Altering the number of surface receptors can rapidly modulate cellular responses to extracellular signals. Some receptors, like the transferrin receptor (TfR), are constitutively internalized and recycled to the plasma membrane. Other receptors, like the epidermal growth factor receptor (EGFR), are internalized after ligand binding and then ultimately degraded in the lysosome. Routing internalized receptors to different destinations suggests that distinct molecular mechanisms may direct their movement. Here, we report that the endosome-associated protein hrs is a subunit of a protein complex containing actinin-4, BERP, and myosin V that is necessary for efficient TfR recycling but not for EGFR degradation. The hrs/actinin-4/BERP/myosin V (CART [cytoskeleton-associated recycling or transport]) complex assembles in a linear manner and interrupting binding of any member to its neighbor produces an inhibition of transferrin recycling rate. Disrupting the CART complex results in shunting receptors to a slower recycling pathway that involves the recycling endosome. The novel CART complex may provide a molecular mechanism for the actin-dependence of rapid recycling of constitutively recycled plasma membrane receptors. INTRODUCTIONEndocytosis is required for the uptake of essential nutrients from the extracellular environment as well as for retrieval of proteins and lipids that are added to the plasma membrane during fusion of regulated and constitutive secretory vesicles (De Camilli and Takei, 1996;Koenig and Ikeda, 1996;Robinson et al., 1996;Mukherjee et al., 1997;Schmid, 1997;Betz and Angleson, 1998;Koenig et al., 1998;Stoorvogel, 1998;D'Hondt et al., 2000;Gruenberg, 2001). The endocytic pathway can be separated into numerous stages based on the movement of cargo and the identification of morphologically defined compartments (De Camilli and Takei, 1996;Koenig and Ikeda, 1996;Robinson et al., 1996;Mukherjee et al., 1997;Schmid, 1997;Betz and Angleson, 1998;Koenig et al., 1998;Stoorvogel, 1998;D'Hondt et al., 2000;Gruenberg, 2001). Early events in the endocytic process include membrane invagination and vesicle budding from the plasma membrane, formation of transport vesicles, and fusion with early endosomes. Later events include cargo sorting, and additional transport/fusion steps, including those responsible for transport to the lysosome for degradation, and those responsible for recycling back to various compartments (De Camilli and Takei, 1996;Koenig and Ikeda, 1996;Robinson et al., 1996;Mukherjee et al., 1997;Schmid, 1997;Betz and Angleson, 1998;Koenig et al., 1998;Stoorvogel, 1998;D'Hondt et al., 2000;Gruenberg, 2001). Although much progress has been made in elucidating the molecular processes involved in early endocytic events, such as those involved in the genesis of clathrin-coated endocytic transport vesicles, an equally clear understanding of later events remains elusive.The early endosome is a crucial point in the endocytic pathway to sort cargo for transport to late endosomes for eventual degradation in the...

It was hypothesized that transplanting human embryonic stem cells (hESCs) lacking human leukocyte antigen class I (HLA‐I) expression would not result in a robust immune rejection by allogeneic recipients. β2‐Microglobulin‐null hESCs transplanted into natural killer cell‐depleted immunocompetent mice developed tumors resembling those from control hESCs in severe combined immunodeficiency mice. β2‐Microglobulin‐null hESCs significantly reduced immunogenicity to CD8+ T cells and might provide a renewable source of cells for tissue regeneration without HLA matching.

Movement through the endocytic pathway occurs principally via a series of membrane fusion and fission reactions that allow sorting of molecules to be recycled from those to be degraded. Endosome fusion is dependent on SNARE proteins, although the nature of the proteins involved and their regulation has not been fully elucidated. We found that the endosome-associated hepatocyte responsive serum phosphoprotein (Hrs) inhibited the homotypic fusion of early endosomes. A region of Hrs predicted to form a coiled coil required for binding the Q-SNARE, SNAP-25, mimicked the inhibition of endosome fusion produced by full-length Hrs, and was sufficient for endosome binding. SNAP-25, syntaxin 13, and VAMP2 were bound from rat brain membranes to the Hrs coiled-coil domain. Syntaxin 13 inhibited early endosomal fusion and botulinum toxin/E inhibition of early endosomal fusion was reversed by addition of SNAP-25(150–206), confirming a role for syntaxin 13, and establishing a role for SNAP-25 in endosomal fusion. Hrs inhibited formation of the syntaxin 13–SNAP-25–VAMP2 complex by displacing VAMP2 from the complex. These data suggest that SNAP-25 is a receptor for Hrs on early endosomal membranes and that the binding of Hrs to SNAP-25 on endosomal membranes inhibits formation of a SNARE complex required for homotypic endosome fusion.

