AIF Shows There's More Than One Way to Kill a Cell

AIF Shows There's More Than One Way to Kill a Cell

by Jeremy Cherfas

WHAT'S HOT IN BIOLOGY...

Rank	Paper	Citations This Period Mar- Apr 00	Rank Last Period Jan- Feb 00
1	S.T. Cole, et al., *"Deciphering the biology of Mycobacterium tuberculosis* from the complete genome sequence,"** Nature, 393(6685):537, 11 June 1998. [Sanger Ctr., Hinxton, England; Inst. Pasteur, Paris, France; NIAID, NIH, Hamilton, MT; Tech. U. Denmark, Lyngby] *ZT988	54	2
2	D.A. Doyle, et al., "Structure of the potassium channel: Molecular basis of K⁺ conduction and selectivity," Science, 280(5360):69-77, 3 April 1998. [Rockefeller U., New York, NY; Howard Hughes Med. Inst., Rockefeller U., NY] *ZF314	52	1
3	T.-C. He, et al., *"Identification of c-MYC* as a target of the APC pathway,"** Science, 281(5382):1509-12, 4 September 1998. [Johns Hopkins U., Baltimore, MD; Howard Hughes Med. Inst., Johns Hopkins U., Baltimore] *116RJ	37	†
4	S.A. Susin, et al., "Molecular characterization of mitochondrial apoptosis-inducing factor," Nature, 397(6718):441-6, 4 February 1999. [CNRS, Villejuif, France; Inst. Pasteur, Paris, France; Amgen Inst., U. Toronto, Canada; U. Washington, Seattle] 164KA	35	†
5	H. Li, et al., "Cleavage of BID by caspase 8 mediates the mitochondrial damage in the Fas pathway of apoptosis," Cell, 94(4):491-501, 21 August 1998. [Harvard Sch. Med., Boston, MA] *113GB	33	4
6	X. Luo, et al., "Bid, a Bcl2 interacting protein, mediates cytochrome c release from mitochondria in response to activation of cell surface death receptors," Cell, 94(4):481-90, 21 August 1998. [Howard Hughes Med. Inst.; U. Texas Southwest. Med. Ctr., Dallas] *113GB	33	†
7	C.-Y. Wang, et al., "NF-kB antiapoptosis: induction of TRAF1 and TRAF2 and c-IAP1 and c-IAP2 to suppress caspase-8 activation," Science, 281(5383):1680-3, 11 September 1998. [U. North Carolina, Chapel Hill; Childrens Hosp. East. Ontario, Canada; Tularik, San Francisco, CA] *118TR	33	†
8	J. Pomerantz, et al., *"The Ink4a* tumor suppressor gene product, p19^Arf, interacts with MDM2 and neutralizes MDM2's inhibition of p53,"** Cell, 1(4):713-23, 20 March 1998. [Albert Einstein Coll. Med., Bronx, NY; Memorial Sloan-Kettering Cancer Ctr., New York, NY] *ZD198	31	†
9	D.G. Wang, et al., "Large-scale identification, mapping, and genotyping of single-nucleotide polymorphisms in the human genome," Science, 280(5366):1077-82, 15 May 1998. [Whitehead Inst., Cambridge, MA; MIT, Cambridge, MA; Affymetrix, Santa Clara, CA] *ZN615	31	†
10	V.R. Iyer, et al., "The transcriptional program in the response of human fibroblasts to serum," Science, 283(5398):83-7, 1 January 1999. [7 U.S. institutions] *155ZH	31	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

f you plan to figure in the annual Science Watch roundup of highly-cited papers, it obviously pays to publish early in the year. That helps explain why this period's paper #9 was last year's second-hottest paper; it hit the stands on 1 January 1999. But it helps even more to study a hot subject: The most-cited paper of 1999 enjoyed only 11 months of exposure, but because it is about cell death it is no surprise to find it appearing this period at #4.

Santos Susin and Guido Kroemer's intercontinental crew describe the purification of a new cellular executioner, called apoptosis inducing factor (AIF). The events underlying programmed cell death have proven to be complex and redundant, with many pathways capable of triggering the cell's suicide, independently and in concert. Several of the crucial chemicals are normally sequestered within the intermembrane space of mitochondria, released only in response to apoptotic signals. AIF now joins the list of killers within the mitochondria.

A key event in apoptosis is the opening of the permeability transition pores in the mitochondrial membrane, which is under the control of genes in the Bcl-2 family. The open pores release chemicals from the intermembrane space into the cytoplasm, among them cytochrome c and three different pro-caspases. Also released is AIF, which had earlier been shown to result in a nucleus that looks distinctly apoptotic. Susin and colleagues purified this factor, which induced apoptosis even in the presence of caspase inhibitors, and used a partial sequence of it to fish for an expressed sequence tag that in turn became the bait for a full-length DNA sequence from human and mouse.

AIF is strongly conserved between the two species, indicating an ancient evolutionary origin. From its structure, it appeared to be an oxido-reductase, but with an additional ability to cause cell suicide. A series of elegant experiments revealed AIF's modus operandi.

Susin and colleagues used antibodies to remove AIF from the pool of intermembrane proteins. When the transition pores were then opened, the nucleus showed no apoptotic changes, which indicates that AIF is the principal mitochondrial factor causing nuclear apoptosis. Furthermore, adding recombinant AIF to purified nuclei resulted in the loss of nuclear DNA. Deletion mutants of the recombinant AIF, which lack the oxido-reductive function, nevertheless cause apoptosis, so the two functions are independent. AIF also exercises positive feedback, though not quite as much as some other apoptotic chemicals. In the presence of AIF, mitochondria swell noticeably; the membrane is being made more permeable, and this is accompanied by the release of cytochrome c and caspases to further drive apoptosis. An intriguing aspect of this is that while Bcl-2 can prevent the release of AIF from the mitochondria, once AIF is present in the cytoplasm the feedback mechanism causes the release of more AIF and other apoptotic compounds, which Bcl-2 is powerless to prevent.

AIF works on whole cells, too. It induces several hallmarks of apoptosis in live cells when the recombinant protein is injected in physiological quantities; the chromatin in the nucleus condenses, DNA is degraded, and the mitochondrial transmembrane potential dissipates. Overexpression of the wild-type gene causes the same effects. Blocking transport into the nucleus prevents degradation of nuclear DNA but not the other effects of AIF.

The general conclusion is that AIF–like cytochrome c–is a Jekyll and Hyde molecule. On the one hand it is an oxido-reductase that is important in energy-transfer reactions. On the other, and quite independently, it causes cell death. Why, though, is the mitochondrion involved? It is normally thought of as the cell's power plant, essential to life, but why would that make it a good place to keep suicide pills? Mitochondria probably evolved from free-living bacteria, but if the effectors of cell death are ancient and derived from bacterial functions, why are so many of the apoptotic molecules missing from bacteria and yeast? And if the apoptotic molecules are not ancient but took up residence in the mitochondria some time after the mitochondria had been fully integrated into the life of the cell, why there? What is so special about the space within the mitochondrial membranes that makes it a good place to store dangerous compounds?

As ever, cutting-edge research raises more questions than it answers, which is what makes it highly cited–no matter when it is published.

Science writer Dr. Jeremy Cherfas
works with the Biotechnology and Biological Sciences
Research Council of the U.K., Swindon.