Double-edged effects of tamoxifen-in-oil-gavage on an infectious murine model for multiple sclerosis

Tamoxifen gavage is a commonly used method to induce genetic modifications in cre-loxP systems. As a selective estrogen receptor modulator (SERM), the compound is known to have immunomodulatory and neuroprotective properties in non-infectious central nervous system (CNS) disorders. It can even cause complete prevention of lesion development as seen in experimental autoimmune encephalitis (EAE). The effect on infectious brain disorders is scarcely investigated. In this study, susceptible SJL mice were infected intracerebrally with Theiler's murine encephalomyelitis virus (TMEV) and treated three times with a tamoxifen-in-oil-gavage (TOG), resembling an application scheme for genetically modified mice, starting at 0, 18, or 38 days post infection (dpi). All mice developed 'TMEV-induced demyelinating disease' (TMEV-IDD) resulting in inflammation, axonal loss, and demyelination of the spinal cord. TOG had a positive effect on the numbers of oligodendrocytes and oligodendrocyte progenitor cells, irrespective of the time point of application, whereas late application (starting 38 dpi) was associated with increased demyelination of the spinal cord white matter 85 dpi. Furthermore, TOG had differential effects on the CD4+ and CD8+ T cell infiltration into the CNS, especially a long lasting increase of CD8+ cells was detected in the inflamed spinal cord, depending of the time point of TOG application. Number of TMEV-positive cells, astrogliosis, astrocyte phenotype, apoptosis, clinical score, and motor function were not measurably affected. These data indicate that tamoxifen gavage has a double-edged effect on TMEV-IDD with the promotion of oligodendrocyte differentiation and proliferation, but also increased demyelination, depending on the time point of application. The data of this study suggest that tamoxifen has also partially protective functions in infectious CNS disease. These effects should be considered in experimental studies using the cre-loxP system, especially in models investigating neuropathologies.

Genetic timestamping of plasma cells in vivo reveals tissue-specific homeostatic population turnover

Plasma cells (PCs) are essential for protection from infection, and at the origin of incurable cancers. Current studies do not circumvent the limitations of removing PCs from their microenvironment and confound formation and maintenance. Also, the investigation of PC population dynamics has mostly relied on nucleotide analog incorporation that does not label quiescent cells, a property of most PCs. The main impediment is the lack of tools to perform specific genetic manipulation in vivo. Here we characterize a genetic tool (JchaincreERT2) in the mouse that permits first-ever specific genetic manipulation in PCs in vivo, across immunoglobulin isotypes. Using this tool, we found that splenic and bone marrow PC numbers remained constant over-time with the decay in genetically labeled PCs being compensated by unlabeled PCs, supporting homeostatic population turnover in these tissues. The JchaincreERT2 tool paves the way for an in-depth mechanistic understanding of PC biology and pathology in vivo, in their microenvironment.

DTR-mediated conditional cell ablation-Progress and challenges

Cell ablation is a valuable complement to mutagenesis for experimentally defining specific cell functions in physiology and pathophysiology in small animal models. One of the most popular ablation strategies involves transgenic expression of a primate diphtheria toxin receptor (DTR) on murine cells that are otherwise resistant to the bacterial exotoxin. The efforts of many laboratories using the DTR approach over the years have yielded numerous valuable insights into specific cell functions. Here, we will discuss the technical aspects of the DTR approach, including the strengths, pitfalls, and future strategies to overcome the shortcomings, highlighting a recent paper published in the European Journal of Immunology [El Hachem et al. Eur. J. Immunol. 2018 https://doi.org/10.1002/eji.201747351]. A particular focus will be given to the application of DTR approach to decipher in vivo functions of the murine myeloid cell compartment.

Osteoblast Production by Reserved Progenitor Cells in Zebrafish Bone Regeneration and Maintenance

Mammals cannot re-form heavily damaged bones as in large fracture gaps, whereas zebrafish efficiently regenerate bones even after amputation of appendages. However, the source of osteoblasts that mediate appendage regeneration is controversial. Several studies in zebrafish have shown that osteoblasts are generated by dedifferentiation of existing osteoblasts at injured sites, but other observations suggest that de novo production of osteoblasts also occurs. In this study, we found from cell-lineage tracing and ablation experiments that a group of cells reserved in niches serves as osteoblast progenitor cells (OPCs) and has a significant role in fin ray regeneration. Besides regeneration, OPCs also supply osteoblasts for normal bone maintenance. We further showed that OPCs are derived from embryonic somites, as is the case with embryonic osteoblasts, and are replenished from mesenchymal precursors in adult zebrafish. Our findings reveal that reserved progenitors are a significant and complementary source of osteoblasts for zebrafish bone regeneration.