Penn's Renal-Electrolyte and Hypertension Division

Faculty members in the Renal-Electrolyte and Hypertension Division engage in basic laboratory research that results in fundamental discoveries to further advance our understanding of kidney diseases.

Russ Carstens, MD

The focus of Dr. Carstens' laboratory is investigation of alternative splicing, whereby a single gene transcript can generate numerous spliced mRNAs, thereby greatly expanding ribonomic and proteomic diversity. Dr. Carstens' lab is particularly interested in the regulation of cell and tissue-specific splicing choices that are important during development and play roles in cancer progression.

The studies in Dr. Carstens' lab previously focused on alternative splicing of fibroblast growth factor receptor 2 (FGFR2) as a model system. Mutually exclusive splicing of two exons, IIIb and IIIc, gives rise to two functionally different receptors, FGFR2-IIIb and FGFR2-IIIc, in epithelial and mesenchymal cells, respectively. These exons encode the C-terminal half of an Ig-like domain in the receptor's extracellular domain and the resulting receptor isoforms exhibit distinct binding preferences for the FGF family of ligands. The exquisite cell type-specific expression of these epithelial or mesenchymal specific splice variants is essential during vertebrate development. Furthermore, a switch in FGFR2 splicing occurs during the epithelial to mesenchymal transition (EMT), implying that this change in splicing is functionally involved in the EMT during development as well as in pathophysiologic conditions such as cancer metastasis and tissue fibrosis.

In collaboration with John Hogenesch in Pharmacology, the Carstens lab recently used luciferase-based splicing reporter assays to carry out a genome-wide, high throughput cDNA screen for factors that regulate FGFR2 splicing. These studies identified a number of novel mammalian splicing regulators, including two epithelial-specific factors that we named Epithelial Splicing Proteins 1 and 2 (ESRP1 and ESRP2). Expression of these splicing regulators is required for expression of the epithelial FGFR2 splice variant and ongoing work has shown that they regulate the splicing of an extensive epithelial-specific splicing program. These regulated targets include functionally relevant splicing switches that are implicated in the EMT and both ESRPs are transcriptionally inactivated during the EMT. The Carstens lab is currently carrying out massively parallel high throughput sequencing (RNA-seq) to identify an even more comprehensive epithelial splicing regulatory network (SRN). They predict that the proteins encoded by transcripts comprising this epithelial splicing signature will also define a protein interaction network that underlies important epithelial cell properties. Furthermore, they also have strong reasons to believe that, like FGFR2, many of these proteins will exhibit previously unrecognized isoform-dependent differences in function between epithelial and mesenchymal cells. These proteins and networks are likely to have biologically coherent functions that are relevant for the EMT in development and cancer metastasis as well as epithelial cell differentiation in diverse tissues and organs. In their ongoing studies The Carstens lab will further investigate the molecular mechanisms by which the ESRPs and several additional novel splicing regulators cause switches in splicing. They are also generating mice carrying conditional knockout alleles for Esrp1 and Esrp2 (and both) that will be used to create tissue-specific Esrp-knockout mice. These tools will allow them to clarify the roles of these splicing regulators during development and also allow us to profile ESRP-regulated splicing targets in vivo.

Rotation Projects

A number of potential projects to investigate the targets and functions of the ESRPs can be discussed. Such projects include biochemical characterization of the binding sites and mechanisms of function of these factors as well as work with the mouse knockout studies. We are also interested in performing similar studies with two other novel splicing regulators that we also recently identified. Projects to identify small molecules and compounds that regulate splicing using high throughput approaches are also available.

Lawrence Holzman, MD

Dr. Holzman is an established and NIH funded laboratory investigator, well recognized for his investigations of mechanisms of cell signaling and also of podocyte biology, where he has contributed importantly to our understanding of the mechanisms that govern podocyte cytoskeletal architecture.

The Holzman lab studies the biochemistry and function of DLK, a member of the mixed lineage kinase family of MAPK kinase kinases. The laboratory initially discovered and cloned DLK and first demonstrated that DLK is a MAP3 kinase capable of activating the mitogen activated protein kinase family of JNK kinases. It showed that DLK is activated by insulin and in soon to be published work demonstrated that deletion of DLK in mice results in a phenotype of resistance to diet induced obesity and cell autonomous increased insulin sensitivity.

Dr. Holzman also investigates the biology of the glomerular podocyte, a unique epithelial cell that appears to play a central role in most forms of glomerular disease. The octopus-like processes of the podocyte interdigitate and form specialized intercellular junctions that function as the kidney glomerular filter. The lab has been a leader in characterizing the molecular components of this intercellular junction and first established evidence to support the hypothesis that these junctional components participate in regulating podocyte morphology by modulating actin cytoskeletal dynamics. The lab has particularly focused on signaling functions of members of the cell adhesion molecules of the Nephrin family and has contributed seminal work on the atypical cadherin FAT1. As part of this work, the lab has developed a transgenic mouse strategy for examining the functional biology of proteins and their interactions specifically in the podocyte that is now used internationally.

Katalin Susztak, MD, PhD

Dr. Susztak's laboratory is aimed towards understanding the molecular pathways that govern chronic kidney disease development. The detailed work performed in Dr. Susztak's lab ranges from hypothesis generating high throughput translational research to fundamental and mechanistic studies.

The Susztak lab has carefully banked a large number of healthy and diseased human kidney tissue samples over the last 10 years, providing a basis for several combined genetic, epigenetic and genomic analyses. They hypothesize that integrative analysis of epigenetic and genetic settings in diseased cells can provide a rational basis for more accurately modeling the critical biological pathways involved in mediating the progressive phenotype in individual patients. They also predict that epigenomic integrative analysis can be used to determine the identity of chromatin and transcription factors that contribute mechanistically to aberrant transcriptional programming in chronic kidney disease, and that this information can be used for designing therapeutic strategies. The Susztak lab is specifically interested in defining cis-regulatory modules (promoters, enhancers and repressors) that govern the normal and altered epithelial phenotype in diseased kidneys. In addition, they use genetic approaches and the mouse as a model organism to test the role of candidate signaling molecules and regulatory pathways directly in vivo. The Cre/loxP and tet inducible transgenic technologies allow them to analyze the function of particular factors by deleting or overexpressing genes that encode them in specific cell types in the kidney. Specifically, we are working on determining the role of the Notch and Wnt/beta-catenin pathway in chronic kidney disease development, renal epithelial cell homeostasis, renal stem or progenitor cell function and differentiation. Their recent results highlight the role of embryonic programs in adult disease development.

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