Developmental effects of ghrelin

Ghrelin is a pleiotropic hormone that was originally described as promoting feeding and stimulating growth hormone release in adults. A growing body of evidence suggests that ghrelin may also exert developmental and organizational effects during perinatal life. The perinatal actions of ghrelin include the regulation of early developmental events such as blastocyst development and perinatal growth. Moreover, alterations in perinatal ghrelin levels result in structural differences in various peripheral organs, such as the pancreas and gastrointestinal tract. Recent data have also suggested that ghrelin acts on appetite-related brain centers in early life. Together, these observations indicate that exposure to factors that alter how ghrelin impacts development may induce lasting effects on physiological regulation.     

The Clathrin Adaptor Proteins ARH,Dab2, and Numb Play Distinct Roles in Niemann-Pick C1-Like 1 Versus Low Density Lipoprotein Receptor-mediated Cholesterol Uptake

The uptake of circulating low density lipoproteins (LDL) is mediated by LDL receptor (LDLR) through clathrin-dependent endocytosis. At the early stage of this process, adaptor proteins ARH and Dab2 specifically bind the endocytic signal motif in LDLR and recruit clathrin/AP2 to initiate internalization. On the other hand, intestinal cholesterol is absorbed by Niemann-Pick C1-Like 1 (NPC1L1) through clathrin-dependent endocytosis. Another adaptor protein, Numb recognizes the endocytic motif in NPC1L1 C terminus and couples NPC1L1 to endocytic machinery. The ARH, Dab2, and Numb proteins contain a homogeneous phosphotyrosine binding (PTB) domain that directly binds endocytic motifs. Because ARH, Dab2, and Numb are all PTB domain family members, the emerging mystery is whether these adaptors act complementally in LDLR and NPC1L1 endocytosis. Here, we found that ARH and Dab2 did not bind NPC1L1 and were not required for NPC1L1 internalization. Similarly, Numb lacked the ability to interact with the LDLR C terminus and was dispensable for LDL uptake. Only the Numb isoforms with shorter PTB domain could facilitate NPC1L1 endocytosis. Besides the reported function in intestinal cholesterol absorption, Numb also mediated cholesterol reabsorption from bile in liver. We further identified a Numb variant with G595D substitution in humans of low blood LDL-cholesterol. The G595D substitution impaired NPC1L1 internalization and cholesterol reabsorption, due to attenuating affinity of Numb to clathrin/AP2. These results demonstrate that Numb specifically regulates NPC1L1-mediated cholesterol absorption both in human intestine and liver, distinct from ARH and Dab2, which selectively participate in LDLR-mediated LDL uptake.