Extraocular muscle is spared despite the absence of an intact sarcoglycan complex in gamma- or delta-sarcoglycan-deficient mice

Fiber Types in Rat Laryngeal Muscles and Their Transformations After Denervation and Reinnervation

The intrinsic laryngeal muscles cricothyroid (CT) and thyroarythenoid (TA) differ in myosin expression. CT expresses limb myosin heavy chains (MyHCs) and TA expresses an MyHC found in extraocular (EO) muscles, in addition to limb isoforms. We used immunohistochemical (IHC) analyses with highly specific monoclonal antibodies (MAbs) against various MyHCs to study muscle fiber types in rat CT and TA and to investigate whether nerves to laryngeal muscles control MyHC expression. CT was found to have the full complement of limb fiber types. TA had three major fiber types: 2b/eo, co-expressing 2B and EO MyHCs, 2x/2b, co-expressing 2X and 2B MyHCs, and 2x, expressing 2X MyHC. Type 2a and slow fibers were absent. TA consisted of two divisions: the external division (TA-X), which is homogeneously 2b/eo, and the vocalis division (TA-V), composed principally of 2x and 2b/eo fibers with a minority of 2x/2b fibers. TA-V had two compartments that differ in fiber type composition. At 4 weeks after cutting and re-uniting the recurrent laryngeal nerve (RLN), many 2b/eo fibers in the TA-X began to express 2X MyHC, while EO and 2B MyHC expression in these fibers progressively declined. By 12 weeks, up to 16.5% of fibers in the TA-X were of type 2x. These findings suggest that nerve fibers originally innervating 2x fibers in TA-V and other muscles have randomly cross-innervated 2b/eo fibers in the TA-X and converted them into 2x fibers. We conclude that CT and TA are distinct muscle allo-types and that laryngeal muscle fibers are subject to neural regulation.

Isolation of Mouse Periocular Tissue for Histological and Immunostaining Analyses of the Extraocular Muscles and Their Satellite Cells

The extraocular muscles (EOMs) comprise a group of highly specialized skeletal muscles controlling eye movements. Although a number of unique features of EOMs including their sparing in Duchenne muscular dystrophy have drawn a continuous interest, knowledge about these hard to reach muscles is still limited. The goal of this chapter is to provide detailed methods for the isolation and histological analysis of mouse EOMs. We first introduce in brief the basic anatomy and established nomenclature of the extraocular primary and accessory muscles. We then provide a detailed description with step-by-step images of our procedure for isolating (and subsequently cryosectioning) EOMs while preserving the integrity of their original structural organization. Next, we present several useful histological protocols frequently used by us, including: (1) a method for highlighting the general organization of periocular tissue, using the MyoD(Cre) × R26(mTmG) reporter mouse that elegantly distinguishes muscle (MyoD(Cre)-driven GFP(+)) from the non-myogenic constituents (Tomato(+)); (2) analysis by H&E staining, allowing for example, detection of the pathological features of the dystrophin-null phenotype in affected limb and diaphragm muscles that are absent in EOMs; (3) detection of the myogenic progenitors (i.e., satellite cells) in their native position underneath the myofiber basal lamina using Pax7/laminin double immunostaining. The EOM tissue harvesting procedure described here can also be adapted for isolating and studying satellite cells and other cell types. Overall, the methods described in this chapter should provide investigators the necessary tools for entering the EOM research field and contribute to a better understanding of this highly specialized muscle group and its complex micro-anatomy.

Sparing of the extraocular muscles in mdx mice with absent or reduced utrophin expression: A life span analysis

Sparing of the extraocular muscles in muscular dystrophy is controversial. To address the potential role of utrophin in this sparing, mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were examined for changes in myofiber size, central nucleation, and Pax7-positive and MyoD-positive cell density at intervals over their life span. Known to be spared in the mdx mouse, and contrary to previous reports, the extraocular muscles from both the mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were also morphologically spared. In the mdx:utrophin(+/)(-) mice, which have a normal life span compared to the mdx:utrophin(-/-) mice, the myofibers were larger at 3 and 12 months than the wild type age-matched eye muscles. While there was a significant increase in central nucleation in the extraocular muscles from all mdx:utrophin(+/)(-) mice, the levels were still very low compared to age-matched limb skeletal muscles. Pax7- and MyoD-positive myogenic precursor cell populations were retained and were similar to age-matched wild type controls. These results support the hypothesis that utrophin is not involved in extraocular muscle sparing in these genotypes. In addition, it appears that these muscles retain the myogenic precursors that would allow them to maintain their regenerative capacity and normal morphology over a lifetime even in these more severe models of muscular dystrophy.

