CRISP2

CRISP2 is a testis-enriched, two-domain secretory protein that functions as a multivalent regulator of sperm assembly, ion channel activity, sterol homeostasis, and gamete fusion. Its C-terminal CRISP domain adopts an ion-channel-toxin-like fold that directly modulates ryanodine receptors (inhibiting RyR2, activating RyR1) and binds the CATSPER1 subunit to govern flagellar waveform and Ca²⁺-dependent hyperactivation, while Crisp2-knockout mice exhibit defective sperm hyperactivation, impaired cumulus/zona penetration, and subfertility (PMID:16339766, PMID:30759213, PMID:26786179). The N-terminal CAP domain mediates spermatogenic cell–Sertoli cell adhesion, Mg²⁺-dependent binding to A1BG, and sterol-binding/export activity that is inhibited by PSP94 in a Ca²⁺-sensitive manner (PMID:10646801, PMID:35063506, PMID:39433128). During spermiogenesis CRISP2 is incorporated into distinct compartments—the acrosome (where it forms stable complexes with acrosin and acrosin-binding protein) and the sperm tail outer dense fibers—and after capacitation it redistributes to the equatorial segment to participate in sperm–egg fusion by engaging complementary sites on the egg surface (PMID:15734896, PMID:17202389, PMID:36815564, PMID:34309660).

1. 1998 Medium

   Establishing that CRISP2 is not merely a structural sperm protein but functions as a cell-adhesion molecule mediating spermatogenic cell–Sertoli cell interactions during spermatogenesis.

   Evidence Antibody inhibition of Tpx-1 blocked spermatogenic cell–Sertoli cell binding in primary rat testicular cell culture

   PMID:9675100

   Open questions at the time

   • Identity of the Sertoli cell surface receptor is unknown
   • Adhesion function not confirmed in vivo or by genetic loss-of-function
2. 1999 Medium

   Mapping the adhesion function to the N-terminal 101 residues (CAP domain) and showing the C-terminal cysteine-rich region is dispensable for adhesion, establishing a modular architecture with separable functions.

   Evidence Deletion mutant expression and secretion assays in cultured cells coupled with primary testicular cell adhesion assay

   PMID:10646801

   Open questions at the time

   • Ligand on Sertoli cell surface remains unidentified
   • No in vivo validation of domain requirements
3. 2001 High

   Demonstrating that CRISP2 localizes to two distinct sperm compartments—acrosome and outer dense fibers—indicating dual structural/functional roles in head and tail.

   Evidence Immunohistochemistry and immunoelectron microscopy on rat spermatids with fractionation biochemistry

   PMID:11144214

   Open questions at the time

   • Functional significance of tail versus head pools not yet separated
   • Mechanism of targeting to ODFs unknown
4. 2005 High

   Revealing the molecular mechanism by which CRISP2 regulates ion channels: the CRISP domain adopts a toxin-like fold and directly modulates ryanodine receptors, providing the first biochemical activity for the protein.

   Evidence NMR solution structure of the isolated CRISP domain; planar lipid bilayer electrophysiology showing inhibition of RyR2 and activation of RyR1

   PMID:16339766

   Open questions at the time

   • Whether ryanodine receptors are the physiological targets in sperm is not established
   • In vivo relevance of differential RyR1/RyR2 modulation unclear
5. 2005 High

   Establishing a role for CRISP2 in sperm–egg fusion: the protein persists on the equatorial segment after acrosome reaction and antibody blockade inhibits oocyte penetration without affecting motility.

   Evidence Immunofluorescence on human sperm at defined capacitation stages; hamster oocyte penetration assay with anti-TPX1 antibody inhibition

   PMID:15734896

   Open questions at the time

   • Egg-surface receptor identity unknown
   • Heterologous (hamster egg) assay may not fully recapitulate human fusion
6. 2007 High

   Demonstrating that CRISP2 binds the fusogenic area of the egg surface and competes with CRISP1 for shared binding sites, placing both CRISPs in a common fusion pathway.

   Evidence Recombinant mouse CRISP2 egg-binding and competition assays; IVF antibody-inhibition with zona-intact eggs

   PMID:17202389

   Open questions at the time

   • Molecular identity of egg-surface binding partner remains unknown
   • Redundancy with CRISP1 not fully resolved
7. 2008 Medium

   Identifying GGN1 as a CRISP2-interacting partner in the sperm tail principal piece, with interaction mapped to the ion-channel-regulatory region of the CRISP domain, and showing the C196R SNP abolishes this interaction.

   Evidence Yeast two-hybrid screen, co-immunoprecipitation, co-localization; point-mutant Y2H assay

   PMID:18502891 PMID:18550510

   Open questions at the time

   • Functional consequence of CRISP2–GGN1 interaction on sperm motility not tested
   • C196R variant not assessed in a loss-of-function animal model
8. 2009 Medium

   Identifying SHTAP as a second sperm tail/head partner that binds both CRISP2 domains and redistributes during capacitation, expanding the CRISP2 interactome.

   Evidence Yeast two-hybrid with domain deletions; immunofluorescence co-localization on sperm

   PMID:19686095

   Open questions at the time

   • No co-IP confirmation of SHTAP–CRISP2 interaction
   • Functional role of this complex is undefined
9. 2013 Medium

   Showing that CRISP2 pre-mRNA splicing is regulated by DAZAP1, which promotes exon 9 inclusion by binding intron 9, linking CRISP2 isoform diversity to a known RNA-binding protein.

   Evidence Microarray exon-usage profiling in Dazap1 knockout testes; splicing minigene reporter assay

   PMID:23965306

   Open questions at the time

   • Functional difference between exon 9-included and exon 9-skipped CRISP2 isoforms not characterized
   • Whether altered splicing affects fertility outcomes is untested
10. 2014 Medium

    Establishing post-transcriptional repression of CRISP2 by miR-27b, with clinical relevance to asthenozoospermia where elevated miR-27b correlates with reduced CRISP2 protein.

