Expansion of capacities for iron transport and sequestration reflects plasma volumes and heart mass among white-blooded notothenioid fishes

Abstract

The family Channichthyidae or “icefishes” (suborder Notothenioidei) represents the only vertebrates lacking hemoglobin (Hb) as adults. Several icefish species also do not express cardiac myoglobin (Mb). We address how levels of proteins involved in iron (Fe) processing (transport, sequestration, and export) vary among white- and red-blooded notothenioids, and whether absence of Hb and/or Mb in channichthyids is accompanied by expansion of contents of Fe-binding proteins to protect against unchaperoned Fe. Levels of transferrin (Tf), ferritin (Ft), ceruloplasmin (Cp), and non-heme Fe were quantified in plasma, serum, and/or nonhematopoietic tissues (cardiac ventricle, skeletal muscle, and liver) from species of white-blooded ( Chaenocephalus aceratus, Champsocephalus gunnari, Chionodraco rastrospinosus, Pseudochaenichthys georgianus) (the first two species not expressing Mb) and red-blooded ( Notothenia coriiceps, Gobionotothen gibberifrons) notothenioids. We also measured levels of ascorbate (Asc), a mediator of Fe uptake. While plasma concentrations of Tf and tissue levels of Asc are similar among species, concentrations of plasma Asc are lower in white-blooded species. Concentrations of Ft and non-heme Fe and activities of Cp are also generally reduced in icefishes compared with red-blooded notothenioids. The presence of cardiac Mb in some icefish species does not appear to influence levels of proteins involved in Fe processing. To address further the question of Fe sequestration within a physiological context, we account for well-characterized differences in blood volume and heart mass among white- and red-blooded notothenioids. We report that total contents of plasma Tf are greater, while ventricle non-heme Fe is at least at parity in white- vs. red-blooded species.

The loss of hemoglobin and myoglobin does not minimize oxidative stress in Antarctic icefishes

“…The mechanism by which heme potentiates the formation of intracellular ROS is unclear, especially given that transcript levels of haptoglobin, ferritin, serotransferrin and hepcidin, which are involved in Fe transport and sequestration, are higher in the Antarctic fish Dissostichus mawsoni than in temperate species (Chen et al, 2008). Also, in our previous work, we found that tissue levels of the proteins ferritin and ceruloplasmin were generally higher in red-blooded species than in icefishes, although transferrin levels were no different among species (Kuhn et al 2016).…”

Growth Limitation of Marine Fish by Low Iron Availability in the Open Ocean

“…Iron is found at the center of hemoglobin and myoglobin, which bind oxygen for vascular transport and storage, respectively (Beard et al, 1996). In vertebrates, as much as 80% of the total iron content can be bound within these two metalloproteins (Kuhn et al, 2016). Although rainbow trout have been shown to take up iron across their gills in ironrich freshwaters, this pathway requires very high dissolved iron concentrations (half-saturation constant of 21 nM) (Cooper and Bury, 2007) so that it should be negligible at the concentrations found in iron-poor seawater (less than 0.2 nM).…”

“…These extremely unusual features have been most often associated with the low temperatures and high dissolved oxygen concentrations of Antarctic waters, which could reduce the requirement for oxygen transportation and storage (Kock, 2005). However, the loss of hemoglobin is accompanied by dramatic adaptations to maintain sufficient oxygen supply to their tissues, including up to a five-fold increase in ventricle size and four-fold increase in total blood volume compared to red-blooded fish of similar size (Kuhn et al, 2016). Even the increased blood volumes cannot overcome what appear to be many profound disadvantages, and explaining why the absence of hemoglobin persists has been recognized as an evolutionary conundrum (Sidell and O'Brien, 2006), even cited as an example of a deleterious adaptation or "disaptation" (Garofalo et al, 2009).…”

Vascular Expression of Hemoglobin Alpha in Antarctic Icefish Supports Iron Limitation as Novel Evolutionary Driver

“…Indeed, without iron-binding hemoglobin, iron content in icefish blood plasma is less than 5% of closely related red-blooded species (di Prisco et al, 2002). Yet there is even further evidence of additional iron minimization beyond loss of hemoglobin and myoglobin: concentrations of non-heme iron in Antarctic icefish plasma are one-sixth of that in closely related red-blood species, and are lower by roughly half across various tissues (Kuhn et al, 2016). With a tissue level reduction in iron content, paired with the knockout of two primary iron binding proteins, the iron requirements of icefish normalized to biomass could be greater than 95% compared to other organisms and awaits confirmation.…”