Altered mechanical properties of titin immunoglobulin domain 27 in the presence of calcium

Abstract

Titin (connectin) based passive force regulation has been an important physiological mechanism to adjust to varying muscle stretch conditions. Upon stretch, titin behaves as a spring capable of modulating its elastic response in accordance with changes in muscle biochemistry. One such mechanism has been the calcium-dependent stiffening of titin domains that renders the spring inherently more resistant to stretch. This transient titin-calcium interaction may serve a protective function in muscle, which could preclude costly unfolding of select domains when muscles elongate to great lengths. To test this idea, fluorescence spectroscopy was performed revealing a change in the microenvironment of the investigated immunoglobulin domain 27 (I27) of titin with calcium. Additionally, an atomic force microscope was used to evaluate the calcium-dependent regulation of passive force by stretching eight linked titin I27 domains until they unfolded. When stretching in the presence of calcium, the I27 homopolymer chain became stabilized, displaying three novel properties: (1) higher stretching forces were needed to unfold the domains, (2) the stiffness, measured as a persistence length (PL), increased and (3) the peak-to-peak distance between adjacent I27 domains increased. Furthermore, a peak order dependence became apparent for both force and PL, reflecting the importance of characterizing the dynamic unfolding history of a polymer with this approach. Together, this novel titin Ig-calcium interaction may serve to stabilize the I27 domain permitting titin to tune passive force within stretched muscle in a calcium-dependent manner.

Titin force is enhanced in actively stretched skeletal muscle

“…3). The predicted force of actively stretched myofibrils without titin binding still exceeded passive forces, presumably due the binding of calcium to the PEVK and Ig domains (Labeit et al, 2003;Joumaa et al, 2008;Ting et al, 2012;DuVall et al, 2013), but only to a small extent. Overall, these simulations conceptually reflect the experimental results and proposed mechanism that titin is bound to the thin filament during activation.…”

“…In the presence of calcium, titin-based stiffness increases (Labeit et al, 2003;DuVall et al, 2013). This calcium-initiated increase in titin-based stiffness was a likely contributor to the enhanced titin force observed during active stretch of myofibrils.…”

“…In addition, the experiments by were unable to resolve an effect of calcium on the increase in stiffness of titin, possibly due to the high variation of their data. This is surprising as calcium has been shown to increase the stiffness of titin (Labeit et al, 2003;Joumaa et al, 2008;DuVall et al, 2013). Thus this study specifically aimed to quantify the contribution of calcium effects to the enhanced force of titin.…”

“…The configuration of the extensible I-band of titin is changed in the presence of calcium (Tatsumi et al, 2001). Specifically, it has been shown that the stiffness of the I-band region of titin increases when calcium ions bind to the PEVK region and Ig domains (Labeit et al, 2003;DuVall et al, 2013). In addition to intrinsic changes to the stiffness of titin, calcium-dependent interactions between titin and the thin filament have also been observed (Kellermayer and Granzier, 1996).…”

Titin force enhancement following active stretch of skinned skeletal muscle fibres

“…It is well established that titin becomes stiffer in the presence of calcium (DuVall et al, 2013;Labeit et al, 2003). However, the calcium-based increase in titin force in intact sarcomeres is too small to explain the force increase after active stretch (Joumaa et al, 2008;Powers et al, 2014).…”