Hypoxia inducible factor-1 α knockout does not impair acute thermal tolerance or heat hardening in zebrafish

Abstract

The rapid increase in critical thermal maximum (CT max ) in fish (or other animals) previously exposed to critically high temperature is termed ‘heat hardening’, which likely represents a key strategy to cope with increasingly extreme environments. The physiological mechanisms that determine acute thermal tolerance, and the underlying pathways facilitating heat hardening, remain debated. It has been posited, however, that exposure to high temperature is associated with tissue hypoxia and may be associated with the increased expression of hypoxia-inducible factor-1 (Hif-1). We studied acute thermal tolerance in zebrafish ( Danio rerio ) lacking functional Hif-1 α paralogs (Hif-1aa and Hif-1ab double knockout; Hif-1 α −/− ), which are known to exhibit markedly reduced hypoxia tolerance. We hypothesized that Hif-1 α −/− zebrafish would suffer reduced acute thermal tolerance relative to wild type and that the heat hardening ability would be lost. However, on the contrary, we observed that Hif-1 α −/− and wild-type fish did not differ in CT max , and both genotypes exhibited heat hardening of a similar degree when CT max was re-tested 48 h later. Despite exhibiting impaired hypoxia tolerance, Hif-1 α −/− zebrafish display unaltered thermal tolerance, suggesting that these traits are not necessarily functionally associated. Hif-1 α is accordingly not required for short-term acclimation in the form of heat hardening.

Acute measures of upper thermal and hypoxia tolerance are not reliable predictors of mortality following environmental challenges in rainbow trout (Oncorhynchus mykiss)

“…This is in contrast to previous studies, which have indicated that there is a relationship between these traits in a variety of species of salmonids ( Anttila et al , 2013 ; Zhang et al , 2018 ), which has been used to suggest that there may be a mechanistic link between variation in these two traits; however, these previous studies compared rank order across families or strain rather than across individuals. Similar to our findings, Joyce and Perry (2020 ) found that CT max was not correlated with hypoxia tolerance across individuals in zebrafish, further supporting the lack of direct mechanistic linkage between these traits. Indeed, evidence is accumulating that the mechanisms underlying acute upper thermal and hypoxia tolerance are likely very different ( Jutfelt et al , 2019 ; Mandic, Best, & Perry, 2020 ).…”

“…The FV, PN and CL strains exhibited little post-trial mortality (1–4% depending on the strains), the BW strain exhibited intermediate mortality (~20%), whereas the HF strain exhibited extreme mortality (~55%). This high level of mortality is unusual following a CT max trial, as post-trial mortality is generally thought to be low (1–5%) in fish ( Anttila et al , 2013 ; Joyce & Perry, 2020 ; Morgan et al , 2018 ; Scott et al , 2015 ; Zhang et al , 2018 ). The main difference between our study and most others is that our fish were exposed to a hypoxia tolerance trial 2–3 weeks previous to the measurement of thermal tolerance, which might contribute to additional mortality.…”

“…Natural environments are complex and involve changes in multiple interacting stressors, and thus environmentally relevant assessments of climate change resilience should involve determining tolerance to multiple stressors. Indeed, it is becoming increasingly common to measure multiple traits in individual fish to obtain a multifaceted view of the responses of organisms to the environment ( Åsheim et al , 2020 ; Gunderson, Armstrong, & Stillman, 2016 ; Joyce & Perry, 2020 ; Nudds, Ozolina, Fenkes, Wearing, & Shiels, 2020 ). However, measuring multiple traits in single individuals results in logistical challenges as prior exposure to a stressful environment has the potential to alter subsequent tolerance, either reducing tolerance due to accumulation of cellular or organismal damage or improving tolerance through phenomena such as heat-hardening and cross-tolerance ( McArley, Hickey, & Herbert, 2020 ; McBryan, Anttila, Healy, & Schulte, 2013 ; Morgan et al , 2018 ; Todgham et al , 2005 ).…”

Hif‐1α is not required for the development of cardiac adrenergic control in zebrafish (Danio rerio)

“…The zebrafish (Danio rerio) has emerged as an indispensable model organism to explore the role of Hifs (Cai et al, 2018;Elks et al, 2015;Joyce & Perry, 2020;Pelster & Egg, 2018), in part because zebrafish lacking both Hif-1α paralogs (hif-1aa −/− and hif1ab −/− double knockout; hereafter referred to as hif-1α −/− ) are viable (Gerri et al, 2017), in contrast to the embryonic lethality observed in germline Hif-1α or Hif-2α knockout in mammals (Compernolle et al, 2002;Iyer et al, 1998;Krishnan et al, 2008).…”

“…As such, many fish species, including zebrafish, are well adapted to developing in hypoxic conditions (Kopp et al, 2014;Rees et al, 2001;Ton et al, 2003), which appears to mitigate them from the deleterious effects of Hif knockout. Zebrafish thus provide a unique opportunity to understand the role that Hifs play in development when survival is not compromised, although Hif-1α knockout does incur some clear phenotypes in zebrafish, including reduced hypoxia tolerance (Joyce & Perry, 2020;. Fish can specifically provide a valuable perspective in which to understand the evolutionary origins of hypoxia-dependent cardiovascular control.…”