Big Job for the Small Titin: Don’t Underestimate the “Little” Guy

Titin is the largest protein in nature, measuring ~3-4 MDa, where a single molecule spans from the sarcomere Z-disk to M-line.It contributes critically to sarcomere assembly and modulates passive tension, recoil, mechanotransduction, and contractility.Titin is extensively alternatively spliced, with three primary full-length isoforms, N2B, N2BA, and N2A that differ in length and elasticity.Shifts in the relative expression of these isoforms is a key underpinning of mechanical dysfunction in cardiomyopathies. 1 Understanding each titin isoform and how they contribute to sarcomere assembly and function has been crucial in efforts towards treating its dysregulation in heart failure.Therefore, research into titin has primarily focused on these "full-length" isoforms for much of the last 50 years.However, there exist two smaller and less abundant isoforms of titin that have been mostly overlooked until recently -Cronos and Novex 3 (Nx3).Nx3 is produced by alternative inclusion of exon 48 containing an early stop codon, producing the smallest titin isoform at ~650 kDa (although it is still larger than more than 99% of mammalian proteins).It is most abundant in cardiomyocytes but is also expressed in skeletal muscle.Surprisingly, Nx3 was first identified in 2001 but has received very little investigation into its role in cardiomyocytes until recently.2 While some speculative hypotheses have been developed around Nx3's form and function in the sarcomere, in this issue of Circulation Research, Linke et al. pursued a more in-depth exploration of the mini titin.3 Other non-canonical titin isoforms, such as Cronos and fetal cardiac titin, exhibit decreased protein expression through embryonic development and after birth.Conversely, this investigation found that overall Nx3 expression progressively increases through early development in humans and mice.Prior studies have found that Nx3 localizes to the nucleus in hypoxic fetal cardiomyocytes and regulates cell proliferation.4,5 However, after birth, when cardiomyocytes are no longer proliferative, Nx3 is exclusively localized to the sarcomere.While Linke et al 3 did not identify nuclear localization, they demonstrated that Nx3 localization to the z-disk in utero is necessary for optimal sarcomere assembly and