Keywords

TRIM9
stroke
neuroinflammation
NF-κB

Introduction

Tripartite motif containing 9 (TRIM9), a brain-specific ubiquitin (Ub) ligase, is primarily expressed in neurons and regulates netrin-dependent axon guidance and morphogenesis through the interaction with vasodilator-stimulated phosphoprotein (VASP), a mechanism conserved between different species (Hao et al., 2010, Menon et al., 2015, Plooster et al., 2017, Winkle et al., 2016a, Winkle et al., 2016c). We have shown that TRIM9 is a potent inhibitor of nuclear factor κB (NF-κB) signaling pathway in in vitro cell culture upon cytokine stimulation (Shi et al., 2014). However, the in vivo role of TRIM9 in NF-κB-mediated neuroinflammation remains elusive.
Ischemic stroke remains a leading cause of mortality and disabilities in the elderly (Benjamin et al., 2017). Excitotoxicity, oxidative and nitrosative stress, necrosis, and inflammation are key pathogenic events that contribute to neuronal injury and cell death after ischemic stroke (Chamorro et al., 2016, Lo et al., 2005). NF-κB is a master regulator of hypoxia-induced inflammation (Eltzschig and Carmeliet, 2011) and plays important roles in neuronal plasticity, aging, and degeneration in CNS diseases (Gabuzda and Yankner, 2013, Mattson and Camandola, 2001, Salminen et al., 2008, Zhang et al., 2013). NF-κB is a dimeric transcription factor consisting members of the Rel family, including Rel-A (p65), c-Rel, Rel-B, p50, and p52, and is often held in the cytoplasm by inhibitor IκB proteins as its latent form (Chen, 2005). Upon stimulation, the IκB kinase (IKK) complex phosphorylates the amino-terminal serine residues (S32 and S36) of IκBα, triggering its ubiquitination and degradation by the Skp-Cullin-F-box (SCF) Ub ligase complex and the 26S-proteasome pathway, respectively (Chen, 2005, Frescas and Pagano, 2008). Consequently, this allows the nuclear translocation of NF-κB p50-p65 complex for transcriptional activation of targeted genes. NF-κB activation in neurons occurs soon after brain ischemia as evidenced by IκBα degradation and p65 phosphorylation (Stephenson et al., 2000), which drives the neuronal expression of inflammatory mediators such as interleukin 6 (IL-6) (Ohtaki et al., 2006) and chemokine C-C motif ligand 2 (CCL2) (Stowe et al., 2012). Genetic and pharmacological studies targeting NF-κB-activating IKK have shown that inhibiting NF-κB is generally beneficial for stroke recovery (Herrmann et al., 2005, Iadecola and Anrather, 2011). However, this has been challenged by stroke studies in mouse models with p50 or cRel deficiency (Harari and Liao, 2010), as well as in systemic injury models (Elsharkawy and Mann, 2007), suggesting that NF-κB-mediated acute inflammatory response is not just deleterious. In fact, the acute inflammation responses triggered by CNS injuries usually resolve within a short period of time, which set up tissue boundaries for subsequent repair process (Buckley et al., 2013, Iadecola and Anrather, 2011, Jin et al., 2010). However, brain-specific factors that govern inflammation resolution have not been well defined (Iadecola and Anrather, 2011). Hence, understanding the brain’s regulatory mechanisms that ensure the timely activation and subsequent inactivation of NF-κB-mediated neuroinflammation is essential to develop a therapeutic strategy for the recovery and repair after ischemic brain injury.
Stroke mostly occurs in elderly people, and outcomes of stroke patients are highly influenced by age, indicating that aging is an inherent risk factor for stroke (Markus et al., 2005, Popa-Wagner et al., 2011). Compared to the young brain, the aged brain displays a compromised ability to resolve stroke-mediated inflammation, causing high susceptibility to ischemia and poor functional recovery (Chen et al., 2010). Indeed, upon middle cerebral artery occlusion (MCAO), aged mice exhibit elevated proinflammatory mediators, large infarction volumes, severe behavioral impairment, and high mortality rates compared to young mice (DiNapoli et al., 2008, Jin et al., 2004, Liu et al., 2009, Rosen et al., 2005, Shapira et al., 2002), reflecting the effects seen in elderly patients who often experience severe functional disabilities following an ischemia (Hankey et al., 2002). However, little is known about brain-specific mechanisms that regulate the resolution of neuroinflammation that are potentially undermined during aging. Hence, investigating the timely regulation of NF-κB-mediated neuroinflammation is key to a better understanding of pathophysiology and repair after acute brain ischemia.
Here, we report that TRIM9 provides an innate mechanism to resolve ischemic-stroke-induced neuroinflammation via fine-tuning of in vivo NF-κB signaling activity in a mouse model, and thus, targeting TRIM9 and its related neuroinflammatory pathway may offer a target for immunomodulatory therapy for stroke.