Nobel Prize for Oxygen Sensing and Hypoxia: the Environmental Relevance of Phenomena

Tiistai 8.10.2019 klo 9:37 - Mikko Nikinmaa

One of the most conspicuous changes that occur in the aquatic environment is the increasing occurrence of hypoxic areas. The Nobel Prize in Physiology and Medicine is this year given to three scientists, Kaelin, Ratcliffe and Semenza, who have studied and discovered the mechanism of how oxygen deficiency controls gene expression in man. Compared to air-breathers, fish and other aquatic animals must get by with 1/30th of the oxygen concentration. They are further faced with marked variations in oxygen level both daily and seasonally (or unknown periods of time). Further, since fish are poikilothermic, temperature changes affect their oxygen requirements conspicuously.

The oxygen sensing and transport system of fish must therefore be more vHIF.bmpersatile than that of mammals. We have studied the oxygen sensing and hypoxia-inducible factor (HIF) system, i.e. the phenomena now awarded Nobel Prize, since the late 1990’s. First, we observed that hypoxia-inducible factor was present in cells already in normal venous oxygen tension, although it increased in hypoxia (in humans and laboratory rodents it is only found in hypoxic conditions). Second, we observed that although the hypoxia-inducible factor level was controlled posttranscriptionally, also gene transcription could be modified. The HIF transcription depended on the number of hypoxic bouts experienced by the animal (in humans the control of HIF level occurs posttranscriptionally). Finally, we observed that HIF level was affected by temperature (something that is irrelevant for us homeotherms). These facts, together with the observations of interactions between HIF and circadian rhythms and environmental pollutants show that the system given the Nobel Prize for is more versatile in poikilothermic water breathers than humans.

Given that oxygen is a limiting factor in aquatic environment, it is no surprise that HIF system in fish has evolved differently in different fish groups depending on their oxygen requirements. In continuation, the possibilities of fish to adapt to climate change and environmental pollution are markedly affected by what their HIF system is. Thus, the Nobel Prize winning studies have a significant environmental angle. This has been reviewed to some extent in Nikinmaa, M. and Rees, B.B. (2005) Oxygen-dependent gene expression in fishes. Am. J. Physiol. - Regul. Integr. Comp. Physiol. 288, 1079-1090 and in Prokkola, JM and Nikinmaa M (2018) Circadian rhythms and environmental disturbances - underexplored interactions. J. Exp. Biol. 221, jeb179267. The evolution of HIF system in animals was explored in Rytkönen KT et al (2011) Molecular Evolution of the Metazoan PHD–HIF Oxygen-Sensing System. Mol. Biol. Evol. 28: 1913-1926.

Kommentoi kirjoitusta. Avainsanat: climate change, temperature, hypoxia, evolution, environmental pollution

Phenotypic plasticity- a key to environmental responses

Maanantai 23.9.2019 klo 16:54 - Mikko Nikinmaa

We have argued that individual variability is very important in responses to toxicants, not just unwanted noise (Nikinmaa and Anttila 2019. Individual variation in aquatic toxicology: Not only unwanted noise. Aquatic Toxicology 207, 29-33; the article is open access, so everyone is free to read it). Reading all the work nowadays with the outset that plasticity is integral to environmental responses convinces me more and more that plasticity of the responses of animals and individual variability are very much overlooked features in how animals can tolerate environmental changes. Just recently, scientists from Israel (Segev et al 2019. Phenotypic plasticity and local adaptations to dissolved oxygen in larvae fire salamander (Salamandra infraimmaculata). Oecologia 190, 737-746) exposed salamander larvae from stream and pond environment to normoxia and hypoxia. The hypothesis was naturally that, since stream larvae never get exposed to hypoxia whereas pond larvae do, pond larvae would respond more to hypoxic conditions than stream larvae. A simple measure of responding, a change in gill size was taken. However, the results show that regardless of the origin of the larvae, the gill size change between normoxia and hypoxia was the same. This is possible only if the animals can respond plastically to oxygen level. If plasticity is great, and is maintained, genetic changes are not required for environmental responses. In fact, genetic changes are a worse alternative than large plasticity within a genotype, since variation achieved by genetic heterogeneity will decrease as soon as conditions are such that some genotypes of the population will not tolerate them. That will not happen, if the variance is a property of one plastic genotype.

Kommentoi kirjoitusta. Avainsanat: environmental adaptation, variability, hypoxia

Hydrogen sulphide - both toxicant and a cell signalling molecule

Perjantai 13.10.2017 klo 11:53 - Mikko Nikinmaa

With increased eutrophication many aquatic bodies have anoxic bottom sediments. They are characterized by high concentration of hydrogen sulphide, smelling of rotten eggs. Hydrogen sulphide is considered to be a highly toxic substance, and together with the lack of oxygen contributing to the death of organisms in the hypoxic areas. This may be so, but the initial effects disturbing the functions of organisms may not be caused by the traditionally considered effect of sulphide disturbing aerobic respiration.

It has recently become clear that hydrogen sulphide is an important cellular signalling molecule, in various cases the "oxygen sensor" of the cells. Thus, variations in its cellular concentrations fine-tune oxygen-dependent effects tp occur appropriately. Consequently, any disturbances in the level of hydrogen sulphide can disturb cellular signalling, and harmful effects can take place because of disturbances of cell signalling even when the concentration would not be adequate to cause breakdown of aerobic respiration and death because of that.

Kommentoi kirjoitusta. Avainsanat: hypoxia, water pollution, cell signalling