Creatine kinase transcript accumulation: Effect of nerve during muscle development

To determine the role of the nerve in regulating the accumulation of cytopl asmic creatine kinase (CK) mRNAs in hindleg muscles of the developing mouse , the lumbosacral spinal cords of 14-day gestation mice (E14) were laser ab lated, and the accumulation of muscle CK (MCK) and brain CK (BCK) mRNAs was evaluated just prior to birth with in situ hybridization. Numbers of molec ules of each of these transcripts/ng total RNA in the soleus and extensor d igitorum longus (EDL) muscles were determined with competitive PCR and comp ared to transcripts found in innervated crural muscles. Data suggest that: 1) the level of BCK mRNA accumulation in innervated hindlimb muscles peaks at E16.5 and remains at fetal levels until the second month postnatal, when it falls to the level found in the adult, Given that MCK transcripts meet or exceed adult levels by day 28 postnatal, the "down-regulation" of the BC K gene and the "up-regulation" of the MCK gene are not tightly coupled; 2) the developmental switch from BCK to MCK, as the dominant cytoplasmic CK mR NA, occurs in innervated and aneural leg muscles between E14 and E16.5, ind icating this switch is not nerve dependent; 3) the absence of innervation h as no effect on BCK mRNA accumulation. MCK transcripts/ng total RNA continu e to increase in aneural muscle throughout the late fetal period, but from E16.5-E19.5 the MCK transcript levels in aneural muscles become progressive ly lower than in age-matched innervated muscles. Thus, the accumulation of the muscle specific cytoplasmic CK, but not BCK, transcripts is affected by the absence of innervation during the fetal period, (C) 1999 Wiley-Liss, I nc.