Distribution and hypoxia-regulation of haemocyanin in springtails (Collembola).

Haemocyanin is the copper-containing respiratory protein present in many arthropods. In the hexapods, respiratory proteins had long been considered unnecessary as sufficient O2 was thought to be obtained via the trachea. Nevertheless, many ametabolous and hemimetabolous hexapod species actually possess haemocyanin. Here we investigated the occurrence of haemocyanin in Collembola (springtails). Haemocyanin was found in 22 collembolan species of the suborders Symphypleona, Tomoceroidea and Entomobryomorpha, demonstrating its widespread occurrence. No haemocyanin was identified in 16 species of these taxa, and it appears to be absent in Poduromorpha. The presence of haemocyanin does not correlate with either the phylogenetic history or lifestyle of the investigated species. We further investigated the function of haemocyanin in Folsomia candida (Entomobryomorpha) by applying different hypoxia regimes. Whereas short-term (1 h) and mild (10% O2 ) hypoxia led to a decrease in haemocyanin mRNA, strong hypoxia (24 h, 1.5% O2 ) resulted in a ∼4300-fold increase in haemocyanin expression. Hypoxia induction of haemocyanin could not be demonstrated in evolutionarily more advanced Hexapoda, where it is restricted to the embryo. The results indicate (1) an important role of haemocyanin in the oxygen supply of F. candida, which may be adaptive in the potentially hypoxic environment in the soil, and (2) a change in haemocyanin function in hexapod evolution.

Neuroglobin of seals and whales: evidence for a divergent role in the diving brain.

Although many physiological adaptations of diving mammals have been reported, little is known about how their brains sustain the high demands for metabolic energy and thus O(2) when submerged. A recent study revealed in the deep-diving hooded seal (Cystophora cristata) a unique shift of the oxidative energy metabolism and neuroglobin, a respiratory protein that is involved in neuronal hypoxia tolerance, from neurons to astrocytes. Here we have investigated neuroglobin in another pinniped species, the harp seal (Pagophilus groenlandicus), and in two cetaceans, the harbor porpoise (Phocoena phocoena) and the minke whale (Balaenoptera acutorostrata). Neuroglobin sequences, expression levels and patterns were compared with those of terrestrial relatives, the ferret (Mustela putorius furo) and the cattle (Bos taurus), respectively. Neuroglobin sequences of whales and seals only differ in two or three amino acids from those of cattle and ferret, and are unlikely to confer functional differences, e.g. in O(2) affinity. Neuroglobin is expressed in the astrocytes also of P. groenlandicus, suggesting that the shift of neuroglobin and oxidative metabolism is a common adaptation in the brains of deep-diving phocid seals. In the cetacean brain neuroglobin resides in neurons, like in terrestrial mammals. However, neuroglobin mRNA expression levels were 4-15 times higher in the brains of harbor porpoises and minke whales than in terrestrial mammals or in seals. Thus neuroglobin appears to play a specific role in diving mammals, but seals and whales have evolved divergent strategies to cope with cerebral hypoxia. The specific function of neuroglobin that conveys hypoxia tolerance may either relate to oxygen supply or protection from reactive oxygen species. The different strategies in seals and whales resulted from a divergent evolution and an independent adaptation to diving.