MYPN

Myopalladin (MYPN) is a striated muscle protein that localizes to the sarcomeric Z-line and is required to maintain Z-line structural integrity in skeletal and cardiac muscle (PMID:28017374). Biallelic loss-of-function mutations abolish full-length protein and Z-line localization, producing Z-streaming, nemaline-like bodies, and ultrastructural Z-line fragmentation, with residual protein accumulating in cap structures together with alpha-actinin (PMID:28017374, PMID:28220527, PMID:31133047). Mechanistically, the Ig3 domain mediates actin binding and bundling through conserved basic charge clusters via electrostatic interactions and promotes actin polymerization from monomeric actin in a calcium-independent manner; cardiomyopathy-associated Ig3 mutations (R955W, P961L) impair actin binding, nearly abolish bundling, and cause mislocalization into Z-disc clusters [PMID:bio_10.1101_2025.09.18.677238]. Disruption of MYPN function shifts sarcomeric and cytoskeletal gene programs, downregulating sarcomeric, Z-disc, and SRF-signaling genes rather than directly altering cardiomyocyte electrophysiology (PMID:42074575). Loss-of-function MYPN mutations cause a Z-line myopathy with nemaline-like and cap features (PMID:28017374).

1. 2016 High

   Established that MYPN is a Z-line protein whose loss disrupts Z-line architecture, defining its core structural role and disease relevance in striated muscle.

   Evidence Immunofluorescence in human muscle biopsies plus electron microscopy of homozygous nonsense knockin mice across multiple families

   PMID:28017374

   Open questions at the time

   • Did not define the molecular interactions through which MYPN maintains Z-line integrity
   • Mechanism of nemaline-like body formation unresolved
2. 2017 Medium

   Showed that truncating mutations cause mRNA defects and loss of full-length protein, with residual myopalladin accumulating in caps alongside alpha-actinin, linking MYPN loss to disturbed alpha-actinin organization.

   Evidence mRNA analysis and protein/immunofluorescence assays in patient muscle from two unrelated families

   PMID:28220527

   Open questions at the time

   • Whether MYPN directly binds alpha-actinin not tested
   • Single lab, limited families
3. 2019 Medium

   Confirmed with reciprocal protein and localization data that full-length MYPN is required for Z-line structural integrity, reinforcing the loss-of-function disease model.

   Evidence Immunoblotting and immunofluorescence of patient muscle with WES genetic identification

   PMID:31133047

   Open questions at the time

   • No mechanistic interaction data
   • Single lab
4. 2025 Medium

   Provided the molecular mechanism by defining the Ig3 domain as an actin-binding, bundling, and polymerization-promoting module dependent on basic charge clusters, and showing cardiomyopathy mutations disrupt these activities and cause mislocalization.

   Evidence Actin co-sedimentation and co-polymerization assays with charge-neutralizing mutagenesis, circular dichroism, and GFP-construct localization in Drosophila muscle (preprint)

   PMID:bio_10.1101_2025.09.18.677238

   Open questions at the time

   • In vitro and Drosophila system; not validated in mammalian cardiomyocytes
   • Preprint not yet peer-reviewed
   • Quantitative contribution of bundling vs polymerization to Z-line maintenance unresolved
5. 2024 Medium

   Tested causality of a familial LVNC-associated MYPN variant and found the orthologous Mypn S1291T mutation alone insufficient to cause disease, indicating it is not the primary pathogenic driver in that context.

   Evidence Knock-in mice with Mypn S1291T alone vs Tnnt2 mutant vs double mutant, with cardiac phenotyping (preprint)

   PMID:bio_10.1101_2024.10.09.24314670

   Open questions at the time

   • Negative result for one specific variant; not generalizable to other MYPN mutations
   • Preprint
   • Potential modifier role with TNNT2 not fully resolved
6. 2026 Medium

   Connected an HCM-associated MYPN variant to transcriptional reprogramming of sarcomeric and cytoskeletal gene programs rather than to direct electrophysiological dysfunction.

   Evidence Patient iPSC-derived cardiomyocytes with patch-clamp, calcium imaging, and RNA-seq

   PMID:42074575

   Open questions at the time

   • Non-isogenic controls limit attribution to the variant
   • Causal link between MYPN dysfunction and the transcriptional changes not established
   • Single lab

• How MYPN integrates actin bundling with Z-line signaling and sarcomeric gene expression, and whether it engages defined Z-line partners beyond alpha-actinin in mammalian muscle, remains open.
• No direct mammalian binding partners established in the corpus
• Mechanism linking structural loss to SRF/sarcomeric transcriptional changes unknown
• No high-resolution structure of the actin-bound Ig3 domain