Neuronal p38 MAPK signalling: an emerging regulator of cell fate and function in the nervous system
Laboratory of Cell Signalling, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, and Laboratory of Cell Signalling, Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Laboratory of Cell Signalling, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, and Laboratory of Cell Signalling, Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
p38 mitogen-activated protein kinases (MAPKs), together with extracellular signal-regulated kinases (ERKs) and c-Jun N-terminal kinases (JNKs), constitute the MAPK family. Multiple intracellular signalling pathways that converge on MAPKs exist in all eukaryotic cells and play pivotal roles in a wide variety of cellular functions. p38 MAPKs and JNKs, also termed stress-activated protein kinases (SAPKs), are preferentially activated by various cytotoxic stresses and cytokines and appear to be potent regulators of stress-induced apoptosis. Whereas JNKs have been shown to play pivotal roles in the regulation of neuronal apoptosis, the role of p38 MAPKs in the nervous system is poorly understood. However, accumulating evidence from mammalian cell culture systems and the strong genetic tool C. elegans suggests that neuronal p38 signalling has diverse functions beyond the control of cell death and survival. This review focuses on possible roles for the p38 pathway in the nervous system, with principal emphasis placed on the roles in neuronal cell fate decision and function.
In neuronal and non-neuronal cells, various intracellular signalling pathways strictly control cell function and fate, i.e. differentiation, death and survival. The MAP kinase cascades are among such signalling pathways and are evolutionarily well conserved in all eukaryotic cells (Ichijo 1999; Kyriakis & Avruch 2001; Widmann et al. 1999). Three MAP kinase cascades that converge on ERKs, JNKs and p38 MAPKs have been extensively characterized, and each consists of three classes of serine/threonine kinases, MAPK, MAPK kinase (MAPKK, also referred to as MEK) and MAPKK kinase (MAPKKK). MAPKKK phosphorylates and thereby activates MAPKK, and activated MAPKK in turn phosphorylates and activates MAPK. Whereas the ERK cascade is generally involved in the control of cell proliferation and differentiation by mitogens and growth factors, the JNK and p38 cascades are preferentially activated by environmental stresses such as UV radiation, X-rays, heat shock and osmotic shock, and by proinflammatory cytokines such as tumour necrosis factor (TNF) and interleukin-1 (IL-1) (Tibbles & Woodgett 1999).
Recently, potential roles for JNKs and p38 MAPKs in neuronal apoptosis have received a great deal of attention. Mice lacking the JNK3 gene, a member of the JNK family, have been reported to exhibit a marked reduction in excitotoxicity-induced apoptosis of hippocampal neurones, clearly demonstrating the requirement of JNK activity for neuronal apoptosis (Yang et al. 1997). Moreover, compound mutant mice lacking the JNK1 and JNK2 genes have been shown to be embryonic lethal due to severe dysregulation of apoptosis in brain, suggesting that JNK1 and JNK2 regulate region-specific apoptosis during early brain development (Kuan et al. 1999). Several lines of evidence have suggested that p38 MAPKs also play roles in neuronal apoptosis (Harper & LoGrasso 2001; Kawasaki et al. 1997; Mielke & Herdegen 2000; Xia et al. 1995). However, since the hypothesis of a pro-apoptotic role of p38 MAPKs has been based on studies mainly using the over-expression of dominant-negative mutants and p38 inhibitors such as SB203580, it remains unclear whether the p38 pathway is involved in neuronal apoptosis in vivo. Indeed, no obvious abnormalities in neuronal development have been reported in mice lacking the p38α gene, which is the only gene that has thus far been reported to be disrupted among the four p38 isoforms (Adams et al. 2000; Allen et al. 2000; Mudgett et al. 2000; Tamura et al. 2000). In this regard, analyses of knockout mice for other p38 isoforms and their compound mutant mice will be needed to research a precise conclusion concerning a pro-apoptotic role of p38 MAPKs, as has been successfully obtained in the case of JNKs.
In addition to the regulation of cell death and survival as stress-activated kinases, p38 MAPKs have been shown to have diverse biological functions including cell fate specification and control of cellular function in various types of cells including neuronal cells (Martin-Blanco 2000; Nebreda & Porras 2000; Ono & Han 2000). In this review, we discuss the possible roles of p38 signalling in neuronal cell fate decision, in light of analyses using mammalian cell culture systems and C. elegans. We also introduce emerging functions of p38 signalling which are relevant to the regulation of neuronal function.
