Innate Immune Pathways Promote Oligodendrocyte Progenitor Cell Recruitment to the Injury Site in Adult Zebrafish Brain.

The oligodendrocyte progenitors (OPCs) are at the front of the glial reaction to the traumatic brain injury. However, regulatory pathways steering the OPC reaction as well as the role of reactive OPCs remain largely unknown. Here, we compared a long-lasting, exacerbated reaction of OPCs to the adult zebrafish brain injury with a timely restricted OPC activation to identify the specific molecular mechanisms regulating OPC reactivity and their contribution to regeneration. We demonstrated that the influx of the cerebrospinal fluid into the brain parenchyma after injury simultaneously activates the toll-like receptor 2 (Tlr2) and the chemokine receptor 3 (Cxcr3) innate immunity pathways, leading to increased OPC proliferation and thereby exacerbated glial reactivity. These pathways were critical for long-lasting OPC accumulation even after the ablation of microglia and infiltrating monocytes. Importantly, interference with the Tlr1/2 and Cxcr3 pathways after injury alleviated reactive gliosis, increased new neuron recruitment, and improved tissue restoration.

Electroporation Method for In Vivo Delivery of Plasmid DNA in the Adult Zebrafish Telencephalon.

Electroporation is a transfection method in which an electrical field is applied to cells to create temporary pores in a cell membrane and increase its permeability, thereby allowing different molecules to be introduced to the cell. In this paper, electroporation is used to introduce plasmids to ependymoglial cells, which line the ventricular zone of the adult zebrafish telencephalon. A fraction of these cells shows stem cell properties and generates new neurons in the zebrafish brain; therefore, studying their behavior is essential to determine their roles in neurogenesis and regeneration. The introduction of plasmids via electroporation enables long-term labeling and tracking of a single ependymoglial cell. Furthermore, plasmids such as Cre recombinase or Cas9 can be delivered to single ependymoglial cells, which enables gene recombination or gene editing and provides a unique opportunity to assess the cell's autonomous gene function in a controlled, natural environment. Finally, this detailed, step-by-step electroporation protocol is used to obtain successful introduction of plasmids into a large number of single ependymoglial cells.

One step generation of customizable gRNA vectors for multiplex CRISPR approaches through string assembly gRNA cloning (STAgR).

Novel applications based on the bacterial CRISPR system make genetic, genomic, transcriptional and epigenomic engineering widely accessible for the first time. A significant advantage of CRISPR over previous methods is its tremendous adaptability due to its bipartite nature. Cas9 or its engineered variants define the molecular effect, while short gRNAs determine the targeting sites. A majority of CRISPR approaches depend on the simultaneous delivery of multiple gRNAs into single cells, either as an essential precondition, to increase responsive cell populations or to enhance phenotypic outcomes. Despite these requirements, methods allowing the efficient generation and delivery of multiple gRNA expression units into single cells are still sparse. Here we present STAgR (String assembly gRNA cloning), a single step gRNA multiplexing system, that obtains its advantages by employing the N20 targeting sequences as necessary homologies for Gibson assembly. We show that STAgR allows reliable and cost-effective generation of vectors with high numbers of gRNAs enabling multiplexed CRISPR approaches. Moreover, STAgR is easily customizable, as vector backbones as well as gRNA structures, numbers and promoters can be freely chosen and combined. Finally, we demonstrate STAgR's widespread functionality, its efficiency in multi-targeting approaches, using it for both, genome and transcriptome editing, as well as applying it in vitro and in vivo.

The Aryl Hydrocarbon Receptor Pathway Defines the Time Frame for Restorative Neurogenesis.

Zebrafish have a high capacity to replace lost neurons after brain injury. New neurons involved in repair are generated by a specific set of glial cells, known as ependymoglial cells. We analyze changes in the transcriptome of ependymoglial cells and their progeny after injury to infer the molecular pathways governing restorative neurogenesis. We identify the aryl hydrocarbon receptor (AhR) as a regulator of ependymoglia differentiation toward post-mitotic neurons. In vivo imaging shows that high AhR signaling promotes the direct conversion of a specific subset of ependymoglia into post-mitotic neurons, while low AhR signaling promotes ependymoglial proliferation. Interestingly, we observe the inactivation of AhR signaling shortly after injury followed by a return to the basal levels 7 days post injury. Interference with timely AhR regulation after injury leads to aberrant restorative neurogenesis. Taken together, we identify AhR signaling as a crucial regulator of restorative neurogenesis timing in the zebrafish brain.