Frizzled related protein deficiency impairs muscle strength, gait and calpain 3 levels

Background: Limb-girdle muscular dystrophy recessive 1 calpain3-related (LGMDR1), previously known as LGMD2A, is a disease caused by mutations in the CAPN3 gene. It is characterized by progressive weakness and muscle degeneration. Frizzled related protein (FRZB), upregulated in LGMDR1, was identified as a key regulator of the crosstalk between Wnt and integrin signalling pathways. FRZB gene silencing showed a recovery in the expression of some of the costamere protein levels in myotubes. Results: Here, we performed a comprehensive characterization of Frzb -/- mice muscles to study the absence of Frzb in skeletal muscle and eventual links with the molecular characteristics of LGMDR1 patient muscles. Frzb -/- mice showed reduced muscle size and strength. Gait analysis showed that Frzb -/- mice moved more slowly but no impaired regeneration capacity was observed after muscle injury. Additionally, Frzb -/- mice muscle showed an increased number of mesoangioblasts. Lack of Frzb gene in Frzb -/- mice and its increased expression in LGMDR1 patients, showed contrary regulation of Rora, Slc16a1, Tfrc and Capn3 genes. The reciprocal regulation of Frzb and Capn3 genes further supports this axis as a potential target for LGMDR1 patients. Conclusions: Our data confirm a role for Frzb in the regulation of Rora, Slc16a1, Tfrc, and Capn3 genes in muscle cells. In vivo, reduced muscle strength and gait in the Frzb -/- mice are intriguing features. The reciprocal relationship between FRZB and CAPN3 further supports a key role for this axis in patients with LGMDR1.