Dynamic extrinsic pacing of the HOX clock in human axial progenitors controls motor neuron subtype specification.

Rostro-caudal patterning of vertebrates depends on the temporally progressive activation of HOX genes within axial stem cells that fuel axial embryo elongation. Whether the pace of sequential activation of HOX genes, the 'HOX clock', is controlled by intrinsic chromatin-based timing mechanisms or by temporal changes in extrinsic cues remains unclear. Here, we studied HOX clock pacing in human pluripotent stem cell-derived axial progenitors differentiating into diverse spinal cord motor neuron subtypes. We show that the progressive activation of caudal HOX genes is controlled by a dynamic increase in FGF signaling. Blocking the FGF pathway stalled induction of HOX genes, while a precocious increase of FGF, alone or with GDF11 ligand, accelerated the HOX clock. Cells differentiated under accelerated HOX induction generated appropriate posterior motor neuron subtypes found along the human embryonic spinal cord. The pacing of the HOX clock is thus dynamically regulated by exposure to secreted cues. Its manipulation by extrinsic factors provides synchronized access to multiple human neuronal subtypes of distinct rostro-caudal identities for basic and translational applications.This article has an associated 'The people behind the papers' interview.

Generation of Storable Retinal Organoids and Retinal Pigmented Epithelium from Adherent Human iPS Cells in Xeno-Free and Feeder-Free Conditions.

Human induced pluripotent stem cells (hiPSCs) are potentially useful in regenerative therapies for retinal disease. For medical applications, therapeutic retinal cells, such as retinal pigmented epithelial (RPE) cells or photoreceptor precursors, must be generated under completely defined conditions. To this purpose, we have developed a two-step xeno-free/feeder-free (XF/FF) culture system to efficiently differentiate hiPSCs into retinal cells. This simple method, relies only on adherent hiPSCs cultured in chemically defined media, bypassing embryoid body formation. In less than 1 month, adherent hiPSCs are able to generate self-forming neuroretinal-like structures containing retinal progenitor cells (RPCs). Floating cultures of isolated structures enabled the differentiation of RPCs into all types of retinal cells in a sequential overlapping order, with the generation of transplantation-compatible CD73+ photoreceptor precursors in less than 100 days. Our XF/FF culture conditions allow the maintenance of both mature cones and rods in retinal organoids until 280 days with specific photoreceptor ultrastructures. Moreover, both hiPSC-derived retinal organoids and dissociated retinal cells can be easily cryopreserved while retaining their phenotypic characteristics and the preservation of CD73+ photoreceptor precursors. Concomitantly to neural retina, this process allows the generation of RPE cells that can be effortlessly amplified, passaged, and frozen while retaining a proper RPE phenotype. These results demonstrate that simple and efficient retinal differentiation of adherent hiPSCs can be accomplished in XF/FF conditions. This new method is amenable to the development of an in vitro GMP-compliant retinal cell manufacturing protocol allowing large-scale production and banking of hiPSC-derived retinal cells and tissues. Stem Cells 2017;35:1176-1188.

Whole-exome sequencing identifies KIZ as a ciliary gene associated with autosomal-recessive rod-cone dystrophy.

Rod-cone dystrophy (RCD), also known as retinitis pigmentosa, is a progressive inherited retinal disorder characterized by photoreceptor cell death and genetic heterogeneity. Mutations in many genes have been implicated in the pathophysiology of RCD, but several others remain to be identified. Herein, we applied whole-exome sequencing to a consanguineous family with one subject affected with RCD and identified a homozygous nonsense mutation, c.226C>T (p.Arg76(∗)), in KIZ, which encodes centrosomal protein kizuna. Subsequent Sanger sequencing of 340 unrelated individuals with sporadic and autosomal-recessive RCD identified two other subjects carrying pathogenic variants in KIZ: one with the same homozygous nonsense mutation (c.226C>T [p.Arg76(∗)]) and another with compound-heterozygous mutations c.119_122delAACT (p.Lys40Ilefs(∗)14) and c.52G>T (p.Glu18(∗)). Transcriptomic analysis in mice detected mRNA levels of the mouse ortholog (Plk1s1) in rod photoreceptors, as well as its decreased expression when photoreceptors degenerated in rd1 mice. The presence of the human KIZ transcript was confirmed by quantitative RT-PCR in the retina, the retinal pigment epithelium, fibroblasts, and whole-blood cells (highest expression was in the retina). RNA in situ hybridization demonstrated the presence of Plk1s1 mRNA in the outer nuclear layer of the mouse retina. Immunohistology revealed KIZ localization at the basal body of the cilia in human fibroblasts, thus shedding light on another ciliary protein implicated in autosomal-recessive RCD.

