Generation of transparency and cellular organization in lens explants

Single-cell RNA sequencing reveals new subtypes of lens superficial tissue in humans

Although the cell atlas of the human ocular anterior segment of the human eye was revealed by single-nucleus RNA sequencing, whether subtypes of lens stem/progenitor cells exist among epithelial cells and the molecular characteristics of cell differentiation of the human lens remain unclear. Single-cell RNA sequencing is a powerful tool to analyse the heterogeneity of tissues at the single cell level, leading to a better understanding of the processes of cell differentiation. By profiling 18,596 cells in human lens superficial tissue through single-cell sequencing, we identified two subtypes of lens epithelial cells that specifically expressed C8orf4 and ADAMTSL4 with distinct spatial localization, a new type of fibre cells located directly adjacent to the epithelium, and a subpopulation of ADAMTSL4+ cells that might be lens epithelial stem/progenitor cells. We also found two trajectories of lens epithelial cell differentiation and changes of some important genes during differentiation.

Soemmerring's Rings Developed around IOLs, in Human Donor Eyes, Can Present Internal Transparent Areas

Soemmerring's rings consist of a ring of lens epithelial derived cells that grow along the periphery of an aphakic lens capsule, or around an intraocular lens. These rings when visualized frontally, appear opaque, however, in some cases the cells that compose these rings are organized in the same fashion as those in normal transparent adult lenses. Thus, our purpose was to test whether any part of the adult Soemmerring's ring could be transparent and how this related to morphological factors. To study this, 16 Soemmerring's rings were extracted from donor eye globes. After imaging, they were thickly sectioned sagittally in order to analyze the degrees of transparency of different areas. All samples were also histologically analyzed using alpha smooth muscle actin, Vimentin, wheat germ agglutinin and DAPI. Our results showed that many samples had some transparent areas, mostly towards the center of their cross-section. Of the factors that we analyzed, only lens fiber organization at the bow region and an increased area of mature lens fiber cells had a significant relation to the degree of transparency at the center. Thus, we can conclude that as Soemmerring's rings mature, they can develop organized and transparent areas of lens cells.

Nicotinamide improves in vitro lens regeneration in a mouse capsular bag model

Background

Mammalian lens regeneration holds great potential as a cataract therapy. However, the mechanism of mammalian lens regeneration is unclear, and the methods for optimization remain in question.

Methods

We developed an in vitro lens regeneration model using mouse capsular bag culture and improved the transparency of the regenerated lens using nicotinamide (NAM). We used D4476 and SSTC3 as a casein kinase 1A inhibitor and agonist, respectively. The expression of lens-specific markers was examined by real-time PCR, immunostaining, and western blotting. The structure of the in vitro regenerated lens was investigated using 3,3′-dihexyloxacarbocyanine iodide (DiOC6) and methylene blue staining, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and transmission electron microscopy.

Results

The in vitro lens regeneration model was developed to mimic the process of in vivo mammalian lens regeneration in a mouse capsular bag culture. In the early stage, the remanent lens epithelial cells proliferated across the posterior capsule and differentiated into lens fiber cells (LFCs). The regenerated lenses appeared opaque after 28 days; however, NAM treatment effectively maintained the transparency of the regenerated lens. We demonstrated that NAM maintained lens epithelial cell survival, promoted the differentiation and regular cellular arrangement of LFCs, and reduced lens-related cell apoptosis. Mechanistically, NAM enhanced the differentiation and transparency of regenerative lenses partly by inhibiting casein kinase 1A activity.

Conclusion

This study provides a new in vitro model for regeneration study and demonstrates the potential of NAM in in vitro mammalian lens regeneration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02862-8.

Lens Stretching Modulates Lens Epithelial Cell Proliferation via YAP Regulation

Purpose

The continuous growth of the lens throughout life may contribute to the onset of age-related conditions in the lens (i.e., presbyopia and cataract). Volumetric growth is the result of continuous proliferation of lens epithelial cells (LECs). The driving factors controlling LEC proliferation are not well understood. This study tested the hypothesis that mechanical stretching modulates LEC proliferation.

Methods

Biomechanical regulation of LEC proliferation was investigated by culturing whole porcine lenses and connective tissues ex vivo under varying physiologically relevant stretching conditions using a bespoke lens stretching device. Additionally, some lenses were treated with a YAP function inhibitor to determine the Hippo signaling pathway's role in regulating lens growth. Resulting changes in LEC labeling index were analyzed using EdU incorporation and flow cytometry for each lens.

Results

LEC proliferation was found to be modulated by mechanical strain. Increasing both the magnitude of static stretching and the stretching frequency in cyclic stretching resulted in a proportional increase in the labeling indices of the LECs. Additionally, treatment with the YAP function inhibitor effectively eliminated this relationship.

