Báo cáo khoa học: ERK and cell death: ERK1⁄2 in neuronal death Srinivasa Subramaniam1 and Klaus Unsicker2 pptx

MINIREVIEW ERK and cell death: ERK1 ⁄ 2 in neuronal death Srinivasa Subramaniam 1 and Klaus Unsicker 2 1 Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD, USA 2 Molecular Embryology, Institute of Anatomy and Cell Biology, University of Freiburg, Germany Neuronal death in the nervous system Neuronal death is a major phenomenon in nervous system development and a hallmark of all neurodegen- erative diseases. Although numerous proteins have been implicated in neuronal death, the detailed mecha- nisms of how neurons succumb to death is far from clear. Signals for cell death can emanate from the cell surface or within the cell cytoplasm, mitochondria or nucleus [1–3]. Caspases are a well-characterized group of proteins that are involved in promoting an apopto- tic mode of cell death involving DNA fragmentation and cell shrinkage. Death receptors, such as tumor necrosis factor-a (TNF-a), on the cell surface can initi- ate a caspase cascade upon binding of ligands such as Fas ligand, TNF-a or TNF-related apoptosis-inducing ligand [3]. Although caspases play a crucial role in cell death, increasing evidence suggests that cells can turn on caspase-independent modes of cell death under a variety of circumstances [4]. Various modes of neuro- nal death have been described and are commonly observed in neurodegenerative disorders and stroke [5]. During development, dying neurons display similar changes in morphology and nuclear DNA degradation through an apoptotic process, although it has been suggested that more than one cell death mechanism may act during development [6]. In addition, both in development and in neurodegenerative diseases, only specific sets of neurons die. For example, in Hunting- ton’s disease, predominantly the striatal neurons degenerate. In Alzheimer’s disease (AD) and Parkin- son’s disease (PD), cholinergic neurons in the basal forebrain and dopaminergic neurons in the nigro-stria- tal system seem to be particularly vulnerable. What Keywords Akt ⁄ PKB; apoptosis; caspase; cell survival; ERK; ischemia; neurodegenerative disease; neuronal death; sustained ERK; transient ERK Correspondence S. Subramaniam, Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Fax: +410 955 3623 Tel: +410 955 2379 E-mail: ssubram9@jhmi.edu (Received 19 June 2009, revised 28 August 2009, accept 4 September 2009) doi:10.1111/j.1742-4658.2009.07367.x Extracellular signal-regulated kinase (ERK) is a versatile protein kinase that regulates many cellular functions. Growing evidence suggests that ERK1 ⁄ 2 plays a crucial role in promoting cell death in a variety of neuro- nal systems, including neurodegenerative diseases. It is believed that the magnitude and the duration of ERK1 ⁄ 2 activity determine its cellular func- tion. In this review, we summarize recent evidence for a role of ERK1 ⁄ 2in neuronal death. Furthermore, we discuss the mechanisms involved in ERK1 ⁄ 2 mediating neuronal death. Abbreviations AD, Alzheimer’s disease; CDK, cyclin-dependent kinase; CGN, cerebellar granule neurons; EGF, epidermal growth factor; ERK, extracellular signal-regulated kinase; JNK, c-JunNH 2 -terminal kinase; MAPK, mitogen-activated protein kinase; MCAO, middle cerebral artery occlusion; NGF, nerve growth factor; PD, Parkinson’s disease; ROS, reactive oxygen species; TNF, tumor necrosis factor. 22 FEBS Journal 277 (2010) 22–29 ª 2009 The Authors Journal compilation ª 2009 FEBS makes these specific sets of neurons particularly sus- ceptible to death stimuli and what signaling mecha- nisms are behind such selective neurodegeneration, are far from clear. However, an understanding of the mechanisms of neuronal death is crucial for designing effective therapeutic strategies for neurodegenerative diseases. Mitogen-activated protein kinase (MAPK) in neuronal death The MAPKs are serine ⁄ threonine protein kinases that promote a large diversity of cellular functions in many cell types. Three major mammalian MAPK subfamilies have been described: the extracellular signal-regulated kinases 1 and 2 (ERK1 ⁄ 2), the c-JunNH 2 -terminal kin- ases (JNK) and the p38 kinases. There is a widely accepted perception that JNK⁄ SAPK (stress-activated protein