Differential processing of the ferritin heavy chain mRNA in human liver and adult human brain

Hyphantria cunea ferritin heavy chain homologue: cDNA sequence and mRNA expression

We have sequenced a cDNA clone encoding a 26-kDa ferritin subunit, which was heavy chain homologue (HCH), in fall webworm, Hyphantria cunea. The HCH cDNA was obtained from the screening of a cDNA library using a PCR product. H. cunea ferritin is composed of 221 amino acid residues and their calculated mass is 26,160 Da. The protein contains the conserved motifs for the ferroxidase center typical for heavy chains of vertebrate ferritin. The iron-responsive element sequence with a predicted stem-loop structure is present in the 5'-untranslated region of ferritin HCH mRNA. The sequence alignment of ferritin HCH shows 68.9 and 68.7% identity with Galleria mellonella HCH (26 kDa ferritin) and Manduca sexta HCH, respectively. While G type insect ferritin vertebrate light chain homologue (LCH) is distantly related to H. cunea ferritin HCH (17.2-20.8%), the Northern blot analysis revealed that H. cunea ferritin HCH was ubiquitously expressed in various tissues and all developmental stages. The ferritin expression of midgut is more responsive to iron-fed, compared to fat body in H. cunea.

Differential regulation of H- and L-ferritin messenger RNA subunits, ferritin protein and iron following focal cerebral ischemia-reperfusion

Iron may catalyse the production of reactive oxygen species during post-ischemic reoxygenation and subsequently lead to brain damage. Ferritin, an iron sequestering and storage protein, can also be a source of iron after ischemic insult. However, its role in ischemia-reperfusion has not been carefully investigated. In the present study, we examined the temporal and spatial induction profiles of both H- and L-ferritin messenger RNA and protein in a well-defined focal cerebral ischemia model. Results of northern blot analysis showed a delayed and prolonged induction of both H- and L-ferritin messenger RNA in the ischemic cortex of rats subjected to 60min ischemic insult. A significant induction of both H- and L-ferritin messenger RNA was observed at 12h and remained elevated for up to 336h after the onset of reperfusion. At the peak level, quantitative analysis of the blot indicated a 2.5-fold and a six-fold increase in H- and L-ferritin messenger RNA, respectively, compared with the sham-operated controls. No apparent change in the levels of either messenger RNA was observed in the contralateral side. Results of in situ hybridization studies revealed constitutive expression of both H- and L-ferritin messenger RNA throughout the brain in sham-operated animals, in particular the hippocampus and the piriform cortex. Nevertheless, the signal intensity of H-ferritin messenger RNA was much higher than that of L-ferritin messenger RNA. Seventy-two hours after 60min ischemia, marked expression of H-ferritin messenger RNA was observed in the area surrounding the middle cerebral artery irrigated cortex, the medial part of the caudoputamen and in the subfield of the CA1 hippocampal region of the ipsilateral hemisphere. Similarly, a large induction of L-ferritin messenger RNA was also noted in several areas, including the middle cerebral artery irrigated cortex, the lateral part of the caudoputamen and the stratum pyramidale of the CA1 hippocampal region, which were totally different from areas where H-ferritin messenger RNA was found. At 336h after ischemia, increased expression of H-ferritin messenger RNA was observed in the peri-necrosis and ipsilateral thalamus regions, while L-ferritin messenger RNA was noted exclusively at the edge within the necrosis. Results of immunohistochemical study further revealed that ferritin immunoreactivity was present in the same areas where increased ferritin messenger RNA was found. Sixty-minute ischemia also led to iron deposition in discrete areas. Iron deposition was highly associated with the induction of ferritin, particularly in the macrophage- and microglia-positive areas where cell death or tissue necrosis was noted.In summary, our initial findings indicate that ischemic insult leads to induction of both H- and L-ferritin messenger RNA. In the present study, although the temporal induction profiles were similar, the major expression areas for these two genes were totally different. Ferritin immunoreactivity was observed in the same areas where increased ferritin messenger RNA was found. Ischemia also resulted in iron deposition, which highly associated with the ferritin immunoreactivity. The exact regulatory mechanism and pathological significance for the differential expression of H- and L-ferritin genes following ischemia/reperfusion remain to be clarified.

Iron availability dramatically alters the distribution of ferritin subunit messages in Drosophila melanogaster

Insect ferritins have subunits homologous to the heavy and light chains of vertebrate ferritins. Cloning and sequence of the heavy chain homologue (HCH) of Drosophila melanogaster ferritin subunit have been reported earlier. When Northern blots of D. melanogaster RNA were probed with a cDNA for this HCH, three bands were observed. It was shown that these represented at least four classes of mRNA of various lengths. The polymorphism results from alternative splicing of an intron in the 5' untranslated region (UTR) that contains the iron-responsive element (IRE) and from two alternative polyadenylation sites in the 3' UTR. This has also been reported by others [Lind, M. I., Ekengren, S., Melefors, O. & Söderhäll, K. (1998) FEBS Lett. 436, 476-482]. By hybridizing Northern blots with specific probes, it has been shown that the relative proportions of the messages vary with the life stage and especially with iron supplementation of the diet. Iron significantly increases the amount of ferritin HCH messages and dramatically shifts the balance toward those messages that lack an IRE and/or have a short 3' UTR. In the larvae this change takes place in the gut, but not in the fat body. We speculate that this dramatic shift in message distribution may result from an effect of iron on the rate of transcription or message degradation, or from an effect on the splicing process itself. Synthesis of ferritin HCH subunit mRNAs that lack an IRE may be important under conditions of iron overload.

