Research in my laboratory is aimed at learning how components of the endomembrane and cytoskeletal systems function in concert to synthesize, modify, compartmentalize, and unidirectionally transport products to the cell surface for secretion. Alterations in endomembrane function caused by tumorigenic transformation also is a subject of interest to my group. In our research we use tissue culture, cell fractionation, electron microscopic, immunocytochemical, and many different biochemical methods.

Much of my research has been with the mammary gland and with epithelial cells from mammary gland. During lactation, mammary epithelial cells secrete a number of products including proteins, lactose and lipids, as well as water and minerals. Products are secreted constitutively and continuously over long time periods, and endomembrane differentiation and flow occurs rapidly in these cells. Proteins and lactose are secreted by the classic Golgi apparatus to secretion vesicle to plasma membrane exocytic pathway, but these cells have evolved a unique mechanism for secretion of lipids. Lipids, primarily triacylglycerols, aggregate into surfactant-surrounded droplets that are discharged from cells by progressive envelopment in differentiated regions of apical plasma membrane. In many respects this mechanism for lipid droplet secretion resembles the process by which enveloped viruses are released from infected cells.

Procedures for isolation of endoplasmic reticulum, Golgi apparatus, transition vesicles, secretion vesicles, plasma membranes, and lipid droplet precursors of milk lipid globules from mammary epithelial cells have been developed in my laboratory. Cell-free systems utilizing these isolated components to reconstitute individual steps in product formation and transit have been developed. These systems have enabled us to identify a number of constituents involved in these processes, to explore the role of signalling molecules and second messengers in these processes, and to begin to understand what controls the rate of formation and secretion of milk constituents.

Morre� D.J., J. Lawler, S. Wang, T.W. Keenan and D. M. Morre� (2002) Entrainment in solution of an oscillating NADH oxidase activity from the bovine milk fat globule membrane with a temperature-compensated period length suggestive of an ultradian time-keeping (clock) function. Biochim. Biophys. Acta 1559:10-20.   [Abstract]

Keenan, T.W. (2001) Milk lipid globules and their surrounding membrane: A brief history and perspectives for future research. J. Mammary Gland Biol. Neoplasia 6:365-371.   [Abstract]

Keenan, T. W., Winter, S., Rackwitz, H.-R., and Heid, H. W. (2000) Nuclear Coactivator Protein P100 is Present in Endoplasmic Reticulum and Lipid Droplets of Milk Secreting Cells. Biochim. Biophys. Acta 1523:84-90.   [Abstract]

Heid, H. W., Moll, R., Schwetlick, I., Rackwitz, H.-R., and Keenan, T. W. (1998) Adipophilin is a specific marker of lipid accumulation in diverse cell types and diseases. Cell Tissue Res. 294:309-321.   [Abstract]

Mather, I. H. and Keenan, T. W. (1998) Origin and secretion of milk lipids. J. Mammary Gland Biol. Neoplasia 3:259-273.   [Abstract]

Mather, I. H. and Keenan, T. W. (1998) The cell biology of milk secretion: Historical notes. J. Mammary Gland Biol. Neoplasis 3:227-232.   [Abstract]

Keenan, T. W. (1998) Biochemistry of the Golgi apparatus. J. Histochem. Cell Biol. 109:505-516.   [Abstract]

Hill, D. R., Keenan, T. W., Helm, R. F., Potts, M., Crowe, L. M., and Crowe, J. H. (1997) Extracellular polysaccharide of Nostoc commune (Cyanobacteria) inhibits fusion of membrane vesicles during dessication. J. Appl. Phycol. 9:237-248.   [Abstract]

Morre, D. J. and Keenan, T. W. (1997) Membrane flow revisited. BioScience 47:489-498.

Heid, H. W., Schnolzer, M., and Keenan, T. W. (1996) Adipocyte differentiation-related protein is secreted into milk as a constituent of milk lipid globule membrane. Biochem. J. 320:1025-1030.   [Abstract]