MCDB Home > Faculty > Thomas Weimbs > Current Research

Research

Our research focus is divided into two related areas. One is the investigation of the role of the SNARE membrane fusion machinery in polarized epithelial cells. The other is the investigation of the function of polycystin-1, the protein affected in autosomal-dominant polycystic kidney disease.

SNAREs and epithelial cell polarity

The majority of human cell types are polarized, i.e. they exhibit asymmetry, which is essential to their function. This includes epithelial cells that form barriers between the outside world and the underlying basement membrane and connective tissue. Epithelial cells are the functional units of most major human organs such as the kidney, liver, lungs, gastrointestinal tract, exocrine glands etc. The function of epithelial cells depends on their ability to form two distinct surfaces: the apical and basolateral plasma membrane domains which differ in their protein composition and function.

Establishment and maintenance of this so-called "cell polarity" depends on the precise targeting of proteins to the apical and basolateral plasma membrane domains using vesicular transport pathways. Understanding the mechanisms that underlie polarized trafficking is of fundamental importance to understand function and dysfunction of polarized cells.

Newly synthesized plasma membrane proteins are sorted inside epithelial cells and are specifically targeted to either the apical or basolateral surface by vesicular transport. We are particularly interested in the mechanism of membrane fusion between a transport vesicle and its target membrane. Membrane fusion involves the so-called "SNARE" machinery which consists of specific proteins on the vesicle membrane (v-SNAREs) and on the target membrane (t-SNAREs).

We have found that two SNAREs of the syntaxin family, syntaxin 3 and 4, specifically localize to the apical and basolateral plasma membrane, respectively. We are investigating how these syntaxins themselves are targeted and what role their function plays for the overall establishment and maintenance of cell polarity.

Polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is one of the most common monogenic inherited human diseases. During disease progression, renal epithelial cells partially lose their cell polarity and proliferate to form fluid-filled cysts that eventually replace most of the normal renal tissue resulting in renal failure in 50% of the patients by age 50. Currently, no treatment exists to prevent or slow cyst formation, and most ADPKD patients require renal transplantation or life-long hemodialysis for survival. Mutations in either of two genes, PKD1 and PKD2, are the underlying cause of ADPKD, with PKD1 mutations accounting for over 85% of the cases. The function of the PKD1 gene product, polycystin-1 (PC1), is poorly understood. PC1 is a large integral membrane protein with putative extracytoplasmic ligand-binding domains, but no physiological ligand has clearly been identified to date.

We have found that PC1 regulates two different signaling processes via different parts of its C-terminal cytoplasmic tail. In one of these novel signaling pathways, PC1 transduces a mechanical signal from primary cilia of renal epithelial cells to changes in gene expression. We found that PC1 undergoes proteolytic cleavage of its cytoplasmic tail followed by nuclear translocation, and have identified the transcriptional machinery that interacts with the PC1 tail and mediates PC1-regulated gene expression. The PC1 tail regulates nuclear transcription in response to the cessation of lumenal/apical fluid flow. A different part of the PC1 tail regulates the activity of the Ser/Thr kinase mTOR via the protein tuberin that is implicated in the disease tuberous sclerosis complex. Aberrant mTOR activation appears to play a critical role in cell growth and proliferation during renal cystogenesis.

We are further investigating these novel functions of PC1 using biochemical and cell biological methods and using animal models. These signaling functions of PC1 may reveal promising targets for strategies for therapeutic intervention for ADPKD.