Biomedical subjects

John D Porter

Publications and source records attributed to John D Porter.

At least 19 recordsLinked to original sources

PubMed2007

Challenges in drug development for muscle disease: a stakeholders' meeting.

Current treatment benefits for patients with muscle disease are limited, but progress in legislative and scientific initiatives have set the stage for the development of new therapies. The MD-CARE Act (Public Law 107-84), which allocates federal resources to muscular dystrophy, was approved by Congress and signed into law by the President of the United States in 2001. This has shifted the emphasis toward translational research. To facilitate a push toward therapy for muscle disorders, the Muscular Dystrophy Association (MDA) sponsored a meeting with representatives from industry, the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and other government agencies and academia. Each contributed in different ways. The FDA helped define the necessary data to support investigational new drug (IND) applications including the design of proof-of-principle studies, outcome measures for clinical trials, and the pathway for developing surrogate measures for fast-tracking promising new drugs. The NIH, other government agencies, and the MDA described potential funding sources for translational research. Industry delineated a complementary role with academia, and academic investigators elucidated the current strengths and weaknesses of available clinical endpoints. The meeting provided a format for communication for diverse disciplines that usually have no common meeting ground, helping to lay the foundation for bringing products to market in a timely fashion.

Academic Medical Centers↗

PubMed2006

Analysis of gene expression differences between utrophin/dystrophin-deficient vs mdx skeletal muscles reveals a specific upregulation of slow muscle genes in limb muscles.

Dystrophin deficiency leads to the progressive muscle wasting disease Duchenne muscular dystrophy (DMD). Dystrophin-deficient mdx mice are characterized by skeletal muscle weakness and degeneration but they appear outwardly normal in contrast to DMD patients. Mice lacking both dystrophin and the dystrophin homolog utrophin [double knockout (dko)] have muscle degeneration similar to mdx mice, but they display clinical features similar to DMD patients. Dko limb muscles also lack postsynaptic membrane folding and display fiber-type abnormalities including an abundance of phenotypically oxidative muscle fibers. Extraocular muscles, which are spared in mdx mice, show a significant pathology in dko mice. In this study, microarray analysis was used to characterize gene expression differences between mdx and dko tibialis anterior and extraocular skeletal muscles in an effort to understand the phenotypic differences between these two dystrophic mouse models. Analysis of gene expression differences showed that upregulation of slow muscle genes specifically characterizes dko limb muscle and suggests that upregulation of these genes may directly account for the more severe phenotype of dko mice. To investigate whether any upregulation of slow genes is retained in vitro, independent of postsynaptic membrane abnormalities, we derived mdx and dko primary myogenic cultures and analyzed the expression of Myh7 and Myl2. Real-time reverse transcriptase-polymerase chain reaction analysis demonstrates that transcription of these slow genes is also upregulated in dko vs mdx myotubes. This data suggests that at least part of the fiber-type abnormality is due directly to the combined absence of utrophin and dystrophin and is not an indirect effect of the postsynaptic membrane abnormalities.

Animals↗

PubMed2006

Biological organization of the extraocular muscles.

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

Animals↗

PubMed2005

Temporal and spatial mRNA expression patterns of TGF-beta1, 2, 3 and TbetaRI, II, III in skeletal muscles of mdx mice.

To address potential regulatory roles of TGF-beta1 in muscle inflammation and fibrosis associated with dystrophin deficiency, we performed quantitative RT-PCR and in situ hybridization to characterize the temporal and spatial mRNA expression patterns of TGF-beta1 and other TGF-beta subfamily members, TGF-beta2 and TGF-beta3, as well as their receptors, in quadriceps and diaphragm muscles of mdx mice. TGF-beta1 mRNA was markedly upregulated in the endomysial inflammatory cells and regenerating fibers of mdx quadriceps and diaphragm, with the mRNA levels correlated with the degree of endomysial inflammation. Upregulation of TGF-beta2, beta3, and their receptors was also appreciated but to a much lesser degree. While high levels of TGF-beta1 mRNA remained in the aging mdx quadriceps but not the diaphragm, progressive fibrosis only occurred in the diaphragm. Our data support a regulatory role for TGF-beta1 in muscle inflammation in mdx mice. It also suggests different susceptibility of quadriceps and diaphragm muscles to fibrosis induced by TGF-beta1 signaling pathway.

