Feng-Quan Zhou PhD
Professor of Orthopedic Surgery and Neuroscience
Professor of Orthopedic Surgery and Neuroscience
The overall goal of our research is to understand the molecular mechanisms underlying development of the mammalian nervous system and neural regeneration after neural injuries. Specifically, we are interested in understanding how neurons generate their complex morphology and form proper circuitries during development and how neurons regenerate to restore connections after brain or spinal cord injuries. Advanced experimental technologies will be used in our study, such as multiomic sequencing (e.g. RNA-seq, ATAC-seq), either at bulk or single cell level, advanced bioinformatics, advanced tissue clearing techniques combined with high resolution 3D imaging (e.g. Light Sheet), Crispr-Cas9/dCas9 based genetic manipulation, and various animal models of neural development and regeneration.
1. Reprogramming CNS neurons for axon regeneration
During development, stem cells undergo many steps to turn into differentiated cells. During such process, the whole gene expression profile changes drastically with stem cell related gene shutting down and only genes relevant for the differentiated cell type expressed. The emerging idea is that regulation of gene expression during cell differentiation is largely achieved through epigenetic modifications, including DNA methylation, histone modifications, and non-coding RNAs. Importantly, differentiated cells (e.g. fibroblast) can be reprogrammed back to induced pluripotent stem cells (iPSC) by overexpressing several reprogramming factors, such as Sox2, c-Myc, KLF4, Oct4, Nanog, and Lin28, which lead to global epigenetic remodeling (7). Besides these pioneering transcription factors, changes in histone or DNA modification also play direct and important roles in cell reprogramming. Do mammalian CNS neurons undergo similar epigenetic changes during maturation to lose their ability to support axon growth? If so, can we reprogram mature CNS neurons back to a cellular state that supports axon regeneration? Indeed, Both KLF4 and c-Myc are important regulators of axon regeneration. Our study showed clearly that Lin28 acted to regulate both sensory axon regeneration and optic nerve regeneration in vivo. Our study also reveals that mammalian neurons undergo an epigenetic transition during maturation that lead to silencing of axon growth-promoting genes and upregulation of axon growth-inhibitory genes. Thus, it is possible to reprogram the mature CNS neurons into a regenerating state via remodeling their epigenetic landscape through reprogramming factors or chromatin modulators.
2. Targeting aging genes and pathways to regulate axon regeneration
Recent studies, including ours, provided strong evidence that neuronal aging might be a key converging process underlying the loss of intrinsic axon growth ability of CNS neurons. Indeed, many identified genes that act to regulate axon regeneration are also hallmark genes of aging, which include genomic instability (c-Myc, P53), telomere attrition (TERT), epigenetic alteration (UTX, EZH2), and nutrient sensing (IIS pathway, Sirt1, LKB1), etc. Recent findings indicated that during iPSC production the reprogramming process can rejuvenate the cells by reversing hallmarks of aging. Thus, targeting aging genes and pathways will not only be a new direction to identify candidate molecules promoting CNS axon regeneration, but also reveal new insights into the molecular mechanisms by which axon regeneration is regulated. Moreover, the strong evidence for cellular rejuvenation during iPSC reprogramming suggests that aging processes are mostly regulated at the epigenetic rather than genetic level and reversible. Thus, in the future it is possible to rejuvenate aged CNS neurons to regain axon regeneration capacity.
3. Remodeling of chromatin and transcriptomic landscape to enhance axon regeneration
Recent studies in other fields have provided strong and clear evidence that during development, the transcriptional profiles of cells change with time to gain specific cellular phenotypes. During such process, it is well known now that the chromatin structures in the nucleus change with some regions of chromatin adopting a closed conformation, whereas in other regions the chromatin becoming open. During the development and maturation of neurons, they gradually lose their intrinsic axon growth ability, likely through changes in gene transcription. We think that such genetic switch in neurons is also mediated by changes in their chromatin structures, which are regulated by epigenetic modulators, such as histone modifications. Although manipulation of several genes, including transcription factors (TFs), have been identified to enhance the intrinsic axon regeneration ability of mature CNS neurons, it is possible that the chromatin state (open or close at specific genomic loci) could affect the access of these genes to the chromatin and their regulation of gene transcription. We think that matching chromatin structures with the expression of TFs would re-switch RGC transcriptomics into a state that supports axon regeneration (RegeNeuron state). The current available multiomics approaches, including RNA-seq, ATAC-seq and ChIP-seq, provide us strong tools to dissect the transcriptome and chromatin structures of neurons at different developing stages, during which they gradually lose their ability to support axon growth.