Activation of caspase‐6 and cleavage of caspase‐6 substrates is an early event in NMDA receptor–mediated excitotoxicity

• Published in: Journal of neuroscience research 2018-03, Vol.96 (3), p.391-406
• Main Authors: Girling, Kimberly D., Demers, Marie‐Josee, Laine, Jean, Zhang, Shu, Wang, Yu Tian, Graham, Rona K.
• Format: Article
• Language: English
• Subjects: Activation; Alzheimer disease; Alzheimer's disease; Antagonists; Apoptosis; Cascades; Caspase; Caspase-3; Caspase-6; Caspase-8; Cell death; cell signaling; DAXX; Disorders; Excitation; Excitotoxicity; Glutamic acid receptors (ionotropic); Huntingtin; Huntington disease; Mortality; mRNA; N-Methyl-D-aspartic acid receptors; neurodegeneration; Neurodegenerative diseases; Neurological diseases; Neurological disorders; NMDA receptor; Pathogenesis; Protein-serine/threonine kinase; Receptors; Rodents; Side effects; STK3 (resource identifiers: RGD_5508397, AB_2068330, AB_10829240, AB_331439, AB_2123255, AB_882734, AB_1310022, SCR_002798); Stroke; Substrates
• ISSN: 0360-4012; 1097-4547
• DOI: 10.1002/jnr.24153

Excitotoxicity, due to overstimulation of N‐methyl D‐aspartate receptors (NMDARs), has a pivotal role in many neurological disorders. However, NMDAR antagonists often cause side effects, and identifying more druggable therapeutic targets for NMDAR excitotoxicity is an important goal. Activation of caspases is a downstream effect of excitotoxicity that may be critically involved in NMDAR‐mediated cell death. Caspase‐6 (casp6) in particular has been shown to play a key role in the pathogenesis of stroke, Huntington disease, and Alzheimer disease. Using N‐methyl D‐aspartate (NMDA)‐induced excitotoxic injuries of primary rat neurons, we demonstrate that there is an early increase in caspase profiles after an excitotoxic event at the level of mRNA, protein, and activity. Casp6 is elevated and activated first, followed by caspase‐8 and caspase‐3. Similarly, known casp6 substrates huntingtin, as well as novel casp6 substrates serine/threonine kinase 3 and death domain‐associated protein, are cleaved in similar temporal patterns post NMDA. On the basis of these data, we propose that casp6 may be an initiator caspase in apoptotic cascades leading to neuronal death after an excitotoxic event and suggest casp6 as a potential therapeutic target for neurological disorders where NMDAR‐mediated excitotoxicity has been shown to play a role. NMDA excitotoxicity has a role in several neurodegenerative diseases. Caspase activity contributes to cell death after an excitotoxic event; however, the specific excitotoxic caspase signaling cascade is unknown. Here, we observe very early expression/activity of caspase‐6 following NMDA excitotoxicity, suggesting it may be an initiator caspase and potential therapeutic target. Proposed signaling pathway leading to activation of caspase‐6 and substrate cleavage in the pathogenesis of neurodegenerative diseases. NMDA receptor signaling leads to increased intracellular calcium levels, induction of p53, casp6 activation, and cleavage of casp6 substrates. Cleavage of NF‐κß is observed in stroke, generating a transcriptionally inactive p65 molecule that acts as a dominant‐negative inhibitor of NF‐κß and promotes apoptosis. These studies suggest that cleavage of other casp6 substrates may contribute to the pathogenic process in stroke and other neurodegenerative disorders. Using an in vitro model of stroke, we show that decreases in the full‐length form of the casp6 substrates DAXX, STK3, and huntingtin occur early during excitotoxicity. The