Identification and expression analysis of two developmentally regulated myosin heavy chain gene transcripts in carp (Cyprinus carpio).

Steven Ennion,David Wilkes,Hélene Alami-Durante,Laurent Gauvry,Geoffrey Goldspink

doi:10.1242/jeb.202.9.1081

Abstract

Whilst developmentally regulated genes for the myosin heavy chain (MyoHC) have been characterised in mammalian, avian and amphibian species, no developmental MyoHC gene has previously been characterised in a species of fish. In this study, we identify two developmentally regulated MyoHC gene transcripts (named Eggs22 and Eggs24) in carp (Cyprinus carpio) and characterise their expression patterns during embryonic and larval development. The transcripts showed an identical temporal pattern of expression commencing 22 h post-fertilisation (18 degrees C incubation temperature), coincident with the switch from exclusive expression of genes for beta-actin to expression of genes for both beta- and alpha-actin, and continuing for 2 weeks post-hatching. No expression of these myosin transcripts was detected in juvenile or adult carp. Wholemount in situ hybridisation showed that both transcripts are expressed initially in the rostral region of the developing trunk and progress caudally. Both are expressed in the developing pectoral fin and protractor hyoideus muscles. However, the muscles of the lower jaw express only the Eggs22 transcript. No expression of either transcript was detected in cardiac or smooth muscle. A distinct chevron pattern of expression was observed in the myotomal muscle. This was shown to be caused by localisation of the mRNAs to the myoseptal regions of the fibres, the sites of new sarcomere addition during muscle growth, suggesting transport of MyoHC mRNA transcripts. The 3' untranslated region of the Eggs24 transcript contains a 10 base pair motif (AAAATGTGAA) which is shown to be also present in the 3' untranslated regions of MyoHC genes from a wide range of species. Possible reasons for the need for developmental isoforms of myosin heavy chain isoforms are discussed.

Highlights

The precursors of muscle fibres are mononucleated embryonic mesodermal cells which do not themselves synthesize any of the muscle-specific myofibrillar proteins
This process initially leads to a ‘rosette’ arrangement of muscle fibres, with the larger primary myotubes surrounded by smaller secondary myotubes
Subsequent to the formation of the inner white and superficial red zones of muscle in the embryo and yolk-sac larvae, a second stage of muscle development occurs in the freeswimming larvae when the adult red and pink fibre types form and a new phase of fibre hyperplasia begins in the white muscle zone

Summary

Introduction

The precursors of muscle fibres are mononucleated embryonic mesodermal cells which do not themselves synthesize any of the muscle-specific myofibrillar proteins During muscle formation, these cells proliferate and differentiate into mononucleated myoblasts, which fuse to form multinucleated myotubes. The first myotubes to form are known as primary myotubes, and these provide a framework along which remaining myotubes orientate themselves longitudinally and subsequently fuse to form the secondary myotubes This process initially leads to a ‘rosette’ arrangement of muscle fibres, with the larger primary myotubes surrounded by smaller secondary myotubes. The difference in size between primary and secondary myotubes gradually diminishes until the two populations are indistinguishable in the adult In fish, this biphasic development of primary and secondary myotubes is not observed.

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