The Z/I region of the sarcomere is a hot spot for protein-protein interaction and mechanotransduction. A major component is the giant protein titin, which spans the half-sarcomere as an elastic molecular spring. Titin has been linked to various myocardial signalling cascades, and force transmission via the titin springs may be central to the mechanosensory function of the Z/I region. Titin stiffness can be regulated by altering the ratio of N2BA:N2B titin isoforms. This ratio shifts in favour of the stiffer N2B isoform during development of the fetal/perinatal heart, but it can switch back to increased N2BA proportions in chronic systolic heart failure. We recently demonstrated that the perinatal isoform shift is greatly affected by changes in thyroid hormone status and can be stalled by inhibition of the PI3K/Akt pathway.

Titin springiness can also be altered on a faster time scale by PKA- or PKG-mediated phosphorylation of a cardiac-specific I-band titin domain. We found that both kinases target a serine residue (S469) in the N2-B_us, causing a significant drop in passive myofibrillar stiffness. Thus, ß-adrenergic agonists, NO or natriuretic peptides affect diastolic mechanical properties by reducing titin stiffness. PKG also phosphorylated the N2-A segment, which may be important for regulating protein-protein interactions. Failing human hearts revealed a deficit for basal titin phosphorylation compared to donor hearts, which may contribute to diastolic dysfunction in heart failure but also affect titin-based signalling in the Z/I region.