Abstract:
The extracellular signal-regulated kinase 1/2 (ERK1/2) and 5 (ERK5) play critical roles in diverse cellular processes and have been shown to be modulated by multiple environmental toxicants. Although the function and regulation of ERK1/2 and ERK5 signaling pathways have been extensively studied in the central nervous system, the underlying mechanisms are still not completely understood. Here, we found that brain-derived neurotrophic factor (BDNF) induces a sustained activation of ERK5 in rat cortical neurons and activates Rap1, a small GTPase, as well as MEKK2, a MEK5 kinase. Our data indicate that activation of Rap1 or MEKK2 is sufficient to stimulate ERK5, while inhibition of either Rap1 or MEKK2 attenuates BDNF activation of ERK5. Furthermore, BDNF stimulation of MEKK2 is regulated by Rap1. Our evidence also indicates that Ras and MEKK3, a MEK5 kinase in non-neuronal cells, do not play a significant role in BDNF activation of ERK5. Taken together, our study identifies Rap1 and MEKK2 as critical upstream signaling molecules mediating BDNF stimulation of ERK5 in central nervous system neurons.Neurotrophin activation of myocyte-enhancer factor (MEF) 2C is one of the strongest pro-survival signaling pathways in developing neurons. To date, neurotrophin stimulation of MEF2C has been largely attributed to its direct phosphorylation by ERK5. Because MEF2C is not directly phosphorylated by ERK1/2 in vitro, it is generally assumed that the ERK1/2 signaling cascade does not regulate MEF2C. Surprisingly, we discovered that ERK1/2 is required for both the transcriptional and neuroprotective activity of MEF2C in cortical neurons stimulated by BDNF. ERK1/2 stimulation of MEF2C is mediated by p90 ribosomal S6 kinase 2 (RSK2), a Ser/Thr protein kinase downstream of ERK1/2. RSK2 strongly phosphorylates purified recombinant MEF2C protein in vitro. Furthermore, RSK2 can directly phosphorylate MEF2C on S192, a consensus RSK2-phosphorylation site located in the transactivation domain of MEF2C. Substitution of S192 with a non-phosphorylatable alanine diminishes both the transcriptional and neuroprotective activity of MEF2C to an extent similar to mutation on S387, an established activating phosphorylation site. Together, our data identifies ERK1/2-RSK2 signaling as a novel mechanism by which neurotrophins activate MEF2C and promote neuronal survival.
URI
http://hdl.handle.net/1773/8455
