When Death Was Young: An Ancestral Apoptotic Network in Bacteria

In this issue of Molecular Cell, Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar characterize a RecA-dependent and ClpXP-regulated pathway that controls the acquisition of several apoptotic markers upon bactericidal treatment of prokaryotes, placing the hypothetical origin of apoptosis further downstream in evolution. In this issue of Molecular Cell, Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar characterize a RecA-dependent and ClpXP-regulated pathway that controls the acquisition of several apoptotic markers upon bactericidal treatment of prokaryotes, placing the hypothetical origin of apoptosis further downstream in evolution. In metazoans, the life span of individual cells is regulated by an integrated suicide system (programmed cell death, PCD) that can be activated when cells become superfluous, accumulate damage, or menace organismal fitness. Among the distinct subroutines constituting PCD, apoptosis represents the best-studied one. Apoptotic death is a structurally and functionally conserved process in thus far that it is also observed in unicellular eukaryotes, such as protozoan parasites or yeast (Carmona-Gutierrez et al., 2010Carmona-Gutierrez D. Eisenberg T. Büttner S. Meisinger C. Kroemer G. Madeo F. Cell Death Differ. 2010; 17: 763-773Crossref PubMed Scopus (384) Google Scholar; Madeo et al., 1997Madeo F. Fröhlich E. Fröhlich K.U. J. Cell Biol. 1997; 139: 729-734Crossref PubMed Scopus (679) Google Scholar). Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar provide phenotypic and mechanistic evidence that may expand the evolutionary conservation frame of apoptosis into the realm of prokaryotes. The authors demonstrate that bacterial cell death induced by treatment with different bactericidal antibiotics is accompanied by several biochemical markers of apoptosis, including DNA fragmentation, chromosomal condensation, exposure of phosphatidylserine to the outer leaflet of the plasma membrane, and dissipation of membrane potential (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar). These results add to previous work by the same group (Dwyer et al., 2007Dwyer D.J. Kohanski M.A. Hayete B. Collins J.J. Mol. Syst. Biol. 2007; 3: 91Crossref PubMed Scopus (341) Google Scholar; Kohanski et al., 2007Kohanski M.A. Dwyer D.J. Hayete B. Lawrence C.A. Collins J.J. Cell. 2007; 130: 797-810Abstract Full Text Full Text PDF PubMed Scopus (1924) Google Scholar) showing that bactericidal antibiotics promote the generation of reactive oxygen species (ROS), which are crucial apoptotic regulators in multicellular as well as in unicellular eukaryotes (Herker et al., 2004Herker E. Jungwirth H. Lehmann K.A. Maldener C. Fröhlich K.U. Wissing S. Büttner S. Fehr M. Sigrist S. Madeo F. J. Cell Biol. 2004; 164: 501-507Crossref PubMed Scopus (453) Google Scholar; Simon et al., 2000Simon H.U. Haj-Yehia A. Levi-Schaffer F. Apoptosis. 2000; 5: 415-418Crossref PubMed Scopus (2259) Google Scholar). In bacteria, ROS seem to play a similar role, since suppressing their formation reduces drug-induced cell death (Dwyer et al., 2007Dwyer D.J. Kohanski M.A. Hayete B. Collins J.J. Mol. Syst. Biol. 2007; 3: 91Crossref PubMed Scopus (341) Google Scholar) as well as DNA fragmentation (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar). Now, Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar identify and characterize RecA, a multifunctional protein crucial for DNA maintenance and repair, as an additional player involved in the antibiotic-triggered apoptotic demise of bacteria. Consistent with this finding, RecA plays a critical role in the recently described apoptosis-like death (ALD) pathway of E. coli (Erental et al., 2012Erental A. Sharon I. Engelberg-Kulka H. PLoS Biol. 2012; 10: e1001281Crossref PubMed Scopus (115) Google Scholar). Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar extend these observations by showing that the cell stress-triggered conversion of RecA into its active form is a prerequisite for its contribution to cell-death induction (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar). The lethal activity of active RecA is thereby negatively regulated by the ClpP protease complex ClpXP. These factors also dampen the LexA-regulated bacterial DNA-damage (or SOS) stress response, which is necessary for the efficient induction of apoptosis in response to cellular stress (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar). In this network of interacting regulators, RecA seems to function in a similar fashion as do caspases, the central executionary cysteine proteases in many scenarios of mammalian apoptosis. Indeed, RecA can bind and hydrolyze synthetic caspase substrates and appears to be the only bacterial enzyme to do so, at least in E. coli (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar). However, RecA is not only involved in lethal signaling; it also harbors multiple essential functions associated with DNA repair and as a regulator of the SOS response. Thus, RecA combines both lethal and vital roles, paralleling the ambiguous involvement of mammalian caspases in multiple signal transduction pathways that may either factor cell death or survival (Galluzzi et al., 2012Galluzzi L. Kepp O. Trojel-Hansen C. Kroemer G. EMBO Rep. 2012; 13: 322-330Crossref PubMed Scopus (90) Google Scholar). It remains elusive whether RecA's vital and lethal functions might be dissociated from each other. This could be explored, for instance by generating specific point mutants that would preferentially affect one or the other function. The double-sided functional nature of RecA may account for the observation that its disruption leads to both (1) reduced acquisition of the apoptotic phenotype (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar) and (2) an increased sensitivity toward bactericidal drugs (Kohanski et al., 2007Kohanski M.A. Dwyer D.J. Hayete B. Lawrence C.A. Collins J.J. Cell. 2007; 130: 797-810Abstract Full Text Full Text