Oxygen sensors and energy sensors act synergistically to achieve a graded alteration in gene expression: consequences for assessing the level of neuroprotection in response to stressors

Abstract

Changes in gene expression are associated with switching to an autoprotected phenotype in response to environmental and physiological stress. Ubiquitous molecular chaperones from the heat shock protein (HSP) superfamily confer neuronal protection that can be blocked by antibodies. Recent research has focused on the interactions between the molecular sensors that affect the increased expression of neuroprotective HSPs above constitutive levels. An examination of the conditions under which the expression of heat shock protein 70 (Hsp70) was up regulated in a hypoxia and anoxia tolerant tropical species, the epaulette shark (Hemiscyllium ocellatum), revealed that up-regulation was dependent on exceeding a stimulus threshold for an oxidative stressor. While hypoxic-preconditioning confers neuroprotective changes, there was no increase in the level of Hsp70 indicating that its increased expression was not associated with achieving a neuroprotected state in response to hypoxia in the epaulette shark. Conversely, there was a significant increase in Hsp70 in response to anoxic-preconditioning, highlighting the presence of a stimulus threshold barrier and raising the possibility that, in this species, Hsp70 contributes to the neuroprotective response to extreme crises, such as oxidative stress. Interestingly, there was a synergistic effect of coincident stressors on Hsp70 expression, which was revealed when metabolic stress was superimposed upon oxidative stress. Brain energy charge was significantly lower when adenosine receptor blockade, provided by treatment with aminophylline, was present prior to the final anoxic episode, under these circumstances, the level of Hsp70 induced was significantly higher than in the pair-matched saline treated controls. An understanding of the molecular and metabolic basis for neuroprotective switches, which result in an up-regulation of neuroprotective Hsp70 expression in the brain, is needed so that intervention strategies can be devised to manage CNS pathologies and minimise damage caused by ischemia and trauma. In addition, the current findings indicate that measurements of HSP expression per se may provide a useful correlate of the level of neuroprotection achieved in the switch to an autoprotected phenotype.

Transcriptional responses to hypoxia are enhanced by recurrent hypoxia (hypoxic preconditioning) in the epaulette shark

“…Of the three chaperone genes (hsc70, dnaja2, cct3) examined for each experimental condition in all tissues, only hypoxia-preconditioned hearts had increased hsc70 transcript levels. Previously, the increase of HSP70 protein level have been detected in response to anoxia, but not to hypoxiapreconditioned epaulette shark brains (36), fitting with the general conclusion of the present study that epaulette sharks probably do not experience a major general stress response to hypoxia alone.…”

“…Adaptive responses to hypoxia and anoxia can be primed by previous exposures to sublethal levels of low oxygen (hypoxic preconditioning) in a number of species including the epaulette shark, which reduces its metabolic rate (38) and enters an adenosine-mediated phase of cerebellar shutdown in response to anoxia (36). Hypoxic preconditioning has been studied in mammals, where hypoxia-inducible factor (HIF) and adenosine pathways are central in protective responses, and a previous experience of low oxygen confers tissue protection in subsequent hypoxic or anoxic situations (6,9,42).…”

Elasmobranch qPCR reference genes: a case study of hypoxia preconditioned epaulette sharks

“…In bladder cancer data sets, heat shock protein cognate B ( hspcb ) clearly outperformed other candidates [26]. In epaulette shark, hsp70 protein levels increased in anoxia but not in hypoxia [29]. However, in other species, heat shock protein levels often increase in hypoxia [30,31].…”

Mechanisms of oxidative stress resistance in the brain: Lessons learned from hypoxia tolerant extremophilic vertebrates

“…Animals that live with chronic oxidative stress are likely to have evolved mechanisms to protect and maintain proteasome function. For example, the proteasome-associated chaperone HSP70 was induced under anoxic conditions in the cerebellum of the epaulette shark ( Hemiscyllium ocellatum ) [119]. This hypoxia and anoxia tolerant species also showed positive modulation of proteasome degradation during oxygen deprivation in the cerebellum perhaps to prevent excitotoxicity and protect cerebellar dendrites from damage during these stress events [120].…”