Mitochondria as a Control in Apoptosis

For this article, Jennifer Fisher Wilson interviewed Guido Kroemer, Research Director, Centre National de la Recherche Scientifique, Institut Gustave Roussy in Villejuif, France. Data from the Web of Science (ISI, Philadelphia) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age.   S.A. Susin, H.K. Lorenzo, N. Zamzami, I. Marzo, B.E. Snow, G.M. Brothers, J. Mangion, E. Jacotot, P. Costantini, M. Loeffler, N. Larochette, D.R. Goodlett,

Feb 5, 2001
Jennifer Fisher Wilson

For this article, Jennifer Fisher Wilson interviewed Guido Kroemer, Research Director, Centre National de la Recherche Scientifique, Institut Gustave Roussy in Villejuif, France. Data from the Web of Science (ISI, Philadelphia) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age.

 

S.A. Susin, H.K. Lorenzo, N. Zamzami, I. Marzo, B.E. Snow, G.M. Brothers, J. Mangion, E. Jacotot, P. Costantini, M. Loeffler, N. Larochette, D.R. Goodlett, R. Aebersold, D.P. Siderovski, J.M. Penninger, G. Kroemer, "Molecular characterization of mitochondrial apoptosis-inducing factor," Nature, 397:441-6, Feb. 4, 1999. (Cited 263 times)

Less than a decade ago, scientists at Centre National de la Recherche Scientifique in Villejuif, France, noticed a connection between decreased mitochondrial transmembrane potential and cell death. Thinking that this might indicate that mitochondria play a larger role in apoptosis than previously thought, they pursued the connection and soon established that mitochondrial alterations defined the point of no return of the death process.1,2

Subsequent investigations revealed that the intermembrane space of mitochondria contains proteins that move through the outer membrane to participate in the degradation phase of apoptosis. Then, in a key finding published in Nature in early 1999, they established that one of these proteins, apoptosis-inducing factor (AIF), constitutes a direct molecular link between mitochondrial membrane permeabilization and nuclear alterations, a morphological hallmark of apoptosis. This report suggested the likelihood that mitochondria might actually control apoptosis.


Guido Kroemer
"Our paper on AIF is part of a series of papers showing that mitochondrial membrane permeabilization is a critical early step of apoptosis," said Guido Kroemer, lead investigator and research director, Institut Gustave Roussy in Villejuif, France. "Our studies suggest that AIF plays a major role in the regulation of the apoptotic process, not only as a downstream effector of nuclear apoptosis but also as true regulator of cell death or survival," he said. In the Nature paper, Kroemer and his co-authors suggest that AIF is a phylogenetically old, bifunctional protein with an election acceptor-donor function and a second, independent apoptogenic function.

Their research shows that during apoptosis, AIF translocates from the mitochondrial intermembrane space through the outer mitochondrial membrane to the cytosol and then to the nucleus. According to their work, AIF is the major caspase-independent effector of nuclear chromatin condensation and large-scale DNA fragmentation into 50 kilobase pairs.

Understanding how apoptosis occurs is key to understanding biology, Kroemer emphasized, since apoptosis is as fundamental of a biological process as cell division. Dysregulations of apoptosis are pathogenic and participate in major diseases including cancer, which is associated with an abnormal resistance to apoptosis, and ischemia and AIDS, which are associated with acute or chronic cell loss.

"The elucidation of the molecular mechanisms of apoptosis is of utmost biomedical interest. Our team has established the central role of mitochondria in apoptosis, and pharmacological interventions on mitochondrial targets, one of which might be AIF, may be useful in correcting an excess or a deficiency in the apoptotic process," Kroemer said. In particular, he noted one possible AIF strategy to overcome the Bcl-2- mediated apoptosis resistance in cancer cells. Ordinarily, Bcl-2 stimulates cell survival by maintaining the barrier function of mitochondrial membranes upstream of AIF, but Kroemer's group found that AIF microinjected into the (extra-mitochondrial) cytoplasm of cells or misdirected to a nonmitochondrial subcellular localization, such as by transfection with an AIF cDNA construct lacking the aminoterminal mitochondrial localization sequence, will kill cells overexpressing Bcl-2 with the same efficiency as control cells.

Kroemer and his group have focused on apoptosis since 1993. Early in their research, two working hypotheses were launched based on their initial observation that the mitochondrial transmembrane potential frequently drops early during the apoptotic process. First, they postulated that permeability transition (PT), a partial and sometimes transient permeabilization of the mitochondrial inner membrane, could be rate-limiting for the apoptotic process. "Indeed, it turned out that pharmacological inhibition of the permeability transition pore complex (PTPC) could, at least in some experimental systems, inhibit cell death. Moreover, members of the prominent Bcl-2/Bax family of apoptosis-regulatory proteins turned out to act on mitochondria and to interact with the PTPC," notes Kroemer.3

Second, they postulated that mitochondria would release downstream death effectors that would explain some of the morphological hallmarks of apoptosis. This hypothesis, too, proved accurate. "We discovered AIF bioactivity by combining purified mitochondria induced to undergo PT, or their supernatant, with purified nuclei in vitro," said Kroemer.4,5 His group termed the activity that was released from mitochondria and that caused chromatin condensation in purified nuclei in vitro as AIF.

Although the work by Kroemer and his laboratory established the central role of mitochondria in apoptosis, many of the molecular details still elude researchers. His lab is pursuing which pro-apoptotic signaling molecules act on mitochondrial membranes to cause their permeabilization and how membranes are actually permeabilized. They are also investigating the structures that form the conduit through which AIF and other apoptogenic proteins escape from mitochondria and the downstream targets of AIF. And they are working to identify which among the different proteins released from the intermembrane space actually participate in the death process. "These are some of the fundamental questions addressed in my laboratory," Kroemer said. "In addition, we attempt to apply our knowledge to investigate apoptosis in the context of cancer and AIDS." S

 

Jennifer Fisher Wilson (jfwilson@snip.net) is a contributing editor for The Scientist.

References

1. N. Zamzami et al., "Reduction in mitochondrial potential constitutes an early irreversible step of programmed lymphocyte death in vivo," Journal of Experimental Medicine, 181(5):1661-72, 1995.

2. N. Zamzami et al., "Sequential reduction of mitochondrial transmembrane potential and generation of reactive oxygen species in early programmed cell death," Journal of Experimental Medicine, 182:367-77, 1995.

3. Marzo et al., "Bax and adenine nucleotide translocator cooperate in the mitochondrial control of apoptosis," Science, 281:2027-31, 1998.

4. N. Zamzami, et al., "Mitochondrial control of nuclear apoptosis," Journal of Experimental Medicine, 183:1533-44, 1996.

5. S.A. Susin et al., "Bcl-2 inhibits the mitochondrial release of an apoptogenic protease," Journal of Experimental Medicine, 184:1331-41, 1996.