Evolutionary Conserved Mechanisms that Control Central Nervous System Development Regeneration and Degeneration

Gradual loss of brain function and neurodegeneration are common features of aging throughout diverse phyla. Senile dementias, including Alzheimer’s Disease (AD), likely involve failures of adult neural stem cell (NSC) number, viability, and/or functions. Our lab studies NSC’s role in central nervous system (CNS) development, adult regeneration, and in onset dementia such as AD. We are establishing a field of research in regenerative medicine and aging as the first lab to prospectively isolate human NSC and use them in published preclinical and clinical trial studies. In this proposal we seek to understand basic principles and evolutionarily conserved elements of NSC involvement in neuronal regeneration, degeneration, and aging in the colonial tunicate Botryllus schlosseri. Botryllus has two reproductive modes: sexual reproduction which produces a primitive chordate with a simple CNS (the chordate brain), that will undergo metamorphosis into an asexually reproducing sessile invertebrate which propagates by budding. We have found that Botryllus buds contain self-renewing germline stem cells, and somatic stem cells which self-organize to form a colony composed of genetically identical individuals. This stage exhibits weekly assayable CNS tissue regeneration from candidate NSC and undergoes repeated neurodegeneration throughout its adult life, a process that resembles adult neurogenesis and neurodegeneration in vertebrates. Thus, Botryllus offers a unique opportunity to study the cellular and molecular mechanisms of CNS generation and degeneration through observing weekly regeneration cycles in young and old colonies (e.g. <3 months vs. >7 years). We aim to identify the mutations and/or epigenetic changes that accumulate in the NSC, and through self-renewal remain present throughout an organism's life. We have undertaken a systematic molecular (brain transcriptomic) analysis of CNS cells of old and young Botryllus colonies, paired with morphological and behavioral characterization of each of their CNS lineage cells. This analysis revealed 93 homologous genes that correlate with Alzheimer’s disease, including APP, GRN, PSEN1, GLUD2, and VPS35 that are differentially expressed between young and old colonies. Furthermore, the brains of old colonies contain a lower number of cells and have reduced neuron-mediated responses to sensory stimuli. Since stem cells are the only cells that self-renew and are maintained throughout the colony’s life, we hypothesize that genetic mutations or epigenetic changes that accumulate over time in NSC and their progenitors are the main cause of age-related neurodegenerative diseases. To test this hypothesis, we plan to characterize the molecular and cellular diversity of the Botryllus brain in chordate larvae and young and old colonies, isolate their NSCs at the single cell level, identify mutations that accumulate in NSC DNA, and test their effect on brain regeneration and function. Morpholino anti-sense RNAs will also be used to manipulate the activity of genes whose genetic variation predicts the onset of Alzheimer’s disease and test their effect on brain regeneration and degeneration capacity, and also attempt to introduce permanent alterations of their genomes by CRISPR.