Structural insight into the molecular mechanism of p53-mediated mitochondrial apoptosis.

The tumor suppressor p53 is mutated in approximately half of all human cancers. p53 can induce apoptosis through mitochondrial membrane permeabilization by interacting with and antagonizing the anti-apoptotic proteins BCL-xL and BCL-2. However, the mechanisms by which p53 induces mitochondrial apoptosis remain elusive. Here, we report a 2.5 Å crystal structure of human p53/BCL-xL complex. In this structure, two p53 molecules interact as a homodimer, and bind one BCL-xL molecule to form a ternary complex with a 2:1 stoichiometry. Mutations at the p53 dimer interface or p53/BCL-xL interface disrupt p53/BCL-xL interaction and p53-mediated apoptosis. Overall, our current findings of the bona fide structure of p53/BCL-xL complex reveal the molecular basis of the interaction between p53 and BCL-xL, and provide insight into p53-mediated mitochondrial apoptosis.

HT-SuMD: making molecular dynamics simulations suitable for fragment-based screening. A comparative study with NMR.

Fragment-based lead discovery (FBLD) is one of the most efficient methods to develop new drugs. We present here a new computational protocol called High-Throughput Supervised Molecular Dynamics (HT-SuMD), which makes it possible to automatically screen up to thousands of fragments, representing therefore a new valuable resource to prioritise fragments in FBLD campaigns. The protocol was applied to Bcl-XL, an oncological protein target involved in the regulation of apoptosis through protein-protein interactions. Initially, HT-SuMD performances were validated against a robust NMR-based screening, using the same set of 100 fragments. These independent results showed a remarkable agreement between the two methods. Then, a virtual screening on a larger library of additional 300 fragments was carried out and the best hits were validated by NMR. Remarkably, all the in silico selected fragments were confirmed as Bcl-XL binders. This represents, to date, the largest computational fragments screening entirely based on MD.

[Expression, purification and activity analyses of three Bcl-2 family proteins].

Bcel-2 family proteins (Bcl-x(L), Bcl-2, Mel-1 etc.) are key regulators of some life processes, including apoptosis and autophagy. They are currently considered as promising targets for developing new anti-tumor therapies. In our study, the human Bcl-2/Bcl-x(L) chimeric gene and the human/mouse Mel-1 chimeric gene were designed and cloned, and the prokaryotic expression vectors for expressing glutathione S-transferase (GST) fusion proteins and histidine tag fusion proteins were constructed respectively. These two proteins as well as the GST-Bcl-x(L) fusion protein were all successfully expressed in E. coli and subsequently purified. In addition, we measured the binding of these Bcl-2 family proteins to the Bid BH3 peptide by fluorescence polarization-based assay. The dissociation constants (Kd) obtained by us were in general agreement with the data reported in literature. The Kd values of all three proteins with or without the GST tag were almost identical. All these results validate the biological functions of these Bcl-2 family proteins obtained by us. These proteins can be used in the experimental screening of small-molecule regulators of Bcl-2 family proteins in vitro.

A general strategy for the bacterial expression of amyloidogenic peptides using BCL-XL-1/2 fusions.

Biophysical studies on amyloidogenic and aggregation-prone peptides often require large quantities of material. However, solid-phase synthesis, handling, and purification of peptides often present challenges on these scales. Recombinant expression is an attractive alternative because of its low cost, the ability to isotopically label the peptides, and access to sequences exceeding approximately 50 residues. However, expression systems that seek to solubilize amyloidogenic peptides suffer from low yields, difficult optimizations, and isolation challenges. We present a general strategy for expressing and isolating amyloidogenic peptides in Escherichia coli by fusion to a polypeptide that drives the expression of attached peptides into bacterial inclusion bodies. This scheme minimizes toxicity during bacterial growth and enables the processing and handling of the peptides in denaturing solutions. Immobilized metal affinity chromatography, reverse phase HPLC, and cyanogen bromide cleavage are used to isolate the peptide, followed by further reverse phase HPLC, which yields milligram quantities of the purified peptide. We demonstrate that driving the peptides into inclusion bodies using fusion to BCL-XL-1/2 is a general strategy for their expression and isolation, as exemplified by the production of 11 peptides species.

High-yield expression and purification of soluble forms of the anti-apoptotic Bcl-x(L) and Bcl-2 as TolAIII-fusion proteins.

