E6020, a synthetic TLR4 agonist, accelerates myelin debris clearance, Schwann cell infiltration, and remyelination in the rat spinal cord

Abstract

Oligodendrocyte progenitor cells (OPCs) are present throughout the adult brain and spinal cord and can replace oligodendrocytes lost to injury, aging, or disease. Their differentiation, however, is inhibited by myelin debris, making clearance of this debris an important step for cellular repair following demyelination. In models of peripheral nerve injury, TLR4 activation by lipopolysaccharide (LPS) promotes macrophage phagocytosis of debris. Here we tested whether the novel synthetic TLR4 agonist E6020, a Lipid A mimetic, promotes myelin debris clearance and remyelination in spinal cord white matter following lysolecithin-induced demyelination. In vitro, E6020 induced TLR4-dependent cytokine expression (TNFα, IL1β, IL-6) and NF-κB signaling, albeit at ~10-fold reduced potency compared to LPS. Microinjection of E6020 into the intact rat spinal cord gray/white matter border induced macrophage activation, OPC proliferation, and robust oligodendrogenesis, similar to what we described previously using an intraspinal LPS microinjection model. Finally, a single co-injection of E6020 with lysolecithin into spinal cord white matter increased axon sparing, accelerated myelin debris clearance, enhanced Schwann cell infiltration into demyelinated lesions, and increased the number of remyelinated axons. In vitro assays confirmed that direct stimulation of macrophages by E6020 stimulates myelin phagocytosis. These data implicate TLR4 signaling in promoting repair after CNS demyelination, likely by stimulating phagocytic activity of macrophages, sparing axons, recruiting myelinating cells, and promoting remyelination. This work furthers our understanding of immunemyelin interactions and identifies a novel synthetic TLR4 agonist as a potential therapeutic avenue for white matter demyelinating conditions such as spinal cord injury and multiple sclerosis.

The fate and function of oligodendrocyte progenitor cells after traumatic spinal cord injury

“…These attributes make OPCs an ideal cell population to target to enhance their differentiation into new oligodendrocytes to accelerate remyelination. Approaches that have improved endogenous OPC differentiation after SCI include attenuating inhibitors of OPC differentiation like BMP signaling (Sellers et al, 2009;Wang et al, 2011) or myelin debris (Church et al, 2017;Plemel et al, 2013). Alternatively, expressing known promoters of oligodendrocyte differentiation like T3 (Shultz, Wang, Nong, Zhang, & Zhong, 2017), or growth factors like NT-3, Horner, Stokes, & Gage, 1998) have also been successful in promoting OPC differentiation into new oligodendrocytes.…”

“…The speed of remyelination following SCI is likely affected by injury‐induced changes in the microenvironment, which have been recently reviewed elsewhere (Alizadeh & Karimi‐Abdolrezaee, ; Plemel et al, ; Pu, Stephenson, & Yong, ; Pukos, Goodus, Sahinkaya, & McTigue, ). Notably, remyelination may be slowed by cytotoxicity near the lesion, insufficient tissue oxygenation (Tsai et al, ; Yuen et al, ), or inhibitory factors in the glial scar and myelin debris (Buss & Schwab, ; Church, Milich, Lerch, Popovich, & McTigue, ; Dyck et al, ; Dyck, Kataria, Akbari‐Kelachayeh, Silver, & Karimi‐Abdolrezaee, ; Keough et al, ; Plemel, Manesh, Sparling, & Tetzlaff, ). Accelerated remyelination is sufficient to preserve axons following T‐cell mediated demyelination (Mei et al, ).…”

Mechanisms of lysophosphatidylcholine‐induced demyelination: A primary lipid disrupting myelinopathy

“…How various elements of the acute immune response mitigate ongoing secondary injury is an area we wish to explore in the future. The relationship between inflammation and injury is complicated also by the importance of inflammation in promoting remyelination (Church, Milich, Lerch, Popovich, & McTigue, 2017;Clarner et al, 2012;Patani, Balaratnam, Vora, & Reynolds, 2007;Setzu et al, 2006).…”

Microglial Interferon Signaling and White Matter

“…Furthermore, such in vitro studies shed light on the complexities of an in vivo system, where multiple innate immune pathways may be engaged, to various outcomes that may be unexpected based on simple in vitro studies examining the activation or inhibition of a sole TLR. A recent in vivo study on the rat spinal cord suggests that a TLR4 agonist promotes macrophage activation and phagocytosis of debris after a demyelinating insult, oligodendrocyte precursor proliferation, and remyelination of damaged axons in the spinal cord white matter [64]. The implications of these studies are that microglia could be engaged through TLR-signaling, perhaps targeting TLR3 and TLR4, in the brain to perform similar functions to promote the remyelination of white matter lesions.…”