Corrigendum to "Generation of cardiomyocytes by atrioventricular node cells in long-term cultures" [Biochem. Biophys. Rep. 26 (2021) 101018].

[This corrects the article DOI: 10.1016/j.bbrep.2021.101018.].

[Transcription Factor MTF-1 Involved in the Cellular Response to Zinc].

Heavy metals, both toxic and essential, have long been an important research focus in life science. To investigate the intracellular actions of heavy metals at the molecular level, I have been exploring protein factors involved in induction of metallothionein (MT) genes by heavy metals that specifically bind to a metal responsive element (MRE) in the region upstream of the human MT-IIA gene. Purification of a zinc-dependent MRE-binding factor, and cloning of its cDNA identified a sequence identical to that of metal-responsive transcription factor-1 (MTF-1). MTF-1, which is characterized by six tandem repeats of the C2H2 type zinc finger motif, is indispensable for induction of MT gene expression by multiple types of heavy metal, but zinc is the only metal that can directly activate MTF-1 binding to the MRE, indicating that other heavy metal signals act through zinc as a second messenger. Functional analysis of various MTF-1 point mutants revealed several cysteine (Cys) residues critical for DNA binding and/or transactivation activity. Interestingly, six finger motifs seem to mediate several MTF-1 functions other than DNA binding. Immunohistochemical analyses of various mouse tissues revealed selective expression of MTF-1 in spermatocytes among the testicular cells, suggesting roles relevant to spermatogenesis. The zinc regulon, under the control of MTF-1, will likely provide good clues to aid in unraveling novel functions of intracellular zinc ions.

Generation of cardiomyocytes by atrioventricular node cells in long-term cultures.

Turnover of cardiac pacemaker cells may occur during the lifetime of the body, and we recently raised the hypothesis that specialized cardiac cells have in common the potential to generate cardiomyocytes from fibroblasts. To examine this hypothesis, we analyzed the ability of atrioventricular node cells (AVNCs) to generate functional cardiomyocytes in long-term culture. AVNCs were isolated from adult guinea pig hearts and cultured for up to three weeks. Under phase-contrast microscopic observation over time, it was found that within a week, a number of fibroblasts gathered around the AVNCs and formed cell clusters, and thereafter the cell clusters started to beat spontaneously. The nascent cell clusters expanded their area gradually by three weeks in culture and expressed specific cardiac genes and proteins. Maturation of newly formed cardiomyocytes seems to be slow in cultures of AVNCs compared with those of sinoatrial node cells. Stimulation of muscarinic receptors with acetylcholine induced a beating rate decrease which was blocked by atropine, and activation of adenylate cyclase activity with forskolin increased the beat rate, while stimulation of beta adrenoceptors by isoproterenol had no effect. These results indicate that AVNCs form a cluster of cells with properties of functional cardiomyocytes and provide evidence to support the hypothesis.

Cardiac Pacemaker Cells Generate Cardiomyocytes from Fibroblasts in Long-Term Cultures.

Because cardiomyocyte generation is limited, the turnover of cardiomyocytes in adult heart tissues is much debated. We report here that cardiac pacemaker cells can generate cardiomyocytes from fibroblasts in vitro. Sinoatrial node cells (SANCs) were isolated from adult guinea pig hearts and were cultured at relatively low cell densities. Within a week, a number of fibroblast-like cells were observed to gather around SANCs, and these formed spontaneously beating clusters with cardiomyocyte structures. The clusters expressed genes and proteins that are characteristic of atrial cardiomyocytes. Pharmacological blocking of pacemaker currents inhibited generation of action potentials, and the spontaneous beating were ceased by physically destroying a few central cells. Inhibition of beating during culture also hampered the cluster formation. Moreover, purified guinea pig cardiac fibroblasts (GCFs) expressed cardiac-specific proteins in co-culture with SANCs or in SANC-preconditioned culture medium under electrical stimulation. These results indicate that SANCs can generate cardiomyocytes from cardiac fibroblasts through the influence of humoral factor(s) and electrophysiological activities followed by intracellular Ca2+ oscillations. This potential of SANCs to generate cardiomyocytes indicates a novel mechanism by which cardiomyocytes turns over in the vicinity of pacemaker cells and could be exploited in the development of strategies for cardiac regenerative therapy in adult hearts.

Analysis of cysteine and histidine residues required for zinc response of the transcription factor human MTF-1.

