Distinct and Conserved Prominin-1/CD133–Positive Retinal Cell Populations Identified across Species

Abstract

Besides being a marker of various somatic stem cells in mammals, prominin-1 (CD133) plays a role in maintaining the photoreceptor integrity since mutations in the PROM1 gene are linked with retinal degeneration. In spite of that, little information is available regarding its distribution in eyes of non-mammalian vertebrates endowed with high regenerative abilities. To address this subject, prominin-1 cognates were isolated from axolotl, zebrafish and chicken, and their retinal compartmentalization was investigated and compared to that of their mammalian orthologue. Interestingly, prominin-1 transcripts—except for the axolotl—were not strictly restricted to the outer nuclear layer (i.e., photoreceptor cells), but they also marked distinct subdivisions of the inner nuclear layer (INL). In zebrafish, where the prominin-1 gene is duplicated (i.e., prominin-1a and prominin-1b), a differential expression was noted for both paralogues within the INL being localized either to its vitreal or scleral subdivision, respectively. Interestingly, expression of prominin-1a within the former domain coincided with Pax-6–positive cells that are known to act as progenitors upon injury-induced retino-neurogenesis. A similar, but minute population of prominin-1–positive cells located at the vitreal side of the INL was also detected in developing and adult mice. In chicken, however, prominin-1–positive cells appeared to be aligned along the scleral side of the INL reminiscent of zebrafish prominin-1b. Taken together our data indicate that in addition to conserved expression of prominin-1 in photoreceptors, significant prominin-1–expressing non-photoreceptor retinal cell populations are present in the vertebrate eye that might represent potential sources of stem/progenitor cells for regenerative therapies.

Spatial Distribution of Prominin-1 (CD133) – Positive Cells within Germinative Zones of the Vertebrate Brain

“…Expression of prominin-1 in terminally differentiated neuroepithelium-derived retinal cells or its detection in myelin sheets of oligodendrocytes further supports this notion [35], [45], [70]. The neuroepithelium-derived cuboidal choroid plexus epithelium, however, lacks prominin-1 consistent with a recent case report showing that choroid plexus carcinoma is negative for prominin-1 [72].…”

“…Expression of putative stem- and progenitor markers is often context dependent. Interestingly, other established markers of neural stem/progenitor cells such as Sox2 and msi1 behave similarly prominin-1 inasmuch as they could also be detected in retinal neurons [35], [71]. Therefore a careful evaluation and a combinatorial use of markers either in positive or negative selection paradigms are advisable in identifying stem- and progenitor cells.…”

“…In situ hybridization (ISH) on PFA-fixed cryosections was performed according to a protocol as described [35]. Briefly, serial sections were hybridized with appropriate species-specific digoxygenin (DIG) labeled cRNA probes (see below) at a concentration of 0.5 ng/µl for 16 h at 70°C.…”

“…Combined with the stringent hybridization and post-hybridization conditions, these preclude any cross-hybridization. Dr musashi-1 probe was previously described [35].…”

“…Taking advantage of the molecular information obtained within the mammalian brain [3.33,34], we have addressed whether characterization of the stem cell marker prominin-1 (CD133) could provide essential information, if not a signature, of germinative (proliferating) neurogenic zones of the non-mammalian vertebrate CNS. Recently, prominin-1 orthologs were isolated from non-mammalian vertebrate model organisms (chick, zebrafish, axolotl and frog) providing unique biological tools for such an investigation [35], [36]. By mapping the distribution of their transcripts, the spatial compartmentalization of prominin-1–positive cells was analyzed in relation to proliferating cells.…”