kinase) and p38 MAPK promote cell death, whereas ERK1 ⁄ 2 opposes cell death [5]. However, this view is overly simplistic. A growing number of studies have suggested a death-promoting role for ERK1 ⁄ 2in both in vitro and in vivo models of neuronal death. Recently, a new member of MAPKs, ERK5 (also called big mitogen-activated kinase 1; BMK1) has been identified and implicated in neuronal survival [7]. Rapid ERK5 activation was observed in the hippo- campal cornu ammonis (CA3) and dentate gyrus regions after cerebral ischemia [8]. In medulloblastoma cell lines, overexpression of ERK5 was shown to promote apoptotic cell death [9]. Most of the pharma- cological studies implicating ERK1 ⁄ 2 have been car- ried out using PD98059 or U0126 (which inhibits mitogen-activated protein kinase/ERK kinase (MEK), an upstream activator of ERK1⁄ 2). Both of these inhibitors also inhibit ERK5 activation [10]. Therefore, it remains to be seen whether ERK5 is also involved in ERK1 ⁄ 2-implicated cell death paradigms. ERK1 ⁄ 2 in cellular models of neuronal death The first evidence for a role of ERK1 ⁄ 2 in cell death was demonstrated in the oligodendroglial cell line, CG4. The addition of H 2 O 2 to CG4 cells resulted in activation of all three major MAPKs. However, H 2 O 2 - induced cell death was prevented by pharmacological blockade of the ERK1 ⁄ 2 pathway (PD98059) inhibitor only [11]. In neuronal cells, glutamate- or camptothe- cin-induced neuronal injury was abolished when ERK1 ⁄ 2 activation was suppressed using U0126 inhibi- tor [12,13]. Neuronal death induced by glutathione depletion was shown to be abolished when reactive oxygen species (ROS)-dependent activation of ERK1 ⁄ 2 was inhibited by either PD98059 or U0126 [14]. Nitric oxide produced by glial cells induced neuronal degener- ation through ERK1 ⁄ 2 activation that had been blocked by PD98059 or U0126 [15]. Another recent study using U0126 showed that death of striatal neu- rons induced by dopamine was associated with ERK1 ⁄ 2 activation [16]. Death of cortical neurons mediated by the transient receptor potential vanilloid 1 channel was abolished when ERK1 ⁄ 2 activation was suppressed by PD98059 [17]. Ho et al. [18] demonstrated that a zinc- dependent pathway of cell death is abolished when ERK1 ⁄ 2 activation is prevented by U0126. Consistent with a promoting role of ERK1 ⁄ 2 in cell death, hippo- campal damage after traumatic brain injury was pre- vented by the inhibition of ERK1 ⁄ 2 by PD98059 [19]. ERK1 ⁄ 2 activation is also implicated in hyperglycemia- mediated cerebral damage [20]. Similarly, ERK1 ⁄ 2 activation is involved in b-amyloid-induced neuronal cell death [21]. Thus, ERK1 ⁄ 2 activation seems to play an active role in several models of neuronal death. Transient versus sustained ERK1 ⁄ 2 activation Mechanisms underlying ERK1 ⁄ 2-mediated neuronal death are only beginning to emerge. It is challenging to understand how ERK1 ⁄ 2 can promote either neuronal survival or neuronal death under different paradigms. Oxidative stress generated by ROS is often linked to an activation of the ERK1 ⁄ 2 pathway. ROS-induced ERK1 ⁄ 2 activation has been demonstrated in a wide variety of cells, including neurons [22]. Oxidants induce neuronal death and in several other paradigms discussed above seem to require a sustained ERK1 ⁄ 2 activation for promoting neuronal death. Luo and DeFranco [23] elegantly demonstrated that a transient ERK1 ⁄ 2 activa- tion induced by glutamate in HT-22 cells reflects a pro- survival response. In contrast, sustained ERK1 ⁄ 2 activation observed after 6 h of glutamate treatment is a prodeath signal. Moreover, this study also demonstrated that sustained ERK1 ⁄ 2 activation alone is not sufficient to promote HT-22 cell death, implying that ERK1 ⁄ 2 must cooperate with other pathways or cellular compo- nents affected by glutamate to elicit cell death [23]. Transient ERK1 ⁄ 2 activation has also been observed upon growth factor stimulation. Brain derived neuro- trophic factor (BDNF) protects hippocampal neurons from glutamate toxicity by transient activation of ERK1 ⁄ 2 [24]. We have demonstrated that insulin-like growth factor-1 transiently induced ERK1 ⁄ 2, but abro- gated the induction of a prodeath sustained ERK1 ⁄ 2 signal in cerebellar granule