Titin kinase ubiquitination aligns autophagy receptors with mechanical signals in the sarcomere

Abstract

Striated muscle undergoes remodelling in response to mechanical and physiological stress, but little is known about the integration of such varied signals in the myofibril. The interaction of the elastic kinase region from sarcomeric titin (A168-M1) with the autophagy receptors Nbr1/p62 and MuRF E3 ubiquitin ligases is well suited to link mechanosensing with the trophic response of the myofibril. To investigate the mechanisms of signal cross-talk at this titin node, we elucidated its 3D structure, analysed its response to stretch using steered molecular dynamics simulations and explored its functional relation to MuRF1 and Nbr1/p62 using cellular assays. We found that MuRF1-mediated ubiquitination of titin kinase promotes its scaffolding of Nbr1/p62 and that the process can be dynamically down-regulated by the mechanical unfolding of a linker sequence joining titin kinase with the MuRF1 receptor site in titin. We propose that titin ubiquitination is sensitive to the mechanical state of the sarcomere, the regulation of sarcomere targeting by Nbr1/p62 being a functional outcome. We conclude that MuRF1/Titin Kinase/Nbr1/p62 constitutes a distinct assembly that predictably promotes sarcomere breakdown in inactive muscle.

Removal of MuRF1 Increases Muscle Mass in Nemaline Myopathy Models, but Does Not Provide Functional Benefits

“…The mechanism that underlies the increased muscle weight that was found in some of the studied muscles upon MuRF1 deletion was not studied, but several possibilities exist. A recently proposed model implicates MuRF1 ubiquitination of the titin kinase (TK) by binding to the A168–170 domains, localized at the N-terminal end of the TK [ 43 , 44 , 45 ]. In this model, ubiquitination of the TK by MuRF1 promotes the recruitment of p62 and its interacting partner Nbr1 to the M-band.…”

“…In this model, ubiquitination of the TK by MuRF1 promotes the recruitment of p62 and its interacting partner Nbr1 to the M-band. P62 and Nbr1 are adaptor molecules that assemble onto polyubiquitinated proteins and promote their removal by selective autophagy [ 45 ]. Importantly, this process appears to be inhibited by muscle contraction, as this results in high forces on the M-band that unfold the N-terminal extension of the TK, increasing the distance between the MuRF1 binding site and its target sites on TK, thereby reducing or preventing TK ubiquitination and reducing the capability of TK to recruit Nbr1 and p62 [ 45 ].…”

An increase in serial sarcomere number induced via weighted downhill running improves work loop performance in the rat soleus

“…It follows that if a muscle is repeatedly forced to operate with overstretched sarcomeres (i.e., with eccentric training), SSN would increase to maintain optimal actin-myosin overlap regions in that position (Koh, 1995; Lynn et al, 1998; Williams and Goldspink, 1978). These adaptations relate to a muscle’s ability to sense a change in tension (in this case increased tension at a long muscle length), then convert that mechanical signal into biochemical events regulating gene expression and protein synthesis (Aoki et al, 2009; Bogomolovas et al, 2021; Franchi et al, 2018; Herring et al, 1984; Soares et al, 2007; Spletter et al, 2018).…”

Protein Quality Control at the Sarcomere: Titin Protection and Turnover and Implications for Disease Development

“…The A-band portion of titin, which is tightly linked to myosin and myosin-binding protein C ( Linke and Hamdani, 2014 ) is thought to be embedded in an autophagosome and degraded after fusion with a lysosome. The hypothesis of domain dependent degradation is supported by recent studies demonstrating localization of proteasome subunits of the 20S core at the Z-disk/I-band region of the sarcomere in adult cardiomyocytes ( Rudolph et al, 2019 ) and localization of the autophagy-related Nbr1/p62/MuRF-1 complex at the titin kinase region upon MuRF-1 mediated ubiquitination of titin ( Lange et al, 2005 ; Bogomolovas et al, 2021 ). Figure 2 summarizes recent findings supporting the hypothesis of a domain specific degradation of titin by the proteasome and autophagy.…”

“…Current evidence suggests that both the proteasome ( Kötter et al, 2016 ; Higashikuse et al, 2019 ) and autophagy ( Lange et al, 2005 ; Bogomolovas et al, 2021 ; Müller et al, 2021 ), are involved in the degradation of titin and that the degradation of the different titin domains might occur specifically via one or the other system. This hypothesis was lately supported by experimental in vitro inhibition of the proteasome, which lead to increased proteasome-dependent K48 polyubiquitination, and inhibition of autophagy, causing increased autophagy-dependent K63 polyubiquitination of titin ( Kötter et al, 2016 ; Müller et al, 2021 ).…”