TRDN

TRDN encodes triadin, a sarcoplasmic reticulum protein of the muscle calcium release complex that is expressed as multiple alternatively spliced isoforms (Trisk 51, Trisk 95, Trisk 32) from a single gene (PMID:12659871). Within skeletal muscle, Trisk 95 is the functionally decisive isoform for excitation-contraction coupling: its overexpression specifically abolishes depolarization-induced calcium release without altering caffeine-induced release or dihydropyridine receptor activation, identifying it as a direct regulator of the calcium release step (PMID:16176928). This regulation is mediated by three lysine residues (K218, K220, K224) in the luminal domain of Trisk 95, which are jointly required for binding and activating the RyR1 channel; the same residues mediate calsequestrin binding, so a single triadin monomer cannot simultaneously engage RyR1 and calsequestrin (PMID:27595738). A distinct isoform, Trisk 32, instead associates with IP3 receptors and potentiates IP3-mediated calcium release independently of ryanodine receptors (PMID:21811790). In the heart, loss-of-function and missense TRDN mutations impair RyR2-mediated calcium release: a homozygous p.L56P substitution alters cardiac TRISK32 dynamics and reduces caffeine-induced calcium release when co-expressed with RyR2 (PMID:31437535), and a deep intronic variant disrupts cardiac triadin splicing to abolish triadin protein in patient-derived iPSC cardiomyocytes (PMID:32402482), establishing TRDN as a cause of recessive arrhythmia (triadin knockout syndrome). Triadin also functions outside striated muscle, where Trisk 95 is transcriptionally induced by glucose and is required for glucose-evoked calcium flux and mitochondrial fragmentation in skin cells (PMID:32699238).

1. 2003 Medium

   Established that human TRDN produces multiple triadin isoforms by alternative splicing, defining the molecular repertoire whose individual roles would later be dissected.

   Evidence Gene cloning, RT-PCR and Western blot of human skeletal muscle

   PMID:12659871

   Open questions at the time

   • Did not assign distinct functions to Trisk 51 versus Trisk 95
   • Cardiac isoform expression not addressed
2. 2005 High

   Showed that Trisk 95, not Trisk 51, specifically governs the excitation-contraction coupling step, resolving which isoform regulates depolarization-induced calcium release.

   Evidence Adenoviral overexpression with antisense rescue, calcium imaging and electrophysiology in rat myotubes

   PMID:16176928

   Open questions at the time

   • Did not define the molecular interface with RyR1
   • Mechanism of the EC-coupling-specific effect unresolved
3. 2011 Medium

   Demonstrated that a separate isoform, Trisk 32, links triadin to IP3-receptor signaling rather than ryanodine receptors, revealing isoform-specific channel partnerships.

   Evidence Overexpression, reciprocal co-immunoprecipitation and calcium measurement with RyR pharmacological exclusion in myoblasts

   PMID:21811790

   Open questions at the time

   • Binding interface on Trisk 32 not mapped
   • Single lab, physiological role of IP3R potentiation untested in vivo
4. 2016 High

   Identified the luminal lysine cluster (K218/K220/K224) as the structural determinant for Trisk 95 binding and activation of RyR1, and showed it is shared with calsequestrin binding, defining mutually exclusive engagement.

   Evidence Peptide binding assays, alanine-scanning mutagenesis and single-channel bilayer recordings

   PMID:27595738

   Open questions at the time

   • Functional competition between RyR1 and CSQ not demonstrated in intact cells
   • Structural basis of triadin oligomerization not resolved
5. 2019 Medium

   Connected a TRDN missense mutation to arrhythmia mechanism by showing p.L56P alters cardiac TRISK32 and reduces RyR2-coupled calcium release.

   Evidence GFP-tagged mutant expression and caffeine-induced calcium release assay with RyR2 co-expression in heterologous cells

   PMID:31437535

   Open questions at the time

   • Single functional readout for the core claim
   • Effect on native cardiomyocyte calcium handling not shown
6. 2020 Medium

   Defined a non-coding pathogenic mechanism in which a deep intronic variant disrupts cardiac triadin splicing and eliminates triadin protein, plus a non-muscle role for glucose-induced Trisk 95.

   Evidence Genome sequencing, RT-PCR and Western blot in patient iPSC-cardiomyocytes; luciferase reporter, calcium imaging and knockout mouse in skin cells

   PMID:32402482 PMID:32699238

   Open questions at the time

   • Mechanism linking triadin loss to arrhythmia at tissue level not fully resolved
   • Transcription factors driving glucose induction of Trisk 95 unidentified

• How the distinct triadin isoforms are coordinated within a single calcium release complex and how loss of triadin produces malignant cardiac arrhythmia mechanistically remain open.
• No integrated structural model of the triadin-RyR-calsequestrin complex
• In vivo arrhythmia mechanism downstream of triadin loss not established