C1QL3

C1QL3 is a secreted, oligomeric gC1q-domain protein that functions as a trans-synaptic organizer in the brain and as a metabolic regulator (adipokine CTRP13) in peripheral tissues. In excitatory neurons, C1QL3 binds the adhesion GPCR BAI3/ADGRB3 and neuronal pentraxins NPTX1/NPTXR to form trans-synaptic adhesion complexes that maintain excitatory synapse density in the amygdala-prefrontal cortex circuit, suprachiasmatic nucleus, and hippocampal mossy fibers, with knockout animals exhibiting reduced synapse numbers, impaired fear memory, disrupted circadian consolidation, hyperactivity, and working memory deficits (PMID:27478018, PMID:28553739, PMID:38379452, PMID:33337553). Peripherally, CTRP13 activates CaMKKβ/AMPK signaling to promote glucose uptake and suppress hepatic glucose output, and loss-of-function studies in knockout mice reveal it acts as a negative metabolic regulator whose absence paradoxically improves glucose tolerance, insulin sensitivity, and triglyceride clearance (PMID:21378161, PMID:37844630). CTRP13 also exerts vascular protective effects by upregulating GCH1/BH4 to preserve eNOS coupling and inhibit endothelial ferroptosis, by promoting autophagy-lysosomal degradation of CD36 to reduce foam cell formation, and by stabilizing NAMPT1 protein to attenuate aortic aneurysm formation (PMID:31676569, PMID:39541845, PMID:30222079, PMID:32966772).

1. 2011 High

   Identification of CTRP13 as a secreted oligomeric adipokine that activates AMPK signaling established C1QL3 as a metabolic hormone capable of promoting glucose uptake and suppressing lipid-induced stress signaling across multiple cell types.

   Evidence Recombinant protein functional assays measuring glucose uptake and AMPK/JNK signaling in adipocytes, myotubes, and hepatocytes

   PMID:21378161

   Open questions at the time

   • Receptor mediating metabolic signaling unidentified
   • No in vivo metabolic phenotyping of loss-of-function models
   • Oligomeric state of native endogenous protein not determined
2. 2013 Medium

   Central administration of CTRP13 demonstrated that C1QL3 acts in the hypothalamus to suppress food intake and reciprocally regulates AgRP, establishing a central anorexigenic role beyond its peripheral metabolic effects.

   Evidence Intracerebroventricular delivery of recombinant protein in mice with qPCR of hypothalamic neuropeptide expression

   PMID:23638159

   Open questions at the time

   • Hypothalamic receptor for CTRP13 not identified
   • No genetic loss-of-function confirmation of feeding phenotype
   • Single lab finding
3. 2016 High

   Discovery that C1QL3 binds BAI3/ADGRB3 and is required for excitatory synapse maintenance in the BLA-PFC circuit transformed understanding of C1QL3 from a purely metabolic factor to a trans-synaptic organizer controlling synapse number and fear memory.

   Evidence Constitutive and conditional C1QL3 knockout mice, electrophysiology, synapse counting, circuit tracing, and behavioral assays

   PMID:27478018

   Open questions at the time

   • Mechanism by which C1QL3–BAI3 interaction promotes synapse formation unknown
   • Whether C1QL3 signals through BAI3 GPCR activity or purely as adhesion molecule unresolved
4. 2017 Medium

   Extension of the synaptic role to the suprachiasmatic nucleus demonstrated that C1QL3's synaptogenic function is not circuit-specific and that synapse loss in the SCN has functional consequences for circadian behavioral consolidation.

   Evidence C1QL3 knockout mice with SCN synapse quantification and circadian phase-shift behavioral assays

   PMID:28553739

   Open questions at the time

   • Molecular mechanism of C1QL3-dependent synapse maintenance in SCN not dissected
   • Single lab replication
5. 2018 Medium

   Identification of autophagy-lysosomal degradation of CD36 as a downstream effector of CTRP13 provided the first specific vascular protective mechanism, explaining how CTRP13 reduces foam cell formation and atherosclerosis.

   Evidence ApoE−/− mice with CTRP13 infusion, peritoneal macrophage assays, pharmacological autophagy blockade

   PMID:30222079

   Open questions at the time

   • Direct receptor or upstream signal linking CTRP13 to autophagy pathway not identified
   • Single lab
6. 2019 Medium

   Three studies collectively delineated CTRP13's vascular protective mechanisms: preservation of eNOS coupling via PKA-PPARα-GCH1/BH4 transcriptional axis, attenuation of vascular calcification via TTP-mediated Runx2 mRNA destabilization, and regulation of cerebrovascular integrity downstream of miR-124.

