Elevated phospholipase A in Duchenne muscle

Low-dose prednisolone treatment in Duchenne and Becker muscular dystrophy

Forty-one boys, aged 4.0-19.4 yr, with Duchenne or Becker muscular dystrophy, took part in a 12-month randomized, double-blind cross-over trial in which the patients received 0.35 mg kg-1 day-1 prednisolone for six months and placebo for six months. One patient stopped the treatment because of excessive weight gain. The boys were assessed every third month with a comprehensive test battery of muscle force and function. The results support earlier reports that prednisolone treatment can either improve muscle force and function or diminish the deterioration of muscle function in both Becker and Duchenne muscular dystrophy.

Phospholipase A2 activity in dystrophinopathies

Phospholipase A2 activity in human muscle with or without dystrophin abnormality was studied. The results showed an increased phospholipase A2 activity in Duchenne muscular dystrophy (DMD) patients (1160 +/- 160, P < 0.01) compared to controls (< 200 U mg-1). DMD fetal muscle showed normal levels, but levels then increased dramatically postnatally. Highest levels were found at 5 yr of age (10 times normal) and then declined to 1.5-2 times normal by age 10. Steroid treatment did not change the phospholipase A2 levels significantly. In patients with abnormal dystrophin, i.e. Becker muscular dystrophy, phospholipase A2 activity was increased in the age group 3-15 (920 +/- 230 U mg-1, P < 0.01), while older patients (17-49) showed a non-significant (220 +/- 60 U mg-1) increase. The lack of phospholipase A2 activation in fetuses with DMD, indicates that activation is not a direct consequence of dystrophin deficiency. Phospholipase A2 activity has been shown to be connected to the formation of several inflammatory mediators such as prostaglandins, leukotriens, platelet activating factor and lysophospholipids. Phospholipase A2 activation may therefore play an important role in the development of inflammation and necrosis, with subsequent fibrosis and massive loss of muscle function, which develops in Duchenne and Becker muscular dystrophy.

Dystrophin-deficient myofibers are vulnerable to mast cell granule-induced necrosis

Duchenne muscular dystrophy is the most common inherited lethal X-linked disorder of mankind and is caused by dystrophin deficiency. The steps involved in the dystrophin-deficiency-induced cascade which lead to myofiber necrosis, progressive muscle wasting in humans and dogs and prominent muscle hypertrophy in mice and cats are obscure. Dystrophin is an intracellular component of the membrane cytoskeleton and its absence would be expected to cause necrosis of isolated myofibers (cell autonomous defect). However, all dystrophin-deficient muscles characteristically show simultaneous degeneration of large groups of muscle fibers (grouped necrosis). This implies that cell death may be mediated by extracellular, non-cell autonomous factors which occur as a secondary consequence of dystrophin deficiency. We have proposed a model where tissue pathology may be mediated by infiltrating mast cells (Gorospe et al., J Neurol Sci 1994). Here we show that intramuscular injections of purified mast cell granules induce widespread myofiber necrosis in dystrophin-deficient mdx mice, but not in normal mice. These data support the hypothesis that dystrophin acts as a plasma membrane stabilizer and that its deficiency renders myofibers more susceptible to damage from mast cell proteases. Moreover, our results support the hypothesis that mast cell degranulation may be a trigger for myofiber death in dystrophin-deficient muscle.

Notexin-induced muscle injury in the dog

Notexin, a myotoxic phospholipase, was used to induce focal necrosis in the sartorius muscles of normal mixed-breed adult dogs and in 12-week-old beagles. Notexin injury caused pathologic changes similar to those of Duchenne muscular dystrophy (DMD) and its canine homologue, golden retriever muscular dystrophy (GRMD). All three conditions are characterized by increased serum creatine kinase (CK) levels, sarcolemmal defects, delta lesions, hyaline degeneration of myofibers, calcium-positive myofibers, and minimal effects on neurovascular structures. Four and 24 h after exposure to notexin, serum CK levels were elevated, and many myofibers were necrotic. In addition, by 24 h the necrotic areas were heavily invaded by mononuclear cells, and calcium-positive myofibers were prominent. Capillaries appeared intact even in areas of marked myonecrosis. Massive cellular infiltrate and myotube formation was evident at 3 days post injury. By 7 days, most affected fascicles were occupied by small immature myofibers. Regeneration was largely complete at 21 days. Our results suggest that notexin-induced muscle injury in dogs will be useful in the evaluation of potential therapies for DMD such as myoblast transplantation.

Molecular pharmacology of manoalide. Inactivation of bee venom phospholipase A2

The marine natural product manoalide (MLD) was shown to directly inactivate bee venom phospholipase A2 (PLA2). Inactivation was pH dependent (maximum inactivation occurred at pH 8.0), time dependent and concentration dependent. The IC50 was estimated at 0.05 microM and virtually complete inactivation of the enzyme occurred at 4.0 microM. The time-dependent loss of PLA2 activity suggested that inactivation does not follow typical Michaelis-Menten kinetics. Reversibility was studied directly by dilution and dialysis; both methods were ineffective in dissociating the MLD-PLA2 complex. A kinetic plot of initial velocity (v) versus [PLA2] supported our hypothesis that MLD apparently inactivates bee venom PLA2 by an irreversible mechanism.