Metal responsive element (MRE)-binding transcription factor-1 (MTF-1) is a zinc finger (ZF) transcription factor that plays a key role in heavy metal homeostasis by regulating relevant genes in response to metals. MTF-1 is known to be activated by heavy metals such as Zn and Cd, but the mechanism of activation remains unclear. In the present study, Cys and His residues of human MTF-1 (hMTF-1), some of which may be involved in interaction with metals or with each other, were screened for their contribution to Zn-dependent transcription. To avoid poor induction ratios of previous transfection assays, we re-examined experimental conditions to establish an assay able to correctly detect Zn-responsive transcription. Using this assay, a series of Cys and/or His substitution mutants were analyzed over the entire hMTF-1 molecule. In five out of the six ZFs (ZF1 to ZF5), Cys mutations that disrupt the ZF structure abolished response to Zn. Of these, ZF5 was shown for the first time to be essential for Zn-responsive transcription, despite it being unnecessary for Zn-induced DNA binding. These results indicate that Zn activation of hMTF-1 involves an additional process besides induction of DNA binding activity. Our assay also confirmed the importance of Cys in the acidic activation domain, as well as those in the C-terminal Cys cluster, implicated in transcription in other studies. The identified Cys residues might contribute to metal response of hMTF-1 through direct metal binding and/or intramolecular interactions, analysis of which will be helpful in understanding the mechanism of metal response.

A versatile transfection assay system to evaluate the biological effects of diverse industrial chemicals.

Gene expression processes are now recognized as important targets of the toxic effects exerted by industrial chemicals. The transient transfection assay is a powerful tool to evaluate such effects. Thus, we developed a versatile assay system by constructing a basic reporter plasmid in which the regulatory DNA sequence to be studied can easily be substituted. To verify the performance of this system, reporter plasmids carrying any of the three distinct regulatory sequences, estrogen responsive element (ERE), glucocorticoid responsive element (GRE) and xenobiotic responsive element (XRE) were constructed. After transfection of human cells, these plasmids successfully expressed the relevant reporter genes in response to specific inducers, ß-estradiol, dexamethasone and 3-methylcholanthrene, respectively. Several industrial chemicals were assayed using these reporter plasmids, and the ability of p-dimethylaminoazobenzene to elevate GRE- and XRE-mediated transcription was detected. α-Naphthylamine and o-tolidine were also observed to increase the XRE-mediated response. The transfection assay system established here will be useful to evaluate the effects of a wide variety of industrial chemicals.

[Mechanism of metallothionein gene activation mediated by heavy-metal dependent transcription factor MTF-1].

Transcriptional activation of metallothionein (MT) genes by heavy metals is a valuable system for understanding the functions of MT as well as the cellular response against heavy metals. Although it is now known that heavy metal signals culminating in MT induction converge upon a transcription factor MTF-1, the mechanism underlying the MTF-1 response to heavy metals has not been elucidated. To address this issue, we investigated various aspects of the in vivo response of MTF-1 against heavy metals. Chromatin immunoprecipitation assay showed that heavy metal-dependent DNA binding of MTF-1 is the critical step in vivo. MTF-1 is primarily localized in the nucleus so that heavy metal-dependent nuclear translocation demonstrated by other groups does not seem to be universal and hence may not be critical for activation of MTF-1. In the six Zn finger motifs, the hallmark of MTF-1, the third and the fourth fingers are essential for the nuclear localization of MTF-1. Furthermore, all fingers except the last are important for transcriptional activation function of MTF-1, suggesting their key role for MTF-1 function. Also, a cysteine cluster structure located in the C-terminal region of MTF-1 is critical for transactivating function of MTF-1. These results suggest a central role of the Zn-finger domain and intramolecular cooperation through a structural change of MTF-1 for its response to heavy metal challenge.

Analysis of human proteins that have an affinity to heavy metals by metal-chelating column chromatography.

To clarify the molecular basis of toxicities of industrial chemicals, it is demanded to develop appropriate methods whereby their cellular target molecules can be directly identified. In the present study, we focused on target proteins of heavy metals and established the method to detect them using a combination of metal-chelating column chromatography and a subsequent analysis by electrophoresis. Protein samples prepared from HeLa cells were applied to the Zn- or Cd-chelating column, and the bound proteins were analyzed by SDS-polyacrylamide gel electrophoresis followed by either silver staining, or fluorography when using radiolabel protein samples. Among several protein species trapped in the columns, a 36-kDa protein apparently has an affinity to both Zn and Cd, indicating the possibility that Cd can exchange essential Zn on this protein. These results suggest that the established method is useful for the target protein screening and further analyses of separated proteins.

Gene expression assay for hazard assessment of chemicals.

Recent progress in our knowledge of gene expression systems provides evidence that many industrial chemicals affect the transcriptional machineries directly or indirectly, and gene expression is now recognized as one of the main targets of many chemicals. In view of the increasing number of man-made chemicals, it is therefore necessary to establish a reliable gene expression assay with rapidity and high sensitivity. Among various gene expression assays, the so-called reporter assay is now accepted as a suitable tool to assess hazardous effects of chemicals on gene expression. This article focuses on the principle and applications of the reporter assay in research on endocrine disrupters.