Extraocular muscle satellite cells are high performance myo-engines retaining efficient regenerative capacity in dystrophin deficiency

Extraocular muscles (EOMs) are highly specialized skeletal muscles that originate from the head mesoderm and control eye movements. EOMs are uniquely spared in Duchenne muscular dystrophy and animal models of dystrophin deficiency. Specific traits of myogenic progenitors may be determinants of this preferential sparing, but very little is known about the myogenic cells in this muscle group. While satellite cells (SCs) have long been recognized as the main source of myogenic cells in adult muscle, most of the knowledge about these cells comes from the prototypic limb muscles. In this study, we show that EOMs, regardless of their distinctive Pax3-negative lineage origin, harbor SCs that share a common signature (Pax7(+), Ki67(-), Nestin-GFP(+), Myf5(nLacZ+), MyoD-positive lineage origin) with their limb and diaphragm somite-derived counterparts, but are remarkably endowed with a high proliferative potential as revealed in cell culture assays. Specifically, we demonstrate that in adult as well as in aging mice, EOM SCs possess a superior expansion capacity, contributing significantly more proliferating, differentiating and renewal progeny than their limb and diaphragm counterparts. These robust growth and renewal properties are maintained by EOM SCs isolated from dystrophin-null (mdx) mice, while SCs from muscles affected by dystrophin deficiency (i.e., limb and diaphragm) expand poorly in vitro. EOM SCs also retain higher performance in cell transplantation assays in which donor cells were engrafted into host mdx limb muscle. Collectively, our study provides a comprehensive picture of EOM myogenic progenitors, showing that while these cells share common hallmarks with the prototypic SCs in somite-derived muscles, they distinctively feature robust growth and renewal capacities that warrant the title of high performance myo-engines and promote consideration of their properties for developing new approaches in cell-based therapy to combat skeletal muscle wasting.

Loss of dystrophin and β-sarcoglycan significantly exacerbates the phenotype of laminin α2 chain-deficient animals

The adhesion molecule laminin α2 chain interacts with the dystrophin-glycoprotein complex, contributes to normal muscle function, and protects skeletal muscles from damage. Complete loss of the laminin α2 chain in mice results in a severe muscular dystrophy phenotype and death at approximately 3 weeks of age. However, it is not clear if the remaining members of the dystrophin-glycoprotein complex further protect laminin α2 chain-deficient skeletal muscle fibers from degeneration. Hence, we generated mice deficient in laminin α2 chain and dystrophin (dy(3K)/mdx) and mice devoid of laminin α2 chain and β-sarcoglycan (dy(3K)/Sgcb). Severe muscular dystrophy and a lack of nourishment inevitably led to massive muscle wasting and death in double-knockout animals. The dy(3K)/Sgcb mice were generally more severely affected than dy(3K)/mdx mice. However, both double-knockout strains displayed exacerbated muscle degeneration, inflammation, fibrosis, and reduced life span (5 to 13 days) compared with single-knockout animals. However, neither extraocular nor cardiac muscle was affected in double-knockout animals. Our results suggest that, although laminin α2 chain, dystrophin, and β-sarcoglycan are all part of the same adhesion complex, they have complementary, but nonredundant, roles in maintaining sarcolemmal integrity and protecting skeletal muscle fibers from damage. Moreover, the double-knockout mice could potentially serve as models in which to study extremely aggressive muscle-wasting conditions.

