Education & Certifications
Postdoctoral, Stanford University, Genetics (2011)
Ph.D., The University of Texas Medical Branch, Cell Biology (2005)
B.S., Grand Valley State University, Biology (1998)
Mertk is a key phagocytic receptor in the immune, male reproductive, and visual systems. In the retinal pigment epithelium, Mertk is required for the daily ingestion of photoreceptor outer segment (OS) tips. Loss of Mertk function causes retinal degeneration in rats, mice, and humans; however, little is known about the mechanism by which Mertk regulates the ingestion phase of retinal pigment epithelial (RPE) phagocytosis. To address this, the authors sought proteins that associated with Mertk during OS phagocytosis.Lysates of RPE-J cells challenged with OS for various times were immunoprecipitated with Mertk antibody. Potential interacting proteins were identified by mass spectrometry and characterized with confocal microscopy, pharmacologic inhibition, and siRNA knockdown coupled with an in vitro phagocytic assay in primary RPE cells.Myh9, the non-muscle myosin II-A heavy chain, was enriched in immunoprecipitates from OS-treated samples. Myosin II-A and II-B isoforms exhibited a striking redistribution in wild-type rat primary RPE cells challenged with OS, moving from the cell periphery to colocalize with ingested OS over time. In contrast, myosin II-A redistribution in response to OS was blunted in primary RPE cells from RCS rats, which lack functional Mertk. Wild-type rat primary RPE cells treated with the myosin II-specific inhibitor blebbistatin or myosin II siRNAs exhibited a significant phagocytic defect.Mertk mobilizes myosin II from the RPE cell periphery to sites of OS engulfment, where myosin II function is essential for the normal phagocytic ingestion of OS.
View details for DOI 10.1167/iovs.08-3058
View details for PubMedID 19117932
Sorting endosomes and the endocytic recycling compartment are critical intracellular stores for the rapid recycling of internalized membrane receptors to the cell surface in multiple cell types. However, the molecular mechanisms distinguishing fast receptor recycling from sorting endosomes and slow receptor recycling from the endocytic recycling compartment remain poorly understood. We previously reported that Rab15 differentially regulates transferrin receptor trafficking through sorting endosomes and the endocytic recycling compartment, suggesting a role for distinct Rab15-effector interactions at these endocytic compartments. In this study, we identified the novel protein Rab15 effector protein (REP15) as a binding partner for Rab15-GTP. REP15 is compartment specific, colocalizing with Rab15 and Rab11 on the endocytic recycling compartment but not with Rab15, Rab4, or early endosome antigen 1 on sorting endosomes. REP15 interacts directly with Rab15-GTP but not with Rab5 or Rab11. Consistent with its localization, REP15 overexpression and small interfering RNA-mediated depletion inhibited transferrin receptor recycling from the endocytic recycling compartment, without affecting receptor entry into or recycling from sorting endosomes. Our data identify REP15 as a compartment-specific protein for receptor recycling from the endocytic recycling compartment, highlighting that the rapid and slow modes of transferrin receptor recycling are mechanistically distinct pathways.
View details for DOI 10.1091/mbc.E05-03-0204
View details for Web of Science ID 000233626000021
View details for PubMedID 16195351
Receptor recycling has emerged as an important regulatory mechanism for cell surface composition, pathogen invasion, and for control over the intensity and duration of receptor signaling in multiple cell types. In the case of the transferrin receptor, receptor recycling is an important step for facilitating iron uptake into the cell, by regulating the availability of the receptor at the cell surface. Following internalization into clathrin-coated pits, the transferrin receptor first enters peripheral sorting endosomes. Here, internalized transferrin receptor is either sorted for recycling back to the cell surface directly, or targeted to a slower route of recycling through a perinuclear population of endosomes termed the endocytic recycling compartment. This chapter describes methodologies to examine the fast and slow modes of transferrin receptor recycling, with a particular emphasis on the function of the novel protein Rab15 effector protein.
View details for DOI 10.1016/S0076-6879(05)03064-8
View details for Web of Science ID 000234855400064
View details for PubMedID 16473635
Rab15 is a novel endocytic Rab that counters the stimulatory effect of Rab5-GTP on early endocytic trafficking. Rab15 may interfere with Rab5 function directly by sequestering Rab5 effectors or indirectly through novel sets of effector interactions. To distinguish between these possibilities, we examined the effector binding properties of Rab15. Rab15 does not interact directly with the Rab5 effectors rabex-5 and rabaptin-5 in a yeast two-hybrid binding assay. Rather mammalian suppressor of Sec4 (Mss4) was identified as a binding partner for Rab15. Mss4 preferentially binds GDP-bound (T22N) and nucleotide-free (N121I) Rab15, consistent with the proposed role of Mss4 as a chaperone that stabilizes target Rabs in their nucleotide-free form. Mutational analysis of Rab15 indicates that lysine at position 48 (K48Q) is important for the binding of Rab15-GDP to Mss4. Moreover, the mutation K48Q counters the inhibitory phenotype of wild type Rab15 on receptor-mediated endocytosis in HeLa cells and homotypic endosome fusion in vitro without altering the relative amount of cell surface-associated transferrin receptor. Together, these data indicate a novel role for Mss4 as an effector for Rab15 in early endocytic trafficking.
View details for DOI 10.1074/jbc.M205101200
View details for Web of Science ID 000177859000043
View details for PubMedID 12105226