Immunohistochemical delineation of the conduction system. I: The sinoatrial node

We have raised a mouse monoclonal antibody that reacts specifically with the myocytes of the sinoatrial node of the bovine heart. By use of this antibody (445-6E10) and antibodies against the gap junction protein connexin43, the periphery of the sinoatrial node and the distribution of gap junctions in the nodal region were studied. The reaction patterns of 445-6E10 and anti-connexin43 are exactly complementary; ie, connexin43 was not detected in the nodal myocytes but was clearly present in the atrial myocytes. Both reaction patterns demonstrate that nodal myocytes and atrial myocytes can unambiguously be distinguished by their characteristic molecular phenotype. The transitional nodal myocytes at the periphery of the node that have intermediate morphological and electrophysiological characteristics could now clearly be defined as nodal by our immunohistochemical criteria. The center of the node is surrounded by a region of interdigitating nodal and atrial bundles. Nodal bundles, coming from the center of the node, penetrate the atrial myocardium aligned at atrial bundles, forming histological connections between nodal and atrial myocytes at regular distances. This interdigitating arrangement of bundles of connexin43-negative nodal and connexin43-positive atrial myocytes is also found in the human and rat heart. We hypothesize that the architecture of the periphery of the node is important to prevent silencing of the pacemaking nodal myocytes by the atrium while ensuring a sufficient source loading of the nodal myocytes.

Identification and functional characterization of regulatory elements of the glutamine synthetase gene from rat liver

Hepatic glutamine synthetase (GS) shows a unique expression pattern limited to a few hepatocytes surrounding the terminal hepatic veins. Starting from the genomic clone of the rat GS gene, lambda GS1 [Van de Zande, L. P. G. W., Labruyère, W. T., Arnberg, A. C., Wilson, R. H., Van den Bogaert, A. J. W., Das, A. T., Frijters, C., Charles, R., Moorman, A. F. M. & Lamers, W. H. (1990) Gene (Amst.) 87, 225-232] additional genomic clones containing up to 9 kb of 5'flanking region were isolated in order to characterize cis-acting elements involved in the regulation of GS expression. Sequence analysis of the 5'flanking region up to -2520 bp revealed a putative AP2-binding site at -223 bp and a second GC box at -2343 bp in addition to the canonical TATA, CCAAT and GC boxes found proximal to the transcription-start site. A possible negative glucocorticoid-responsive element (GRE) and regions with very weak similarity to a GRE and to a known silencer element were noted at -506 bp, -406 bp and at -798 bp, respectively. Within the sequenced part of the 5'flanking region no known regulatory elements associated with liver-specific gene expression were found except for a putative HNF3-binding site at -896 bp. Functional analysis by transient transfection assays using constructs with the pSSCAT or the pXP1 vector revealed that the elements present within the first 153 bp and particularly the first 368 bp of upstream sequence constitute an active promoter the activity of which is decreased by additional sequences up to -2148 bp. The presence of dexamethasone led to a 2-4-fold increase in the promoter activity of all these constructs. Using the heterologous truncated thymidine-kinase-gene promoter of the plasmid pT81-luc a strong enhancer element was located between -2520 bp and -2148 bp. Its activity was not affected by dexamethasone but was negatively influenced by flanking sequences in both directions. This enhancer was also effective with the homologous GS promoter (-153 to +59 bp) and the heterologous full thymidine-kinase-gene promoter (pT109luc). No further enhancers were found up to -6200 bp. Using the same approach, a second enhancer was found between +259 bp and +950 bp within the first intron. Deoxyribonuclease-I hypersensitivity studies confirmed the presence of a hypersensitive site between +350 bp and +550 bp and suggested a second site between +850 bp and +1200 bp.(ABSTRACT TRUNCATED AT 400 WORDS)