Delayed synapse elimination in mouse levator palpebrae superioris muscle

At birth, synaptic sites in developing rodent muscles are innervated by numerous motor axons. During subsequent weeks, this multiple innervation disappears as one terminal strengthens, and all the others are eliminated. Experimental perturbations that alter neuromuscular activity affect the rate of synaptic refinement, with more activity accelerating the time to single innervation and neuromuscular blockade retarding it. However, it remains unclear whether patterns of muscle use (driven by endogenous neuronal activity) contribute to the rate of synapse elimination. For this reason we examined the timing of supernumerary nerve terminal elimination at synapses in extraocular muscles (EOMs), a specialized set of muscles controlling eye movements. On the basis of their exceptionally high patterns of activity, we hypothesized that synaptic refinement would be greatly accelerated at these synapses. We found, however, that rates of synaptic refinement were only modestly accelerated in rectus and oblique EOMs compared with synapses in somite-derived skeletal muscle. In contrast to these results, we observed a dramatic delay in the elimination of supernumerary nerve terminals from synapses in the levator palpebrae superioris (LPS) muscle, a specialized EOM that initiates and maintains eyelid elevation. In mice, natural eye opening occurs at the end of the second postnatal week of development. Thus, although synapse elimination is occurring in most EOMs and somite-derived skeletal muscles, it appears to be dramatically delayed in a set of specialized eyelid muscles that remain immobile during early postnatal development.

Barx homeobox family in muscle development and regeneration

Homeobox transcription factors are key intrinsic regulators of myogenesis. In studies spanning several years, we have characterized the homeobox factor Barx2 as a novel marker for muscle progenitor cells and an important regulator of muscle growth and repair. In this review, we place the expression and function of Barx2 and its paralogue Barx1 in context with other muscle-expressed homeobox factors in both embryonic and adult myogenesis. We also describe the structure and regulation of Barx genes and possible gene/disease associations. The functional domains of Barx proteins, their molecular interactions, and cellular functions are presented with particular emphasis on control of genes and processes involved in myogenic differentiation. Finally, we describe the patterns of Barx gene expression in vivo and the phenotypes of various Barx gene perturbation models including null mice. We focus on the Barx2 null mouse model, which has demonstrated the critical roles of Barx2 in postnatal myogenesis including muscle maintenance during aging, and regeneration of acute and chronic muscle injury.

Sparing of extraocular muscle in aging and muscular dystrophies: a myogenic precursor cell hypothesis

The extraocular muscles (EOM) are spared from pathology in aging and many forms of muscular dystrophy. Despite many studies, this sparing remains an enigma. The EOM have a distinct embryonic lineage compared to somite-derived muscles, and we have shown that they continuously remodel throughout life, maintaining a population of activated satellite cells even in aging. These data suggested the hypothesis that there is a population of myogenic precursor cells (mpcs) in EOM that is different from those in limb, with either elevated numbers of stem cells and/or mpcs with superior proliferative capacity compared to mpcs in limb. Using flow cytometry, EOM and limb muscle mononuclear cells were compared, and a number of differences were seen. Using two different cell isolation methods, EOM have significantly more mpcs per mg muscle than limb skeletal muscle. One specific subpopulation significantly increased in EOM compared to limb was positive for CD34 and negative for Sca-1, M-cadherin, CD31, and CD45. We named these the EOMCD34 cells. Similar percentages of EOMCD34 cells were present in both newborn EOM and limb muscle. They were retained in aged EOM, whereas the population decreased significantly in adult limb muscle and were extremely scarce in aged limb muscle. Most importantly, the percentage of EOMCD34 cells was elevated in the EOM from both the mdx and the mdx/utrophin(-/-) (DKO) mouse models of DMD and extremely scarce in the limb muscles of these mice. In vitro, the EOMCD34 cells had myogenic potential, forming myotubes in differentiation media. After determining a media better able to induce proliferation in these cells, a fusion index was calculated. The cells isolated from EOM had a 40% higher fusion index compared to the same cells isolated from limb muscle. The EOMCD34 cells were resistant to both oxidative stress and mechanical injury. These data support our hypothesis that the EOM may be spared in aging and in muscular dystrophies due to a subpopulation of mpcs, the EOMCD34 cells, that are retained in significantly higher percentages in normal, mdx and DKO mice EOM, appear to be resistant to elevated levels of oxidative stress and toxins, and actively proliferate throughout life. Current studies are focused on further defining the EOMCD34 cell subtype molecularly, with the hopes that this may shed light on a cell type with potential therapeutic use in patients with sarcopenia, cachexia, or muscular dystrophy.

