Thomas K. Shires, Ph.D.

David Sheff, M.D., Ph.D.
Assistant Professor
Ph.D. (biochemistry)
Iowa, 1992
M.D., Iowa, 1992
E-mail: david-sheff@uiowa.edu
Office: 2-570 BSB
Phone: (319) 335-7705

Receptor Traffic in the Endosomal System

The mechanisms by which cells generate and maintain polarity are among the most fundamental in all of biology. Especially intriguing, and pharmacologically important, is the question of how cells maintain the different compositions of their apical and basolateral surfaces in the face of rapid and continuous internalization of the plasma membrane. My research is focused on how proteins internalized from either surface navigate the endosomal system to return to the cell surface. To approach these problems, I use a combination of cutting edge digital microscopy, proteomic analysis and biochemical assays.

Most drugs, hormones, chemical transmitters and other ligands operate by binding to receptors on the plasma membrane of target cells. Receptors are continuously internalized and delivered to early endosomes. To maintain receptor levels at the cell surface, the receptors are recycled to the plasma membrane. Receptors recycle either directly to the plasma membrane or through recycling endosomes. This second pathway is important because receptors and membrane are stored in recycling endosomes.

Polarized cells such as epithelial cells, neurons, or hepatocytes are more complex. The plasma membranes of these cells have apical and basolateral domains with different protein and lipid compositions. Receptors from each domain are internalized to separate populations of early endosomes, but traffic from both surfaces meets in a single set of recycling endosomes on the apical side of the nucleus. Surprisingly, receptors from this common compartment are sorted and delivered back to the correct plasma membrane domain. A receptor may pass through recycling endosomes over 100 times during its lifetime, so that even minimal mis-sorting as occurs in familial hypercholesterolemia, familial hypertension and some forms of cystic fibrosis, causes life-threatening disease.

The process by which recycling endosomes sort proteins for apical or basolateral delivery is little understood. While the signals governing sorting are known, the mechanism for interpreting those signals is not. We have recently observed that apical and basolateral receptors segregate within recycling endosomes to form distinct apical and basolateral subdomains. Other proteins, such as Rab11 and Rab8 GTPases colocalize with the apical and basolateral subdomains suggesting that they may play a role in sorting of the receptors. These findings allow us for the first time to correlate morphological data within the recycling endosomes with quantitative kinetic data derived from the effects of mutant Rab proteins on receptor recycling traffic. As a result we can begin dissect what role each protein may play in sorting of apical from basolateral traffic.
While the aim of this research is a fundamental understanding of basic cellular processes, many of the proteins involved in recycling endosome traffic regulation may themselves be useful targets for new therapeutic drugs. Such drugs would change levels of key receptors by altering traffic at the endosomal level or by retargeting receptors to the opposite plasma membrane domain.

Representative Publications:

Pelletier, L., Stern, C.A., Pypaert, M., Sheff, D., Ngo, H.M., Roper, N., He, C.Y., Hu, K., Toomre, D., Coppens, I., Roos, D.S., Joiner, K.A. and Warren, G.: Golgi biogenesis in Toxoplasma gondii. Nature 418:548-552, 2002.

Sheff, D.R., Pelletier, L., O'Connell, C., Warren, G. and Mellman, I.: Transferrin receptor recycling in the absence of perinuclear recycling endosomes. J Cell Biol 156:797-804, 2002.

Sheff, D.R., Krochewski, R. and Mellman, I.: Actin dependence of polarized receptor recycling in MDCK cell endosomes. Mol Biol Cell 13:262-275, 2002.

Sheff, D.R., Daro, E.A. and Mellman, I.: The receptor recycling pathway contains two distinct populations of early endosomes with different sorting functions. J Cell Biol 145:123-139, 1999.


	David Sheff, M.D., Ph.D. Assistant Professor Ph.D. (biochemistry) Iowa, 1992 M.D., Iowa, 1992 E-mail: david-sheff@uiowa.edu Office: 2-570 BSB Phone: (319) 335-7705
Receptor Traffic in the Endosomal System
The mechanisms by which cells generate and maintain polarity are among the most fundamental in all of biology. Especially intriguing, and pharmacologically important, is the question of how cells maintain the different compositions of their apical and basolateral surfaces in the face of rapid and continuous internalization of the plasma membrane. My research is focused on how proteins internalized from either surface navigate the endosomal system to return to the cell surface. To approach these problems, I use a combination of cutting edge digital microscopy, proteomic analysis and biochemical assays. Most drugs, hormones, chemical transmitters and other ligands operate by binding to receptors on the plasma membrane of target cells. Receptors are continuously internalized and delivered to early endosomes. To maintain receptor levels at the cell surface, the receptors are recycled to the plasma membrane. Receptors recycle either directly to the plasma membrane or through recycling endosomes. This second pathway is important because receptors and membrane are stored in recycling endosomes. Polarized cells such as epithelial cells, neurons, or hepatocytes are more complex. The plasma membranes of these cells have apical and basolateral domains with different protein and lipid compositions. Receptors from each domain are internalized to separate populations of early endosomes, but traffic from both surfaces meets in a single set of recycling endosomes on the apical side of the nucleus. Surprisingly, receptors from this common compartment are sorted and delivered back to the correct plasma membrane domain. A receptor may pass through recycling endosomes over 100 times during its lifetime, so that even minimal mis-sorting as occurs in familial hypercholesterolemia, familial hypertension and some forms of cystic fibrosis, causes life-threatening disease. The process by which recycling endosomes sort proteins for apical or basolateral delivery is little understood. While the signals governing sorting are known, the mechanism for interpreting those signals is not. We have recently observed that apical and basolateral receptors segregate within recycling endosomes to form distinct apical and basolateral subdomains. Other proteins, such as Rab11 and Rab8 GTPases colocalize with the apical and basolateral subdomains suggesting that they may play a role in sorting of the receptors. These findings allow us for the first time to correlate morphological data within the recycling endosomes with quantitative kinetic data derived from the effects of mutant Rab proteins on receptor recycling traffic. As a result we can begin dissect what role each protein may play in sorting of apical from basolateral traffic. While the aim of this research is a fundamental understanding of basic cellular processes, many of the proteins involved in recycling endosome traffic regulation may themselves be useful targets for new therapeutic drugs. Such drugs would change levels of key receptors by altering traffic at the endosomal level or by retargeting receptors to the opposite plasma membrane domain. Representative Publications: Pelletier, L., Stern, C.A., Pypaert, M., Sheff, D., Ngo, H.M., Roper, N., He, C.Y., Hu, K., Toomre, D., Coppens, I., Roos, D.S., Joiner, K.A. and Warren, G.: Golgi biogenesis in Toxoplasma gondii. Nature 418:548-552, 2002. Sheff, D.R., Pelletier, L., O'Connell, C., Warren, G. and Mellman, I.: Transferrin receptor recycling in the absence of perinuclear recycling endosomes. J Cell Biol 156:797-804, 2002. Sheff, D.R., Krochewski, R. and Mellman, I.: Actin dependence of polarized receptor recycling in MDCK cell endosomes. Mol Biol Cell 13:262-275, 2002. Sheff, D.R., Daro, E.A. and Mellman, I.: The receptor recycling pathway contains two distinct populations of early endosomes with different sorting functions. J Cell Biol 145:123-139, 1999.