Antiviral Drug Ganciclovir Is a Potent Inhibitor of the Proliferation of Müller Glia-Derived Progenitors During Zebrafish Retinal Regeneration

A comparative analysis of gene and protein expression in chronic and acute models of photoreceptor degeneration in adult zebrafish

Background: Adult zebrafish are capable of photoreceptor (PR) regeneration following acute phototoxic lesion (AL). We developed a chronic low light (CLL) exposure model that more accurately reflects chronic PR degeneration observed in many human retinal diseases. Methods: Here, we characterize the morphological and transcriptomic changes associated with acute and chronic models of PR degeneration at 8 time-points over a 28-day window using immunohistochemistry and 3'mRNA-seq. Results: We first observed a differential sensitivity of rod and cone PRs to CLL. Next, we found no evidence for Müller glia (MG) gliosis or regenerative cell-cycle re-entry in the CLL model, which is in contrast to the robust gliosis and proliferative response from resident MG in the AL model. Differential responses of microglia between the models was also observed. Transcriptomic comparisons between the models revealed gene-specific networks of PR regeneration and degeneration, including genes that are activated under conditions of chronic PR stress. Finally, we showed that CLL is at least partially reversible, allowing for rod and cone outer segment outgrowth and replacement of rod cell nuclei via an apparent upregulation of the existing rod neurogenesis mechanism. Discussion: Collectively, these data provide a direct comparison of the morphological and transcriptomic PR degeneration and regeneration models in zebrafish.

EZH2-Myc driven glioblastoma elicited by cytomegalovirus infection of human astrocytes

Mounting evidence is identifying human cytomegalovirus (HCMV) as a potential oncogenic virus. HCMV has been detected in malignant gliomas. EZH2 and Myc play a potential oncogenic role, correlating with the glioma grade. Herewith, we present the first experimental evidence for HCMV as a reprogramming vector, straight through the dedifferentiation of mature human astrocytes, and generation of CMV-Elicited Glioblastoma Cells (CEGBCs) possessing glioblastoma-like traits. HCMV counterparts the progression of the perceived cellular and molecular mechanisms succeeding the transformation and invasion processes with CEGBCs involved in spheroid formation and invasiveness. Glioblastoma multiforme (GBM) biopsies were characterized by an elevated EZH2 and Myc expression, possessing a strong positive correlation between the aforementioned markers in the presence of HCMV. From GBM tissues, we isolated HCMV clinical strains that transformed HAs toward CEGBCs exhibiting upregulated EZH2 and Myc. Spheroids generated from CEGBCs possessed invasion potential and were sensitive to EZH2 inhibitor, ganciclovir, and temozolomide triple therapy. HCMV clinical strains transform HAs and fit with an HCMV-induced glioblastoma model of oncogenesis, and supports the tumorigenic properties of Myc and EZH2 which might be highly pertinent in the pathophysiology of astrocytic brain tumors and thereby paving the way for new therapeutic strategies.

Sox11b regulates the migration and fate determination of Müller glia-derived progenitors during retina regeneration in zebrafish

The transcription factor Sox11 plays important roles in retinal neurogenesis during vertebrate eye development. However, its function in retina regeneration remains elusive. Here we report that Sox11b, a zebrafish Sox11 homolog, regulates the migration and fate determination of Müller glia-derived progenitors (MGPCs) in an adult zebrafish model of mechanical retinal injury. Following a stab injury, the expression of Sox11b was induced in proliferating MGPCs in the retina. Sox11b knockdown did not affect MGPC formation at 4 days post-injury, although the nuclear morphology and subsequent radial migration of MGPCs were altered. At 7 days post-injury, Sox11b knockdown resulted in an increased proportion of MGPCs in the inner retina and a decreased proportion of MGPCs in the outer nuclear layer, compared with controls. Furthermore, Sox11b knockdown led to reduced photoreceptor regeneration, while it increased the numbers of newborn amacrines and retinal ganglion cells. Finally, quantitative polymerase chain reaction analysis revealed that Sox11b regulated the expression of Notch signaling components in the retina, and Notch inhibition partially recapitulated the Sox11b knockdown phenotype, indicating that Notch signaling functions downstream of Sox11b. Our findings imply that Sox11b plays key roles in MGPC migration and fate determination during retina regeneration in zebrafish, which may have critical implications for future explorations of retinal repair in mammals.

