RESEARCH ARTICLE Stochastic cell-cycle entry and cell-state- dependent fate outputs of injury- reactivated tectal radial glia in zebrafish Shuguang Yu1,2, Jie He1* 1State Key Laboratory of Neuroscience, Institute of Neuroscience, Shanghai Institutes for Biological Sciences, Center for Excellence in Brain Science and Intelligence Technology, Chinese Academy of Sciences, Shanghai, China; 2University of Chinese Academy of Sciences, Beijing, China Abstract Gliosis defined as reactive changes of resident glia is the primary response of the central nervous system (CNS) to trauma. The proliferation and fate controls of injury-reactivated glia are essential but remain largely unexplored. In zebrafish optic tectum, we found that stab injury drove a subset of radial glia (RG) into the cell cycle, and surprisingly, proliferative RG responding to sequential injuries of the same site were distinct but overlapping, which was in agreement with stochastic cell-cycle entry. Single-cell RNA sequencing analysis and functional assays further revealed the involvement of Notch/Delta lateral inhibition in this stochastic cell-cycle entry. Furthermore, the long-term clonal analysis showed that proliferative RG were largely gliogenic. Notch inhibition of reactive RG, not dormant and proliferative RG, resulted in an increased production of neurons, which were short-lived. Our findings gain new insights into the proliferation and fate controls of injury-reactivated CNS glia in zebrafish. DOI: https://doi.org/10.7554/eLife.48660.001 *For correspondence: Introduction [email protected] Traumatic brain injury (TBI) is one clinically principal type of central nervous system insults (Burda and Sofroniew, 2014). Gliosis defined as reactive changes of resident macroglia (e.g., mam- Competing interests: The malian astrocytes) is a primary CNS response to TBI in mammals (Barres, 2008; Burda and Sofro- authors declare that no niew, 2014). In mammals, gliosis undergoes three significant stages: Glial cells initially become competing interests exist. reactive, hypertrophic, and inflammatory, with characteristic upregulation of GFAP and vimentin Funding: See page 24 (Liddelow and Barres, 2017; Zamanian et al., 2012); subsequently, a subset of reactive glia re- Received: 21 May 2019 enter the cell cycle and become proliferative (Gallo and Deneen, 2014); finally, proliferative glia Accepted: 02 August 2019 undergo gliogenesis, the process of glial cell production, and form structures known as glial scars Published: 23 August 2019 (Burda and Sofroniew, 2014). Earlier studies have demonstrated both protective and detrimental roles of the gliosis in the injured CNS (Faulkner et al., 2004; Li et al., 2008; Silver and Miller, Reviewing editor: Marianne E 2004; Sofroniew and Vinters, 2010; Wanner et al., 2013). For instance, the blockage of initial glia Bronner, California Institute of Technology, United States reactivation worsened the injury (Faulkner et al., 2004; Li et al., 2008; Wanner et al., 2013), whereas glia scars hindered neuronal regeneration (Silver and Miller, 2004). In the process, the pro- Copyright Yu and He. This liferation and fate controls of injury-reactivated RG are essential but remain elusive in vivo. article is distributed under the In contrast to the mammalian CNS, teleost fish exhibit a superior neural regeneration in response terms of the Creative Commons Attribution License, which to TBI beyond embryonic development (Baumgart et al., 2012; Grandel et al., 2006; permits unrestricted use and Kishimoto et al., 2012; Reimer et al., 2008; Than-Trong and Bally-Cuif, 2015). Radial glia (RG), redistribution provided that the the primary form of macroglia in teleost fish, are the main cell source for injury-mediated regenera- original author and source are tion (Than-Trong and Bally-Cuif, 2015). For instance, RG of different brain regions in zebrafish, credited. including the telencephalon, the hypothalamus, and the spinal cord can produce newborn neurons Yu and He. eLife 2019;8:e48660. DOI: https://doi.org/10.7554/eLife.48660 1 of 27 Research article Developmental Biology Stem Cells and Regenerative Medicine eLife digest The brain contains networks of cells known as neurons that rapidly relay information from one place to another. Other brain cells called glial cells perform several roles to support and protect the neurons including holding them in position and supplying them with oxygen and other nutrients. Damage to the brain as a result of physical injuries is one of the leading causes of death and disability in people worldwide. Brain injuries generally stimulate glial cells to enter a “reactive” state to help repair the damage. However, some glial cells may start to divide and produce more glial cells instead, leading to scar-like structures in the brain that hinder the repair process. To investigate why brain injuries trigger some glial cells to divide, Yu and He systematically examined glial cells in the part of the zebrafish brain that handles vision, known as the optic tectum. The experiments showed that a physical injury stimulated some