Differences in erythropoiesis in normal chicken and quail embryos

Using antibodies against the fetal and adult forms of alpha- and beta-globin, it has been shown that erythropoiesis in the para-aortic foci (PAF) constitutes a major species-specific difference between chicken and quail embryos. In quail embryos, para-aortic foci are rare, small and rather heterogeneous with regard to their erythropoietic and haemopoietic cell composition. In contrast, the PAFs in chicken embryos are abundant and consist of large numbers of erythropoietic cells. In both species a time difference (approximately 1 day) is observed between the first expression of the fetal alpha- and beta-globin and the adult alpha- and beta-globin in erythropoietic cells. Adult erythropoiesis in both species can be detected first in the stalk of the yolk sac; this is similar to the situation in mammalian and amphibian species. From this time onward the number of circulating adult erythrocytes increases steadily. Whereas in chicken, large intraembryonic foci that can serve as sources for these adult cells arise concomitantly, no such foci can be detected in quail embryos, suggesting that the quail yolk sac is a major source for these adult red blood cells.

Isolation and characterization of the rat gene encoding glutamate dehydrogenase

The concentration of glutamate dehydrogenase (GDH) varies strongly between different organs and between different regions within organs. To permit further studies on the regulation of GDH expression, we isolated and characterized the rat gene encoding the GDH protein. This gene contains 13 exons and spans approximately 34 kbp. The GDH gene is present as a single, autosomally located copy in the Wistar rat genome, but shows an extensive restriction-fragment-length polymorphism for several enzymes. Promoter activity of the 5'-flanking sequence is shown by transient transfection experiments. The 5'-flanking sequence contains a TTAAAA sequence at position -29, instead of a consensus TATA box and, like many other TATA-less promoters, is characterized by a very high G + C content. In addition, consensus sequences for the binding sites of the transcription factors Sp1 and Zif268 are present in the G + C-rich upstream region.

[Application of combined DNA and dystrophin protein analysis in the diagnosis of Duchenne's and Becker's muscular dystrophy in 102 Dutch patients]

The recent progress in molecular genetic studies on Duchenne and Becker muscular dystrophy (DMD, BMD) had an important spin-off for our diagnostic abilities of both muscle disease. The mapping and isolation of the DMD gene which codes for the 427 kD cytoskeletal protein dystrophin made it possible to diagnose 80-85% of the patients by means of DNA analysis. At present, most of the remaining 15-20% of the patients can be diagnosed by protein analysis. In this report we describe the analysis of dystrophin in a group of 102 Dutch patients with muscular dystrophies. An immunohistochemical and immunobiochemical study of dystrophin was performed on muscle tissue, partly integrated with DNA analysis. In this study we underline the value of dystrophin analysis in all patients suspected of DMD, BMD or other muscular dystrophies, particularly in those without detectable DNA mutations. By means of integrated DNA/dystrophin analysis 98% of the DMD patients and 90% of th BMD patients and their families can now be provided with an unambiguous diagnosis. In particular, discrimination between BMD and other muscular dystrophies has strongly improved.

Lactase gene expression during early development of rat small intestine

Expression of lactase messenger (m) RNA and protein in rat small intestine during fetal and postnatal development was analyzed using in situ hybridization and immunohistochemistry. Lactase mRNA was first identified at 18 days of development, and lactase protein was first detected at day 20. Lactase mRNA and protein were present along the entire villus. Lactase mRNA increased, reaching a maximum at day 20. Just before birth a decrease in lactase mRNA was observed. In newborn intestine, lactase mRNA was present only from the base of the villus up to the mid-villus region and was undetectable up to the villus tips. Lactase protein continued to be expressed along the entire villus. These data show that expression of lactase mRNA and protein do not parallel, indicating a posttranscriptional control in fetal development. Lactase gene transcription is initiated late in gestation concomitant with villus formation and is exclusively seen in villus epithelial cells. The restriction after birth of lactase mRNA expression to cells at the villus base suggests the occurrence of a previously unknown step in postnatal differentiation of the enterocyte.

New findings concerning ventricular septation in the human heart. Implications for maldevelopment

BACKGROUND

The mechanics involved in development of the inlet component of the morphologically right ventricle are, as yet, undecided. Some argue that this component is derived from the descending limb of the ventricular loop, and that the inlet and apical trabecular components of the muscular ventricular septum have separate developmental origins. Others state that the entirety of the right ventricle grows from the ascending limb of the loop, and that the muscular septum, apart from its outer component, has a unitary origin. We now have material from human embryos at our disposal, which, we believe, solves this conundrum.