PDF PubMed Scopus (1924) Google Scholar), possibly as a result of the combined (1) loss of the main apoptotic executioner and (2) the fatal removal of the protein's vital functions. In the latter case, cell death might be accomplished through an alternative, nonapoptotic PCD subroutine like the mazEF-mediated death pathway, whose execution has been shown to be recA independent (Erental et al., 2012Erental A. Sharon I. Engelberg-Kulka H. PLoS Biol. 2012; 10: e1001281Crossref PubMed Scopus (115) Google Scholar). In fact, bacterial apoptosis and the mazEF pathway seem to be intertwined, the latter one possibly suppressing the first one by reducing recA mRNA levels (Erental et al., 2012Erental A. Sharon I. Engelberg-Kulka H. PLoS Biol. 2012; 10: e1001281Crossref PubMed Scopus (115) Google Scholar). ClpXP is known to be involved in the synthesis of the quorum-sensing signaling factor EDF (extracellular death factor) (Kolodkin-Gal and Engelberg-Kulka, 2006Kolodkin-Gal I. Engelberg-Kulka H. J. Bacteriol. 2006; 188: 3420-3423Crossref PubMed Scopus (76) Google Scholar), which is required for mazEF-mediated cell death and induces the endoribonucleolytic activities of MazF and ChpBK (Belitsky et al., 2011Belitsky M. Avshalom H. Erental A. Yelin I. Kumar S. London N. Sperber M. Schueler-Furman O. Engelberg-Kulka H. Mol. Cell. 2011; 41: 625-635Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar). Thus, it is conceivable that ClpXP, which suppresses RecA apoptotic activity (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar), might partly do so by promoting MazF-mediated recA mRNA cleavage/splicing (Figure 1). Such indirect mechanisms might represent an additional regulatory process beyond the proposed direct proteolytic regulation and modification of RecA by ClpXP (Dwyer et al., 2012Dwyer D.J. Camacho D.M. Kohanski M.A. Callura J.M. Collins J.J. Mol. Cell. 2012; 46 (this issue): 561-572Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar) that further refine the crosstalk between the mazEF- and apoptotic pathways. The existence of mutually inhibitory PCD pathways—like in mammalian cells, in which some catabolic pathways reciprocally suppress each other—might function as a backup system to genetically ensure programmed demise even upon loss of one of the pathways and/or as a regulatory toolkit to modulate the cellular reaction to different types and intensities of stress. The presence of a regulated suicide network in unicellular bacteria might—in analogy to PCD of eukaryotic microorganisms—increase the fitness of populations by facilitating the elimination of unwanted cells (Herker et al., 2004Herker E. Jungwirth H. Lehmann K.A. Maldener C. Fröhlich K.U. Wissing S. Büttner S. Fehr M. Sigrist S. Madeo F. J. Cell Biol. 2004; 164: 501-507Crossref PubMed Scopus (453) Google Scholar). To address the possible advantage of maintaining intact PCD programs in bacteria, it would be interesting to perform competition assays confronting wild-type, apoptosis-deficient, and/or mazEF mutant cells under conditions of antibiotic stress. Such experiments could be carried out not only in vitro but also in vivo, for instance in suitable mouse models of bacterial infection. It can be anticipated that the identification and detailed characterization of bacterial PCD pathways that respond to drug-induced cell killing will be instrumental to understand and counteract the surge of bacterial strains resistant to available antibiotic treatments. Of note, both the mazEF and apoptotic pathways permit survival of a small fraction of the bacterial population responding to external stress (Erental et al., 2012Erental A. Sharon I. Engelberg-Kulka H. PLoS Biol. 2012; 10: e1001281Crossref PubMed Scopus (115) Google Scholar). Hence, elucidating the interweavement of distinct PCD pathways might pave the way for the development of new strategies of antibiosis. The existence of morphological and biochemical signs of apoptosis in prokaryotes has broad evolutionary implications. Given the core role of mitochondria in the eukaryotic apoptotic machinery, it is possible that eukaryotic apoptosis evolved during endosymbiosis, when the prokaryotic protomitochondrion was introduced into the primitive protoeukaryotic cell. One intriguing scenario predicts that eukaryotes obliged to develop a control system that would avoid lysis of the endosymbiont activating its endogenous PCD machinery. The development of pathways to control these imported lethal mechanisms might have also promoted the eukaryotic ability to make use of them when required. Hence, the phylogenetic origin of PCD in mitochondrial precursors might explain the architecture of eukaryotic cell-death control (which involves mitochondrial permeabilization as a central gateway to death) as well as its complex regulation. However paradoxical it appears, the emergence of life might have promoted the advent of regulated death to maintain evolutionary progress and ecological balance, thus ultimately assuring the long-time establishment of life itself. If the lethal programs that we harbor in our own cells constitute a modern, adapted version of ancestral pathways that we can still recapitulate in prokaryotes, a more detailed comprehension of when death was young will have a profound scientific, medical, and even philosophical impact. Antibiotic-Induced Bacterial Cell Death Exhibits Physiological and Biochemical Hallmarks of ApoptosisDwyer et al.Molecular CellMay 24, 2012In BriefProgrammed cell death is a gene-directed process involved in the development and homeostasis of multicellular organisms. The most common mode of programmed cell death is apoptosis, which is characterized by a stereotypical set of biochemical and morphological hallmarks. Here we report that Escherichia coli also exhibit characteristic markers of apoptosis—including phosphatidylserine exposure, chromosome condensation, and DNA fragmentation—when faced with cell death-triggering stress, namely bactericidal antibiotic treatment. Full-Text PDF Open Archive