Animals↗

PubMed2005

Distinctive morphological and gene/protein expression signatures during myogenesis in novel cell lines from extraocular and hindlimb muscle.

Skeletal muscles are not created equal. The underutilized concept of muscle allotypes defines distinct muscle groups that differ in their intrinsic capacity to express novel traits when exposed to a facilitating extrinsic environment. Allotype-specific traits may have significance as determinants of the preferential involvement or sparing of muscle groups that is observed in a variety of neuromuscular diseases. Little is known, however, of the developmental mechanisms underlying the distinctive skeletal muscle allotypes. The lack of appropriate in vitro models, to dissociate the cell-autonomous and non-cell-autonomous mechanisms behind allotype diversity, has been a barrier to such studies. Here, we derived novel cell lines from the extraocular and hindlimb muscle allotypes and assessed their similarities and differences during early myogenesis using morphological and gene/protein expression profiling tools. Our data establish that there are fundamental differences in the transcriptional and cellular signaling pathways used by the two myoblast lineages. Taken together, these data show that myoblast lineage plays a significant role in the divergence of the distinctive muscle groups or allotypes.

Animals↗

PubMed2005

FGF-2 enhances the mitotic and chondrogenic potentials of human adult bone marrow-derived mesenchymal stem cells.

Human mesenchymal stem cells (hMSCs) expanded with and without fibroblast growth factor (FGF) supplementation were compared with respect to their proliferation rate, ability to differentiate along the chondrogenic pathway in vitro, and their gene expression profiles. hMSCs expanded in FGF-supplemented medium were smaller and proliferated more rapidly than hMSCs expanded in control conditions. Chondrogenic cultures made with FGF-treated cells were larger and contain more proteoglycan than those made with control cells. Furthermore, aggregates of FGF-treated cells lacked the collagen type I-positive and collagen type II-negative outer layer characteristic of aggregates of control cells. A total of 358 unique transcripts were differentially expressed in FGF-treated hMSCs. Of these, 150 were upregulated and 208 downregulated. Seventeen percent of these genes affect proliferation. Known genes associated with cellular signaling functions comprised the largest percentage ( approximately 20%) of differentially expressed transcripts. Eighty percent of differentially expressed extracellular matrix-related genes were downregulated. The present findings that FGF-2 enhances proliferation and differentiation of hMSCs adds to a growing body of evidence that cytokines modulate the differentiation potential and, perhaps, the multipotentiality of adult stem cells. With the generation of gene expression profiles of FGF-treated and control cells we have taken the first steps in the elucidation of the molecular mechanism(s) behind these phenomena.

Age Factors↗

PubMed2004

Conserved and muscle-group-specific gene expression patterns shape postnatal development of the novel extraocular muscle phenotype.

Current models in skeletal muscle biology do not fully account for the breadth, causes, and consequences of phenotypic variation among skeletal muscle groups. The muscle allotype concept arose to explain frank differences between limb, masticatory, and extraocular (EOM) muscles, but there is little understanding of the developmental regulation of the skeletal muscle phenotypic range. Here, we used morphological and DNA microarray analyses to generate a comprehensive temporal profile for rat EOM development. Based upon coordinate regulation of morphologic/gene expression traits with key events in visual, vestibular, and oculomotor system development, we propose a model that the EOM phenotype is a consequence of extrinsic factors that are unique to its local environment and sensory-motor control system, acting upon a novel myoblast lineage. We identified a broad spectrum of differences between the postnatal transcriptional patterns of EOM and limb muscle allotypes, including numerous transcripts not traditionally associated with muscle fiber/group differences. Several transcription factors were differentially regulated and may be responsible for signaling muscle allotype specificity. Significant differences in cellular energetic mechanisms defined the EOM and limb allotypes. The allotypes were divergent in many other functional transcript classes that remain to be further explored. Taken together, we suggest that the EOM allotype is the consequence of tissue-specific mechanisms that direct expression of a limited number of EOM-specific transcripts and broader, incremental differences in transcripts that are conserved by the two allotypes. This represents an important first step in dissecting allotype-specific regulatory mechanisms that may, in turn, explain differential muscle group sensitivity to a variety of metabolic and neuromuscular diseases.

Animals↗

PubMed2004

Genome-wide transcriptional profiles are consistent with functional specialization of the extraocular muscle layers.

