Identification of a short, single site matriglycan that maintains neuromuscular function in the mouse.

Matriglycan (-1,3-ß-glucuronic acid-1,3-α-xylose-) is a polysaccharide that is synthesized on α-dystroglycan, where it functions as a high-affinity glycan receptor for extracellular proteins, such as laminin, perlecan and agrin, thus anchoring the plasma membrane to the extracellular matrix. This biological activity is closely associated with the size of matriglycan. Using high-resolution mass spectrometry and site-specific mutant mice, we show for the first time that matriglycan on the T317/T319 and T379 sites of α-dystroglycan are not identical. T379-linked matriglycan is shorter than the previously characterized T317/T319-linked matriglycan, although it maintains its laminin binding capacity. Transgenic mice with only the shorter T379-linked matriglycan exhibited mild embryonic lethality, but those that survived were healthy. The shorter T379-linked matriglycan exists in multiple tissues and maintains neuromuscular function in adult mice. In addition, the genetic transfer of α-dystroglycan carrying just the short matriglycan restored grip strength and protected skeletal muscle from eccentric contraction-induced damage in muscle-specific dystroglycan knock-out mice. Due to the effects that matriglycan imparts on the extracellular proteome and its ability to modulate cell-matrix interactions, our work suggests that differential regulation of matriglycan length in various tissues optimizes the extracellular environment for unique cell types.

A common disease-associated missense mutation in alpha-sarcoglycan fails to cause muscular dystrophy in mice.

Limb-girdle muscular dystrophy type 2D (LGMD2D) is caused by autosomal recessive mutations in the alpha-sarcoglycan gene. An R77C substitution is the most prevalent cause of the disease, leading to disruption of the sarcoglycan-sarcospan complex. To model this common mutation, we generated knock-in mice with an H77C substitution in alpha-sarcoglycan. The floxed neomycin (Neo)-cassette retained at the targeted H77C alpha-sarcoglycan locus caused a loss of alpha-sarcoglycan expression, resulting in muscular dystrophy in homozygotes, whereas Cre-mediated deletion of the floxed Neo-cassette led to recovered H77C alpha-sarcoglycan expression. Contrary to expectations, mice homozygous for the H77C-encoding allele expressed both this mutant alpha-sarcoglycan and the other components of the sarcoglycan-sarcospan complex in striated muscle, and did not develop muscular dystrophy. Accordingly, conditional rescued expression of the H77C protein in striated muscle of the alpha-sarcoglycan-deficient mice prevented the disease. Adding to the case that the behavior of mutant alpha-sarcoglycan is different between humans and mice, mutant human R77C alpha-sarcoglycan restored the expression of the sarcoglycan-sarcospan complex when introduced by adenoviral vector into the skeletal muscle of previously created alpha-sarcoglycan null mice. These findings indicate that the alpha-sarcoglycan with the most frequent missense mutation in LGMD2D is correctly processed, is transported to the sarcolemma, and is fully functional in mouse muscle. Our study presents an unexpected difference in the behavior of a missense-mutated protein in mice versus human patients, and emphasizes the need to understand species-specific protein quality control systems.