M74 syndrome in Baltic salmon and the possible role of oxidative stresses in its development: present knowledge and perspectives for future studies

Baltic salmon suffer from maternally transmitted yolk-sac fry mortality syndrome--M74. The incidence of M74 varies considerably on a year to year basis. In the 1990s the mortalities were 50-80% but in 2003-2005, below 10%. Before death, M74-affected fry have several typical symptoms. M74-eggs are characterized by low thiamine and carotenoid content, and affected fry show signs of oxidative stress. Although M74 is associated with thiamine deficiency and the symptoms of the fry can be alleviated with thiamine, the underlying causes of the syndrome have remained a mystery. We have studied the symptoms of M74 at the molecular level by investigating the global gene expression patterns using cDNA microarray and have quantified the changes in transcriptional regulation in M74-affected and healthy yolk-sac fry. Our and previous results suggest that M74 in Baltic salmon yolk-sac fry results from oxidative stresses disturbing several different developmental molecular pathways. Because the M74 syndrome is of maternal origin, factors in the Baltic Sea during salmon feeding and migration, i.e., the chemical composition of food, may be decisive in the development of M74. The possible mechanisms by which oxidative stresses may develop in adult salmon are discussed in the review.

Comparison of hypoxia-inducible factor-1 alpha in hypoxia-sensitive and hypoxia-tolerant fish species

Levels of oxygen can vary dramatically in aquatic environments. Aquatic organisms, including fishes, have adapted accordingly to survive. As there are both phylogenetically closely related fish species with differing oxygen requirements and distantly related species with similar oxygen requirements, fishes are good candidates for examining oxygen-related functions in vertebrates. We set out to investigate if sequence variation in the hypoxia-inducible factor-1 alpha (HIF-1alpha) gene is associated with variations in oxygen requirements. Since the teleost HIF-1alpha sequences available in databases represent a very limited dataset both phylogenetically and with regard to oxygen requirements, we have sequenced the protein coding sequence for HIF-1alpha from an additional 9 fish species. Our results indicate that the deduced HIF-1alpha proteins of teleost fishes are somewhat shorter than those of tetrapods. Additionally, the results suggest that tetrapod sequences more closely resemble the ancestral form of the protein than do teleost sequences. No clear signatures which could be associated with the oxygen requirements of the species were found. This study suggests that if species-specific differences in HIF-1alpha function with regards to oxygen dependence have evolved, they do not occur in the protein coding sequence but at other levels of the HIF-1alpha pathway.

Baltic salmon (Salmo salar) yolk-sac fry mortality is associated with disturbances in the function of hypoxia-inducible transcription factor (HIF-1alpha) and consecutive gene expression

Baltic salmon (Salmo salar) suffer from abnormally high yolk-sac fry mortality designated as M74-syndrome. In 1990s, 25-80% of salmon females, which ascended rivers to spawn, produced yolk-sac fry suffering from the syndrome. Symptoms of M74-affected fry include neurological disturbances, impaired vascular development and abnormal haemorrhages. The latter symptoms are observed in mammalian embryos if the function of hypoxia inducible transcription factor (HIF-1alpha), its dimerization partner aryl hydrocarbon nuclear translocator (ARNT) or target gene vascular endothelial growth factor (VEGF) is disturbed. To study the possible involvement of HIF-1alpha and its target gene VEGF in the development of the syndrome, we collected healthy and M74-affected wild Baltic salmon yolk-sac fry and analyzed HIF-1alpha mRNA and protein expression, HIF-1alpha DNA-binding, target gene VEGF protein expression, and blood vessel density in both groups at different stages of yolk-sac fry development. In addition, since Baltic salmon females contain organochlorine contaminants, which have been suggested to be the cause of M74 syndrome via the aryl hydrocarbon receptor (AhR)-dependent gene expression pathway, we studied AhR protein expression, AhR DNA-binding and target gene CYP1A protein expression. Since the parents of both healthy and M74-affected wild fry will have experienced the organochlorine load from the Baltic Sea, hatchery-reared fry were included in the studies as an additional control. The results show that the vascular defects observed in fry suffering from M74 are associated with reduced DNA-binding activity of HIF-1alpha and subsequent downregulation of its target gene vascular endothelial growth factor (VEGF). In addition, also AhR function is decreased in diseased fry making it unlikely that symptoms of M74-affected fry would be caused by an upregulation of xenobiotically induced AhR-dependent gene expression pathway.