HOX and TALE signatures specify human stromal stem cell populations from different sources.

Human stromal stem cell populations reside in different tissues and anatomical sites, however a critical question related to their efficient use in regenerative medicine is whether they exhibit equivalent biological properties. Here, we compared cellular and molecular characteristics of stromal stem cells derived from the bone marrow, at different body sites (iliac crest, sternum, and vertebrae) and other tissues (dental pulp and colon). In particular, we investigated whether homeobox genes of the HOX and TALE subfamilies might provide suitable markers to identify distinct stromal cell populations, as HOX proteins control cell positional identity and, together with their co-factors TALE, are involved in orchestrating differentiation of adult tissues. Our results show that stromal populations from different sources, although immunophenotypically similar, display distinct HOX and TALE signatures, as well as different growth and differentiation abilities. Stromal stem cells from different tissues are characterized by specific HOX profiles, differing in the number and type of active genes, as well as in their level of expression. Conversely, bone marrow-derived cell populations can be essentially distinguished for the expression levels of specific HOX members, strongly suggesting that quantitative differences in HOX activity may be crucial. Taken together, our data indicate that the HOX and TALE profiles provide positional, embryological and hierarchical identity of human stromal stem cells. Furthermore, our data suggest that cell populations derived from different body sites may not represent equivalent cell sources for cell-based therapeutical strategies for regeneration and repair of specific tissues.

Expression of a retinal homeobox (Rx) gene during planarian regeneration.

Retinal homeobox (Rx) genes, with representatives in vertebrates and invertebrates, encode fundamental regulators of early eye and brain formation. Here we describe the spatio-temporal expression profile of a candidate planarian orthologue of Rx during regeneration in Dugesia japonica and Schmidtea mediterranea. Although low levels of Rx transcripts were found throughout the body of intact planarians, high levels of Rx expression were specific to regenerating tissue in both head and tail fragments. We also observed that Rx was never expressed in the simple rhabdomeric planarian eyes, supporting the notion that only formation of eyes that use the ciliary type of photoreceptors requires Rx function.