Single-cell RNA sequencing reveals the heterogeneity and interactions of immune cells and Müller glia during zebrafish retina regeneration.

ABSTRACT: Inflammation plays a crucial role in the regeneration of fish and avian retinas. However, how inflammation regulates Müller glia (MG) reprogramming remains unclear. Here, we used single-cell RNA sequencing to investigate the cell heterogeneity and interactions of MG and immune cells in the regenerating zebrafish retina. We first showed that two types of quiescent MG (resting MG1 and MG2) reside in the uninjured retina. Following retinal injury, resting MG1 transitioned into an activated state expressing known reprogramming genes, while resting MG2 gave rise to rod progenitors. We further showed that retinal microglia can be categorized into three subtypes (microglia-1, microglia-2, and proliferative) and pseudotime analysis demonstrated dynamic changes in microglial status following retinal injury. Analysis of cell-cell interactions indicated extensive crosstalk between immune cells and MG, with many interactions shared among different immune cell types. Finally, we showed that inflammation activated Jak1-Stat3 signaling in MG, promoting their transition from a resting to an activated state. Our study reveals the cell heterogeneity and crosstalk of immune cells and MG in zebrafish retinal repair, and may provide valuable insights into future mammalian retina regeneration.

Transcriptomic profiling across human serotonin neuron differentiation via the FEV reporter system.

BACKGROUND: The detailed transcriptomic profiles during human serotonin neuron (SN) differentiation remain elusive. The establishment of a reporter system based on SN terminal selector holds promise to produce highly-purified cells with an early serotonergic fate and help elucidate the molecular events during human SN development process. METHODS: A fifth Ewing variant (FEV)-EGFP reporter system was established by CRISPR/Cas9 technology to indicate SN since postmitotic stage. FACS was performed to purify SN from the heterogeneous cell populations. RNA-sequencing analysis was performed for cells at four key stages of differentiation (pluripotent stem cells, serotonergic neural progenitors, purified postmitotic SN and purifed mature SN) to explore the transcriptomic dynamics during SN differentiation. RESULTS: We found that human serotonergic fate specification may commence as early as day 21 of differentiation from human pluripotent stem cells. Furthermore, the transcriptional factors ZIC1, HOXA2 and MSX2 were identified as the hub genes responsible for orchestrating serotonergic fate determination. CONCLUSIONS: For the first time, we exposed the developmental transcriptomic profiles of human SN via FEV reporter system, which will further our understanding for the development process of human SN.

Corticosterone Impairs Hippocampal Neurogenesis and Behaviors through p21-Mediated ROS Accumulation.

Stress is known to induce a reduction in adult hippocampal neurogenesis (AHN) and anxiety-like behaviors. Glucocorticoids (GCs) are secreted in response to stress, and the hippocampus possesses the greatest levels of GC receptors, highlighting the potential of GCs in mediating stress-induced hippocampal alterations and behavior deficits. Herein, RNA-sequencing (RNA-seq) analysis of the hippocampus following corticosterone (CORT) exposure revealed the central regulatory role of the p21 (Cdkna1a) gene, which exhibited interactions with oxidative stress-related differentially expressed genes (DEGs), suggesting a potential link between p21 and oxidative stress-related pathways. Remarkably, p21-overexpression in the hippocampal dentate gyrus partially recapitulated CORT-induced phenotypes, including reactive oxygen species (ROS) accumulation, diminished AHN, dendritic atrophy, and the onset of anxiety-like behaviors. Significantly, inhibiting ROS exhibited a partial rescue of anxiety-like behaviors and hippocampal alterations induced by p21-overexpression, as well as those induced by CORT, underscoring the therapeutic potential of targeting ROS or p21 in the hippocampus as a promising avenue for mitigating anxiety disorders provoked by chronic stress.

Uncovering the role of FOXA2 in the Development of Human Serotonin Neurons.

