Complete cDNA sequence of rat atrial myosin light chain 1: patterns of expression during development and with hypertension.

Distinct atrial and ventricular isoforms of myosin light chain 1 (LC1) exist in mammals. The atrial LC1 is also expressed in fetal ventricular and skeletal muscle. Here we present a full length cDNA encoding a rat atrial LC1, based upon homology with previously reported LC1 sequences and its atrial-specific pattern of RNA hybridization in adult cardiac muscle. Atrial and ventricular RNA expression were studied during rat development and with chronic hypertension. Atrial LC1 mRNA was expressed in rat atria throughout development, and was coexpressed with ventricular LC1 mRNA in the hearts of 12-day and 16-day embryos, and in the ventricles of newborn rats (less than 24 hours). In 9 day-old neonates, atrial LC1 mRNA expression was restricted to rat atrium. In adult rats exhibiting renovascular hypertension, the expression of the atrial and ventricular LC1 mRNAs was unchanged from that seen in normal control animals.     

Small RNA molecules related to the Alu family of repetitive DNA sequences.

A rodent 4.5S RNA molecule with extensive homology to the Alu family of interspersed repetitive DNA sequences has been found physically associated with polyadenylated nuclear and cytoplasmic RNAs (W. Jelinek and L. Leinwand, Cell 15:205-214, 1978; S. Haynes et al., Mol. Cell. Biol. 1:573-583, 1981). In this report, we describe a 4.5S RNA molecule in rat cells whose RNase fingerprints are identical to those of the equivalent mouse molecule. We show that the rat 4.5S RNA is part of a small family of RNA molecules, all sharing sequence homology to the Alu family of DNA sequences. These RNAs are synthesized by RNA polymerase III and are developmentally regulated and short-lived in the cytoplasm. Of this family of small RNAs, only the 4.5S RNA is found associated with polyadenylated RNA.     

The Chinese hamster Alu-equivalent sequence: a conserved highly repetitious, interspersed deoxyribonucleic acid sequence in mammals has a structure suggestive of a transposable element.

A consensus sequence has been determined for a major interspersed deoxyribonucleic acid repeat in the genome of Chinese hamster ovary cells (CHO cells). This sequence is extensively homologous to (i) the human Alu sequence (P. L. Deininger et al., J. Mol. Biol., in press), (ii) the mouse B1 interspersed repetitious sequence (Krayev et al., Nucleic Acids Res. 8:1201-1215, 1980) (iii) an interspersed repetitious sequence from African green monkey deoxyribonucleic acid (Dhruva et al., Proc. Natl. Acad. Sci. U.S.A. 77:4514-4518, 1980) and (iv) the CHO and mouse 4.5S ribonucleic acid (this report; F. Harada and N. Kato, Nucleic Acids Res. 8:1273-1285, 1980). Because the CHO consensus sequence shows significant homology to the human Alu sequence it is termed the CHO Alu-equivalent sequence. A conserved structure surrounding CHO Alu-equivalent family members can be recognized. It is similar to that surrounding the human Alu and the mouse B1 sequences, and is represented as follows: direct repeat-CHO-Alu-A-rich sequence-direct repeat. A composite interspersed repetitious sequence has been identified. Its structure is represented as follows: direct repeat-residue 47 to 107 of CHO-Alu-non-Alu repetitious sequence-A-rich sequence-direct repeat. Because the Alu flanking sequences resemble those that flank known transposable elements, we think it likely that the Alu sequence dispersed throughout the mammalian genome by transposition.     

Comparative sequence analysis of the complete human sarcomeric myosin heavy chain family: implications for functional diversity.

The conventional myosin motor proteins that drive mammalian skeletal and cardiac muscle contraction include eight sarcomeric myosin heavy chain (MyHC) isoforms. Six skeletal MyHCs are encoded by genes found in tightly linked clusters on human and mouse chromosomes 17 and 11, respectively. The full coding regions of only two out of six mammalian skeletal MyHCs had been sequenced prior to this work. In an effort to assess the extent of sequence diversity within the human MyHC family we present new full-length coding sequences corresponding to four additional human genes: MyHC-IIb, MyHC-extraocular, MyHC-IIa and MyHC-IIx/d. This represents the first opportunity to compare the full coding sequences of all eight sarcomeric MyHC isoforms within a vertebrate organism. Sequence variability has been analyzed in the context of available structure/function data with an emphasis on potential functional diversity within the family. Results indicate that functional diversity among MyHCs is likely to be accomplished by having small pockets of sequence diversity in an otherwise highly conserved molecule.     

Skeletal muscle adaptations to microgravity exposure in the mouse.

To investigate the effects of microgravity on murine skeletal muscle fiber size, muscle contractile protein, and enzymatic activity, female C57BL/6J mice, aged 64 days, were divided into animal enclosure module (AEM) ground control and spaceflight (SF) treatment groups. SF animals were flown on the space shuttle Endeavour (STS-108/UF-1) and subjected to approximately 11 days and 19 h of microgravity. Immunohistochemical analysis of muscle fiber cross-sectional area revealed that, in each of the muscles analyzed, mean muscle fiber cross-sectional area was significantly reduced (P < 0.0001) for all fiber types for SF vs. AEM control. In the soleus, immunohistochemical analysis of myosin heavy chain (MHC) isoform expression revealed a significant increase in the percentage of muscle fibers expressing MHC IIx and MHC IIb (P < 0.05). For the gastrocnemius and plantaris, no significant changes in MHC isoform expression were observed. For the muscles analyzed, no alterations in MHC I or MHC IIa protein expression were observed. Enzymatic analysis of the gastrocnemius revealed a significant decrease in citrate synthase activity in SF vs. AEM control.     

