Retinal differentiation, inherited degenerations, and regeneration

• Elucidate the molecular and cellular mechanisms of retinal differentiation using engineered mice
• Model human retinal differentiation and inherited degenerations using pluripotent stem cells

The neuroretina, retinal pigment epithelium (RPE), ciliary body, and iris are structurally and functionally connected in the human adult retina. Inherited degenerations of any tissue will affect the others, leading to blinding retinal disease such as retinitis pigmentosa, age-related macular degenerations, and glaucoma. Macular degenerations affect vision the most, since the macula is responsible for central vision and visual acuity. Human adult neuroretina does not naturally regenerate. Regenerative medicine of the retina holds a promise to save and restore vision.

Elucidating the mechanisms of retinal differentiation is a prerequisite for retinal regeneration. Embryonic development of the neuroretina, RPE, ciliary body, and iris is an integrated process under the regulation of transcription factors and signal transduction molecules. In mice, morphogenesis of optic-cups leads to the specification of neuroretinal and RPE progenitor cells in the inner and outer layer of optic cups at E10.5, respectively. Neuroretina is continuous with RPE via epithelial sheet bending. Close to the bending region, peripheral neuroretina gradually reduces its thickness to form a tapered zone, which is subsequently specified as ciliary margin at E12.5. Neuroretinal progenitor cells are multipotent, producing all retinal neurons and Müller glial cells. Ciliary margin differentiates into ciliary body and iris. How multipotent retinal progenitor cells are regulated in coordination with ciliary margin specification is underexplored. We address the critical knowledge gap by dissecting the molecular functions of homeodomain transcription factors and signaling transduction molecules in retinal differentiation using engineered mice.

The macula is enriched for cone photoreceptors and is unique to primates. The availability and high cost of non-human primates limit their use in retinal disease studies. Macular degenerations are often not closely recapitulated in mouse models because mice do not have the macula. Notably, we recently generated and characterized cone-rich human retinal organoids reminiscent of the macula based on the ratio of cones to rods and single-cell transcriptomes. As a recognition by the field, we recently received an NEI prize for progress toward developing lab-made retinas. We now utilize retinal organoids to model human retinal differentiation and inherited degenerations.

Current projects in my lab:

• To elucidate the mechanisms underlying the regulation of multipotent neuroretinal progenitor cells;
• To determine the mechanisms of photoreceptor cell differentiation;
• To model human retinal differentiation and inherited degenerations using human embryonic stem cells.

Our studies will decipher the mechanisms of retinal differentiation and inherited degenerations, leading to therapeutic development for blinding retinal disease.

NIH-funded positions are available for highly self-motivated postdoctoral research fellow/graduate student.

1. Ferrena, A., Zhang, X., Shrestha, R., Zheng, D., and Liu, W. (2023). Six3 and Six6 jointly regulate the identities and developmental trajectories of multipotent retinal progenitor cells in the mouse retina. BioRxiv https://doi.org/10.1101/2023.05.03.539288.

2. Liu, W¶., Shrestha, R., Lowe, A., Zhang, X., and Spaeth, L. (2023). Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons. Elife, 12. doi:10.7554/eLife.87306. ¶Corresponding author.

    

3. Guo X, Zhou J, Starr C, Mohns EJ, Li Y, Chen E, Yoon Y, Kellner CP, Tanaka K, Wang H, Liu W, LR, Demb JB, Crair MC, and Chen B. “Preservation of vision after CaMKII-mediated protection of retinal ganglion cells.” Published online July 22, 2021 in Cell. DOI: 10.1016/j.cell.2021.06.031

4. Li, B., Zhang, T., Liu, W., Wang, Y., Xu, R., Zeng, S., Zhang, R., Zhu, S., Gillies, M.C., Zhu, L., et al. (2020). Metabolic Features of Mouse and Human Retinas: Rods versus Cones, Macula versus Periphery, Retina versus RPE. iScience 23, 101672.

5. Kim S, Lowe A, Dharmat R…Zhou Z, Chen R, Liu W (2019). PNAS, doi.org/10.1073/pnas.1901572116.
6. Diacou R, Zhao Y, Zheng D, Cvekl A, Liu W (2018) Six3 and Six6 are jointly required for the maintenance of multipotent retinal progenitors through both positive and negative regulation. Cell Reports (2018) 25: 2510-2523. https://doi.org/10.1016/j.celrep.2018.10.106
7. Liu W^¶, Cvekl A (2017) Six3 in a small population of progenitors at E8.5 is required for neuroretinal specification via regulating cell signaling and survival in mice. Developmental Biology. http://dx.doi.org/10.1016/j.ydbio.2017.05.026.        ^¶Corresponding author. 
8. Lowe A, Harris R, Bhansali P, Cvekl A, Liu W. Intercellular adhesion-dependent cell survival and ROCK-regulated actomyosin-driven forces mediate self-formation of a retinal organoid. Stem Cell Reports (2016), http://dx.doi.org/0.1016/j.stemcr.2016.03.011.
9. Liu W, LagutinO, SwindellE, JamrichM and OliverG. Neuroretina specification in mouse embryos requires Six3-mediated suppression of Wnt8b in the anterior neural plate. J of Clin Invest,120: 3568-77, 2010.
10. Liu W. Focus on Molecules: Wnt8b: A suppressor of early eye and retinal progenitor formation. Exp Eye Res 2011 Jan 8. [Epub ahead of print].
11. Liu W, Lagutin OV, Mende M, Streit A and Oliver G. Six3 activation of Pax6 expression is essential for mammalian lens induction and specification. EMBO J 25: 5383-5395, 2006.
12. Geng X, Speirs C, Lagutin O, Inbal A, Liu W, Solnica-Krezel L, Jeong Y, Epstein DJ, Oliver G. Haploinsufficiency of Six3 fails to activate Sonic hedgehog expression in the ventral forebrain and causes holoprosencephaly. Dev Cell 15: 236-247, 2008.
13. Geng X, Lavado A, Lagutin OV, Liu W, Oliver, G. Expression of Six3 Opposite Strand (Six3OS) during mouse embryonic development. Gene Expression Patterns 7: 252-257, 2006.