Iron Homeostasis in the Injured Spinal Cord

Iron is essential for life, but its redox activity can render it toxic under certain conditions. Mammalian cells and the organism as a whole have evolved several mechanisms to acquire and utilize iron. The dysregulation of these iron homeostatic mechanisms can cause a number of human diseases and is likely to be a contributing factor in many disorders of the nervous system. Traumatic injuries to the central nervous system (CNS) that result in hemorrhage and cellular disruption can also be associated with impaired iron homeostasis. However, little work has been done to understand the molecular control of iron homeostasis after CNS trauma. The main aim of my thesis research was to investigate the mechanisms underlying the handling of iron after spinal cord injury (SCI) and its impact on secondary pathology and locomotor recovery.;In Chapter 3, I sought to further investigate how inflammatory signals (cytokines) may regulate iron homeostasis in astrocytes and microglia. As SCI is known to be associated with a robust inflammatory response involving TNF-alpha and TGF-beta1, I assessed the effects of these cytokines on iron homeostasis in astrocytes and microglia. The studies showed that these two glial cell types exhibit distinct iron homeostatic responses to TNF-alpha and TGF-beta1, and help explain some of the in vivo results seen in SCI. The SCI work also revealed that macrophages phagocytose red blood cells (RBC) at the injury site. I therefore assessed the effects of RBC phagocytosis on the cytokine expression profile of macrophages in vitro. These results (presented in chapter 3) show that RBC phagocytosis results in a switch from pro-inflammatory to anti-inflammatory cytokine expression; thus suggesting that macrophages that have phagocytosed RBCs in SCI may be anti-inflammatory and pro-fibrogenic in nature. Finally in Chapter 4, I examined the role of the iron binding protein Lipocalin 2 (Lcn2) in SCI. In this chapter I show that the expression of Lcn2 and its receptor are increased in CNS cells, as well as certain types of invading immune cells after SCI. Using Lcn2 -/- mice, I show that Lcn2 plays a detrimental role, and that it contributes to inflammation and secondary cell death after SCI.;Together the results presented in this thesis shed light on the iron homeostatic response and its interplay with inflammation in spinal cord injury.;In Chapter 2, I carried out a detailed assessment of the localization of iron, and the expression of proteins involved in its trafficking and storage after SCI in mice. This data revealed important and distinct roles for macrophages and astrocytes in iron homeostasis after SCI. In addition, the work showed that iron-loaded macrophages remain at the lesion site for extended periods of time and eventually release their iron, contributing to delayed toxicity. I also examined the role of ceruloplasmin in the iron homeostatic response to SCI using Cp-/- mice. These studies demonstrated that CP plays an important protective role in the injured spinal cord.