HIV and apoptosis death and the mitochondrion.

Abstract

T cells due to apoptosis, the mechanisms of apop-tosis are highly controversial (1, 2). Several HIV-1 proteinshave been implicated in apoptosis regulation, among them viral protein R (Vpr), a small ( z 14 kD) HIV-1 accessoryprotein that is packed in the nucleocapsid through its inter-action with the Pr55 Gag precursor (3). In addition to itsroles in apoptosis and virus assembly, several other activitieshave been ascribed to Vpr–protein interaction, including:(a) translocation of the HIV-1 preintegration complexthrough the nuclear pore, a necessary step for the replica-tion of HIV-1 in nondividing cells—Vpr appears to partic- ipate in this process by binding to kariopherin a ; (b) induc-tion of cell cycle arrest, likely by Vpr binding to andinactivating MOV34, an upstream positive regulator of thep34–cyclin B complex shown to be essential for the G2–Mphase transition; (c) stimulation of viral gene expressionthrough physical interaction of Vpr with transcription fac-tors and/or as a consequence of its effect on cell cycle. Theability of Vpr to exert so many effects through direct pro-tein–protein interactions, followed by changes in targetprotein activity, can be explained by thinking of Vpr as achaperone, as recently suggested by the fact that Vpr cansubstitute for hsp70, a cellular chaperone (4). Thus, Vprseems to possess structural features that allow for binding tomore than one protein with sufficient energy to causechanges in activity (presumably in conformation) of targetproteins. In this issue (5), Jacotot et al. extend their previous find-ing (6) that the mitochondrial adenine nucleotide transloca-tor (ANT), a proposed component of the permeabilitytransition pore, constitutes a novel Vpr target. Here, theypresent a large number of experiments to test the idea thatthis physical interaction of Vpr and ANT is central to Vpr-induced apoptosis. First, in pure lipid bilayer membranes,they demonstrate that adding an apoptogenic peptide de-rived from Vpr (Vpr 52–96) and ANT together leads tochannel formation. The channels they measure could easilybe large enough to permeabilize (although channel selectiv-ity remains to be determined) the inner mitochondrialmembrane, leading to uncoupling of mitochondrial respira-tion, loss of transmembrane potential, swelling of the ma-trix, and release of intermembrane proteins, activities ofVpr 52–96 that they independently demonstrate on isolatedmitochondria. Second, based on the ability of PA10, a volt-age-dependent anion channel (VDAC) inhibitor to impairVpr binding to mitochondria, they argue that Vpr targetsANT by passing through VDAC. Third, they show thatBcl-2 can displace Vpr 52–96 from ANT with recombinantproteins and that thereby, Bcl-2 can inhibit both the bind-ing of Vpr to ANT in mitochondrial membranes and theeffects of Vpr on mitochondria. According to Jacotot et al.,the mechanism of Vpr-induced apoptosis that emerges fromthis multidisciplinary approach is as follows (Fig. 1, toprow: Vpr enters the intermembrane space through VDAC,binds to the intermembrane face of ANT (this is the stage atwhich Bcl-2 would inhibit apoptosis), and opens ANT topermeabilize the inner mitochondrial membrane. This leadsto inner membrane swelling and rupture of the outer mem-brane to release apoptogenic factors.While this scheme is attractive, there are alternative hy-potheses and many details to worry about. If Vpr crossesthe outer mitochondrial membrane through VDAC, this isvery important because Vpr oligomerizes at very low (asyet undetectable) concentrations, and the lumen of VDACis already close to that of an a helix (7, 8). More work isneeded to confirm their proposal for the following reasons:(a) since Vpr binds mitochondrial VDAC (6), the inhibi-tory effect of PA10 on Vpr binding to mitochondria mayreflect direct binding to VDAC; (b) PA10 generally de-creases mitochondrial membrane permeability, and PA10efficiently inhibits pores other than VDAC (our unpub-lished observations), so PA10 is not specific for VDAC; (c)NADH does not impair Vpr 52–96 from reaching ANTdespite the fact that NADH induces VDAC closure in wayssimilar to those of PA10 (9). As extracellular Vpr crossesthe plasma membrane before gaining access to the mito-chondria, Vpr can cross membranes through alternative,VDAC-independent pathways, and these transport mecha-nisms may be at work in the outer mitochondrial mem-brane as well (Fig. 1 A, second from top).The fact that Vpr can cross membranes may be related tothe fact that Vpr is membrane active and perforates lipid bi-