Quantification of Exercise‐Induced Sarcomeric Damage in R349P Desmin Knock‐In Mice: A New Approach in Myofibrillar Myopathy Research

The classical morphological hallmarks of the clinically and genetically diverse group of human myofibrillar myopathies are signs of myofibrillar degeneration and desmin-positive protein aggregates. The local sarcomeric enrichment of filamin-C and xin actin-binding repeat-containing proteins 1 and 2 (xirp 1 and xirp 2) is a marker of myofibrillar damage. This work aimed to (i) address filamin-C- and xirp 1/2-positive sarcomeric lesions in desminopathy mouse models, (ii) develop an approach to quantifying xirp 1/2-positive sarcomeric lesions, and (iii) study the effects of acute physical exercise on sarcomeric lesion formation in R349P desmin knock-in mice, which are a model of human R350P desminopathy. Sarcomeric lesions were visualised by xirp 1/2 and filamin-C immunofluorescence in soleus muscles from R349P and R405W desmin knock-in, desmin knock-out and W2711X filamin-C knock-in mice. The open-source software QuPath was used to analyse xirp 1/2 confocal immunofluorescence images of soleus muscle from nonexercised and treadmill-exercised R349P desmin knock-in mice. Filamin-C and xirp 1/2 stained muscles revealed the presence of congenerous sarcomeric lesions in heterozygous and homozygous R349P and R405W desmin knock-in, homozygous desmin knock-out and heterozygous and homozygous W2711X filamin-C knock-in mice. Quantitative analysis of R349P desmin knock-in mice showed (i) significantly more lesions in nonexercised heterozygous mice, with an even more pronounced increase in homozygous animals, as compared to wild-type, and (ii) a significant increase in sarcomeric lesions per mm2 in heterozygous mice and wild-type siblings subjected to strenuous treadmilling. A QuPath workflow to quantify sarcomeric lesions using xirp 1/2 immunofluorescence images showed augmented densities of lesions in R349P desminopathy mice and after treadmill exercise. Eccentric and high-intensity physical activity may exhibit a disease-promoting effect on skeletal muscles in desminopathy.