Regulation of mitochondria morphology

National Heart, Lung, And Blood Institute

Investigators

Linked publications & trials

Abstract

Our research group has a continuing interest in understanding the molecular mechanisms responsible for morphologic changes in mitochondria. We are focusing on specific molecular interactions that regulate these mitochondria changes following a cell death (apoptotic) initiation. We are concentrating our study on the Bcl-2 family of proteins, in particular the Bax protein, which represents a non-reversible trigger for apoptosis. There are many challenges in studying Bax that have to be overcome. This is due to its mitochondria membrane association after apoptosis initiation, multi-component complexes that it can form, and the time dependent changes in its conformation and physiological properties following the apoptosis signal. This requires new methodologies and approaches in Nuclear Magnetic Resonance in order to allow us to study this protein in its various physiological forms at a detailed molecular level and with a high efficiency. We developed a new protocol in order to measure conformational change of the protein Bax in living cell. We devised a scheme in order to incorporate three different fluorophores into the protein without disturbing its structure. We chose a pair of fluorophores that can allow us to measure FRET distance, while the extra fluorophore was used as a control for environmental change. With this set up we have been able to measure distances in Bax while it is in the cytosol and after its translocated into the mitochondria membrane following apoptosis trigger. Based on our measurements we are proposing the conformations of Bax in the cytosol as well as in the mitochondria membrane after following translocation. Our data showed that Bax C-terminal helix is not tucked into the hydrophobic core even in the cytosol. In contrast other helices in the protein adopt conformations similar to the structure of Bax that was determined in solution. We followed the Bax conformational study in live cell using FRET and FCS measurements to look at Bid conformational changes after initiation of apoptosis. We showed that tBid, Bid that has been cleaved by a caspace, translocates to the mitochondria membrane and adopts an extended structure similar to its NMR structure that we determined in a detergent micelle. We also showed if we cross linked parts of Bid, thus preventing it from converting to an extended conformation, it is no longer translocates to the mitochondria membrane and is inactive. We further probe inter-molecular contacts between tBid molecules as well as between tBid and Bax after the initiation of apoptosis. The tBid forms head to tail oligomer and it furthers form contacts involving the BH3 domain of Bax oligomers. Our data therefore suggests a formation of network of protein oligomer that promotes the permeabilization of the mitochondria outer membrane. In order to determine the orientation of a certain domain relative to the membrane bilayer we developed a hybrid FRET method to detect location of a fluorophore associated with a protein relative to the membrane. We chose to use dipricylamine (DPA) as a FRET acceptor that can be modulated by a pulsed electric field to be close to the inner or outer leaflet of the membrane. Using short acquisition time confocal imaging we can readily determine the location of the fluorophore relative to the bilayer. We used this method to determine the orientation of helix-6 in Bax after it is incorporated into a vesicle. We confirmed that conformational heterogeneity in Bax in solution, that was proposed as key in its activation, does not exist. We used a battery of biophysical techniques including paramagnetic relaxation enhancement (PRE), residual dipolar coupling (RDC), and double electron-electron resonance (DEER) data. All of our data can fit the original NMR structure of Bax that we first determined. Since all of the Bcl-2 family of proteins contain mostly alpha-helical structure, analyzing their NMR resonances can be a challenge because they don't show large dispersion. We recently developed a new method in employing the additional of lanthanide group to a specific site in a protein that can introduce large dispersion in their NMR resonances. We showed an example where we used such an approach to study NMR signals from unstructured proteins which have very little resonance dispersion. This new technology can be widely used in NMR applications. One of the research targets that we are interested in is the Humanin peptide (HN). HN has been shown to play a key role for neuronal cell survival in patients with Amyloid-Beta. Eventhough the interaction between Humanin and Bax has been shown previously, but the mechanism for interaction was not established. We showed using a series of biophysical techniques that HN can sequester Bax into a stable fibril. In the fiber Bax loses all of its helical composition and it has a different fold that its structure in the cytoplasm. Using gold-labeled antibody to Bax, we showed that the protein is embedded within the fiber in almost a regularly spaced arrangement. We used two variants of Bax, a C-terminal truncated and S184V mutant to show that they form fibril at a faster rate than the WT Bax. Similarly, using HN variants we showed a correlation where the HN mutant that has less neuro-protective effect doesn't form as regular fibril with Bax as the WT HN or its mutant that has higher neuro-protection. This sequestration seems to be its mechanism of action, preventing Bax from translocation into the mitochondria membrane to trigger cell death. We also have data to show that another Bcl-2 protein, Bid can form fibril in the presence of HN. The reaction rate and the size of the fibers are quite different than those formed by HN and Bax. We currently hypothesize that sequestration of pro-apoptotic members of Bcl-2 protein might be general mechanism for inhibiting apoptosis in general. We expanded our study on Human (HN) peptide interaction. We found that two other pro-apoptotic Bcl-2 proteins interacted with HN and form fibers, while an anti-apoptotic member, Bcl-xL did not form fibers. This general distinction between the different groups of the family may hold an important functional clue. We are gearing up to expand our research into two areas, one is to slow the rate of fiber formation when HN interact with these Bcl-2 proteins, with the goal for us to study the specific molecular interaction by NMR. The expectation is to be able to design small compound mimic of this interaction to either activate or inhibit these Bcl-2 proteins. Since these proteins are associated with a number of human diseases, identifying HN interaction site and use that as a target for therapeutic development offers a new alternative to what's been pursued so far. Considering the rapid triggering for fiber formation and the stability of the proteins being incorporated, this site potentially can be a more potent site to target as well.