Components of the p38 MAPK signalling pathway
p38 MAPK was originally identified as a kinase phosphorylated in response to endotoxic lipopolysaccharide (LPS), and was found to be the mammalian MAPK homologue structurally and functionally related to HOG1, the osmo-sensing MAPK of S. cerevisiae (Han et al. 1994). p38 MAPK was also identified as a molecular target of a class of experimental pyridinyl-imidazole compounds, the cytokine-suppressive anti-inflammatory drugs (CSAIDs) represented by SB203580 (Lee et al. 1994). Four isoforms of p38 MAPK have been identified to date, and all are thought to play roles in cellular responses to environmental stresses and proinflammatory cytokines. The original isoform is p38α, also referred to as CSAIDs binding protein (CSBP) and SAPK2a, and the other three isoforms are p38β/SAPK2b (Jiang et al. 1996; Stein et al. 1997), p38γ/SAPK3/ERK6 (Goedert et al. 1997b; Lechner et al. 1996), and p38δ/SAPK4 (Goedert et al. 1997a; Jiang et al. 1997; Wang et al. 1997). Transcripts of the p38α and p38β genes are ubiquitously expressed in tissues, whereas that of p38γ gene is highly expressed in muscle and that of p38δ gene is enriched in lung and kidney (Jiang et al. 1997; Wang et al. 1997). Due to the unavailability of antibodies that clearly distinguish the four isoforms, it remains to be determined whether the distribution of mRNA expression among tissues corresponds to that of functional protein expression of the p38 isoforms.
p38 MAPK inhibitors have been extensively studied as anti-inflammatory drugs, since p38 MAPKs critically control the production of TNF and IL-1, which play important roles in regulation of the inflammatory response (Lee et al. 2000). SB203580 is the most commonly used compound among various p38 MAPK inhibitors, and its availability has greatly contributed to understanding of the biological functions of p38 MAPKs. Nevertheless, it is important to note that pyridinyl-imidazole compounds inhibit only p38α and p38β (English & Cobb 2002). The biological significance of p38γ and p38δ has therefore been less well characterized, since no effective and specific inhibitors of them have been available.
The rat pheochromocytoma cell line PC12 is a culture system useful for the study of neuronal differentiation (Greene & Tischler 1976). Following treatment with nerve growth factor (NGF), PC12 cells differentiate with sympathetic neurone-like characteristics including neurite outgrowth. In the NGF-induced neuronal differentiation of PC12 cells, the ERK pathway is thought to mediate signals necessary and sufficient for differentiation, since constitutively active MEK induces neurite outgrowth and selective blockade of the ERK pathway using dominant-negative MEK or MEK inhibitors results in the inhibition of NGF-induced neurite outgrowth (Cowley et al. 1994; Fukuda et al. 1995; Pang et al. 1995). However, recent studies have suggested that p38 signalling also participates in the NGF-induced differentiation of PC12 cells (Fig. 1). It has been reported that NGF induces sustained activation of p38 and that selective blockade of the p38 pathway results in inhibition of NGF-induced neurite outgrowth in PC12 cells (Morooka & Nishida 1998). Surprisingly, neurite outgrowth induced by the expression of constitutively active MEK, which has been recognized as a specific MAPKK of the ERK pathway, also partly depends on p38 activity, since MEK activates not only ERKs but also p38 MAPKs, and MEK-induced neurite outgrowth is sensitive to SB203580 in PC12 cells (Morooka & Nishida 1998). These findings suggest that p38 signalling is necessary for the NGF-induced differentiation of PC12 cells. Other evidence for the involvement of the p38 pathway in NGF signalling is that NGF-induced phosphorylation at the critical site of regulation of the transcription factor CREB is mediated in concert with the ERK and p38 pathways (Xing et al. 1998). Since CREB appears to be one of the transcription factors critical for growth factor-mediated cell fate decision (Mayr & Montminy 2001), a well-balanced activation of the ERK and p38 pathways may be necessary for neuronal differentiation in response to NGF.
