Signal Transduction

T. Fernandes-Alnemri, R.C. Armstrong, J. Krees, S.M. Srinivasula, L. Wang, F. Bullrich, L.C. Fritz, J.A. Trapani, K.J. Tomaselli, G. Litwack, E.S. Alnemri, "In vitro activation of CPP32 and Mch3 by Mch4, a novel human apoptotic cysteine protease containing two FADD-like domains," Proceedings of the National Academy of Sciences, 93:7464-9, 1996. (Cited in more than 185 papers since publication) Comments by Emad S. Alnemri, associate professor and deputy director of the Center for Apoptosis Research, The Kimmel Cancer Center at Thomas Jefferson University in Philadelphia To understand the function of a caspase, researchers must look at the long and short of it. All caspases--aspartate-specific cystein proteases that mediate apoptosis and proinflammatory cytokine production--share structural similarities: a prodomain, a large subunit, and a small subunit. However, differences in the lengths of caspase prodomains may be especially significant, notes Emad S. Alnemri, whose lab has discovered seven of the 14 known caspases. "What is the function of the prodomain?" Alnemri asks rhetorically. "The ones that have short prodomains appear to be downstream, or effector, caspases." Those inactive zymogens depend on the upstream caspases to convert them into active proteases. So far, caspase 3, (CPP32), caspase 6 (Mch2) and caspase 7 (Mch3) have been identified as downstream effector caspases with short prodomains. Short prodomains prevent these downstream caspases from playing an earlier role in the signaling cascade, because the lack of length denies the proteins a binding site. "They cannot interact with signaling molecules," Alnemri explains. "So they depend on something to get activated." That something The upstream, or initiator caspases, which have longer prodomains. The caspases with longer prodomains include caspase 2 (Ich-1), caspase 8 (Mch5), caspase 9 (Mch6) and caspase10 (Mch4). This paper primarily describes the identification and activities of caspase 10, although it also discusses caspase 8, which was known as both FLICE (Hot Papers, The Scientist, 12[11]:11, May 25, 1998); and MACH (Hot Papers, The Scientist, 12[3]:13, Feb. 2, 1998). The signaling activity involving those two caspases begins with the death receptor and the receptor interacting molecule FADD. FADD's two chemical arms allow the molecule to act as an adapter. One arm binds to the receptor. The second arm links with the long prodomain of either caspase 8 or caspase 10 precursors (also known as zymogens). "When one of them gets recruited, that process of having three caspase precursor molecules coming together causes auto-activation." In this case, the self-activation means cleaving the caspase precursor between the large subunit and small subunit. "Once they get cleaved, they assume a conformation that allows them to have enzymatic activity." That ensuing cascade of enzymatic activity eventually includes the cleaving of the downstream, short-prodomain caspase precursors. After those caspase precursors are chemically "ripped in half," their separate components fold into an active enzyme complex that attacks the cell's substrates, leading to the cell's eventual dismantling, or apoptosis. Alnemri suspects the paper, which describes this process, has received citation attention because it answers some early questions about this apoptotic machinery. "Everyone wanted to know how the death receptors--FAS, TNFR1--triggers activation of the apoptotic program and triggers activation of the caspases," he notes. Before the discovery of caspase 8 and caspase 10, nobody knew how death receptors activate the downstream components. Still many questions about this machinery remain unanswered. "There are some gaps in the pathways that are not yet known," Alnemri notes. "For example, another adapter molecule called RAIDD or CRADD recruits caspase 2." Caspase 2, in turn, somehow triggers apoptosis. Caspase 2 has a long prodomain like caspase 8 and caspase 10. But researchers don't yet know that protein's exact physiological function or how it transmits an apoptotic signal downstream. Questions also remain about the roles of both caspase 8 and caspase 10, since they share structural similarities. "They both could be recruited at the same time to the death receptor or one of them could be preferentially recruited to one type of death receptor and one to another, or maybe some cell types use one and some cell types use the other." To really understand the importance of these proteins, researchers will turn from in vitro studies such as this one, to in vivo studies, Alnemri predicts: "The next thing [to learn] is how important is caspase 8 and caspase 10 in physiology. The only way to know is by the generation of mouse knockouts of these genes." He already has a hypothesis about the outcome of such a study: "I predict that if these caspases are really the initiator caspases--that they are essential for development--then knockouts of these genes will be lethal." Filling in the gaps in the pathway and understanding each protein's role in the signaling process has huge implications for drug discovery, Alnemri notes. He adds that IDUN Pharmaceuticals Inc., of San Diego, who sponsored and participated in this study, wants to have an impact on pathways of diseases, such as muscular dystrophy and Alzheimer's disease, that are characterized, in part, by massive apoptosis. "They are trying to use drugs that would interfere with these caspases or inhibit the effects of these caspases." Alnemri says of the company. "If you know all the steps in the pathway, it makes it easier to design a drug or find a drug that could be targeted to any of these steps."

Signal Transduction

The Scientist Staff

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