Bozho Todorich

Oligodendrocytes and myelination: The role of iron

Iron is an essential trophic factor that is required for oxygen consumption and ATP production. Thus it plays a key role in vital cell functions. Although the brain has a relatively high rate of oxygen consumption compared to other organs, oligodendrocytes are the principal cells in the CNS that stain for iron under normal conditions. The importance of iron in myelin production has been demonstrated by studies showing that decreased availability of iron in the diet is associated with hypomyelination. The timing of iron delivery to oligodendrocytes during development is also important because hypomyelination and the associated neurological sequelae persist long after the systemic iron deficiency has been corrected. Therefore, identifying the molecular roles of iron in oligodendrocyte development and myelin production, and the mechanisms and timing of iron acquisitions are important prerequisites to developing effective therapies for dysmyelinating disorders. It is the purpose of this review to give a comprehensive overview of the existing literature on role of iron in oligodendrocytes and the mechanisms of iron acquisition and intracellular handling.

Tim-2 is the receptor for H-ferritin on oligodendrocytes.

Oligodendrocytes stain more strongly for iron than any other cell in the CNS, and they require iron for the production of myelin. For most cell types transferrin is the major iron delivery protein, yet neither transferrin receptor protein nor mRNA are detectable in mature oligodendrocytes. Thus an alternative iron delivery mechanism must exist. Given the significant long term consequences of developmental iron deficiency and the iron requirements for normal myelination, identification of the iron delivery mechanism for oligodendrocytes is important. Previously we have reported that oligodendrocytes bind H‐ferritin and that H‐ferritin binds to white matter tracts in vivo. Recently, T cell immunoglobulin and mucin domain‐containing protein‐2 (Tim‐2) was shown to bind and internalize H‐ferritin. In the present study we show that Tim‐2 is expressed on oligodendrocytes both in vivo and in vitro. Further, the onset of saturable H‐ferritin binding in CG4 oligodendrocyte cell line is accompanied by Tim‐2 expression. Application of a blocking antibody to the extracellular domain of Tim‐2 significantly reduces H‐ferritin binding to the differentiated CG4 cells and primary oligodendrocytes. Tim‐2 expression on CG4 cells is responsive to iron; decreasing with iron loading and increasing with iron chelation. Taken together, these data provide compelling evidence that Tim‐2 is the H‐ferritin receptor on oligodendrocytes suggesting it is the primary mechanism for iron acquisition by these cells.

Cytokine toxicity to oligodendrocyte precursors is mediated by iron.

Inflammatory processes play a key role in the pathogenesis of a number of common neurodegenerative disorders such as Alzheimers disease (AD), Parkinsons disease (PD), and multiple sclerosis (MS). Abnormal iron accumulation is frequently noted in these diseases and compelling evidence exists that iron is involved in inflammatory reactions. Histochemical stains for iron repeatedly demonstrate that oligodendrocytes, under normal conditions, stain more prominently than any other cell type in the brain. Therefore, we examined the hypothesis that cytokine toxicity to oligodendrocytes is iron mediated. Oligodendrocytes in culture were exposed to interferon‐γ (IFN‐γ), interleukin‐1β (IL‐1β), and tumor necrosis factor‐α (TNF‐α). Toxicity was observed in a dose‐dependent manner for IFN‐γ and TNF‐α. IL‐1β was not toxic in the concentrations used in this study. The toxic concentration of IFN‐γ, and TNF‐α was lower if the cells were iron loaded, but iron loading had no effect on the toxicity of IL‐1β. These data provide insight into the controversy regarding the toxicity of cytokines to oligodendrocytes by revealing that iron status of these cells will significantly impact the outcome of cytokine treatment. The exposure of oligodendrocytes to cytokines plus iron decreased mitochondrial membrane potential but activation of caspase 3 is limited. The antioxidant, TPPB, which targets mitochondria, protected the oligodendrocytes from the iron‐mediated cytotoxicity, providing further support that mitochondrial dysfunction may underlie the iron‐mediated cytokine toxicity. Therapeutic strategies involving anti‐inflammatory agents have met with limited success in the treatment of demyelinating disorders. A better understanding of these agents and the contribution of cellular iron status to cytokine toxicity may help develop a more consistent intervention strategy.

