Ayman M. Saleh

Bio: Ayman M. Saleh is an academic researcher from King Saud bin Abdulaziz University for Health Sciences. The author has contributed to research in topics: Apoptosis & Paraoxon. The author has an hindex of 22, co-authored 57 publications receiving 4803 citations. Previous affiliations of Ayman M. Saleh include Thomas Jefferson University & Lake Erie College of Osteopathic Medicine.

A conserved XIAP-interaction motif in caspase-9 and Smac/DIABLO regulates caspase activity and apoptosis

Abstract: X-linked inhibitor-of-apoptosis protein (XIAP) interacts with caspase-9 and inhibits its activity, whereas Smac (also known as DIABLO) relieves this inhibition through interaction with XIAP. Here we show that XIAP associates with the active caspase-9-Apaf-1 holoenzyme complex through binding to the amino terminus of the linker peptide on the small subunit of caspase-9, which becomes exposed after proteolytic processing of procaspase-9 at Asp315. Supporting this observation, point mutations that abrogate the proteolytic processing but not the catalytic activity of caspase-9, or deletion of the linker peptide, prevented caspase-9 association with XIAP and its concomitant inhibition. We note that the N-terminal four residues of caspase-9 linker peptide share significant homology with the N-terminal tetra-peptide in mature Smac and in the Drosophila proteins Hid/Grim/Reaper, defining a conserved class of IAP-binding motifs. Consistent with this finding, binding of the caspase-9 linker peptide and Smac to the BIR3 domain of XIAP is mutually exclusive, suggesting that Smac potentiates caspase-9 activity by disrupting the interaction of the linker peptide of caspase-9 with BIR3. Our studies reveal a mechanism in which binding to the BIR3 domain by two conserved peptides, one from Smac and the other one from caspase-9, has opposing effects on caspase activity and apoptosis.

Negative regulation of the Apaf-1 apoptosome by Hsp70.

Abstract: Release of cytochrome c from mitochondria by apoptotic signals induces ATP/dATP-dependent formation of the oligomeric Apaf-1-caspase-9 apoptosome. Here we show that the documented anti-apoptotic effect of the principal heat-shock protein, Hsp70, is mediated through its direct association with the caspase-recruitment domain (CARD) of Apaf-1 and through inhibition of apoptosome formation. The interaction between Hsp70 and Apaf-1 prevents oligomerization of Apaf-1 and association of Apaf-1 with procaspase-9. On the basis of these results, we propose that resistance to apoptosis exhibited by stressed cells and some tumours, which constitutively express high levels of Hsp70, may be due in part to modulation of Apaf-1 function by Hsp70.

Applications of nanoparticle systems in drug delivery technology

Abstract: The development of nanoparticle-based drug formulations has yielded the opportunities to address and treat challenging diseases. Nanoparticles vary in size but are generally ranging from 100 to 500 nm. Through the manipulation of size, surface characteristics and material used, the nanoparticles can be developed into smart systems, encasing therapeutic and imaging agents as well as bearing stealth property. Further, these systems can deliver drug to specific tissues and provide controlled release therapy. This targeted and sustained drug delivery decreases the drug related toxicity and increase patient’s compliance with less frequent dosing. Nanotechnology has proven beneficial in the treatment of cancer, AIDS and many other disease, also providing advancement in diagnostic testing.

Negative regulation of cytochrome c‐mediated oligomerization of Apaf‐1 and activation of procaspase‐9 by heat shock protein 90

Abstract: The release of cytochrome c from mitochondria results in the formation of an Apaf-1–caspase-9 apoptosome and induces the apoptotic protease cascade by activation of procaspase-3. The present studies demonstrate that heat shock protein 90 (Hsp90) forms a cytosolic complex with Apaf-1 and thereby inhibits the formation of the active complex. Immunodepletion of Hsp90 depletes Apaf-1 and thereby inhibits cytochrome c-mediated activation of caspase-9. Addition of purified Apaf-1 to Hsp90-depleted cytosolic extracts restores cytochrome c-mediated activation of procaspase-9. We also show that Hsp90 inhibits cytochrome c-mediated oligomerization of Apaf-1 and thereby activation of procaspase-9. Furthermore, treatment of cells with diverse DNA-damaging agents dissociates the Hsp90–Apaf-1 complex and relieves the inhibition of procaspase-9 activation. These findings provide the first evidence for a negative cytosolic regulator of cytochrome c-dependent apoptosis and for involvement of a chaperone in the caspase cascade.

