Angiomotin

Expression of angiomotin p80 in endothelial cells increases the random migration of endothelial cells, as well as the migration of endothelial cells toward growth factors, e.g. bFGF, VEGF and LPA etc. Angiomotin also mediates tube formation of endothelial cells.[5][13] Angiomotin promotes angiogenesis by both stimulating cell spreading and stabilizing established tubes, e.g. in mouse aortic endothelial (MAE) cells the tubes remained stable for over 30 days, while control tubes started to regress after 3 days.[14] In the presence of angiostatin, endothelial cells expressing angiomotin p80 exhibit reduction in migration as well as reduction in tubules formation in vitro. These observations are consistent with the localization of angiomotin in the leading edge of migrating cells. Angiostatin therefore, is an inhibitor of angiomotin.

Angiomotin p80 locates and binds angiostatin on the cell surface. In primary endothelial of Chinese hamster ovary, it localizes to cell-cell junction, recruits ZO-1 and interacts with MAGI-1. It may play a role in the assembly of endothelial cell-cell junctions, as well.[6]

Angiomotin p130 does not promote cell migration, nor responds to angiostatin. It localizes to cell-cell junction like p80 and regulates paracellular permeability. Its N-terminal domain localizes to actin fibers and stabilizes them, and this effect is not affected by angiostatin. Transfection of p130 angiomotin into MAE cells results in change in cell shape, increased average cell size and stress fiber formation. So p80 is involved in cell migration and expressed during migratory phase. While p130 controls cell shape by interaction with actin, and is expressed during the period of blood vessel stabilization and maturation.[11][15] The relative expression levels of p80 and p130 regulate a switch between a migratory and a non-migratory cell phenotype, where homo-oligomerization of p80 and hetero-oligomerization of both isoforms are critical for this regulation.[15]

AMOT, AMOTL1 and AMOTL2 play critical roles in the Hippo signaling pathway by regulating the subcellular localization of the co-activators YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ-binding motif),[16] and activating LATS2 through a novel conserved domain.[17] The activity of YAP and TAZ can be restricted through their interaction with AMOT and AMOTL1, and such interaction depends on the WW domain of TAZ and the Proline-Proline-x–Tyrosine motif at the N-terminus of AMOT.[18]

In position-dependent Hippo signaling, where the outer and inner cells are polar and nonpolar respectively, AMOT and AMOTL2 are essential for Hippo pathway activation and appropriate cell fate specification. In the nonpolar inner cells, AMOT localizes to adherens junctions (AJs), and Ser-176 at the N-terminal domain is phosphorylated by LATS downstream of GPCR signaling, which inhibits actin binding activity and stabilizes the AMOT-LATS interaction to activate the Hippo pathway. Thus, AMOT is a direct substrate of LATS and its phosphorylation at Ser-176 inhibits cell migration and angiogenesis. In the outer cells, the cell polarity sequesters AMOT from basolateral adherens junctions to apical domains, thereby suppressing Hippo signaling.[19][20] It is therefore proposed AMOT acts as a molecular switch for Hippo pathway and links F-actin with LATS activity.[21]

Along the Hippo pathway, AMOT's binding to Merlin releases its auto-inhibition and promotes Merlin's binding to LATS1/2. Phosphorylation of Ser-518 outside the Merlin's auto-inhibitory tail prevents binding and thus inhibits Hippo pathway kinase activation.[22] USP9x regulates the ubiquitin-mediated turnover of AMOT, and the deubiquitylation of AMOT results in its stabilization and lower YAP/TAZ activity.[23]