Directed differentiation of serotonin neurons (SNs) from human pluripotent stem cells (hPSCs) provides a valuable tool for uncovering the mechanism of human SN development and the associated neuropsychiatric disorders. Previous studies report that FOXA2 is expressed by serotonergic progenitors (SNPs) and functioned as a serotonergic fate determinant in mouse. However, in the routine differentiation experiments, it is accidentally found that less SNs and more non-neuronal cells are obtained from SNP stage with higher percentage of FOXA2-positive cells. This phenomenon prompted them to question the role of FOXA2 as an intrinsic fate determinant for human SN differentiation. Herein, by direct differentiation of engineered hPSCs into SNs, it is found that the SNs are not derived from FOXA2-lineage cells; FOXA2-knockout hPSCs can still differentiate into mature and functional SNs with typical serotonergic identity; FOXA2 overexpression suppresses the SN differentiation, indicating that FOXA2 is not intrinsically required for human SN differentiation. Furthermore, repressing FOXA2 expression by retinoic acid (RA) and dynamically modulating Sonic Hedgehog (SHH) signaling pathway promotes human SN differentiation. This study uncovers the role of FOXA2 in human SN development and improves the differentiation efficiency of hPSCs into SNs by repressing FOXA2 expression.

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.

Generation of a TPH2-EGFP reporter cell line for purification and monitoring of human serotonin neurons in vitro and in vivo.

Generation of serotonin neurons (SNs) from human pluripotent stem cells (hPSCs) provides a promising platform to explore the mechanisms of serotonin-associated neuropsychiatric disorders. However, neural differentiation always yields heterogeneous cell populations, making it difficult to identify and purify SNs in vitro or track them in vivo following transplantation. Herein, we generated a TPH2-EGFP reporter hPSC line with insertion of EGFP into the endogenous tryptophan hydroxylase 2 (TPH2) locus using CRISPR-Cas9-mediated gene editing technology. This TPH2-reporter, which faithfully indicated TPH2 expression during differentiation, enabled us to obtain purified SNs for subsequent transcriptional analysis and study of pharmacological responses to antidepressants. In addition, the reporter system showed strong EGFP expression to indicate SNs, which enabled us to explore in vitro and ex vivo electrophysiological properties of SNs. In conclusion, this TPH2-EGFP reporter cell line might be of great significance for studies on human SN-related development and differentiation, drug screening, disease modeling, and cell replacement therapies.

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.

Corticosterone induces obesity partly via promoting intestinal cell proliferation and survival.

Introduction: A vicious cycle ensues whereby prolonged exposure to social stress causes increased production of glucocorticoids (GCs), leading to obesity even further. Understanding the role of GCs, the key element in the vicious circle, might be helpful to break the vicious circle. However, the mechanism by which GCs induce obesity remains elusive. Methods: Corticosterone (CORT) was administered to mice for 8 weeks. Food and water intake were recorded; obesity was analyzed by body-weight evaluation and magnetic resonance imaging (MRI); intestinal proliferation and survival were evaluated by H&E staining, EdU-progression test, TUNEL assay and immunofluorescence staining of Ki67 and CC3; RNA-seq was performed to analyze transcriptional alterations in small intestines and livers. Results: Chronic CORT treatment induced obesity, longer small intestines, hepatic steatosis and elevated levels of serum insulin and leptin in mice; CORT-treated mice showed increased cell proliferation and decreased apoptosis of small intestines; RNA-seq results indicate that differentially expressed genes (DEGs) were enriched in several cell growth/death-associated signaling pathways. Discussion: Herein we find that administration of CORT to mice promotes the proliferation and survival of intestinal cells, which might contribute to the longer small intestines and the elongated intestinal villi, thus leading to increased nutrient absorption and obesity in mice. Understanding CORT-induced alterations in intestines and associated signaling pathways might provide novel therapeutic clues for GCs or stress-associated obesity.

Elimination of Serotonergic Neurons by Stereotaxic Injection of 5,7-Dihydroxytryptamine in the Dorsal Raphe Nuclei of Mice.