Mechanisms underlying ubiquitination.

Abstract: ▪ Abstract The conjugation of ubiquitin to other cellular proteins regulates a broad range of eukaryotic cell functions. The high efficiency and exquisite selectivity of ubiquitination reactions reflect the properties of enzymes known as ubiquitin-protein ligases or E3s. An E3 recognizes its substrates based on the presence of a specific ubiquitination signal, and catalyzes the formation of an isopeptide bond between a substrate (or ubiquitin) lysine residue and the C terminus of ubiquitin. Although a great deal is known about the molecular basis of E3 specificity, much less is known about molecular mechanisms of catalysis by E3s. Recent findings reveal that all known E3s utilize one of just two catalytic domains—a HECT domain or a RING finger—and crystal structures have provided the first detailed views of an active site of each type. The new findings shed light on many aspects of E3 structure, function, and mechanism, but also emphasize that key features of E3 catalysis remain to be elucidated.

Selective recognition of methylated lysine 9 on histone H3 by the HP1 chromo domain.

Abstract: Heterochromatin protein 1 (HP1) is localized at heterochromatin sites where it mediates gene silencing. The chromo domain of HP1 is necessary for both targeting and transcriptional repression. In the fission yeast Schizosaccharomyces pombe, the correct localization of Swi6 (the HP1 equivalent) depends on Clr4, a homologue of the mammalian SUV39H1 histone methylase. Both Clr4 and SUV39H1 methylate specifically lysine 9 of histone H3 (ref. 6). Here we show that HP1 can bind with high affinity to histone H3 methylated at lysine 9 but not at lysine 4. The chromo domain of HP1 is identified as its methyl-lysine-binding domain. A point mutation in the chromo domain, which destroys the gene silencing activity of HP1 in Drosophila, abolishes methyl-lysine-binding activity. Genetic and biochemical analysis in S. pombe shows that the methylase activity of Clr4 is necessary for the correct localization of Swi6 at centromeric heterochromatin and for gene silencing. These results provide a stepwise model for the formation of a transcriptionally silent heterochromatin: SUV39H1 places a 'methyl marker' on histone H3, which is then recognized by HP1 through its chromo domain. This model may also explain the stable inheritance of the heterochromatic state.

Mammalian caspases: structure ,a ctivation ,s ubstrates, and functions during apoptosis

Abstract: ■ Abstract Apoptosis is a genetically programmed, morphologically distinct form of cell death that can be triggered by a variety of physiological and pathological stimuli. Studies performed over the past 10 years have demonstrated that proteases play critical roles in initiation and execution of this process. The caspases, a family of cysteine-dependent aspartate-directed proteases, are prominent among the death proteases. Caspases are synthesized as relatively inactive zymogens that become activated by scaffold-mediated transactivation or by cleavage via upstream proteases in an intracellular cascade. Regulation of caspase activation and activity occurs at several different levels: ( a) Zymogen gene transcription is regulated; ( b) antiapoptotic members of the Bcl-2 family and other cellular polypeptides block proximity-induced activation of certain procaspases; and ( c) certain cellular inhibitor of apoptosis proteins (cIAPs) can bind to and inhibit active caspases. Once activated, caspases cleave a variety of intracellular polypeptides, including major structural elements of the cytoplasm and nucleus, components of the DNA repair machinery, and a number of protein kinases. Collectively, these scissions disrupt survival pathways and disassemble important architectural components of the cell, contributing to the stereotypic morphological and biochemical changes that characterize apoptotic cell death.

Regulation of cell death: the calcium-apoptosis link.

Abstract: To live or to die? This crucial question eloquently reflects the dual role of Ca2+ in living organisms--survival factor or ruthless killer. It has long been known that Ca2+ signals govern a host of vital cell functions and so are necessary for cell survival. However, more recently it has become clear that cellular Ca2+ overload, or perturbation of intracellular Ca2+ compartmentalization, can cause cytotoxicity and trigger either apoptotic or necrotic cell death.