of the glial cells to divide. Repeated injuries to the same part of the brain did not always stimulate the same glial cells to divide, suggesting that this process happens in random cells. Further experiments revealed that molecules involved in a signaling pathway known as Notch signaling were released from some brain cells and inhibited neighboring glial cells from dividing to make new glial cells. Unexpectedly, inhibiting Notch signaling after a brain injury caused some of the glial cells that were in the reactive state to divide to produce neurons instead of glial cells. Understanding how the brain responds to injury may help researchers develop new therapies that may benefit human patients in future. The next steps following on from this work will be to find out whether glial cells in humans and other mammals work in the same way as glial cells in zebrafish. DOI: https://doi.org/10.7554/eLife.48660.002 in response to the injury (Duncan et al., 2016; Goldshmit et al., 2012; Johnson et al., 2016; Kizil et al., 2012; Kroehne et al., 2011; Than-Trong and Bally-Cuif, 2015). Also, retinal mu¨ ller glia (MG) can be reactivated by the injury, giving rise to newborn retinal neurons (Goldman, 2014; Gorsuch and Hyde, 2014). Molecular mechanisms underlying the proliferation and fate controls of injury-reactivated RG have been examined in zebrafish for many years (Dias et al., 2012; Goldman, 2014). As to the pro- liferation control, Notch signaling is involved but is somehow context-dependent. For instance, in the zebrafish spinal cord, glial cells have low levels of Notch activity when they are in the dormant state, and enter the cell cycle by increased Notch activity after the injury (Dias et al., 2012). In con- trast, dormant RG of zebrafish telencephalon exhibit high Notch activity and become proliferative by a rapid decrease of Notch activity (Chapouton et al., 2010). Notch signaling has also been reported to regulate fate outputs of reactivated MG in the injured zebrafish retina, that is, Notch inhibition leads to gliogenesis, whereas Notch over-activation results in the production of photoreceptor cells (Wan et al., 2012). Zebrafish optic tectum, the higher sensory integration center, possesses a large population of RG (Galant et al., 2016; Ito et al., 2010). Unlike other brain regions where RG present as both dormant and proliferative forms at physiological conditions, tectal RG has been reported to be dormant, and are reactivated by injury to give rise to newborn neurons via Wnt signaling as well as Notch signaling (Shimizu et al., 2018; Ueda et al., 2018). Interestingly, a recent study showed that tectal RG pro- duced a significant number of glial cells (~25%) but not neurons (Lindsey et al., 2019). It is essential to resolve this inconsistency on the fate potentials of injury-reactivated tectal RG. In this study, we set out to investigate the mechanism controlling injury responses of tectal RG in vivo. We found that stab injury drove a subset of tectal RG into the cell cycle. Surprisingly, prolifer- ative tectal RG responding to the sequential injuries at the same injury site were distinct but overlap- ping. Quantitative analysis showed the probability of proliferative RG responding to both sequential stab injuries could be well explained by a model incorporating stochastic cell-cycle entry at the fixed probability of ~25%. Single-cell RNA-seq and functional analysis revealed this stochastic cell-cycle entry was dependent on Notch/Delta lateral inhibition. The clonal analysis showed that proliferative tectal RG underwent gliogenesis. Interestingly, post-injury notch inhibition drove reactive RG into Yu and He. eLife 2019;8:e48660. DOI: https://doi.org/10.7554/eLife.48660 2 of 27 Research article Developmental Biology Stem Cells and Regenerative Medicine the cell cycle and resulted in increased neurogenesis. Interestingly, the over-produced neurons mostly diminished by approximately 25 days post-injury (dpi). Results Stab injury induces the proliferation of dormant tectal RG Consistent with earlier studies (Galant et al., 2016; Ito et al., 2010; Jung et al., 2012), our results showed that Tg(gfap:GFP) (Bernardos and Raymond, 2006), Tg(her4.1:dRFP) (Yeo et al., 2007), and the antibody against glutamine synthetase (GS) specifically labeled RG that line the tectal ventri- cle and extend their basal processes into the superficial neuropils (Figure 1A–B3 and Figure 1—fig- ure supplement 1A-C2). To examine the dormancy of tectal RG, we quantified the proliferative RG in the optic tectum of Tg(gfap:GFP) fish using the antibodies against proliferating cell nuclear anti- gen (PCNA), glutamine synthetase (GS), and GFP (Figure 1A–B3). Only 1.4 ± 0.2% (n = 5, mean ± SEM) of GFP+/GS+ RG at the bottom of the periventricular gray zone (PGZ) were PCNA+ (Figure 1A–B3). Bromodeoxyuridine (BrdU) is a nucleotide analog that incorporates into new synthe- sized DNA of dividing cells in the S phase. We noticed that 1 day’s administration of BrdU labeled very few tectal RG under physiological conditions (Figure 1—figure supplement 1D-F1).