Stereotaxic injection has been widely used for direct delivery of compounds or viruses to targeted brain areas in rodents. Direct targeting of serotonergic neurons in the dorsal raphe nucleus (DRN) can cause excessive bleeding and animal death, due to its location below the superior sagittal sinus (SSS). This protocol describes the generation of a DRN serotonergic neuron-lesioned mouse model (>90% survival rate) with stable loss of >70% 5-HT-positive cells in the DRN. The lesion is induced by stereotaxic injection of a selective serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the DRN using an angled approach (30° in the anterior/posterior direction) to avoid injury to the SSS. DRN serotonergic neuron-lesioned mice display anxiety-associated behavior alterations, which helps to confirm success of the DRN lesion surgery. This method is used here for DRN lesions, but it can also be used for other stereotaxic injections that require angular injections to avoid midline structures. This DRN serotonergic neuron-lesioned mouse model provides a valuable tool for understanding the role of serotonergic neurons in the pathogenesis of psychiatric disorders, such as generalized anxiety disorder and major depressive disorder.

Materials for Neural Differentiation, Trans-Differentiation, and Modeling of Neurological Disease.

Neuron regeneration from pluripotent stem cells (PSCs) differentiation or somatic cells trans-differentiation is a promising approach for cell replacement in neurodegenerative diseases and provides a powerful tool for investigating neural development, modeling neurological diseases, and uncovering the mechanisms that underlie diseases. Advancing the materials that are applied in neural differentiation and trans-differentiation promotes the safety, efficiency, and efficacy of neuron regeneration. In the neural differentiation process, matrix materials, either natural or synthetic, not only provide a structural and biochemical support for the monolayer or three-dimensional (3D) cultured cells but also assist in cell adhesion and cell-to-cell communication. They play important roles in directing the differentiation of PSCs into neural cells and modeling neurological diseases. For the trans-differentiation of neural cells, several materials have been used to make the conversion feasible for future therapy. Here, the most current applications of materials for neural differentiation for PSCs, neuronal trans-differentiation, and neurological disease modeling is summarized and discussed.

Characterization of Induced Pluripotent Stem Cell-derived Human Serotonergic Neurons.

In the brain, the serotonergic neurons located in the raphe nucleus are the unique resource of the neurotransmitter serotonin, which plays a pivotal role in the regulation of brain development and functions. Dysfunction of the serotonin system is present in many psychiatric disorders. Lack of in vitro functional human model limits the understanding of human central serotonergic system and its related diseases and clinical applications. Previously, we have developed a method generating human serotonergic neurons from induced pluripotent stem cells (iPSCs). In this study, we analyzed the features of these human iPSCs-derived serotonergic neurons both in vitro and in vivo. We found that these human serotonergic neurons are sensitive to the selective neurotoxin 5, 7-Dihydroxytryptamine (5,7-DHT) in vitro. After being transplanted into newborn mice, the cells not only expressed their typical molecular markers, but also showed the migration and projection to the host's cerebellum, hindbrain and spinal cord. The data demonstrate that these human iPSCs-derived neurons exhibit the typical features as the serotonergic neurons in the brain, which provides a solid foundation for studying on human serotonin system and its related disorders.

Detecting Aß deposition and RPE cell senescence in the retinas of SAMP8 mice.

AIM: Our previous study indicated that Aß-induced Retinal Pigment Epithelial (RPE) cell senescence may be associated with chronic inflammation in age-related macular degeneration (AMD). The present study was designed to explore whether Aß deposition and RPE senescence could be found in the senescence-prone mouse strain 8 (SAMP8), which is an animal model for AMD. METHODS: Eyes of both SAMP8 and age-matched SAMR1 (SAM resistant) mice were examined in vivo by fundus photography and electroretinography (ERG). Retinal morphological features were assessed using light and electron microscopy. Aß deposition and p16-positive senescent RPE cells were traced using immunofluorescence labeling. P16 expression was detected using western blot. Expressions of IL-6 and IL-8 in RPE/choroid were analyzed using RT-PCR. RESULTS: In fundus of SAMP8, age-dependent increase of drusen-like lesions and the increase of granular autofluorescent spots were respectively detected using IR (near-infrared) and AF (autofluorescence) imaging of confocal scanning laser ophthalmoscope. The amplitude of the ERGs declined with age in SAMP8 and these changes were paralleled with the significant changes in retinal morphological features examined by funduscopy. Histopathological analysis found significant loss of photoreceptor outer segments (OS) and abnormal localization of RPE cells in aged SAMP8 mice. Degenerative changes in RPE cells of aged SAMP8 mice, including massive vacuoles, thickened Bruch's membrane (BrM), and loss of basal infoldings were further confirmed by electron microscopy. Increased Aß deposits in OS layer and p16-positive senescent RPE cells were observed using immunofluorescence microscopy. Western blot confirmed that P16 expression was significantly increased in RPE cells of aged SAMP8 mice. Expressions of proinflammatory IL-6 and IL-8 were significantly upregulated in RPE/choroid of aged SAMP8 mice. CONCLUSIONS: Our results showed that aged SAMP8 mice developed ocular pathology similar to some features of human AMD. In this AMD mouse model, Aß deposition and RPE senescence may be associated with AMD development, and RPE senescence is likely a mechanistic link between Aß deposition and inflammation.

