Elevated post-ischemic ubiquitination results from suppression of deubiquitinase activity and not proteasome inhibition
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Cellular and Molecular Life Sciences
ORIGINAL ARTICLE
Elevated post‑ischemic ubiquitination results from suppression of deubiquitinase activity and not proteasome inhibition Timo Kahles1,2 · Carrie Poon1 · Liping Qian1 · Victoria Palfini1 · Shanmukha Priya Srinivasan1 · Shilpa Swaminathan1 · Ismary Blanco1 · Reunet Rodney‑Sandy1 · Costantino Iadecola1 · Ping Zhou1 · Karin Hochrainer1 Received: 21 May 2020 / Revised: 28 July 2020 / Accepted: 18 August 2020 © Springer Nature Switzerland AG 2020
Abstract Cerebral ischemia–reperfusion increases intraneuronal levels of ubiquitinated proteins, but the factors driving ubiquitination and whether it results from altered proteostasis remain unclear. To address these questions, we used in vivo and in vitro models of cerebral ischemia–reperfusion, in which hippocampal slices were transiently deprived of oxygen and glucose to simulate ischemia followed by reperfusion, or the middle cerebral artery was temporarily occluded in mice. We found that post-ischemic ubiquitination results from two key steps: restoration of ATP at reperfusion, which allows initiation of protein ubiquitination, and free radical production, which, in the presence of sufficient ATP, increases ubiquitination above pre-ischemic levels. Surprisingly, free radicals did not augment ubiquitination through inhibition of the proteasome as previously believed. Although reduced proteasomal activity was detected after ischemia, this was neither caused by free radicals nor sufficient in magnitude to induce appreciable accumulation of proteasomal target proteins or ubiquitin–proteasome reporters. Instead, we found that ischemia-derived free radicals inhibit deubiquitinases, a class of proteases that cleaves ubiquitin chains from proteins, which was sufficient to elevate ubiquitination after ischemia. Our data provide evidence that free radical-dependent deubiquitinase inactivation rather than proteasomal inhibition drives ubiquitination following ischemia–reperfusion, and as such call for a reevaluation of the mechanisms of post-ischemic ubiquitination, previously attributed to altered proteostasis. Since deubiquitinase inhibition is considered an endogenous neuroprotective mechanism to shield proteins from oxidative damage, modulation of deubiquitinase activity may be of therapeutic value to maintain protein integrity after an ischemic insult. Keywords Cerebral ischemia–reperfusion · Ubiquitin · Free radical production · Deubiquitinase inhibition
Introduction Stroke is the second leading cause of death worldwide and a major cause of disability for which therapeutic interventions are very limited [1]. Most strokes result from embolism or thrombosis leading to occlusion of a major cerebral artery (ischemic stroke) [2]. While early reperfusion by intravenous thrombolysis or mechanical thrombectomy ameliorates stroke outcome [3], only a minority of patients can benefit from these interventions due to exclusion criteria, fear of * Karin Hochrainer [email protected] 1
Feil Family Brain and Mind Research Inst
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