METHODS AND RESULTS

We used a monoclonal antibody against an antigen to neural tissue from the chick to demarcate a ring of cells separating the descending (inlet) and ascending (outlet) limbs of the developing ventricular loop of the human heart. Preparation of serial sections of graded human embryos enabled us to trace the fate of this ring, and hence the formation of the inlet of the right ventricle, to the completion of cardiac septation. Eight embryos were studied, encompassing stages 14-23 of the Carnegie classification. The ring of cells initially separating the ascending and descending limbs of the ventricular loop were, at the conclusion of ventricular septation, located within the atrioventricular junction, sequestrated for the most part in the terminal segment of atrial myocardium.

CONCLUSIONS

Our study conclusively shows that the inlet component of the morphologically right ventricle is derived from the ascending limb of the embryonic ventricular loop, and that the inlet and apical trabecular components of the muscular septum are derived from the same primary ventricular septum.

Construction of dystrophin fusion proteins to raise targeted antibodies to different epitopes

For the study of the structure and function relationship of dystrophin, defective in DMD, and for diagnostic purposes it is important to dispose of antibodies against different parts of the protein. We have made five different constructs for the expression of fusion proteins containing parts of the four domains of dystrophin. Two different recombinant expression vectors, pATH2 and pEX1, were used. Rabbits were immunized with the fusion products and several polyclonal antibodies were raised. At a later stage, monoclonal antibodies were also raised to some of the fusion proteins. One polyclonal antibody, named P20 AB, is directed against the region covering amino acid sequence 1749-2248 or the nucleotide sequence 5456-6953 of the mRNA, which corresponds to the major deletion-prone region of the DMD gene. We show the particular value, sensitivity and specificity of the P20 AB in dystrophin analysis.

Persisting zones of slow impulse conduction in developing chicken hearts

We performed a correlative electrophysiological and immunohistochemical study of embryonic chicken hearts during the septational period (Hamburger and Hamilton stages 13-31 [2-7 days of incubation]). The analyses yield conclusive evidence for slow conduction, up to 7 days of development, in the outflow tract, in the atrioventricular canal, and in the sinoatrial junction. The conduction velocity remains approximately 1 cm/sec in the outflow tract and increases in the ventricle 20-fold to approximately 20 cm/sec between 2 and 7 days of development. Transmembrane potentials of myocytes in the outflow tract and atrioventricular canal slowly rise (less than 5 V/sec), whereas in the atrium and ventricle, the upstroke velocity is eightfold to 13-fold higher. In the outflow tract, repolarization is completed only after the start of the next cycle. Because of the persistence of slow conduction, the myocardium flanking the developing atria and ventricle is thought to represent segments of persisting "primary" myocardium, whereas the more rapidly conducting "working" myocardium of the ventricle and atria is thought to represent more advanced stages of myocardial differentiation. The persisting primary myocardium was characterized by a continued coexpression of both the atrial and ventricular isoforms of myosin heavy chain. The developing atria and ventricle could be demarcated morphologically from the primary myocardium because the free walls of these segments only express their respective isoforms of myosin heavy chain. The slowly conducting myocardial zones appear to be essential for the function of the embryonic heart because 1) they provide the septating heart with alternating segments of slow and relatively fast conduction necessary for consecutive contraction of the atrial and ventricular segments and 2) their sphincterlike prolonged peristaltic contraction pattern can substitute for the adult type of one-way valves that start to develop at the end of septation.

