Characterizing the phenotype of murine epidermal progenitor cells: complementary whole-mount visualization and flow cytometry strategies.

The epidermis and its appendages, the hair follicle and sebaceous gland, have the capacity to constantly regenerate throughout adult life. Postnatal hair follicles undergo a cyclic mode of tissue homeostasis, defined by periods of growth, degeneration, and rest. A multipotent population of stem cells residing within the hair follicle bulge not only generates the hair lineages during each hair cycle, but also transiently contributes to the repair of epidermis following wounding. In this chapter, we provide methods for identifying epidermal stem cells and investigating their proliferative and apoptotic characteristics. We introduce whole-mount and flow cytometry techniques, which complement each other by permitting visualization of the epidermal stem cell compartment in situ and assessment of the phenotype of purified cells. These techniques can easily be adapted to characterize novel putative epidermal stem or progenitor cell populations. By applying whole-mount and flow cytometry techniques to characterize normal and genetically modified mice with skin defects, we expect to learn more about the factors that regulate stem cell self-renewal and differentiation.

NKX6 transcription factor activity is required for alpha- and beta-cell development in the pancreas.

In diabetic individuals, the imbalance in glucose homeostasis is caused by loss or dysfunction of insulin-secreting beta-cells of the pancreatic islets. As successful generation of insulin-producing cells in vitro could constitute a cure for diabetes, recent studies have explored the molecular program that underlies beta-cell formation. From these studies, the homeodomain transcription factor NKX6.1 has proven to be a key player. In Nkx6.1 mutants, beta-cell numbers are selectively reduced, while other islet cell types develop normally. However, the molecular events downstream of NKX6.1, as well as the molecular pathways that ensure residual beta-cell formation in the absence of NKX6.1 are largely unknown. Here, we show that the Nkx6.1 paralog, Nkx6.2, is expressed during pancreas development and partially compensates for NKX6.1 function. Surprisingly, our analysis of Nkx6 compound mutant mice revealed a previously unrecognized requirement for NKX6 activity in alpha-cell formation. This finding suggests a more general role for NKX6 factors in endocrine cell differentiation than formerly suggested. Similar to NKX6 factors, the transcription factor MYT1 has recently been shown to regulate alpha- as well as beta-cell development. We demonstrate that expression of Myt1 depends on overall Nkx6 gene dose, and therefore identify Myt1 as a possible downstream target of Nkx6 genes in the endocrine differentiation pathway.

Endodermal expression of Nkx6 genes depends differentially on Pdx1.

Nkx family members are essential for normal development of many different tissues such as the heart, lungs, thyroid, prostate, and CNS. Here, we describe the endodermal expression pattern of three Nkx6 family genes of which two shows conserved expression in the early pancreatic epithelium. In chicken, Nkx6.1 expression is not restricted to the presumptive pancreatic area but is more broadly expressed in the endoderm. In mice, expression of Nkx6.1 is restricted to the pancreatic epithelium. In both mice and chicken, Nkx6.2 and Pdx1 are expressed in very similar domains, identifying Nkx6.2 as a novel marker of pancreas endoderm. Additionally, our results show that Nkx6.3 is expressed transiently in pancreatic endoderm in chicken but not mouse embryos. At later stages, Nkx6.3 is found in the caudal stomach and rostral duodenum in both species. Finally, we demonstrate that Pdx1 is required for Nkx6.1 but not Nkx6.2 expression in mice and that ectopic Pdx1 can induce Nkx6.1 but not Nkx6.2 or Nkx6.3 expression in anterior chicken endoderm. These results demonstrate that Nkx6.1 lies downstream of Pdx1 in a genetic pathway and that Pdx1 is required and sufficient for Nkx6.1 expression in the early foregut endoderm.