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The six extraocular muscles (EOMs) are arranged around the eyeball as agonist-antagonist pairs performing the eye movements. The EOMs comprise a distinct muscle group that is fundamentally different from other skeletal muscle, which is reflected on many levels, such as functionality, anatomy as well as in their molecular make-up. Physiologically EOMs are considered superfast, high endurance muscles that are continuously active. In addition, EOMs contain unusual slow-tonic fibers that share features with amphibian and avian slow-tonic fibers. EOMs also express slow/cardiac isoforms of proteins and genes along with the typical isoforms of fast muscle fibers. Another striking hallmark of EOM is their differential involvement in a number of diseases. For instance, EOMs are preferentially spared in Duchenne Muscular Dystrophy (DMD). DMD is the most common fatal, genetic disease in males clinically characterized by progressive muscle wasting. Mutations in the dystrophin gene result in a destabilization of the muscle membrane causing muscle fiber damage. While all other skeletal muscles deteriorate the EOMs remain morphologically and functionally healthy. In the pathogenesis of DMD elevated Ca2+ levels are believed to be an early event and it has been shown that EOMs are protected from pharmacologically induced Ca2+ damage. The goal of this study was to characterize the spared EOMs, in particular their Ca2+ homeostasis, in the context of DMD pathology to reveal new potential therapeutic targets for the disease. A combination of physiological, molecular and biochemical methods was used to investigate the Ca2+ homeostasis of EOMs to demonstrate clear differences compared with the fast limb muscle tibialis anterior (TA). Ca2+ handling of stimulated cultured EOM myotubes suggested more efficient Ca2+ removal from the cytoplasm after induced Ca2+ influx compared with cultured myoblasts from TA. Subsequent mRNA and protein expression analyses of myoblasts and adult muscle tissue revealed high expression levels of many key Ca2+ regulating and buffering proteins in rodent EOMs compared with TA. Among these Ca2+ proteins were slow/cardiac proteins, which normally are not found in fast muscles. For instance, the sarcoplasmic Ca2+ ATPase SERCA2 was elevated along with its regulator phospholamban (PLN). Further, PLN was preferentially endogenously phosphorylated at Thr17 suggesting continuous activation of SERCA2 and possibly the fast isoform SERCA1, the main Ca2+ pumps responsible for removing Ca2+ from the cytoplasm after muscle contraction. Furthermore, Ca2+ buffers, such as calsequestrin (CASQ2) and parvalbumin (PARV) were elevated. These results suggest that EOMs are endowed with a unique and superior Ca2+ homeostasis that facilitates efficient Ca2+ buffering and removal from the cytoplasm. This is in agreement with their continuous and fast activation cycles, as well as with a potential protective mechanism in prevention of Ca2+ overload in DMD. The extreme activity patterns of EOM suggested that a high activity of store-operated Ca2+ entry (SOCE) plays a critical part to replenish Ca2+ for rapid and continuous cycles of contractions. To extend the data on general Ca2+ homeostasis and because of possible implications of store-operated Ca2+ influx and other Ca2+ influx pathways in DMD, the expression patterns of group 1 transient receptor potential (TRP) channels and the proteins Orai1 and STIM1 were studied. The TRP channels, TRPC1, TRPC6 and TRPV4 channel proteins in addition to STIM1 showed higher expression in EOM compared with TA. High TRPC1, TRPV4 and STIM1 levels could play a significant role in the high fatigue resistance, muscle differentiation and SOCE in EOM. In addition, tissue from the mdx mouse model of DMD was investigated. The only channels differentially expressed in mdx EOM compared with normal EOM were TRPM4 and TRPM7 (decreased in mdx EOM) and TRPV4 (increased in mdx EOM). Although, these changes in mdx EOM were of small magnitude, they could point toward subtle compensatory changes related to the disease process. In general, EOMs seem to be unaffected by the disease and inherently protected. In conclusion, the results in this thesis have improved the understanding of the Ca2+ homeostasis in EOMs and suggest that EOM may be better able to prevent prolonged elevation of cytoplasmic Ca2+ levels. These data may help to design new therapeutic approaches targeting Ca2+ handling proteins to ameliorate muscular dystrophy.
