Age-related macular degeneration (AMD) and glaucoma are degenerative conditions of the retina and a significant cause of irreversible blindness in developed countries. Alzheimer’s disease (AD), the most common dementia of the elderly, is often associated with AMD and glaucoma. The cardinal features of AD include extracellular accumulation of amyloid β (Aβ) and intracellular deposits of hyper-phosphorylated tau (p-tau). Neuroinflammation and brain iron dyshomeostasis accompany Aβ and p-tau deposits and, together, lead to progressive neuronal death and dementia. The accumulation of Aβ and iron in drusen, the hallmark of AMD, and Aβ and p-tau in retinal ganglion cells (RGC), the main retinal cell type implicated in glaucoma, and accompanying inflammation suggest overlapping pathology. Visual abnormalities are prominent in AD and are believed to develop before cognitive decline. Some are caused by degeneration of the visual cortex, while others are due to RGC loss or AMD-associated retinal degeneration. Here, we review recent information on Aβ, p-tau, chronic inflammation, and iron dyshomeostasis as common pathogenic mechanisms linking the three degenerative conditions, and iron chelation as a common therapeutic option for these disorders. Additionally discussed is the role of prion protein, infamous for prion disorders, in Aβ-mediated toxicity and, paradoxically, in neuroprotection.

Background: Accumulation of iron is a consistent feature of Alzheimer’s disease (AD) brains. The underlying cause, however, remains debatable. Objective: To explore whether local hepcidin synthesized by brain cells contributes to iron accumulation in AD brains. Methods: Brain tissue from the cingulate cortex of 33 cases of AD pre-assigned to Braak stage I-VI, 6 cases of non-dementia, and 15 cases of non-AD dementia were analyzed for transcriptional upregulation of hepcidin by RT-qPCR and RT-PCR. Change in the expression of ferritin, ferroportin (Fpn), microglial activation marker Iba1, IL-6, and TGFβ2 was determined by western blotting. Total tissue iron was determined by colorimetry. Results: Significant transcriptional upregulation of hepcidin was observed in Braak stage III-VI relative to Braak stage I and II, non-AD dementia, and non-dementia samples. Ferritin was increased in Braak stage V, and a significant increase in tissue iron was evident in Braak stage III-VI. The expression of Iba1 and IL-6 was also increased in Braak stage III-VI relative to Braak stage I and II and non-AD dementia samples. Amyloid-β plaques were absent in most Braak stage I and II samples, and present in Braak stage III-VI samples with few exceptions. Conclusion: These observations suggest that upregulation of brain hepcidin is mediated by IL-6, a known transcriptional activator of hepcidin. The consequent downregulation of Fpn on neuronal and other cells results in accumulation of iron in AD brains. The increase in hepcidin is disease-specific, and increases with disease progression, implicating AD-specific pathology in the accumulation of iron.

Purpose: Elevated levels of transforming-growth-factor (TGF)-β2 in the trabecular meshwork (TM) and aqueous humor are associated with primary open-angle glaucoma (POAG). The underlying mechanism includes alteration of extracellular matrix homeostasis through Smad-dependent and independent signaling. Smad4, an essential co-Smad, upregulates hepcidin, the master regulator of iron homeostasis. Here, we explored whether TGF-β2 upregulates hepcidin, implicating iron in the pathogenesis of POAG. Methods: Primary human TM cells and human and bovine ex vivo anterior segment organ cultures were exposed to bioactive TGF-β2, hepcidin, heparin (a hepcidin antagonist), or N-acetyl carnosine (an antioxidant), and the change in the expression of hepcidin, ferroportin, ferritin, and TGF-β2 was evaluated by semiquantitative RT-PCR, Western blotting, and immunohistochemistry. Increase in reactive oxygen species (ROS) was quantified with dihydroethidium, an ROS-sensitive dye. Results: Primary human TM cells and bovine TM tissue synthesize hepcidin locally, which is upregulated by bioactive TGF-β2. Hepcidin downregulates ferroportin, its downstream target, increasing ferritin and iron-catalyzed ROS. This causes reciprocal upregulation of TGF-β2 at the transcriptional and translational levels. Heparin downregulates hepcidin, and reduces TGF-β2-mediated increase in ferritin and ROS. Notably, both heparin and N-acetyl carnosine reduce TGF-β2-mediated reciprocal upregulation of TGF-β2. Conclusions: The above observations suggest that TGF-β2 and hepcidin form a self-sustained feed-forward loop through iron-catalyzed ROS. This loop is partially disrupted by a hepcidin antagonist and an anti-oxidant, implicating iron and ROS in TGF-β2-mediated POAG. We propose that modification of currently available hepcidin antagonists for ocular use may prove beneficial for the therapeutic management of TGF-β2-associated POAG.

