SPEM2

SPEM2 is a testis-enriched transmembrane protein essential for spermiogenesis and spermiation. It localizes to the perinuclear theca of spermatids, where it physically interacts with actin-related perinuclear theca proteins (ACTRT1, ACTRT2, ACTL7A) and sperm surface/acrosomal proteins (ZPBP, ADAM2, ADAM3, PRSS21, PRSS54, PRSS55), and is required for their processing and maturation (PMID:38421455, PMID:41668650). SPEM2 also engages planar cell polarity components VANGL2, PRICKLE3, and DVL3 through its transmembrane domain, linking it to the cytoplasmic remodeling that drives sperm individualization, excess cytoplasm shedding, acrosome formation, and residual body resolution during late spermiation (PMID:42028965). Loss of SPEM2 in mice causes oligoasthenoteratozoospermia and male infertility, with defective sperm unable to migrate to the oviduct or fertilize oocytes even by IVF (PMID:38421455, PMID:42028965).

1. 2024 High

   Establishing that SPEM2 is required for multiple steps of spermiogenesis resolved its essential role in male fertility: its loss causes infertility through combined defects in sperm individualization, cytoplasm shedding, acrosome formation, and sperm-egg interaction.

   Evidence Spem2-knockout mouse model with phenotypic, histological, in vivo fertilization, and IVF analyses

   PMID:38421455

   Open questions at the time

   • How SPEM2 mechanistically coordinates these distinct spermiogenic processes is unresolved
   • Whether SPEM2 functions cell-autonomously in spermatids versus Sertoli cells is not determined
   • The IVF failure suggests a sperm-egg recognition defect but the specific molecular step is unknown
2. 2024 Medium

   Identifying SPEM2 as a physical interaction partner of sperm surface and acrosomal proteins (ZPBP, ADAM2, ADAM3, PRSS21, PRSS54, PRSS55) and showing their defective processing in KO sperm linked SPEM2 to sperm surface protein maturation.

   Evidence Co-immunoprecipitation and protein maturation analysis in wild-type versus Spem2-null epididymal sperm

   PMID:38421455

   Open questions at the time

   • Reciprocal pull-downs and independent replication are lacking for several partners
   • Whether SPEM2 acts as a chaperone, scaffold, or enzymatic regulator in processing is unknown
   • Stoichiometry and temporal order of these interactions during spermiogenesis are not defined
3. 2026 High

   Demonstrating that SPEM2 deficiency specifically disrupts residual body formation during late spermiation pinpointed its role at the cytoplasmic elimination step, explaining the oligoasthenoteratozoospermia phenotype.

   Evidence Histological and morphological analysis of spermiation in Spem2-deficient mice

   PMID:42028965

   Open questions at the time

   • The signaling pathway downstream of SPEM2 that drives residual body resolution is not mapped
   • Whether the spermiation defect is secondary to the polarity defect or an independent function is unclear
4. 2026 Medium

   Identifying SPEM2 interactions with planar cell polarity factors VANGL2, PRICKLE3, and DVL3, with the transmembrane domain mediating VANGL2 binding, connected SPEM2-dependent spermiation to PCP-pathway-mediated cytoplasmic remodeling.

   Evidence Co-immunoprecipitation and in vitro domain-mapping experiments

   PMID:42028965

   Open questions at the time

   • Single-lab co-IP without independent validation; reciprocal and in vivo proximity assays needed
   • Whether PCP pathway disruption phenocopies Spem2 loss has not been tested in this context
   • The downstream effectors of PCP signaling in spermatid cytoplasmic remodeling are not identified
5. 2026 Medium

   Detecting SPEM2 as a component of the perinuclear theca scaffold interacting with ACTRT1, ACTRT2, ACTL7A, and ZPBP placed it within a structural framework linking the nucleus to the acrosome in spermatids.

   Evidence Co-immunoprecipitation in an ACTRT3-focused study identifying SPEM2 among PT-associated proteins

   PMID:41668650

   Open questions at the time

   • Single co-IP study; SPEM2's direct versus bridged association with each PT protein is not resolved
   • Whether SPEM2 is required for perinuclear theca assembly or only recruited to a pre-formed structure is unknown
   • Structural basis of SPEM2 integration into the PT lattice is not defined

• Key unresolved questions include: the precise enzymatic or scaffolding mechanism by which SPEM2 promotes sperm surface protein maturation; how SPEM2 integrates PCP signaling with perinuclear theca assembly; whether SPEM2 mutations cause human male infertility; and whether the transmembrane domain is sufficient for all SPEM2 functions.
• No human genetic studies linking SPEM2 variants to infertility
• No structural or biochemical reconstitution of SPEM2 function
• Relative contributions of PCP interaction versus PT scaffolding to the infertility phenotype are unseparated