Hypotheses and recent findings concerning aetiology and pathogenesis of the muscular dystrophies

Abnormal vascular development in zebrafish models for fukutin and FKRP deficiency

Fukutin and fukutin-related protein (FKRP) are involved in the glycosylation of α-dystroglycan, a key receptor for basement membrane proteins. Aberrant α-dystroglycan glycosylation leads to a broad spectrum of disorders, ranging from limb girdle muscular dystrophy to Walker-Warburg syndrome. This is the first study investigating a role of fukutin and FKRP-mediated glycosylation in angiogenesis. Transgenic zebrafish expressing enhanced green fluorescent protein in blood vessels were treated with morpholino antisense oligonucleotides that blocked the expression of fukutin, FKRP and dystroglycan. All morphant fish showed muscle damage and vascular abnormalities at day 1 post-fertilization. Intersegmental vessels of somites failed to reach the dorsal longitudinal anastomosis and in more severe phenotypes retracted further or were in some cases even completely missing. In contrast, the eye vasculature was distorted in both fukutin and FKRP morphants, but not in dystroglycan morphants or control fish. The eye size was also smaller in the fukutin and FKRP morphants when compared with dystroglycan knockdown fish and controls. In general, the fukutin morphant fish had the most severe skeletal muscle and eye phenotype. Our findings suggest that fukutin and FKRP have functions that affect ocular development in zebrafish independently of dystroglycan. Despite anecdotal reports about vascular abnormalities in patients affected by dystroglycanopathies, the clinical relevance of such lesions remains unclear and should be subject to further review and investigations.

Ultrastructure of the developing myotendinous junction of genetic dystrophic chickens

This paper presents an ultrastructural study of the changes occurring within the tendon and at the insertion of the developing gastrocnemius of White Leghorn chickens and the dystrophic lines 413 and 423 from Davis, CA. The study revealed that the Davis lines contained abnormalities in these areas as early as 13 d in ovo with progressive deterioration to 19 d in ovo. The fibroblasts, collagen, and myofibers showed significant alterations as early as 13 in ovo. Fibroblasts contained abnormal mitochondria and altered Golgi bodies. At day 19, many were ruptured. The average diameter of collagen fibrils was smaller in the dystrophic chickens and myofibers showed a variety of alterations, some of which were severe. The details of these alterations are described and their possible relationship with the etiology of genetic muscular dystrophy is discussed.

In vivo morphometric analysis of muscle microcirculation in dystrophic mice

In order to test the vascular hypothesis of muscular dystrophy, the gracilis muscle in 6- to 7-week-old C57BL/6J-dy2J normal and dystrophic mice was studied using in vivo quantitative morphometric techniques to determine the total length and surface area of capillaries in which blood was flowing per unit volume of muscle. Individual capillary lengths, diameters, and red blood cell velocities were also quantified. During resting conditions, the capillary density(length per unit volume of muscle) and surface area are increased significantly in dystrophic muscle compared to normal muscle. Under fully vasodilated conditions, the capillary density and surface area are similar in normal and dystrophic muscle. Individual capillary lengths, diameters, and red blood cell velocities are also similar in normal and dystrophic muscle under resting conditions. These results indicate that, contrary to the vascular hypothesis, dystrophic muscle at rest has increased capillary density, surface area, and blood flow. It is postulated that the increased capillary density in dystrophic muscle at rest is secondary to muscle fiber breakdown.