    Evidence Luciferase 3′ UTR reporter assay; miR-27b mimic/inhibitor transfection; Western blot; clinical correlation in asthenozoospermic samples

    PMID:25505194

    Open questions at the time

    • Causal relationship between miR-27b-mediated CRISP2 reduction and motility defects not proven in vivo
    • Other miRNA regulators not surveyed
11. 2016 High

    Genetic loss-of-function definitively established CRISP2 as a regulator of Ca²⁺-dependent sperm hyperactivation, zona/cumulus penetration, and egg fusion, with knockout mice showing subfertility under competitive conditions.

    Evidence Crisp2 knockout mouse with CASA, flow cytometry Ca²⁺ measurement, IVF, acrosome reaction assay

    PMID:26786179

    Open questions at the time

    • Molecular targets downstream of CRISP2 that mediate Ca²⁺ regulation not identified in this study
    • Redundancy with CRISP1/CRISP3/CRISP4 not fully dissected
12. 2019 High

    Identifying CATSPER1 as a direct binding partner of CRISP2, connecting its ion-channel-regulatory activity to the principal Ca²⁺ channel controlling sperm motility and explaining the stiff-midpiece/abnormal waveform phenotype of Crisp2-deficient sperm.

    Evidence Yeast two-hybrid screen; co-immunoprecipitation; flagellum waveform analysis in Crisp2 loss-of-function sperm

    PMID:30759213

    Open questions at the time

    • Whether CRISP2 activates, inhibits, or allosterically modulates CatSper channel conductance is unknown
    • Structural basis of CRISP2–CATSPER1 interaction not resolved
13. 2021 Medium

    Demonstrating that CRISP2 forms structurally distinct oligomeric assemblies in the sperm tail versus head, with different biochemical stabilities, revealing compartment-specific quaternary organization.

    Evidence Native/non-reducing gel electrophoresis; immunogold EM; sequential extraction of porcine sperm

    PMID:34309660

    Open questions at the time

    • Stoichiometry and exact composition of oligomers not determined
    • Functional significance of distinct oligomeric states untested
14. 2022 High

    Uncovering a sterol-binding/export function for CRISP2 that is inhibited by PSP94 in a Ca²⁺-sensitive manner, revealing a lipid-handling role and a physiological regulatory switch tied to the ionic milieu of the reproductive tract.

    Evidence Yeast sterol-export assay; in vitro sterol-binding assay; mutagenesis of PSP94 interaction interface; Ca²⁺-dependent complex dissociation

    PMID:35063506

    Open questions at the time

    • Whether sterol binding is relevant in mammalian sperm membranes in vivo is untested
    • Identity of physiological sterol cargo unclear
15. 2022 Medium

    Tracking CRISP2 redistribution through capacitation, acrosome reaction, and fertilization showed rapid post-fertilization dispersal from the perinuclear theca, likely driven by disulfide reduction, linking CRISP2 complex disassembly to early zygote remodeling.

    Evidence Confocal immunofluorescence on porcine sperm at defined stages; IVF followed by CRISP2 immunostaining of zygotes

    PMID:36054334

    Open questions at the time

    • Reductase responsible for disulfide reduction not identified
    • Functional consequence of perinuclear theca dispersal for paternal chromatin remodeling unknown
16. 2023 High

    Identifying acrosin and acrosin-binding protein as the primary stable partners of CRISP2 in the acrosomal ~150 kDa complex under non-capacitating, capacitated, and acrosome-reacted conditions, defining the core acrosomal CRISP2 interactome.

    Evidence Blue-native gel; co-IP plus LC-MS/MS; proximity ligation assay on boar sperm

    PMID:36815564

    Open questions at the time

    • Functional role of the CRISP2–acrosin–ACRBP complex in acrosome reaction or zona penetration untested
    • Whether this complex exists in human sperm not confirmed
17. 2024 High

    Demonstrating that A1BG inhibits CRISP2 sterol function through Mg²⁺-dependent binding to conserved CAP domain residues, establishing a second serum protein (after PSP94) that regulates CRISP2 lipid-handling activity via divalent cation-dependent interactions.

    Evidence Yeast sterol-export assay; in vitro sterol-binding assay; A1BG domain-deletion and Mg²⁺-coordination mutagenesis

    PMID:39433128

    Open questions at the time

    • Physiological context where A1BG and CRISP2 interact (seminal plasma vs. serum) not established
    • Whether A1BG regulation is relevant to sperm function in vivo is unknown
18. 2025 Medium

    Mapping the dynamic localization of human CRISP2 from nuclear in spermatocytes through cytoplasmic/flagellar in elongated spermatids to equatorial/flagellar in ejaculated sperm, and showing that human CRISP2 forms high-MW homo-oligomeric complexes with minimal post-translational modification.

    Evidence Immunofluorescence across human spermatogenesis; native gel electrophoresis; mass spectrometry of purified complexes

    PMID:40079119

    Open questions at the time

    • Function of nuclear CRISP2 in spermatocytes is unknown
    • Whether homo-oligomers are functionally equivalent to the porcine heteromeric complexes containing acrosin is unclear

• The egg-surface receptor for CRISP2, the structural basis of its CATSPER1 modulation, the in vivo relevance of its sterol-binding activity in mammalian sperm, and the function of nuclear CRISP2 in spermatocytes remain unresolved.
• Egg-surface receptor identity unknown
• No structure of CRISP2–CATSPER1 complex
• In vivo sterol-handling function not demonstrated in mammalian sperm
• Role of nuclear CRISP2 in meiotic spermatocytes undefined