Darren P. Wallace

Kidney Institute – Faculty
Nephrology – Associate Professor
University of Kansas Cancer Center Cancer Biology – Full Member
Department of Medicine

Dr. Wallace is a Research Associate Professor in the Department of Medicine and the Kidney Institute at the University of Kansas Medical Center. He received his PhD from the Department of Molecular and Integrative Physiology at KUMC in 1998 and joined the faculty of the Department of Medicine in 2002. Dr. Wallace’s research is funded by an R01 grant from the National Institutes of Health (NIDDK). He is the Director of the PKD Research Biomaterials and Cellular Models Core funded by the PKD Foundation. Additional research support is derived from private foundations and biotechnology companies. Dr. Wallace is also a member of the Scientific Advisory Committee for the PKD Foundation.


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Research Focus

Autosomal Dominant Polycystic Kidney Disease (ADPKD), Renal Physiology

ADPKD is a lethal, hereditary disorder characterized by the formation of 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 for polycystin-1 (PC-1) and polycystin-2 (PC-2), respectively.   Progressive expansion of cysts involves aberrant cell proliferation, loss of planar cell polarity and accumulation of fluid within the cavity due to active transepithelial fluid secretion.  There is elevated extracellular matrix turn-over and tremendous accumulation of extracellular matrix proteins around the cysts.  Basement membranes that line the cystic cells become laminated and fragmented, and mononuclear cells infiltrate the interstitial space adjacent to the cysts.  These changes are forerunners of functional loss of nephrons and chronic kidney disease. 

 

1. Abnormal cell proliferation.  The function of the polycystins remains unclear; however, several studies have indicated that PC-1 and PC-2 interact to form a Ca2+ permeable cation channel.  Recently, 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 renal cell hyperplasia in ADPKD.  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 stimulating the proliferation of cystic cells.  Our central hypothesis is that in ADPKD, dysfunctional Ca2+ metabolism uncovers a cellular pathway leading to cAMP-dependent activation of B-Raf, a key kinase in MEK/ERK activation and cell proliferation. cAMP inhibits ERK and proliferation of normal renal cells through protein kinase A phosphorylation of Raf-1 at a key inhibitory site.  B-Raf is thought to be repressed in normal cells by Ca2+ dependent kinases, possibly Akt.  Treatment of normal renal cells with Ca2+ channel blockers de-repressed B-Raf and caused a phenotypic switch such that cAMP stimulates B-Raf, ERK and cell proliferation, mimicking the ADPKD phenotype. Conversely, treatment of ADPKD cells with Ca2+ channel activator or ionophore repressed B-Raf and prevented cAMP activation of ERK and cell proliferation.  In an animal model of ADPKD, we showed that treatment with a Ca2+ channel blocker increased the cystic disease and accelerated the decline in renal function.  Current studies in my laboratory are focused on the investigation of cellular and molecular mechanisms involved in the regulation of the Raf/MEK/ERK pathway by cAMP and Ca2+ signaling pathways in normal and ADPKD cells. The identification of key regulatory components of this pathway will assist in the design of small-molecule therapies to slow the proliferation of cystic cells and retard disease progression. 

 

2. Epithelilal fluid secretion.  ADPKD cysts can expand to several centimeters in diameter due to the accumulation of fluid within the enlarging cavity involving active transepithelial fluid secretion. We used modified Ussing chambers and transepithelial voltage clamp devices to measure the electrical properties of human ADPKD cell monolayers.  These studies showed that fluid secretion by the cyst-lining cells was driven by cAMP-dependent transepithelial Cl- secretion involving Cl- uptake via basolateral Na+,K+,2Cl- cotransporter (NKCC1) and Cl- loss across the apical membrane via CFTR Cl- channels.  We are currently investigating cellular mechanisms that regulate anion and fluid secretion by human ADPKD cells and testing compounds that target key components in the transport process.   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.

 

3. Matrix remodeling.  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 nephrons.  Cystic cells are characterized by incomplete cellular differentiation and aberrant expression of proteins, including ECM molecules.  To uncover potential pathways involved in cyst formation, we used differential microarray analysis to compare autonomous gene expression in cultured ADPKD and normal human kidney cells.  We found that the aberrant expression of ECM molecules was remembered by the ADPKD cystic cells 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 (osteoblast-specific factor-2), a protein originally thought to be a bone factor, was highly overexpressed in the ADPKD cells. Periostin binds αV-integrins leading to activation of integrin-linked kinase (ILK).  ILK phosphorylates GSK-3β causing β-catenin stabilization, increased nuclear β-catenin and activation of transcription factors involved in cell proliferation.  Our hypothesis is that aberrant expression of ECM molecules, including periostin, accelerates cyst growth and contributes importantly to structural changes in the kidneys, including interstitial remodeling and fibrosis. 

Renal Physiology

My research interest is also in the investigation of renal mechanisms for salt and water balance, in particular the role of the collecting duct system in urine formation. Mammalian kidneys regulate salt and water excretion with extraordinary precision, balancing urinary excretion to salt and water intake. Despite the clinical importance, there is incomplete understanding of how the kidney precisely controls salt balance around a net urinary NaCl excretion equal to 0.5 to 1 % of the filtered load. The classical view of salt and water regulation focuses on mechanisms that control renal blood flow, glomerular filtration and salt absorption. It is well established that collecting ducts reabsorb vast quantities of salt and water. We discovered, unexpectedly, that collecting ducts have an intrinsic capacity for salt and fluid secretion, thus harboring a counteractive force to NaCl absorption. cAMP plays a central role in controlling net transport by activating transepithelial Cl- secretion. Ammonium which is normally found in relatively high concentrations within the interstitial fluid of the renal inner medulla amplifies cAMP mediated anion and fluid secretion. Our central hypothesis is that inner medullary collecting ducts, the last nephron segments in contact with the tubular fluid, make the final adjustments to the tubular fluid before it is excreted as urine.

Last modified: Apr 29, 2013

D Wallace

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Darren P. Wallace
Kidney Institute – Faculty
Nephrology – Associate Professor
University of Kansas Cancer Center Cancer Biology – Full Member

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