The oxoglutarate dehydrogenase complex is involved in myofibril growth and Z-disc assembly in Drosophila.

Myofibrils are long intracellular cables specific to muscles, composed mainly of actin and myosin filaments. The actin and myosin filaments are organized into repeated units called sarcomeres, which form the myofibrils. Muscle contraction is achieved by the simultaneous shortening of sarcomeres, which requires all sarcomeres to be the same size. Muscles have a variety of ways to ensure sarcomere homogeneity. We have previously shown that the controlled oligomerization of Zasp proteins sets the diameter of the myofibril. Here, we looked for Zasp-binding proteins at the Z-disc to identify additional proteins coordinating myofibril growth and assembly. We found that the E1 subunit of the oxoglutarate dehydrogenase complex localizes to both the Z-disc and the mitochondria, and is recruited to the Z-disc by Zasp52. The three subunits of the oxoglutarate dehydrogenase complex are required for myofibril formation. Using super-resolution microscopy, we revealed the overall organization of the complex at the Z-disc. Metabolomics identified an amino acid imbalance affecting protein synthesis as a possible cause of myofibril defects, which is supported by OGDH-dependent localization of ribosomes at the Z-disc.

The insect perspective on Z-disc structure and biology.

Myofibrils are the intracellular structures formed by actin and myosin filaments. They are paracrystalline contractile cables with unusually well-defined dimensions. The sliding of actin past myosin filaments powers contractions, and the entire system is held in place by a structure called the Z-disc, which anchors the actin filaments. Myosin filaments, in turn, are anchored to another structure called the M-line. Most of the complex architecture of myofibrils can be reduced to studying the Z-disc, and recently, important advances regarding the arrangement and function of Z-discs in insects have been published. On a very small scale, we have detailed protein structure information. At the medium scale, we have cryo-electron microscopy maps, super-resolution microscopy and protein-protein interaction networks, while at the functional scale, phenotypic data are available from precise genetic manipulations. All these data aim to answer how the Z-disc works and how it is assembled. Here, we summarize recent data from insects and explore how it fits into our view of the Z-disc, myofibrils and, ultimately, muscles.

The nebulin repeat protein Lasp regulates I-band architecture and filament spacing in myofibrils.

Mutations in nebulin, a giant muscle protein with 185 actin-binding nebulin repeats, are the major cause of nemaline myopathy in humans. Nebulin sets actin thin filament length in sarcomeres, potentially by stabilizing thin filaments in the I-band, where nebulin and thin filaments coalign. However, the precise role of nebulin in setting thin filament length and its other functions in regulating power output are unknown. Here, we show that Lasp, the only member of the nebulin family in Drosophila melanogaster, acts at two distinct sites in the sarcomere and controls thin filament length with just two nebulin repeats. We found that Lasp localizes to the Z-disc edges to control I-band architecture and also localizes at the A-band, where it interacts with both actin and myosin to set proper filament spacing. Furthermore, introducing a single amino acid change into the two nebulin repeats of Lasp demonstrated different roles for each domain and established Lasp as a suitable system for studying nebulin repeat function.

Integrin-dependent apposition of Drosophila extraembryonic membranes promotes morphogenesis and prevents anoikis.

BACKGROUND: Two extraembryonic tissues form early in Drosophila development. One, the amnioserosa, has been implicated in the morphogenetic processes of germ band retraction and dorsal closure. The developmental role of the other, the yolk sac, is obscure. RESULTS: By using live-imaging techniques, we report intimate interactions between the amnioserosa and the yolk sac during germ band retraction and dorsal closure. These tissue interactions fail in a subset of myospheroid (mys: betaPS integrin) mutant embryos, leading to failure of germ band retraction and dorsal closure. The Drosophila homolog of mammalian basigin (EMMPRIN, CD147)-an integrin-associated transmembrane glycoprotein-is highly enriched in the extraembryonic tissues. Strong dominant genetic interactions between basigin and mys mutations cause severe defects in dorsal closure, consistent with basigin functioning together with betaPS integrin in extraembryonic membrane apposition. During normal development, JNK signaling is upregulated in the amnioserosa, as midgut closure disrupts contact with the yolk sac. Subsequently, the amnioserosal epithelium degenerates in a process that is independent of the reaper, hid, and grim cell death genes. In mys mutants that fail to establish contact between the extraembryonic membranes, the amnioserosa undergoes premature disintegration and death. CONCLUSIONS: Intimate apposition of the amnioserosa and yolk sac prevents anoikis of the amnioserosa. Survival of the amnioserosa is essential for germ band retraction and dorsal closure. We hypothesize that during normal development, loss of integrin-dependent contact between the extraembryonic tissues results in JNK-dependent amnioserosal disintegration and death, thus representing an example of developmentally programmed anoikis.

Molecular mechanisms of epithelial morphogenesis.

Epithelial morphogenesis comprises the various processes by which epithelia contribute to organ formation and body shape. These complex and diverse events play a central role in animal development and regeneration. Recently, the characterization of some of the molecular mechanisms involved in epithelial morphogenesis has provided an abundance of new information on the role and regulation of the cytoskeleton, cell-cell adhesion, and cell-matrix adhesion in these processes. In this review, we discuss our current understanding of the molecular mechanisms driving cell shape changes, cell intercalation, fusion of epithelia, ingression, egression, and cell migration. Our discussion is mostly focused on results from Drosophila and mammalian tissue culture but also draws on the insights gained from other organisms.