Intrinsic laryngeal muscles are spared from degeneration in the dy3K/dy3K mouse model of congenital muscular dystrophy type 1A

Deficiency of laminin alpha2 chain leads to a severe form of congenital muscular dystrophy (MDC1A). Here, we analyzed whether the intrinsic laryngeal muscles (ILM) are spared in the dy(3K)/dy(3K) mouse model of complete laminin alpha2 chain absence. No muscle degeneration was evident; expression of various laminin chains was similar to that of limb muscles, and sustained integrin alpha7B expression was noted in laminin alpha2 chain-deficient ILM. We conclude that ILM are spared in MDC1A.

Microutrophin delivery through rAAV6 increases lifespan and improves muscle function in dystrophic dystrophin/utrophin-deficient mice

Duchenne muscular dystrophy (DMD), the most prevalent lethal genetic disorder in children, is caused by mutations in the 2.2-MB dystrophin gene. Absence of dystrophin and the dystrophin-glycoprotein complex (DGC) from the sarcolemma leads to severe muscle wasting and eventual respiratory and/or cardiac failure. There is presently no effective therapy for DMD. Several lines of evidence have suggested that methods to increase expression of utrophin, a dystrophin paralog, show promise as a treatment for DMD. Adeno-associated viral (AAV) vectors are a promising vehicle for gene transfer to muscle, but microutrophin transgenes small enough to be carried by AAV have not been tested for function. In this study, we intravenously administered recombinant AAV (rAAV2/6) harboring a murine codon-optimized microutrophin (DeltaR4-R21/DeltaCT) transgene to adult dystrophin(-/-)/utrophin(-/-) (mdx:utrn(-/-)) double-knockout mice. Five-month-old mice demonstrated localization of microutrophin to the sarcolemma in all the muscles tested. These muscles displayed restoration of the DGC, increased myofiber size, and a considerable improvement in physiological performance when compared with untreated mdx:utrn(-/-) mice. Overall, microutrophin delivery alleviated most of the pathophysiological abnormalities associated with muscular dystrophy in the mdx:utrn(-/-) mouse model. This approach may hold promise as a treatment option for DMD because it avoids the potential immune responses that are associated with the delivery of exogenous dystrophin.

Nebulin isoforms of extraocular muscle

The extraocular muscles (EOMs), which are responsible for reflexive and voluntary eye movements, have many unique biochemical, physiological, and ultrastructural features that set them apart from other skeletal muscles. For example, rodent EOMs lack M-lines and express EOM-specific myosin heavy chain (MYH13) and alpha-cardiac myosin heavy chain. Recent gene-expression profiling studies indicate the presence of other cardiac-specific proteins in adult EOMs. This interesting mixture of myofibrillar and cytoskeletal proteins poses the questions as to whether nebulette, as opposed to nebulin, might be expressed in EOM, and what isoforms of titin are expressed in the EOM. We have performed gel electrophoresis and immunological analyses to determine the titin and nebulin isoforms expressed in the EOM. We have found that the mass of the titin isoforms expressed in the EOM most closely resemble those found in the skeletal muscles tested, viz., the soleus and extensor digitorum longus (EDL). We also demonstrate that, although the EOM expresses cardiac isoforms of myosin, it does not express nebulette and contains a nebulin isoform with a mass consistent with that found in the prototypical fast hindlimb muscle EDL.

Frequency of dystrophic muscle abnormalities in chronic progressive external ophthalmoplegia: analysis of 86 patients

BACKGROUND

There are few reports describing the coexistence of dystrophic features with those typical of mitochondrial myopathies in muscle biopsy. A recent study suggested that dystrophic features are frequent in patients with chronic progressive external ophthalmoplegia (CPEO) with a high mutation load, but the actual frequency of these abnormalities in CPEO remains undetermined.

OBJECTIVE

To review the occurrence of dystrophic abnormalities in a large series of patients with CPEO to assess the frequency of such abnormalities and to verify whether they are correlated with specific mitochondrial DNA (mtDNA) mutations.

METHODS

Retrospective survey of case series (86 patients with CPEO).

RESULTS

Only three cases with dystrophic abnormalities were found: two with a large scale mtDNA deletion and one with the A3251G mutation. All three patients showed predominantly proximal muscular weakness resembling limb girdle muscular dystrophy.

