AXL Receptor Tyrosine Kinase

Protein Structure

AXL (Anexelekto; Greek: uncontrolled) is a single-pass transmembrane receptor tyrosine kinase encoded by the AXL gene on chromosome 19q13.2. The mature protein (894 amino acids in humans; ∼104 kDa apparent MW under reducing SDS-PAGE conditions due to N-glycosylation) consists of three functionally distinct regions:

Extracellular Domain

The extracellular portion is organized into a characteristic domain architecture from N-terminus to membrane:

• Two immunoglobulin-like (Ig-like) domains (Ig1 and Ig2): These adopt a C-type Ig-fold and mediate direct contact with the ligand Gas6. The Ig-like domains are the primary determinant of ligand binding affinity; Ig1 makes the principal contacts with the LG modules of Gas6, while Ig2 contributes additional binding surface and participates in receptor dimerization. The Ig1–Ig2 region is highly disulfide-stabilized.
• Two fibronectin type III (FNIII) repeats (FN1 and FN2): Membrane-proximal spacer domains that position the Ig-like binding modules at the appropriate distance from the cell surface to accommodate the Gas6 bridging complex; FN domains also contribute to homotypic receptor–receptor contacts during dimerization.

Transmembrane Domain and Intracellular Kinase Domain

A single α-helical transmembrane segment connects the ectodomain to the intracellular region. The cytoplasmic region contains a juxtamembrane domain followed by the canonical bilobed tyrosine kinase domain (with the characteristic DFG-motif in the activation loop). Key autophosphorylation sites include Y779 (juxtamembrane; adaptor recruitment) and Y821 (activation loop; kinase activation). A C-terminal extension contains additional regulatory residues.

TAM Family Comparison

Ligands: Gas6 and Protein S

Gas6 (Growth Arrest-Specific Gene 6)

Gas6 is the primary, highest-affinity ligand for AXL. It is a 75-kDa secreted vitamin K-dependent protein structurally homologous to Protein S. Gas6 contains the following domains from N-terminus to C-terminus:

• Gla domain (N-terminal): Contains 11–13 γ-carboxyglutamic acid (Gla) residues generated by vitamin K-dependent γ-carboxylase; these Gla residues coordinate Ca²⁺ ions and mediate high-affinity binding to phosphatidylserine (PS) exposed on the outer leaflet of apoptotic cell membranes; without γ-carboxylation (e.g., in warfarin treatment), Gas6 cannot bind PS and loses efferocytosis-bridging function
• EGF-like domain: Short spacer connecting Gla to the LDL-receptor binding region
• LDL receptor-related domain (LDL-R): Intermediate spacer and structural domain
• Two LG (laminin G-like) modules (LG1 and LG2): C-terminal; these bind directly to the Ig-like domains of AXL (and to lesser extents Tyro3 and MerTK); the LG1–LG2 tandem adopts a bilobed domain organization that engages two Ig domains simultaneously, promoting receptor dimerization

The functional geometry of Gas6 action is that of a bridging molecule: the N-terminal Gla domain binds PS on an apoptotic cell surface, and the C-terminal LG modules bind AXL (or another TAM receptor) on the phagocyte membrane, physically tethering the two cells together and clustering AXL for signaling.

Protein S and Receptor Selectivity

Protein S (encoded by PROS1) is a structurally related vitamin K-dependent protein that also contains a Gla domain (PS-binding) and C-terminal SHBG-like (sex hormone-binding globulin-like) modules that bind TAM receptors. Protein S binds MerTK > Tyro3 >> AXL with substantially lower affinity for AXL compared to Gas6. Protein S is the dominant efferocytic ligand for MerTK in macrophages. This ligand selectivity profile allows the TAM receptor sub-family to have partially distinct physiological roles despite their structural similarities.

Intracellular Signaling

Gas6 binding to AXL promotes receptor dimerization (Gas6 LG modules can crosslink two Ig-domain pairs), followed by trans-autophosphorylation of the activation loop tyrosines (Y779, Y821) and juxtamembrane sites. The activated receptor recruits multiple downstream effectors:

  Apoptotic cell (PS-exposed) ──Gas6-Gla binds PS──
                                        │
                              Gas6-LG binds AXL ectodomain
                                        │
                              AXL dimerization + Y779/Y821 phosphorylation
                                        │
                    ┌───────────────────┼───────────────────┐
                    │                   │                   │
            GRB2/SH2 → Sos         PI3K → PIP3          SOCS1/SOCS3
            → Ras → Erk1/2        → Akt (survival)     → suppresses
              (proliferation)        Rac1 → lamellipodia   JAK-STAT
              NF-κB (survival)       (cytoskeletal)        IFN-α/β
                                     remodeling            signaling
                    │
             efferocytosis → TGF-β1 / IL-10 induction
                           → NF-κB suppression
                           → anti-inflammatory milieu