PURPOSE: Compartmentalization of the extraocular muscles into well-defined orbital and global layers is highly conserved. Recently, the active pulley hypothesis correlated the anatomic properties of orbital-global muscle layers with layer-specific division of labor. Microarray technology was used to identify muscle-layer-specific transcriptional profiles and, thereby, extend understanding of the structure-function characteristics of extraocular muscle layers. METHODS: Laser capture microdissection was used to obtain muscle layer samples from monkey medial rectus muscles. RNA was linearly amplified and hybridized to human U133 series microarrays (Affymetrix, Santa Clara, CA), which have sufficient sequence homology for use in subhuman primates. Data was analyzed using Affymetrix and Robust Multichip Average (RMA) algorithms. Select transcripts were verified by quantitative PCR and in situ hybridization. RESULTS: A broad spectrum of transcriptional differences (> 181 transcripts) was identified between the two extraocular muscle layers. Patterned differences in the sarcomeric contractile machinery and cytoskeleton were suggestive of key layer differences in contraction speed. Differentially expressed transcript identities, however, extended well beyond those traditionally associated with muscle-fiber-group differences. CONCLUSIONS: Muscle layer transcriptional profiles correlated with the different loads and usage patterns of extraocular muscle layers, as proposed in the active pulley hypothesis. The magnitude and breadth of orbital-global layer expression differences strongly suggests that oculomotor control systems may drive two distinct motor output pathways, each comprising separate motoneurons and muscle fibers, with one output path adapted to determining pulley position and the other to movement of the eye.

Animals↗

PubMed2004

The development of longitudinal variation of Myosin isoforms in the orbital fibers of extraocular muscles of rats.

PURPOSE: To examine the appearance of longitudinal variation of extraocular and embryonic myosin heavy chain (MyHC) isoforms during the development of orbital singly innervated fibers of rat extraocular muscles (EOMs). METHODS: EOMs were dissected from rat pups of various ages and stained with isoform-specific monoclonal antibodies to the embryonic and extraocular MyHC isoforms and to neurofilaments, as well as with labeled alpha-bungarotoxin. The orbital layers of whole muscles were examined by confocal microscopy. RNase protection assays for the embryonic (Myh3) and extraocular (Myh13) MyHC isoform mRNAs were also performed. RESULTS: At 10 days postpartum, the EOM MyHC RNA was first detected by RNase protection assay. At 11 days postpartum, the extraocular isoform was detected in the orbital fibers as two thin stripes just proximal and distal to the neuromuscular junction (NMJ). Over the next few weeks, the area occupied by the extraocular isoform increased to include the entire central region of the orbital fibers at and surrounding the NMJ. At the same time, the embryonic isoform became excluded from the region of the NMJ. CONCLUSIONS: The orbital layer fibers of rat EOMs contain a longitudinal variation in MyHC isoforms not seen in other skeletal muscles. Development of this longitudinal variation begins as a late event postpartum; and the first appearance of it may be closely linked to neural contact. This targeting of MyHC isoforms to distinct domains is unique to EOMs.

Animals↗

PubMed2003

Temporal gene expression profiling of dystrophin-deficient (mdx) mouse diaphragm identifies conserved and muscle group-specific mechanisms in the pathogenesis of muscular dystrophy.

Mutations in dystrophin are the proximate cause of Duchenne muscular dystrophy (DMD), but pathogenic mechanisms linking the absence of dystrophin from the sarcolemma to myofiber necrosis are not fully known. The muscular dystrophies also have properties not accounted for by current disease models, including the temporal delay to disease onset, broad species differences in severity, and diversity of skeletal muscle responses. To address the mechanisms underlying the differential targeting of muscular dystrophy, we characterized temporal expression profiles of the diaphragm in dystrophin-deficient (mdx) mice between postnatal days 7 and 112 using oligonucleotide microarrays and contrasted these data with published hindlimb muscle data. Although the diaphragm and hindlimb muscle groups differ in severity of response to dystrophin deficiency, and exhibited substantial divergence in some transcript categories including inflammation and muscle-specific genes, our data show that the general mechanisms operative in muscular dystrophy are highly conserved. The two muscle groups principally differed in expression levels of differentially regulated genes, as opposed to the non-conserved induced/repressed transcripts defining fundamentally distinct mechanisms. We also identified a postnatal divergence of the two wild-type muscle group expression profiles that temporally correlated with the onset and progression of the dystrophic process. These findings support the hypothesis that conserved disease mechanisms interacting with baseline differences in muscle group-specific transcriptomes underlie their differential responses to DMD. We further suggest that muscle group-specific transcriptional profiles contribute toward the muscle targeting and sparing patterns observed for a variety of metabolic and neuromuscular diseases.

