There was an error published in Development 137, 2807-2817.On p. 2808, StripeB was incorrectly referred to as an epidermal growth factor (Egf)-like, rather than an early growth response (Egr)-like, transcription factor. The corrected sentence appears below.One of the earliest genes that induce tendon progenitor cells within the ectoderm encodes the early growth response (Egr)-like transcription factor StripeB, one of the two isoforms produced by the stripe gene.We apologise to the authors and readers for this mistake. SummaryThe formation of the musculoskeletal system represents an intricate process of tissue assembly involving heterotypic inductive interactions between tendons, muscles and cartilage. An essential component of all musculoskeletal systems is the anchoring of the force-generating muscles to the solid support of the organism: the skeleton in vertebrates and the exoskeleton in invertebrates. Here, we discuss recent findings that illuminate musculoskeletal assembly in the vertebrate embryo, findings that emphasize the reciprocal interactions between the forming tendons, muscle and cartilage tissues. We also compare these events with those of the corresponding system in the Drosophila embryo, highlighting distinct and common pathways that promote efficient locomotion while preserving the form of the organism.

The KASH proteins Klar and MSP-300 cooperate to promote even myonuclear spacing by linking the MSP-300 nuclear ring to the astral microtubule network.

A Drosophila FGF receptor homolog (DFGF-R2/DFRI) termed Heartless (Htl) is expressed in the embryonic mesoderm. The phenotypes of null mutant embryos demonstrated that Htl is a central player that is required for the development of several mesodermal lineages. No abnormalities in the primary specification of the mesoderm were observed. The first defects were seen as irregular migration and spreading of the mesoderm over the ectoderm. Subsequently, cell fates were not induced in several lineages including the visceral mesoderm, heart, and the dorsal somatic muscles. The defects in the induction of cell fates are likely to result from failure of the mesoderm to spread over the ectoderm and receive patterning signals. The defective spreading could be circumvented in htl mutant embryos by providing an ectopic Dpp patterning signal, leading to the formation of heart and dorsal muscle cells. Htl appears to be required also subsequently during the migration and morphogenesis of the different lineages. Expression of a dominant-negative ht/construct after the initial induction of cell fates gave rise to aberrant migration and organization of the visceral mesoderm, heart, and somatic muscles. Thus, a common role for Htl in cell migration and tissue organization may account for the pleiotropic defects of the htl mutation.

We describe here the subcellular distributions of three junctional proteins in different adherens-type contacts. The proteins examined include vinculin, talin, and a recently described 135-kD protein (Volk, T., and B. Geiger, 1984, EMBO (Eur. Mol. Biol. Organ.) J., 10:2249-2260). Immunofluorescent localization of the three proteins indicated that while vinculin was ubiquitously present in all adherens junctions, the other two showed selective and mutually exclusive association with either cell-substrate or cell-cell adhesions. Talin was abundant in focal contacts and in dense plaques of smooth muscle, but was essentially absent from intercellular junctions such as intercalated disks or adherens junctions of lens fibers. The 135-kD protein, on the other hand, was present in the latter two loci and was apparently absent from membrane-bound plaques of gizzard or from focal contacts. Radioimmunoassay of tissue extracts and immunolabeling of cultured chick lens cells indicated that the selective presence of talin and of the 135-kD protein in different cell contacts is spatially regulated within individual cells.On the basis of these findings it was concluded that adherens junctions are molecularly heterogeneous and consist of at least two major subgroups. Contacts with noncellular substrates contain talin and vinculin but not the 135-kD protein, whereas their intercellular counterparts contain the latter two proteins and are devoid of talin. The significance of these results and their possible relationships to contact-induced regulation of cell behavior are discussed.Adherens junctions consist of a family of stable cell contacts in which actin is characteristically associated with the endofacial surfaces of the plasma membrane (16,20,23,40). Typical examples of adherens junctions are the zonula and fascia adherentes of polarized epithelia and cardiac myocytes (32, 42), small adhesions of fibroblasts (29), focal contacts in cell cultures (3, 18), dense plaque of smooth muscle (19), etc. The major justification for the reference to all these morphologically diverse structures as a closely related group of cell contacts was the apparent molecular homology between them. Studies in several laboratories indicated that all adherens junctions contain vinculin at their cytoplasmic aspects, and it was thus postulated that vinculin is involved in the linkage of actin to the membrane in these sites. The presence of a ubiquitous "plaque" component in all adherens junctions and their apparent association with actin filaments have raised the possibility that, in spite of considerable morphological variability, the molecular homology between all adherensjunctions may be quite extensive. To study this aspect directly we have tried to identify additional components of adherens junctions and study their spatial distributions in cells and tissues.Recently two relevant proteins were described which are specifically bound to adherens junctions, including talin (9, 34) and a 135-kD protein (45). The former is a 215-kD protein i...

In the Drosophila embryo, the correct association of muscles with their specific tendon cells is achieved through reciprocal interactions between these two distinct cell types. Tendon cell differentiation is initiated by activation of the EGF-receptor signaling pathway within these cells by Vein, a neuregulin-like factor secreted by the approaching myotube. Here, we describe the cloning and the molecular and genetic analyses of kakapo, a Drosophila gene, expressed in the tendons, that is essential for muscle-dependent tendon cell differentiation. Kakapo is a large intracellular protein and contains structural domains also found in cytoskeletal-related vertebrate proteins (including plakin, dystrophin, and Gas2 family members). kakapo mutant embryos exhibit abnormal muscle-dependent tendon cell differentiation. A major defect in the kakapo mutant tendon cells is the failure of Vein to be localized at the muscle–tendon junctional site; instead, Vein is dispersed and its levels are reduced. This may lead to aberrant differentiation of tendon cells and consequently to the kakapo mutant deranged somatic muscle phenotype.