Signaling of the receptor tyrosine kinase Axl and the role of TAM family members in CNS demyelinating disease

Abstract

The TAM family of receptor tyrosine kinases (Tyro3, Axl, and Mer), and their ligand Gas6 have important roles in cell survival, maturation, and phagocytosis. Signaling molecules that can modulate disease-induced central nervous system (CNS) damage and maintain cell survival are highly sought after. In several CNS in vitro models, Axl is associated with survival. Axl-/- mice have altered recovery in the cuprizone toxicity model, and Axl is upregulated in EAE, a mouse model of multiple sclerosis (MS). MS is a debilitating neurological disease involving infiltration of inflammatory cells across the blood brain barrier, resulting in demyelinated lesions, axonal damage, neuronal loss, and oligodendrocyte cell death. Using wildtype (WT) and mutant Axl transfected cells we determined that Axl directly bound Grb2 at p821YVN and the p85 subunit of PI3-kinase at pY779ALM and pY 821VNM. Further, p85 indirectly bound to Axl via Grb2. One minute Gas6-stimulation resulted in recruitment of active PI3-kinase to Axl and Akt phosphorylation. Interactions between Axl, p85 and Grb2 were confirmed in O4+ oligodendrocytes immunopanned from day 10 mouse brain homogenates.;Examination of Axl-/- and WT C57B16 mice during the acute phase of MOG-induced EAE showed spinal cords from Axl-/- mice had more immune infiltrates and myelin loss. Also, Axl-/- mice had significantly fewer activated microglia/macrophages adjacent to lesions, and visibly more myelin/cellular debris and axonal damage, all contributing to higher clinical scores. Our studies of human brain sections from MS patients suggest that aberrant Gas6/TAM signaling is associated with lesion progression. Full-length membrane-bound Mer, and soluble forms of Axl and Mer were significantly elevated in homogenates from chronic active and chronic silent (established) MS lesions, relative to normal-appearing white matter controls. In addition, soluble Axl and Mer negatively correlated with Gas6 in established MS lesions. Increased soluble Axl and Mer were associated with increased mature ADAM17, mature ADAM10 and Furin, proteins associated with Axl and Mer solubilization. Soluble Axl and Mer are known to act as decoy receptors that block Gas6 binding to membrane-bound receptors. Thus, EAE and MS tissue studies demonstrated that changes or loss in Gas6/TAM receptor signaling contributed to lesion pathology.