Irwin L. Flink

Expression of α and β integrins during terminal differentiation of cardiomyocytes

Background: In the myocardium, myocyte cell division is irreversibly blocked shortly after birth. The signal that initiates cell cycle withdrawal is unknown. The purpose of this study was to relate changes in expression of β1 integrin and its associated α subunits to cardiomyocyte cell cycle progression during the fetal-to-neonatal developmental transition in rat. Methods and results: The developmental expression pattern and function of β1 integrin and several of its associated α subunits were examined using reverse transcription (RT) polymerase chain reaction (PCR) and β1 blocking antibodies. During the fetal to neonatal transition, a dramatic shift occurred in the levels of β1 and α isoforms. At the 17-day fetal stage only β1A was present, which remained relatively constant until immediately after birth then decreased by 30% at the adult stage. By contrast, β1D appeared at fetal day 18, increased at neonatal day 2, and afterwards remained constant. This resulted in a ratio of β1A to β1D of about 1:1 in the adult heart. The integrin β1-associated subunits, α3, α6, and α7, were expressed at extremely low levels in 17-day fetal cardiomyocytes. After birth α3 and α6 transiently increased at the 2-day neonatal stage, while α7 isoforms B, C, and X2 progressively increased to the adult stage. Unlike skeletal muscle cells, fluorescence-activated cell sorting analysis (FACS) showed no down regulation of the α5β1 fibronectin receptor during cell cycle withdrawal. Treatment of cultured cardiomyocytes with β1 blocking antibody inhibited the cell cycle in fetal but not in neonatal cells. Conclusion: These results suggest that progression through the cardiomyocyte cell cycle may be dependent upon cell attachment via integrin β1 and correlate with changes that occur in β1 spliced variants and their respective α isoforms.

The hepatic biosynthesis of rat thyroxine binding globulin (TBG): Demonstration, ontogenesis, and UP-regulation in experimental hypothyroidism

Using a human thyroxine binding globulin (TBG) cDNA probe, we demonstrate that rat liver contains two TBG mRNA species of different length, consisting of about 1.8 Kb and 2.4 Kb respectively. Slot blot analysis of the hepatic mRNAs from rats of different age reveals a fair correlation between the developmental trend of the messengers and that of the TBG circulating levels. Finally Northern blot and slot studies demonstrate that the increase of serum TBG induced in adults by thyroidectomy actually reflects an enhanced hepatic biosynthesis of the protein.

Characterization of cellular nucleic acid binding protein from Xenopus laevis: Expression in all three germ layers during early development

The Xenopus CNBP homologue (XCNBP) has been cloned from stage 14 neurula. XCNBP encodes a 18.4‐kDa protein containing seven highly conserved zinc finger (Zn‐finger) repeats (CX2CX4HX4CX2), with sequence similarity to human, mouse, rat, and yeast CNBP. A unique feature of XCNBP is that it contains a 10 amino acid (aa) deletion in the linker region between Zn‐fingers 1 and 2, immediately downstream from an alternatively spliced exon of human CNBP isoforms. A similar deletion is found in mouse and yeast CNBP proteins. The deleted region lacks potential PEST and casein kinase II phosphorylation sites. Because CNBP proteins from a variety of species contain deletions in a similar region, these results suggest that the pattern of alternative processing of CNBP isoforms is highly conserved among metazoa and unicellular eukaryotes. XCNBP RNA is initially maternally derived and is widely expressed throughout early development at the gastrula, neurula, and tailbud stages. At the early gastrula stage, XCNBP is expressed in ectodermal, endodermal, and mesodermal germ layers. Previous data have demonstrated the presence of CNBP in the cytoplasm and nucleus. The interactions of CNBP with single‐stranded DNA and RNA suggest that CNBP may serve dual functions in transcriptional and translational regulation in a wide variety of tissues during development. Dev. Dyn. 1998;211:123–130.