DLK2

DLK2 (EGFL9) is a membrane-bound EGF-like protein that acts as a non-canonical inhibitory ligand of NOTCH1, binding the same NOTCH1 extracellular region as the canonical ligand DLK1 and suppressing basal NOTCH1 signaling in preadipocytes and fibroblasts (PMID:21419176). DLK2 homodimerizes and physically associates with DLK1, and the two proteins mutually antagonize each other's NOTCH-inhibitory activity, providing a reciprocal switch that tunes signaling output (PMID:21419176). Through this NOTCH-modulating activity DLK2 controls mesenchymal cell fate decisions, acting opposite to DLK1 to influence adipogenesis (PMID:17320102) and to promote osteogenic differentiation with concomitant ERK1/2 MAPK activation (PMID:39473022). Its expression is set transcriptionally: Sp1 binds a proximal GC-box to drive basal transcription (PMID:22185379), while KLF4 binds the promoter to induce DLK2 during early adipogenesis (PMID:22306741). Beyond NOTCH, DLK2 engages distinct partners to activate kinase cascades — it binds Syap1 to drive Akt(Ser473)/ERK1/2/p38 signaling and promote osteoclastogenesis, with osteoclast-specific deletion producing a high-bone-mass phenotype (PMID:37669921), and in cancer it binds cMET and the COX assembly factor COA3 to activate downstream signaling and reprogram mitochondrial metabolism toward a Warburg-like state in triple-negative breast cancer (PMID:31695034) and drives hepatocellular carcinoma progression via EGFR/PI3K/AKT signaling and amino acid metabolic rewiring (PMID:41003932). A Dlk2 knockout mouse additionally links the gene to NOTCH-target and GABA-A receptor expression in the brain and to anxiety- and depressive-like behavior (PMID:28863347).

1. 2006 Medium

   Established that DLK2 is a functional regulator of mesenchymal cell fate, acting in opposition to its paralog DLK1 during adipogenesis and revealing the two genes are coordinately co-regulated.

   Evidence Overexpression and knockdown in 3T3-L1 and C3H10T1/2 cell lines with adipogenesis assays

   PMID:17320102

   Open questions at the time

   • Did not define the molecular mechanism linking DLK2 to adipogenic outcome
   • Cross-regulation of DLK1/DLK2 expression not mechanistically explained
2. 2011 Medium

   Defined the molecular basis of DLK2 action by showing it homodimerizes, binds DLK1, and engages the same NOTCH1 extracellular region as DLK1 to act as a non-canonical inhibitory NOTCH ligand, with the two paralogs reciprocally antagonizing each other.

   Evidence Reciprocal Co-IP and NOTCH reporter assays with DLK1/DLK2 co-overexpression in preadipocytes and MEFs

   PMID:21419176

   Open questions at the time

   • Structural details of the DLK2-NOTCH1 interface not resolved
   • Whether inhibition is competitive with canonical DSL ligands not directly tested
3. 2011 Medium

   Identified Sp1 as the transcription factor setting basal DLK2 expression, mapping its action to a proximal GC-box promoter element.

   Evidence 5' RACE, ChIP, and promoter reporter assays in mouse cell lines

   PMID:22185379

   Open questions at the time

   • Does not address stimulus-dependent or tissue-specific regulation
4. 2012 Medium

   Connected DLK2 expression to adipogenic signaling by showing KLF4 directly binds the promoter and induces DLK2 in response to IBMX during early adipogenesis.

   Evidence ChIP, promoter reporter assays, KLF4 overexpression and knockdown in 3T3-L1 and C3H10T1/2 cells

   PMID:22306741

   Open questions at the time

   • Functional consequence of KLF4-driven DLK2 induction on adipocyte differentiation not directly traced
5. 2014 Medium

   Extended DLK2's NOTCH-inhibitory function to cancer, showing it modulates melanoma proliferation in a dose-dependent manner in vitro and in vivo.

   Evidence NOTCH reporter assays, DLK2 overexpression/knockdown in SK-MEL-2 cells, proliferation and xenograft assays

   PMID:25093684

   Open questions at the time

   • Biphasic dependence on NOTCH inhibition level not mechanistically explained
   • Downstream effectors of the proliferative response not identified
6. 2017 Medium

   Linked DLK2 to neural function in vivo, showing knockout alters NOTCH-target and GABA-A receptor expression in brain and produces anxiety/depressive behaviors with loss of alprazolam response.

   Evidence Dlk2 knockout mouse behavioral assays, regional gene expression analysis, pharmacological challenge

   PMID:28863347

   Open questions at the time

   • Causal chain from DLK2 loss to GABA-A subunit changes not established
   • Cell-autonomous vs systemic origin of phenotype unresolved
7. 2019 High

   Revealed a NOTCH-independent arm of DLK2 function in cancer: binding cMET and the COX assembly factor COA3 to activate signaling and drive Warburg-like metabolic reprogramming.

   Evidence Reciprocal Co-IP, confocal co-localization, COX activity and metabolic assays, in vivo metastasis assays in breast cancer cells

   PMID:31695034

   Open questions at the time

   • How a single EGF-like protein localizes to both membrane and mitochondria not fully explained
   • Stoichiometry of the cMET/COA3 interactions unresolved
8. 2023 High

   Identified Syap1 as a direct DLK2 partner that links the protein to Akt(Ser473)/ERK1/2/p38 activation and osteoclastogenesis, with genetic deletion establishing a causal high-bone-mass phenotype.

   Evidence Reciprocal Co-IP, osteoclast-specific conditional knockout, phospho-western blots, in vitro differentiation and in vivo bone phenotyping including ovariectomized mice

   PMID:37669921

   Open questions at the time

   • Mechanism by which DLK2-Syap1 binding triggers Akt phosphorylation not defined
   • Relationship to DLK2's NOTCH-inhibitory role in the same lineage not reconciled
9. 2024 Medium

   Showed DLK2 promotes osteogenic differentiation of mesenchymal cells via ERK1/2 MAPK and NOTCH inhibition, acting opposite to DLK1.

   Evidence DLK1/DLK2 gain/loss-of-function in C3H10T1/2 cells, osteogenesis assays, phospho-westerns, DAPT NOTCH inhibitor treatment

   PMID:39473022

   Open questions at the time

   • Direct demonstration that NOTCH inhibition mediates the osteogenic effect remains correlative
10. 2025 Medium

    Extended the cancer pro-tumor arm to hepatocellular carcinoma, implicating EGFR/PI3K/AKT signaling and amino acid metabolic reprogramming in DLK2-driven proliferation and metastasis.

    Evidence EGFL9 knockdown/overexpression in HCC lines, functional assays, xenografts, multi-omics, inhibitor rescue

    PMID:41003932

    Open questions at the time

    • Direct receptor partner driving EGFR/PI3K/AKT activation in HCC not identified
    • Whether metabolic rewiring is cause or consequence of growth not resolved

• How DLK2's distinct molecular modes — NOTCH1 inhibition versus direct activation of cMET/EGFR/Syap1-driven kinase and metabolic programs — are integrated within a single cell, and the structural basis for its dual membrane/mitochondrial localization, remain unresolved.
• No structural model of any DLK2 interaction interface
• Mechanism switching DLK2 between NOTCH ligand and kinase-activating partner unknown
• Mitochondrial targeting mechanism uncharacterized