Presence of cardiac alpha-myosin correlates with histochemical myosin Ca2+ ATPase activity in rabbit masseter muscle

A combined enzyme-histochemical (ATPase reactivity) and immunohistochemical study has been performed on sections of rabbit masseter muscle. The majority of the fibres previously designated as type IIC and/or type I according to their ATPase activity were found to contain 'cardiac' alpha-myosin heavy chain in addition to other myosin heavy chains. All alpha-myosin heavy chain-containing fibres reveal ATPase activity after pre-incubation at pH 4.2-4.6 similar to that of the classical type I fibres, while, in that pH range, limb type IIC fibres show intermediate ATPase activity. One group of these fibres reveal ATPase activity after pre-incubation at pH 10.1-10.3 as well, but not at pH 10.4-10.5. These fibres contain exclusively either alpha- or alpha- and I-myosin heavy chains but do not contain the IIA-myosin heavy chain. The second part of the fibres reveals ATPase activity after treatment within the whole alkaline pre-incubation range (pH 10.1-10.5) and these fibres contain alpha-myosin and IIA-myosin but no I-myosin heavy chain. It is concluded that the classical IIC fibre type is not present in the rabbit masseter muscle. Furthermore, ATPase reactivity does not allow us to distinguish fibres on their myosin heavy chain content in rabbit masseter muscle.

Distribution of the nuclear thyroid-hormone receptor in extraocular and skeletal muscles

The correlation between the occurrence of Graves' ophthalmopathy and Graves' hyperthyroidism may indicate a role for tri-iodothyronine (T3) hormone in the pathogenesis of Graves' ophthalmopathy. In Graves' ophthalmopathy the recti eye muscles are greatly enlarged whereas skeletal muscles seem unaffected. The distribution of the nuclear T3 receptor was studied in normal human and rat eye and skeletal muscles with immunohistochemistry using mouse (monoclonal) antibodies, and by in-situ hybridization for the detection of mRNA encoding the T3-receptor protein. Nuclear staining with T3-receptor antibodies was found in all types of tissues studied. Cytoplasmic staining occurred predominantly in the muscle fibres of the orbital layer of the eye muscles and was generally absent or very low in skeletal muscle fibres and hepatocytes. Immunostaining could be inhibited by preabsorbing the antibodies with bacterially expressed T3-receptor protein, implying specificity. The presence of nuclear and cytoplasmic hormone-free T3 receptor sites was indicated after preincubation of sections with T3 hormone; T3-receptor immunostaining decreased and T3-hormone staining increased. In-situ hybridization clearly revealed the presence of alpha-1 and beta-1 forms of the T3-receptor mRNA in liver, skeletal muscles, and orbital and intermediate layers of the eye muscles. The data demonstrate the presence of T3 hormone-receptor molecules in the extraocular and skeletal muscles. The different susceptibilities of these muscles to Graves' hyperthyroidism may relate to the quantitative differences in T3 hormone-receptor distribution.

Myosin heavy chain expression in rabbit masseter muscle during postnatal development

The expression of isoforms of myosin heavy chain (MHC) during postnatal development was studied in the masseter muscle of the rabbit. Evidence is presented that in addition to adult fast and slow myosin, the rabbit masseter contains neonatal and 'cardiac' alpha-MHC. During postnatal growth myosin transitions take place from neonatal and fast (IIA, IIA/IIB--referring to a fibre containing both IIA and IIB MHCs) MHC to adult 'cardiac' alpha-MHC and I/alpha-MHC. Since there is a temporary population of fibres containing IIA/alpha-MHC during the first 4 wk of development with a peak in the 3rd to 4th wk, the transition from IIA-MHC to alpha-MHC may occur in these IIA/alpha-MHC-containing fibres. The appearance of 'cardiac' alpha-MHC coincides with the timing of weaning, suggesting that the changes in MHC content, that probably result in a transition to a lower speed of contraction, have functional significance related to weaning. The finding of neonatal MHC in adult rabbits indicates that the masseter develops at a rate and in a way that is distinct from most other skeletal muscles. A spatiotemporal variation in expression of myosin isozymes within the masseter was observed, with many fibres containing more than one myosin type, indicating developmentally regulated spatial differences in function.

