An FHL1-containing complex within the cardiomyocyte sarcomere mediates hypertrophic biomechanical stress responses in mice

Abstract

The response of cardiomyocytes to biomechanical stress can determine the pathophysiology of hypertrophic cardiac disease, and targeting the pathways regulating these responses is a therapeutic goal. However, little is known about how biomechanical stress is sensed by the cardiomyocyte sarcomere to transduce intracellular hypertrophic signals or how the dysfunction of these pathways may lead to disease. Here, we found that fourand-a-half LIM domains 1 (FHL1) is part of a complex within the cardiomyocyte sarcomere that senses the biomechanical stress-induced responses important for cardiac hypertrophy. Mice lacking Fhl1 displayed a blunted hypertrophic response and a beneficial functional response to pressure overload induced by transverse aortic constriction. A link to the Gαq (Gq) signaling pathway was also observed, as Fhl1 deficiency prevented the cardiomyopathy observed in Gq transgenic mice. Mechanistic studies demonstrated that FHL1 plays an important role in the mechanism of pathological hypertrophy by sensing biomechanical stress responses via the N2B stretch sensor domain of titin and initiating changes in the titin-and MAPK-mediated responses important for sarcomere extensibility and intracellular signaling. These studies shed light on the physiological regulation of the sarcomere in response to hypertrophic stress.

Titin‐based mechanosensing modulates muscle hypertrophy

“…There are multiple previous studies that support titin mechanosensing activity through the elastic cardiac‐specific N2B element, where hypertrophy seems to be regulated through the four‐and‐a‐half LIM domains proteins (FHL) and the mitogen‐activated protein kinases (MAPK) 4, 5, 37, 38. Also deletion of the serine/threonine kinase (TK) domain of titin results in hypertrophy in the heart39 and deletion of part of the skeletal muscle elastic N2A domain results in severe dystrophy,40 highlighting the importance of the various titin signalling hubs in muscle growth.…”

“…Whether calcium‐independent mechanisms play a role in muscle hypertrophy is poorly understood. Such mechanisms might involve mechanosensory proteins, of which the giant protein titin is an attractive candidate because it forms a continuous filament along the myofibril and contains elastic segments that have been proposed to sense muscle stretch 3, 4, 5. Titin‐based mechanosensing might involve titin‐binding proteins that form several signalling ‘hubs’ along the titin molecule.…”

“…Furthermore, decreasing titin stiffness in the heart blunts hypertrophy development during mechanical overload6 and increasing titin's stiffness has the opposite effect,7, 8 processes that presumably involve titin‐binding proteins (e.g. FHL1/2,5 MARP1–39, 10, 11, 12 and CAPN313, 14). However, the results of these studies are confounded by the fact that not only was titin‐based stiffness altered, so was active force generation,15, 16, 17 which itself modulates muscle growth.…”

The Giant Protein Titin: A Regulatory Node That Integrates Myocyte Signaling Pathways

“…2). N2-Bus interacts with the four-anda-half LIM domain proteins FHL1 and FHL2 (82,83), which are transcriptional coactivators able to translocate from the sarcomere and cytosol to the nucleus. Note that another binding site for FHL2 is in the longest unique sequence insertion of M-band titin, Mis-2 (82).…”

Single Molecule Force Spectroscopy of the Cardiac Titin N2B Element

“…These functions include providing elasticity toward the upper limit of the physiological sarcomere length range (12) and binding two members of the FHL (four-and-a-half LIM domain) protein family, FHL1 (31) and FHL2 (32,33). Recent evidence indicates that FHL1 functions in the hypertrophic biomechanical stress response (31) and that FHL2 functions in localizing various metabolic enzymes (32). In addition, the N2B-Us contains sequences that can be phosphorylated by PKA (34) and PKG (35) and that thereby tune the compliance of the N2B-Us spring.…”