Affinage

SMPX

Small muscular protein · UniProt Q9UHP9

Length
88 aa
Mass
9.6 kDa
Annotated
2026-06-10
34 papers in source corpus 13 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SMPX encodes a small (~88-amino-acid) cytoskeleton-associated protein expressed preferentially in striated muscle and inner ear hair cells that functions as a mechanosensitive scaffold (PMID:10598820, PMID:11401441). In muscle it localizes to focal adhesion complexes and to repetitive double stripes flanking the Z-disc while being excluded from nuclei, consistent with a sarcomeric/costameric mechanosensor rather than a transcription factor (PMID:15893749, PMID:24936977). SMPX co-immunoprecipitates with vinculin and co-localizes with paxillin, integrin β1, and Rac1; its overexpression activates Rac1, promotes vinculin–Arp2/3 association, and drives cell spreading and lamellipodia formation through a Rac1–p38 signaling axis, linking it directly to actin dynamics (PMID:15893749). Its expression is directly transcriptionally activated by the nuclear receptor NOR-1 (PMID:27181368), and its interaction partner MUSTN1 binds and stabilizes the SMPX protein, with SMPX's pro-myogenic activity depending on MUSTN1 (PMID:39828423). In hair cells SMPX localizes to stereocilia, kinocilia, and the primary cilium, where it maintains hair bundle integrity and mechanotransduction (PMID:34722533, PMID:34204426, PMID:38570547). Loss-of-function nonsense and splice-site mutations cause X-linked progressive hearing loss through stereocilia degeneration and impaired mechanotransduction (PMID:21549336, PMID:21549342, PMID:34722533), while missense mutations increase aggregation propensity, sequester SMPX into stress granules, and cause a distinct X-linked distal myopathy with amyloid-like sarcoplasmic inclusions (PMID:33974137).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 1999 Medium

    Establishing that SMPX is a small protein expressed abundantly in striated muscle defined the tissue context in which all later mechanistic work would be interpreted.

    Evidence cDNA cloning, genomic sequencing, and tissue expression profiling mapping the gene to Xp22.1

    PMID:10598820

    Open questions at the time
    • No functional or subcellular role assigned
    • Hair-cell expression not yet recognized
  2. 2001 Medium

    The question of whether SMPX responds to mechanical input was addressed by showing its mRNA is induced by passive muscle stretch, framing it as a candidate mechanosensing gene.

    Evidence In vivo stretch model of tibialis anterior with mRNA differential display and sequence analysis

    PMID:11401441

    Open questions at the time
    • Transcriptional induction does not establish protein-level mechanosensory function
    • Predicted NLS and CKII sites not functionally validated
  3. 2005 High

    The molecular mechanism was advanced from correlation to a defined signaling axis by placing SMPX at focal adhesions in physical association with vinculin and driving cytoskeletal remodeling via Rac1 and p38.

    Evidence Tagged overexpression in C2C12 cells with reciprocal Co-IP, co-localization, Rac1 pull-down activity assay, dominant-negative and pharmacological rescue

    PMID:15893749

    Open questions at the time
    • Direct vs indirect vinculin binding not resolved
    • Effects shown by overexpression rather than endogenous loss-of-function
    • How mechanical stimulus couples to Rac1 activation unknown
  4. 2011 Medium

    The disease relevance of SMPX was established by linking nonsense loss-of-function mutations to X-linked progressive hearing loss and localizing the protein in cochlear hair and supporting cells.

    Evidence Linkage analysis, targeted sequencing, co-segregation in two families, heterologous mutant overexpression, and mouse cochlear immunolocalization

    PMID:21549336 PMID:21549342

    Open questions at the time
    • Cellular mechanism of hearing loss not defined
    • No animal model of deafness yet
  5. 2014 Medium

    The localization question was resolved in adult muscle fibers, showing SMPX sits at the Z-disc/costamere and is excluded from nuclei, ruling out a transcription-factor role despite its predicted NLS.