Messenger RNA sorting in enterocytes. Co-localization with encoded proteins

This study describes the intracellular compartmentalization of three different mRNAs in the polarized rat fetal enterocyte. They encode proteins that are known to be localized within different regions of the epithelial cell namely (i) the apical, membrane-bound glycoprotein, lactase-phlorizin hydrolase (lactase), (ii) the mitochondrially localized enzyme, carbamoylphosphate synthetase (CPS), and (iii) the cytoplasmically localized enzyme, phosphoenolpyruvate carboxykinase (PEPCK). These mRNAs are found in close proximity to their respective protein products, i.e. the apical membrane, mitochondria and cytoplasm, respectively. The significance of these observations is twofold; (i) they indicate that mRNAs are sorted into specific domains of the cytosol of intestinal epithelial cells; and (ii) they imply the presence of two distinct pathways of mRNA targeting one that allows transport of mRNAs that are translated on ribosomes associated with the rough endoplasmic reticulum (lactase mRNA), and the other that allows sorting of mRNAs that are translated on free polysomes (CPS and PEPCK mRNA).

Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. III. An immunohistochemical analysis of the distribution of the neural tissue antigen G1N2 in the embryonic heart; implications for the development of the atrioventricular conduction system

A monoclonal antibody raised against an extract from the Ganglion Nodosum of the chick and designated G1N2 proves to bind specifically to a subpopulation of cardiomyocytes in the embryonic human heart. In the youngest stage examined (Carnegie stage 14, i.e., 4 1/2 weeks of development) these G1N2-expressing cells are localized in the myocardium that surrounds the foramen between the embryonic left and right ventricle. In the lesser curvature of the cardiac loop this "primary" ring occupies the lower part of the wall of the atrioventricular canal. During subsequent development, G1N2-expressing cells continue to identify the entrance to the right ventricle, but the shape of the ring changes as a result of the tissue remodelling that underlies cardiac septation. During the initial phases of this process the staining remains recognizable as a continuous band of cells in the myocardium that surrounds the developing right portion of the atrioventricular canal, subendocardially in the developing interventricular septum and around the junction of the embryonic left ventricle with the subaortic portion of the outflow tract. During the later stages of cardiac septation, the latter part of the ring discontinues to express G1N2, while upon the completion of septation, no G1N2-expressing cardiomyocytes can be detected anymore. The topographic distribution pattern of G1N suggests that the definitive ventricular conduction system derives from a ring of cells that initially surrounds the "primary" interventricular foramen. The results indicate that the atrioventricular bundle and bundle branches develop from G1N2-expressing myocytes in the interventricular septum, while the "compact" atrioventricular node develops at the junction of the band of G1N2-positive cells in the right atrioventricular junction (the right atrioventricular ring bundle) and the ("penetrating") atrioventricular bundle. A "dead-end tract" represents remnants of conductive tissue in the anterior part of the top of the interventricular septum. The location of the various components of the avian conduction system is topographically homologous with that of the G1N2-ring in the human embryonic heart, indicating a phylogenetically conserved origin of the conduction system in vertebrates.

Hepatocyte heterogeneity in the metabolism of amino acids and ammonia

With respect to hepatocyte heterogeneity in ammonia and amino acid metabolism, two different patterns of sublobular gene expression are distinguished: 'gradient-type' and 'strict- or compartment-type' zonation. An example for strict-type zonation is the reciprocal distribution of carbamoylphosphate synthase and glutamine synthase in the liver lobule. The mechanisms underlying the different sublobular gene expressions are not yet settled but may involve the development of hepatic architecture, innervation, blood-borne hormonal and metabolic factors. The periportal zone is characterized by a high capacity for uptake and catabolism of amino acids (except glutamate and aspartate) as well as for urea synthesis and gluconeogenesis. On the other hand, glutamine synthesis, ornithine transamination and the uptake of vascular glutamate, aspartate, malate and alpha-ketoglutarate are restricted to a small perivenous hepatocyte population. Accordingly, in the intact liver lobule the major pathways for ammonia detoxication, urea and glutamine synthesis, are anatomically switched behind each other and represent in functional terms the sequence of the periportal low affinity system (urea synthesis) and a previous high affinity system (glutamine synthesis) for ammonia detoxication. Perivenous glutamine synthase-containing hepatocytes ('scavenger cells') act as a high affinity scavenger for the ammonia, which escapes the more upstream urea-synthesizing compartment. Periportal glutaminase acts as a pH- and hormone-modulated ammonia-amplifying system in the mitochondria of periportal hepatocytes. The activity of this amplifying system is one crucial determinant for flux through the urea cycle in view of the high Km (ammonia) of carbamoylphosphate synthase, the rate-controlling enzyme of the urea cycle. The structural and functional organization of glutamine and ammonia-metabolizing pathways in the liver lobule provides one basis for the understanding of a hepatic role in systemic acid base homeostasis. Urea synthesis is a major pathway for irreversible removal of metabolically generated bicarbonate. The lobular organization enables the adjustment of the urea cycle flux and accordingly the rate of irreversible hepatic bicarbonate elimination to the needs of the systemic acid base situation, without the threat of hyperammonemia.

