Edward J. Benz

Characterization of the interaction between protein 4.1R and ZO-2. A possible link between the tight junction and the actin cytoskeleton.

Multiple isoforms of the red cell protein 4.1R are expressed in nonerythroid cells, including novel 135-kDa isoforms. Using a yeast two-hybrid system, immunocolocalization, immunoprecipitation, and in vitro binding studies, we found that two 4.1R isoforms of 135 and 150 kDa specifically interact with the protein ZO-2 (zonulaoccludens-2). 4.1R is colocalized with ZO-2 and occludin at Madin-Darby canine kidney (MDCK) cell tight junctions. Both isoforms of 4.1R coprecipitated with proteins that organize tight junctions such as ZO-2, ZO-1, and occludin. Western blot analysis also revealed the presence of actin and α-spectrin in these immunoprecipitates. Association of 4.1R isoforms with these tight junction and cytoskeletal proteins was found to be specific for the tight junction and was not seen in nonconfluent MDCK cells. The amino acid residues that sustain the interaction between 4.1R and ZO-2 reside within the amino acids encoded by exons 19–21 of 4.1R and residues 1054–1118 of ZO-2. Exogenously expressed 4.1R containing the spectrin/actin- and ZO-2-binding domains was recruited to tight junctions in confluent MDCK cells. Taken together, our results suggest that 4.1R might play an important role in organization and function of the tight junction by establishing a link between the tight junction and the actin cytoskeleton.

Detection of Early Cardiac Dysfunction in Patients with Severe Beta-Thalassemia and Chronic Iron Overload

To detect early left ventricular dysfunction, we used radionuclide cineangiography to determine left ventricular ejection fraction during exercise in 24 patients with transfusion-dependent, congenital anemias, 21 of whom had severe beta thalassemia. Ejection fraction at rest was normal in 21 patients (greater than 45 per cent) and in all patients was 53 +/- 2 per cent (mean +/- S.E.M.)--not significantly different from the value in normal subjects. However, ejection fraction during exercise was normal in only 11 patients (53 +/- 3 per cent in all patients, P less than 0.001 as compared with the normal value). All eight patients who had received fewer than 100 transfusions but only three of 16 (19 per cent, P less than 0.001) who had received 100 or more transfusions had normal responses during exercise. Whereas echocardiographic fractional shortening at rest was normal in 16 of 19 patients studied, eight patients with normal fractional shortening had abnormal ejection-fraction responses to exercise. Thus, radionuclide cineangiography during exercise is a highly sensitive technique for detecting preclinical myocardial dysfunction in patients with systemic iron overload.

Rat-brain Na,K-ATPase beta-chain gene: primary structure, tissue-specific expression, and amplification in ouabain-resistant HeLa C+ cells.

We deduced the complete amino acid sequence of the rat brain Na,K-ATPase beta-subunit from cDNA. The rat brain beta-subunit exhibits a high degree of primary sequence and secondary structural homology with the human and Torpedo beta-subunit polypeptides. Analysis of rat tissue RNA reveals that the beta-subunit gene encodes four separate mRNA species which are expressed in a tissue-specific fashion. In ouabain-resistant HeLa C+ cells, beta-subunit DNA sequences are amplified (approximately 20-fold) and beta-subunit mRNAs are overproduced relative to levels in parental HeLa cells. These results suggest that the beta-subunit plays an important role in Na,K-ATPase structure-function and in the mechanism underlying cellular resistance to the cardiac glycosides.

