The cellular and subcellular localization of neuroglobin and cytoglobin -- a clue to their function?

Neuroglobin and cytoglobin are recently discovered respiratory proteins of vertebrates with yet ill-defined physiological functions. Neuroglobin is widely expressed in neurons, but not glia, in the vertebrate central and peripheral nervous systems. Other major expression sites are the retina and endocrine tissues. This distribution is indicative of a function of neuroglobin in metabolically most active, oxygen-consuming cell types, but does not yet allow to safely distinguish between different cellular roles, such as oxygen homeostasis, scavenging of reactive oxygen species or sustaining energy metabolism. Cytoglobin is predominantly expressed in connective tissue fibroblasts and related cell types in the body organs. Its main function may therefore be related to the specific amounts of extracellular matrix. Cytoglobin may hypothetically be involved in the oxygen-consuming maturation of collagen proteins. Cytoglobin is also expressed in distinct cell types of brain and retina. Its distribution strikingly differs from neuroglobin, suggesting an independent, yet unknown function.