Collectively our studies have demonstrated

Collectively, our studies have demonstrated a paradigm for direct conversion of multipotent iENPs from human somatic somatostatin receptor through overexpression of hESC-NP-enriched TFs. This system will allow generation of expandable iNP populations with desirable neural differentiation propensities (Figure 6G) and also facilitate the discovery of disease mechanisms and drugs for treatment of neurodegenerative diseases and use in regenerative medicine.
Experimental Procedures
Author Contributions
Introduction Neonatal and fetal retina sheet transplants are reported to restore activity in host retinal ganglion cells or superior colliculus, a midbrain visual center for motor commands, in some mice and rats with retinal degeneration (Woch et al., 2001; Radner et al., 2001; Sagdullaev et al., 2003), and fetal retinas have been transplanted together with retinal pigment epithelium into patients with retinal degeneration and improved vision in some patients (Radtke et al., 2008). Mechanisms have been suggested as neurotrophic effect (Radner et al., 2001) or synaptic connections between unspecified inner retinal cells of host and graft (Seiler et al., 2008, Seiler et al., 2010), but there is no conclusive evidence that photoreceptors in these retina transplants can form functional synapses with host bipolar cells. Recently, Pearson et al. (2012) demonstrated that postnatal photoreceptor precursor cells can functionally integrate into the outer nuclear layer (ONL) of Gnat−/− mice and restore visual function; in these mice, rod photoreceptors are not functional but transplanted rod cells can restore scotopic visual function in a dose-dependent manner. Gonzalez-Cordero et al. (2013) then reported that mouse embryonic stem cell (mESC)-derived photoreceptor precursors can integrate into the ONL of mice retina. These reports, together with a number of reports describing protocols to differentiate retinal tissue from human ESCs or induced pluripotent stem cells (iPSCs) (Kuwahara et al., 2015; Nakano et al., 2012; Zhong et al., 2014), provided a basis for developing cell-based therapies for retinal degenerative diseases. More recently, however, Pearson’s and another group have reinterpreted their work, as new evidence has emerged that the functional restoration after photoreceptor transplantation had been more likely the result of biomaterial transfer from the transplanted cells to the local photoreceptor cells in the host ONL, rather than direct integration of the graft cells (Pearson et al., 2016; Santos-Ferreira et al., 2016). These reports brought us back to the initial question of whether the transplanted photoreceptors could make synapses with adult host retinal cells. In clinical practice, cell-based therapies would primarily target end-stage retinas that have lost the ONL, leaving the secondary retinal neurons missing their partners for signal input. Therefore, the end-stage retinas can be considered to be in a different environment for graft cells from those of disease models retaining ONL that were used in the previous studies. Recent studies using cell suspensions of postnatal mouse photoreceptor precursors or human ES/iPS derived photoreceptor precursors in end-stage retinas, which have lost the ONL, indicated possible light response by pupillary reflex and behavior tests, although direct evidence of light response from the graft cells or synaptic function is still lacking. These graft photoreceptors did not develop mature morphology with outer segments or organized ONL structure that is important for photoreceptors to efficiently respond to light (Barnea-Cramer et al., 2016; Singh et al., 2013). In addition, rat fetal retina sheet grafts (Sagdullaev et al., 2003) apparently survive longer than mouse cell suspensions (Mandai et al., 2012; West et al., 2010), and a fetal retinal graft sheet in a clinical trial was observed to survive 3 years after the transplantation, while transplants in the form of microaggregates were no longer detected (del Cerro et al., 2000). Reconstruction of a structured ONL would definitely be ideal in these cases, but it has not been clearly demonstrated that an ES/iPS-derived structured, retina-like sheet can restore visual function. The difficulty of proving that visual function is present in mice and rats adds to the challenge of developing effective therapies for retinal degeneration.