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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ZF-BrainReg (Regeneration of the adult zebrafish brain)

Teaser

Summary

Age-related cognitive decline and neurodegenerative diseases like Parkinsonâ€™s are associated with neuronal cell loss and represent huge unsolved biomedical problems in western societies. Controlled brain regeneration, either by supplying programmed cells, or by activating endogenous progenitor cells, might provide a solution. The objective of this project is to unravel the underlying mechanisms for the ability of the adult zebrafish brain to regenerate itself after a severe lesion, and to compare these mechanisms with the mammalian brain which is unable to regenerate. The project aims to identify the critical cell sources as well as the genes and mechanisms controlling regeneration in the adult zebrafish brain. Furthermore, selected identified genes and mechanisms underlying regeneration in zebrafish will be activated in rodents and in human neural tissue culture to test their ability to rekindle regeneration in mammals. Taken together this work will provide fundamental insights into the process of brain regeneration in vertebrates, and might provide avenues for future progenitor cell-based therapies of the injured or diseased human brain.

Work performed

In the adult zebrafish forebrain, we identified ventricular radial glia cells as the critical stem cell source that proliferates after lesion and gives rise to mature neurons. Besides radial glia cells, several other cell types including pericytes, oligodendrocyte precursor cells and endothelial cells react to injury, but the contribution of these reactively proliferating cells and their fate towards a neural lineage was unknown. To address this question, we used hematopoietic stem cell transplantation and genetic lineage tracing and found that all three cell types remain committed to their respective lineage and do not convert to a neuronal fate. Hence, our results indicate that radial glia cells are the key source of newly generated neurons during regeneration. Comparison of the transcriptome of radial glia cells from regenerating and uninjured brains revealed numerous genes and signaling pathways associated with regeneration. Among others, we found increased expression of the thyroid hormone precursor thyroglobulin (tg) indicating an important role for thyroid hormones in regeneration. Applying gain- and loss-of-function experiments, we could show that thyroid hormones are required for reactive proliferation and reactive neurogenesis suggesting that they represent a likely prerequisite for regeneration to occur. Our demonstration of thyroid hormones potential in regenerating zebrafish brain, and the association of hypothyroidism with impaired recovery after traumatic brain injury in patients, suggests that thyroid hormone activity may be of therapeutic relevance. To obtain evidence for thyroid hormones ability to rekindle a regeneration program in mammals, we will employ the first adult astrocyte cell culture system that we developed. This adult astrocyte cell culture system shows all features of adult in vivo astrocytes and can be easily cultured as well as manipulated to allow subsequent screening of reprogramming factors.

Final results

All work packages of the project have been initiated. They build on the knowledge and tools that my group has assembled in the past 10 years. However, most of them go beyond the current state of the art, by taking full advantage of the excellent cellular, genomic and genetic toolbox in zebrafish to study its astounding regenerative ability, and with its determination to translate the zebrafish findings into the mammalian context. With the recent advent of CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein 9) technology tailor-made genomic modifications are now feasible in zebrafish, which allow achieving the specific aims 1-4 in a more precise and also faster manner. The translation of our findings into the mammalian context in aim 6 is clearly still the most challenging part. Despite an initial setback in the in vivo reprogramming of mammalian stem cells into new neurons using genes identified in zebrafish brain regeneration, the approach is still promising and will be continued. I hence foresee to achieve most of the aims of the project.