Gerard Apodaca, Ph.D.

Stretch-regulated membrane traffic in bladder umbrella cells

Umbrella cells are the outermost cell layer of the the urothelium, a stratified epithelium that lines the renal pelvis, ureters, and bladder. These cells form a tight barrier that prevents solute flow and toxic metabolites in the urine from entering the underlying blood supply. Components of this barrier include tight junctions and an apical membrane with a specialized lipid and protein composition. An important aspect of the umbrella cell barrier is that it must accommodate large changes in volume as the bladder fills and empties. One mechanism used by these cells is massive exocytosis of a large population of subapical discoidal/fusiform vesicles (DFVs), which significantly increases the apical surface area of the cell. Coupled with changes in cell shape and expansion of the tight junction ring, the increased apical surface area allows for increased urine storage. Upon voiding, the added apical membrane must be recovered, a process we have shown is mediated by a clathrin-independent endocytic pathway that is regulated by the dynamin GTPase, RhoA, and the beta1 integrin. Some of our current research projects include: defining the mechanosensors and signal transduction cascades that occur in response to stretch; exploring the regulatory Rab GTPases, their effectors, and how stretch regulates these Rab-dependent signaling cascades; revealing how membrane stretch is coupled to integrin signaling and stimulation of the endocytic machinery; exploring the fate of endocytosed membrane and fluid in umbrella cells.

Regulation of tight junction morphology and function in response to stretch

Tight junctions (TJs) encircle adjacent epithelial cells, forming a seal that prevents the unregulated flux of ions, organic solutes, and water across the paracellular space. TJs are formed by the close apposition of anastomosing filamentous strands between the plasma membranes of neighboring cells. Claudins, major components of the strands, promote cell-cell adhesion and control paracellular permeability by forming anion-selective, cation-selective, or occluding pores. A critical feature of TJs is that they must maintain function in the face of mechanical stretch or shear stress, forces that are commonplace as air fills the lungs, as solids move along the epithelial surfaces of the gastrointestinal tract, or as fluid passes through the nephrons of the kidney or accumulates in sac-like organs such as the bladder. At present, surprisingly little is understood about how mechanical forces affect TJ activity (i.e. permeability), remodeling, and stability. Ongoing research projects in the laboratory include identification of umbrella cell claudins and defining their contribution to the umbrella cell TJ barrier, understanding the dynamics of the TJ during bladder filling and voiding, and understanding how solute flow across the TJs controls bladder function by affecting the activity of the afferent nerve processes that innervate the urothelium.

Role of uroplakins in urinary tract development and congenital anomalies of the kidney and urinary tract (CAKUT)

CAKUT are developmental disorders that occur in 1 out of every 500 live births, yet the cellular or molecular basis of these malformations is not well understood. One gene targeted in CAKUT is UPK3a, which encodes the type I transmembrane protein uroplakin 3a (UPK3a). For example, a substitution of a Pro residue for Leu (P273L) in the cytoplasmic domain of human UPK3a leads to renal adysplasia and other urinary tract defects. To better understand why UPK3a expression is important, we have examined the function of Upk3l (aka UPK3d), the UPK3a-like ortholog in zebrafish. Upk3l is expressed at the apical surfaces of the pronephric tubule-associated epithelial cells that form the zebrafish larval urinary tract (i.e., pronephros). Strikingly, loss of Upk3l expression leads to altered epithelial differentiation, including the aberrant expression and distribution of polarity proteins, as well as defects in morphogenesis, including loss of apical microvilli. Current projects in the lab include determining whether UPK3a/Upk3l interacts with Par polarity proteins, identifying how polarity proteins contribute to the formation of the microvillar brush border, and defining whether the P273L mutation of UPK3a causes disease as a result of aberrant and/or defective