Modulation of embryonic and muscle‐specific enolase gene products in the developing mouse hindlimb

Abstract During striated muscle development, the glycolytic enzyme enolase (EC 4.2.1.11) undergoes an isozymic transition, from the embryonic αα form towards the muscle-specific forms αβ and ββ. The regulation of this transition was analyzed in mouse hindlimb muscles from embryonic day 15 (E15) to the adult stage. The quantitative modulations of the levels of the transcripts and subunits of α and β enolase genes were determined. The absolute amounts of α and β enolase mRNAs were estimated using in vitro synthesized transcripts as calibration standards, thus allowing an evaluation of their relative contribution at each stage examined. The muscle-specific β enolase mRNA is already present at E15. Its level then increases and, from E17, this transcript becomes predominant. This accumulation is biphasic: a steep prenatal rise, corresponding to a net increase per fiber, accompanies the formation of secondary myofibers and the development of innervation; a second rise, beginning at postnatal day 5, is temporally correlated with the definitive specialization of the myofibers. Most of the decrease in α mRNA level occurs postnatally. No temporal or quantitative correlation between the up-regulation of β mRNA and the down-regulation of α mRNA levels is observed throughout hindlimb muscle development. Quantitative immunoblotting analyses carried out in parallel show that the enolase isozymic transition is mainly controlled at the mRNA level. The developmental profile of α-skeletal actin mRNA matches the biphasic pattern of β enolase mRNA. Interestingly, the second phase of transcript increases coincides with that of the mRNA of an adult-specific myosin heavy chain, the accumulation of which depends on increasing thyroid hormone levels. It has been proposed that thyroid hormones could coordinate the expression of contractile proteins and enzymes of energy metabolism during the postnatal muscle development. Our results support this notion and suggest a role of thyroid hormones in the regulation of the β enolase gene.