Skeletal muscle adaptations in response to voluntary wheel running in myosin heavy chain null mice.

10.1152/ japplphysiol.00832.2001.-To examine the effects of gene inactivation on the plasticity of skeletal muscle, mice null for a specific myosin heavy chain (MHC) isoform were subjected to a voluntary wheel-running paradigm. Despite reduced running performance compared with nontransgenic C57BL/6 mice (NTG), both MHC IIb and MHC IId/x null animals exhibited increased muscle fiber size and muscle oxidative capacity with wheel running. In the MHC IIb null animals, there was no significant change in the percentage of muscle fibers expressing a particular MHC isoform with voluntary wheel running at any time point. In MHC IId/x null mice, wheel running produced a significant increase in the percentage of fibers expressing MHC IIa and MHC I and a significant decrease in the percentage of fibers expressing MHC IIb. Muscle pathology was not affected by wheel running for either MHC null strain. In summary, despite their phenotypes, MHC null mice do engage in voluntary wheel running. Although this wheel-running activity is lessened compared with NTG, there is evidence of distinct patterns of muscle adaptation in both null strains.     

Assignment of the gene for adenosine kinase to chromosome 14 in Mus musculus by somatic cell hybridization.

Evidence is presented for the assignment of the gene for adenosine kinase to Mus musculus chromosome 14 by synteny testing and karyotypic analysis of mouse X Chinese hamster somatic cell hybrid clones. ADOK and two enzymes previously mapped to mouse chromosome 14, NP and ES-10, were expressed concordantly in 29 hybrid clones. Chromosome analysis confirmed this assignment. Syntenic evidence is also presented using several clones of a gene transfer system in which the gene for human HPRT had integrated into modified mouse chromosome 14's.     

Impaired Skeletal Muscle Regeneration in the Absence of Fibrosis during Hibernation in 13-Lined Ground Squirrels

Skeletal muscle atrophy can occur as a consequence of immobilization and/or starvation in the majority of vertebrates studied. In contrast, hibernating mammals are protected against the loss of muscle mass despite long periods of inactivity and lack of food intake. Resident muscle-specific stem cells (satellite cells) are known to be activated by muscle injury and their activation contributes to the regeneration of muscle, but whether satellite cells play a role in hibernation is unknown. In the hibernating 13-lined ground squirrel we show that muscles ablated of satellite cells were still protected against atrophy, demonstrating that satellite cells are not involved in the maintenance of skeletal muscle during hibernation. Additionally, hibernating skeletal muscle showed extremely slow regeneration in response to injury, due to repression of satellite cell activation and myoblast differentiation caused by a fine-tuned interplay of p21, myostatin, MAPK, and Wnt signaling pathways. Interestingly, despite long periods of inflammation and lack of efficient regeneration, injured skeletal muscle from hibernating animals did not develop fibrosis and was capable of complete recovery when animals emerged naturally from hibernation. We propose that hibernating squirrels represent a new model system that permits evaluation of impaired skeletal muscle remodeling in the absence of formation of tissue fibrosis.     

Akt and MAPK signaling mediate pregnancy-induced cardiac adaptation

Although the signaling pathways underlying exercise-induced cardiac adaptation have been extensively studied, little is known about the molecular mechanisms that result in the response of the heart to pregnancy. The objective of this study was to define the morphological, functional, and gene expression patterns that define the hearts of pregnant mice, and to identify the signaling pathways that mediate this response. Mice were divided into three groups: nonpregnant diestrus control, midpregnancy, and late pregnancy. Both time points of pregnancy were associated with significant cardiac hypertrophy. The prosurvival signaling cascades of Akt and ERK1/2 were activated in the hearts of pregnant mice, while the stress kinase, p38, was decreased. Given the activation of Akt in pregnancy and its known role in cardiac hypertrophy, the hypertrophic response to pregnancy was tested in mice expressing a cardiac-specific activated (myristoylated) form of Akt (myrAkt) or a cardiac-specific constitutively active (antipathologic hypertrophic) form of its downstream target, glycogen synthase kinase 3β (caGSK3β). The pregnancy-induced hypertrophic responses of hearts from these mice were significantly attenuated. Finally, we tested whether pregnancy-associated sex hormones could induce hypertrophy and alter signaling pathways in isolated neonatal rat ventricular myocytes (NRVMs). In fact, progesterone, but not estradiol treatment increased NRVM cell size via phosphorylation of ERK1/2. Inhibition of MEK1 effectively blocked progesterone-induced cellular hypertrophy. Taken together, our study demonstrates that pregnancy-induced cardiac hypertrophy is mediated by activation of Akt and ERK1/2 pathways.