CONCLUSIONS

Dystrophic abnormalities are rare in CPEO and are not correlated with a specific molecular defect.

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.

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.

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.

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.

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.

Microarray analysis of normal and dystrophic skeletal muscle

The development and increasingly common use of DNA microarrays for comprehensive RNA expression analysis has had a substantial impact on the study of molecular pathology. DNA microarrays are orderly, high-density arrangements of nucleic acid spots that can be used as substrates for global gene expression analysis. Prior to their development, technical limitations necessitated that the molecular mechanisms underlying biological processes be broken down into their component parts and each gene or protein studied individually. This approach, focused as it is on a single aspect of a scientific phenomenon, does not allow appreciation or understanding of the fact that biological pathways do not exist in isolation, but are influenced by numerous factors. Enormous technological advances have been made over the past decade and now high-density DNA microarrays can provide rapid measurement of thousands of distinct transcripts simultaneously. These experiments raise the exciting opportunity to examine biological pathways in all their complexity and to compare the hypotheses deduced from the study of histological pathology with the findings of molecular pathology. This review focuses on how microarray technology has been used to interrogate muscular gene expression and, in particular, on how data generated from differential expression analysis of dystrophic and normal skeletal muscle has contributed to understanding the molecular pathophysiological pathways of muscular dystrophy.

Bupivacaine myotoxicity is mediated by mitochondria

We have investigated the effects of the myotoxic local anesthetic bupivacaine on rat skeletal muscle mitochondria and isolated myofibers from flexor digitorum brevis, extensor digitorum longus, soleus, and from the proximal, striated portion of the esophagus. In isolated mitochondria, bupivacaine caused a concentration-dependent mitochondrial depolarization and pyridine nucleotide oxidation, which were matched by an increased oxygen consumption at bupivacaine concentrations of 1.5 mm or less at pH 7.4, whereas respiration was inhibited at higher concentrations. As a consequence of depolarization, bupivacaine caused the opening of the permeability transition pore (PTP), a cyclosporin A-sensitive inner membrane channel that plays a key role in many forms of cell death. In intact flexor digitorum brevis fibers bupivacaine caused mitochondrial depolarization and pyridine nucleotides oxidation that were matched by increased concentrations of cytosolic free Ca(2+), release of cytochrome c, and eventually, hypercontracture. Both mitochondrial depolarization and cytochrome c release were inhibited by cyclosporin A, indicating that PTP opening rather than bupivacaine as such was responsible for these events. Similar responses to bupivacaine were observed in the soleus, which is highly oxidative. In contrast, fibers from the esophagus (which we show to be more fatigable than flexor digitorum brevis fibers) and from the highly glycolytic extensor digitorum longus didn't undergo pyridine nucleotide oxidation upon the addition of bupivacaine and were resistant to bupivacaine toxicity. These results suggest that active oxidative metabolism is a key determinant in bupivacaine toxicity, that bupivacaine myotoxicity is a relevant model of mitochondrial dysfunction involving the PTP and Ca(2+) dysregulation, and that it represents a promising system to test new PTP inhibitors that may prove relevant in spontaneous myopathies where mitochondria have long been suspected to play a role.

Extraocular muscle: cellular adaptations for a diverse functional repertoire

Oculomotor control systems are considerably more complex and diverse than are spinal skeletomotor systems. Moreover, individual skeletal muscles are frequently functional role-specific, while all extraocular muscles operate across a very wide dynamic range. We contend that the novel phenotype of the extraocular muscles is a direct consequence of the functional demands imposed upon this muscle group by the central eye movement controllers. This review highlights five basic themes of extraocular muscle biology that set them apart from more typical skeletal muscles, specifically, the (a) novel innervation pattern, (b) heterogeneity in contractile proteins, (c) structural and functional compartmentalization of the rectus and oblique muscles, (d) diversity of extraocular muscle fiber types, and (e) relationship between the novel muscle phenotype and the differential response of these muscles in neuromuscular and endocrine disease. Finally, new data from broad genome-wide profiling studies are reviewed, with global gene expression patterns lending substantial support to the notion that the extraocular muscles are fundamentally different from traditional skeletal muscle. This novel eye muscle phenotype represents an adaptation that exploits the full range of variability in skeletal muscle to meet the needs of visuomotor systems.

