Multiple mitochondrial tRNAMLeu[UUR] mutations associated with infantile myopathy

We have identified a cluster of mitochondrial tRNA^Leu[UUR], mutations in a severe case of infantile myopathy. There were A to G transitions found at mtDNA positions 3259, 3261, 3266 and 3268. These point mutations change the anticodon arm and the anticodon UAA, normally found in tRNA^Leu[UUR], to UGA which is the one of the tRNAs^Ser[UCN]. This is the first anticodon alteration described in this tRNA. Another swap straight to the anticodon of tRNA^Pro alone was recently described in a less severe case [1]. Until now infantile myopathies have not been attributed to defined mtDNA alterations. This study reports for the first time mtDNA point mutations causing this early onset of a mitochondrial disorder. The apparent homoplasmy of these mutations and especially the location in the anticodon must be considered lethal, if the child would not have been respirated for 5 years from its birth. (Mol Cell Biochem 174: 231–236, 1997)     

Defects in the mitochondrial energy metabolism outside the respiratory chain and the pyruvate dehydrogenase complex

Disturbances in substrate oxidations in muscle mitochondria from patients with a suspicion of a mitochondrial myopathy may arise from a deficiency of one or more of the complexes of the respiratory chain or of the pyruvate dehydrogenase complex. However, we found no clear-cut defect in a substantial part of such patients. In this report we discuss some of the other possibilities which could account for the disturbed substrate oxidation rates. Special attention will be paid to defects which are localized outside the respiratory chain, such as defects in post-respiratory chain enzymes, defects in transport mechanisms of the mitochondrial inner or outer membrane, deficiency of cofactors and deficiency of heat-shock protein. (Mol Cell Biochem 174: 243–247, 1997)     

A1603P and K1617del,Mutations in β-Cardiac Myosin Heavy Chain that Cause Laing Early-Onset Distal Myopathy,Affect Secondary Structure and Filament Formation In Vitro and In Vivo

Over 20 mutations in β-cardiac myosin heavy chain (β-MHC), expressed in cardiac and slow muscle fibers, cause Laing early-onset distal myopathy (MPD-1), a skeletal muscle myopathy. Most of these mutations are in the coiled-coil tail and commonly involve a mutation to a proline or a single-residue deletion, both of which are predicted to strongly affect the secondary structure of the coiled coil. To test this, we characterized the effects of two MPD-1 causing mutations: A1603P and K1617del in vitro and in cells. Both mutations affected secondary structure, decreasing the helical content of 15 heptad and light meromyosin constructs. Both mutations also severely disrupted the ability of glutathione S-transferase–light meromyosin fusion proteins to form minifilaments in vitro, as demonstrated by negative stain electron microscopy. Mutant eGFP-tagged β-MHC accumulated abnormally into the M-line of sarcomeres in cultured skeletal muscle myotubes. Incorporation of eGFP-tagged β-MHC into sarcomeres in adult rat cardiomyocytes was reduced. Molecular dynamics simulations using a composite structure of part of the coiled coil demonstrated that both mutations affected the structure, with the mutation to proline (A1603P) having a smaller effect compared to K1617del. Taken together, it seems likely that the MPD-1 mutations destabilize the coiled coil, resulting in aberrant myosin packing in thick filaments in muscle sarcomeres, providing a potential mechanism for the disease.     

Mitochondrial dysfunction in human skeletal muscle biopsies of lipid storage disorder

