, 2010). The first step in the endocytic trafficking

of a 7TMR is its removal from the plasma membrane by packaging into an endocytic vesicle. Mammalian cells express multiple endocytic mechanisms (McMahon and Boucrot, 2011; Sandvig et al., 2011) that individual 7TMRs can potentially engage (Tsao and von Zastrow, 2001; Wolfe and Trejo, 2007). Many neuromodulatory 7TMRs are internalized by clathrin-coated pits (CCPs), which are complex and highly versatile endocytic machines capable of internalizing a wide variety of membrane cargoes in addition to 7TMRs (McMahon and Boucrot, 2011; Conner and Schmid, 2003). In studies that have carefully examined the endocytic process, 7TMRs primarily undergo activation-induced accumulation in previously formed CCPs and only rarely appear to initiate CCP formation on their own; accordingly, a major determinant of 7TMR endocytic see more rate is the degree to which receptors concentrate

in CCPs (Goodman et al., 1998; Puthenveedu and von Zastrow, 2006; Krupnick et al., 1997; Kang et al., 2009). For many neuromodulatory 7TMRs that undergo regulated endocytosis via CCPs, receptor concentration in them is stimulated by activation-induced phosphorylation of receptors followed by phosphorylation-promoted association of receptors with beta-arrestins, as reviewed previously elsewhere (Goodman et al., 1998; Gainetdinov et al., 2004). Beta-arrestins bind both to activated 7TMRs

and to components of the CCP (including clathrin heavy chain, the endocytic adaptor protein AP-2, and phosphatidylinositol 4,5-bisphosphate), Selleck SB203580 thereby functioning as regulated endocytic adaptors (Goodman et al., 1996; Laporte et al., 1999; Gaidarov et al., 1999). Beta-arrestins Ketanserin can associate with CCPs after assembly of major structural components has already occurred (Santini et al., 2000; Puthenveedu and von Zastrow, 2006), explaining how 7TMRs concentrate in CCPs after their formation and in the presence of other endocytic cargoes. While there is presently no evidence for 7TMR packaging into specialized CCPs a priori, 7TMRs can associate with pre-existing CCPs apparently in a cooperative manner, producing a receptor-enriched CCP subset, and their presence can influence the kinetics of subsequent CCP maturation events. This appears to be a means by which some 7TMRs, including beta-adrenergic catecholamine receptors (Puthenveedu and von Zastrow, 2006) and mu opioid neuropeptide receptors (Henry et al., 2012), locally modify the properties of their enclosing CCP after the fact. 7TMR clustering in previously formed CCPs has been directly demonstrated in neurons (Yu et al., 2010) but subsequent “customization” of CCP dynamics by locally accumulated 7TMRs has been shown only in nonneural cell models, and its functional significance remains largely unexplored in any system.