Bone morphogenetic protein (BMP)-2, a member of the TGF-β superfamily, also induces the neuronal differentiation of PC12 cells (Iwasaki et al. 1996; Paralkar et al. 1992). In this process, p38 MAPKs but not ERKs are activated upon BMP treatment. Consistent with this, SB203580 and dominant-negative MKK3 or MKK6 effectively inhibit BMP-induced differentiation (Iwasaki et al. 1999). BMPs, originally identified as bone- and/or cartilage-inducing factors, have recently been recognized as an emerging group of neurotrophic factors (Mehler et al. 1997). Thus, the p38 pathway may play a common role in neurotrophic factor-mediated cell fate decision. Furthermore, it has recently been reported that activation of the cyclic AMP (cAMP) pathway by stimulation with forskolin induces neuronal differentiation of PC12 cells in a p38 activity-dependent manner (Hansen et al. 2000), suggesting that the p38 pathway may also participate in growth factor-independent signalling for differentiation.
Nevertheless, it has not been fully determined how the activity of p38 MAPKs is regulated in neuronal differentiation, especially at the level of MAPKKK. TAK1 is a candidate for the mediator of BMP-induced neuronal differentiation, since dominant-negative TAK1 has been shown to inhibit p38 activation and neurite outgrowth induced by BMP (Yanagisawa et al. 2001). We have recently shown that expression of constitutively active ASK1 effectively induces neurite outgrowth and the up-regulation of neuronal marker proteins in PC12 cells (Takeda et al. 2000). Together with the findings that ASK1 induces a sustained activation of p38 MAPKs but not of ERKs, and that the neurite outgrowth induced by ASK1 is inhibited by the treatment with SB203580, these results suggest that ASK1-induced differentiation is dependent on p38 activity. Interestingly, it is likely that not all MAPKKKs which can activate the p38 pathway necessarily induce neuronal differentiation in PC12 cells, since expression of constitutively active MEKK1 induces apoptosis instead of differentiation (Le-Niculescu et al. 1999). In PC12 cells, MEKK1 activates the JNK and p38 pathways to nearly equal extents, whereas ASK1 preferentially activates p38 MAPKs, and the extent of JNK activation induced by ASK1 is limited. Therefore, MAPKKK that preferentially activates the p38 pathway may be a necessary component in differentiation signalling in PC12 cells.
Involvement of the p38 pathway in neuronal differentiation has also been reported in other cell culture systems. Under serum-free conditions, N1E-115 neuroblastoma cells are known to exhibit neurite outgrowth, which is enhanced by integrin-mediated signal (Sarner et al. 2000). It has recently been shown that Integrin-linked kinase (ILK), a cytoplasmic serine/threonine kinase that binds to the cytoplasmic domain of β1 integrin, is a candidate mediator of the integrin-induced neurite outgrowth in N1E-115 cells, and that the p38 pathway is one of the downstream effectors of ILK-mediated signalling (Ishii et al. 2001). In MN9D dopaminergic neuronal cells, expression of calbindin-D28K, a member of the calmodulin superfamily of calcium-binding proteins, induces neurite outgrowth in a p38 activity-dependent manner (Choi et al. 2001). These findings further support the possibility of a role for p38 signalling in the differentiation of various types of neuronal cells.
A novel type of cell fate decision via the ASK1-p38 axis in C. elegans
Recently, strong evidence supporting the above-mentioned roles for the p38 pathway in mammalian neuronal differentiation has emerged from an excellent genetic tool, C. elegans. In C. elegans, the bilaterally symmetric pair of olfactory neurones, AWC, senses several odourants (Bargmann et al. 1993). Although the two bilateral AWC neurones are structurally similar to each other, the laterally asymmetric fate of the two neurones is developmentally determined through a process that requires normal axon guidance and axon contact (Troemel et al. 1999). Expression of the candidate odourant receptor STR-2 is restricted to either the left or the right AWC neurone in a stochastic manner (Fig. 2), and this asymmetric expression is required for odour discrimination and odour chemotaxis (Wes & Bargmann 2001). Genetic screening for mutants defective in STR-2 asymmetry has revealed that calcium signalling through UNC-2 and UNC-36, which are subunits of a voltage-dependent calcium channel, and UNC-43, calcium/calmodulin-dependent protein kinase II (CaMKII), acts as a negative regulator of STR-2 expression (Troemel et al. 1999). Furthermore, MAPKKK NSY-1 and MAPKK SEK-1, which are most similar to mammalian ASK1 and MKK3/MKK6, respectively, have been found to repress STR-2 expression downstream of UNC-43/CaMKII (Sagasti et al. 2001; Tanaka-Hino et al. 2002) (Fig. 2). Candidate MAPKs that act as determinants of AWC cell fate downstream of the NSY-1-SEK-1 pathway appear to be p38 MAPKs but not JNKs, since the C. elegans JNK homologue JNK-1 does not affect the asymmetric expression of STR-2 in AWC neurones (Tanaka-Hino et al. 2002). These findings strongly suggest that calcium signals can determine cell fate during neuronal development by modulating the p38 pathway. Consistent with this, in mammals as well, CaMKII recruits and activates ASK1 by phosphorylation in calcium signalling, and calcium influx evoked by membrane depolarization induces the strong activation of p38, which is impaired in cells derived from ASK1-deficient mice (Tobiume et al. 2001; our unpublished data). Thus, ASK1 is a functionally conserved intermediate of calcium signalling between CaMKII and p38. Together with the potential of ASK1 to induce neuronal differentiation in vitro as noted above, the ASK1-p38 axis may thus play a role in the determination of neuronal cell fate.
