglobulin (MAM) and a monomeric murinoglobulin (MUG). Heterogeneity in their reactions was observed in murine T. cruzi-infected plasma A2M levels despite an overall increase. In addition, up-regulation of the A2M receptor (A2MR/LRP) was observed in peritoneal macrophages during T. cruzi infection. Here, we show that during T. cruzi infection (Y strain), the MAM and MUG hepatic mRNA levels and the corresponding plasma protein levels were up-regulated in C3H and C57BL/6 (B6) mice, but with different kinetics. On the contrary, A2MR/LRP mRNA levels increased in acutely infected C3H mice, but decreased in B6 mice, in both liver and heart. Immunocytochemistry of infected B6 heart cryosections confirmed a less intense endothelium labeling by the fluoresceinated ligand for A2MR/LRP. On the other hand, infected B6 spleen cells displayed higher F-A2M-FITC binding and MAC1 expression, confirming higher A2MR/LRP expression in macrophages. In uninfected mice, as well as after T. cruzi infection, higher A2M plasma levels were measured in C3H mice than in B6 mice. The lower tissue T. cruzi parasitism found in C3H-infected mice could reflect an inhibitory effect of A2M on parasite invasion. Our present data further contribute to clarifying aspects of the role of A2MR/LRP in a model of acute Chagas' disease in different mouse strains.

Alpha-2-macroglobulin receptor is differently expressed in peritoneal macrophages from C3H and C57/B16 mice and up-regulated during Trypanosoma cruzi infection

Chagas' disease is caused by the protozoan Trypanosoma cruzi. The acute phase of T. cruzi infection, which can be conveniently studied in mouse models, is thought to be a determinant of survival and of the pathological features of the chronic phase. With regard to the occurrence of early death and parasitaemia levels C3H and C57/B16 mice are classically classified as 'susceptible' and 'resistant' to T. cruzi infection, respectively. Alpha-2-macroglobulin (A2M) is a physiological proteinase inhibitor found in tissues and in the plasma of mammals. Previous studies showed that A2M plasma levels increase in C3H mice acutely infected by T. cruzi but do not change in C57/B16 mice. This difference might involve two possible phenomena, concerning A2M synthesis and/or clearance by its receptor (A2M-R). In this study, we examined by flow cytometry the binding of A2M-trypsin conjugated with FITC to macrophages from normal and T. cruzi-infected C3H and C57/B16 mice. Our present results show for the first time that A2M-R is expressed more (by approximately 33%) in the surface of cells from normal C57/B16 as compared to C3H mice. We also show that A2M-R expression is up-regulated in both strains during acute T. cruzi infection, but at higher levels and earlier in C57/B16 mice. At the same time, peritoneal cells become activated as judged by: (1) increase of their size and granularity; (2) gradual increase of Fc gamma RII/III expression assayed by 2.4G2 binding; (3) down-modulation of F4/80 binding, a mAb that recognizes an antigen typically expressed in resident macrophages. Finally, our results indicate that as macrophages become activated in vivo a higher expression of A2M-R is observed.

Distribution of the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein in human tissues

The hepatic alpha 2-macroglobulin receptor (alpha 2MR)/low density lipoprotein receptor-related protein (LRP) binds and endocytoses alpha 2-macroglobulin-proteinase complexes in plasma. In addition, it binds lipoproteins, a novel 40 kDa protein, and complexes between plasminogen activators and plasminogen activator inhibitor type-1. This study shows, for the first time, the tissue distribution of alpha 2MR/LRP as determined by immunohistochemistry with specific monoclonal antibodies. The analysis revealed alpha 2MR/LRP-expression in a restricted spectrum of cell types, including neurons and astrocytes in the central nervous system, epithelial cells of the gastrointestinal tract, smooth muscle cells, fibroblasts, Leydig cells in testis, granulosa cells in ovary, and dendritic interstitial cells of kidney. Monocyte-derived cells displayed marked alpha 2MR/LRP expression in the phagocytes of liver, lung and lymphoid tissues, but no or low expression in antigen-presenting cells including Langerhans' cells of the skin. The high abundance of alpha 2MR/LRP in certain cell types of most organs suggests two main routes for alpha 2MR/LRP-mediated ligand clearance: (1) systemic removal in liver of circulating ligands, and (2) non-hepatic interstitial removal in different organs, including the brain.

Localization of anti-clathrin antibody in the sarcomere and sensitivity of myofibril structure to chloroquine suggest a role for clathrin in myofibril assembly

Immunofluorescence microscopy has been used to demonstrate that X22, a monoclonal antibody specific for clathrin heavy chain, localizes in repetitive bands that appear soon after the fusion of skeletal myoblasts into multinucleate fibers. This organization has been found in cultures containing myotubes that develop in vitro from explants of newborn rat hindlimb cells and in myotubes derived from the L8E63 myogenic line. Bands were also prominent in skinned fibers prepared from adult rat soleus muscle and in cardiac myocytes grown in vitro from 4-day heart ventricles. Immunofluorescence banding was localized in the sarcomere as a doublet, with one element on either side of the Z line. Evidence that supports the conclusion that the reaction with X22 antibody is specific and indicative of the localization of clathrin in the sarcomere includes: (1) Identical titration of X22 antibody reactivity with the determinant in coated vesicles and in the sarcomere. (2) Conditions (eg., pH and Tris) that disrupt clathrin baskets or prevent its assembly likewise disrupt the localization of X22 in bands. (3) Chloroquine inhibits both the normal trafficking of clathrin in the cell and X22 banding in the sarcomere. (4) Immunoblot analysis of myotube lysates reveals a single band with an electrophoretic mobility identical to the 180,000-Da clathrin heavy chain. (5) The assembly of clathrin into sarcomeric bands occurs early in the development of the myofibrillar apparatus. Quantitation of the appearance of X22 banding in primary cultures of myotubes indicates that it precedes that of other myofibrillar proteins and that assembly takes place in the following order: X22, titin, myosin heavy chain, actin, and desmin. The assembly of myosin, titin, and actin into sarcomeric bands, as well as X22, is inhibited by chloroquine. Upon prolonged exposure to chloroquine previously assembled proteins are drastically reduced or no longer evident in the sarcomere. On the basis of these results and considering the role of clathrin in intracellular transport and its capacity to interact with actin and alpha-actinin, we suggest that clathrin may have diverse roles in the assembly, integrity, and functioning of the sarcomere and its integration with the sarcolemma. The early organization of X22 into bands further suggests that clathrin may also function early in the assembly of the contractile system.

