Effects of thyroid hormone receptor gene disruption on myosin isoform expression in mouse skeletal muscles

Abstract

Skeletal muscle is known to be a target for the active metabolite of thyroid hormone, i.e., 3,5,3'-triiodothyronine (T(3)). T(3) acts by repressing or activating genes coding for different myosin heavy chain (MHC) isoforms via T(3) receptors (TRs). The diverse function of T(3) is presumed to be mediated by TR-alpha(1) and TR-beta, but the function of specific TRs in regulating MHC isoform expression has remained undefined. In this study, TR-deficient mice were used to expand our knowledge of the mechanisms by which T(3) regulates the expression of specific MHC isoforms via distinct TRs. In fast-twitch extensor digitorum longus (EDL) muscle, TR-alpha(1)-, TR-beta-, or TR-alpha(1)beta-deficient mice showed a small but statistically significant decrease (P < 0.05) of type IIB MHC content and an increased number of type I fibers. In the slow-twitch soleus, the beta/slow MHC (type I) isoform was significantly (P < 0. 001) upregulated in the TR-deficient mice, but this effect was highly dependent on the type of receptor deleted. The lack of TR-beta had no significant effect on the expression of MHC isoforms. An increase (P < 0.05) of type I MHC was observed in the TR-alpha(1)-deficient muscle. A dramatic overexpression (P < 0.001) of the slow type I MHC and a corresponding downregulation of the fast type IIA MHC (P < 0.001) was observed in TR-alpha(1)beta-deficient mice. The muscle- and fiber-specific differences in MHC isoform expression in the TR-alpha(1)beta-deficient mice resembled the MHC isoform transitions reported in hypothyroid animals, i.e., a mild MHC transition in the EDL, a dramatic but not complete upregulation of the beta/slow MHC isoform in the soleus, and a variable response to TR deficiency in different soleus muscle fibers. Thus the consequences on muscle are similar in the absence of thyroid hormone or absence of thyroid hormone receptors, indicating that TR-alpha(1) and TR-beta together mediate the known actions of T(3). However, it remains unknown how thyroid hormone exerts muscle- and muscle fiber-specific effects in its action. Finally, although developmental MHC transitions were not studied specifically in this study, the absence of embryonic and fetal MHC isoforms in the TR-deficient mice indicates that ultimately the transition to the adult MHC isoforms is not solely mediated by TRs.

Aberrant expression of myosin isoforms in skeletal muscles from mice lacking therev-erbAαorphan receptor gene

“…TR␣ 1 expression and thyroid hormone ␤-receptor expression have strong impacts on skeletal muscle expression of MyHC isoforms, especially in the slow-twitch soleus (54). The influence of the nonligand-binding TR␣ 2 on skeletal muscle MyHC isoform expression, on the other hand, remains unknown.…”

“…Furthermore, rodent soleus muscle contains different populations of type I fibers that are indistinguishable with respect to enzyme histochemical fiber type and MyHC isoform expression but differ from each other in the response to denervation and to thyroid hormone (49). The variable expression of the rev-erbA␣ gene between and within soleus fibers and the modulatory role of the reverbA␣ gene in thyroid hormone action on MyHC synthesis may accordingly contribute to, or cause, the variable response to circulating thyroid hormone levels and nuclear thyroid hormone receptor deficiency in the slow-twitch soleus (25,28,29,53,54).…”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…The MHC isoforms, as well as the MLC isoforms, can be separated by high-resolution gel electrophoresis. Silver staining and subsequent densitometric analysis allows accurate determination of the relative expression levels of the various isoforms in samples as small as single fibers (679,692,699,701,702). Antibodies for Western analysis are available for a large number of skeletal muscle proteins encoded by genes which are directly or indirectly regulated by thyroid hormone, including MyoD, myogenin, MHC isoforms, SERCA1a, SERCA2a, myoglobin, PGC-1a, RYR, and Na + -K + -ATPase subunits (92,691,701,703,704).…”

“…Antibodies discriminating between these isoforms have been described (708) and although some caution is warranted when using currently available commercial antibodies (709), multicolor immunofluorescence analysis has been described for the simultaneous assessment of expression of all MHC isoforms in rat and mouse skeletal muscle using these antibodies (710). More typically, a histochemical method for myofibrillar ATPase expression is widely used which distinguishes between Type I fibers (MHC I), Type IIB fibers (MHC IIb), and intermediate fibers (MHC IIa, MHC IIx/d) (684,692,700,702,711). Muscles are clamped at their approximate in situ length and frozen in liquid N 2 -cooled freon or isopentane.…”

Concerted regulation of myofiber-specific gene expression and muscle performance by the transcriptional repressor Sox6

“…Chronic muscle stimulation induces a reversible transition in myofiber phenotype from fast to slow, which occurs in a sequential manner through several fast myofiber intermediates (type IIb → type IIx/d → type IIa → type I) (7). Previous studies have implicated multiple transcriptional pathways in the regulation of basal-muscle fiber-type specific gene expression, stress-induced fiber-type remodeling, and myofiber metabolism (8)(9)(10)(11)(12)(13)(14)(15)(16)(17).…”