Just a few moments without oxygen can do terrible damage to a mammal's heart. As the oxygen rushes back, toxic reactive oxygen species cause untold damage to cellular structures. But many amphibians often experience periods of oxygen deprivation, and suffer few ill effects. Isidoros Beis is intrigued by the mechanisms that protect amphibian hearts from these potentially fatal situations. Working with his team in Athens, Greece, he has focused on the cellular stress pathways that are activated by oxidative stress, and in this issue of the J. Exp. Biol., he describes how the oxygen free radical,hydrogen peroxide, triggers the protective p38-MAPK signalling cascade(p. 2759).
Beis explains that he chose to study Rana ridibunda hearts,because he knew that the frogs' hearts respond to certain types of oxidative stress very differently to mammalian hearts; they seem almost immune to oxidative damage after a period of anoxia! But reoxygenating a mammal's heart after a period of ischemia triggers one of the mitogen activated protein kinase (MAPK) signalling pathways; specifically the p38-MAPK pathway, often culminating in cell death and permanent damage to the tissue. When he tested the frog's heart, reoxygenation failed to activate the signalling cascade. Beis explains that shortly before a period of anoxia, the amphibian produces high levels of antioxidant enzymes, ready to mop up any free radicals produced as oxygen returns to the animal's tissues. So, the anoxia-tolerant animal doesn't need to resort to protection from cellular signalling pathways after a bout of anoxia.
But oxidative stress is caused by many factors, other than anoxia. Beis wondered whether these other forms of oxidative stress might activate one of the three major MAPK stress-signalling pathways.
After exposing frog hearts to short pulses of hydrogen peroxide, Beis' team began searching for evidence of stress signalling pathways that had been activated. Both ERK and JNK, two of the MAPKstress signalling pathways, were activated by short exposures to hydrogen peroxide, but the p38-MAPK pathway was activated to a much greater extent, yet didn't seem to trigger tissue damage. Knowing that one form of p38-MAPK stimulated a pathway that protects cells from stress, they decided to follow the signalling cascade to find out which proteins it activated.
By probing tissue extracts with antibodies that recognise proteins downstream of the p38-MAPK signal, the team realised that the pathway ultimately activated a heat shock protein; HSP27. Many heat shock proteins mediate a variety of cellular responses to different environmental stresses by protecting proteins from degradation, but whether HSP27 acts as a chaperone to protect the heart, is not obvious.
Having identified the cascade that is triggered by hydrogen peroxide oxidative damage, Beis is keen to find the antioxidant enzymes that the cascade in turn activates, to protect the animals from oxidative stress.