Myotonic dystrophy (DM) is commonly associated with CTG repeat expansions within the gene for DM-protein kinase (DMPK). The effect of altered expression levels of DMPK, which is ubiquitously expressed in all muscle cell lineages during development, was examined by disrupting the endogenous Dmpk gene and overexpressing a normal human DMPK transgene in mice. Nullizygous (-/-) mice showed only inconsistent and minor size changes in head and neck muscle fibres at older age, animals with the highest DMPK transgene expression showed hypertrophic cardiomyopathy and enhanced neonatal mortality. However, both models lack other frequent DM symptoms including the fibre-type dependent atrophy, myotonia, cataract and male-infertility. These results strengthen the contention that simple loss- or gain-of-expression of DMPK is not the only crucial requirement for development of the disease.

The myosin heavy chain (HC) composition of various rabbit muscles was analysed at both the mRNA and the protein level. S1-nuclease mapping was performed with a cDNA probe specific for myosin HCIIa, yielding a fully protected sequence for HCIIa, a partially protected sequence for HCIIb, and an additional signal putatively assigned to HCIId. At the protein level, three fast myosin HC isoforms, HCIIa, HCIIb and HCIId, were separated by gradient PAGE. The results obtained at the protein level were in agreement with the findings at the mRNA level. The expression of appreciable amounts of myosin HCIIb, the predominating isoform of fast-twitch muscles in rat and mouse, was restricted in the rabbit to only a few muscles, i.e. adductor magnus, gastrocnemius, latissimus dorsi and vastus lateralis. Typical fast-twitch muscles such as extensor digitorum longus, tibialis anterior and psoas contained only minute amounts of HCIIb. The HCIId isoform, demonstrated in the present study for the fist time in rabbit, is the predominating fast myosin HC isoform in this species. Electrophoretic analyses of myosin HC in histochemically defined single fibers confiied the lack of fibers expressing only HCIIb in tibialis anterior, whereas such fibers were found in the adductor rnagnus. In addition to fiber types IIB, IID, and IIA expressing HCIIb, HCIId, and HCIIa, respectively, an appreciable amount of hybrid fibers coexpressing two HC isoforms at various ratios were found: HCIIb > HCIId; HCIId > HCIIb; HCIId > HCIIa; HCIIa > HCIId; HCIIa > HCI; HCI > HCIIa. This fiber-type spectrum indicates possible fiber-type transitions in the order IIB t)A third fast myosin heavy chain (HC) isofom has been demonstrated in rat limb musculature by two laboratories [l -51 (for review see [6]), termed HC2x by Schiaffho et al. [l] and, because of its abundance in rat diaphragm, HCIId by Biir and Pette [4]. Assuming that HC2x and HCIId are the same, this brings the total number of fast myosin HC isoforms in rat limb musculature to three, i.e. HCIIa, HCIIb and HCIId/HC2x. Subsequent studies have revealed the presence of HCIId also in mouse [7-91 and guinea pig [8]. In light of the fact that numerous data have accumulated on the physiological and biochemical properties of rabbit muscle, it was of interest to search for the possible presence and distribution of this new isoform also in this species.In the present investigation, the myosin HC isoforms were studied at the mRNA and protein levels of wholemuscle extracts combined with protein analysis in single fibers. The presence of different myosin HC mRNA isoforms was studied by S1-nuclease mapping using a cDNA probe specific for fast myosin HC mRNA isoforms HCHa and HCIIb. At the protein level, the fast myosin HC composition was assessed by the use of an improved electrophoretic technique [5]. By combining these methods, it has become clear that HCIId, and not HCIIb, is the predominant fast isoform in rabbit fast-twitch muscles. The same conclusion has been drawn from an independent histochemical study com...

Time-dependent changes in myosin heavy chain (MHC) isoform expression were investigated in rat soleus muscle unloaded by hindlimb suspension. Changes at the mRNA level were measured by RT-PCR and correlated with changes in the pattern of MHC protein isoforms. Protein analyses of whole muscle revealed that MHCI decreased after 7 days, when MHCIIa had increased, reaching a transient maximum by 15 days. Longer periods led to inductions and progressive increases of MHCIId(x) and MHCIIb. mRNA analyses of whole muscle showed that MHCIId(x) displayed the steepest increase after 4 days and continued to rise until 28 days, the longest time period investigated. MHCIIb mRNA followed a similar time course, although at lower levels. MHCIalpha mRNA, present at extremely low levels in control soleus, peaked after 4 days, stayed elevated until 15 days, and then decayed. Immunohistochemistry of 15-day unloaded muscles revealed that MHCIalpha was present in muscle spindles but at low amounts also in extrafusal fibers. The slow-to-fast transitions thus seem to proceed in the order MHCIbeta --> MHCIIa --> MHCIId(x) --> MHCIIb. Our findings indicate that MHCIalpha is transiently upregulated in some fibers as an intermediate step during the transition from MHCIbeta to MHCIIa.

To investigate the plasticity of slow and fast muscles undergoing slow-to-fast transition, rat soleus (SOL), gastrocnemius (GAS), and extensor digitorum longus (EDL) muscles were exposed for 14 days to 1) unweighting by hindlimb suspension (HU), or 2) treatment with the beta(2)-adrenergic agonist clenbuterol (CB), or 3) a combination of both (HU-CB). In general, HU elicited atrophy, CB induced hypertrophy, and HU-CB partially counteracted the HU-induced atrophy. Analyses of myosin heavy (MHC) and light chain (MLC) isoforms revealed HU- and CB-induced slow-to-fast transitions in SOL (increases of MHCIIa with small amounts of MHCIId and MHCIIb) and the upregulation of the slow MHCIa isoform. The HU- and CB-induced changes in GAS consisted of increases in MHCIId and MHCIIb ("fast-to-faster transitions"). Changes in the MLC composition of SOL and GAS consisted of slow-to-fast transitions and mainly encompassed an exchange of MLC1s with MLC1f. In addition, MLC3f was elevated whenever MHCIId and MHCIIb isoforms were increased. Because the EDL is predominantly composed of type IID and IIB fibers, HU, CB, and HU-CB had no significant effect on the MHC and MLC patterns.