Membrane-cytoskeleton associations during myogenesis deviate from traditional definitions

Plasma membrane-cytoskeleton associations involving four membrane proteins (A5, H58, H36, and I20) were studied in developing L8E63 rat skeletal muscle cells using immunofluorescence microscopy and photometry on the basis of three criteria: Triton-insolubility, colocalization with cytoskeletal components, and sensitivity to cytoskeleton-directed drugs. The results presented demonstrate that there are developmental stage-specific associations between membrane proteins and the cytoskeleton during skeletal myogenesis. Several inconsistencies were found with traditional expectations of membrane-cytoskeleton associations. For example, although A5 is Triton-insoluble and sensitive to cytochalasin, its distribution generally does not correspond with any known cytoskeletal structure. Furthermore, the topography of A5 is dependent on the integrity of the plasma membrane. H36 and I20 are completely soluble in Triton and therefore by accepted definitions would not be expected to be associated with any cytoskeletal component. Yet H36 and actin codisrupt in the presence of cytochalasin, while I20, whose distribution does not correspond with microtubules, is uniquely sensitive to their disruption. These results demonstrate that (i) neither Triton-solubility nor colocalization alone predicts all membrane-cytoskeleton associations; some associations between the membrane and cytoskeleton are unstable in nonionic detergent; (ii) the native distribution of proteins in the membrane may not reflect their cytoskeletal associations; and (iii) the topography of some membrane proteins with no apparent association with the cytoskeleton may be greatly influenced by the cell cytoskeleton.

Replicating myoblasts express a muscle-specific phenotype

During the terminal stage of skeletal myogenesis, myoblasts stop replicating, fuse to form multinucleate fibers, and express the genes that encode the proteins that convey contractile capacity. Because of this dramatic shift in proliferative state, morphology, and gene expression, it has been possible to readily identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the proliferating cells that give rise to postmitotic myoblasts are equally distinct in their phenotype and in fact whether distinct stages in skeletal myogenesis precede the onset of terminal differentiation. To address these questions, monoclonal antibodies and immunofluorescence microscopy were used to determine that replicating myoblasts from newborn rats do express a muscle-specific phenotype. To identify replicating cells, incorporation of 5-bromo-2'-deoxyuridine (BrdUrd) into DNA was assayed by using anti-BrdUrd antibody. The developmentally regulated, muscle-specific, integral membrane protein H36 and the intermediate-filament protein desmin were scored as markers of the myogenic phenotype. The percentage of BrdUrd+ (i.e., proliferative) cells among H36+ and desmin+ myoblasts was equal to the percentage of BrdUrd+ cells in the entire population, indicating that the expression of H36 and desmin is uniformly characteristic of replicating myoblasts. Inhibition of protein synthesis before and during growth in BrdUrd did not alter the frequency of desmin and H36 immunofluorescence in BrdUrd+ cells. Thus, desmin and H36 were present in the replicating myoblasts prior to the onset of growth in BrdUrd. These results were confirmed using H36+ cells selected by flow cytometry: these purified H36+ myoblasts replicate, express desmin, and differentiate. Similar results were obtained with mouse myoblasts. Desmin expression in these mammalian cells differs from that in chicken embryo myoblasts: only a small proportion of replicating chicken embryo myoblasts express desmin. That replicating mammalian myoblasts have a muscle-specific phenotype serves to define a distinct stage in myogenic development and a specific cell in the myogenic lineage. Further, it implies that there is a regulatory event activated during myogenesis that precedes terminal differentiation and that is required for expression of those genes whose products distinguish the replicating myoblast.

Fibroblasts modulate expression of Thy-1 on the surface of skeletal myoblasts

Thy-1 antigen is a well-characterized cell-surface glycoprotein known to be variably expressed in many different tissues, including lymphocytes, brain, and muscle. Its function remains unknown. In skeletal muscle, both in vivo and in vitro, the antigen has been reported on immature but not on adult tissue, and its disappearance corresponds roughly to the time of myoblast fusion. Using monoclonal H36 antibody to identify myoblasts unambiguously, we demonstrate here that Thy-1 is expressed only on a small (less than 1%) fraction of rat skeletal muscle myoblasts in heterogeneous primary cultures, but the number of myoblasts that express Thy-1 rises to a steady level of about 70% when fibroblasts are removed from secondary cultures. Restitution of fibroblasts or growth of purified myoblasts in medium conditioned by fibroblasts greatly suppresses this increase in myoblast Thy-1 expression. Thus an interaction between fibroblasts and myoblasts, mediated by a soluble nondialyzable molecule, modulates expression of Thy-1 on the myoblast outer membrane.