• PI3K/Akt pathway: PI3K is recruited to pY docking sites → phosphorylation of PIP2 to PIP3 → Akt activation → cell survival, anti-apoptotic signals (BCL-2 upregulation); Rac1 GTPase activation downstream of PI3K drives actin polymerization and phagocytic cup formation for apoptotic corpse engulfment
• Ras/Erk/NF-κB pathway: GRB2 SH2 domain docks to pY779 → SOS exchange factor → Ras-GTP → Raf → MEK → Erk1/2 → NF-κB nuclear translocation → transcription of survival and proliferative genes; this pathway is hyperactivated in AXL-overexpressing cancers
• SOCS1/3 induction and IFN suppression: AXL activation induces Suppressor Of Cytokine Signaling 1 and 3 (SOCS1/3), which inhibit JAK1/2 → STAT1 signaling downstream of type I interferon (IFN-α/β) receptors; this creates an immunosuppressive feedback loop that links efferocytosis (Gas6/AXL-mediated clearance of apoptotic cells) to dampened innate antiviral signaling — exploited by ZIKA virus and other enveloped viruses
• RET co-receptor signaling: In some cell types AXL forms heterodimeric complexes with RET receptor tyrosine kinase, transactivating RET and modifying downstream signaling specificity

Role in Efferocytosis and Immune Homeostasis

Efferocytosis (from Latin efferre: to carry out) is the phagocytic clearance of apoptotic cells by professional phagocytes (primarily macrophages and dendritic cells) and non-professional phagocytes (epithelial cells, mesothelial cells). It is a non-inflammatory process that prevents secondary necrosis and the release of pro-inflammatory damage-associated molecular patterns (DAMPs) from uncleared apoptotic corpses.

AXL and MerTK are the two dominant efferocytic receptors on tissue-resident macrophages and dendritic cells. They function in an overlapping and partially redundant manner: MerTK is the primary efferocytic receptor in resting tissue macrophages and retinal pigment epithelium (RPE), while AXL is more prominent in monocyte-derived macrophages, dendritic cells, and endothelial cells. Together, they ensure the >10⁸ apoptotic cells generated daily in the adult human body (from hematopoiesis, intestinal epithelial turnover, immune activation) are cleared efficiently without triggering inflammation.

Consequences of TAM Receptor Deficiency

Mice with targeted deletion of all three TAM receptors (Tyro3^-/-/Axl^-/-/MerTK^-/- triple knockout) develop a severe lupus-like autoimmune syndrome by 3–6 months of age, characterized by:

• Accumulation of secondary necrotic cells in lymph nodes, spleen, and peripheral tissues (consequence of inefficient efferocytosis)
• High-titer autoantibodies against nuclear antigens (anti-dsDNA, anti-Smith), consistent with systemic lupus erythematosus (SLE)-like pathology
• Lymphocyte hyperactivation and glomerulonephritis
• Retinal photoreceptor degeneration (MerTK critical for RPE phagocytosis of shed photoreceptor outer segments)

These phenotypes establish that TAM-receptor-mediated efferocytosis is not merely housekeeping but is an active mechanism of immune tolerance to self-antigens. Dysfunction in this pathway is implicated in human SLE susceptibility (MerTK/AXL variants in GWAS) and in the accumulation of nuclear debris that drives anti-nuclear antibody production.

AXL and Zika Virus Entry into Neural Progenitor Cells

Zika virus (ZIKV; Flavivirus genus) tropism for neural progenitor cells (NPCs) of the developing fetal cortex underlies its capacity to cause microcephaly following intrauterine infection. AXL plays a critical role in this tropism.

AXL Expression in Neural Progenitor Cells

AXL is highly expressed on the apical surface of outer radial glia (oRG) and ventricular radial glia (vRG) — the two NPC subtypes primarily responsible for cortical neurogenesis. It is also expressed on astrocytes and microglia. This expression pattern makes NPCs uniquely susceptible to ZIKV infection compared to differentiated neurons, which express lower AXL levels.

