A wide variety of extracellular stimuli are capable of activating the ERK1/2 cascade. Mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)1 and 2 are the immediate upstream kinases that phosphorylate and activate ERK1/2. MEK1 and 2 are dual-specificity protein kinases that mediate the phosphorylation of tyrosine and threonine residues. The activity of MEK1/2 is also regulated by phosphorylation, as MEK1 and 2 are phosphorylated by mitogen-activated protein kinase kinase kinases (MAP3Ks). The most extensively studied MAP3Ks are the Raf proteins, including A-Raf, B-Raf, C-Raf and Raf-1 (10). They are activated by MAP4K proteins, such as Rap1, Ras, PKA and Rho (Fig. 1). ERK1/2 is a ubiquitously expressed hydrophilic non-receptor protein that participates in the Ras-Raf-MEK-ERK signal transduction cascade, which is involved in various diseases, including cancer, cardiac hypertrophy, pain and neuroinflam-mation (11–14). Therefore, this cascade is an interesting target for basic and translational research, including the development of drugs for therapeutic purposes.

Once activated, p-ERK1/2 can translocate into the nucleus to activate a wide array of transcription factors or can simply remain in the cytoplasm, where it regulates other subcellular functions (Fig. 1). ERK1 and 2 have more than 175 documented cytoplasmic and nuclear substrates (15). ERK1/2 nuclear targets include the ternary complex factor family of transcription factors. These proteins mediate the expression of immediate early genes, whose products contribute to cell survival, division and motility (16,17). Elk1 is one of the most thoroughly studied targets of the ERK1/2 MAPK kinase cascade, and Elk1 activation leads to increased transcriptional activity (15). Members of the ERK1/2 family of protein kinases participate in a wide variety of cellular processes. To date, more than 50 cytoplasmic substrates have been identified, including the ribosomal S6 kinase (RSK) family of protein kinases, apop-totic proteins and cytoskeletal proteins. The RSK family consists of four human RSK isoforms (RSK1-4), mitogen- and stress-activated kinase (MSK)1 and 2, which are directly activated by ERK1/2 in response to stimuli. RSK1-4 are key components downstream of the Raf-MEK-ERK signaling cascade. The RSK family regulates transcription by mediating the phosphorylation of various types of transcription factors, including nuclear factor-κB (NF-κB), serum response factor (SRF) and transcription initiation factor (TIF), in cells (18).

Scaffolds are proteins that bind to multiple components of signaling modules. Scaffolds regulate and integrate overall signal transduction and play a pivotal role in the spatial and temporal regulation of the ERK1/2 signaling cascade. In response to stimulus exposure, ERK1/2 binds to a variety of cytoplasmic scaffold and anchor proteins, including the suppressor of Ras (KSR1/2), MEK partner 1 (MP1), IQ motif-containing GTPase activating protein 1 (IQGAP1) and MAP/ERK kinase kinase 1 (MEKK1) (19,20). MP1, which is also known as MAP kinase scaffold protein 1 and LAMTOR3, was identified as a scaffold protein that potentiates MAPK signaling by binding to MEK1 and ERK1. MP1 is localized to endomembrane compartments as part of larger signaling complexes and modulates the Raf-MKK1/2-ERK1/2 pathway together with its partner, p14 (21,22). In fact, IQGAP1 is a well-known regulator of signaling events involved in the MAPK pathway. The interaction between IQGAP1 and ERK1/2 plays a critical role in tumor formation, as competition for ERK1/2 binding between IQGAP1 and a peptide that encompasses the WW domain inhibits Ras and Raf-driven tumorigenesis (23). MEKK1, a MAP3 kinase, catalyzes the phosphorylation of MEK1 and 2, which are components of the ERK pathway. Xu et al (24) and Karandikar et al (25) both showed that MEKK1 binds to C-Raf, MEK1 and ERK2 of the ERK1/2 MAPK signaling module. Recent studies have suggested that KSR1 and 2 possess catalytic activity and that KSR2 participates in the assembly of a MEK1/KSR2/B-Raf ternary complex that is responsible for promoting rabbit MEK1 phosphorylation by mouse B-Raf (26,27).