Novel Splicing Factor RBM25 Modulates Bcl-x Pre-mRNA 5′ Splice Site Selection

ABSTRACT RBM25 has been shown to associate with splicing cofactors SRm160/300 and assembled splicing complexes, but little is known about its splicing regulation. Here, we characterize the functional role of RBM25 in alternative pre-mRNA splicing. Increased RBM25 expression correlated with increased apoptosis and specifically affected the expression of Bcl-x isoforms. RBM25 stimulated proapoptotic Bcl-xS 5′ splice site (5′ ss) selection in a dose-dependent manner, whereas its depletion caused the accumulation of antiapoptotic Bcl-xL. Furthermore, RBM25 specifically bound to Bcl-x RNA through a CGGGCA sequence located within exon 2. Mutation in this element abolished the ability of RBM25 to enhance Bcl-xS 5′ ss selection, leading to decreased Bcl-xS isoform expression. Binding of RBM25 was shown to promote the recruitment of the U1 small nuclear ribonucleoprotein particle (snRNP) to the weak 5′ ss; however, it was not required when a strong consensus 5′ ss was present. In support of a role for RBM25 in modulating the selection of a 5′ ss, we demonstrated that RBM25 associated selectively with the human homolog of yeast U1 snRNP-associated factor hLuc7A. These data suggest a novel mode for Bcl-xS 5′ ss activation in which binding of RBM25 with exonic element CGGGCA may stabilize the pre-mRNA-U1 snRNP through interactions with hLuc7A.

Chromosomal localization of human Na+,K+-ATPase α- and β-subunit genes

Abstract Na+,K+-ATPase is a heterodimeric enzyme responsible for the active maintenance of sodium and potassium gradients across the plasma membrane. Recently, cDNAs for several tissue-specific isoforms of the larger catalytic α-subunit and the smaller β-subunit have been cloned. We have hybridized rat brain and human kidney cDNA probes, as well as human genomic isoform-specific DNA fragments, to Southern filters containing panels of rodent × human somatic cell hybrid lines. The results obtained have allowed us to assign the loci for the ubiquitously expressed α-chain (ATP1A1) to human chromosome 1, region 1p21→cen, and for the α2 isoform that predominates in neural and muscle tissues (ATP1A2) to chromosome 1, region cen→q32. A common PstI RFLP was detected with the ATP1A2 probe. The α3 gene, which is expressed primarily in neural tissues (ATP1A3), was assigned to human chromosome 19. A fourth α gene of unknown function (αD) that was isolated by molecular cloning (ATP1AL1) was mapped to chromosome 13. Although evidence to date had suggested a single gene for the β-subunit, we found hybridizing restriction fragments derived from two different human chromosomes. On the basis of knowledge of conserved linkage goups on human and murine chromosomes, we propose that the coding gene ATP1B is located on the long arm of human chromosome 1 and that the sequence on human chromosome 4 (ATP1BL1) is either a related gene or a pseudogene.

Comparison of nonerythroid alpha-spectrin genes reveals strict homology among diverse species.

The spectrins are a family of widely distributed filamentous proteins. In association with actin, spectrins form a supporting and organizing scaffold for cell membranes. Using antibodies specific for human brain alpha-spectrin (alpha-fodrin), we have cloned a rat brain alpha-spectrin cDNA from an expression library. Several closely related human clones were also isolated by hybridization. Comparison of sequences of these and other overlapping nonerythroid and erythroid alpha-spectrin genes demonstrated that the nonerythroid genes are strictly conserved across species, while the mammalian erythroid genes have diverged rapidly. Peptide sequences deduced from these cDNAs revealed that the nonerythroid alpha-spectrin chain, like the erythroid spectrin, is composed of multiple 106-amino-acid repeating units, with the characteristic invariant tryptophan as well as other charged and hydrophobic residues in conserved locations. However, the carboxy-terminal sequence varies markedly from this internal repeat pattern and may represent a specialized functional site. The nonerythroid alpha-spectrin gene was mapped to human chromosome 9, in contrast to the erythroid alpha-spectrin gene, which has previously been assigned to a locus on chromosome 1.

Absence of messenger RNA and gene DNA for β-globin chains in hereditary persistence of fetal hemoglobin

The relative amounts of alpha-amd beta-globin mRNA and globin gene DNA were measured in reticulocyte RNA and lymphocyte DNA of an individual with homozygous hereditary persistence of fetal hemoglobin whose red blood cells contain 100% fetal hemoglobin (hb F: alpha2gamma2.) Molecular hybridization assays used as probes full-length DNA copies of human alpha- and beta-globin messenger RNA. The results of these hybridization assays demonstrated the expected amounts of alpha-globin mRNA and gene DNA, but absence of beta-globin mRNA and absence of beta-globin gene DNA. In the individual studied, hereditary persistence of fetal hemoglobin is associated with total deletion of the beta-globin structural gene.

