Oligodendrocyte Development in the Absence of Their Target Axons In Vivo

Oligodendrocytes form myelin around axons of the central nervous system, enabling saltatory conduction. Recent work has established that axons can regulate certain aspects of oligodendrocyte development and myelination, yet remarkably oligodendrocytes in culture retain the ability to differentiate in the absence of axons and elaborate myelin sheaths around synthetic axon-like substrates. It remains unclear the extent to which the life-course of oligodendrocytes requires the presence of, or signals derived from axons in vivo. In particular, it is unclear whether the specific axons fated for myelination regulate the oligodendrocyte population in a living organism, and if so, which precise steps of oligodendrocyte-cell lineage progression are regulated by target axons. Here, we use live-imaging of zebrafish larvae carrying transgenic reporters that label oligodendrocyte-lineage cells to investigate which aspects of oligodendrocyte development, from specification to differentiation, are affected when we manipulate the target axonal environment. To drastically reduce the number of axons targeted for myelination, we use a previously identified kinesin-binding protein (kbp) mutant, in which the first myelinated axons in the spinal cord, reticulospinal axons, do not fully grow in length, creating a region in the posterior spinal cord where most initial targets for myelination are absent. We find that a 73% reduction of reticulospinal axon surface in the posterior spinal cord of kbp mutants results in a 27% reduction in the number of oligodendrocytes. By time-lapse analysis of transgenic OPC reporters, we find that the reduction in oligodendrocyte number is explained by a reduction in OPC proliferation and survival. Interestingly, OPC specification and migration are unaltered in the near absence of normal axonal targets. Finally, we find that timely differentiation of OPCs into oligodendrocytes does not depend at all on the presence of target axons. Together, our data illustrate the power of zebrafish for studying the entire life-course of the oligodendrocyte lineage in vivo in an altered axonal environment

Effects of Spaceflight on the Immune System

The immune system belongs to the most affected systems during spaceflight, and sensitivity of cells of the human immune system to reduced gravity has been confirmed in numerous studies in real and simulated microgravity. Immune system dysfunction during spaceflight represents a substantial risk for exploration class mission knowledge about the clinical, cellular, and genetic basis of immune system response, and adaptation to altered gravity will provide key information for appropriate risk management, efficient monitoring, and countermeasures against existing limiting factors for human health and performance during manned exploration of the solar system. In spite of the immune system dysregulation, studies indicate an adaptation reaction of the immune system to the new microgravity environment, at least for the T-cell system, starting after 2 weeks and continuing until 6 months or longer, reflected by cytokine concentrations in blood plasma or in stimulation assays. At the cellular level, rapid adaptation responses could be detected as early as after seconds until minutes in T cells and macrophages. Therefore, adaptive responses of cells and the whole organism could be expected under microgravity and altered gravity in general. Preventive countermeasures should therefore consider support and stabilization of the endogenous adaptation programs. Potential countermeasures for risk mitigation are summarized in this chapter. We assume that the immune systems not only have a significant adaptation potential when challenged with low gravitational environments but also provide interesting preventive and therapeutic options for long-term space missions