Phosphorylation at Serines 157 and 161 Is Necessary for Preserving Cardiac Expression Level and Functions of Sarcomeric Z-Disc Protein Telethonin

Aims: In cardiac myocytes, the sarcomeric Z-disc protein telethonin is constitutively bis-phosphorylated at C-terminal residues S157 and S161; however, the functional significance of this phosphorylation is not known. We sought to assess the significance of telethonin phosphorylation in vivo, using a novel knock-in (KI) mouse model generated to express non-phosphorylatable telethonin (Tcap ^S157/161A). Methods and Results: Tcap ^S157/161A and wild-type (WT) littermates were characterized by echocardiography at baseline and after sustained β-adrenergic stimulation via isoprenaline infusion. Heart tissues were collected for gravimetric, biochemical, and histological analyses. At baseline, Tcap ^S157/161A mice did not show any variances in cardiac structure or function compared with WT littermates and mutant telethonin remained localized to the Z-disc. Ablation of telethonin phosphorylation sites resulted in a gene-dosage dependent decrease in the cardiac telethonin protein expression level in mice carrying the S157/161A alleles, without any alteration in telethonin mRNA levels. The proteasome inhibitor MG132 significantly increased the expression level of S157/161A telethonin protein in myocytes from Tcap ^S157/161A mice, but not telethonin protein in myocytes from WT mice, indicating a role for the ubiquitin-proteasome system in the regulation of telethonin protein expression level. Tcap ^S157/161A mice challenged with sustained β-adrenergic stimulation via isoprenaline infusion developed cardiac hypertrophy accompanied by mild systolic dysfunction. Furthermore, the telethonin protein expression level was significantly increased in WT mice following isoprenaline stimulation but this response was blunted in Tcap ^S157/161A mice. Conclusion: Overall, these data reveal that telethonin protein turnover in vivo is regulated in a novel phosphorylation-dependent manner and suggest that C-terminal phosphorylation may protect telethonin against proteasomal degradation and preserve cardiac function during hemodynamic stress. Given that human telethonin C-terminal mutations have been associated with cardiac and skeletal myopathies, further research on their potential impact on phosphorylation-dependent regulation of telethonin protein expression could provide valuable mechanistic insight into those myopathies.