Mitochondria regulate the balance between lipid metabolism and storage in the skeletal muscle. Altered lipid transport, metabolism and storage influence the bioenergetics, redox status and insulin signalling, contributing to cardiac and neurological diseases. Lipid storage disorders (LSD s) are neurological disorders which entail intramuscular lipid accumulation and impaired mitochondrial bioenergetics in the skeletal muscle causing progressive myopathy with muscle weakness. However, the mitochondrial changes including molecular events associated with impaired lipid storage have not been completely understood in the human skeletal muscle. We carried out morphological and biochemical analysis of mitochondrial function in muscle biopsies of human subjects with LSD s (n  = 7), compared to controls (n  = 10). Routine histology, enzyme histochemistry and ultrastructural analysis indicated altered muscle cell morphology and mitochondrial structure. Protein profiling of the muscle mitochondria from LSD samples (n  = 5) (vs. control, n  = 5) by high‐throughput mass spectrometric analysis revealed that impaired metabolic processes could contribute to mitochondrial dysfunction and ensuing myopathy in LSD s. We propose that impaired fatty acid and respiratory metabolism along with increased membrane permeability, elevated lipolysis and altered cristae entail mitochondrial dysfunction in LSD s. Some of these mechanisms were unique to LSD apart from others that were common to dystrophic and inflammatory muscle pathologies. Many differentially regulated mitochondrial proteins in LSD are linked with other human diseases, indicating that mitochondrial protection via targeted drugs could be a treatment modality in LSD and related metabolic diseases.

Cover Image for this Issue: doi: 10.1111/jnc.14177 .

     

Proteomic profiling of muscle fibre type shifting in neuromuscular diseases

Introduction: Mature skeletal muscles are composed of a complex assembly of slow-twitching, fast-twitching and hybrid fibres. Since muscle fibres exhibit a high degree of cellular plasticity, changed physiological conditions or pathophysiological disturbances have generally a substantial impact on fibre specification.

Areas covered: This article reviews the findings from comparative proteomic profiling studies that have focused on neuromuscular diseases and discusses the identified protein changes of fibre type shifting. The reviewed literature on weight loss, obesity, diabetes, cancer cachexia, disuse atrophy, motor neuron disease, myotonia, inflammatory myopathies, myofibrillar myopathies, muscular dystrophies and sarcopenia of old age suggests that proteome-wide alterations occur in the expression of distinct protein families, encompassing especially contractile and regulatory proteins of the acto-myosin apparatus.

Expert commentary: The systematic determination of proteome-wide changes in neuromuscular disorders can now be used to design novel diagnostic and therapy-monitoring tools for evaluating fibre transitions in pathological muscles.     

Dynamin‐2 in nervous system disorders

Dynamin‐2 is a pleiotropic GTPase whose best‐known function is related to membrane scission during vesicle budding from the plasma or Golgi membranes. In the nervous system, dynamin‐2 participates in synaptic vesicle recycling, post‐synaptic receptor internalization, neurosecretion, and neuronal process extension. Some of these functions are shared with the other two dynamin isoforms. However, the involvement of dynamin‐2 in neurological illnesses points to a critical function of this isoform in the nervous system. In this regard, mutations in the dynamin‐2 gene results in two congenital neuromuscular disorders. One of them, Charcot‐Marie‐Tooth disease, affects myelination and peripheral nerve conduction, whereas the other, Centronuclear Myopathy, is characterized by a progressive and generalized atrophy of skeletal muscles, yet it is also associated with abnormalities in the nervous system. Furthermore, single nucleotide polymorphisms located in the dynamin‐2 gene have been associated with sporadic Alzheimer's disease. In the present review, we discuss the pathogenic mechanisms implicated in these neurological disorders.

     

Evaluating Ungulate Mortality Associated With Helicopter Net-Gun Captures in the Northern Great Plains

ABSTRACT Ungulate mortality from capture-related injuries is a recurring concern for researchers and game managers throughout North America and elsewhere. We evaluated effects of 7 variables to determine whether ungulate mortality could be reduced by modifying capture and handling procedures during helicopter net-gunning. During winter 2001–2006, we captured 208 white-tailed deer (Odocoileus virginianus) and 281 pronghorn (Antilocapra Americana) by helicopter net-gunning throughout the Northern Great Plains. Of 281 pronghorn, 25 (8.9%) died from capture-related injuries; 12 were from direct injuries during capture, and 13 occurred postrelease. Of 208 deer, 3 (1.4%) died from injuries sustained during helicopter captures, with no mortalities documented postrelease. We used logistic regression to evaluate the probability that ungulates would die of injuries associated with helicopter net-gun captures by analyzing effects of snow depth, transport distance, ambient and rectal temperatures, pursuit and handling times, and whether individuals were transported to processing sites. The probability of capture-related mortality postrelease decreased 58% when transport distance was reduced from 14.5 km to 0 km and by 69% when pursuit time decreased from 9 minutes to <1 minute. Wildlife managers and researchers using helicopter capture services in landscapes of the Midwest should limit pursuit time and eliminate animal transport during pronghorn and white-tailed deer capture operations to minimize mortality rates postrelease.     

