Non-Canonical Activation of CREB Mediates Neuroprotection in aC. elegansModel of Excitotoxic Necrosis

Abstract

Excitotoxicity, caused by exaggerated neuronal stimulation by Glutamate (Glu), is a major cause of neurodegeneration in brain ischemia. While we know that neurodegeneration is triggered by overstimulation of Glu-Receptors (GluRs), the subsequent mechanisms that lead to cellular demise remain controversial. Surprisingly, signaling downstream of GluRs can also activate neuroprotective pathways. The strongest evidence involves activation of the transcription factor cAMP Response Element Binding-protein (CREB), widely recognized for its importance in synaptic plasticity. Canonical views describe CREB as a phosphorylation-triggered transcription factor, where transcriptional activation involves CREB phosphorylation and association with CREB Binding Protein (CBP). However, given CREB’s ubiquitous cross-tissue expression, the multitude of cascades leading to CREB phosphorylation, and its ability to regulate thousands of genes, it remains unclear how CREB exerts closely-tailored, differential neuroprotective responses in excitotoxicity. A non-canonical, alternative cascade for activation of CREB-mediated transcription involves the CREB co-factor cAMP-regulated transcriptional co-activator (CRTC), and may be independent of CREB phosphorylation. To identify cascades that activate CREB in excitotoxicity we use a C. elegans model of neurodegeneration by excitotoxic necrosis. We demonstrate that CREB’s neuroprotective effect is conserved, and seems most effective in neurons with moderate Glu exposure. We find that factors mediating canonical CREB activation are not involved. Instead, phosphorylation-independent CREB activation in nematode excitotoxic necrosis hinges on CRTC. CREB-mediated transcription that depends on CRTC, but not on CREB phosphorylation, might lead to expression of a specific subset of neuroprotective genes. Elucidating conserved mechanisms of excitotoxicity-specific CREB activation can help us focus on core neuroprotective programs in excitotoxicity.

Novel Strategies for Glutamate Clearance in the Glia-Deprived Synaptic Hub ofC. elegans

“…Indeed, we observe that inhibiting fluid agitation (using two different paralyzing agents) obstructs chemical stimulation of AVA (either directly from ASH or by spillover from ASE, Figures 7 & 8). This notion is further supported by our recent observation that under conditions that cause glt-3 KO–induced excitotoxicity, the most severely affected neurons are not those that express the most GluRs, but rather those that face the innermost rim of the nerve ring (Feldmann et al, 2019).…”

“…We therefore hypothesize that mechanical agitation of body fluids and perfusion of the inner rim of the nerve ring might underlie the privileged effect of distal GluTs on the ASH -> AVA synapses (an effect seen in Figures 1-3) and affect the clearance of Glu that reach AVA by spillover (Figures 4 and 5). In line with this hypothesis, we recently saw that under conditions where Glu accumulation leads to nematode excitotoxicity (Mano and Driscoll, 2009), the neurons that are most severely affected by neurodegeneration are not those that express the most GluRs, but those who form Glu synapses in the innermost face of the nerve ring (Feldmann et al, 2019).…”

Excitotoxicity: Still Hammering the Ischemic Brain in 2020

“…Lowering extracellular Ca 2+ or reducing membrane Ca 2+ channel opening induces apoptosis in a wide range of cell types [reviewed in Canzoniero et al (2004)]. Many signaling pathways transduce the ability of moderate Ca 2+ levels to promote cell survival, including the activation of PI3K/Akt/mTOR, Ras/Raf/ERK, and AMPK pathways, as well as modulation of gene expression by CREB and NFAT family transcription factors downstream of Ca 2+ / calmodulin (Pinto et al, 2015;Feldmann et al, 2018;Varghese et al, 2019).…”