The Michael Forgac Lab
V-ATPase Structure, Function and Regulation
The focus of our laboratory is on understanding the function and regulation of a novel family of ATP-dependent proton pumps known as the vacuolar ATPases (or V-ATPases). Acidification of intracellular compartments by the V-ATPases is important for such processes as receptor-mediated endocytosis, intracellular membrane traffic, protein processing and degradation and the coupled transport of small molecules, such as neuro-transmitters (Fig.1). Acidic compartments also provide the entry point for disease causing agents such as influenza virus and anthrax toxin. V-ATPases in the plasma membrane of certain cells function in bone resorption, renal acidification and tumor cell invasion. Our laboratory is interested in both the regulation of V-ATPase activity and the role of V-ATPases in the processes of energy homeostasis and tumor growth and metastasis.
Figure 1. Shown are the main functions of intracellular V-ATPases. Taken from Nishi & Forgac, 2002.
The V-ATPases are large, multi-subunit complexes composed of a peripheral V1 domain that hydrolyzes ATP and an integral V0 domain that transports protons. An important mechanism of regulating V-ATPase activity is through control of assembly of these two domains (Fig. 2). We have identified the Ras/cAMP/PKA pathway as a crucial regulator of assembly in yeast and have shown that PI-3 kinase and mTORC1 are important in regulating V-ATPase assembly and activity during dendritic cell maturation, a process that is essential for antigen processing. We have also recently identified amino acid starvation as a novel activator of assembly and activity of lysosomal V-ATPases in mammalian cells, a change that likely functions in amino acid homeostasis. We are exploring the signaling pathways controlling V-ATPase assembly and activity in response to changes in nutrient availability and the role of V-ATPases in controlling nutrient sensing signaling molecules, including AMPK and mTORC1.
Figure 2. Regulated assembly of the V-ATPase represents an important mechanism of controlling V-ATPase activity in cells. Taken from Forgac, 2007.
The other major focus of our laboratory is understanding the role of V-ATPases in tumor growth and metastasis (Fig.3). We have demonstrated that highly invasive human breast tumor cells express high levels of a V-ATPase subunit a isoform (a3) that directs the V-ATPase to the plasma membrane. Importantly, knockdown of a3 reduces both in vitro invasion and plasma membrane expression of V-ATPases, while overexpression of a3 has the opposite effects. Most recently we have shown overexpression of a3 in human breast tumors. We are testing the role of a3 in promoting breast tumor growth and metastasis in an in vivo mouse model with the goal of developing the V-ATPase as a novel and effective target for the treatment of breast cancer.
Figure 3. Plasma membrane V-ATPases may promote tumor cell invasion by activating secreted proteases. Taken from McGuire, et al 2017.