    Evidence In vivo confocal imaging of SMPX-EGFP and in vivo overexpression with fiber-type and cross-sectional area measurements in adult muscle

    PMID:24936977

    Open questions at the time
    • Ectopic overexpression produced no measurable phenotype, leaving the in vivo muscle function unresolved
    • Predicted NLS function unexplained
  6. 2016 Medium

    The upstream regulation of SMPX was defined by identifying the nuclear receptor NOR-1 as a direct transcriptional activator binding a non-consensus NBRE in its promoter.

    Evidence NOR-1 gain- and loss-of-function in VSMC and myoblasts, reporter assays, DNA-protein binding assays, and RT-qPCR

    PMID:27181368

    Open questions at the time
    • SMPX itself dispensable for myotube differentiation, so its functional output downstream of NOR-1 remains unclear
    • Other transcriptional regulators not surveyed
  7. 2021 High

    Two distinct disease mechanisms were established: a knockout deafness model defining progressive stereocilia degeneration, and missense mutations defining a toxic aggregation myopathy.

    Evidence CRISPR Smpx knockout mouse with ABR, cochlear EM and live hair-cell localization; plus aggregation prediction, stress-granule co-localization, and patient biopsy histopathology/EM

    PMID:33974137 PMID:34722533

    Open questions at the time
    • Molecular function of SMPX within stereocilia not defined
    • How missense aggregation links to muscle pathology mechanistically unresolved
    • Relationship between deafness loss-of-function and myopathy gain-of-toxicity within one protein not reconciled
  8. 2021 Medium

    The hair-cell role was extended cross-species by showing zebrafish Smpx loss impairs kinocilia/stereocilia structure and mechanotransduction, generalizing the mechanosensory function.

    Evidence Morpholino and CRISPR knockout in zebrafish with immunofluorescence and FM1-43 mechanotransduction assays plus muscle morphology

    PMID:34204426

    Open questions at the time
    • Molecular interaction partners in hair cells not identified
    • Mechanism linking SMPX to ciliary maintenance unknown
  9. 2024 Medium

    The lateral-line study refined the mechanosensory role, localizing Smpx to the cytoplasm and primary cilium of neuromast hair cells and showing loss impairs hair-cell differentiation and mechanotransduction.

    Evidence In situ hybridization, immunofluorescence localization, morpholino and CRISPR F0 knockout, and FM1-43 mechanotransduction assay in zebrafish neuromasts

    PMID:38570547

    Open questions at the time
    • Ciliary protein partners unidentified
    • Direct molecular activity at the cilium not established
  10. 2025 Medium

    A protein partner controlling SMPX stability was identified, with MUSTN1 binding and stabilizing SMPX and SMPX's pro-myogenic activity depending on MUSTN1.

    Evidence Reciprocal Co-IP, MUSTN1 knockout mouse phenotyping, and myoblast knockdown/overexpression with protein stability assays

    PMID:39828423

    Open questions at the time
    • Structural basis of the MUSTN1–SMPX interaction unknown
    • How stabilization relates to the costameric/mechanosensor role not connected

Open questions

Synthesis pass · forward-looking unresolved questions
  • The core biochemical activity of SMPX and how a single small scaffold protein produces both loss-of-function deafness and gain-of-toxicity myopathy remain unresolved.
  • No defined enzymatic or direct molecular activity for SMPX
  • Mechanism coupling mechanical stimulus to Rac1/p38 signaling not reconstituted
  • No structural model of SMPX or its complexes

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060089 molecular transducer activity 3 GO:0008092 cytoskeletal protein binding 1
Localization
GO:0005856 cytoskeleton 2 GO:0005929 cilium 2 GO:0005829 cytosol 1 GO:0005886 plasma membrane 1
Pathway
R-HSA-9709957 Sensory Perception 3 R-HSA-1266738 Developmental Biology 2
Partners
ITGB1MUSTN1PXNRAC1VCL
Complex memberships
costamere/Z-discfocal adhesion complex

