Commuting (to) suicide: An update on nucleocytoplasmic transport in apoptosis Patricia Grote, Karin Schaeuble, Elisa Ferrando-May" Ulliversity of KOllstanz, Departmellt of Biology, Molecular Toxicology, P.O. Box X911, D 78457 KOllstall z, GermallY Abstract Commuting is the process of travelling between a place of residence and a place of work. In the context of biology, this expression evokes the continuous movement of macromolecules between different compartments of a eukaryotic cell. Transport in and out of the nucleus is a major example of intracellular commuting. This article discusses recent findings that substantiate the emerging link between nucleocytoplasmic transport and the signalling and execution of cell death. KeYlVords: Caspase; Nuclear pore; DNA damage; Nuclear per~eability; Cellular stress; Ran; NFKB; GAPDH; PIDD; Virus Interorganellar cross-talk has become a fundamental of the nuclear pore complex (NPC) I in dying cells, (2) issue in the field of cell death by apoptosis. Distinct cel­ mechanisms of nuclear uptake/release of individual apo­ lular organelles and compartments such as the mitochon­ ptosis mediators, and (3) stress-induced changes in global dria, the endoplasmic reticulum, the nucleus and the transport activity. The present mini review highlights plasma membrane can be the source of primary signals some recent results related to each of these three topics. that lead to cell killing [1,2]. Conversely, downstream For a more detailed introduction into the molecular effectors of cell death, primarily caspases, act at different biology of the NPC and the mechanisms of nuclear subcellular sites to accomplish the dismantling of cellular transport, several excellent review articles are available structures. This dual role as initiator and victim of death (e.g. [5 7]). signalling cascades is probably best exemplified by the cell nucleus, being on the one hand the origin of the cell's response to DNA damage, and on the other hand, Stress- and pathogen-induced NPC dysfunction a major substrate for the demolishing activity of the apoptotic execution machinery. Thus, the apparatus gov­ A recent comprehensive analysis of the fate of the NPC erning the movement of molecules across the nuclear in cell death by apoptosis [8] has confirmed and extended membrane has the potential to influence the cell death previous data on the caspase-mediated degradation of process on multiple levels. In fact, connections between nuclear pore proteins [8 II]. According to these studies, nucleocytoplasmic transport and apoptosis have emerged both peripherally located nuclear pore proteins (Nups) as in the past and have been summarized in two recent arti­ well as two Nups within the NPC core are degraded, while cles [3,4]. In general, studies in the field have developed the overall structure of the pore remains intact. This is along three lines: (I) structural and functional alterations I Abbreviations used: NPC, nuclear pore complex; PARP, poly (ADP) ribose polymerase; AIF, apoptosis inducing factor; GAPDH, glyceralde • Corresponding author. Fax: + 49 7531 884033. hyde 3 phosphate dehydrogenase; PIDD, p53 induced protein with a Email address:[email protected] (E. Ferrando May). death domain; NO, nitric oxide; CA V, chicken anaemia virus. 157 thought to cause the collapse of the permeability barrier as disruption of the PARP-l gene, confer neuroprotection and to inhibit active transport, but conclusive data on in cellular and animal models of ischemia and stroke the functional consequences of caspase-mediated NPC [27,28]. Two papers have now pinpointed the molecular cleavage are still lacking. Other studies have proposed that mechanism of PAR-mediated cytotoxicity: the PAR poly­ NPC permeability may be altered in cells committed to die mer itself acts as a death signal causing neuronal demise in the absence of any apparent disruption of nuclear pore · when its concentration is increased either by transfection proteins [1 2,13] suggesting the existence of other mecha­ or by downregulating the expression of PARG, the glyco­ nisms of NPC dysfunction. hydrolase responsible for PAR catabolism [29]. A key Examples for such alternative, caspase-independent mediator of PAR-induced cell death is Apoptosis-Inducing pathways for breaking the nucleocytoplasmic barrier come Factor (AI F), a mitochondrial flavoprotein which was pre­ from virus-infected cells. The poliovirus 2A protease, for viously shown to move to the nucleus during apoptosis and example, mediates, directly or indirectly, the cleavage of trigger chromatin condensation as well as high-molecular the nucleoporins Nupl53 and p62. Most remarkably and weight DNA fragmentation [30]. In the absence of in contrast to what has been reported for apoptotic cells, PARP-l, the mitochondrial release of AIF is abrogated virus infection is accompanied by structural disruption of and cells are protected from death triggered by DNA dam­ the NPC as shown by electron microscopy [14 16]. In age, oxidative stress and excitotoxic insults [31]. Dawson yeast, degradation of nucleoporins as a