Animals↗

PubMed2003

Dissection of temporal gene expression signatures of affected and spared muscle groups in dystrophin-deficient (mdx) mice.

Although dystrophin mutations are the proximate cause of Duchenne muscular dystrophy (DMD), interactions among heterogeneous downstream mechanisms may be key phenotypic determinants. Temporal gene expression profiling was used to identify and correlate diverse transcriptional patterns to one another and to the disease course, for both affected and spared muscle groups, in postnatal day 7-112 dystrophin-deficient (mdx) mice. While 719 transcripts were differentially expressed at one or more ages in leg muscle, only 56 genes were altered in the spared extraocular muscles (EOM). Contrasting molecular signatures of affected versus spared muscles provide compelling evidence that the absence of dystrophin alone is necessary but not sufficient to cause the patterned fibrosis, inflammation and failure of muscle regeneration characteristic of dystrophinopathy. Dystrophic and adaptive changes in the microarray profiles were further quantified using an aggregate disease load index (DLI) to measure stage-dependent transcriptional impact in both muscles. DLI analysis highlighted the divergent responses of EOM and leg muscle groups. Cellular process-specific DLIs in leg muscle identified positively correlated temporal expression profiles for some gene classes, and the independence of others, that are linked to major disease components. Data also showed a previously unrecognized transient and selective developmental delay in pre-necrotic mdx skeletal muscle that was confirmed by qPCR. Taken together, validation and targeting of signaling pathways responsible for the coordination of the fibrotic, proteolytic and inflammatory mechanisms shown here for mdx muscle may yield new therapeutic means of mitigating the devastating consequences of DMD.

Animals↗

PubMed2003

Paradoxical absence of M lines and downregulation of creatine kinase in mouse extraocular muscle.

The M lines are structural landmarks in striated muscles, necessary for sarcomeric stability and as anchoring sites for the M isoform of creatine kinase (CK-M). These structures, especially prominent in fast skeletal muscles, are missing in rodent extraocular muscle, a particularly fast and active muscle group. In this study, we tested the hypotheses that 1). myomesin and M protein (cytoskeletal components of the M lines) and CK-M are downregulated in mouse extraocular muscle compared with the leg muscles, gastrocnemius and soleus; and 2). the expression of other cytosolic and mitochondrial CK isoforms is correspondingly increased. As expected, mouse extraocular muscles expressed lower levels of myomesin, M protein, and CK-M mRNA than the leg muscles. Immunocytochemically, myomesin and M protein were not detected in the banding pattern typically seen in other skeletal muscles. Surprisingly, message abundance for the other known CK isoforms was also lower in the extraocular muscles. Moreover, total CK activity was significantly decreased compared with that in the leg muscles. Based on these data, we reject our second hypothesis and propose that other energy-buffering systems may be more important in the extraocular muscles. The downregulation of major structural and metabolic elements and relative overexpression of two adenylate kinase isoforms suggest that the extraocular muscle group copes with its functional requirements by using strategies not seen in typical skeletal muscles.

Adenylate Kinase↗

PubMed2003

Constitutive properties, not molecular adaptations, mediate extraocular muscle sparing in dystrophic mdx mice.

Extraocular muscle (EOM) is spared in Duchenne muscular dystrophy. Here, we tested putative EOM sparing mechanisms predicted from existing dystrophinopathy models. Data show that mdx mouse EOM contains dystrophin-glycoprotein complex (DGC)-competent and DGC-deficient myofibers distributed in a fiber type-specific pattern. Up-regulation of a dystrophin homologue, utrophin, mediates selective DGC retention. Counter to the DGC mechanical hypothesis, an intact DGC is not a precondition for EOM sarcolemmal integrity, and active adaptation at the level of calcium homeostasis is not mechanistic in protection. A partial, fiber type-specific retention of antiischemic nitric oxide to vascular smooth muscle signaling is not a factor in EOM sparing, because mice deficient in dystrophin and alpha-syntrophin, which localizes neuronal nitric oxide synthase to the sarcolemma, have normal EOMs. Moreover, an alternative transmembrane protein, alpha7beta1 integrin, does not appear to substitute for the DGC in EOM. Finally, genomewide expression profiling showed that EOM does not actively adapt to dystrophinopathy but identified candidate genes for the constitutive protection of mdx EOM. Taken together, data emphasize the conditional nature of dystrophinopathy and the potential importance of nonmechanical DGC roles and support the hypothesis that broad, constitutive structural cell signaling, and/or biochemical differences between EOM and other skeletal muscles are determinants of differential disease responsiveness.

