Fonction de la dystrophine dans le muscle strié squelettique

Duchene muscular dystrophy (DMD)-is a lethal inherited disease, affecting 1 in 3500 boys. DMD is characterized by muscle fibre necrosis and continuous cycles of muscle degeneration/regeneration. However, the process of regeneration fails to compensate adequately for the degeneration and muscle fibers are progressively replaced by fat and connective tissue leading to progressive muscle weakening.
The disease is caused by a gene deletion in the p21 regions of the X-chromosome, resulting in the absence of a protein named dystrophin. During the last decade, progress has been made on the localization and sequencing of dystrophin. The protein is normally expressed in muscles and brain, and is localized just underneath the plasma membrane. The amino-acid sequence of dystrophin predicts a rod-shaped structure with four distinct domains: an N-terminal domain which can bind actin, a central domain constituted of spectrin-like repeats, a cysteine-rich domain and a C-terminal domain which can bind to a plasma membrane glycoprotein complex. These biochemical findings together with the histochemical localization studies have led to the hypothesis that dystrophin may form a link between the cytoskeleton and the extracellular matrix.
However, many questions remain and several hypotheses have been proposed concerning pathopsychological processes leading to DMD, and the function of dystrophin in normal muscle. (1) Dystrophin may be involved in control of the intercellular calcium homeostasis (2) The absence of dystrophin may cause alterations of the muscle precursor cells leading to an ineffective regeneration process. (3) Dystrophin may be part of a cytoskeletal network, strengthening the plasma membrane when submitted to mechanical stress.
We briefly review the first hypothesis and present results from our experiments testing the two latter ones. All experiments were performed on an animal model for DMD, the mdx mouse. As in DMD, the mdx mouse lacks dystrophin and, during the a period of its life (about 100 days postnatal), its muscles are subject to intensive necrosis with cycles of degeneration/regeneration. However, unlike DMD, fat infiltrations and proliferation of connective tissue are limited and mdx mice have a normal lifespan.
(1) Dystrophin could be involved in the regulation of the intracellular calcium content. Indeed, an accumulation of intracellular free calcium has been observed in muscles fibres of DMD and mdx mice. This accumulation could lead to the necrosis of the muscle fibres following activation of proteolytic processes. However, differences in free calcium concentrations between normal and dystrophic muscles were small and recent work from our laboratory (Gailly et at. in press) shows that there are no differences in free calcium concentration between normal and mdx mice.
(2) The absence of dystrophin may cause alterations of the muscle precursor cells leading to an ineffective regeneration process. During postnatal growth and the late phase of degeneration, the muscle precursor cells (mpc) are activated and proliferate. Fusion of mpc results in the formation of myotubes, which mature to new muscle fibres. Thus, alterations of mpc due to dystrophin deficiency could greatly affect muscle development and repair of muscle fibres. A cycle of degeneration/regeneration can be experimentally induced by performing whole muscle grafting. To investigate whether dystrophin deficiency modifies regeneration of the muscle fibers, we have performed orthotopical transplantation of slow (soleus) and fast (extensor digitorum longus : EDL) in normal and dystrophic mice (by grafting mdx muscles into mdx host mice and C57 muscles into C57 host mice). In both strains, the transplantation induces similar modifications: a decrease of force together with an increase of the velocity of contraction in soleus muscles and a decrease of force without modifications of the velocity of contraction in EDL muscles. Thus, we conclude that the regeneration process is not affected by dystrophin deficiency.
(3) Dystrophin could be part of a cytoskeletal network stabilizing the plasma membrane when submitted to mechanical stress. Indeed, dystrophin may form a link between the cytoplasmic actin filaments and the extracellular matrix. It has been shown on the one hand, that the cysteine-rich domain and the first half of the C-terminal domain of dystrophin are tightly associated with a complex of six intramembranous glycoproteins which may connect with extracellular matrix via laminin, and on the other hand, that dystrophin’s N-terminal domain binds to F-actin and may connect with the contractile apparatus through the extra actin filaments arising from the M and Z lines of the myofibrils. The results of our mechanical experiments show that mdx EDL muscles are much more damaged than normal EDL muscles following a series of contractions with stretch. This supports a mechanical role for dystrophin on skeletal muscles.
In conclusion, it is possible that the lack of dystrophin weakens the submembranous network and causes plasma membrane lesions when muscles are submitted to high mechanical stress. These lesions then induce a massive influx of calcium which activates the calcium-dependent proteases, leading to muscle fibre death and stimulating the process of regeneration. In DMD, continuous regeneration might finally lead to depletion of the mpc and to replacement of the muscle fibres by fat and connective tissue, resulting in progressive muscle weakening

La dystrophine est une protéine localisée sous le membrane plasmique des muscles squelettiques. L’absence de cette protéine serait responsable de la nécrose des fibres musculaire, provoquant des cycles continus de dégénération/régénération des fibres et conduisant à la faiblesse musculaire progressive caractéristique de la dystrophie musculaire de Duchenne (DMD).
Plusieurs hypothèses ont été proposées concernant les mécanismes responsables de la faiblesse progressive des muscles squelettiques et le rôle qu’y pourrait jouer la dystrophine :
(1) la dystrophine pourrait intervenir dans le contrôle de l’homéostasie calcique intracellulaire. En effet, le contenu en calcium total et la concentration en calcium libre des cellules musculaires de souris mdx (modèle animal de la DMD) et de DMD, seraient plus élevés que dans les cellules musculaires normales provoquant l’activation de protéases et la nécrose des cellules. Cependant, les différences de concentration en calcium entre souris normales et dystrophiques sont faibles et des travaux récents n’ont pas confirmé de différence de concentration en calcium libre entre les deux souches de souris.
(2) La régénération musculaire serait inefficace suite à une altération des cellules myogéniques. Nous avons montré que la capacité de régénération des cellules myogéniques n’était pas diminuée par l’absence de dystrophine du muscle : pour cela, nous avons réalisé des transplantations directes de muscles entiers en greffant des muscles normaux dans des souris normales et de muscle mdx dans des souris mdx. L’analyse des propriétés mécaniques et le composition en isoformes de myosine ont montré que la dégénération/régénération induite par transplantation des muscles normaux et dystrophiques est similaire et provoque dans les deux souches de souris, une diminution de la force et une augmentation de la vitesse de contraction du muscle dans le cas des muscles lents (soleus) et une diminution de la force sans modification de la vitesse de contraction dans la cas des muscles rapides (extenseur long des doigts : EDL).
(3) La dystrophine pourrait s’intégrer dans un réseau protéique sous-membranaire permettant au muscle de soutenir une contrainte mécanique importante. Nos expériences d’étirement des muscles pendant une contraction tétanique étayent cette hypothèse. En effet, nous avons observé que suite à des contractions avec étirement, les lésions observées dans les muscles EDL n’exprimant pas la dytrophine sont beaucoup plus importantes que dans les muscles normaux.
La dystrophine semble intervenir dans le maintien de l’intégrité de la membrane plasmique lorsque celle-ci est soumise à des contraintes mécaniques importantes