Molecular structure and functional adaptations of hemoglobins from Antarctic marine organisms

Atomic-Level View of the Functional Transition in Vertebrate Hemoglobins: The Case of Antarctic Fish Hbs

Tetrameric hemoglobins (Hbs) are prototypal systems for studies aimed at unveiling basic structure-function relationships as well as investigating the molecular/structural basis of adaptation of living organisms to extreme conditions. However, a chronological analysis of decade-long studies conducted on Hbs is illuminating on the difficulties associated with the attempts of gaining functional insights from static structures. Here, we applied molecular dynamics (MD) simulations to explore the functional transition from the T to the R state of the hemoglobin of the Antarctic fish Trematomus bernacchii (HbTb). Our study clearly demonstrates the ability of the MD technique to accurately describe the transition of HbTb from the T to R-like states, as shown by a number of global and local structural indicators. A comparative analysis of the structural states that HbTb assumes in the simulations with those detected in previous MD analyses conducted on HbA (human Hb) highlights interesting analogies (similarity of the transition pathway) and differences (distinct population of intermediate states). In particular, the ability of HbTb to significantly populate intermediate states along the functional pathway explains the observed propensity of this protein to assume these structures in the crystalline state. It also explains some functional data reported on the protein that indicate the occurrence of other functional states in addition to the canonical R and T ones. These findings are in line with the emerging idea that the classical two-state view underlying tetrameric Hb functionality is probably an oversimplification and that other structural states play important roles in these proteins. The ability of MD simulations to accurately describe the functional pathway in tetrameric Hbs suggests that this approach may be effectively applied to unravel the molecular and structural basis of Hbs exhibiting peculiar functional properties as a consequence of the environmental adaptation of the host organism.

Oogenesis at subzero temperatures: A comparative study of the oocyte morphology in nine species of Notothenioids

Oogenesis was examined in nine species of Antarctic fish to verify the existence of morphological peculiarities. The analyses were carried out on specimens belonging to three different families of Notothenioids (Nototheniidae, Channichthyidae and Bathydraconidae), all captured in the Ross Sea, in front of the Italian Station of Terra Nova Bay. Following dissection, the ovaries were processed and examined at the light and electron microscopes to determine the oocyte gross and fine morphology. The attention, in particular, was focused on the presence of cytoplasmic round bodies and on the organization of the cortical alveoli and the vitelline envelope. Results reveal significant specie-specific differences that could be partly correlated to the phylogenetic radiation but not to the peculiar environmental conditions being essentially comparable to those observed among temperate species.

Cytology of lymphomyeloid head kidney of Antarctic fishes Trematomus bernacchii (Nototheniidae) and Chionodraco hamatus (Channicthyidae)

Species that live in extreme conditions have specially adapted physiology and tissue/organ organisation. The adaptation of lymphoid organs to low temperatures in polar species could be an original field of study, indicating how the immune system works under extreme conditions. In fishes, the head kidney is a key organ for immunity and here the cytology of this organ is studied in two common Antarctic species: Trematomus bernacchii and Chionodraco hamatus. Ultrastructural analysis revealed heterogeneity of epithelial cells, with reticular cells, subcapsular- and perivascular-limiting cells. Differences in the size and morphology of epithelial cells were observed between the polar species and warm water species of fish. Intermingled with epithelial cell leucocytes, such as lymphocytes, thrombocytes and macrophages, had comparable morphology in both species, contrary to sharp differences observed in the morphology of erythrocytes and granulocytes. The functional adaptation of the head kidney to the low temperatures of polar water is discussed.