Context-dependent effects of inflammation on retina regeneration

Inflammation is required for the proliferation of Müller glia (MG) into multipotent progenitors (MGPCs) in the injured fish and avian retinas. However, its function in retina regeneration has not been fully understood. Here we investigated the role of inflammation in three different retinal regeneration paradigms in zebrafish (stab-injury, NMDA-injury and insulin treatment). We first show that different types of immune cells and levels of inflammatory cytokines were found in the retinas of these paradigms. Though zymosan injection alone was insufficient to induce MG proliferation in the uninjured retina, immune suppression significantly inhibited MGPC formation in all three paradigms. Enhancing inflammation promoted MGPC formation after stab-injury, while exhibiting a context-dependent role in the NMDA or insulin models. We further show that proper levels of inflammation promoted MG reprogramming and cell cycle re-entry after stab- or NMDA-injury, but excessive inflammation also suppressed MG proliferation in the latter model. Finally, inflammation differentially affected neuronal regeneration in various injury paradigms. Our study reveals the complex and context-dependent role of inflammation during retinal repair in fish and suggests accurate inflammation management may be crucial for successful retina regeneration in mammals.

Ganciclovir attenuates the onset and progression of experimental autoimmune uveitis by inhibiting infiltration of Th17 and inflammatory cells into the retina

Noninfectious (autoimmune and immune-mediated) uveitis is one of the primary diseases leading to blindness in the world. Due to the limitation of current first-line drugs for clinical uveitis, novel drugs and targets against uveitis are urgently needed. Ganciclovir (GCV), an FDA-approved antiviral drug, is often used to treat cytomegalovirus-induced retinitis in clinical patients. Recently, GCV was found to suppress neuroinflammation via targeting STING signaling because the STING pathway plays a pivotal role in autoimmune diseases. However, until now, the effect of GCV on non-infectious uveitis has never been explored. In this work, using the rat experimental autoimmune uveitis (EAU) model, we first found STING to be highly expressed in infiltrating cells (CD68⁺, CD45⁺, and CD4⁺) and retinal glial cells (Iba1⁺ and GFAP⁺) of the immunized retina. More importantly, GCV treatment can significantly suppress the initiation and progression of EAU by inhibiting infiltration of Th17 and inflammatory cells into the retina. Mechanistically, we found that GCV could reverse the levels of pro-inflammatory factors (such as IL-1β) and chemokine-related factors (such as Cxcr3), possibly via targeting the STING pathway. The present results suggest that GCV may be considered as a novel therapeutic strategy against human uveitis.

Development and characterization of a chronic photoreceptor degeneration model in adult zebrafish that does not trigger a regenerative response

Zebrafish (Danio rerio) have become a highly-utilized model system in the field of regenerative biology because of their endogenous ability to regenerate many tissues and organs, including the retina. The vast majority of previous research on retinal regeneration in adult zebrafish utilizes acute methodologies for retinal damage. Acute retinal cell death triggers a reactive gliosis response of Müller glia (MG), the resident macroglia of the retina. In addition, each activated MG undergoes asymmetric cell division to produce a neuronal progenitor, which continues to divide and ultimately gives rise to new retinal neurons. Studies using these approaches have uncovered many crucial mechanisms by which MG respond to acute damage. However, they may not adequately mimic the chronic neuronal degeneration observed in many human retinal degenerative diseases. The current study aimed to develop a new long-term, chronic photoreceptor damage and degeneration model in adult zebrafish. Comparing the subsequent cellular responses to that of the commonly-used acute high-intensity model, we found that low, continuous light exposure damaged the outer segments of both rod and cone photoreceptors, but did not result in significant apoptotic cell death, MG gliosis, or MG cell-cycle re-entry. Instead, chronic light nearly completely truncated photoreceptor outer segments and resulted in a recruitment of microglia to the area. Together, these studies present a chronic photoreceptor model that can be performed in a relatively short time frame (21 days), that may lend insight into the cellular events underlying non-regenerative photoreceptor degeneration observed in other model systems.