Subretinal injection of amyloid-ß peptide accelerates RPE cell senescence and retinal degeneration.

Drusen are considered a hallmark characteristic of age-related macular degeneration (AMD). In our previous study, we found that amyloid-ß (Aß) peptide, a component of drusen, induced the cells of the retinal pigment epithelium (RPE; RPE cells) to enter senescence; however, its effects in vivo remain unknown. Thus, the present study was carried out to explore the in vivo effects of Aß peptide on RPE cell senescence and senescence-associated inflammation in C57BL/6 mice. C57BL/6 mice received a subretinal injection of Aß(1-42) peptide; on day 7 post-injection, the mice were anesthetized and subjected to whole-body perfusion with 4% paraformaldehyde (PFA) in PBS and the whole eyes were then enucleated. Retinal function was assessed by electroretinography (ERG), and the morphological characteristics of the retina were examined by light and electron microscopy. Fundus autofluorescence (FAF) was examined by confocal scanning laser ophthalmoscopy (cSLO). The expression of p16INK4a, a marker of cellular senescence, was examined by immunofluorescence staining and western blot analysis. The RPE-choroid was analyzed for cytokine expression by RT-PCR. In Aß(1-42)-injected mice, scotopic ERG responses declined. Degenerative alterations, including the disruption of the inner segment (IS)/outer segment (OS) junction and extensive vacuolation and thickness of Bruch's membrane (BrM) were observed under a a light microscope. The accumulation of vacuoles and the loss of basal infoldings in the RPE were identified using an electron microscope. FAF and p16INK4a expression increased in Aß(1-42)-injected mice. In addition, Aß(1-42) upregulated interleukin (IL)-6 and IL-8 gene expression in the RPE-choroid. In conclusion, our results confirm the effects of Aß(1-42) peptide on RPE senescence in vivo. The Aß-injected mice developed AMD-like ocular pathology. It is thus suggested that RPE cell senescence is a potential mechanistic link between inflammation and retinal degeneration.

[Alterations of retinal tissue induced by amyloid ß(1- 42) subretinal injection in mice].

OBJECTIVE: To investigate the alterations of retinal tissue induced by OAß(1-42) subretinal injection in normal C57BL/6N mice. METHODS: Experimental study. One hundred and twenty C57BL/6N mice aged 8 weeks participated in this study. Among them, 60 eyes received subretinal injection of OAß(1-42); another 60 received double distilled water as control. The concentration of OAß(1-42) was 0.312 5 mmol/L. We took fundus photograph and autofluorescence, electroretinogram(ERG), haematoxylineosin staining of retinal paraffin sections, ß-galactosidase staining of neural retinal flatmounts, immunofluorescence of senescence marker P16INK4a of RPE/choroidal flatmounts, Western blot and RT-PCR of senescence relative cytokines IL-6, TGF-ß1 before and after the injection. The data were analyzed by independent sample t-test. RESULTS: Comparing with controls, 1 week post injection, the experimental group eyes had atrophied area with high autofluorescence site, and showed evident decrease in amplitudes of a wave (39.94±7.75)µV comparing with the controls (225.27±28.94)µV(t=12.45, P<0.001) in scotopia ERG, so did the amplitude of b wave (185.55±4.62)µV comparing with the controls (873.78±43.80)µV(t=27.06, P<0.001), retinal structures appeared significant loss of retinal thickness (t=75.13, P<0.001) along with hypo- or hyperpigmentation; the experimental eyes had high frequency of blue-green deposits in retinal ß-gal staining; the green fluorescence sites of RPE/choroidal flatmounts were clear and bright, indicating higher expression of P16INK4a in the nucleus; these results were the same till 8 weeks post operation. Western blot and RT-PCR vertified conspicuous increases in expression of TGF-ß1, IL-6 in retinas of experimental eyes compared with the control ones 1 week post injection. CONCLUSIONS: OAß(1-42) subretinal injection could induce visual function deficiency and retinal tissue senescence. All the manifestations were AMD-like retinopathy, which implied that Aß(1-42) might be an essential clue in human AMD pathology.