Mechanism of AXL-Mediated ZIKV Entry

ZIKV does not bind AXL directly with high affinity. Instead, it exploits a Gas6-bridged indirect mechanism:

1. ZIKV virions (enveloped, ∼50 nm diameter) expose phosphatidylserine on the outer leaflet of the viral lipid envelope — a property termed "apoptotic mimicry" that enveloped viruses exploit to co-opt efferocytic pathways
2. Gas6 in the extracellular environment binds both PS on the viral envelope (via Gla domain) and AXL on the NPC surface (via LG domains), acting as a molecular bridge
3. AXL clustering and signaling triggers clathrin-mediated endocytosis of the virion, delivering it to acidified endosomes where the ZIKV fusion peptide activates at low pH
4. ZIKV envelope protein E also interacts directly with DC-SIGN, Tim-1, and Tyro3 as alternative co-receptors, explaining why AXL blockade does not completely abolish infection

Evidence and Therapeutic Implications

Hamel et al. (Cell Host Microbe 2015) demonstrated AXL upregulation in ZIKV-infected cells and identified AXL as a permissivity factor. Nowakowski et al. (Cell Stem Cell 2016) showed that AXL is expressed on the specific oRG subtype that preferentially expands the human cortex, linking AXL tropism to the disproportionate cortical surface disruption in congenital Zika. Blockade of AXL using:

• Cabozantinib (XL184): Multi-kinase inhibitor (MET/VEGFR2/AXL/RET); reduces ZIKV infection of NPCs in vitro by >50%
• R428 (bemcentinib/BGB324): Selective AXL inhibitor; reduces ZIKV entry and NPC infection in organoid models
• Soluble AXL (sAXL) decoy receptor: Competes with membrane AXL for Gas6-bridged virion binding; reduces ZIKV infection in iPSC-derived NPC cultures

AXL in Cancer: EMT, Metastasis, and Therapeutic Resistance

AXL is overexpressed or genetically amplified in a broad range of human cancers and has emerged as a clinically significant mediator of epithelial-mesenchymal transition (EMT), invasion, metastasis, and acquired resistance to targeted therapies.

Cancers with AXL Overexpression

AXL and Epithelial-Mesenchymal Transition (EMT)

Gas6/AXL signaling activates Twist1 and Slug (SNAI2) transcription factors via NF-κB and Erk1/2, which repress E-cadherin (CDH1) and upregulate vimentin and fibronectin — the molecular hallmarks of EMT. EMT is associated with acquisition of migratory and invasive phenotypes (loss of apicobasal polarity, cytoskeletal reorganization, basement membrane degradation) that enable cancer cells to enter the vasculature and seed distant metastases.

EGFR Inhibitor Resistance in NSCLC

In EGFR-mutant NSCLC treated with tyrosine kinase inhibitors (TKIs), AXL upregulation is one of the most common mechanisms of acquired resistance to both first/second-generation (erlotinib/afatinib) and third-generation (osimertinib) EGFR inhibitors. AXL activates PI3K/Akt and Erk survival pathways independently of EGFR, allowing cancer cells to bypass EGFR blockade. Combination of EGFR-TKI with AXL inhibitors is under clinical investigation to delay or overcome this resistance.

Therapeutic Targeting of AXL

Connections

• viral entryPathogen Atlas — AXL is an entry factor for Zika virus, Dengue virus (partial), Ebola virus (GP/PS-TIM1 mechanism also relevant), and other enveloped viruses; exploited via Gas6/PS apoptotic mimicry
• diseaseMicrocephaly — AXL expressed on outer radial glia (oRG) and ventricular RG; ZIKV entry via Gas6-AXL bridge disrupts NPC division and cortical neurogenesis; primary mechanism linking ZIKV to congenital microcephaly
• vectorAedes aegypti — primary vector for Zika virus transmission; AXL is the NPC entry factor for ZIKV delivered via the hematogenous route after mosquito bite; understanding vector biology essential for prevention
• zoonosisZoonosis — Zika virus is a zoonotic flavivirus with a sylvatic NHP reservoir; AXL expression patterns in neural tissue explain the specific human fetal brain tropism that distinguishes ZIKV from related flaviviruses
• viral antigenSpike Protein — SARS-CoV-2 spike protein has been reported to interact with AXL as an alternative host-cell receptor; AXL co-expression enhances SARS-CoV-2 entry in some cell types; conceptual parallel with ZIKV PS-AXL mechanism
• innate immunityNLRP3 Inflammasome — AXL-SOCS1/3 signaling suppresses IFN-α/β and dampens JAK-STAT signaling; cross-talk with NLRP3 inflammasome in macrophages during efferocytosis; balance between anti-inflammatory AXL signaling and pro-inflammatory NLRP3 activation determines inflammatory outcome of apoptotic cell clearance