Inflammation-induced mammalian target of rapamycin signaling is essential for retina regeneration

Upon retina injury, Müller glia in the zebrafish retina respond by generating multipotent progenitors to repair the retina. However, the complete mechanisms underlying retina regeneration remain elusive. Here we report inflammation-induced mammalian target of rapamycin (mTOR) signaling in the Müller glia is essential for retina regeneration in adult zebrafish. We show after a stab injury, mTOR is rapidly activated in Müller glia and later Müller glia-derived progenitor cells (MGPCs). Importantly, mTOR is required for Müller glia dedifferentiation, as well as the proliferation of Müller glia and MGPCs. Interestingly, transient mTOR inhibition by rapamycin only reversibly suppresses MGPC proliferation, while its longer suppression by knocking down Raptor significantly inhibits the regeneration of retinal neurons. We further show mTOR promotes retina regeneration by regulating the mRNA expression of key reprogramming factors ascl1a and lin-28a, cell cycle-related genes and critical cytokines. Surprisingly, we identify microglia/macrophage-mediated inflammation as an important upstream regulator of mTOR in the Müller glia and it promotes retina regeneration through mTOR. Our study not only demonstrates the important functions of mTOR but also reveals an interesting link between inflammation and the mTOR signaling during retina regeneration.

Opposing Actions of Fgf8a on Notch Signaling Distinguish Two Muller Glial Cell Populations that Contribute to Retina Growth and Regeneration

The teleost retina grows throughout life and exhibits a robust regenerative response following injury. Critical to both these events are Muller glia (or, Muller glial cells; MGs), which produce progenitors for retinal growth and repair. We report that Fgf8a may be an MG niche factor that acts through Notch signaling to regulate spontaneous and injury-dependent MG proliferation. Remarkably, forced Fgf8a expression inhibits Notch signaling and stimulates MG proliferation in young tissue but increases Notch signaling and suppresses MG proliferation in older tissue. Furthermore, cessation of Fgf8a signaling enhances MG proliferation in both young and old retinal tissue. Our study suggests that multiple MG populations contribute to retinal growth and regeneration, and it reveals a previously unappreciated role for Fgf8a and Notch signaling in regulating MG quiescence, activation, and proliferation.

Prospective purification and characterization of Müller glia in the mouse retina regeneration assay

Reactive gliosis is an umbrella term for various glia functions in neurodegenerative diseases and upon injury. Specifically, Müller glia (MG) in some species readily regenerate retinal neurons to restore vision loss after insult, whereas mammalian MG respond by reactive gliosis-a heterogeneous response which frequently includes cell hypertrophy and proliferation. Limited regeneration has been stimulated in mammals, with a higher propensity in young MG, and in vitro compared to in vivo, but the underlying processes are unknown. To facilitate studies on the mechanisms regulating and limiting glia functions, we developed a strategy to purify glia and their progeny by fluorescence-activated cell sorting. Dual-transgenic nuclear reporter mice, which label neurons and glia with red and green fluorescent proteins, respectively, have enabled MG enrichment up to 93% purity. We applied this approach to MG in a mouse retina regeneration ex vivo assay. Combined cell size and cell cycle analysis indicates that most MG hypertrophy and a subpopulation proliferates which, over time, become even larger in cell size than the ones that do not proliferate. MG undergo timed differential genomic changes in genes controlling stemness and neurogenic competence; and glial markers are downregulated. Genes that are potentially required for, or associated with, regeneration and reactive gliosis are differentially regulated by retina explant culture time, epidermal growth factor stimulation, and animal age. Thus, MG enrichment facilitates cellular and molecular studies which, in combination with the mouse retina regeneration assay, provide an experimental approach for deciphering mechanisms that possibly regulate reactive gliosis and limit regeneration in mammals.