HIV-1 Tat-Mediated Effects on Focal Adhesion Assembly and Permeability in Brain Microvascular Endothelial Cells

Cell death in HIV dementia

Many patients infected with human immunodeficiency virus type-1 (HIV-1) suffer cognitive impairment ranging from mild to severe (HIV dementia), which may result from neuronal death in the basal ganglia, cerebral cortex and hippocampus. HIV-1 does not kill neurons by infecting them. Instead, viral proteins released from infected glial cells, macrophages and/or stem cells may directly kill neurons or may increase their vulnerability to other cell death stimuli. By binding to and/or indirectly activating cell surface receptors such as CXCR4 and the N-methyl-D-aspartate receptor, the HIV-1 proteins gp120 and Tat may trigger neuronal apoptosis and excitotoxicity as a result of oxidative stress, perturbed cellular calcium homeostasis and mitochondrial alterations. Membrane lipid metabolism and inflammation may also play important roles in determining whether neurons live or die in HIV-1-infected patients. Drugs and diets that target oxidative stress, excitotoxicity, inflammation and lipid metabolism are in development for the treatment of HIV-1 patients.

HIV-1 Tat Is a Natively Unfolded Protein

Tat (transactivator of transcription) is a small RNA-binding protein that plays a central role in the regulation of human immunodeficiency virus type 1 replication and in approaches to treating latently infected cells. Its interactions with a wide variety of both intracellular and extracellular molecules is well documented. A molecular understanding of the multitude of Tat activities requires a determination of its structure and interactions with cellular and viral partners. To increase the dispersion of NMR signals and permit dynamics analysis by multinuclear NMR spectroscopy, we have prepared uniformly 15N- and 15N/13C-labeled Tat-(1-72) protein. The cysteine-rich protein is unambiguously reduced at pH 4.1, and NMR chemical shifts and coupling constants suggest that it exists in a random coil conformation. Line broadening and multiple peaks in the Cys-rich and core regions suggest that transient folding occurs in two of the five sequence domains. NMR relaxation parameters were measured and analyzed by spectral density and Lipari-Szabo approaches, both confirming the lack of structure throughout the length of the molecule. The absence of a fixed conformation and the observation of fast dynamics are consistent with the ability of Tat protein to interact with a wide variety of proteins and nucleic acid and support the concept of a natively unfolded protein.

Endothelial focal adhesions and barrier function

Focal adhesions composed of integrins provide an important structural basis for anchoring the endothelial lining to its surrounding matrices in the vascular wall. Complex molecular reactions occur at the endothelial cell-matrix contact sites in response to physical and chemical stress present in the circulatory system. Recent experimental evidence points to the importance of focal adhesions in the regulation of microvascular barrier function. On one hand, the adhesive interaction between integrins and their extracellular ligands is essential to the maintenance of endothelial barrier properties, and interruption of integrin-matrix binding leads to leaky microvessels. On the other hand, focal adhesion assembly and activation serve as important signalling events in modulating endothelial permeability under stimulatory conditions in the presence of angiogenic factors, inflammatory mediators, or physical forces. The focal responses show distinctive patterns with different temporal characteristics, whereas focal adhesion kinase (FAK) plays a central role in initiating and integrating various signalling pathways that ultimately affect the barrier function. The molecular basis of focal adhesion-dependent microvascular permeability is currently under investigation, and advances in the technologies of computerized quantitative microscopy and intact microvessel imaging should aid the establishment of a functional significance for focal adhesions in the physiological regulation of microvascular permeability.

αvβ3-integrin-dependent activation of focal adhesion kinase mediates NF-κB activation and motogenic activity by HIV-1 Tat in endothelial cells

Once in the extracellular environment, the transactivator protein HIV-1 Tat exerts several pleiotropic effects by interacting with different cellular receptors, including integrin αvβ3. Real-time surface plasmon resonance analysis reveals that Tat/αVβ3 interaction occurs with rapid kinetics (association and dissociation rates equal to 1.16×107 M-1 s-1 and 3.78×10-1 s-1, respectively) and high affinity (dissociation constant = 32 nM). Through this interaction, substratum-immobilized Tat promotes adhesion and motogenic activity in endothelial cells. Also, αvβ3/Tat interaction triggers the activation of focal adhesion kinase, RhoA and pp60src. Overexpression of the dominant negative form of focal adhesion kinase, but not of an inactive Leu1034Ser substitution mutant isoform, impairs the activation of focal adhesion kinase and RhoA, but not that of pp60src, without affecting endothelial cell adhesion and spreading. αvβ3/Tat interaction triggers the activation of NF-κB in endothelial cells in a focal adhesion kinase-, RhoA- and pp60src-dependent manner, as shown in dominant negative focal adhesion kinase transfectants or using specific pharmacological inhibitors. Finally, the activation of focal adhesion kinase, RhoA, NF-κB and pp60src are required to mediate the motogenic activity of Tat in endothelial cells.

Since Tat accumulates in an immobilized form in the extracellular matrix, these results provide new biochemical and biological insights about αvβ3/Tat interaction exploitable for the design of anti-Tat strategies.