neurons (CGN). We showed S. Subramaniam and K. Unsicker ERK1 ⁄ 2 in neuronal death FEBS Journal 277 (2010) 22–29 ª 2009 The Authors Journal compilation ª 2009 FEBS 23 that this inhibition is mediated via the phosphatidylino- sitol 3-kinase ⁄ protein kinase A ⁄ C-raf pathway [25]. In addition, we observed that transforming growth factor- b, a prodeath factor for CGN, also transiently enhanced ERK1 ⁄ 2 activation in CGN. However, transforming growth factor-b requires the sustained p38 pathway to induce CGN cell death [26]. These studies have strength- ened the notion that both the magnitude and the dura- tion of ERK1 ⁄ 2 activation determine the cellular outcomes, and that growth factors may exert regulatory functions with regard to the death-promoting capacity of the ERK1 ⁄ 2 pathway. In PC12 cells it is well known that epidermal growth factor (EGF) transiently induces ERK1 ⁄ 2 activity, which stimulates cell growth, whereas nerve growth fac- tor (NGF)-mediated sustained ERK1 ⁄ 2 activity leads to neurite outgrowth and cell survival. It has been pro- posed that this differential effect of ERK1 ⁄ 2 may depend on the specific receptor availability. It is well known that EGF receptors are downregulated faster than NGF receptors upon respective ligand binding. In addition, stimulation of endogenous EGF receptors promotes transient ERK1 ⁄ 2 and proliferation, and EGF receptor overexpression induces sustained ERK1 ⁄ 2 and promotion of differentiation. Thus, in PC12 cells, the duration of ERK1 ⁄ 2 activation seems to depend on surface receptor availability [27]. However, an implica- tion of surface receptor availability in regulating ERK1 ⁄ 2 is not the only mechanism determining the duration of ERK1 ⁄ 2 activation. For example, macro- phage migration inhibitory factor can induce sustained ERK1 ⁄ 2 via Rho and in a Rho kinase-dependent man- ner [28]. Alternatively, it can induce transient ERK1 ⁄ 2 via Src-type tyrosine kinase [29]. Overactivation of the EGF receptor in drosophila neurons, or cultured cortical neurons, leading to activation of the ERK1 ⁄ 2 pathway can promote neuronal degeneration [5]. Thus, temporal regulation of ERK1 ⁄ 2 not only depends on receptor availability, but also possibly on differential regulation of other signaling pathways (Fig. 1). It is noteworthy that a sustained ERK1 ⁄ 2 activation does not always promote cell death. As discussed above, in PC12 cells a sustained ERK1 ⁄ 2 activation induced by NGF promotes differentiation and cell sur- vival. Thus, the decision by sustained ERK1 ⁄ 2to induce cell death or survival possibly depends on addi- tional factors. NGF, in addition to inducing sustained ERK1 ⁄ 2, might also activate other parallel signaling pathways. For example, NGF might activate Akt ⁄ pro- tein kinase B or ERK5 for cell survival [30,31] and such prosurvival signals might suppress ERK1 ⁄ 2 cell death function in PC12 cells, as observed in CGN [25]. In addition, a recent study suggested that sustained ERK1 ⁄ 2 recruits micro-RNA to promote PC12 cell survival by blocking the expression of the proapoptotic BH3-only protein Bcl2-interacting mediator of cell death (BIM) [32]. Similarly, whether micro-RNA are involved in ERK1 ⁄ 2-dependent neuronal death is not known. Thus, sustained ERK1 ⁄ 2 may recruit differen- tial downstream factors to promote survival or death through yet unknown mechanisms. Mechanisms of ERK1 ⁄ 2-promoted cell death Oxidants can activate ERK1 ⁄ 2 either through acting on receptors, calcium channels, or directly on Src-tyrosine Fig. 1. Model of ERK1 ⁄ 2 in life and death. Both survival and death signals can activate ERK1 ⁄ 2. The mechanisms involved in such dif- ferential ERK1 ⁄ 2 activation and how ERK1 ⁄ 2 interacts with other cellular components are not yet clear. It is believed that the dura- tion, magnitude and ⁄ or compartmentalization of active ERK1 ⁄ 2 dic- tate the cellular outcome. For example, ERK1 ⁄ 2 may be transiently induced by growth factors, resulting in promotion of neuronal sur- vival (dotted arrow), whereas oxidative stress may result in a sus- tained induction of ERK1 ⁄ 2, which may promote neuronal death. However, for promoting cell death ERK1 ⁄ 2 induction must not always be sustained. In an MCAO model, ERK1 ⁄ 2 was shown to be transiently induced, but