ISM1

ISM1 is a secreted immunomodulatory and developmental signaling protein that acts through multiple cell-surface receptors to control apoptosis, inflammation, and tissue morphogenesis. In the lung, ISM1 selectively induces apoptosis of alveolar macrophages expressing high levels of cell-surface GRP78 (csGRP78), thereby maintaining immune homeostasis and protecting against emphysema, while simultaneously suppressing NF-κB–driven pro-inflammatory cytokine production to limit acute lung injury and fibrosis (PMID:35046017, PMID:35752760). During embryonic development, ISM1 functions as an extracellular NODAL antagonist by binding NODAL and its type I receptor ACVR1B through the AMOP domain to block SMAD2 phosphorylation and regulate left-right asymmetry, and separately engages integrin α8β1 to promote cell-cell adhesion and sustain GDNF/RET signaling required for kidney morphogenesis (PMID:31171630, PMID:37185772). ISM1 also triggers apoptosis of podocytes via GRP78 and integrin αvβ5 through both caspase-dependent and AIF-mediated caspase-independent pathways, and modulates adipocyte progenitor differentiation and macrophage inflammatory responses in adipose tissue (PMID:36769045, PMID:40051329).

1. 2018 Medium

   Establishing that ISM1 has a conserved role in blood cell development: zebrafish ism1 morphant knockdown revealed a requirement for ISM1 in generating hematopoietic stem and progenitor cells and their myeloid/erythroid progeny, providing the first loss-of-function evidence for ISM1 in a developmental hematopoietic context.

   Evidence Morpholino knockdown in zebrafish with transgenic lineage-specific reporters and clonal HSPC assays

   PMID:29758043

   Open questions at the time

   • Mammalian hematopoietic phenotype not examined
   • Receptor and downstream signaling pathway mediating HSPC effects unknown
   • No genetic mutant confirmation of morpholino specificity
2. 2019 High

   Defining ISM1 as a specific extracellular NODAL antagonist resolved how ISM1 participates in embryonic patterning: ISM1 binds NODAL and ACVR1B through its AMOP domain, competitively blocking NODAL-induced SMAD2 phosphorylation without affecting TGF-β1, Activin-A, or BMP4 signaling, and ectopic ISM1 disrupts left-right asymmetry in chick embryos.

   Evidence Co-immunoprecipitation, SMAD2 phosphorylation assays, AMOP domain mutagenesis, and ectopic expression in chick embryos

   PMID:31171630

   Open questions at the time

   • Endogenous loss-of-function for left-right asymmetry not shown in mammals
   • Structural basis of AMOP–NODAL interaction unresolved
   • Whether ISM1 antagonism of NODAL operates in adult tissues is unknown
3. 2022 High

   Identifying csGRP78 as the receptor through which ISM1 selectively eliminates pro-inflammatory alveolar macrophages established the molecular basis for ISM1's lung-protective function: Ism1-knockout mice showed AM expansion and spontaneous emphysema, and recombinant ISM1 rescued this by depleting csGRP78-high AMs.

   Evidence Ism1−/− mouse model, intratracheal recombinant ISM1, flow cytometry, lung function and histology in both genetic and cigarette-smoke COPD models

   PMID:35046017

   Open questions at the time

   • Precise signaling cascade downstream of csGRP78 leading to AM apoptosis not fully dissected
   • Whether csGRP78 acts alone or requires a co-receptor on AMs unknown
4. 2022 High

   Demonstrating ISM1's anti-inflammatory mechanism via NF-κB suppression extended its role beyond apoptosis to cytokine regulation: ISM1 deficiency exacerbated LPS-induced neutrophil/macrophage infiltration, TGF-β upregulation, and post-injury lung fibrosis, all rescued by recombinant ISM1.

   Evidence Ism1−/− mice with intratracheal LPS, Western blot for NF-κB, recombinant ISM1 rescue in cultured macrophages, flow cytometry, IHC

   PMID:35752760

   Open questions at the time

   • Direct molecular target through which ISM1 inhibits NF-κB not identified
   • Whether NF-κB suppression and AM apoptosis are mechanistically coupled or independent pathways is unresolved
5. 2023 High

   Identifying integrin α8β1 as an ISM1 receptor in the developing kidney and linking it to GDNF/RET signaling explained the renal agenesis phenotype of Ism1-knockout mice, establishing ISM1 as a cell-adhesion factor critical for ureteric bud branching and mesenchyme condensation.

   Evidence HRP-induced proximity labelling, single-cell RNA-seq, Ism1−/− mouse embryos, functional adhesion assay

   PMID:37185772

   Open questions at the time

   • Binding site on ISM1 for integrin α8β1 not mapped
   • Whether ISM1-integrin α8β1 interaction is relevant outside kidney development is unknown
6. 2023 Medium

   Demonstrating dual apoptotic pathways (caspase-dependent and AIF-mediated caspase-independent) in podocytes exposed to ISM1 via GRP78 and integrin αvβ5 extended the cell-type repertoire of ISM1-induced death beyond alveolar macrophages.

   Evidence Confocal co-localization, recombinant ISM1 treatment of cultured human podocytes, caspase activation and mitochondrial membrane potential assays, AIF nuclear translocation

   PMID:36769045

   Open questions at the time

   • Genetic confirmation of GRP78 or integrin αvβ5 requirement in podocyte apoptosis not performed
   • In vivo podocyte-specific relevance not demonstrated
   • Whether dose-dependent switch between caspase-dependent and AIF pathways occurs in vivo is unknown
7. 2025 Medium

   Revealing ISM1's role in adipose tissue — impairing preadipocyte differentiation, promoting myofibroblast-like conversion, and amplifying macrophage inflammatory responses — broadened ISM1 function to metabolic tissue remodeling.

   Evidence Stable overexpression in 3T3-F442A preadipocytes, differentiation and cytokine assays, AAV-mediated WAT-specific overexpression in mice on HFD

   PMID:40051329

   Open questions at the time

   • Receptor mediating ISM1 effects in adipocyte progenitors not identified
   • Loss-of-function adipose phenotype not reported
   • Whether adipose ISM1 acts in an autocrine or paracrine manner is unresolved

• A unified structural and signaling model explaining how ISM1 engages its multiple receptors (GRP78, integrin αvβ5, integrin α8β1, ACVR1B/NODAL) in different tissues and the molecular determinants that dictate apoptotic versus adhesive versus anti-inflammatory outcomes remains unestablished.
• No crystal or cryo-EM structure of ISM1 or ISM1–receptor complexes
• Relative contributions of distinct receptor engagements to each biological outcome not genetically resolved
• Physiological concentrations and regulation of circulating versus local ISM1 poorly characterized