Darren P. Wallace, Ph.D.
Departments of Internal Medicine
Molecular and Integrative Physiology (secondary appointment)
Director, PKD Biomarkers and Biomaterials Core
Associate Director, Kansas PKD Research and Translation Core Center
Jared Grantham Kidney Institute, Division of Nephrology and Hypertension
School of Medicine, Internal Medicine
University of Kansas Medical Center - Ph.D. (physiology)
University of Kansas Medical Center - Postdoctoral Fellowship
B.A., University of Kansas, Lawrence, KS
Autosomal dominant polycystic kidney disease (ADPKD) is a lethal, inherited disorder characterized by the formation and growth of numerous fluid-filled cysts, massively enlarged kidneys, and progressive loss of renal function. The disease is caused by mutations in PKD1 or PKD2, genes that encode polycystin-1 (PC-1) and polycystin-2 (PC-2), respectively. The proteins are thought to directly interact to form a Ca2+ channel that is important in intracellular Ca2+ homeostasis and signaling. Progressive expansion of cysts involves aberrant cell proliferation and the accumulation of fluid within the developing cyst cavity. As cysts grow, extracellular matrix (ECM) remodeling and the accumulation of EMC proteins lead to tissue fibrosis and a decline in kidney function.
Abnormal cell proliferation. We showed that epithelial cells isolated from human ADPKD cysts have reduced intracellular Ca2+ levels compared to normal human kidney cells, suggesting that dysfunctional intracellular Ca2+ regulation was associated with cyst formation. Adenosine 3', 5' cyclic monophosphate (cAMP) agonists, including arginine vasopressin (AVP), accelerate cyst growth by activating the MEK/extracellular signal-regulated kinase (ERK) pathway and proliferation of cystic cells. Our central hypothesis is that dysfunctional Ca2+ metabolism, secondary to mutations in the PKD gene, uncovers a cellular pathway leading to cAMP-dependent activation of BRAF, a kinase that activates MEK/ERK signaling. cAMP inhibits ERK and proliferation of normal renal cells through protein kinase A phosphorylation of RAF-1. In normal cells, BRAF is repressed by Ca2+ -dependent kinases; therefore, cAMP is not mitogenic. We showed that treatment of normal human kidney cells with a Ca2+ channel blocker de-repressed BRAF and caused a phenotypic switch such that cAMP stimulated BRAF, ERK and cell proliferation, mimicking the ADPKD phenotype. Conversely, treatment of ADPKD cells with Ca2+ channel activator or ionophore repressed BRAF and prevented cAMP activation of ERK and cell proliferation. In an animal model of ADPKD, treatment with a Ca2+ channel blocker increased cystic disease and accelerated the decline in renal function. Recently, we found that specific expression of activated BRAF (V600E) in collecting ducts was sufficient to induce cyst formation in otherwise normal mice and to accelerate disease progression in PKD mice. Identification of key regulatory components of the cAMP, Ca2+ and the BRAF/MEK/ERK pathways will assist in the design of small-molecule therapies to slow the proliferation of cystic cells.
Epithelilal fluid secretion. ADPKD cysts expand to several centimeters in diameter due to the accumulation of fluid within the cyst cavity involving transepithelial fluid secretion. We showed that fluid secretion by cyst-lining cells was driven by cAMP-dependent transepithelial Cl- secretion, involving Cl- uptake via basolateral Na+,K+,2Cl- cotransporter (NKCC1) and Cl- efflux across the apical membrane via CFTR Cl- channels. Development of therapies that selectively inhibit fluid secretion by cystic cells may reduce the size of the cysts, thus limiting nephron loss and preserving normal parenchyma.
ECM/integrin signaling. During renal development, nephron formation involves a coordinated orchestration of cell proliferation, differentiation, apoptosis, and angiogensis; processes that are regulated by diverse proteins expressed by tubule epithelial cells. Extracellular matrix (ECM) molecules and autocrine factors play important roles in the growth and elongation of these structures to form renal tubules with a relatively uniform diameter of ~40 microns. In ADPKD, cysts expand to several centimeters in diameter and disrupt the function of neighboring nephrons. We found that the aberrant expression of ECM molecules was remembered by ADPKD cells grown in culture. There was overexpression of structural ECM proteins, including type I and III collagens and laminin, and soluble ECM-associated proteins, including metalloproteinases, a disintegrin and metalloproteinases (ADAMs) and TGF-β. Surprisingly, mRNA for periostin (POSTN), a repair molecule, was highly overexpressed by cystic epithelial cells from ADPKD, autosomal recessive PKD and several murine models of renal cystic disease. Periostin binds αV-integrins leading to activation of integrin-linked kinase (ILK), a scaffold protein that forms a complex with PINCH and α-parvin to regulate the actin cytoskeleton and pathways involved in tissue repair. Our hypothesis is that aberrant expression of periostin accelerates cyst growth and contributes to structural changes in the kidneys, including interstitial remodeling and fibrosis.
PKD Biomarkers and Biomaterials Core. This Core, directed by Dr. Wallace, is part of the Kansas PKD Research and Translational Core Center funded by a NIH P30 grant. The Core provides a comprehensive list of biomaterials for PKD research, including serum, plasma, urine, urinary exosomes and clinical information such as GFR and total kidney volume (TKV) from individuals enrolled in an observational clinical trial "Early-Stage Polycystic Kidney Disease Biomarkers Repository Study" (NCT02936791; D.P. Wallace, P.I.), as well as serum, plasma, urine and clinical information from patients with established disease who attend the KUMC PKD clinic. In addition, the Core provides frozen and para-formaldehyde-fixed-paraffin blocked tissues, microscope slides with tissue slices, primary ADPKD and normal human kidney cells, individual and pooled cyst fluids, and RNA and protein lysates from fresh ADPKD and normal kidneys.
Complete list of publications: Click Here