Stefan Riedl's Research Focus
All cellular processes, including cell death, proliferation, motility and inflammation, are governed by molecular signaling pathways. At the heart of these pathways are protein signaling complexes and platforms that act as regulatory nodes to orchestrate activation, inhibition or crosstalk of different signaling networks. The goal of the Riedl lab is to elucidate the structure and mechanisms underlying the regulation of such signaling proteins and complexes in respect to their function in cell death, proliferation, motility and inflammation. To achieve this, the Riedl lab uses X-ray crystallography in combination with biochemistry, chemistry and cell biology. The unique and strongly collaborative environment at Sanford-Burnham is ideally suited for this multi-disciplinary approach. Ultimately, the knowledge gained will help us understand cellular signaling units and can additionally help to develop compounds that combat diseases such as cancer, immune deficiencies and neurological disorders.
Stefan Riedl's Research Report
Modulators of cell death and inflammation: death domain agglomerates
Members of the death domain protein superfamily can form large signaling agglomerates, such as the Death Inducing Signaling Complexes (DISCs), which are key players in the balance of cell death, inflammation and proliferative signaling. Understanding the regulation of these signaling platforms is a main goal of our lab. Our structure of a conditional Fas/FADD death domain complex, which is a key component in the Fas/FADD DISC, led to a new understanding of how the initiation of the extrinsic pathway of apoptosis can be triggered solely by receptor clustering. Another example of assemblies mediated by death domain components are the so-called ASC inflammasomes. Apoptosis speck-like protein (ASC) is a small modulator protein that consists of a pyrin (PYD) domain and a caspase activation and recruitment domain (CARD). ASC can form speck-like agglomerates termed inflammasome foci, which are capable of activating caspase-1, a key inflammatory caspase. ASC inflammasome formation represents a key element of innate immunity and is also involved in a variety of other cellular processes, including specific forms of cell death. We seek to understand the mechanisms underlying ASC inflammasome formation and have adapted a bimolecular fluorescence complementation (BiFC) assay system to monitor this process in cells, since the highly oligomeric and difficult nature of ASC inflammasomes limits investigation by classic in vitro and structural approaches.
Nod-like receptors: initiators of inflammasomes
While ASC foci can be formed solely by ASC, the initiation of many inflammasome agglomerates is triggered by members of the Nod-like receptor (NLR) family. These multi-domain proteins are relatives of the apoptosome forming apoptotic protease-activating factor 1 (Apaf-1), whose structure was elucidated by Dr. Riedl. Apaf-1 is a member of the extended ATPases Associated with various cellular Activities (AAA+) superfamily of proteins and exists as an auto-inhibited monomer, which when triggered by a cell death signal undergoes conformational changes to form a heptameric oligomer with a defined architecture. This heptameric Apaf-1 represents the central unit of the apoptosome and subsequently uses CARD domains to recruit and activate caspase-9, ultimately leading to the controlled apoptotic demise of the cell. Unlike the case of Apaf-1 and the apoptosome, high resolution insight is lacking for members of the NLR family or their oligomeric assemblies, which are the central trigger of infammasome formation. We aim to elucidate the structure of NLR members in both monomeric and oligomeric states, and combine this with complementary biochemical and biological investigations to understand the exact mechanism underlying NLR signaling.
ERK cascade: regulation by the death effector domain protein PEA-15
As an extension of our interest in the regulation of critical enzymes in cell signaling cascades, we aim to investigate key regulatory complexes in the ERK MAP kinase pathway, which is a crucial mediator of proliferation and survival. While significant insight at atomic resolution is available for the upstream RAF/KSR/MEK platforms, assemblies involving the key effector of this pathway, ERK, have largely evaded structural elucidation. Indeed, the available information is currently limited to a few complex structures featuring short peptides bound to ERK. The ubiquitously expressed “15kDa phosphoprotein enriched in astrocytes” (PEA-15) is a direct ERK regulator capable of blocking ERK’s activity, thus acting as a tumor suppressor. However, PEA-15 also acts to promote tumors in certain circumstances. Our interest in PEA-15 is further kindled by the fact that PEA-15 contains a death effector domain and is therefore also a member of the death domain superfamily. We seek to understand how PEA-15 can achieve ERK regulation and how death effector domains in general can directly bind and affect kinases. We have recently solved the three dimensional structure of the ERK–PEA-15 regulon, which gives surprising insight into the function of this death effector domain protein and general principles in ERK regulation.
Kinase pathways: targeting Eph receptor ligand binding domains
Receptor tyrosine kinases are at the apex of many cellular signaling pathways, and the Ephrin receptors (Eph receptors) represent the largest family of receptor tyrosine kinases. In collaboration with Dr. Elena Pasquale at SBMRI, we are investigating the mechanism of action of peptidic and small molecule antagonists targeting the ligand-binding domain of the Eph receptors, with particular focus on EphA4. These agents where initially identified by Dr. Pasquale and have been used in models of neurological diseases, such as spinal cord injury and Amyotrophic Lateral Sclerosis (ALS), to specifically antagonize EphA4 in order to gain insight into EphA4 function and its potential as a therapeutic target. We seek to obtain insight into the three dimensional structure of EphA4 in complex with these targeting agents to understand and improve their activity.
Pathways crosstalk: NSP-Cas modules
We are interested in the downstream modulation of receptor tyrosine kinase and integrin adhesion receptor signals by members of the Crk-associated substrate (CAS) and novel SH2-containing protein (NSP) families, which have been of long-standing interest at SBMRI. Signaling platforms formed by CAS and NSP family members are prime examples of multi-domain protein complexes that mediate communication between different signaling pathways. Both CAS and NSP proteins contain several protein-interaction domains and motifs, and bind to each other through their respective C-terminal domains. This interaction is mediated by a FAT-type domain in CAS proteins and a unique CDC25-homology exchange factor domain in NSP proteins, making this linkage remarkable in several respects. Our recent structural work featuring these domains and a NSP-CAS complex structure shows that the members of the two families engage in a tight and sophisticated interaction that allows permutations of the family members to modulate different signaling pathways. Thus, NSP-CAS modules evolved to integrate various combinations of signaling systems, which is illustrated by their diverse roles in cellular processes spanning from breast cancer antiestrogen resistance to immune cell chemotaxis.
Stefan Riedl's Bio
Dr. Riedl studied biochemistry at the University of Bayreuth, Germany and went on to spend the first two years of his doctoral work at the Burnham Institute, La Jolla, California. He received his Ph.D. in the group of Dr. Robert Huber at the Max Planck Institute, Munich, Germany, for his work on the activation and inhibition of caspases. His postdoctoral work on the structure of Apaf-1 was carried out at Princeton University, New Jersey. He joined the faculty at Sanford-Burnham as an Assistant Professor in 2006. He is currently an Associate Professor in the Sanford-Burnham program on Cell Death and Survival Networks and the Inflammatory Diseases program. His laboratory focuses on the structure and mechanism of signaling platforms and signaling complexes regulating cell death, survival, inflammation and immune response.
Honors and Recognition
V Foundation for Cancer Research Scholar, 2006