A myogenic factor from sea urchin embryos capable of programming muscle differentiation in mammalian cells.

Abstract

Using the basic helixloop-helix domain of the myogenic factor myogenin as a probe, we identified a clone from a sea urchin cDNA library with considerable sequence similarity to the vertebrate myogenic factors. This cDNA, sea urchin myogenic factor 1 (SUM-1), transactivated a muscle creatine kinase-chloramphenicol acetyltransferase reporter gene in 1OTI/2 fibroblasts to a level comparable to that of the vertebrate myogenic factors. In addition, bacterially expressed fl-galactosidase-SUM-1 fusion protein interacted directly with the KE-2 site in the muscle creatine kinase enhancer core as assayed by electrophoretic mobility shift assays. Stably transfected SUM-1 activated the muscle differentiation program and converted 10TI/2 cells from fibroblasts to myotubes. In sea urchin embryos, SUM-1 RNA was not detected before gastrulation. It accumulated to its highest levels during the prism stage when myoblasts were first detected by myosin immunostaining and then diminished as myocytes differentiated. SUM-1 protein was localized in secondary mesenchyme cells when they could first be identified as muscle cells by myosin immunostaining. These results implicate SUM-1 as a regulatory factor involved in the early decision of a pluripotent secondary mesenchyme cell to convert to a myogenic fate. SUM-1 is an example of an invertebrate myogenic factor that is capable of functioning in mammalian cells.

Functional conservation of nematode and vertebrate myogenic regulatory factors

“…This is not the first reported instance of a non-mammalian myogenic factor acting in the 10T1/2 cell line. Venuti et al (1991) isolated a sea urchin analogue of the vertebrate myogenic factors called SUM-1. Transfection of EMSV SUM-1 can convert 10T1/2 cells, and irans-activate muscle-specific reporter gene expression, as efficiently as EMSV mouse MyoD.…”

Identification of a muscle factor related to MyoD in a fish species

“…In multipotential 10T1/2 cells, transfection experiments have shown that forced expression of these exogenous myogenic factors is sufficient to drive them down the muscle differentiation pathway suggesting their functions in myogenic lineage determination [1][2][3][4][5]. In contrast to vertebrates whose genome encodes multiple members of the MyoD family, invertebrates, including Sea urchin [7], C. elegans [8] and Drosophila [9], appear to contain only a single myogenic factor encoding gene. However, the myogenic factor for Sea urchin and C. elegans activates myogenesis in 10T1/2 cells indicating a highly conserved mechanism for muscle genes activation.…”

Analysis of the myogenin promoter reveals an indirect pathway for positive autoregulation mediated by the muscle-specific enhancer factor MEF-2.

“…Members of this muscle regulatory gene family include MyoD (21), myogenin (25,77), myf5 (9), and MRF4/herculin/myf6 (8,48,61), each of which can activate myogenesis when introduced into a wide range of nonmuscle cell types. Related myogenic factors have also been identified in a variety of vertebrate and invertebrate species (22,30,35,39,44,47,57,65,72). An additional gene, myd, that appears to be structurally unrelated to the MyoD family, has been identified as a genomic clone that can induce myogenesis in 1OT1/2 cells, but it has not yet been characterized at the molecular level (59).…”