Synaptic plasticity is thought to be crucial for information processing in the brain and to underlie many complex behaviours. The best studied forms of synaptic plasticity in the CNS are long-term potentiation (LTP) and long-term depression (LTD). The regulation of protein phosphorylation has an important role in the process of LTP and LTD.

Several recent studies have implicated the ERK1/2 pathway in the control of synaptic plasticity in the adult nervous system (29,30). English and Sweatt (31) investigated the role of MAPKs in regulating synaptic plasticity in adult rat neurons, with a particular focus on the modulatory role of ERK1/2 in hippocampal LTP. They provided the first demonstration of N-methyl-D-aspartate (NMDA)-receptor dependent activation of ERK2 in rat hippocampal area CA1 in response to LTP-inducing high-frequency stimulation and suggested a crucial regulatory role of ERK2 in synaptic plasticity. Kanterewicz et al (32) further confirmed the role of ERK1/2 in NMDA receptor-independent LTP in the hippocampus. Over the past few years, a number of studies have demonstrated that ERK1/2 activity is required for several forms of synaptic plasticity in the amygdala which is associated with fear-dependent learning (33,34). Ratto and Pizzorusso (35) offered evidence, both in vivo and in vitro, that ERK1/2 plays a crucial role in controlling synaptic plasticity in the visual cortex. Inhibition of ERK1/2 can prevent the induction of various forms of LTP and LTD in the hippocampus and amygdala (33,36). These studies indicated that a requirement for ERK1/2 activation is common to many forms of synaptic plasticity but that the precise targets of ERK1/2 may differ between different types of plasticity.

Evidence has shown that total ERK1/2 activity controls the proliferation of certain late-born progenitor cells and the differentiation of neurons and glia during fetal brain development and that the two may compensate for each other during this process, at least in part, due to their overlapping functions (37,38). Samuels et al (39,40) also found that mutations that increase ERK1/2 activity can result in macrocephaly, while mutations that decrease ERK1/2 activity can result in microcephaly, suggesting that the ERK1/2 pathway is involved in the expansion of human neural progenitor cells. Furthermore, evidence indicates that ERK1/2 also takes part in regulating the proliferation and differentiation of astrocytes in the developing brain. Li et al (41) found that MEK/ERK signaling regulated the generation of glia from radial progenitors in the developing cortex, leading to a major increase in the number of astrocytes in the brain. This finding provides insight into the mechanisms involved in ERK1/2-mediated regulation of normal and abnormal astrocyte function during brain development. Recent evidence has consistently demonstrated that the ERK1/2 pathway is one of the dominant intracellular pathways for the regulation of oligodendroglial development, myelination and remyelination (38,42–44).

Although ERK1/2 activation has generally been associated with brain cell differentiation and proliferation, a number of studies have shown that the activation of ERK1/2 can mediate cell death in several neuronal systems (45,46). The different effects of ERK1/2 on brain cells may be owing to the various stimuli and cell types involved. The activation of ERK1/2 was observed in glutamate- and heme-induced neuronal cell death and the neuronal injury (47,48) and loss of function (49,50) were reduced when suppressing ERK1/2 activation. ERK1/2 was found to play a caspase-independent role in promoting neuronal cell death in several other models. Okadaic acid has been shown to induce pyramidal cell death in hippocampal area CA3 in a manner dependent on ERK1/2 activation but not consistent with apoptosis (51). These findings may help us design strategies that can specifically attenuate ERK1/2-promoted neuronal pathologies.

ERK1/2 is expressed in microglia, astrocytes and oligodendrocytes. Microglial cells are the primary immune cells in the CNS and promote host defense by destroying invading pathogens (52). Intra-glial signaling, including ERK1/2 pathway cascades, controls the regulation of inflammatory cytokine production and iNOS expression in activated microglia. Many in vitro experiments have demonstrated that the ERK1/2 signaling pathway contributes to the inflammatory response in microglia that is induced upon stimulation with radiation, thrombin or LPS (53–55). ERK1/2 is also involved in the inflammatory response in astrocytes (56,57). Furthermore, accumulating evidence indicates that many of the pharmaceutical-based therapies used to reduce neuroinflammation in stroke, neurodegenerative disorders, intracranial infections and other diseases act by suppressing the ERK1/2 pathway (58–61).

