C1QL2

C1QL2 (CTRP10) is a secreted C1q-family glycoprotein that acts as a transsynaptic organizer in the brain and as a sex-biased peripheral regulator of body weight and motor function (PMID:27133466, PMID:40126547). The protein assembles into trimers as its basic structural unit, with each protomer adopting a jelly-roll globular C1q fold; trimers coordinate multiple Ca2+ ions along their symmetry axis and are further assembled into hexameric and higher-order oligomers through disulfide bonds formed by N-terminal cysteine residues (PMID:18783346, PMID:25752542). Beyond homo-oligomerization, C1QL2 forms heteromeric complexes with related CTRP proteins through distinct interfaces—via C-terminal globular domains in the case of CTRP11 and through a cysteine-independent interface with CTRP13 (PMID:21378161, PMID:23449976). At hippocampal mossy fiber–CA3 synapses, C1QL2 produced by mossy fibers bridges the synaptic cleft by binding presynaptic neurexin-3 containing exon 25b and the amino-terminal domains of postsynaptic kainate receptor subunits GluK2 and GluK4, thereby recruiting functional KAR complexes; loss of C1ql2/3 abolishes the slow KAR-mediated synaptic response (PMID:27133466). Its synaptic expression is a direct functional target of the transcription factor Bcl11b, and C1QL2 mediates Bcl11b-dependent synaptic vesicle recruitment and long-term potentiation through its interaction with Nrxn3(25b+) (PMID:38358390). C1QL2 also binds the adhesion-type GPCR BAI3 (PMID:25752542). Peripherally, C1QL2 negatively regulates female body weight, with Ctrp10 knockout females developing obesity uncoupled from metabolic dysfunction, and is required for optimal motor coordination, with its loss altering cerebellar and motor cortex synaptic and mitochondrial pathways and reducing motor cortex mitochondrial respiration (PMID:40126547, PMID:41933728).

1. 2008 Medium

   Established the basic structural architecture of the secreted protein, showing it builds from trimers into higher-order oligomers via N-terminal cysteine disulfide bonds.

   Evidence Heterologous mammalian cell expression with SDS-PAGE, size exclusion, and cysteine mutagenesis

   PMID:18783346

   Open questions at the time

   • Functional consequence of oligomerization not addressed
   • No physiological binding partners identified
2. 2010 Medium

   Confirmed secretion and extended the oligomerization model to include homomeric, heteromeric, and hexameric assemblies governed by N-terminal cysteines.

   Evidence Heterologous cell expression, biochemical fractionation, and co-immunoprecipitation

   PMID:20525073

   Open questions at the time

   • Heteromeric partners not yet defined in this work
   • In vivo relevance of complexes untested
3. 2011 Medium

   Identified CTRP13 as a heteromeric partner and showed this association uses an interface distinct from the conserved N-terminal cysteines, indicating multiple oligomerization modes.

   Evidence Co-expression in heterologous cells with co-immunoprecipitation and mutagenesis

   PMID:21378161

   Open questions at the time

   • Physiological significance of the heteromer unknown
   • Single-lab co-IP without reciprocal in vivo validation
4. 2013 Medium

   Mapped a second heteromeric partnership with CTRP11 to the C-terminal globular domains, defining domain-specific assembly rules for the family.

   Evidence Co-expression in heterologous cells with co-immunoprecipitation and domain mapping

   PMID:23449976

   Open questions at the time

   • Functional output of the CTRP11 heteromer not established
   • No endogenous tissue confirmation
5. 2015 High

   Resolved the atomic structure of the globular C1q domain and identified Ca2+-binding sites essential for protein stability, while also showing C1QL2 binds the adhesion-GPCR BAI3.

   Evidence X-ray crystallography, site-directed mutagenesis, biophysical stability assays, and in vitro binding

   PMID:25752542

   Open questions at the time

   • Downstream signaling consequence of BAI3 binding not defined
   • Role of Ca2+ sites in receptor engagement untested
6. 2016 High

   Defined C1QL2's core synaptic function as a transsynaptic organizer bridging presynaptic neurexin-3(25b+) and postsynaptic GluK2/GluK4 to recruit functional kainate receptors at mossy fiber–CA3 synapses.

   Evidence In vitro pulldown with recombinant proteins, C1ql2/3 double-knockout mice, and electrophysiology

   PMID:27133466

   Open questions at the time

   • Stoichiometry of the transsynaptic complex unresolved
   • Contribution of oligomeric state to bridging unaddressed
7. 2024 High

   Placed C1QL2 downstream of the transcription factor Bcl11b and demonstrated that its Nrxn3(25b+) interaction is required for synaptic vesicle recruitment and long-term potentiation, establishing a transcription-to-synapse axis.

   Evidence Conditional knockout and Nrxn3 deletion in mice, non-binding mutant expression, in vivo/in vitro electrophysiology, and interaction assays

   PMID:38358390

   Open questions at the time

   • Other Bcl11b targets contributing to phenotype not excluded
   • Mechanism coupling KAR recruitment to LTP not fully detailed
8. 2025 High

   Revealed a peripheral role: C1QL2 negatively regulates female body weight in a sexually dimorphic manner, uncoupling obesity from metabolic dysfunction.

   Evidence Constitutive knockout mice with comprehensive metabolic phenotyping and multi-tissue transcriptomics

   PMID:40126547

   Open questions at the time

   • Tissue source and receptor mediating the metabolic effect unknown
   • Mechanistic basis of sexual dimorphism unresolved
9. 2026 High

   Extended the loss-of-function phenotype to motor behavior, linking C1QL2 to cerebellar/motor cortex synaptic organization and mitochondrial respiration.

   Evidence Constitutive knockout mice, behavioral testing, transcriptomics, and mitochondrial respiration assays

   PMID:41933728

   Open questions at the time

   • Direct molecular link between C1QL2 and mitochondrial respiration not established
   • Whether motor deficit is cell-autonomous to specific neuron types unknown

• How C1QL2's biochemical features—oligomeric state, Ca2+ coordination, and receptor binding—are integrated to produce both its central synaptic organizing function and its peripheral metabolic/motor roles remains unresolved.
• No unified receptor map across CNS and peripheral tissues
• Mechanism of sex-biased peripheral action undefined
• Functional relevance of CTRP heteromers in vivo untested