ERK1 ⁄ 2 inhibition significantly reduced the ischemic damage. In addition to ERK1 ⁄ 2, death signals can also activate stress kinases, such as p38 ⁄ JNK, which may further potentiate neuronal death (thick arrow). On the other hand, survival signals, such as protein kinase B ⁄ Akt, can inhibit sustained ERK1 ⁄ 2 and thereby promote neuronal survival. ERK1 ⁄ 2 in neuronal death S. Subramaniam and K. Unsicker 24 FEBS Journal 277 (2010) 22–29 ª 2009 The Authors Journal compilation ª 2009 FEBS kinase. Activated ERK1 ⁄ 2 can interact with cytoplasmic components or can translocate to the nucleus. Evidence has shown that sustained ERK1 ⁄ 2 is translocated to the nucleus [33,34] and nuclear translocated ERK1 ⁄ 2 can promote neuronal cell death, regulating transcription [5,35]. Although caspases have been implicated as pre- dominant inducers of apoptotic cell death, numerous studies have shown that apoptotic mechanisms can operate without the involvement of caspases [36–38]. Several studies have also demonstrated that caspase activation and the subsequent development of biochemi- cal or morphological features of apoptotic cell death are not mutually interdependent. Caspase-independent pathways can operate to promote apoptotic cell death and, conversely, cells dying through a nonapoptotic mode may recruit caspase-dependent pathways [5]. In the CGN model of neuron death, we observed activa- tion and nuclear translocalization of ERK1 ⁄ 2 after withdrawal of the survival signal. This sustained ERK1 ⁄ 2 activation promoted plasma membrane dam- age, whereas caspase-3 activation observed in a subset of CGN promoted DNA damage [34]. Biochemical and morphological features of plasma membrane-damaged CGN resembled neither necrosis nor apoptosis, but rather represented a mixture of apoptotic and necrotic features, including plasma membrane damage and apop- totic-like nuclear condensation. This ‘necro-apoptotic’ mode of neuron death could not be blocked by caspase inhibitors. Thus, ERK1 ⁄ 2 seems to play a crucial role in pro- moting this unique kind of cell death independent of caspase activation [34]. Similarly, ERK1 ⁄ 2 was shown to promote neuronal death in several other models independently of caspase. Thus, substance P and its receptor, neurokinin-1, mediate an alternative, nona- poptotic form of cell death in hippocampal, striatal and cortical neurons via ERK1 ⁄ 2 activation [39,40]. 17beta-E2, a steroid hormone, induces oncotic ⁄ necro- tic, but not apoptotic, programmed cell death in a sub- population of developing granule cells by activating the ERK1 ⁄ 2 pathway [41]. Okadaic acid-induced death of pyramidal cells in the CA3 region, which was not consistent with apoptotic features, is dependent on ERK1 ⁄ 2 activation [42]. In addition, it was demon- strated that neurotrophin-aggravated necrotic neuronal death was mediated by ERK1 ⁄ 2 [43]. Together, these data suggest that ERK1 ⁄ 2-mediated features of neuro- nal death may differ depending on cell type and death stimulus, but in several cell death models, ERK1 ⁄ 2 seems to promote predominantly a nonapoptotic mode of death independently of caspases. The identity of the molecular players associated with ERK1 ⁄ 2 in caspase- independent neuronal death still remains to be estab- lished [44]. Whether ERK1 ⁄ 2 directly activates cell death path- ways or whether it changes the prodeath gene expres- sion profile is not completely understood. Sustained ERK1 ⁄ 2 was shown to translocate to the nuclei, sug- gesting it may regulate prodeath gene expression [33,34]. When sustained ERK1 ⁄ 2 is retained in the cytoplasm, neuronal death is no longer observed, sug- gesting a requirement of nuclear retention for prodeath function [33]. On the other hand, in non-neuronal cells, cytoplasmic retention of ERK1 ⁄ 2 is required for death-associated protein kinase-mediated cell death [35]. Thus, ERK1 ⁄ 2 might promote cell death depend- ing upon the cell type. Considering the fact that ERK1 ⁄ 2 has multiple activators and targets, it is possi- ble that activation and its cell death-promoting func- tion may involve multiple partners and regulations [5]. ERK1 ⁄ 2 in neurodegeneration and ischemia ERK1 ⁄ 2-induced neuronal degeneration has also been extended to various models of neurodegenerative dis- ease. In