Complementary distribution of carbamoylphosphate synthetase (ammonia) and glutamine synthetase in rat liver acinus is regulated at a pretranslational level.

Abstract

We studied the distribution of the mRNAs for carbamoylphosphate synthetase (ammonia) and glutamine synthetase in frozen sections of adult rat liver by in situ hybridization to [35S]-labeled cDNA probes. The density of silver grains resulting from hybridization to the labeled cDNA probe for carbamoylphosphate synthetase is highest around the portal venules, decreases towards the central venule, and is virtually absent from an area two to three cells wide that lines the central venules in which mRNA for glutamine synthetase is predominantly localized. Therefore, both mRNAs show the same complementary distribution within the liver acinus that was found for the proteins they encode, demonstrating that compartmentalization of the expression of these enzymes is controlled at a pretranslational level. In addition, we found that carbamoylphosphate synthetase mRNA is present mainly in the epithelium of the crypts of the proximal part of the small intestine, whereas carbamoylphosphate synthetase protein is present in the epithelium of both crypts and villi.

Heterogeneous expression of glutamine synthetase mRNA in rat liver parenchyma revealed by in situ hybridization and Northern blot analysis of RNA from periportal and perivenous hepatocytes

“…fig.lB). This finding corroborates the results of a study by Moorman et al [1] who showed that, in rat liver, GS and CPS mRNA have a complementary distribution pattern. The hybridization signal, i.e.…”

“…Thus, the heterogeneous distribution of GS in rat liver parenchyma seems to be due to differences in gene expression m hepatocytes from different acinar localizations. A similar conclusion was drawn by Moorman et al [1] for rat and by Smith and Campbell [8] for mouse liver based on in situ hybridization only. This interpretation is supported by the results of different experimental approaches such as induction of hepatocyte growth in vitro [13], induction of hepatocarcinogenesis [15], and hepatocyte transplantation [27], suggesting that this unique program of gene expression may be altered only under certain circumstances provided that the hepatocytes have undergone mitosis.…”

“…The glutamine synthetase probe used was the 1.5 kb cDNA insert isolated from clone pBR-GS2 [1,5]. It was labeled with [35S]dCTP and [32p]dCTP (all from Amersham/Buchler, Braunschweig) to a specific activity of 107-108 and 108 cpm/,ag, respectively, using the multi-prime DNA labeling kit (Amersham/Buchler, Braunschweig).…”

“…1C). Preincubation of the liver sections with pronase up to 20 rain followed by a second fixation with paraformaldehyde as suggested by [1,16] did not improve the hybridization, but rather reduced the amount of silver grains in contrast to the findings of Moorman et al [1]. Attempts to combine immunocytochemistry and in situ hybridization on one and the same section, with immunocytochemistry performed either before or after hybridization with the cDNA, were not satisfactory, since the density of silver grains was strongly reduced under these conditions (not shown).…”

Arginine metabolism: nitric oxide and beyond

“…Thus arginine-dependent glutamate synthesis is highly restricted within the liver. As glutamine synthetase is also selectively expressed in perivenous hepatocytes [267,268], the co-expression of all of these enzymes would support the perivenous synthesis of glutamine in the intercellular glutamine cycle proposed by Hau$ ssinger [269].…”