Differential susceptibility of the ocular motor system to disease

This review summarizes an alternative approach to the understanding of neuromuscular disease. By contrasting disease susceptibility of extraocular muscle and ocular motor neurons, it is hoped that unique insights into disease mechanisms may be identified. Disorder of eye movements leads to dramatic symptoms for patients and the ocular motor system is relatively limited in its ability to compensate rapidly for such disruptions. However, more profound reasons exist as to why myasthenia gravis compromises neuromuscular transmission at ocular muscle synapses as well as why Graves' ophthalmopathy exists. In contrast, muscular dystrophies spare the eye muscles while devastating all other skeletal muscles; the same is true for motor neuron diseases. It is hoped that this review will encourage others to view the world of neuromuscular diseases as delineated into those that spare the ocular motor system and those that do not.

The neuroanatomical basis of oculomotor disorders: the dual motor control of extraocular muscles and its possible role in proprioception

Current investigations show that two separate sets of motoneurons control the extraocular eye muscles, and that is there is a dual final common pathway. We propose that one set of motoneurons are the major source of tension generating eye movements, whereas the other may participate in a proprioceptive system concerned more with the exact alignment and stabilization of the eyes. In this article we discuss the structures that may participate in the proprioceptive circuits; and consider several recent publications in the light of this sensory feedback hypothesis, emphasizing the relevance to eye movement disorders.

Extraocular muscle is defined by a fundamentally distinct gene expression profile

Skeletal muscle fibers are defined by patterned covariation of key traits that determine contractile and metabolic characteristics. Although the functional properties of most skeletal muscles result from their proportional content of a few conserved muscle fiber types, some, typically craniofacial, muscles exhibit fiber types that appear to lie outside the common phenotypic range. We analyzed gene expression profiles of three putative muscle classes, limb, masticatory, and extraocular muscle (EOM), in adult mice by high-density oligonucleotide arrays. Pairwise comparisons using conservative acceptance criteria identified expression differences in 287 genes between EOM and limb and/or masticatory muscles. Use of significance analysis of microarrays methodology identified up to 400 genes as having an EOM-specific expression pattern. Genes differentially expressed in EOM reflect key aspects of muscle biology, including transcriptional regulation, sarcomeric organization, excitation-contraction coupling, intermediary metabolism, and immune response. These patterned differences in gene expression define EOM as a distinct muscle class and may explain the unique response of these muscles in neuromuscular diseases.

Analysis of human sarcospan as a candidate gene for CFEOM1

BACKGROUND

Congenital fibrosis of the extraocular muscles type 1 (CFEOM1) is an autosomal dominant eye movement disorder linked to the pericentromere of chromosome 12 (12p11.2 - q12). Sarcospan is a member of the dystrophin associated protein complex in skeletal and extraocular muscle and maps to human chromosome 12p11.2. Mutations in the genes encoding each of the other components of the skeletal muscle sarcospan-sarcoglycan complex (alpha - delta sarcoglycan) have been shown to cause limb girdle muscular dystrophy (LGMD2C-F). To determine whether mutations in the sarcospan gene are responsible for CFEOM1 we: (1) attempted to map sarcospan to the CFEOM1 critical region; (2) developed a genomic primer set to directly sequence the sarcospan gene in CFEOM1 patients; and (3) generated an anti-sarcospan antibody to examine extraocular muscle biopsies from CFEOM1 patients.

RESULTS

When tested by polymerase chain reaction, sarcospan sequence was not detected on yeast or bacterial artificial chromosomes from the CFEOM1 critical region. Sequencing of the sarcospan gene in CFEOM1 patients from 6 families revealed no mutations. Immunohistochemical studies of CFEOM1 extraocular muscles showed normal levels of sarcospan at the membrane. Finally, sarcospan was electronically mapped to bacterial artificial chromosomes that are considered to be outside of the CFEOM1 critical region.

CONCLUSIONS

In this report we evaluate sarcospan as a candidate gene for CFEOM1. We have found that it is highly unlikely that sarcospan is involved in the pathogenesis of this disease. As of yet no sarcospan gene mutations have been found to cause muscular abnormalities.