Article

Expression and Regulation of Heme Oxygenase Isozymes in the Developing Mouse Cortex

Department of Pediatrics , Stanford University, Palo Alto, California, United States
Pediatric Research (Impact Factor: 2.31). 12/2006; 60(5):518-23. DOI: 10.1203/01.PDR.0000242374.21415.f5
Source: PubMed

ABSTRACT

Heme oxygenase (HO), the rate-limiting enzyme in heme degradation, plays a role in neonatal jaundice. Understanding the regulation of the developmental expression patterns of the two HO isozymes, HO-1 and HO-2, is essential for targeting HO to control pathologic jaundice, and uncovering the fundamental role that they play in mammalian development. Here we characterized the ontogeny of HO-1 and HO-2 expression in the developing mouse cortex by in vivo bioluminescence imaging, quantitative RT-PCR, and Western blot. HO-2, the predominant isoform in the adult cortex, was relatively stable throughout all ages. HO-1 was observed to be progressively down-regulated in an age-related manner. HO-1 expression in the adult cortex was also the lowest among the eight adult tissues analyzed. Because there is a 283-bp CpG island region in the HO-1 promoter, we hypothesized that methylation of the island is responsible for the age-related HO-1 down-regulation in the cortex. Methylation status was assessed using regular and quantitative methylation-specific PCR and the CpG island was found to be hypomethylated at all ages. Therefore, we conclude that HO-1 gene expression in the cortex is developmentally-regulated and that methylation of the HO-1 CpG island is not associated with the down-regulation of the gene.

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Available from: Christopher H Contag, May 20, 2014
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    • "On the contrary, HO2 has been less studied likely due to its apparent constitutive nature. Nevertheless, its particular abundance in the brain emphasizes the relevance of HO2 function [27, 28]. HO2 is constitutively expressed by most brain cells, notably neurons, and endothelial and glial cells293031 and accounts for the majority of HO activity in the brain [32]. "
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    ABSTRACT: Following intracerebral hemorrhage (ICH), red blood cells release massive amounts of toxic heme that causes local brain injury. Hemopexin (Hpx) has the highest binding affinity to heme and participates in its transport, while heme oxygenase 2 (HO2) is the rate-limiting enzyme for the degradation of heme. Microglia are the resident macrophages in the brain; however, the significance and role of HO2 and Hpx on microglial clearance of the toxic heme (iron-protoporphyrin IX) after ICH still remain understudied. Accordingly, we postulated that global deletion of constitutive HO2 or Hpx would lead to worsening of ICH outcomes. Intracerebral injection of stroma-free hemoglobin (SFHb) was used in our study to induce ICH. Hpx knockout (Hpx −/− ) or HO2 knockout (HO2 −/− ) mice were injected with 10 μL of SFHb in the striatum. After injection, behavioral/functional tests were performed, along with anatomical analyses. Iron deposition and neuronal degeneration were depicted by Perls’ and Fluoro-Jade B staining, respectively. Immunohistochemistry with anti-ionized calcium-binding adapter protein 1 (Iba1) was used to estimate activated microglial cells around the injured site. This study shows that deleting Hpx or HO2 aggravated SFHb-induced brain injury. Compared to wild-type littermates, larger lesion volumes were observed in Hpx −/− and HO2 −/− mice, which also bear more degenerating neurons in the peri-lesion area 24 h postinjection. Fewer Iba1-positive microglial cells were detected at the peri-lesion area in Hpx −/− and HO2 −/− mice, interestingly, which is associated with markedly increased iron-positive microglial cells. Moreover, the Iba1-positive microglial cells increased from 24 to 72 h postinjection and were accompanied with improved neurologic deficits in Hpx −/− and HO2 −/− mice. These results suggest that Iba1-positive microglial cells could engulf the extracellular SFHb and provide protective effects after ICH. We then treated cultured primary microglial cells with SFHb at low and high concentrations. The results show that microglial cells actively take up the extracellular SFHb. Of interest, we also found that iron overload in microglia significantly reduces the Iba1 expression level and resultantly inhibits microglial phagocytosis. This study suggests that microglial cells contribute to hemoglobin-heme clearance after ICH; however, the resultant iron overloads in microglia appear to decrease Iba1 expression and to further inhibit microglial phagocytosis.
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    • "On the other hand, HO-2 has been considered as the constitutive isoform. High expression is observed in testis, brain, and endothelial and smooth cells from cerebral vessels [9] [10]. HO-2 is the abundant isoform in rat adult brain. "
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    ABSTRACT: Hemeoxygenase (HO) system is responsible for cellular heme degradation to biliverdin, iron, and carbon monoxide. Two isoforms have been reported to date. Homologous HO-1 and HO-2 are microsomal proteins with more than 45% residue identity, share a similar fold and catalyze the same reaction. However, important differences between isoforms also exist. HO-1 isoform has been extensively studied mainly by its ability to respond to cellular stresses such as hemin, nitric oxide donors, oxidative damage, hypoxia, hyperthermia, and heavy metals, between others. On the contrary, due to its apparently constitutive nature, HO-2 has been less studied. Nevertheless, its abundance in tissues such as testis, endothelial cells, and particularly in brain, has pointed the relevance of HO-2 function. HO-2 presents particular characteristics that made it a unique protein in the HO system. Since attractive results on HO-2 have been arisen in later years, we focused this review in the second isoform. We summarize information on gene description, protein structure, and catalytic activity of HO-2 and particular facts such as its cellular impact and activity regulation. Finally, we call attention on the role of HO-2 in oxygen sensing, discussing proposed hypothesis on heme binding motifs and redox/thiol switches that participate in oxygen sensing as well as evidences of HO-2 response to hypoxia.
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    • "In the adult brain, HO-2 is the isoenzyme predominantly expressed. In contrast, HO-1 expression in the brain is developmentally regulated, being highest in the early gestational ages and progressively decreasing during the perinatal period to adulthood (Zhao et al., 2006). The constitutive HO-2 isoform is important in the maintenance of neuronal function, whereas HO-1 is believed to play a protective role (Snyder et al., 1998; Maines, 2000). "
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