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2001 SMPX (Smpx) mRNA is upregulated in skeletal muscle in response to 7 days of passive stretch in vivo, and the encoded protein contains a nuclear localization signal and two overlapping casein kinase II phosphorylation sites, suggesting a role in mechanosensing. In vivo stretch model of tibialis anterior muscle followed by mRNA differential display and sequence analysis of the novel cDNA Genomics Medium 11401441
1999 SMPX encodes an 88-amino-acid protein (86 aa in rodents) expressed preferentially and abundantly in heart and skeletal muscle; the gene maps to Xp22.1 and consists of five exons spanning 52.1 kb. cDNA cloning, library screening, database alignment, complete genomic sequencing, and tissue expression analysis by reciprocal probing Human genetics Medium 10598820
2005 Chisel/Smpx localizes to focal adhesion complexes and co-immunoprecipitates with vinculin from C2C12 myoblasts and native muscle; it also co-localizes with paxillin, integrin β1, and Rac1 at peripheral membranes. Overexpression induces cell spreading and lamellipodia formation via a Rac1-dependent mechanism, increases Rac1 activity, and promotes vinculin–p34 (Arp2/3 subunit) association. Spreading is also dependent on p38 kinase activity downstream of Rac1. MYC-epitope-tagged overexpression in C2C12 cells, co-immunoprecipitation, immunofluorescence co-localization, Rac1 activity assay (pull-down), dominant-negative Rac1 rescue, p38 inhibitor treatment Experimental cell research High 15893749
2014 In adult rat skeletal muscle fibers, SMPX-EGFP fusion protein localizes to repetitive double stripes flanking the Z-disc and is excluded from all nuclei, consistent with a sarcomeric/costameric mechanosensor role and inconsistent with a transcription factor role. Overexpression of ectopic SMPX in adult mouse skeletal muscle produces no significant changes in fiber type distribution or cross-sectional area. In vivo confocal imaging of SMPX-EGFP in adult rat muscle fibers; in vivo overexpression in adult mice with fiber-type and cross-sectional area measurements PloS one Medium 24936977
2016 The nuclear receptor NOR-1 directly transcriptionally activates SMPX through a non-consensus NBRE site in the human SMPX promoter. NOR-1 silencing in human skeletal muscle myoblasts (HSMM) prevents SMPX expression; NOR-1 overexpression in vascular smooth muscle cells upregulates SMPX. SMPX upregulation itself is dispensable for HSMM differentiation into myotubes, but NOR-1 silencing prevents both SMPX expression and myotube differentiation. NOR-1 overexpression in VSMC, NOR-1 siRNA knockdown in HSMM, transcriptional reporter assays, DNA-protein binding assays (EMSA/ChIP), RT-qPCR Scientific reports Medium 27181368
2021 Missense mutations in SMPX increase the aggregation propensity of the SMPX protein; overexpressed mutant SMPX localizes to stress granules and slows their clearance in cell culture. Patient muscle biopsies show sarcoplasmic inclusions with amyloid-like characteristics and rimmed vacuoles. In silico aggregation prediction, cell culture overexpression of wild-type and mutant SMPX with stress granule marker co-localization, electron microscopy and histopathology of patient biopsies Acta neuropathologica Medium 33974137
