Aberrant Protein Turn-Over Associated With Myofibrillar Disorganization in FHL1 Knockout Mice

Abstract

Mutations in the FHL1 gene, and FHL1 protein deletion, are associated with rare hereditary myopathies and cardiomyopathies. FHL1-null mice develop age-dependent myopathy and increased autophagic activity. However, the molecular pathway involved in contractile function and increased autophagic activity in the FHL1-null mouse has not yet been fully elucidated. In this study, FHL1 protein was knocked out in mice using Transcription Activator-like Effector Nucleases (TALENs) and the IRS1-FOXO1/mTOR signaling pathway was investigated in skeletal muscles and heart. TALEN constructs caused targeted mutations in 30% of newborn mice; these mutations caused a deletion of 1–13 base pairs which blocked synthesis of the full-length FHL1 protein. Furthermore, 2.5-month old FHL1-null male mice were not prone to global muscular fatigue when compared with WT littermates, but histological analysis and ultrastructural analysis by transmission electron microscopy confirmed the presence of myofibrillar disorganization and the accumulation of autophagosome or autolysosome-like structures in FHL1-null mice. Moreover, autophagy and mitophagy were both activated in FHL1 KO mice and the degradation of autophagic lysosomes was impeded. Enhanced autophagic activity in FHL1 KO mice was induced by FOXO1 up-regulation and protein synthesis was increased via mTOR. The cytoskeletal proteins, MYBPC2 and LDB3, were involved in the formation of pathological changes in FHL1 KO mice. Markers of early differentiation (MEF2C and MYOD1) and terminal differentiation (total MYH) were both up-regulated in tibialis anterior (TA) muscles in FHL1 KO mice. The number of type I and type II fibers increased in FHL1-null TA muscles, but the number of type| | b, and type | | d fibers were both reduced in FHL1-null TA muscles. The results obtained from the heart were consistent with those from the skeletal muscle and indicated autophagic activation by FOXO1 and an increase in protein synthesis via mTOR also occurred in the heart tissue of FHL1 knockout mice. In conclusion, aberrant protein turn-over associated with myofibrillar disorganization in FHL1 knockout mice. the up-regulation of FOXO1 was associated with enhanced autophagic activity and pathological changes in the muscle fibers of FHL1 KO mice. These results indicated that autophagy activated by FOXO1 is a promising therapeutic target for hereditary myopathies and cardiomyopathies induced by FHL1.

MiR-96-5p Induced by Palmitic Acid Suppresses the Myogenic Differentiation of C2C12 Myoblasts by Targeting FHL1

“…Four-and-a-half LIM domains protein 1 (FHL1) is encoded by the FHL1 gene and is a member of the FHL family of proteins involved in nuclear–cytoplasmic networking, muscle development, and sarcomere assembly [ 18 ]. Since FHL1 is highly expressed in skeletal muscle, an increasing number of studies have focused on the role of FHL1 in skeletal muscle integrity and myopathy [ 19 ], and several recent studies have demonstrated that FHL1 is a crucial regulator of skeletal myogenesis and muscle maintenance [ 20 , 21 , 22 , 23 ]. Mutations in the FHL1 gene have been reported to be associated with various human myopathies, including X-linked myopathy with postural muscle atrophy [ 24 ], reducing body myopathy [ 25 ], scapuloperoneal myopathy [ 26 ], and Emery–Dreifuss muscular dystrophy [ 27 ].…”

“…Moreover, FHL1 overexpression increased myotube fusion and caused myotube hypertrophy in C2C12 cells, indicating that it has the potential to enhance skeletal myogenesis and muscle mass [ 20 , 21 ]. FHL1 knockout in mice, on the other hand, increased skeletal muscle autophagic activity and induced myopathies with irregular structure and muscle fiber size, suggesting an important role of FHL1 in myogenesis and muscle integrity [ 21 , 22 , 23 ]. However, the association between FHL1 expression and SFA accumulation in myocytes has not been explored.…”

FHL1‐mutated reducing body myopathy

“…Their study revealed that forkhead box O1 (FOXO1)‐activated autophagic activity, and protein synthesis was promoted by mammalian target of rapamycin (mTOR) in FHL1 KO mice. The authors suggested that FOXO1 and mTOR can be therapeutic targets in FHL1 ‐mutant muscular dystrophy …”

“…Ding et al's study showed that FHL1 protein knockedout (KO) mice showed myofibrillar disorganization and the accumulation of autophagosome or autolysosome-like structures in the skeletal muscle and myocardium. 24 Their study revealed that forkhead box O1 (FOXO1)-activated autophagic activity, and protein synthesis was promoted by mammalian target of rapamycin (mTOR) in FHL1 KO mice. The authors suggested that FOXO1 and mTOR can be therapeutic targets in FHL1-mutant muscular dystrophy.…”

“…The authors suggested that FOXO1 and mTOR can be therapeutic targets in FHL1-mutant muscular dystrophy. 24 We have reported a case of FHL1-mutated RBM to demonstrate the importance of phenotype-genotype correlation to obtain a precise diagnosis and strategy for future targeted therapy.…”