Conclusions

These data demonstrate that LEC proliferation is regulated in part, by the mechanotransduction of stresses induced in the lens capsule and that YAP plays an important role in mechanosensing. These results have important implications for understanding lens growth and morphogenesis. The model may also be used to identify and evaluate targets for modulating lens growth.

Light-focusing human micro-lenses generated from pluripotent stem cells model lens development and drug-induced cataract in vitro

Cataracts cause vision loss and blindness by impairing the ability of the ocular lens to focus light onto the retina. Various cataract risk factors have been identified, including drug treatments, age, smoking and diabetes. However, the molecular events responsible for these different forms of cataract are ill-defined, and the advent of modern cataract surgery in the 1960s virtually eliminated access to human lenses for research. Here, we demonstrate large-scale production of light-focusing human micro-lenses from spheroidal masses of human lens epithelial cells purified from differentiating pluripotent stem cells. The purified lens cells and micro-lenses display similar morphology, cellular arrangement, mRNA expression and protein expression to human lens cells and lenses. Exposing the micro-lenses to the emergent cystic fibrosis drug Vx-770 reduces micro-lens transparency and focusing ability. These human micro-lenses provide a powerful and large-scale platform for defining molecular disease mechanisms caused by cataract risk factors, for anti-cataract drug screening and for clinically relevant toxicity assays.

Identification and characterization of FGF2-dependent mRNA: microRNA networks during lens fiber cell differentiation

MicroRNAs (miRNAs) and fibroblast growth factor (FGF) signaling regulate a wide range of cellular functions, including cell specification, proliferation, migration, differentiation, and survival. In lens, both these systems control lens fiber cell differentiation; however, a possible link between these processes remains to be examined. Herein, the functional requirement for miRNAs in differentiating lens fiber cells was demonstrated via conditional inactivation of Dicer1 in mouse (Mus musculus) lens. To dissect the miRNA-dependent pathways during lens differentiation, we used a rat (Rattus norvegicus) lens epithelial explant system, induced by FGF2 to differentiate, followed by mRNA and miRNA expression profiling. Transcriptome and miRNome analysis identified extensive FGF2-regulated cellular responses that were both independent and dependent on miRNAs. We identified 131 FGF2-regulated miRNAs. Seventy-six of these miRNAs had at least two in silico predicted and inversely regulated target mRNAs. Genes modulated by the greatest number of FGF-regulated miRNAs include DNA-binding transcription factors Nfib, Nfat5/OREBP, c-Maf, Ets1, and N-Myc. Activated FGF signaling influenced bone morphogenetic factor/transforming growth factor-β, Notch, and Wnt signaling cascades implicated earlier in lens differentiation. Specific miRNA:mRNA interaction networks were predicted for c-Maf, N-Myc, and Nfib (DNA-binding transcription factors); Cnot6, Cpsf6, Dicer1, and Tnrc6b (RNA to miRNA processing); and Ash1l, Med1/PBP, and Kdm5b/Jarid1b/Plu1 (chromatin remodeling). Three miRNAs, including miR-143, miR-155, and miR-301a, down-regulated expression of c-Maf in the 3′-UTR luciferase reporter assays. These present studies demonstrate for the first time global impact of activated FGF signaling in lens cell culture system and predicted novel gene regulatory networks connected by multiple miRNAs that regulate lens differentiation.

Intrinsic lens forming potential of mouse lens epithelial versus newt iris pigment epithelial cells in three-dimensional culture

Adult newts (Notophthalmus viridescens) are capable of complete lens regeneration that is mediated through dorsal iris pigment epithelial (IPE) cells transdifferentiation. In contrast, higher vertebrates such as mice demonstrate only limited lens regeneration in the presence of an intact lens capsule with remaining lens epithelial cells. To compare the intrinsic lens regeneration potential of newt IPE versus mouse lens epithelial cells (MLE), we have established a novel culture method that uses cell aggregation before culture in growth factor-reduced Matrigel. Dorsal newt IPE aggregates demonstrated complete lens formation within 1 to 2 weeks of Matrigel culture without basic fibroblast growth factor (bFGF) supplementation, including the establishment of a peripheral cuboidal epithelial cell layer, and the appearance of central lens fibers that were positive for αA-crystallin. In contrast, the lens-forming potential of MLE cell aggregates cultured in Matrigel was incomplete and resulted in the formation of defined-size lentoids with partial optical transparency. While the peripheral cell layers of MLE aggregates were nucleated, cells in the center of aggregates demonstrated a nonapoptotic nuclear loss over a time period of 3 weeks that was representative of lens fiber formation. Matrigel culture supplementation with bFGF resulted in higher transparent bigger-size MLE aggregates that demonstrated increased appearance of βB1-crystallin expression. Our study demonstrates that bFGF is not required for induction of newt IPE aggregate-dependent lens formation in Matrigel, while the addition of bFGF seems to be beneficial for the formation of MLE aggregate-derived lens-like structures. In conclusion, the three-dimensional aggregate culture of IPE and MLE in Matrigel allows to a higher extent than older models the indepth study of the intrinsic lens-forming potential and the corresponding identification of lentogenic factors.