SummaryHere we report the successful generation and long-term expansion of SOX9-expressing CD271+PDGFRα+CD73+ chondrogenic ectomesenchymal cells from the PAX3/SOX10/FOXD3-expressing MIXL1−CD271hiPDGFRαloCD73− neural crest-like progeny of human pluripotent stem cells in a chemically defined medium supplemented with Nodal/Activin/transforming growth factorβ (TGFβ) inhibitor and fibroblast growth factor (FGF). When “primed” with TGFβ, such cells efficiently formed translucent cartilage particles, which were completely mineralized in 12 weeks in immunocompromized mice. The ectomesenchymal cells were expandable without loss of chondrogenic potential for at least 16 passages. They maintained normal karyotype for at least 10 passages and expressed genes representing embryonic progenitors (SOX4/12, LIN28A/B), cranial mesenchyme (ALX1/3/4), and chondroprogenitors (SOX9, COL2A1) of neural crest origin (SOX8/9, NGFR, NES). Ectomesenchyme is a source of many craniofacial bone and cartilage structures. The method we describe for obtaining a large quantity of human ectomesenchymal cells will help to model craniofacial disorders in vitro and potentially provide cells for the repair of craniofacial damage.

Rationale: Postmenopausal atherosclerosis (AS) has for decades been attributed to estrogen deficiency. Although the follicular stimulating hormone (FSH) levels rise sharply in parallel, the direct effect of FSH on AS has never been investigated. In this study, we explored the possible role of FSH in the development of AS.Methods: This was a prospective cohort study of 48 healthy premenopausal and 15 postmenopausal women. ApoE knockout mice were used as atherosclerosis model and human umbilical vascular endothelial cells (HUVECs) were cultured as cell model. Serum hormones and vascular cell adhesion molecule-1 (VCAM-1) levels were measured. Real-time PCR, histology for atherosclerotic lesions, immunofluorescence, luciferase assay, transfection experiments, flow chamber adhesion assay and western blot were performed.Results: In ApoE knockout mice, administration of FSH increased the atherosclerotic lesions and serum VCAM-1 concentration. Importantly, in blood samples of postmenopausal women, we detected significantly higher levels of FSH and VCAM-1 compared with those from premenopausal women, and there was a positive correlation between these two molecules. In cultured HUVECs, FSH receptor (FSHR) mRNA and protein expression were detected and FSH enhanced VCAM-1 expression. This effect was mediated by the activation of nuclear factor κB (NF-κB), which was sequentially enhanced by the activation of PI3K/Akt/mTOR cascade. FSH first enhanced GαS activity resulting in elevated cAMP level and PKA activity, which relayed the signals from FSHR to the PI3K/Akt/mTOR cascade. Furthermore, FSHR was detected in endothelial caveolae fraction and interacted with caveolin-1 and GαS. The disruption of caveolae or the silencing of caveolin-1 blocked FSH effects on signaling activation and VCAM-1 expression, suggesting the existence of a functional signaling module in membrane caveolae. Finally, FSH increased human monocyte adhesion to HUVECs which was reversed by the VCAM-1 neutralizing antibody.Conclusion: FSHR was located in the membrane caveolae of HUVECs and FSH promoted VCAM-1 expression via FSHR/GαS /cAMP/PKA and PI3K/Akt/mTOR/NF-κB pathway. This may contribute to the deleterious role of FSH in the development of AS in postmenopausal women.