ERK1/2 is localized in the soma and dendritic trees of neurons in the neocortex, hippocampus, striatum and cerebellum (62). An increase in ERK1/2 activation, as measured as the ratio of phosphorylated to total (phosphorylated and non-phosphorylated) ERK1/2, is necessary for learning and the formation of memory as well as for affect and arousal. In a seminal paper, Atkins et al (63) were the first to show that ERK1/2 is involved in memory processing in rat after fear conditioning. Later studies showed that activation of the ERK1/2 pathway is also required for the development of short-term memory and long-term memory consolidation (64,65). Treatment with ERK1/2 inhibition can impair long-term memory retention and prevents the formation of lasting memories of an event or association, including object recognition memory (66,67). Spatial learning and fear conditioning are the types of long-term memory in which the involvement of ERK1/2 has been best characterized (68). Studies of ERK1/2 KO mice demonstrated that ERK1/2 is involved in various aspects of learning and memory formation (69). ERK1 KO mice at first appear to be neurologically normal, whereas ERK2 KO is embryonic lethal; ERK2 KO mice die at embryonic day 6.5 (5,70,71). Selcher et al (70) showed that the ERK1 isoform is not required for associative learning in mice; instead, they found that the ERK2 isoform plays a predominant role in the synaptic plasticity that underlies learning and memory. Short-term memory is retained in ERK1-KO mice, but a marked enhancement of long-term memory was found in a one-trial inhibitory avoidance task (72). The results of a re-consolidation study also support the pivotal role of ERK2 in memory process (73). These results suggest that ERK1/2 may be a target for therapeutics to treat disorders of learning and memory.

ERK1/2 pathway activation is also known to play physiological and pathological roles post-development, and a large body of evidence suggests that ERK1/2 also contributes to the regulation of inflammatory responses, cytokines, cell apoptosis and death in ischemic and hemorrhagic brain injury (78–82). Several pharmacological studies have also demonstrated that suppression of ERK1/2 activation frequently downregulates features of apoptosis and inflammation and reduces neurological damage after stroke (49,81–83). Madami and Edvinsson showed that the elevated microvascular pro-inflammatory cytokine expression observed following focal ischemia in MCAO models also involved the ERK1/2 pathway (82). Moreover, Shioda et al (3) found that ERK1/2 signaling plays an important role in neurogenesis following brain ischemia. Substantial evidence has suggested that the ERK1/2 pathway is involved in regulating the changes in inflammation, cyto toxicity and cerebral vasospasm that occur after hemorrhagic stroke (76,84). Recently, Feng et al (85) showed that Ras/Raf/ERK signals participate in the neuronal apoptosis observed in the hippocampus in early post-subarachnoid hemorrhage brain injury. Taken together, these results suggest that therapies targeted at suppression of the ERK1/2 pathway may be beneficial in stroke.

PD is the second most prevalent neurodegenerative disease after AD and is characterized by selective dopaminergic neuronal loss in the substantial nigra. The ERK1/2 pathway is known to play a major regulatory role in PD-related cellular processes. Accumulating evidence indicates that microglial cells play a crucial role in the degeneration of dopaminergic neurons in animal models of PD. Recent studies have shown that the oxidative stress response plays a central role in the etiology of PD (86,87). The oxidative stress response that occurs in microglia is mediated by the activation of the ERK1/2 signaling pathway upon stimulation with pro-inflammatory stimuli. Furthermore, ERK1/2 has been shown to participate in L-DOPA-induced dyskinesia through striatal synaptic plasticity (75,88). In addition, in the dopamine-depleted striatum, ERK1/2 plays an important role in the development of L-DOPA-induced dyskinesia in both mouse and non-human primate models of PD (75,89). The inhibition of ERK1/2 attenuated LID and completely inhibited all markers of angiogenesis in rat and mouse models of LID (75,90). Therefore, the modulation of ERK1/2 in response to dopamine in PD patients may be therapeutic for motor complications.

