C1QL4

C1QL4 (CTRP11/C1QTNF11) is a secreted C1q-family protein that assembles into disulfide-linked homo- and heteromeric oligomers and acts as a signaling ligand across multiple tissues (PMID:20525073, PMID:23449976). It binds with high affinity to the extracellular thrombospondin-repeat domain of the adhesion GPCR BAI3 through its globular C1q domain, and this interaction underlies its repressive activities on synapse density in neurons (PMID:21262840) and on BAI3-mediated myoblast fusion (PMID:30367035). Beyond BAI3 antagonism, the recombinant globular domain drives c-Raf/MEK1/2/ERK1/2 (p90RSK/MSK1) signaling to stimulate endothelial migration and capillary tube formation (PMID:27734226) and to upregulate StAR and steroidogenic enzymes for testosterone production in Leydig cells, where BAI3 mediates only part of the response, indicating an additional unidentified receptor (PMID:30608882). In adipocyte differentiation C1QL4 is inhibitory, suppressing PPARγ and C/EBPα, dampening p42/44-MAPK signaling, and blocking mitotic clonal expansion (PMID:23449976). Despite these activities defined with recombinant protein, knockout mice have normal muscle and testosterone but show sexually dimorphic exacerbation of obesity-induced glucose intolerance, defining a physiological role in metabolic homeostasis (PMID:35579659). Heteromeric partners include CTRP10, CTRP13, and CRF/CTRP14, formed via the C-terminal globular domains, while N-terminal Cys-28/Cys-32 modulate secretion (PMID:23449976).

1. 2010 Medium

   Establishing that C1QL4 is a secreted oligomeric protein answered the basic question of its biochemical form, framing it as a multimeric extracellular factor rather than a cell-intrinsic protein.

   Evidence Secretion and oligomerization assays in heterologous expression cells

   PMID:20525073

   Open questions at the time

   • Did not assign a receptor or downstream function
   • Single lab characterization
2. 2011 High

   Identifying BAI3 as a high-affinity receptor bound via the C1q globular domain converted C1QL4 from an orphan secreted protein into a defined ligand with a synaptic function.

   Evidence Domain-mapped binding/pulldown assays and neuronal culture with competitor thrombospondin-repeat fragment

   PMID:21262840

   Open questions at the time

   • Downstream BAI3 signaling not resolved in this study
   • Specificity among C1QL family members for distinct neuronal contexts unclear
3. 2013 High

   Mapping the determinants of oligomerization/secretion and demonstrating anti-adipogenic activity extended C1QL4 function beyond the nervous system into metabolic regulation.

   Evidence Cys mutagenesis, co-IP for heteromeric complexes, and 3T3-L1 differentiation assays with signaling readouts

   PMID:23449976

   Open questions at the time

   • Receptor mediating anti-adipogenic effect not identified
   • Effects shown in cell line, not in vivo
4. 2016 Medium

   Showing the globular domain activates Raf/MEK/ERK to drive angiogenesis established a pro-angiogenic, kinase-coupled signaling output for C1QL4.

   Evidence Recombinant protein HUVEC tube/migration assays, kinase phospho-Western blots, U0126 inhibition, chick yolk sac assay

   PMID:27734226

   Open questions at the time

   • BAI3 involvement inferred only from immunoreactivity, not functional testing
   • Single lab
5. 2019 Medium

   Demonstrating C1QL4-stimulated steroidogenesis through Raf/MEK/ERK and cAMP/PKA/CREB, only partly BAI3-dependent, revealed an endocrine role and implied a second unidentified receptor.

   Evidence Recombinant C1QL4 on TM3 Leydig cells, testosterone ELISA, signaling Westerns, Bai3 siRNA knockdown

   PMID:30608882

   Open questions at the time

   • Identity of the BAI3-independent receptor unknown
   • In vitro Leydig cell system only
6. 2018 High

   Genetic dissection of the BAI3 axis placed C1QL4 as a repressor of BAI3-driven myoblast fusion, opposite to the Stabilin-2 agonist arm that activates G-protein/Elmo signaling.

   Evidence BAI3 knockout mice with cardiotoxin injury, proteomic interactor ID, GPCR activity assays, co-IP

   PMID:30367035

   Open questions at the time

   • C1QL4-specific (versus pan-C1QL) contribution to muscle phenotype not isolated
   • Mechanism of repression on BAI3 signaling not fully defined
7. 2022 Medium

   Loss-of-function in mice tested whether the in vitro muscle/steroidogenic roles are required in vivo, showing dispensability there but a genuine metabolic role.

   Evidence Ctrp11 knockout mice with metabolic phenotyping (GTT, calorimetry, fasting-refeeding)

   PMID:35579659

   Open questions at the time

   • Molecular basis of the metabolic and sexually dimorphic phenotype unresolved
   • Receptor mediating metabolic effect not identified
8. 2023 Low

   Cancer cell experiments proposed a pro-tumorigenic role via PI3K/AKT/NF-κB, broadening the signaling repertoire to inflammatory/stemness programs.

   Evidence C1QL4 knockdown/overexpression in breast cancer lines, flow cytometry, migration/invasion assays, xenografts, pathway inhibition

   PMID:37324011

   Open questions at the time

   • No receptor or complex identified; pathway placement rests on pharmacological inhibition only
   • Single lab, not independently confirmed

• The identity of the BAI3-independent receptor(s) and how a single secreted ligand selects among synaptic, metabolic, steroidogenic, and angiogenic outputs in different tissues remains unresolved.
• No structural model of the C1QL4-receptor complex
• Tissue-specific receptor repertoire unknown
• Mechanism linking oligomer state to signaling output undefined