The maintenance of developmental potential and cell fate plasticity in a C. elegans stem cell model

Tissue-specific stem cells divide to replace cells lost due to injury or normal wear and tear, replenishing the tissue as needed. The development of many stem cells and progenitor cells is interrupted by a period of quiescence, which is a reversible non-proliferating state. Quiescent progenitor cells must “remember” their precise place in their developmental program, neither differentiating prematurely, nor losing their tissue identity. We use the microscopic nematode C. elegans to investigate how this is accomplished. When young C. elegans larvae are cultured in adverse environmental conditions they pause their development by entering the quiescent and stress-resistant dauer larva stage. If conditions improve, dauer larvae recover and complete development normally. This indicates that progenitor cells in wild-type dauer larvae maintain their developmental potential, or the ability to give rise to all proper cell types. We are using two different cell types in C. elegans to study how cells maintain developmental potential during dauer. Our work is funded by NIH and NSF.

Why use C. elegans as a model?

Caenorhabditis elegans is a free-living, microscopic nematode. C. elegans is a popular model system due to its simplicity, ease of handling, and amenability to genetic analysis. Because genetic pathways are so well conserved from worms to mammals, much of what is discovered in the worm is relevant to human disease. To date, three Nobel Prizes have been awarded for C. elegans research (2002, 2006, 2008)

One major advantage to using C. elegans as a model system is their essentially invariant cell lineage. Adult C. elegans hermaphrodites have precisely 959 somatic cells that arise from nearly identical cell divisions in each individual. Because C. elegans is transparent, each cell division has been observed and documented. Furthermore, fluorescent cell fate markers highlight particular cells at particular stages in their developmental program. Thus, cell fate can be followed at the single cell level, and any deviations from wild-type can be readily observed.

C. elegans as a stem cell model

Adult stem cells typically remain quiescent for long periods, dividing only as needed to maintain tissue integrity. In order to contribute to tissue renewal, stem cells remain multipotent and plastic, meaning they can give rise to multiple, specific cell-types. Certain C. elegans progenitor cells share similar properties, including quiescence and multipotency. C. elegans larvae develop continuously through four larval stages punctuated by molts. During each stage, progenitor cells divide and contribute to the growth of the worm. In adverse environments (crowding, insufficient food, higher temperatures), C. elegans larvae can arrest their development in the stress-resistant dauer larva stage. Dauer formation occurs immediately after the second larval molt. Since larval development is incomplete at this point, dauer larvae possess progenitor cells with the capacity to give rise to various tissues, including the lateral hypodermis (skin), and the vulva, among others. All cells within dauer larvae are quiescent throughout dauer arrest, which can last from hours to months. This time span is remarkable given that the normal C. elegans lifespan is only a few weeks. If dauer larvae encounter favorable environments they will recover, complete development normally, and live a normal lifespan on top of the time they spent in dauer.

Lateral hypodermis: post-dauer timing and microRNA activity

One cell-type used as a stem-cell model is the lateral hypodermal seam cells that form part of the worm skin. These cells divide each larval stage in a characteristic pattern that includes self-renewal as well as the production of other skin cell types. The stage at which each cell division pattern occurs is called “stage-specific cell fate”. Stage-specific cell fate is controlled by a gene network called “heterochronic”. Mutations in heterochronic genes cause two types of defect: precocious and reiterative. Often, the precocious genes encode transcription factors that specify early cell fate, whereas reiterative genes encode microRNAs that downregulate expression of the transcription factors.

Schematic of stage-specific cell fates in wild-type, precocious and reiterative mutants. After dauer, mutant phenotypes are corrected.

Surprisingly, many heterochronic genes that are important during continuous development are dispensable after dauer. Some of the differences between phenotypes during continuous and post-dauer development can be explained by modulation of microRNA activity, making dauer an excellent system to probe the biological relevance of microRNA pathways and their regulation. We are interested in unraveling the gene network that operates in post-dauer worms, including microRNA pathways. To accomplish this we are probing the role of candidate genes as well as performing unbiased genetic screens for mutants defective in post-dauer timing.

Vulval precursor cells: signal transduction pathways and cell fate plasticity

A second cell type studied in the Karp lab is vulval precursor cells. These six cells are born in the L1 stage and remain multipotent until specification occurs during the L3 stage. During dauer, a mechanism exists to maintain plasticity by blocking the activity of signal transduction pathways, and erasing prior specification. This mechanism involves DAF-16, the C. elegans ortholog of FOXO transcription factors, proteins with conserved roles in longevity and stress-resistance. We are interested in elucidating the mechanisms by which daf-16 promotes cell fate plasticity during dauer, including discovering the genes that DAF-16 regulates controlling plasticity.