From confluent human iPS cells to self-forming neural retina and retinal pigmented epithelium.

Progress in retinal-cell therapy derived from human pluripotent stem cells currently faces technical challenges that require the development of easy and standardized protocols. Here, we developed a simple retinal differentiation method, based on confluent human induced pluripotent stem cells (hiPSC), bypassing embryoid body formation and the use of exogenous molecules, coating, or Matrigel. In 2 wk, we generated both retinal pigmented epithelial cells and self-forming neural retina (NR)-like structures containing retinal progenitor cells (RPCs). We report sequential differentiation from RPCs to the seven neuroretinal cell types in maturated NR-like structures as floating cultures, thereby revealing the multipotency of RPCs generated from integration-free hiPSCs. Furthermore, Notch pathway inhibition boosted the generation of photoreceptor precursor cells, crucial in establishing cell therapy strategies. This innovative process proposed here provides a readily efficient and scalable approach to produce retinal cells for regenerative medicine and for drug-screening purposes, as well as an in vitro model of human retinal development and disease.

Modeling PRPF31 retinitis pigmentosa using retinal pigment epithelium and organoids combined with gene augmentation rescue.

Mutations in the ubiquitously expressed pre-mRNA processing factor (PRPF) 31 gene, one of the most common causes of dominant form of Retinitis Pigmentosa (RP), lead to a retina-specific phenotype. It is uncertain which retinal cell types are affected and animal models do not clearly present the RP phenotype observed in PRPF31 patients. Retinal organoids and retinal pigment epithelial (RPE) cells derived from human-induced pluripotent stem cells (iPSCs) provide potential opportunities for studying human PRPF31-related RP. We demonstrate here that RPE cells carrying PRPF31 mutations present important morphological and functional changes and that PRPF31-mutated retinal organoids recapitulate the human RP phenotype, with a rod photoreceptor cell death followed by a loss of cones. The low level of PRPF31 expression may explain the defective phenotypes of PRPF31-mutated RPE and photoreceptor cells, which were not observed in cells derived from asymptomatic patients or after correction of the pathogenic mutation by CRISPR/Cas9. Transcriptome profiles revealed differentially expressed and mis-spliced genes belonging to pathways in line with the observed defective phenotypes. The rescue of RPE and photoreceptor defective phenotypes by PRPF31 gene augmentation provide the proof of concept for future therapeutic strategies.

Establishment of an induced pluripotent stem (iPS) cell line from dermal fibroblasts of an asymptomatic patient with dominant PRPF31 mutation.

A human iPS cell line was generated from fibroblasts of a phenotypically unaffected patient from a family with PRPF31-associated retinitis pigmentosa (RP). The transgene-free iPS cells were generated with the human OSKM transcription factors using the Sendai-virus reprogramming system. iPS cells contained the expected c.709-734dup substitution in exon 8 of PRPF31, expressed the expected pluripotency markers, displayed in vivo differentiation potential to the three germ layers and had normal karyotype. This cellular model will provide a powerful tool to study the unusual pattern of inheritance of PRPF31-associated RP.

Generation of an induced pluripotent stem cell (iPSC) line from a patient with autosomal dominant retinitis pigmentosa due to a mutation in the NR2E3 gene.