All types of muscles use Ca2+ as their main intracellular messenger. In skeletal muscle fibers abnormal levels of intracellular calcium result in altered contractile properties, altered energy metabolism, and altered gene expression. Moreover, long term failure of normal Ca2+ homeostasis can lead to cell death of muscle fibers by necrosis and apoptosis. Elevations of intracellular Ca2+ levels are more and more regarded as the reason for pathological changes and muscle fiber damage in Duchenne Muscular Dystrophy (DMD). DMD is a severe recessive x-linked muscle disease caused by mutations in the dystrophin gene. The characteristics of DMD are muscle tissue wasting and fibrosis. Both muscle wasting and intracellular Ca2+ are to be reflected in changes of muscle force. Several Ca2+ conducting channels including transient receptor potential (TRP) channels are supposed to account for the abnormal Ca2+ homeostasis in DMD. Gene expressions of TRP channels have been studied in human and mouse skeletal muscle and among others TRPC3, TRPC6 and TRPV4 channels were found to occur in skeletal muscles. The present study followed the hypothesis that TRPC3, TRPC6 and TRPV4 are functional in skeletal muscle fibers and that they contribute to muscular Ca2+ homeostasis. Further, it was assumed that dysfunction of the mentioned TRP channels contributes to abnormal contractile properties and pathology and of dystrophin-deficient muscle. To study Ca2+ changes in mouse skeletal muscle fibers the fluorescent calcium indicator Fura-2 was used. Further, the technique of Mn2+ quench of Fura-2 fluorescence was applied. Muscle force measurements of mouse soleus and diaphragm strips were performed. To elucidate abnormalities of TRP channel function in dystrophin-deficient muscle, muscles and muscle fibers of mdx mice were studied. Hyperforin, an activator of TRPC6 channels elicited increases of calcium levels in wildtype muscle fibers. These increases were partly inhibited by the TRPC6 inhibitor 1-(5-chloronaphthalenesulfonyl) homopiperazine hydrochloride (ML-9). The TRPC3/TPRC6 activator 1-oleoyl-2-acetyl-sn-glycerol (OAG) resulted in increased calcium entry, which was attenuated by ML-9. 2-aminoethoxydiphenylborane (2-APB), an unspecific TRP channel inhibitor, suppressed calcium entry in muscle fibers under basal conditions. In addition, the specific TRPC3 inhibitor Pyr3, strongly inhibited background calcium entry. The TRPV4 activator 4α-phorbol 12,13-didecanoate (4α-PDD) induced significant increased calcium entry and this increase could be inhibited by the TRPV4 inhibitor HC 067047. During muscle force recordings ML-9 significantly inhibited twitches and tetani and accelerated muscle fatigue during sustained repetitive stimulation. The results indicate that TRPC3, TRPC6 and TRPV4 are functionally expressed in mouse muscle fibers. TRPC3 stays active under the basal conditions and contributes to background calcium entry. In contrast, TRPC6 and TRPV4 did not seem to be active at resting conditions, but could be pharmacologically activated. TRPC6 may play a role to counteract the calcium loss under long-term muscle fatigue. Though TRPC3 and C6 play a role for muscular Ca2+ homeostasis, it is unclear whether and how the two channels associate and cross-talk with each other in skeletal muscle cells. In mdx fibers Pyr3 inhibited background calcium influx stronger that in WT fibers, implying a possible over-activation of TRPC3 channels in mdx muscle fibers. At later stages mdx muscle showed marked decrease in force reflecting muscle wasting. Soleus showed moderate decrease and diaphragm showed severe decrease (more than 60%) in force. Resistance to muscle fatigue was shown in mdx soleus muscle when compared with WT soleus muscle. Diaphragm segments of mdx mice showed very strong resistance to muscle fatigue. The results indicate a substantial loss of muscle mass, an increase in oxidative fiber types and a reduction of fast fatigable muscle fibers. It is concluded that the hypothesis of functional expression of TRPC3, TRPC6 and TRPV4 in mouse skeletal muscle has been confirmed. The results give improved knowledge about the relation of Ca2+ homeostasis, mdx pathology and TRP channels. Diaphragms of old mdx mice show severe muscle weakness but the remaining fibers of the diaphragm showed strong fatigue-resistance. The application of a TRPC3 inhibitor may be a promising treatment to prevent high Ca2+ mediated muscle damage in muscular dystrophy.