Singh Ratnesh K, Nasonkin Igor O
Lineage Cell Therapeutics, Alameda, CA, United States.
Front Cell Neurosci. 2020 Sep 10;14:179. doi: 10.3389/fncel.2020.00179. eCollection 2020.
The self-formation of retinal tissue from pluripotent stem cells generated a tremendous promise for developing new therapies of retinal degenerative diseases, which previously seemed unattainable. Together with use of induced pluripotent stem cells or/and CRISPR-based recombineering the retinal organoid technology provided an avenue for developing models of human retinal degenerative diseases "in a dish" for studying the pathology, delineating the mechanisms and also establishing a platform for large-scale drug screening. At the same time, retinal organoids, highly resembling developing human fetal retinal tissue, are viewed as source of multipotential retinal progenitors, young photoreceptors and just the whole retinal tissue, which may be transplanted into the subretinal space with a goal of replacing patient's degenerated retina with a new retinal "patch." Both approaches (transplantation and modeling/drug screening) were projected when Yoshiki Sasai demonstrated the feasibility of deriving mammalian retinal tissue from pluripotent stem cells, and generated a lot of excitement. With further work and testing of both approaches and , a major implicit limitation has become apparent pretty quickly: the absence of the uniform layer of Retinal Pigment Epithelium (RPE) cells, which is normally present in mammalian retina, surrounds photoreceptor layer and develops and matures first. The RPE layer polarize into apical and basal sides during development and establish microvilli on the apical side, interacting with photoreceptors, nurturing photoreceptor outer segments and participating in the visual cycle by recycling 11-trans retinal (bleached pigment) back to 11-cis retinal. Retinal organoids, however, either do not have RPE layer or carry patches of RPE mostly on one side, thus directly exposing most photoreceptors in the developing organoids to neural medium. Recreation of the critical retinal niche between the apical RPE and photoreceptors, where many retinal disease mechanisms originate, is so far unattainable, imposes clear limitations on both modeling/drug screening and transplantation approaches and is a focus of investigation in many labs. Here we dissect different retinal degenerative diseases and analyze how and where retinal organoid technology can contribute the most to developing therapies even with a current limitation and absence of long and functional outer segments, supported by RPE.
多能干细胞自我形成视网膜组织为开发视网膜退行性疾病的新疗法带来了巨大希望,而这在以前似乎是无法实现的。结合诱导多能干细胞的使用或基于CRISPR的重组技术,视网膜类器官技术为在培养皿中开发人类视网膜退行性疾病模型提供了一条途径,用于研究病理学、阐明机制,并建立大规模药物筛选平台。同时,高度类似于发育中的人类胎儿视网膜组织的视网膜类器官,被视为多能视网膜祖细胞、年轻光感受器以及整个视网膜组织的来源,这些组织可移植到视网膜下间隙,目的是以新的视网膜“补丁”替代患者退化的视网膜。当笹井芳树证明从多能干细胞中获得哺乳动物视网膜组织的可行性时,这两种方法(移植和建模/药物筛选)都被提了出来,并引起了极大的轰动。随着对这两种方法的进一步研究和测试,一个主要的潜在限制很快变得明显:缺少视网膜色素上皮(RPE)细胞的均匀层,而这在哺乳动物视网膜中是正常存在的,它包围着光感受器层,并且首先发育和成熟。RPE层在发育过程中极化形成顶端和基底侧,并在顶端侧形成微绒毛,与光感受器相互作用,滋养光感受器外段,并通过将11-反式视黄醛(漂白色素)循环回11-顺式视黄醛参与视觉循环。然而,视网膜类器官要么没有RPE层,要么大多在一侧带有RPE斑块,从而直接使发育中的类器官中的大多数光感受器暴露于神经培养基中。到目前为止,在顶端RPE和光感受器之间重建关键的视网膜微环境是无法实现的,这对建模/药物筛选和移植方法都造成了明显的限制,并且是许多实验室研究的重点。在这里,我们剖析不同的视网膜退行性疾病,并分析即使在当前存在局限性且缺乏由RPE支持的长而功能性外段的情况下,视网膜类器官技术如何以及在何处能够为开发治疗方法做出最大贡献。