A precise control of neuronal activity is a prerequisite for the development of synaptic connections and the maintenance of the nervous system. Calcium signalling is an important mediator of neuronal survival during development. Although first characterized as a myogenic factor, MEF2 has been shown to be a calcium-regulated transcription factor and to act as a mediator of neuronal activity-dependent cell survival (Mao et al. 1999). The p38 pathway appears to activate MEF2 in response to calcium influx into cerebellar granule cells, and activated MEF2 regulates neuronal survival by stimulating MEF2-dependent gene transcription. Since MEF2 is primarily expressed in differentiating neurones but not in actively dividing neuronal precursor cells, it may act in newly differentiated neurones to support neuronal activity-dependent maturation of the nervous system in vivo. Another relevant finding is that MEF2C, the predominant form in the mammalian cerebral cortex, and p38 activity are required for cell survival during the retinoic acid-induced neuronal differentiation of P19 embryonal carcinoma cells, suggesting a positive role for the p38-MEF2 cascade in neuronal differentiation (Okamoto et al. 2000). As concerns cell death/survival control through the p38 pathway, however, quite a few findings have suggested a pro-apoptotic role for p38 MAPKs in neuronal cells (Harper & LoGrasso 2001; Kawasaki et al. 1997; Mielke & Herdegen 2000; Xia et al. 1995). One plausible explanation for this discrepancy is that the physiological level of neuronal activity might control the p38 pathway as a mediator of cell survival. In contrast, once cells suffer strong stress beyond the physiological level, the p38 pathway might act as a stress-activated mediator of neuronal apoptosis. In this regard, ASK1 is a candidate for the upstream regulator of these opposite effects of p38 MAPKs, since ASK1 appears to function as both a pro- and an anti-apoptotic signalling intermediate depending on cell type and/or cellular context (Kanamoto et al. 2000; Matsuzawa & Ichijo 2001; Takeda et al. 2000).
During development, the migration of certain types of neuronal cells to their precise positions is necessary for establishing the integrity of the nervous system. Neurones producing gonadotropin-releasing hormone (GnRH neurones) are among such cells, and migrate from the olfactory placode to the hypothalamus during development (Wray et al. 1994; Yoshida et al. 1995). Recently, the critical role of the membrane receptor adhesion-related kinase (Ark) and its ligand encoded by growth arrest-specific gene 6 (Gas6) in the migration of GnRH neurones has been reported (Allen et al. 2002). Gas6/Ark signalling appears to promote the migration of GnRH neuronal cells by activating Rho family GTPase Rac-dependent actin cytoskeletal reorganization. Importantly, p38 MAPKs are activated downstream of Ark and Rac, and blockade of the p38 pathway inhibits Gas6/Ark-mediated migration, strongly suggesting that the p38 pathway is involved in neuronal migration. Recent studies suggest that the p38 pathway also regulates migration of non-neuronal cells. The growth factor- and stress-stimulated migration of endothelial and smooth muscle cells (Hedges et al. 1999; Huot et al. 1997; Matsumoto et al. 1999; Rousseau et al. 1997) and the chemokine- and bacterial chemoattractant-stimulated chemotaxis of neutrophils (Heuertz et al. 1999; Nick et al. 2000) have been shown to depend on the activity of p38 MAPKs. In these non-neuronal cells, a proposed possible mechanism by which the p38 pathway regulates cell migration is the control of cytoskeletal remodelling. Heat shock protein 27 (HSP27), depending on its phosphorylation state, influences the modulation of actin polymerization as an F-actin cap binding protein (Lavoie et al. 1995). MAPKAPK-2, -3, and PRAK when activated by p38 MAPKs phosphorylate HSP27 (McLaughlin et al. 1996; New et al. 1998; Stokoe et al. 1992b), and phosphorylated HSP27 appears to play a central role in cytoskeletal remodelling (Huot et al. 1997; Rousseau et al. 1997). In GnRH neurones as well, the p38-MAPKAPK-2-HSP27 axis has been shown to regulate cell migration (Allen et al. 2002). The p38 pathway may thus control neuronal migration by a common mechanism irrespective of cell type.