   Evidence Diabetic mouse aortic relaxation and HUVEC ChIP assays (GCH1); CRF rat VSMC calcification with TTP binding and Runx2 overexpression rescue; APP/PS1 mice with lentiviral miR-124 overexpression and BBB assays

   PMID:31145871 PMID:31499089 PMID:31676569

   Open questions at the time

   • Whether these vascular pathways converge on a common receptor unknown
   • TTP regulation mechanism (which kinase/phosphatase) not fully resolved
   • miR-124–C1QL3 link lacks direct target validation by reporter assay
7. 2020 Medium

   Discovery that CTRP13 stabilizes NAMPT1 by preventing its ubiquitination and that CTRP13 signals through CaMKKβ/AMPK in liver sinusoidal endothelial cells expanded the catalogue of downstream effectors and confirmed AMPK as a conserved signaling node across tissues.

   Evidence ApoE−/− AAA mouse models with NAMPT1 siRNA epistasis and ubiquitination assays; rLSEC overexpression with CaMKKβ/AMPK pharmacological inhibition

   PMID:32554851 PMID:32966772

   Open questions at the time

   • Direct CTRP13 receptor on vascular SMCs and LSECs unidentified
   • Whether NAMPT1 stabilization is AMPK-dependent not tested
8. 2021 Medium

   Identification of NPTX1 and NPTXR as C1QL3 binding partners that co-assemble into a trans-synaptic adhesion complex with BAI3 provided a molecular framework for how C1QL3 bridges pre- and post-synaptic compartments at excitatory synapses.

   Evidence In vivo co-immunoprecipitation/pulldown, cell–cell adhesion assays, single-cell RNA-seq co-expression analysis

   PMID:33337553

   Open questions at the time

   • Stoichiometry and structure of the C1QL3–NPTX–BAI3 complex not determined
   • Functional necessity of NPTX1/NPTXR for C1QL3 synaptogenic activity not tested by combined loss-of-function
9. 2023 High

   Comprehensive metabolic phenotyping of Ctrp13 knockout mice revealed that CTRP13 paradoxically acts as a negative metabolic regulator in vivo, resolving the apparent contradiction between gain-of-function studies showing metabolic benefit and the actual physiological role.

   Evidence Ctrp13 KO mouse metabolic phenotyping (body composition, glucose/insulin tolerance, triglyceride clearance), RNA-seq, integration with human METSIM cohort

   PMID:37844630

   Open questions at the time

   • Tissue-specific contributions (adipose vs. brain vs. liver) to the metabolic phenotype not dissected
   • Mechanism by which CTRP13 restrains metabolic fitness not identified
10. 2024 Medium

    Studies in 2024 extended C1QL3's neural functions: C1QL3 knockout rats showed altered microglial morphology, dendritic damage, and cognitive deficits, while BLA-specific knockdown revealed C1QL3 modulates PSD95 ubiquitination during morphine withdrawal memory, linking synapse remodeling to substance-use-related behavior.

    Evidence CRISPR/Cas9 C1ql3 KO rats with Golgi staining, microglial immunohistochemistry, MRI, behavioral tests; BLA viral knockdown with conditioned place aversion, PSD95 ubiquitination assays

    PMID:38379452 PMID:38772224

    Open questions at the time

    • Whether PSD95 ubiquitination is a direct or indirect effect of C1QL3 unknown
    • Microglial phenotype could be secondary to synapse loss rather than direct C1QL3 action on microglia
    • Rat KO findings not yet replicated across species
11. 2024 Medium

    Demonstration that CTRP13 inhibits endothelial ferroptosis through GCH1/BH4-dependent upregulation of GPX4 unified the earlier eNOS-coupling mechanism with a new cell-death modality, establishing GCH1/BH4 as a central node in CTRP13's vascular protection.

    Evidence ApoE−/− mice with C1ql3 AAV overexpression, MAEC culture with GCH1 siRNA and BH4 inhibitor rescue, ferroptosis marker quantification

    PMID:39541845

    Open questions at the time

    • Whether ferroptosis inhibition operates through AMPK or an independent pathway not tested
    • Single lab

• The identity of the peripheral receptor(s) mediating CTRP13's metabolic and vascular signaling remains unknown, and the structural basis and stoichiometry of the C1QL3–NPTX–BAI3 trans-synaptic complex have not been determined.
• No receptor identified for CTRP13 in metabolic or vascular tissues
• No high-resolution structure of C1QL3 or its complexes
• Tissue-specific conditional knockouts needed to decouple neural vs. peripheral phenotypes