2021 In an Smpx knockout mouse model (CRISPR-Cas9), male null mice show progressive hearing loss starting at high frequencies from the 3rd month; hair cell stereocilia degenerate progressively from the shortest row, followed by cellular edema and cell death. Female knockouts show milder, later-onset loss. Fluorescently tagged Smpx expressed in living hair cells localizes to stereocilia, and noise exposure experiments indicate Smpx participates in maintaining hair cell bundle integrity. CRISPR-Cas9 Smpx knockout mouse; ABR testing; cochlear morphological analysis by confocal and electron microscopy; live-cell fluorescent Smpx expression in hair cells; noise exposure paradigm Frontiers in cell and developmental biology High 34722533
2021 In Smpx-deficient zebrafish, inner ear hair cells show a marked decrease in kinocilia number and structural alterations of stereocilia and kinocilia, and mechanotransduction by hair cells is impaired. Additionally, muscle fiber organization and function are defective. Morpholino knockdown and CRISPR/Cas9 knockout in zebrafish; whole-mount immunofluorescence; mechanotransduction assays (FM1-43 dye uptake); muscle fiber morphology analysis International journal of molecular sciences Medium 34204426
2024 Smpx is expressed in neuromast hair cells of the zebrafish posterior lateral line and localizes to the cytoplasm and primary cilium of these cells. Loss of Smpx (morpholino knockdown or CRISPR F0 knockout) results in fewer properly differentiated neuromasts, a smaller lateral line primordium, structurally and numerically altered kinocilia, and significantly reduced mechanotransduction activity in neuromast hair cells. Whole-mount in situ hybridization, immunofluorescence subcellular localization; morpholino knockdown; CRISPR/Cas9 F0 knockout; mechanotransduction assay (FM1-43 uptake) Scientific reports Medium 38570547
2025 MUSTN1 physically interacts with SMPX (identified as an SMPX-interacting protein), stabilizes the SMPX protein, and promotes myogenic differentiation through SMPX; SMPX's promotion of myogenic differentiation depends on MUSTN1. MUSTN1 knockout mice show reduced muscle mass, fiber cross-sectional area, exercise endurance, and delayed muscle regeneration. Co-immunoprecipitation (protein interaction), MUSTN1 knockout mice (phenotypic analysis), knockdown and overexpression in myoblasts, protein stability assays Cell proliferation Medium 39828423
2011 Loss-of-function nonsense mutations in SMPX (detected in two independent families) cause X-linked progressive hearing loss; heterologous overexpression studies of truncated SMPX proteins were compatible with a loss-of-function mechanism. SMPX protein is present in hair cells and supporting cells of the murine cochlea. Genome-wide linkage analysis, targeted DNA capture with high-throughput sequencing, Sanger validation, co-segregation analysis, heterologous overexpression of mutant proteins, mouse cochlea immunolocalization American journal of human genetics Medium 21549336 21549342
2017 A donor splice-site variant (c.132+1G>A) in SMPX leads to four aberrant RNA transcripts via alternative splicing including non-canonical splice sites, generating a frameshift and premature termination codon; the resulting mRNA is degraded by nonsense-mediated mRNA decay (NMD), establishing NMD as a loss-of-function mechanism for this allele. RT-PCR and Sanger sequencing of aberrant transcripts from patient-derived material; qPCR for mRNA quantification to confirm NMD Molecular genetics & genomic medicine Medium 31478598