consequence of oxi­ and co-workers have now demonstrated that the PAR dative insult was shown to be mediated by Pep4p, a cathep­ polymer acts as an AIF releasing factor [32]. In agreement sin 0 homolog [17]. Here again, nuclear envelope with its role as nuclear-mitochondrial messenger, PAR permeability increased prior to the degradat'ion of nucleo­ appears in the cytosol and at the mitochondria of cortical porins, supporting the notion that NPC function may be neurons after NMDA receptor stimulation. An open and altered without damage to its components. In fact, these very relevant question arising in this context concerns the observations are consistent with the notion of the NPC mechanism of PAR's nuclear extrusion. Only PAR poly­ as a dynamic, adaptive channel, whose conformation and mers of at least 60 ADP-ribose units, corresponding to a functional properties are highly sensitive to changes in cel­ minimum molecular weight of 33.6 kDa, display significant lular activity [5]. Considering the diversity of pathways toxicity. Larger polymers can arise in vivo as a consequence engaged by pathogens, by environmental noxa and by of PARP-l activation [33,34]. In addition, it is not yet clear developmental cues, all potentially culminating in cell whether the deadly polymers exit the nucleus as free mole­ death, it is not surprising to observe variations in the mode cules or associated with a (protein) carrier. In any case, of NE permeabilization depending on the death model sys­ propagation of this signal is likely to be restricted by the tem. Beside proteases, alterations of NPC structure and size exclusion limit of the NPC which under normal condi­ function during apoptosis could arise e.g. via calcium tions is in the range of about 40 kDa. Therefore, small vari­ [18], or hydrophobic compounds such as proapoptotic lipid ations in NPC structure could have a huge impact on the mediators [19,20] or the actin cytoskeleton [21,22]. The fact efficiency of PAR signalling to the mitochondria. It is inter­ that multiple and possibly overlapping pathways may lead esting in this respect that an increase in NPC permeability to the breakdown of the nucleocytoplasmic barrier in has been recently observed in cerebellar granule neurons apoptosis marks a distinction between this process and undergoing cell death by excitotoxicity (Bano, D., and . mitotic nuclear envelope disassembly, the latter ensuing Nicotera P., pers. communication) . from a specifically triggered, precisely orchestrated, and unique sequence of signalling events [23]. p53-induced protein with a death domain (PIDD) Apoptotic players moving in and out of the nucleus PlOD is a p53-inducible gene encoding a novel death domain-containing protein [35,36]. Initially characterized Poly-( ADP)-ribose (PAR) as a proapoptotic factor involved in the activation of cas­ pase-2 after genotoxic damage [37,38], PlOD was subse­ PAR is one of the most interesting newcomers to the quently determined to participate in cell survival via the growing family of molecules that commute from and to NFK-B pathway [39]. The molecular basis for these oppos­ the nucleus in cell death. Protein modification by poly­ ing roles of PlOD lies in the interaction with different (ADP)-ribose results from the activity of the poly-(ADP)­ adaptors resulting in the formation of two distinct macro­ ribose polymerase (PARP) family of enzymes. It has been molecular complexes, also termed PIDDosomes: RAIDD initially described as an immediate response to DNA dam­ is essential for building the platform for activation of cas­ age involved in the activation of several DNA repair path­ pase-2, while RIPI bridges the interaction of PlOD with ways. Additionally, and in particular in the nervous NEMO, a regulatory subunit of the IkB kinase complex. system, poly-(ADP)-ribosylation has been shown to medi­ The latter results in augmented SUMO-modification of ate cell injury in response to ischemia and oxidative stress NEMO and subsequent activation of NFK-B which then (for reviews see [24 26]) . Pharmacological inhibitors of triggers the expression of antiapoptotic genes (for review PARP-l, the founding member of the PARP family, as well see also [40]). The activity of PlOD as a molecular switch 158 between life and death is controlled by autoproteolysis and primarily in the nucleus [46]. As demonstrated by recently by the nucleocytoplasmic distribution of PIDD fragments, published heterokaryon experiments [47], Apoptin is a bona as described by a recent paper from J. Tschopp's group fide nucleocytoplasmic shuttling protein possessing signals [41]. According to this model, PIDD is cleaved by autopro­ for nuclear import (NLS) and export (NES). Transformed teolysis in two fragments with distinct functions: PIDD-C, cells display a cellular activity which negatively regulates which is generated by an initial autoproteolytic step and the NES and favours the NLS, resulting in nuclear accumu­ PIDD-CC a shorter fragment, whose formation requires lation. Phosphorylation of a threonine residue adjacent to a second, delayed cleavage event.