In this report, we evaluated the role of iron in sodium iodate (NaIO₃)-induced model of age-related macular degeneration (AMD) in ARPE-19 cells in-vitro, and mouse models in-vivo. ARPE-19 cells, a human retinal pigmented epithelial cell line, were exposed to 10 mM of NaIO₃ for 24 h, and the expression and localization of major iron modulating proteins was evaluated by Western blotting (WB) and immunostaining. Synthesis and maturation of cathepsin-D (cat-D), a lysosomal enzyme, was evaluated by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR) and WB respectively. For in-vivo studies, C57BL/6 mice were injected with 40 mg/kg mouse body weight of NaIO₃ intraperitoneally, and their retina was evaluated after 3 weeks as above. We observed that NaIO₃ induced a 10-fold increase in ferritin in ARPE-19 cells, which co-localized with LC3II, an autophagosomal marker, and LAMP-1, a lysosomal marker. A similar increase in ferritin was noted in retinal lysates and retinal sections of NaIO₃-injected mice by WB and immunostaining. Impaired synthesis and maturation of cat-D was also noted. Accumulated ferritin was loaded with iron, and released from retinal pigmented epithelial (RPE) cells in Perls’ and LAMP-1 positive vesicles. These observations suggest that NaIO₃ impairs lysosomal degradation of ferritin by decreasing the transcription and maturation of cat-D in RPE-19 cells. Iron-loaded ferritin accumulates in lysosomes and is released in lysosome membrane-enclosed vesicles in the extracellular milieu. Accumulation of ferritin in RPE-19 cells and fusion of ferritin-containing vesicles with adjacent photoreceptor cells is likely to create iron overload, compromising their viability. Moreover, reduced activity of cat-D is likely to promote the accumulation of other cellular debris in lysosomal vesicles, contributing to AMD-like pathology.

Vertebrate sexual dimorphism is ascribed to the presence of testes or ovaries, and, hence, to the secretion of gonad-specific hormones. However, mounting evidence indicates that sex differences in tissues and organs also stem from the presence of sex chromosomes (XX or XY). To tease out the contribution of gonads from sex chromosomes to the musculoskeletal system, we used the Four-Core Genotypes (FCG) mouse model, in which the Sry gene, which dictates testis formation, was either deleted in the Y chromosome, resulting in XY mice with ovaries (XY^-SryO), or overexpressed in XX mice, resulting in XX mice with testes (XXT), together with gonadal males XY^-SryT (Sry deletion and overexpression of the Sry transgene in chromosome 3) and females XXO. The FCG mice are generated by crossing XXO with XY^-SryT mice, all of C57BL/6 J background. We now show that the musculoskeletal phenotype of 2- to 4-month-old FCG mice varies based on both gonads and sex chromosomes, depending on the age and the organ/tissue/cell analyzed. The effect of sex chromosomes on body weight, fat and lean/skeletal muscle mass, and bone mass and structure is minor in 2-/3-month-old mice, soon after sexual maturation. The contribution of sex chromosomes (XX versus XY^-Sry in mice with the same gonads and sex hormones) in several of our measurements becomes apparent in adult 4-month-old mice. Contribution of 1X and 1Y^-Sry versus 2X chromosomes varies among different measurements in gonadal males or females, and mice with XY^-Sry chromosomes might have higher or lower values that XX mice. Our study shows XX versus XY^-Sry chromosome contribution to the musculoskeletal phenotype, which becomes more evident as the animals reach peak bone mass, suggesting that while gonadal sex has a major role, sex chromosomes are a so far unrecognized contributor to musculoskeletal mass and bone strength. Sexual dimorphism, or the morphological differences between males and females, is often attributed to the hormones produced by the testes or ovaries. However, evidence suggests that sex chromosomes (XX or XY) also play a significant role. To separate the effects of these two factors on bone health, this work used a unique mouse model known as the Four-Core Genotypes (FCG). The FCG model allows us to manipulate the expression of the Sry gene, which determines testis development, resulting in four groups: XX mice with ovaries (XXO), XX mice with testes (XXT), XY mice with ovaries (XY^-SryO), and XY mice with testes (XY^-SryT). The results presented in this study revealed that the influence of sex chromosomes on the development of body weight, fat and lean/skeletal muscle mass, and bone mass becomes evident in an age-dependent manner, specifically after sexual maturity. Our evidence suggests that there are distinct differences in musculoskeletal development between mice with XX or XY chromosomes. We conclude that our findings uncovered sex chromosomes as a previously unacknowledged factor contributing to the sexual differences observed in musculoskeletal health.