Aß-induced senescent retinal pigment epithelial cells create a proinflammatory microenvironment in AMD.

PURPOSE: Chronic inflammation is implicated in the pathogenesis of AMD. The source of chronic inflammation is often attributed to the progressive activation of immune cells over time. However, recent studies have shown that senescent cells can alter tissue microenvironment via secretion of growth factors, proteases, and inflammatory cytokines and might be an additional source of chronic inflammation. We hypothesized that altered secretory pattern in Aß-induced senescent RPE cells may contribute to compromised RPE barrier integrity and chronic inflammation in AMD. METHODS: Senescence was assessed by measuring the SA-ß-Galactosidase activity, the expressions of p16(INK4a) and ATM, and cell cycle analysis. Expressions of IL-8 and MMPs were analyzed by RT-PCR, ELISA, and gelatin zymography. The barrier structures of RPE cells were detected by actin-tracker, ZO-1, claudin-19, occludin immunochemistry, and Western blot; barrier function was analyzed by measuring transepithelial resistance (TER) and transepithelial diffusion rate of FITC-dextran. For inhibitory studies, MMP-9 was inhibited by RNA interference strategy. RESULTS: Aß promotes RPE cells to enter senescence and secrete higher concentrations of IL-8 and MMP-9. Secretion of MMP-9 is associated with compromised barrier integrity and with processing of IL-8 to a more activated form. Silence of MMP-9 preserved the barrier integrity of senescent RPE cells. CONCLUSIONS: The altered secretory phenotype of senescent RPE cells may contribute to age-related inflammation in AMD. Chinese Abstract.

Amyloid-ß-induced matrix metalloproteinase-9 secretion is associated with retinal pigment epithelial barrier disruption.

Local and chronic inflammation induced by amyloid-ß (Aß) plays a central role in the development of age-related macular degeneration. The retina is an immune-privileged site due to local tissue barrier. Yet, the manner by which immune cells pass through this barrier and accumulate in the retina remains unclear. Matrix metalloproteinases (MMPs) induce barrier disruption via proteolysis of epithelial tight junction (TJ) proteins. We hypothesized that Aß-induced MMP secretion causes disruption of epithelial barrier integrity. To test this hypothesis, human adult retinal pigment epithelial (haRPE) cells were exposed to Aß, and the expression of MMP-2 and MMP-9 was detected using gelatin zymography. To demonstrate the key role of MMPs in modulating epithelial barrier structure, the MMP agonist 4-aminophenylmercuric acetate (APMA), an MMP inhibitor (GM6001) and siRNA against MMP-9 were employed for comparison. We found that MMP-9, secreted by Aß- or APMA-stimulated cells, mediated low transepithelial electrical resistance (TER) and high transepithelial permeability by disrupting TJ proteins. However, these alterations were reduced by the MMP inhibitor GM6001 or by silencing of the MMP-9 gene. Our findings suggest that the degradation of TJ proteins such as zonula occludens-1, occludin and F-actin by MMP-9 secreted by Aß-stimulated cells constitutes an important mechanism in the breakdown of the barrier which contributes to chronic inflammation in the retina of age-related macular degeneration.