Tubby and tubby-like protein 1 are new MerTK ligands for phagocytosis

Abstract

Tubby and tubby‐like protein 1 (Tulp1) are newly identified phagocytosis ligands to facilitate retinal pigment epithelium (RPE) and macrophage phagocytosis. Both proteins without classical signal peptide have been demonstrated with unconventional secretion. Here, we characterized them as novel MerTK ligands to facilitate phagocytosis. Tulp1 interacts with Tyro3, Axl and MerTK of the TAM receptor tyrosine kinase subfamily, whereas tubby binds only to MerTK. Excessive soluble MerTK extracellular domain blocked tubby‐ or Tulp1‐mediated phagocytosis. Both ligands induced MerTK activation with receptor phosphorylation and signalling cascade, including non‐muscle myosin II redistribution and co‐localization with phagosomes. Tubby and Tulp1 are bridging molecules with their N‐terminal region as MerTK‐binding domain and C‐terminal region as phagocytosis prey‐binding domain (PPBD). Five minimal phagocytic determinants (MPDs) of K/R(X)1–2KKK in Tulp1 N‐terminus were defined as essential motifs for MerTK binding, receptor phosphorylation and phagocytosis. PPBD was mapped to the highly conserved 54 amino acids at the C‐terminal end of tubby and Tulp1. These data suggest that tubby and Tulp1 are novel bridging molecules to facilitate phagocytosis through MerTK.

Receptor Tyrosine Kinases, TYRO3, AXL, and MER, Demonstrate Distinct Patterns and Complex Regulation of Ligand-induced Activation

“…Therefore, one of the important utilities of these TAM reporter cell lines is to assess the activity of functional TAM ligands, as current conventional ELISA technology only provides information on the ligand concentrations, independent of whether the ligand is active or inactive. These cell lines can be also used to determine whether recently identified non-conventional TAM ligands (29,30) can signal through TAM homodimers. In addition, these cell lines can be applied for screening potential agonists or receptor specific inhibitors in the form of neutralizing antibodies or drugs.…”

“…Moreover, activation of TAMs leads to the suppression of NF-␤ signaling; thus, TAMs act as immuno-regulatory receptors that dampen inflammation (28). In recent years, several additional TAM ligands, including TUBBY, TULP-1, and Galectin-3, have been reported; however the functions of these new ligands are still emerging (29,30).…”

Molecular Pathways: MERTK Signaling in Cancer

“…Both ligands are secreted by multiple cell types and are present in human blood, although total protein S levels are 1,000-fold higher; however, modifications of protein S may be necessary for it to activate Mer (39)(40)(41). More recently, a novel phagocytosis-based functional cloning screening strategy identified 3 new MERTK ligands: tubby, tubby-like protein 1 (Tulp1), and galectin-3 (42,43). All known MERTK ligands induce MERTK autophosphorylation, although to date, roles for tubby, Tulp1, and galectin-3 have not been studied in MERTK-driven cancers.…”

MerTK inhibition in tumor leukocytes decreases tumor growth and metastasis

“…These RTKs are widely expressed in many epithelial tissues and in cells of the immune, nervous, and reproductive systems (2,6). The MerTK ligands include growth arrest-specific 6 (GAS6) (7,8), protein-S (9, 10), tubby and tubbylike protein-1 (TULP1) (11), and galectin-3 (12). Several of these ligands are present in serum, and all are expressed locally in some tissues.…”