M.J.B. van den Hoff

The cardiac sodium channel displays differential distribution in the conduction system and transmural heterogeneity in the murine ventricular myocardium.

Cardiac sodium channels are responsible for conduction in the normal and diseased heart. We aimed to investigate regional and transmural distribution of sodium channel expression and function in the myocardium. Sodium channel Scn5a mRNA and Nav1.5 protein distribution was investigated in adult and embryonic mouse heart through immunohistochemistry and in situ hybridization. Functional sodium channel availability in subepicardial and subendocardial myocytes was assessed using patch-clamp technique. Adult and embryonic (ED14.5) mouse heart sections showed low expression of Nav1.5 in the HCN4-positive sinoatrial and atrioventricular nodes. In contrast, high expression levels of Nav1.5 were observed in the HCN4-positive and Cx43-negative AV or His bundle, bundle branches and Purkinje fibers. In both ventricles, a transmural gradient was observed, with a low Nav1.5 labeling intensity in the subepicardium as compared to the subendocardium. Similar Scn5a mRNA expression patterns were observed on in situ hybridization of embryonic and adult tissue. Maximal action potential upstroke velocity was significantly lower in subepicardial myocytes (meanxa0±xa0SEM 309xa0±xa032xa0V/s; nxa0=xa014) compared to subendocardial myocytes (394xa0±xa032xa0V/s; nxa0=xa011; Pxa0<xa00.05), indicating decreased sodium channel availability in subepicardium compared to subendocardium. Scn5a and Nav1.5 show heterogeneous distribution patterns within the cardiac conduction system and across the ventricular wall. This differential distribution of the cardiac sodium channel may have profound consequences for conduction disease phenotypes and arrhythmogenesis in the setting of sodium channel disease.

Developmental changes in the expression of the liver-enriched transcription factors LF-B1, C/EBP, DBP and LAP/LIP in relation to the expression of albumin, α-fetoprotein, carbamoylphosphate synthase and lactase mRNA

SummaryExpression of α-fetoprotein, carbamoylphosphate synthase and albumin, that are generally accepted markers for the hepatic phenotype, require a distinct set of transcription factors. We investigated by in situ hybridization whether this set of transcription factors, LF-B1, C/EBP, DBP and LAP/LIP, is expressed coordinately in the liver during embryonic development and to what extent they are also expressed elsewhere. Our results demonstrate that mRNA levels of all transcription factors tested are significantly above background in the whole embryo and are either reduced or enhanced in expression during subsequent development. Interestingly, cardiac mesoderm, which induces prehepatic endoderm to liver formation, is temporarily permissive to its own signals, showing enhanced expression of these transcription factors and, as a result, the hepatocyte-specific genes α-fetoprotein and carbamoylphosphate synthase. In addition, these transcription factors and many liver-specific structural genes rise concomitantly in intestine and kidney just before birth, suggesting the expression of hepatogenic factors in these tissues as well. Despite the extrahepatic expression of these transcription factors, expression of albumin remains confined to the liver at all developmental stages.

Measure is treasure

See article by Ribadeau-Dumas et al. [1] (pages 426–436) in this issue. nnThe paper of Ribadeau-Dumas et al. in this issue [1] deals with the interesting, but complex regulation of the SERCA2 gene. This gene encodes the sarcoplasmatic calcium pump, which is a crucial enzyme in the calcium handling of the cardiomyocyte. Insight into the regulation of this gene is therefore of paramount importance. A proper understanding of the regulation of the expression of a gene in adult, developing, experimental or diseased (cardiac) tissue requires precise knowledge of the control at each step from gene to the function of the encoded protein, comprising RNA and protein accumulation (being the product of synthesis and degradation) and biological activity of the studied protein. This list can of course be extended (Fig. 1). If we are to appreciate at which step in this pathway control is predominantly exerted, it is important to quantify as many of the parameters as possible. It is equally important to relate the disparate parameters with one another, to evaluate, for example, whether or not the increase in biological activity is proportional with the increase in the amount of protein, or whether the increase in protein follows a proportionate increase in the encoding mRNA. The vital role of quantification of the distinct parameters for our understanding of the regulation of (cardiac) gene expression is beyond any dispute. The lightheartedness concerning this topic is therefore surprising.nnnnFig. 1 nPotential steps at which control of gene expression can be exerted in the … nnnn* Corresponding author. Tel.: +31-20-5669111; fax: +31-20-6976177 m.j.vandenhoff{at}amc.uva.nl a.f.moorman{at}amc.uva.nl