The 3R principle: advancing clinical application of human pluripotent stem cells

The first derivation of human embryonic stem cells brought with it a clear understanding that animal models of human disease might be replaced by an unlimited supply of human cells for research, drug discovery, and drug development. With the advent of clinical trials using human pluripotent stem cell-based therapies, it is both timely and relevant to reflect on factors that will facilitate future translation of this technology. Human pluripotent cells are increasingly being used to investigate the molecular mechanisms that underpin normal and pathological human development. Their differentiated progeny are also being used to identify novel pharmaceuticals, to screen for toxic effects of known chemicals, and to investigate cell or tissue transplantation strategies. The intrinsic assumption of these research efforts is that the information gained from these studies will be more accurate, and therefore of greater relevance, than traditional investigations based on animal models of human disease and injury. This review will therefore evaluate how animals and animal-derived products are used for human pluripotent stem cell research, and will indicate how efforts to further reduce or remove animals and animal products from this research will increase the clinical translation of human pluripotent stem cell technologies through drug discovery, toxicology screening, and cell replacement therapies.

Development and use of the lens epithelial explant system to study lens differentiation and cataractogenesis

Over the last two decades much progress has been made in identifying and characterizing many of the molecules involved in understanding normal lens biology and its pathology. Much of this has been made possible through the establishment and use of the lens epithelial explant system. This simplistic tissue culture model, comprised of a sheet of lens epithelium on its native substratum, has been used effectively to study many cellular processes, including lens epithelial cell proliferation, fiber cell differentiation, cell apoptosis as well as epithelial-to-mesenchymal transformation of cells. In doing so, a number of key growth factors and cytokines, including members of the FGF, Wnt and TGFbeta family have been shown to play essential roles in many of these cellular events. This has led to further studies exploring the signaling pathways downstream of these molecules in the lens, paving the way for the development of a number of in situ models (primarily transgenic mouse lines) to further explore in more detail the nature of these molecular and cellular interactions. To reciprocate, the lens epithelial explant system is increasingly being used to further characterize the nature of many complex phenotypes and pathologies observed in these in situ models, allowing us to selectively isolate and examine the direct impact of an individual molecule on a specific cellular response in lens cells. There is no question that the lens epithelial explant system has served as a powerful tool to further our understanding of lens biology and pathology, and there is no doubt that it will continue to serve in such a capacity, as new developments are realized and putative treatments for aberrant lens cell behavior are to be trialed.

Growth factor signaling in vitreous humor-induced lens fiber differentiation

PURPOSE. Although some of the factors and signaling pathways that are involved in induction of fiber differentiation have been defined, such as FGF-mediated MAPK/ERK and PI3-K/Akt signaling, the factors in the vitreous that regulate this differentiation process in vivo have yet to be identified. The purpose of this study was to better understand the role of growth factors in vitreous that regulate this process by further characterizing the signaling pathways involved in lens fiber differentiation. METHODS. Rat lens epithelial explants were used to compare the ability of vitreous, IGF-1, PDGF-A, EGF, and FGF-2 to stimulate the phosphorylation of ERK1/2 and Akt leading to fiber differentiation, in the presence or absence of selective receptor tyrosine kinase (RTK) inhibitors. RESULTS. Similar to vitreous, FGF induced a sustained ERK1/2 signaling profile, unlike IGF, PDGF, and EGF, which induced a more transient (shorter) activation of ERK1/2. For Akt activation, IGF was the only factor that induced a profile similar to vitreous. IGF, PDGF, and EGF potentiated the effects of a low dose of FGF on lens fiber differentiation by extending the duration of ERK1/2 phosphorylation. In the presence of selective RTK inhibitors, although the sustained vitreous-induced ERK1/2 signaling profile and subsequent fiber differentiation was perturbed, the results also showed that, although prolonged ERK1/2 phosphorylation was necessary, it was not sufficient for fiber differentiation to proceed. CONCLUSIONS. These results are consistent with FGF's being the key growth factor involved in vitreous-induced signaling leading to lens fiber differentiation; however, they also indicate that other vitreal growth factors such as IGF may be involved in fine-tuning ERK1/2- and Akt-phosphorylation to the level that is necessary for initiation and/or maintenance of lens fiber differentiation in vivo.