p38 signalling in the control of neuronal function
Among the diverse functions of the nervous system, the mechanisms of synaptic plasticity that are associated with learning and memory are particularly intriguing. The important role of the ERK pathway in the control of neuronal plasticity has been well characterized. The ERK pathway appears to regulate long-term potentiation (LTP) in the mammalian hippocampus and long-term facilitation in Aplysia (Impey et al. 1999). Several experiments using the in vivo systemic administration or infusion of MEK inhibitors further suggests a role for the ERK pathway in long-term memory formation (Mazzucchelli & Brambilla 2000). By contrast, involvement of the p38 pathway in the control of synaptic function has only recently been demonstrated. It has been shown that the p38 pathway serves as a signal mediator in the induction of metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) at excitatory synapses between CA3 and CA1 pyramidal neurones in the mammalian hippocampus (Bolshakov et al. 2000). This observation suggests that the ERK and p38 pathways act in opposing fashions in regulating synaptic plasticity. Accordingly, the p38 pathway may participate in the impairment of LTP in perforant path-granule cell synapses when the concentration of IL-1β increases in the dentate gyrus (Vereker et al. 2000). Moreover, involvement of p38 MAPKs in associative learning, as measured by classical conditioning of the rabbit's eye-blink response has also been reported (Zhen et al. 2001). In this section, we discuss possible mechanisms by which the p38 pathway may control neuronal function.
The synaptic synthesis of specific proteins is thought to play a role in the functional and morphological modification of synapses. CaMKIIα, a key molecule of synaptic plasticity, is one such protein; mRNAs for CaMKIIα are located in dendrites (Burgin et al. 1990) and the synaptic mRNAs appear to be actively translated in response to synaptic activity (Bagni et al. 2000). Translation is a highly regulated process that permits rapid cellular responses to diverse stimuli independently of transcription. Initiation of protein synthesis and elongation of peptides are regulated by eukaryotic initiation factors (eIFs) and elongation factors (eEFs), respectively, and the critical roles of these factors in translational control have been elucidated (Dever 1999; Rhoads 1999). Although the molecular mechanism of local protein synthesis in synapses is unclear, it has begun to be elucidated by a recent study in which signalling through NMDA receptors appeared to control protein synthesis by modulating the phosphorylation state of eEF2 at developing synapses (Scheetz et al. 2000). eEF2 kinase (eEF2K), which inactivates eEF2 by phosphorylation, has recently been shown to be a direct substrate of p38δ, but not of other p38 isoforms (Knebel et al. 2001). p38δ activated by cellular stresses phosphorylates eEF2K at Ser359 and thus appears to inactivate its ability to suppress eEF2 activity (Fig. 3). It will be of great interest to determine whether p38δ can be regulated not only by cellular stresses but also by synaptic activity and serve as a regulator of local protein synthesis by regulating eEF2K in synapses. To test this hypothesis, it must be determined whether functional p38δ is expressed in neurones, especially in synapses.
