Specialized cells exploit adaptor protein complexes for unique post-Golgi sorting events, providing a unique model system to specify adaptor function. Here, we show that AP-3 and AP-1 function independently in sorting of the melanocyte-specific protein tyrosinase from endosomes to the melanosome, a specialized lysosome-related organelle distinguishable from lysosomes. AP-3 and AP-1 localize in melanocytes primarily to clathrin-coated buds on tubular early endosomes near melanosomes. Both adaptors recognize the tyrosinase dileucine-based melanosome sorting signal, and tyrosinase largely colocalizes with each adaptor on endosomes. In AP-3-deficient melanocytes, tyrosinase accumulates inappropriately in vacuolar and multivesicular endosomes. Nevertheless, a substantial fraction still accumulates on melanosomes, concomitant with increased association with endosomal AP-1. Our data indicate that AP-3 and AP-1 function in partially redundant pathways to transfer tyrosinase from distinct endosomal subdomains to melanosomes and that the AP-3 pathway ensures that tyrosinase averts entrapment on internal membranes of forming multivesicular bodies.

The regulation of blood vessel formation is of fundamental importance to many physiological processes, and angiogenesis is a major area for novel therapeutic approaches to diseases from ischemia to cancer. A poorly understood clinical manifestation of pathological angiogenesis is angiodysplasia, vascular malformations that cause severe gastrointestinal bleeding. Angiodysplasia can be associated with von Willebrand disease (VWD), the most common bleeding disorder in man. VWD is caused by a defect or deficiency in von Willebrand factor (VWF), a glycoprotein essential for normal hemostasis that is involved in inflammation. We hypothesized that VWF regulates angiogenesis. Inhibition of VWF expression by short interfering RNA (siRNA) in endothelial cells (ECs) caused increased in vitro angiogenesis and increased vascular endothelial growth factor (VEGF) receptor-2 (VEGFR-2)-dependent proliferation and migration, coupled to decreased integrin ␣v␤3 levels and increased angiopoietin (Ang)-2 release. ECs expanded from blood- IntroductionAngiogenesis, the formation of new vessels from pre-existing ones, occurs physiologically in specific circumstances such as wound healing and the menstrual cycle. Dysregulated angiogenesis contributes to the pathogenesis of many disorders, including diabetes, cancer, and macular degeneration (reviewed in Carmeliet 1 ). Angiogenic factors such as vascular endothelial growth factor (VEGF) and the angiopoietins (Ang) orchestrate signaling pathways that promote endothelial cell (EC) migration, proliferation, and ultimately the formation of a new vessel. VEGF-A is a major regulator of angiogenesis (reviewed in Grothey and Galanis 2 ) and acts on ECs mainly through VEGF receptor-2 (VEGFR-2), a tyrosine kinase receptor (reviewed in Olsson 3 ), to promote endothelial proliferation, migration, and sprouting of tip cells in the early stages of angiogenesis (reviewed in Gerhardt 4 ). Ang-1 and Ang-2, which bind to the endothelial Tie-2 receptor, act in the later stages of blood vessel formation and are essential for the maturation of a stable vascular network and for the maintenance of endothelial integrity (reviewed in Thomas and Augustin 5 ). Ang-1 and Ang-2 were originally identified as agonist and antagonist of Tie-2 signaling, respectively, with Ang-1 supporting EC survival and endothelial integrity 6 and Ang-2 promoting blood vessel destabilization and regression. 7 However, recent data suggest a more complex picture that includes cross-talk between the VEGF and the Ang pathways. 8 Growth factor signaling pathways are influenced by surface adhesion molecules that mediate cell-cell or cell-matrix interactions, particularly by members of the integrin superfamily. The integrin that has received most attention in ECs is ␣v␤3 (reviewed in Hodivala-Dilke 9 ), which mediates binding to several extracellular matrix proteins and growth factor receptors including VEGFR-2, thus influencing VEGFR-2 signaling (reviewed in Somanath et al 10 ). ␣v␤3 plays a complex role in angiogenesis. Although the origina...