Adaptation, Physiological↗

PubMed2003

Persistent over-expression of specific CC class chemokines correlates with macrophage and T-cell recruitment in mdx skeletal muscle.

Prior studies and the efficacy of immunotherapies provide evidence that inflammation is mechanistic in pathogenesis of Duchenne muscular dystrophy. To identify putative pro-inflammatory mechanisms, we evaluated chemokine gene/protein expression patterns in skeletal muscle of mdx mice. By DNA microarray, reverse transcription-polymerase chain reaction, quantitative polymerase chain reaction, and immunoblotting, convergent evidence established the induction of six distinct CC class chemokine ligands in adult MDX: CCL2/MCP-1, CCL5/RANTES, CCL6/mu C10, CCL7/MCP-3, CCL8/MCP-2, and CCL9/MIP-1gamma. CCL receptors, CCR2, CCR1, and CCR5, also showed increased expression in mdx muscle. CCL2 and CCL6 were localized to both monocular cells and muscle fibers, suggesting that dystrophic muscle may contribute toward chemotaxis. Temporal patterns of CCL2 and CCL6 showed early induction and maintained expression in mdx limb muscle. These data raise the possibility that chemokine signaling pathways coordinate a spatially and temporally discrete immune response that may contribute toward muscular dystrophy. The chemokine pro-inflammatory pathways described here in mdx may represent new targets for treatment of Duchenne muscular dystrophy.

Animals↗

PubMed2003

Comprehensive expression profiling by muscle tissue class and identification of the molecular niche of extraocular muscle.

Muscle tissue is an elegant model for biologic integration of structure with function and is frequently affected by a variety of inherited diseases. Traditional muscle classes--skeletal, cardiac, and smooth--share basic aspects of contractile and energetics mechanisms but also have distinctive role-specific adaptations. We used large-scale oligonucleotide microarrays to broaden knowledge of the adaptive expression patterns underlying muscle tissue differences and to identify transcript subsets that are most likely to represent candidate disease genes. Using stringent analysis criteria, we found >or=95 transcripts, which were preferentially expressed by each muscle class and were validated by inclusion of known muscle class-specific and inherited disease-related genes. Differentially expressed transcripts not previously identified as class-specific extend understanding of muscle class transcriptomes and may represent novel muscle-specific disease genes. We also analyzed the expression profile of extraocular muscle, which is divergent from other skeletal muscles, in the broader context of all major muscle classes. Data show that the extraocular muscle phenotype results from the combination of tissue-specific transcripts, novel expression levels of skeletal muscle transcripts, and partial sharing of gene expression patterns with cardiac and smooth muscle. These, and additional proteomic data, establish that extraocular muscle does not constitute a distinctive muscle class but that it does occupy a novel niche within the skeletal muscle class.

Animals↗

PubMed2003

Molecular organization of the extraocular muscle neuromuscular junction: partial conservation of and divergence from the skeletal muscle prototype.

