Adult limbal neurosphere cells: a potential autologous cell resource for retinal cell generation

Niche regulation of limbal epithelial stem cells: HC-HA/PTX3 as surrogate matrix niche

Homeostasis of the corneal epithelium is ultimately maintained by stem cells that reside in a specialized microenvironment within the corneal limbus termed palisades of Vogt. This limbal niche nourishes, protects, and regulates quiescence, self-renewal, and fate decision of limbal epithelial stem/progenitor cells (LEPCs) toward corneal epithelial differentiation. This review focuses on our current understanding of the mechanism by which limbal (stromal) niche cells (LNCs) regulate the aforementioned functions of LEPCs. Based on our discovery and characterization of a unique extracellular matrix termed HC-HA/PTX3 (Heavy chain (HC1)-hyaluronan (HA)/pentraxin 3 (PTX3) complex, "-" denotes covalent linkage; "/" denotes non-covalent binding) in the birth tissue, i.e., amniotic membrane and umbilical cord, we put forth a new paradigm that HC-HA/PTX3 serves as a surrogate matrix niche by maintaining the in vivo nuclear Pax6+ neural crest progenitor phenotype to support quiescence and self-renewal but prevent corneal fate decision of LEPCs. This new paradigm helps explain how limbal stem cell deficiency (LSCD) develops in aniridia due to Pax6-haplotype deficiency and further explains why transplantation of HC-HA/PTX3-containing amniotic membrane prevents LSCD in acute chemical burns and Stevens Johnson syndrome, augments the success of autologous LEPCs transplantation in patients suffering from partial or total LSCD, and assists ex vivo expansion (engineering) of a graft containing LEPCs. We thus envisage that this new paradigm based on regenerative matrix HC-HA/PTX3 as a surrogate niche can set a new standard for regenerative medicine in and beyond ophthalmology.

The Neural Differentiation Potential of Limbal Stem Cells: A Model for Studying the Multipotency of Limbal Stem Cells

PURPOSE

To investigate the multipotency, especially the neural differentiation potential, of limbal stem cells (LSCs) using a coculture system and to provide a relevant literature review.

METHODS

Limbal tissue was harvested from adult New Zealand white rabbits and treated with collagenase A. Small pieces of the resulting limbal epithelial sheets were cocultured with a neuroblastoma cell line (Neuro-2A) in transwells. Morphological observation of the growing epithelial sheets was accomplished by microscopy, and marker expression was detected by immunocytochemistry for ßIII-tubulin and microtubule-associated protein 2. A literature review of associated studies was performed.

RESULTS

In the coculture group, directly adherent colonies of neuron-like (DACN) cells were observed among the growing limbal epithelial sheets from day 3. The DACN cells exhibited neuron-like morphology. The control group comprising limbal cell sheets cultured alone showed a very small number of DACN cells at the end of the culture period (day 14). Immunocytochemical staining revealed that the DACN cells were positive for ßIII-tubulin and microtubule-associated protein 2, confirming the neuronal phenotype of the neuron-like cells. By contrast, the DACN cells in the control group produced negative results. In previous reports, LSCs and niches exhibited neural potential, but most differentiated neural cells were observed as floating spheres, in contrast to the DACN cells observed in the present study.

CONCLUSIONS

We developed a coculture system of LSCs and Neuro-2A neuroblastoma cells and obtained DACN cells with neural differentiation potential. Our findings confirm the neural potential of LSCs, consistent with previous reports, but in a form other than floating spheres.

Pax 6 Controls Neural Crest Potential of Limbal Niche Cells to Support Self-Renewal of Limbal Epithelial Stem Cells

On ocular surface, corneal epithelial stem cells (SC) reside in limbus between cornea and conjunctiva. Pax6, an evolutionally conserved transcription factor essential for eye development, is expressed in post-natal corneal and limbal epithelia progenitors (LEPC) but not in underlying stroma. Because Pax6 is transiently expressed in developing corneal stroma and a subset of limbal and corneal stromal progenitors, we examined the role of Pax6 in limbal niche cells (LNC) in maintaining the phenotype of neural crest (NC) progenitors to support LEPC. Our results showed that nuclear Pax6 staining was found in freshly isolated LNC but not corneal stromal cells. Serial passaged LNC resulted in gradual loss of nuclear Pax6 (46 kDa) staining and neural crest progenitor status defined by the expression of embryonic SCs and NC markers, neurosphere formation, and differentiation into neurons, oligodendrocytes and astrocytes. Gain of function of 46 kDa Pax6 in late-passaged LNC resulted in nuclear Pax6 staining and promotion of the aforementioned NC progenitor status. In an in vitro reunion assay, early passaged LNC and late passaged LNC with overexpression of Pax6 inhibited the expression of corneal epithelial differentiation marker and promoted holoclone by LEPC. Therefore, expression of nuclear 46 kDa Pax6 in LNC plays an important developmental role in maintaining NC progenitor status to support self-renewal of corneal epithelial SCs in the limbal niche.

Müller Cells Derived from Adult Chicken and Mouse Retina Neurospheres Acquire the Dopaminergic Phenotype

Neurospheres prepared from multipotent progenitors in the retina obtained from postnatal mice differentiate into neurons and Müller glia (De Melo Reis et al., in Cell Mol Neurobiol 31:835-846, 2011). Here, we investigated whether neurospheres prepared from adult chickens (ciliary marginal zone, CMZ) or (ciliary body) retina could also lead to differentiated neurons and glia. Neurospheres were prepared from post-hatched chickens or from adult mice after 7 days in the presence of mitogenic factors (FGFb, insulin, and EGF), generating neurons and glial cells. In addition, Müller (2M6 or glutamine synthetase positive cells) derived from post-hatch chicken CMZ neurospheres displayed the dopaminergic phenotype. Furthermore, we observed that Müller cells derived from adult chickens and mice retina neurospheres released significant amounts of dopamine as well as of its metabolites. Taken together, our data lead us to conclude that as for embryonic (chick) or newborn (mouse), the dopaminergic phenotype is a default condition of Müller glial cells obtained from neurospheres prepared from mature retina. Our data raise the possibility that Müller cells from differentiated tissue could be used to ameliorate neurodegenerative diseases involving dopaminergic dysfunction as in Parkinson's disease as shown previously (Stutz et al., in J Neurochem 128:829-840, 2014).