H‐ferritin is the major source of iron for oligodendrocytes

There is a critical relationship between oligodendrocyte development, myelin production, and iron bioavailability. Iron deficiency leads to hypomyelination both in humans and animal models, and the neurological sequelae of hypomyelination are significant. Therefore, understanding molecular mechanisms of iron import into oligodendrocytes is necessary for devising effective strategies for iron supplementation. Although transferrin has been considered as an essential component of oligodendrocyte media in culture, oligodendrocytes in vivo lack transferrin receptors. We have established that receptors for H‐ferritin (HF) exist on cells of oligodendroglial lineage and that uptake of extracellular HF by oligodendrocyte progenitors is via receptor mediated endocytosis. These data strongly argue that ferritin is a major source of iron for oligodendrocytes. In this study, we demonstrate that media deficient in transferrin results in loss of viability of oligodendrocyte progenitors in culture. Cell loss could be prevented by supplementing the media with HF. Moreover, the addition of extracellular HF stimulates development of oligodendrocyte progenitor cells (OPCs) by increasing expression of myelin basic protein (MBP) and olig2 proteins without increasing their proliferation. The effect of HF on the OPCs could be mimicked by addition of membrane permeable 3,5,5‐trimethylhexanoyl ferrocene (TMH‐Fe) as an iron source to the media, but not membrane‐impermeable ferric ammonium citrate. Overall, therefore, our results demonstrate the importance of iron for OPCs viability and differentiation and identify extracellular HF as a critical source of iron for oligodendrocytes. Given that ferritin receptors, but not transferrin receptors can be demonstrated on oligodendrocytes in vivo, the delivery of iron to oligodendrocytes viaferritin may be the more biological relevant delivery system.

Redox metals in Alzheimer's disease.

Abstract: Redox metals in the brain play many important roles in maintenance of cellular function. The maintenance of their homeostasis is of paramount importance to a number of diseases such as Alzheimers disease and multiple sclerosis. Iron, copper, and zinc are metals of special interest in the pathogenesis of these disorders. This review will focus primarily on iron.

The Mechanism of Vanadium-Mediated Developmental Hypomyelination Is Related to Destruction of Oligodendrocyte Progenitors Through a Relationship with Ferritin and Iron

The second post-natal week in rat is the period of the most intense oligodendrocyte development and myelination. This period coincides with peak iron import by oligodendrocytes. During that time oligodendrocyte progenitors (OPCs) are sensitive to agents that may disturb normal iron homeostasis and assimilation of iron into these cells. One mechanism by which iron homeostasis can be disrupted is by environmental exposure to other metals. Vanadium is a transition metal, and exposure to vanadium during early brain development produces hypomyelination with variety of related neuro-behavioral phenotypes. In the current study, we investigated mechanisms of hypomyelination induced by vanadium exposure in developing rat brain. We demonstrate that both in vivo and in vitro, OPCs are more sensitive to vanadium exposure than astrocytes or mature oligodendrocytes. Vanadium exposure in OPCs resulted in increased ROS generation and increased annexinV labeling suggestive of apoptosis. Because ferritin is a major iron delivery protein for oligodendrocytes, we exposed the cells to recombinant ferritin and iron both of which exacerbated vanadium cytotoxicity, while the iron chelator desferroxamine (DFO) prevented cytotoxic/apoptotic effects of vanadium. To illustrate relationship between ferritin and vanadium, we demonstrate that vanadium exacerbated DNA nicking produced by iron-rich spleen ferritin, but not iron-poor apoferritin, resulting in a single and double strand breaks in a DNA relaxation assay. We propose that developmental exposure to vanadium interferes with normal iron assimilation into oligodendrocytes resulting in oxidative stress and apoptosis. Therefore, depletion of OPCs due to vanadium exposure in early post-natal period may be an important mechanism of vanadium-induced hypomyelination.

