SPEM2

SPEM2 is a testis-enriched transmembrane protein essential for late-stage spermiogenesis and spermiation, where its loss abolishes male fertility (PMID:38421455). Spem2-null mice exhibit insufficient sperm individualization, failure to shed excess cytoplasm, defective acrosome formation, reduced sperm count and motility, and an inability to fertilize oocytes both in vivo and in vitro (PMID:38421455), with the underlying defect localized to disrupted residual body formation and incomplete cytoplasmic removal during late spermiation, while earlier spermatogenic stages and elongated spermatid generation proceed normally (PMID:42028965). Mechanistically, SPEM2 acts through its transmembrane domain, which it uses to bind the planar cell polarity regulators VANGL2, PRICKLE3, and DVL3, implicating SPEM2 in PCP-mediated control of spermiation (PMID:42028965). In parallel, SPEM2 physically interacts with the sperm surface and protease proteins ZPBP, PRSS21, PRSS54, PRSS55, ADAM2, and ADAM3 and is required for their proper processing and maturation in epididymal sperm, linking SPEM2 to the maturation of factors needed for sperm-egg interaction (PMID:38421455).

1. 2024 High

   Establishing whether SPEM2 has a non-redundant role in male reproduction, the knockout defined it as essential for spermiogenesis and fertilization rather than a dispensable testis protein.

   Evidence Spem2 knockout mouse with sperm morphology/motility analysis and in vivo and in vitro fertilization assays

   PMID:38421455

   Open questions at the time

   • Does not resolve which molecular defect (cytoplasm shedding vs. surface protein maturation) is the primary cause of infertility
   • No structural or biochemical characterization of SPEM2 itself
2. 2024 Medium

   To identify SPEM2's biochemical role, it was shown to bind and be required for maturation of a defined set of sperm surface and protease proteins, connecting SPEM2 loss to a molecular processing defect.

   Evidence Co-immunoprecipitation and protein maturation assays in Spem2-null epididymal sperm

   PMID:38421455

   Open questions at the time

   • Whether SPEM2 acts directly on these substrates or indirectly via an upstream processing event is unresolved
   • Single lab; reciprocal validation of each interaction not detailed
   • No enzymatic activity assigned to SPEM2
3. 2026 Medium

   Pinpointing the developmental window of SPEM2 action, stage-specific analysis localized the defect to residual body formation and cytoplasmic removal during late spermiation, sparing earlier spermatogenesis.

   Evidence Histological and morphological analysis of spermiation stages in Spem2 knockout mice

   PMID:42028965

   Open questions at the time

   • Mechanism linking SPEM2 to residual body cytoplasm shedding not established
   • Single lab, single study
4. 2026 Medium

   Addressing how SPEM2 controls spermiation at the molecular level, it was found to bind PCP regulators via its transmembrane domain, implicating planar cell polarity signaling and identifying the functional domain.

   Evidence Co-immunoprecipitation with VANGL2/PRICKLE3/DVL3 and domain deletion/mutagenesis mapping

   PMID:42028965

   Open questions at the time

   • Functional requirement of the SPEM2-VANGL2 interaction for spermiation not tested by separation-of-function mutant in vivo
   • How PCP signaling connects to cytoplasm removal not defined
5. 2026 Low

   Probing SPEM2's structural context in spermatids, a Co-IP placed it among perinuclear theca interactors alongside ACTRT1/2, ACTL7A, and ZPBP.

   Evidence Co-immunoprecipitation using ACTRT3 as bait identifying SPEM2 as an interactor

   PMID:41668650

   Open questions at the time

   • Single Co-IP from an ACTRT3-focused study with no SPEM2-specific functional validation
   • Whether SPEM2 is a stable PT subunit or a transient interactor is unclear

• Whether SPEM2 possesses intrinsic enzymatic activity or acts purely as a scaffold coordinating PCP signaling and surface protein maturation remains unresolved.
• No molecular activity assigned to SPEM2
• Causal relationship between PCP-pathway binding and substrate maturation defects not established
• No human disease or patient-level evidence in the corpus