Fig. 2. The effects of HIV proteins on BMECs.

(A) Cellular responses to Tat resulting in increased blood-brain barrier (BBB) permeability. Tat activates NF-kappaB and mitogen-activated protein kinase through an unknown mechanism, thereby causing a decrease in TNF-alpha neutralization. The resulting accumulation of TNF-alpha causes an increase in MMP-9, which can function locally or may be secreted into the circulation. It is also possible that NF-kappaB and mitogen-activated protein kinase activity directly causes the increase in MMP-9 levels, but the mechanism is unknown. Active MMP-9 degrades laminin and type IV collagen, essential components of the BBB. Tat has been observed to increase ROS within the cell, an effect amplified in the presence of TNF-alpha and alcohol metabolism. ROS causes increased phosphorylation of occludin, claudin-5, and ZO-1 and causes a general decrease in tight junction (TJ) mRNA and protein levels. Tat and TNF-alpha lead to the activation of COX-2 at caveolae in the cell membrane, leading to a decrease in protein levels of occludin and ZO-1. Tat also activates Ras through an unknown mechanism requiring CAV-1, leading to a decrease in protein levels of occludin, ZO-1, and ZO-2. Overall, occludin, claudin-5, ZO-1, and ZO-2 are mislocalized in the cytoplasm, where they are likely degraded by the proteasome. (B) As gp120 binds to either CXCR4 or CCR5, changes within the cell lead to TJ dysfunction, a decrease in TEER, and an increase in BBB permeability. A resulting increase in intracellular concentrations of Ca2+ and activation of protein kinase C and myosin light chain kinase have been observed following gp120 binding. Surface levels of ICAM-1 and VCAM-1 increase significantly, leading to an increase in recruitment and transendothelial migration of lymphocytes and monocytes from the peripheral circulation. Although protein levels of claudin-1 and -5 are unaffected by gp120, the proteins ZO-1, ZO-2, and occludin are targeted to the proteasome for degradation. With loss of connection to the cytoskeleton, claudin-1 and -5 are unable to maintain TJ function. An increase in intracellular ROS is also observed as a result of decreased levels of glutathione peroxidase and glutathione reductase following exposure to gp120. (C) HIV-1 infection of astrocytes is characterized by expression of Tat, high levels of Nef, and increased secretion of proinflammatory cytokines TNF-alpha and IL-6. Nef expression is associated with astrocyte activation, as measured by increased CD88 surface expression and elevated glial fibrillary acidic protein. Myristoylated Nef anchored to the cell membrane can interact with calmodulin and has been associated with increased secretion of MCP-1/CCL2, a potent monocyte chemoattractant. MCP-1/CCL2 secretion decreases protein levels of occludin, claudin-5, ZO-1, and ZO-2 at the TJs. An increase in secretion of IL-2 and IL-8 has also been observed, along with a decrease in secretion of stromal cell-derived factor-1 and IL-10.