Endocytic trafficking of GAS6–AXL complexes is associated with sustained AKT activation

Abstract

AXL, a TAM receptor tyrosine kinase (RTK), and its ligand growth arrest-specific 6 (GAS6) are implicated in cancer metastasis and drug resistance, and cellular entry of viruses. Given this, AXL is an attractive therapeutic target, and its inhibitors are being tested in cancer and COVID-19 clinical trials. Still, astonishingly little is known about intracellular mechanisms that control its function. Here, we characterized endocytosis of AXL, a process known to regulate intracellular functions of RTKs. Consistent with the notion that AXL is a primary receptor for GAS6, its depletion was sufficient to block GAS6 internalization. We discovered that upon receptor ligation, GAS6–AXL complexes were rapidly internalized via several endocytic pathways including both clathrin-mediated and clathrin-independent routes, among the latter the CLIC/GEEC pathway and macropinocytosis. The internalization of AXL was strictly dependent on its kinase activity. In comparison to other RTKs, AXL was endocytosed faster and the majority of the internalized receptor was not degraded but rather recycled via SNX1-positive endosomes. This trafficking pattern coincided with sustained AKT activation upon GAS6 stimulation. Specifically, reduced internalization of GAS6–AXL upon the CLIC/GEEC downregulation intensified, whereas impaired recycling due to depletion of SNX1 and SNX2 attenuated AKT signaling. Altogether, our data uncover the coupling between AXL endocytic trafficking and AKT signaling upon GAS6 stimulation. Moreover, our study provides a rationale for pharmacological inhibition of AXL in antiviral therapy as viruses utilize GAS6–AXL-triggered endocytosis to enter cells.

Gas6/TAM Axis Involvement in Modulating Inflammation and Fibrosis in COVID-19 Patients

“…Despite the different binding affinities to TAM receptors, the downstream activation mechanism is similar: once the ligand is bound, the receptor dimerizes and its tyrosine kinase domains become activated, mediating various cellular responses, such as cell growth and proliferation, the regulation of apoptosis, as well as the modulation of vascular and inflammatory responses in a cell and tissue-dependent manner [ 4 , 15 , 18 , 20 , 24 ]. The different cellular responses triggered by the Gas6/TAM axis activation mainly rely on well-known intracellular signaling mediators such as the p38/MAPK, the PI3K/Akt, the ERK1/2 and the JAK/STAT pathways, which need to work in a coordinated manner to ensure homeostasis in the whole organism [ 25 , 26 , 27 ].…”

“…Even if the majority of the research papers are focused on the Gas6/TAM axis involvement in COVID-19, interesting results also come from in vitro research. Since the first decade of this century, different studies demonstrated, in vitro, that TAM receptors and their ligands, by acting as a bridge with PtdSer, could promote different enveloped virus infections (i.e., filovirus such as Ebola, and flaviviruses such as Dengue and West Nile) [ 22 , 25 , 92 , 98 ]. In particular, the Axl role in lung viral infections has also been studied in a murine model, where it has been demonstrated that its inhibition by monoclonal antibodies locally enhanced innate and adaptive immunity, suggesting Axl-targeted inhibition as an interesting clinical approach to treat viral lung diseases [ 99 ].…”

Protrudin-mediated ER-endosome contact sites promote phagocytosis

“…Alternatively, mTORC1 activity has been implicated in phagocytosis [ 52 , 53 ], and overexpressed Protrudin promotes mTORC1 signalling from peripherally localised LELys [ 54 ]. It is also tempting to speculate that Protrudin might increase the vesicle-mediated surface localisation of phagocytic receptors, such as TAM receptor tyrosine kinases and integrins [ 23 , 55 ]. Moreover, Protrudin has a role in the shaping of the tubular ER network, which extends to the periphery of the cells [ 56 ].…”

COVID-19 Biomarkers at the Crossroad between Patient Stratification and Targeted Therapy: The Role of Validated and Proposed Parameters

“…Gas6 biological activities depend on its binding to one of the three members of a family of tyrosine kinase receptors, collectively named TAM (for Tyro-3, Axl, MerTK), which in turn activates different intracellular signaling pathways (i.e., the p38/MAPK, the ERK1/2, the JAK/STAT, and the PI3K/Akt pathways) [ 131 , 132 , 133 , 134 , 135 , 136 , 137 , 138 ].…”