Selective defect in myeloid cell lactoferrin gene expression in neutrophil specific granule deficiency.

Neutrophil specific granule deficiency (SGD) is a congenital disorder associated with an impaired inflammatory response and a deficiency of several granule proteins. The underlying abnormality causing the deficiencies is unknown. We examined mRNA transcription and protein synthesis of two neutrophil granule proteins, lactoferrin and myeloperoxidase in SGD. Metabolically labeled SGD nucleated marrow cells produced normal amounts of myeloperoxidase, but there was no detectable synthesis of lactoferrin. Transcripts of the expected size for lactoferrin were detectable in the nucleated marrow cells of two SGD patients, but were markedly diminished in abundance when compared with normal nucleated marrow cell RNA. Because lactoferrin is secreted by the glandular epithelia of several tissues, we also assessed lactoferrin in the nasal secretions of one SGD patient by ELISA and immunoblotting. Nasal secretory lactoferrin was the same molecular weight as neutrophil lactoferrin and was secreted in normal amounts. From these data, we conclude that lactoferrin deficiency in SGD neutrophils is tissue specific and is secondary to an abnormality of RNA production. We speculate that the deficiency of several granule proteins is due to a common defect in regulation of transcription that is responsible for the abnormal myeloid differentiation seen in SGD patients.

Role of RBM25/LUC7L3 in Abnormal Cardiac Sodium Channel Splicing Regulation in Human Heart Failure

Background— Human heart failure is associated with decreased cardiac voltage-gated Na+ channel current (encoded by SCN5A), and the changes have been implicated in the increased risk of sudden death in heart failure. Nevertheless, the mechanism of SCN5A downregulation is unclear. A number of human diseases are associated with alternative mRNA splicing, which has received comparatively little attention in the study of cardiac disease. Splicing factor expression profiles during human heart failure and a specific splicing pathway for SCN5A regulation were explored in this study. Methods and Results— Gene array comparisons between normal human and heart failure tissues demonstrated that 17 splicing factors, associated with all major spliceosome components, were upregulated. Two of these splicing factors, RBM25 and LUC7L3, were elevated in human heart failure tissue and mediated truncation of SCN5A mRNA in both Jurkat cells and human embryonic stem cell–derived cardiomyocytes. RBM25/LUC7L3-mediated abnormal SCN5A mRNA splicing reduced Na+ channel current 91.1±9.3% to a range known to cause sudden death. Overexpression of either splicing factor resulted in an increase in truncated mRNA and a concomitant decrease in the full-length SCN5A transcript. Conclusions— Of the 17 mRNA splicing factors upregulated in heart failure, RBM25 and LUC7L3 were sufficient to explain the increase in truncated forms and the reduction in full-length Na+ channel transcript. Because the reduction in channels was in the range known to be associated with sudden death, interruption of this abnormal mRNA processing may reduce arrhythmic risk in heart failure.

Regulation of Hemoglobin Synthesis during the Development of the Red Cell

In this article we have surveyed the current state of knowledge regarding the accumulation of globin mRNA and hemoglobin in red cells. We have attempted to examine the interplay of numerous processes that seem to be necessary to achieve this highly differentiated state. Finally, we have made an effort to formulate some of the mechanisms whereby individual red cells may come to contain varying proportions of specific hemoglobins. The past several years have been characterized by a veritable explosion of knowledge concerning the globin structure genes, and the structure, transcription, processing and function of globin mRNA in erythroid cells. It now seems possible to analyze the earlier stages of erythropoiesis by cultivation and examination of erythroid colonies in vitro. The primary differentiation events leading to the production of specific globins, especially for hemoglobin F production in man, are now experimentally accessible. There is good reason to hope that these advances will soon permit achievement of the long desired therapeutic goal of enhancing hemoglobin F synthesis in patients with severe beta-chain hemoglobinopathies. Our aim has been to review the scientific information that might provide the rationable for amelioration of the clinical phenotypes in patients inheriting abnormal globin genes.