Live volumetric (4D) visualization and guidance of in vivo human ophthalmic surgery with intraoperative optical coherence tomography

Minimally-invasive microsurgery has resulted in improved outcomes for patients. However, operating through a microscope limits depth perception and fixes the visual perspective, which result in a steep learning curve to achieve microsurgical proficiency. We introduce a surgical imaging system employing four-dimensional (live volumetric imaging through time) microscope-integrated optical coherence tomography (4D MIOCT) capable of imaging at up to 10 volumes per second to visualize human microsurgery. A custom stereoscopic heads-up display provides real-time interactive volumetric feedback to the surgeon. We report that 4D MIOCT enhanced suturing accuracy and control of instrument positioning in mock surgical trials involving 17 ophthalmic surgeons. Additionally, 4D MIOCT imaging was performed in 48 human eye surgeries and was demonstrated to successfully visualize the pathology of interest in concordance with preoperative diagnosis in 93% of retinal surgeries and the surgical site of interest in 100% of anterior segment surgeries. In vivo 4D MIOCT imaging revealed sub-surface pathologic structures and instrument-induced lesions that were invisible through the operating microscope during standard surgical maneuvers. In select cases, 4D MIOCT guidance was necessary to resolve such lesions and prevent post-operative complications. Our novel surgical visualization platform achieves surgeon-interactive 4D visualization of live surgery which could expand the surgeon’s capabilities.

Hybrid 25- and 27-Gauge Vitrectomy for Complex Vitreoretinal Surgery.

BACKGROUND AND OBJECTIVE The authors report the technique of using the 27-gauge (G) vitreous cutter through 25-G valved cannulas to allow hybrid instrumentation of both gauges. PATIENTS AND METHODS Vitrectomy is initiated with standard placement of 25-G valved cannulas, followed by insertion of a 27-G vitreous cutter through the 25-G cannulas. RESULTS The hybrid procedure emphasizes the advantages of both platforms: The 25-G cutter is more efficient for core vitrectomy and is more rigid to facilitate peripheral vitrectomy; the 25-G platform enjoys a wider armamentarium of instrumentation options; and the smaller profile of the 27-G cutter can be maneuvered more easily into tight surgical planes to act analogous to vertical scissors, with the added benefits of aspiration and spatula-like features. The authors illustrate this technique in three cases: diabetic tractional retinal detachment with dense plaques, posterior stage 4B retinopathy of prematurity, and sutureless scleral fixation of an intraocular lens. CONCLUSIONS Hybrid use of the 25-G and 27-G platforms offers greater versatility for the management of complex vitreoretinal conditions.

Sutureless Intrascleral Fixation of Secondary Intraocular Lens Using 27-Gauge Vitrectomy System

Transconjunctival sutureless intrascleral fixation is an important surgical option for secondary intraocular lens (IOL) implantation. In this report, the authors describe the technique of using a 27-gauge platform to perform pars plana vitrectomy, lensectomy, and sutureless scleral IOL fixation in a patient with crystalline lens dislocation secondary to Marfans syndrome. Case report and detailed description of the surgical technique are discussed.

Current and investigational pharmacotherapeutic approaches for modulating retinal angiogenesis

Retinal vascular development is a carefully orchestrated developmental process during which retinal and choroidal vasculature form to provide a dual vascular supply to the neurosensory retina and retinal pigment epithelium. The most common causes of vision loss in children and adults involve at least in part perturbation of the normal vascular physiology or development. Vascular endothelial growth factor has emerged as a key molecular regulator of retinal vascular development as well as retinal and choroidal neovascularization, which underlie the pathophysiology of many retinal diseases. Over the past decade, the advent of injectable pharmacotherapeutic agents into the vitreous cavity of the eye has revolutionized our management of neovascular age-related macular degeneration and other retinal diseases and has, for the first time, offered an opportunity to improve vision rather than just slow the progression of disease processes. The transient duration of these agents, however, requires chronic treatment with repeated intraocular injections and significant treatment burden for patients and the healthcare system. Novel treatments modulating retinal angiogenesis offer the promise of improved efficacy, decreased treatment burden and improved cost–effectiveness.