Intraspinal TLR4 activation promotes iron storage but does not protect neurons or oligodendrocytes from progressive iron-mediated damage

Abstract

Iron is essential for basic cellular functions but in excess is highly toxic. For this reason, free iron and iron storage are controlled in the periphery by elaborate regulatory mechanisms. In contrast, iron regulation in the central nervous system (CNS) is not well defined. Given that excess iron is present after trauma, hemorrhagic stroke and neurodegeneration, understanding normal iron regulation and promoting iron uptake in CNS pathology is crucial. Peripherally, toll-like receptor 4 (TLR4) activation promotes iron sequestration by macrophages. Notably, iron-rich sites of CNS pathology typically contain TLR4 agonists, which may promote iron uptake. Indeed, our recent work showed impaired iron storage after acute spinal cord injury in mice with TLR4 deficiency. Here we used a reductionist model to ask if TLR4 activation in the CNS stimulates iron uptake and promotes neuroprotection from iron-induced toxicity. For this, we measured the ability of microglia/macrophages to sequester exogenous iron and prevent pathology with and without concomitant intraspinal TLR4 activation. Results show that, similar to the periphery, activating intraspinal TLR4 via focal LPS injection increased mRNA encoding iron uptake and storage proteins and promoted iron sequestration into ferritin-expressing macrophages. However, this did not prevent oligodendrocyte and neuron loss. Moreover, replacement of oligodendrocytes by progenitor cells – a normally robust response to in vivo macrophage TLR4 activation – was significantly reduced if iron was present concomitant with TLR4 activation. Thus, while TLR4 signaling promotes CNS iron uptake, future work needs to determine ways to enhance iron removal without blocking the reparative effects of innate immune receptor signaling.

Glial Cells Shape Pathology and Repair After Spinal Cord Injury

“…Similarly, microglia can contribute to OPC remyelination failure and toxicity [283,284]. Focal LPS-elicited activation of microglia-driven inflammation caused loss of oligodendrocytes, with subsequent replenishment by OPC division [285]. Indeed, while activated microglia exacerbate oligodendrocyte/OPC death, microglial factors have seemingly paradoxical effects by promoting OPC proliferation, differentiation, and remyelination [286].…”

Myelin status and oligodendrocyte lineage cells over time after spinal cord injury: What do we know and what still needs to be unwrapped?

“…Iron is highly reactive and promotes oxidative cell death through the Fenton reaction (Winterbourn, 1995). TLR4 activation stimulates iron uptake and storage by macrophages in vivo (Goldstein et al, 2017;Schonberg & McTigue, 2009;Schonberg, Popovich, & McTigue, 2007), and there are many TLR4 ligands within the injury site, such as heme, high mobility group box 1 (HMGB1) and fibronectin (Kigerl et al, 2009;Kigerl, de Rivero Vaccari, Dietrich, Popovich, & Keane, 2014). Thus, acute TLR4 signaling after SCI appears to enhance iron sequestration in the injury site which would be protective to oligodendrocytes and other cells.…”

“…Over the first 7 days postinjury, edema at the epicenter resolves and monocytederived macrophages accumulate in large numbers (Popovich & Hickey, 2001). Microglia and macrophages engulf red blood cells (Rathore et al, 2008) and become loaded with intracellular iron and ferritin (Sahinkaya et al, 2014;Sauerbeck et al, 2013;Schonberg et al, 2012), which can alter their functional state (Goldstein et al, 2017;Schonberg et al, 2012;Schonberg & McTigue, 2009;Zhang, Surguladze, Slagle-Webb, Cozzi, & Connor, 2006). For instance, ironloaded microglia enhance oligodendrocyte survival, while iron-loaded lipopolysaccharide-activated microglia kill oligodendrocytes, revealing that the iron status of microglia modulates their effects on other cells (Zhang et al, 2006).…”

The Dual Role of Hepcidin in Brain Iron Load and Inflammation

“…The increase in brain iron content in this setting might occur because during BBB disruption iron leakage to brain parenchyma cannot be accompanied with reactive iron efflux since hepcidin blocks excess iron from getting out of brain cells. In this respect, hepcidin further enhances the effect of inflammatory signals on cellular iron accumulation ( Sansing et al, 2011 ; Urrutia et al, 2013 ; Zhou et al, 2014 ; Xiong et al, 2016 ; Goldstein et al, 2017 ). This occurs because during brain inflammation, cytokines, and hepcidin have agonistic effects in suppressing cellular iron efflux ( Urrutia et al, 2013 ; Xiong et al, 2016 ; Zhao Y. et al, 2018 ).…”