A method is presented to produce large amounts of Bcl-2 and Bcl-x(L), two anti-apoptotic proteins of considerable biomedical interest. Expression constructs were prepared in which the Escherichia coli protein TolAIII, known to promote over expression of soluble product, was added to the N-terminus of Bcl-2 or Bcl-x(L) proteins, which had their C-terminal hydrophobic anchors deleted. Here the expression of these TolAIII-fusion constructs, followed by a two-step metal-affinity based purification protocol is described. The method delivers at least 20 and 10mg of more than 90% pure TolAIII-Bcl-x(L)DeltaC and TolAIII-Bcl-2(2)DeltaC proteins, respectively, per liter of E. coli cell culture. The proteins are released by proteolysis with thrombin providing > 12 mg of Bcl-x(L)DeltaC or > 6 mg of Bcl-2(2)DeltaC per liter of E. coli cell culture with a purity of more than 95%. Whereas Bcl-x(L)DeltaC is soluble both before and after TolAIII removal, Triton X-100 can significantly increase the extraction of TolAIII- Bcl-2(2)DeltaC from the bacterial cells and its subsequent solubility. Far-UV CD spectroscopy demonstrated that they both have an alpha-helical structure. Fluorescence spectroscopy was used to quantitatively analyze the binding of the respiratory inhibitor antimycin A to recombinant Bcl-2 and Bcl-x(L) proteins as well as the displacement of this ligand from the hydrophobic pocket with BH3 Bad-derived peptide. Purified Bcl-x(L)DeltaC and Bcl-2(2)DeltaC both protect isolated mitochondria from Bax-induced release of cytochrome c. The ensemble of data shows that the expressed proteins are correctly folded and functional. Therefore, the TolAIII-fusion system provides a convenient tool for functional characterization and structural studies of anti-apoptotic proteins.

Bcl-XL modulates the differentiation of immortalized human neural stem cells.

Understanding basic processes of human neural stem cell (hNSC) biology and differentiation is crucial for the development of cell replacement therapies. Bcl-X(L) has been reported to enhance dopaminergic neuron generation from hNSCs and mouse embryonic stem cells. In this work, we wanted to study, at the cellular level, the effects that Bcl-X(L) may exert on cell death during differentiation of hNSCs, and also on cell fate decisions and differentiation. To this end, we have used both v-myc immortalized (hNS1 cell line) and non-immortalized neurosphere cultures of hNSCs. In culture, using different experimental settings, we have consistently found that Bcl-X(L) enhances neuron generation while precluding glia generation. These effects do not arise from a glia-to-neuron shift (changes in fate decisions taken by precursors) or by only cell death counteraction, but, rather, data point to Bcl-X(L) increasing proliferation of neuronal progenitors, and inhibiting the differentiation of glial precursors. In vivo, after transplantation into the aged rat striatum, Bcl-X(L) overexpressing hNS1 cells generated more neurons and less glia than the control ones, confirming the results obtained in vitro. These results indicate an action of Bcl-X(L) modulating hNSCs differentiation, and may be thus important for the future development of cell therapy strategies for the diseased mammalian brain.

HIV-1 transgenic rat CD4+ T cells develop decreased CD28 responsiveness and suboptimal Lck tyrosine dephosphorylation following activation.

Impaired CD4+ T cell responses, resulting in dysregulated T-helper 1 (Th1) effector and memory responses, are a common result of HIV-1 infection. These defects are often preceded by decreased expression and function of the alpha/beta T cell receptor (TCR)-CD3 complex and of co-stimulatory molecules including CD28, resulting in altered T cell proliferation, cytokine secretion and cell survival. We have previously shown that HIV Tg rats have defective development of T cell effector function and generation of specific effector/memory T cell subsets. Here we identify abnormalities in activated HIV-1 Tg rat CD4+ T cells that include decreased pY505 dephosphorylation of Lck (required for Lck activation), decreased CD28 function, reduced expression of the anti-apoptotic molecule Bcl-xL, decreased secretion of the mitogenic lympokine interleukin-2 (IL-2) and increased activation induced apoptosis. These events likely lead to defects in antigen-specific signaling and may help explain the disruption of Th1 responses and the generation of specific effector/memory subsets in transgenic CD4+ T cells.