Expression of ferritin protein and subunit mRNAs in normal and iron deficient rat brain

In non-neuronal tissue, ferritin subunit mRNAs are regulated by post-transcriptional mechanisms leading to decreased ferritin protein synthesis during iron deficiency. Biochemical studies have demonstrated that the cerebral ferritin concentration declines during iron deficiency, suggesting that expression of ferritin subunit mRNAs in the brain may be regulated by mechanisms similar to those of non-neuronal tissue. However, as ferritin expression has been only vaguely studied in brain, this hypothesis remains to be tested. We investigated the influence of dietary iron deficiency on the cellular distribution of ferritin protein using immunohistochemistry and H- and L-ferritin subunit mRNAs by non-radioactive in situ hybridization. Pregnant rats were subjected to an iron depleted diet (6.4 mg/kg) from the day of conception. Litters were kept on the same diet until euthanized at the postnatal age of 10 weeks. This treatment reduced brain iron levels from approximately 57 to 26 microgram/g. Reducing the iron stores reduced histochemical detectable iron and the expression of ferritin immunoreactivity in neurons, oligodendrocyte-like and microglia-like cells. In normal rats, H- and L-ferritin subunit mRNAs were expressed in virtually all neurons and non-neuronal cells. The cerebral expression of the ferritin subunit mRNAs was not affected by iron deficiency. The levels of ferritin subunit mRNAs in the brain were also unaltered from iron deficiency when examined by Northern blotting. In conclusion, brain levels of iron and ferritin protein are highly susceptible to dietary iron deficiency, whereas the cerebral expression of H- and L-ferritin subunit mRNAs remains unchanged.

Alternative poly(A) site selection in complex transcription units: means to an end?

Many genes have been described and characterized which result in alternative polyadenylation site use at the 3'-end of their mRNAs based on the cellular environment. In this survey and summary article 95 genes are discussed in which alternative polyadenylation is a consequence of tandem arrays of poly(A) signals within a single 3'-untranslated region. An additional 31 genes are described in which polyadenylation at a promoter-proximal site competes with a splicing reaction to influence expression of multiple mRNAs. Some have a composite internal/terminal exon which can be differentially processed. Others contain alternative 3'-terminal exons, the first of which can be skipped in some cells. In some cases the mRNAs formed from these three classes of genes are differentially processed from the primary transcript during the cell cycle or in a tissue-specific or developmentally specific pattern. Immunoglobulin heavy chain genes have composite exons; regulated production of two different Ig mRNAs has been shown to involve B cell stage-specific changes in trans -acting factors involved in formation of the active polyadenylation complex. Changes in the activity of some of these same factors occur during viral infection and take-over of the cellular machinery, suggesting the potential applicability of at least some aspects of the Ig model. The differential expression of a number of genes that undergo alternative poly(A) site choice or polyadenylation/splicing competition could be regulated at the level of amounts and activities of either generic or tissue-specific polyadenylation factors and/or splicing factors.

A novel ferritin heavy chain messenger ribonucleic acid in the human brain

In the aging human brain, the concentrations of iron and its major storage protein, ferritin, rise but the distribution of metal and protein remains non-uniform. More ferritin could be isolated from the brains of humans who died of Alzheimer's disease (AD) than from age- and sex-matched controls. Also, brain ferritin of rats chronically exposed to aluminum chloride in their drinking water contained more aluminum and iron. Based on these earlier observations, a more detailed study of human brain ferritin was initiated. The results showed that ferritin is a component of neuritic (senile) plaques in AD. Ferritin obtained from normal or AD brains is composed of 24 subunits (70% heavy (H) chain; 30% light (L) chain). With high performance liquid chromatography, the subunits resolved into a cluster of four H-chain peaks and one major L-chain peak. Western blot analysis confirmed the identity of H- and L-fractions. The techniques of molecular biology revealed the presence of an additional ferritin messenger ribonucleic acid (mRNA) species for the H subunit which was more abundant in the brain than in other human tissues. It contained the entire sequence of 919 nucleotides of H chain mRNA from liver but also an additional segment of 279 nucleotides in the 3'-untranslated region. The two mRNA seemed to arise by the use of an alternate polyadenylation site of the same primary transcript. Ribonuclease protection assays revealed that the concentrations of the longer mRNA in the normal hippocampus and the hippocampus of patients with AD brains were similar.