Extraocular Myosin Heavy Chain Research Articles

Sarcomeric Myosin Expression in the Tongue Body of Humans, Macaques and Rats

Expression of developmental and unconventional myosin heavy chain (MHC) isoforms in some adult head and neck muscles is thought to reflect specific contractile demands of muscle fibers active during kinematically complex movements. Mammalian tongue muscles are active during oromotor behaviors that encompass a wide range of tongue movement speeds and tongue shape changes (e.g. respiration, oral transport, swallowing, rejection), but the extent to which tongue muscles express developmental and unconventional MHC is not known. Quantitative PCR was used to determine the mRNA content of conventional MHC-beta, MHC-2a, MHC-2b and MHC-2x, the developmental isoforms embryonic MHC and neonatal MHC and the unconventional isoforms atrial/cardiac-α MHC (MHC-alpha), extraocular MHC, masseter MHC and slow tonic MHC in tongue body muscles of the rat, macaque and human. In all species, conventional MHC isoforms predominate. MHC-2b and MHC-2x account for 98% of total MHC mRNA in the rat. MHC-2a, MHC-2x and MHC-beta account for 94% of total MHC mRNA in humans and 96% of total MHC mRNA in macaque. With the exception of MHC-alpha in humans (5%), developmental and unconventional MHC mRNA represents less than 0.3% of total MHC mRNA. We conclude that in these species, there is limited expression of developmental and unconventional MHC and that diversity of tongue body muscle fiber contractile properties is achieved primarily by MHC-beta, MHC-2a, MHC-2x and MHC-2b. Whether expression of MHC-alpha mRNA in tongue is unique to humans or present in other hominoids awaits further investigation.

Phylogenetic implications of the superfast myosin in extraocular muscles.

Extraocular muscle exhibits higher-velocity and lower-tension contractions than other vertebrate striated muscles. These distinctive physiological properties are associated with the expression of a novel extraocular myosin heavy chain (MYH). Encoded by the MYH13 gene, the extraocular myosin heavy chain is a member of the fast/developmental MYH gene cluster on human chromosome 17 and the syntenic MYH cluster on mouse chromosome 11. Comparison of cDNA sequences reveals that MYH13 also encodes the atypical MYH identified in laryngeal muscles, which have similar fast contractile properties. Comparing the MYH13 sequence with the other members of the fast/developmental cluster, the slow/cardiac MYH genes and two orphan skeletal MYH genes in the human genome provides insights into the origins of specialization in striated muscle myosins. Specifically, these studies indicate (i) that the extraocular myosin is not derived from the adult fast skeletal muscle myosins, but was the first member of the fast/developmental MYH gene cluster to diverge and specialize, (ii) that the motor and rod domains of the MYH13 have evolved under different selective pressures and (iii) that the MYH13 gene has been largely insulated from genomic events that have shaped other members of the fast/developmental cluster. In addition, phylogenetic footprinting suggests that regulation of the extraocular MYH gene is not governed primarily by myogenic factors, but by a hierarchical network of regulatory factors that relate its expression to the development of extraocular muscles.

Autosomal dominant myopathy: missense mutation (Glu-706 --> Lys) in the myosin heavy chain IIa gene.

We here report on a human myopathy associated with a mutation in a fast myosin heavy chain (MyHC) gene, and also the genetic defect in a hereditary inclusion body myopathy. The disorder has previously been described in a family with an "autosomal dominant myopathy, with joint contractures, ophthalmoplegia, and rimmed vacuoles." Linkage analysis and radiation hybrid mapping showed that the gene locus (Human Genome Map locus name: IBM3) is situated in a 2-Mb region of chromosome 17p13, where also a cluster of MyHC genes is located. These include the genes encoding embryonic, IIa, IIx/d, IIb, perinatal, and extraocular MyHCs. Morphological analysis of muscle biopsies from patients from the family indicated to us that the type 2A fibers frequently were abnormal, whereas other fiber types appeared normal. This observation prompted us to investigate the MyHC-IIa gene, since MyHC-IIa is the major isoform in type 2A fibers. The complete genomic sequence for this gene was deduced by using an "in silico" strategy. The gene, found to consist of 38 exons, was subjected to a complete mutation scan in patients and controls. We identified a missense mutation, Glu-706 --> Lys, which is located in a highly conserved region of the motor domain, the so-called SH1 helix region. By conformational changes this region communicates activity at the nucleotide-binding site to the neck region, resulting in the lever arm swing. The mutation in this region is likely to result in a dysfunctional myosin, compatible with the disorder in the family.