AD is a neurodegenerative disease that is characterized by progressive cognitive decline and memory dysfunction as well as the presence of neurofibrillary tangles (NFTs) and senile plaques composed primarily of β-amyloid. ERK1/2 is one of the kinases known to phosphorylate tau and has been shown to be associated with NFTs and senile plaques (74). Increased levels of activated ERK1/2 have been found in AD brains, and inhibition of the pathway can reduce β-amyloid neurotoxicity (91–93). Activated ERK1/2 is found specifically in intracytoplasmic punctate structures and intracellular NFTs, primarily in the subpopulation of neurons that exhibits early AD-related protein deposition. As mentioned above, ERK1/2 is known to play a critical role in hippocampus synaptic plasticity and learning and memory. Abnormal ERK1/2 activation in the hippocampus may impair hippocampal function and contribute to memory deficits in AD patients. Therefore, improving regulation of the ERK1/2 pathway may be a central facet for the development of potential treatments for AD.

Drug addiction is recognized as a type of neuroadaptive disorder. Because the ERK1/2 pathway plays an important role in neuronal plasticity in the adult brain, understanding of the role of this pathway is critical for overall understanding of the molecular mechanisms underlying drug addiction and relapse. Exposure to a variety of substances with abuse potential, including nicotine, alcohol, amphetamine and cocaine, acutely active ERK1/2 in the striatum and other brain areas (94–97). Many of the enduring behavioral effects of acute drug exposure depend on ERK1/2 signaling. Studies have suggested that ERK1/2 is dynamically regulated following repeated drug exposure and withdrawal and that changes in ERK1/2 activation directly affect striatal cell excitability (98,99). These effects may be responsible for the expression of addictive behavior, and alterations of this pathway may contribute to the drug's rewarding effects and to the long-term maladaptation induced by drug abuse. Evidence indicates that ERK1/2 plays a dual role in gene regulation and drug addiction through direct activation of transcription factors, including Elk1 and cAMP response element-binding protein (CREB), and by chromatin remodeling via MSK1 and histone H3 phosphorylation (100,101). Because ERK1/2 activation is a key molecular process in drug self-administration, targeting it may be a potential treatment strategy for drug addiction.

Amyotrophic lateral sclerosis (ALS) is a CNS disease that causes the death of motor neurons and that can be either sporadic or familial origin. Mutant SOD1 is one of the genetic factors that contribute to the etiology of ALS, and mutant SOD1 induces motor neuron vulnerability. Phosphorylated ERK1/2 has been shown to be increased in the hippocampus and cerebellum in SOD1 G93A transgenic models (102). Apolloni et al (103) showed that ERK1/2 also participates in P2X7 receptor-induced enhancement of oxidative stress in ALS microglia, together with the NOX2 pathway. A previous study also identified ERK1/2 as a novel player in the pathogenesis of ALS associated with transactive response DNA-binding protein 43 (TDP-43) (77). A recent study also showed that depletion of TDP-43 in microglia strikingly upregulated the production of COX-2 and PGE2 through the activation of ERK1/2 signaling (61).

Huntington's disease (HD), a devastating neurodegenerative disease that is characterized by progressive and severe cognitive, psychiatric and motor dysfunction, is caused by an expanded CAG repeat in the huntingtin (Htt) gene. MAPK signaling, and particularly the Ras-ERK cascade, is among the pathways that have been implicated in HD. In response to mutant huntingtin, ERK1/2 is activated and directs a protective transcriptional response and inhibits apoptotic caspase-3 and -7 activation (104,105). Data from different model systems indicate that ERK1/2 is involved in HD excitotoxicity at both the intercellular and intracellular level (106–108). Pharmacological interventions that promote ERK1/2 activation could suppress the adverse effects of mutant Htt by activating pro-survival mechanisms and suppressing apoptotic responses. Thus, studies in both cells and animal models suggest that the ERK1/2 cascade may be a potential target for therapeutic interventions for currently untreatable disorders.