A human iPSC line was generated from fibroblasts of a patient affected with autosomal dominant Retinitis Pigmentosa (RP) carrying the mutation p.Gly56Arg in the NR2E3 gene. The transgene-free iPSCs were generated with the human OSKM transcription factors using the Sendai-virus reprogramming system. iPSCs contained the expected c.166G>A substitution in exon 2 of NR2E3, expressed the expected pluripotency markers, displayed in vivo differentiation potential to the three germ layers and had normal karyotype. This cellular model will provide a powerful tool to study the pathogenesis of NR2E3-associated RP. Resource table.

Further Insights into the Ciliary Gene and Protein KIZ and Its Murine Ortholog PLK1S1 Mutated in Rod-Cone Dystrophy.

We identified herein additional patients with rod-cone dystrophy (RCD) displaying mutations in KIZ, encoding the ciliary centrosomal protein kizuna and performed functional characterization of the respective protein in human fibroblasts and of its mouse ortholog PLK1S1 in the retina. Mutation screening was done by targeted next generation sequencing and subsequent Sanger sequencing validation. KIZ mRNA levels were assessed on blood and serum-deprived human fibroblasts from a control individual and a patient, compound heterozygous for the c.52G>T (p.Glu18*) and c.119_122del (p.Lys40Ilefs*14) mutations in KIZ. KIZ localization, documentation of cilium length and immunoblotting were performed in these two fibroblast cell lines. In addition, PLK1S1 immunolocalization was conducted in mouse retinal cryosections and isolated rod photoreceptors. Analyses of additional RCD patients enabled the identification of two homozygous mutations in KIZ, the known c.226C>T (p.Arg76*) mutation and a novel variant, the c.3G>A (p.Met1?) mutation. Albeit the expression levels of KIZ were three-times lower in the patient than controls in whole blood cells, further analyses in control- and mutant KIZ patient-derived fibroblasts unexpectedly revealed no significant difference between the two genotypes. Furthermore, the averaged monocilia length in the two fibroblast cell lines was similar, consistent with the preserved immunolocalization of KIZ at the basal body of the primary cilia. Analyses in mouse retina and isolated rod photoreceptors showed PLK1S1 localization at the base of the photoreceptor connecting cilium. In conclusion, two additional patients with mutations in KIZ were identified, further supporting that defects in KIZ/PLK1S1, detected at the basal body of the primary cilia in fibroblasts, and the photoreceptor connecting cilium in mouse, respectively, are involved in RCD. However, albeit the mutations were predicted to lead to nonsense mediated mRNA decay, we could not detect changes upon expression levels, protein localization or cilia length in KIZ-mutated fibroblast cells. Together, our findings unveil the limitations of fibroblasts as a cellular model for RCD and call for other models such as induced pluripotent stem cells to shed light on retinal pathogenic mechanisms of KIZ mutations.

Human ESC-derived retinal epithelial cell sheets potentiate rescue of photoreceptor cell loss in rats with retinal degeneration.

Replacing defective retinal pigment epithelial (RPE) cells with those derived from human embryonic stem cells (hESCs) or human-induced pluripotent stem cells (hiPSCs) is a potential strategy for treating retinal degenerative diseases. Early clinical trials have demonstrated that hESC-derived or hiPSC-derived RPE cells can be delivered safely as a suspension to the human eye. The next step is transplantation of hESC/hiPSC-derived RPE cells as cell sheets that are more physiological. We have developed a tissue-engineered product consisting of hESC-derived RPE cells grown as sheets on human amniotic membrane as a biocompatible substrate. We established a surgical approach to engraft this tissue-engineered product into the subretinal space of the eyes of rats with photoreceptor cell loss. We show that transplantation of the hESC-RPE cell sheets grown on a human amniotic membrane scaffold resulted in rescue of photoreceptor cell death and improved visual acuity in rats with retinal degeneration compared to hESC-RPE cells injected as a cell suspension. These results suggest that tissue-engineered hESC-RPE cell sheets produced under good manufacturing practice conditions may be a useful approach for treating diseases of retinal degeneration.