Oral administration of insulin in winter-acclimatized carp (Cyprinus carpio) induces hepatic ultrastructural changes

Specific binding of the endocytosis tracer horseradish peroxidase to intestinal fatty acid-binding protein (I-FABP) in apical membranes of carp enterocytes

In a previous study we had demonstrated that a 15-kDa protein present in carp intestinal brush-border membrane vesicles (BBMV) was able to bind the endocytosis tracer horseradish peroxidase (HRP) with high specificity. Here we show that this protein corresponds to a peripheral membrane protein, identified by partial amino acid sequence analysis as the intestinal fatty acid-binding protein (I-FABP), a member of the small cytosolic fatty acid binding protein family (FABPs). The presence of I-FABP and its HRP-binding activity was demonstrated both in the cytosolic and membrane-associated fractions of intestinal mucosa by Western and ligand blot analyses, respectively. Also, both fractions displayed significant capacity to bind [(3)H]palmitic acid, a known ligand for I-FABP. Immunohistochemical analysis showed that I-FABP localizes both in the cytosol and in the brush-border membranes of epithelial cells. Taken together the unusual extra-cellular localization of I-FABP as well as its ability to interact with HRP suggests a novel function for this protein in the intestinal mucosa.

Gene structure of the carp fish ribosomal protein L41: seasonally regulated expression

The seasonal acclimatization of the carp fish demands physiological compensatory responses. The process involves profound nucleolar adjustments and remarkable changes in rRNA synthesis, which affect ribosomal biogenesis. We have documented that protein kinase CK2, whose activity is related to ribosomal protein L41 and the regulation of rRNA synthesis, was expressed in notably higher amounts in summer-acclimatized carp compared to the cold-season adapted fish. Thus, we approached the study of the functional genomics of carp L41 protein. We report the first cloning of a fish L41 gene encoding the highly conserved 25 amino acids, including approximately 1700 bp regulatory upstream region and the 3(') polyadenylation signal, plus the isolation and characterization of two different L41 cDNAs. We found a clear differential expression of L41, which follows the same pattern as protein kinase CK2beta that transcribes at higher levels in the summer-acclimatized carp than it does in the winter-adapted fish.

Horseradish peroxidase binding to intestinal brush-border membranes of Cyprinus carpio. Identification of a putative receptor

Morphologic studies have shown that the classic endocytosis tracer horseradish peroxidase (HRP) is actively internalized by vesicular transport in the carp intestine, suggesting the existence of specific binding sites in the apical membrane of enterocytes. The aim of the present study was to develop an in vitro binding assay using isolated carp intestinal brush-border membranes (BBM) to demonstrate and characterize these specific HRP binding sites. The results obtained show that HRP binding to BBM exhibits a saturable mode and high affinity (K(d) = 22 nM). In addition, HRP binding sites are highly enriched in BBM compared to basolateral membranes. On the other hand, HRP interaction with these sites is apparently of an ionic character because binding increased concomitantly with decreasing NaCl concentrations in the assay, reaching a maximum in the absence of NaCl. Other proteins that are also internalized in carp intestine did not significantly inhibit HRP binding to BBM. A lectin-type of interaction was discarded because neither manan nor ovoalbumin inhibited HRP binding. Proteinase K treatment of BBM reduced HRP binding by 70%, suggesting a proteic nature for this binding site. Finally, ligand blotting assays showed that HRP binds specifically to a 15.3-kDa protein. Taken together, these results are consistent with the existence of a functional receptor for HRP in carp intestinal mucosa that could mediate its internalization.