AD, phosphorylated ERK1 ⁄ 2 immunoreactiv- ity in a granular appearance has been described in a subpopulation of hippocampal neurons with neurofi- brillary degeneration [45]. Tau, a microtubule-associ- ated protein, and its abnormal hyperphosphorylation have been linked to neuronal death in AD [46]. ERK1 ⁄ 2 is known to regulate tau hyperphosphoryla- tion [47,48], and it has been reported that activation of ERK1 ⁄ 2 in AD links oxidative stress to abnormal tau phosphorylation [45]. In addition, an upregulation of ERK1 ⁄ 2 has been shown to be associated with the progression of neurofibrillary degeneration in AD [49]. Considering that the mode and the mechanism of neuronal death in AD has not been fully resolved as yet [50], it is conceivable that ERK1 ⁄ 2 might play a crucial role in yet incompletely understood mecha- nisms of tau-mediated AD pathology. Furthermore, b-amyloid-induced sustained ERK1 ⁄ 2 activation has been shown to contribute to b-amyloid-induced tau phosphorylation and neurite degeneration [51]. 6-Hydroxy-dopamine (OHDA) and MPTP, neurotoxins commonly used in animal models of PD, have been shown to induce cell death through ERK1 ⁄ 2 activation [52,53]. More recently, granular cytoplasmic aggregates of activated ERK1 ⁄ 2 have been observed in the substan- tia nigra of PD patients with Lewy bodies [54]. Neuronal cell-specific cyclin-dependent kinase 5 (CDK5), a known regulator of neurodegenerative disorders such as AD and PD, can be a direct target for ERK1 ⁄ 2. Studies with S. Subramaniam and K. Unsicker ERK1 ⁄ 2 in neuronal death FEBS Journal 277 (2010) 22–29 ª 2009 The Authors Journal compilation ª 2009 FEBS 25 non-neuronal cells have suggested that ERK1 ⁄ 2 can regulate CDKs [55]. For example, ERK1 ⁄ 2 mediates DNA damage-induced breast cancer cell death via CDK5 regulation [56]. Whether CDKs are involved in ERK1 ⁄ 2-mediated death of neuronal cells is not known. ERK1 ⁄ 2 activation also plays a major role in ische- mia-induced cell death. Alessandrini et al. [57] have demonstrated a transient activation of ERK1 ⁄ 2 in the middle cerebral artery occlusion (MCAO) model. Inhi- bition of ERK1 ⁄ 2 activation reduced the infarct size by 55% compared with the control. Various other groups have reported an involvement of ERK1 ⁄ 2in MCAO, hypoxia–ischemia and other ischemic models [58–61]. ERK1 ⁄ 2 activation has also been reported in permanent MCAO [62], although the causal link between ERK1 ⁄ 2 activation and neuronal death still has to be proven in this model of permanent MCAO. So far, only the temporal pattern of activation of ERK1 ⁄ 2 in permanent MCAO suggests a role for ERK1 ⁄ 2 in neuronal death [63,64]. Conclusions and perspectives Initially, ERK1 ⁄ 2 activation was considered as a pro- moter of neuronal survival and memory [65–67]. How- ever, it is now clear that ERK1 ⁄ 2 activation can also participate in a variety of neuronal death signals. Such differential functions can be attributed to the duration of ERK1 ⁄ 2 signaling and association with other molecular players [68,69]. This association may elicit a unique pattern of molecular organization and may also result in a differential gene expression profile, which consequently results in different cellular functions. The opposing roles of ERK1 ⁄ 2 activation are perhaps best illustrated by its implication in the protection of CGN survival in N-methyl-d-aspartate-mediated excito- toxicty and its association with cortical neuron death following glutamate exposure [12,70]. Thus, the stimu- lus, the cell type and, probably most importantly, the duration of the activation of ERK1 ⁄ 2 decide the life or death of neurons. The opposing roles of ERK1 ⁄ 2 in neuron survival and death make it difficult to exploit it for cell survival strategies. 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Snyder Department of Neuroscience, Johns Hopkins University School of Medicine,. whether ERK5 is also involved in ERK1 ⁄ 2-implicated cell death paradigms. ERK1 ⁄ 2 in cellular models of neuronal death The first evidence for a role of ERK1 ⁄ 2 in cell death was demonstrated in. may result in a sus- tained induction of ERK1 ⁄ 2, which may promote neuronal death. However, for promoting cell death ERK1 ⁄ 2 induction must not always be sustained. In an MCAO model, ERK1 ⁄ 2