cDNA sequence of the long mRNA for human glutamine synthase

Screening a human liver cDNA library in lambda ZAP revealed several clones for the mRNA of glutamine synthase. The longest clone was completely sequenced and consists of a 109 bp 5' untranslated region, a 1119 bp protein coding region, a 1498 bp 3' untranslated region and a poly(A) tract of 12 bp.

Detection of CAIII mRNA in rat skeletal muscle and liver by in situ hybridization

We carried out a variety of in situ methods of hybridization on rat liver and rat skeletal muscle using 35S-labeled or biotin-labeled rat carbonic anhydrase III (CAIII) cDNA clone. The methods were compared and evaluated. Use of the biotin system produced defined but nonspecific results which were shown not to be due to the biotinylated cDNA probe binding to the mRNA in the muscle sections. This artifact was shown to persist despite various attempts to eliminate it. Alternatively, using 35S-labeled cDNA gave reproducible results which were shown to be consistent with probe binding specifically to mRNA in the muscle section.

The dynamics of the expression of C/EBP mRNA in the adult rat liver lobulus qualifies it as a pericentral mRNA

A hybridocytochemical approach has been applied to establish whether the gene for the C/EBP mRNA might be involved in the topographical regulation of gene expression in adult rat liver. To that end the spatial distribution of the mRNA of C/EBP has been compared to that of the mRNAs of glutamine synthetase (GS), phosphoenolpyruvate carboxykinase (PEPCK) and glucokinase (GK) in normal adult livers, in livers from dexamethasone-treated animals and in livers from starved animals refed with glucose for 4 h. In normal rat liver, in situ hybridization with a probe for C/EBP mRNA revealed a low density of apparently homogeneously distributed grains, indicating low levels of C/EBP mRNA. In contrast, the livers of the experimentally-treated animals revealed a zonal distribution of the mRNA of C/EBP with the highest density of grains around the central venules. The dynamics of the pattern of expression of C/EBP mRNA are virtually identical to that of the GK mRNA. These data qualify C/EBP mRNA as a pericentral mRNA and suggest a role for the C/EBP protein in the topographical regulation of the expression of the GK mRNA.

Pericentral expression pattern of glucokinase mRNA in the rat liver lobulus

The spatial distribution of glucokinase mRNA (GK mRNA) in rat liver was studied by in situ hybridization under normal and inducing conditions. GK mRNA was first detectable in the liver parenchyma of neonatal rats of 1.5 days. The density of grains decreases in a central-portal direction. This pattern remains essentially unchanged up to 15 days, after which the adult type of distribution gradually starts to develop, i.e. low density of grains indicating low levels of GK mRNA, in which no gradient of expression could be visualized. Within 2 h after an oral glucose load to starved animals, the GK mRNA expression pattern changed from hardly detectable to a clear gradient with the highest grain density around the terminal central venules. Within 6 h relatively high levels of grains, almost homogeneously distributed across the liver lobule, were observed. Glucocorticosteroid treatment also induced GK mRNA in the pericentral area. It is concluded that the observed induction pattern qualifies GK mRNA as a pericentral mRNA suggesting that the pericentral expression pattern of the protein is primarily regulated at the pretranslational level.