A computational method for developing hierarchical large deformation viscoelastic models of the contracting heart

In this study, a new computational method for modelling the contracting heart is described. Using this method, the cardiac wall is constructed from basic, repeating contractile units that represent individual myocardial units and collagen, each with its own set of parameters, including orientation, passive and active behaviour and stimulation propagation. The method allows individual control of each structural unit (e.g. at the level of a single myocardial unit). Feasibility of modelling dynamic heart contraction using this method is demonstrated using 2D cross-sections and simplified 3D geometries. Effects of non-contractile scar and myopathic tissue were also tested in these geometrical configurations. Results from the 2D and 3D simulations were, overall, in agreement with well-established physiological data. The present method holds promise for modelling complex heart pathologies, abnormal mechanical properties (e.g. myocardial infarcts) and electrical conduction properties (branch blocks), and their spatial distributions across the myocardial tissues.     

Lipolysis - a highly regulated multi-enzyme complex mediates the catabolism of cellular fat stores

Lipolysis is the biochemical pathway responsible for the catabolism of triacylglycerol (TAG) stored in cellular lipid droplets. The hydrolytic cleavage of TAG generates non-esterified fatty acids, which are subsequently used as energy substrates, essential precursors for lipid and membrane synthesis, or mediators in cell signaling processes. Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, it is most abundant, however, in white and brown adipose tissue. Over the last 5years, important enzymes and regulatory protein factors involved in lipolysis have been identified. These include an essential TAG hydrolase named adipose triglyceride lipase (ATGL) [annotated as patatin-like phospholipase domain-containing protein A2], the ATGL activator comparative gene identification-58 [annotated as α/β hydrolase containing protein 5], and the ATGL inhibitor G0/G1 switch gene 2. Together with the established hormone-sensitive lipase [annotated as lipase E] and monoglyceride lipase, these proteins constitute the basic "lipolytic machinery". Additionally, a large number of hormonal signaling pathways and lipid droplet-associated protein factors regulate substrate access and the activity of the "lipolysome". This review summarizes the current knowledge concerning the enzymes and regulatory processes governing lipolysis of fat stores in adipose and non-adipose tissues. Special emphasis will be given to ATGL, its regulation, and physiological function.     

Differential and restricted expression of novel collagen VI chains in mouse

Recently, three novel collagen VI chains, α4, α5 and α6, were identified. These are thought to substitute for the collagen VI α3 chain, probably forming α1α2α4, α1α2α5 or α1α2α6 heterotrimers. The expression pattern of the novel chains is so far largely unknown. In the present study, we compared the tissue distribution of the novel collagen VI chains in mouse with that of the α3 chain by immunohistochemistry, immunoelectron microscopy and immunoblots. In contrast to the widely expressed α3 chain, the novel chains show a highly differential, restricted and often complementary expression. The α4 chain is strongly expressed in the intestinal smooth muscle, surrounding the follicles in ovary, and in testis. The α5 chain is present in perimysium and at the neuromuscular junctions in skeletal muscle, in skin, in the kidney glomerulus, in the interfollicular stroma in ovary and in the tunica albuginea of testis. The α6 chain is most abundant in the endomysium and perimysium of skeletal muscle and in myocard. Immunoelectron microscopy of skeletal muscle localized the α6 chain to the reticular lamina of muscle fibers. The highly differential and restricted expression points to the possibility of tissue-specific roles of the novel chains in collagen VI assembly and function.