Fibre types and myosin heavy chain expression in the ocular medial rectus muscle of the adult rat.

Myosin heavy chain (MHC) expression was determined immunohistochemically in individual muscle fibre types characterised by activities of ATPase and the key oxidative and glycolytic enzymes in rat ocular medial rectus (MR) muscles. In the global layer (GL), glycolytic activity of muscle fibres was higher and oxidative activity lower, than in the orbital layer (OL). Muscle fibres in the former displayed rosette-like organisation with a slow fibre surrounded by several fast fibres, which expressed either MHCIIa or MHCIIb, but many co-expressed both isoforms. In the OL some slow fibres co-expressed MHCIIa. Extraocular MHC isoform (MHCeom) could not be determined immunohistochemically and no pure MHCIIx/d containing fibres were found, suggesting that these isoforms, demonstrated electrophoretically, are co-expressed with others. Slow muscle fibres in both layers co-expressed MHCbeta slow, MHCalpha cardiac and MHC-slow tonic. Neonatal isoform (MHCneo) was co-expressed in several fast and slow muscle fibres in the orbital, but not global layer. Slow fibres in the GL displayed very low oxidative activity. Electrophoretic analysis of ocular MR muscle homogenates revealed that about 50% of total MHC was MHCIIb, MHCeom was quite prominent (25%), and MHCIIa, MHCIIx/d and MHCI contributed each about 8%. MHCneo, MHCslow tonic and MHCalpha cardiac could not be identified as separate bands.

Quantification of myosin heavy chain mRNA in somatic and branchial arch muscles using competitive PCR.

The purpose of this study was to quantify the type and amount of myosin heavy chain (MHC) mRNA within muscles of different developmental origins to determine whether the regulation of gene expression is comparable. Seven MHC isoforms were analyzed in rat adult limb (extensor digitorum longus, tibialis anterior, and soleus) and nonlimb (extraocular, thyroarytenoid, diaphragm, and masseter) muscles using a competitive PCR assay. An exogenous template that included oligonucleotide sequences specific for seven rat sarcomeric MHC isoforms (beta-cardiac, 2A, 2X, 2B, extraocular, embryonic, and neonatal) as well as beta-actin was constructed and used as the competitor. Only the extraocular muscle contained all seven isoforms. All seven muscles contained type 2A and type 2X MHC transcripts in varying percentages. As expected, the soleus muscle contained primarily beta-cardiac MHC (87.8 +/- 2.6%). Extraocular MHC was found only in the extraocular and thyroarytenoid muscles and in relatively small proportions (7.4 +/- 1.5% and 4.0 +/- 0.7%, respectively). Neonatal MHC was identified in extraocular (7.9 +/- 0. 3%), thyroarytenoid (4.4 +/- 0.4%), and masseter (1.0 +/- 0.2%) muscles, and embryonic MHC was identified both in extraocular (1.2 +/- 0.5%) and, unexpectedly, in soleus (0.6 +/- 0.1%) muscles. Absolute MHC mRNA mass was greatest in the masseter (106 pg/0.5 microg RNA) and least for the tibialis anterior (64 pg/0.5 microg RNA). These values suggest that MHC mRNA represents from 4 to 17% of the total mRNA pool in various skeletal muscles. Differences in MHC profile between somatic and branchial arch muscles suggest that the developmental origin of a muscle may, at least in part, be responsible for the MHC expression program that is implemented in the adult. An inverse relationship between the expression of beta-cardiac and type 2B MHC transcripts across muscles was noted, suggesting that the expression of these two isoforms may be reciprocally regulated.