The Ubiquitin E3 Ligase TRAF6 Exacerbates Ischemic Stroke by Ubiquitinating and Activating Rac1

Abstract

Stroke is one of the leading causes of morbidity and mortality worldwide. Inflammation, oxidative stress, apoptosis, and excitotoxicity contribute to neuronal death during ischemic stroke; however, the mechanisms underlying these complicated pathophysiological processes remain to be fully elucidated. Here, we found that the expression of tumor necrosis factor receptor-associated factor 6 (TRAF6) was markedly increased after cerebral ischemia/reperfusion (I/R) in mice. TRAF6 ablation in male mice decreased the infarct volume and neurological deficit scores and decreased proinflammatory signaling, oxidative stress, and neuronal death after cerebral I/R, whereas transgenic overexpression of TRAF6 in male mice exhibited the opposite effects. Mechanistically, we demonstrated that TRAF6 induced Rac1 activation and consequently promoted I/R injury by directly binding and ubiquitinating Rac1. Either functionally mutating the TRAF6 ubiquitination site on Rac1 or inactivating Rac1 with a specific inhibitor reversed the deleterious effects of TRAF6 overexpression during I/R injury. In conclusion, our study demonstrated that TRAF6 is a key promoter of ischemic signaling cascades and neuronal death after cerebral I/R injury. Therefore, the TRAF6/Rac1 pathway might be a promising target to attenuate cerebral I/R injury. Stroke is one of the most severe and devastating neurological diseases globally. The complicated pathophysiological processes restrict the translation of potential therapeutic targets into medicine. Further elucidating the molecular mechanisms underlying cerebral ischemia/reperfusion injury may open a new window for pharmacological interventions to promote recovery from stroke. Our study revealed that ischemia-induced tumor necrosis factor receptor-associated factor 6 (TRAF6) upregulation binds and ubiquitinates Rac1 directly, which promotes neuron death through neuroinflammation and neuro-oxidative signals. Therefore, precisely targeting the TRAF6-Rac1 axis may provide a novel therapeutic strategy for stroke recovery.

Poly(I:C) preconditioning ameliorates cognitive dysfunction after cerebral I/R injury by inhibiting TRAF6 signaling

“…TRAF6 expression has been reported to be upregulated in the peripheral blood of ischemic stroke patients, as well as in neurons in a rat model of cerebral I/R. Recently, we and others revealed the functions of TRAF6 in ischemic stroke [17]. Here, we observed the effect of poly(I:C) preconditioning on TRAF6 following cerebral I/R.…”

“…In recent years, tumor necrosis factor receptor-associated factor (TRAF) family members have received much interest from the research community; these receptor-associated factors are adaptor proteins that are important for the assembly of receptor-associated signaling networks that link upstream receptors to downstream effector molecules [17]. Among them, tumor necrosis factor receptor-associated factor 6 (TRAF6) is an important multifunctional intracellular adaptin of the tumor necrosis factor superfamily and Toll/IL-1 receptor (TIR) superfamily.…”

“…Meanwhile, TRAF6 expression has been reported to be upregulated in the peripheral blood of ischemic stroke patients, as well as in neurons from a rat model of cerebral I/R [21]. Moreover, as a ubiquitin E3 ligase, TRAF6 exacerbates cerebral I/R, and its ablation has been shown to decrease the infarct volume and neurological deficit scores by alleviating proinflammatory signaling after cerebral I/R [17]. These results suggest that TRAF6 participates in inflammatory injury after cerebral I/R and that TRAF6 is an important treatment target.…”

The Role of Ubiquitin-Proteasome Pathway and Autophagy-Lysosome Pathway in Cerebral Ischemia

“…Parkin promotes Drp1 degradation and exerts neuroprotective effect in oxygen-glucose deprivation/reperfusion injury [26]. However, the ischemia-induced ubiquitin E3 ligase tumor necrosis factor receptor-associated factor 6 (TRAF6) exacerbates cerebral ischemia injury through ubiquitinating and activating Rac1 [27] (Table 1).…”

“…The herb-derived compound, Britanin, selectively binds to a conserved cysteine residue, cysteine 151, of Keap1 and reduces Keap1-mediated ubiquitination of Nrf2, which results in activation of the Nrf2 pathway and attenuating cerebral ischemia-reperfusion-induced damage [42] (Table 2). In vivo [27] Abbreviations: PTEN: phosphatase and tensin homology deleted on chromosome ten; USP14: ubiquitin-specific protease 14; MLKL: mixed lineage kinase domain-like; REST: RE1-silencing transcription factor; Drp1: dynamin-related protein 1; TRAF6: tumor necrosis factor receptor-associated factor 6.…”