Source papers

Stage 0 corpus · 34 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2019 Stereotactic ablative radiotherapy versus standard radiotherapy in stage 1 non-small-cell lung cancer (TROG 09.02 CHISEL): a phase 3, open-label, randomised controlled trial. The Lancet. Oncology 479 30770291
2020 Characterizing allele- and haplotype-specific copy numbers in single cells with CHISEL. Nature biotechnology 103 32879467
2011 Next-generation sequencing identifies mutations of SMPX, which encodes the small muscle protein, X-linked, as a cause of progressive hearing impairment. American journal of human genetics 73 21549342
2011 Nonsense mutations in SMPX, encoding a protein responsive to physical force, result in X-chromosomal hearing loss. American journal of human genetics 62 21549336
2020 DNA Chisel, a versatile sequence optimizer. Bioinformatics (Oxford, England) 50 32647895
2001 Identification of a novel stretch-responsive skeletal muscle gene (Smpx). Genomics 42 11401441
1996 A novel locus for non-syndromic sensorineural deafness (DFN6) maps to chromosome Xp22. Human molecular genetics 32 8872482
1999 Identification, mapping, and genomic structure of a novel X-chromosomal human gene (SMPX) encoding a small muscular protein. Human genetics 24 10598820
2005 Muscle costameric protein, Chisel/Smpx, associates with focal adhesion complexes and modulates cell spreading in vitro via a Rac1/p38 pathway. Experimental cell research 23 15893749
2012 A novel deletion in SMPX causes a rare form of X-linked progressive hearing loss in two families due to a founder effect. Human mutation 22 22911656
2019 In silico analysis of nonsynonymous single-nucleotide polymorphisms (nsSNPs) of the SMPX gene. Annals of human genetics 18 31583691
2017 A novel frameshift mutation of SMPX causes a rare form of X-linked nonsyndromic hearing loss in a Chinese family. PloS one 17 28542515
2016 The nuclear receptor NOR-1 regulates the small muscle protein, X-linked (SMPX) and myotube differentiation. Scientific reports 16 27181368
2021 Programmed Cell Death Not as Sledgehammer but as Chisel: Apoptosis in Normal and Abnormal Craniofacial Patterning and Development. Frontiers in cell and developmental biology 14 34692678
2011 Variable degrees of hearing impairment in a Dutch DFNX4 (DFN6) family. Hearing research 14 21893181
2021 Missense mutations in small muscle protein X-linked (SMPX) cause distal myopathy with protein inclusions. Acta neuropathologica 12 33974137
2019 Whole-exome sequencing identifies a donor splice-site variant in SMPX that causes rare X-linked congenital deafness. Molecular genetics & genomic medicine 12 31478598
2018 A novel mutation in the SMPX gene associated with X-linked nonsyndromic sensorineural hearing loss in a Chinese family. Journal of human genetics 12 29559740
2018 Skewed X-chromosome inactivation and next-generation sequencing to identify a novel SMPX variants associated with X-linked hearing loss in a Chinese family. International journal of pediatric otorhinolaryngology 10 30174017
2017 A novel splicing mutation in SMPX is linked to nonsyndromic progressive hearing loss. International journal of pediatric otorhinolaryngology 10 29287879
2014 Overexpression of SMPX in adult skeletal muscle does not change skeletal muscle fiber type or size. PloS one 10 24936977
2020 Expression pattern of the small muscle protein, X-linked (smpx) gene during zebrafish embryonic and larval developmental stages. Gene expression patterns : GEP 8 32197943
2025 MUSTN1 Interaction With SMPX Regulates Muscle Development and Regeneration. Cell proliferation 7 39828423
2021 SMPX Deficiency Causes Stereocilia Degeneration and Progressive Hearing Loss in CBA/CaJ Mice. Frontiers in cell and developmental biology 7 34722533
2021 Inner Ear and Muscle Developmental Defects in Smpx-Deficient Zebrafish Embryos. International journal of molecular sciences 6 34204426
2014 Benefit of piezoosteotomy in cranioplasties for craniosynostosis correction versus conventional saw-and-chisel osteotomy: a pilot study. The Cleft palate-craniofacial journal : official publication of the American Cleft Palate-Craniofacial Association 6 24878345
2006 Cloning and identification of porcine SMPX differentially expressed in F1 crossbreds and their parents. Acta biochimica et biophysica Sinica 6 17091191
2023 Comparison of Changes in Pulmonary Function After Stereotactic Body Radiation Therapy Versus Conventional 3-Dimensional Conformal Radiation Therapy for Stage I and IIA Non-Small Cell Lung Cancer: An Analysis of the TROG 09.02 (CHISEL) Phase 3 Trial. International journal of radiation oncology, biology, physics 5 37087060
2021 A novel missense mutation in SMPX causes a rare form of X-linked postlingual sensorineural hearing loss in a Chinese family. Translational pediatrics 4 33708524
2021 System for quantitative evaluation of DAB&H-stained breast cancer biopsy digital images (CHISEL). Scientific reports 4 33927266
2024 Differentiation and functioning of the lateral line organ in zebrafish require Smpx activity. Scientific reports 1 38570547
2024 A cost-effectiveness analysis of stereotactic ablative radiotherapy versus conventionally fractionated radiotherapy in the management of stage 1 non-small-cell lung cancer: Results from the TROG 09.02 CHISEL study. Journal of medical imaging and radiation oncology 1 39382099
2021 Derivation of iPSC line UMi029-A bearing a hearing-loss associated variant in the SMPX gene. Stem cell research 1 34052664
2025 [The pleiotropic role of X-linked SMPX gene mutations: Exploration of mechanism from deafness to myopathy]. Zhonghua yi xue yi chuan xue za zhi = Zhonghua yixue yichuanxue zazhi = Chinese journal of medical genetics 0 40947416

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