Several lines of evidence have suggested that the p38 pathway also regulates eIFs (Fig. 3). MNK-1 and -2 are downstream kinases regulated by the ERK and p38 pathways, and are thought to be the physiological kinases for eIF-4E, a protein binding to capped mRNAs (Waskiewicz et al. 1997, 1999). In the Aplysia nervous system, it has recently been reported that the physiological phosphorylation of eIF-4E at the equivalent site in mammalian orthologs may be regulated by p38 MAPKs but not by ERKs, suggesting the possibility of involvement of the p38 pathway in translational control in the nervous system (Dyer & Sossin 2000). In addition, p38 MAPKs and their downstream kinase MSK-1 appear to be required for the mediation of ultraviolet B (UVB) irradiation-induced phosphorylation of eIF4E-binding protein 1 (4E-BP1), which binds to and inactivates eIF-4E in resting cells (Liu et al. 2002). These findings strongly suggest that the p38 pathway plays roles in the translational machinery at multiple steps, although there is not yet sufficient evidence for this in neurones. The above-mentioned factors related to the translational control have been well characterized as targets of other signalling molecules such as mammalian target of rapamycin (mTOR) and ERKs in insulin and mitogen signalling (Rhoads 1999; Wang et al. 2001). Taken together, these findings suggest that when the ERK and p38 pathways act in opposing fashions in regulating synaptic plasticity, one possible mechanism may be differential control of protein synthesis by the two signalling pathways.
It has recently been demonstrated that p38 MAPKs regulate endocytic trafficking (Cavalli et al. 2001) (Fig. 4). The small GTPase Rab5, one of the key regulators of early endocytic trafficking, cycles between GTP- and GDP-bound states (Martinez & Goud 1998). In addition, Rab5 also cycles between membrane-bound and cytosolic states, and this cycling requires guanyl-nucleotide dissociation inhibitor (GDI) (Wu et al. 1996). GDI extracts the GDP-bound Rab proteins from membranes and forms a cytosolic GDI:Rab complex. GDI functions as a vehicle, which delivers Rab proteins to the appropriate target membrane where Rab proteins are reloaded by a GDI displacement factor (Dirac-Svejstrup et al. 1997). Consistent with the suggestion that cytosolic kinases control the Rab cytosolic cycle (Steele-Mortimer et al. 1993), p38 MAPKs have been identified as activators of GDI, perhaps by phosphorylation (Cavalli et al. 2001). The p38-dependent response to stresses such as hydrogen peroxide and UV irradiation stimulates the formation of the GDI:Rab5 complex and accelerates endocytosis, while inhibition of the p38 pathway has the opposite effects. These findings have been consolidated using cells from p38α-deficient mice.
A possible role for this p38-dependent endocytic trafficking in the control of neuronal function might be the regulation of glutamate receptors of the AMPA subtype (AMPARs). It has been shown that AMPARs are internalized in response to various extracellular stimuli through processes which are similar to the agonist stimulation-dependent internalization of G protein-coupled receptors and receptor tyrosine kinases, and are recycled back to the plasma membrane or targeted to the degradative pathway, probably depending on the type of stimuli (Carroll et al. 2001). AMPARs are internalized via clathrin-coated pits into endosomal compartments (Man et al. 2000). Rab5 appears to be involved in the early endocytic trafficking of AMPARs, since co-localization of the two molecules can be transiently observed in response to agonist stimulation (Ehlers 2000). The physical transport of AMPARs in and out of the synaptic membrane is thought to play a critical role in synaptic plasticity (Carroll et al. 2001). Particularly relevant to the possible involvement of p38 MAPKs in mGluR-dependent LTD, rapidly accumulating evidence has suggested that the down-regulation of AMPARs via endocytosis in response to glutamate receptor activation contributes to the reduction in synaptic strength observed with LTD (Beattie et al. 2000; Man et al. 2000; Matsuda et al. 2000). Consistent with this, a selective agonist of mGluR (R,S)-3,5-dihydroxyphenylglycine (DHPG), stimulates internalization and thus induces loss of synaptic surface AMPARs (Snyder et al. 2001). Although no direct evidence for it has yet emerged, involvement of the p38 pathway in endocytic trafficking of glutamate receptors is an interesting hypothesis.
The p38 MAPK pathway is recognized as a critical regulator of cellular response to a wide variety of stresses. However, roles of the p38 pathway in neuronal cell fate decisions, together with roles in the differentiation of other types of cells, suggest that p38 signalling has a broad range of biological activities beyond the stress response. Accumulating evidence suggesting possible roles in the control of neuronal function further supports this notion. Since the nervous system is known to be highly vulnerable to various stresses, p38 signalling may control neuronal function while monitoring the extent of external stresses for maintenance of the nervous system. Understanding the physiological functions of p38 MAPK signalling may give us the opportunity to elucidate the undisclosed complexity of the nervous system.
We thank A. M. Watabe and all members of the Cell Signalling Laboratories for valuable discussion.