PURPOSE: The phenotypically novel extraocular muscles (EOMs) exhibit fundamental differences in innervation and neuromuscular junction (NMJ) morphology from other skeletal muscles. In the current study, the morphology and molecular organization of NMJs of EOM singly innervated (SIF) and multiply innervated (MIF) fiber types were evaluated and the distribution of molecules involved in formation and maintenance of NMJs were specifically characterized. METHODS: Adult mouse EOM NMJ organization was examined by immunofluorescence and confocal microscopy. Differential cellular localization of components of two established synaptic signaling pathways, (1) neuregulin and erbB receptors 2, 3, and 4 and (2) agrin, MuSK, and rapsyn and select NMJ-associated structural proteins were studied for EOM SIF and MIF populations. Endplate topography and structure were also studied, using both confocal microscopy and transmission electron microscopy, with NMJ morphologic organization correlated with specific EOM fiber types. RESULTS: Confocal fluorescence microscopy demonstrated that, for NMJs of both EOM SIFs and MIFs, components of neuregulin and agrin pathways and the major components of the junctional dystrophin-glycoprotein complex (DGC) colocalized with acetylcholine receptor (AChR) aggregates. However, EOM exhibited novel fiber-type-specific extrasynaptic localization of two key DGC signaling-related molecules: alpha-dystrobrevin 1 (global MIFs) and syntrophin beta1 (global MIFs and orbital MIFs and SIFs). CONCLUSIONS: The data establish that the molecular organization of EOM SIF and MIF NMJs includes the same signaling and structural molecules previously characterized for other skeletal muscles. By contrast, divergence in other aspects of the synaptic and nonsynaptic sarcolemmal organization of EOM fiber types may underlie the unique responses of these muscles in a variety of neuromuscular disorders.

Animals↗

PubMed2003

Comprehensive evaluation of the extraocular muscle critical period by expression profiling in the dark-reared rat and monocularly deprived monkey.

PURPOSE: To address the consequences of visual deprivation paradigms in rat (dark rearing) and monkey (monocular deprivation) on extraocular muscle (EOM) development using genome-wide expression profiling. METHODS: Serial analysis of gene expression (SAGE) was used to determine alterations in the EOM transcriptome induced by dark rearing of rats from birth to postnatal day 45. Data were compared with previously published normal EOM SAGE library. DNA microarray similarly assessed changes in gene expression patterns of EOMs of monkeys reared from birth to 4 months of age with monocular deprivation. RESULTS: Dark rearing produced changes in expression of 280 transcripts in rat EOM. Of these, 71 were known genes representing functional categories that included energy metabolism/mitochondrial-related (21%), protein synthesis and modification (14%), lipid metabolism (13%), and muscle-related (6%) transcripts. Together, the predominant pattern reflected an energetic shift toward fatty acid beta-oxidation and integrated alterations in both myofibers and supportive tissues. The response of monkey rectus muscles to monocular deprivation was considerably less severe. CONCLUSIONS: The visual deprivation paradigms used in this study mimic alterations that are associated with the common disorders of strabismus, congenital cataract, and amblyopia. These data show that postnatal EOM maturation is broadly susceptible to changes in activity patterns that are a consequence of visuomotor maldevelopment. The data extend the concept of an EOM-critical period and establish that activity patterns in developing eye movement systems play vital determinant roles in the novel EOM phenotype.

Animals↗

PubMed2003

Postnatal suppression of myomesin, muscle creatine kinase and the M-line in rat extraocular muscle.

The M-line and its associated creatine kinase (CK) M-isoform (CK-M) are ubiquitous features of skeletal and cardiac muscle. The M-line maintains myosin myofilaments in register, links the contractile apparatus to the cytoskeleton for external force transfer and localizes CK-based energy storage and transfer to the site of highest ATP demand. We establish here that the muscle group responsible for movements of the eye, extraocular muscle (EOM), is divergent from other striated muscles in lacking both an M-line and its associated CK-M. Although an M-line forms during myogenesis, both in vivo and in vitro, it is actively repressed after birth. Transcripts of the major M-line structural proteins, myomesin 1 and myomesin 2, follow the same pattern of postnatal downregulation, while the embryonic heart-specific EH-myomesin 1 transcript is expressed early and retained in adult eye muscle. By immunocytochemistry, myomesin protein is absent from adult EOM sarcomeres. M-line suppression does not occur in organotypic co-culture with oculomotor motoneurons, suggesting that the mechanism for suppression may lie in muscle group-specific activation or workload patterns experienced only in vivo. The M-line is, however, still lost in dark-reared rats, despite the developmental delay this paradigm produces in the visuomotor system and EOMs. EOM was low in all CK isoform transcripts except for the sarcomeric mitochondrial (Ckmt2) isoform. Total CK enzyme activity of EOM was one-third that of hindlimb muscle. These findings are singularly unique among fast-twitch skeletal muscles. Since EOM exhibits isoform diversity for other sarcomeric proteins, the M-line/CK-M divergence probably represents a key physiological adaptation for the unique energetics and functional demands placed on this muscle group in voluntary and reflexive eye movements.

Adaptation, Physiological↗