Totipotent stem cells and germ line origin in flatworms

An apparently totipotent stem cell system is known to exist in all Platyhelminthes a the recently from them separated Acoelomorpha. Pluri- or totipotent stem cells (neoblasts) are diploid and remain mitotically active throughout adult life and are responsible for physiological cell renewal and the exceptional flatworm regenerative abilities, which are well-known in medicine from tapeworms and flukes. The embryonic origin of the stem cell system remains unresolved. In the phylum Platyhelminthes two totipotent cell lines have to be produced during development of sexually reproducing flatworms - germ cells and totipotent stem cells (neoblasts). In many organisms primordial germ cells are specified early in embryogenesis to generate gametes during later development. From these germline stem cells a new organism develops by fusion of gamets and gives rise to a new generation. However, studies on the embryonic development of turbellarian flatworms have not been able to identify a segregation of a germline. Instead, all indirect evidence suggests that the neoblast may indeed be the scource for both germline and somatic stem cells. We characterize 15 stem cell and germline specific genes of Macrostomum lignano and the acoel Convoluta pulchra. Partial sequences of these genes are available form PCR/RACE experiments, from the sequencing of 8010 or 9393 ESTS of Macrostomum and Convoluta and from a subtraction library of Macrostomum. Six additional genes will be selected from EST sequencing of embryonic libraries of Macrostomum and Convoluta. We will study the expression of these stem cell and germ cell markers by in situ hybridization and antibody stainings. Preliminary results on the germline marker vasa showed strong expression in both gonads, specific antibodies to study the presence of the respective proteins will be available soon. the production of additional antibodies is planned. Functional studies by RNA interference are presently performed during development and regeneration. In summary, the morphological and functional analysis of highly conserved stem cell and germ cell markers in two very basal bilaterians will provide insight on the evolutionary origin of stem cell systems. Because of the high degree of conservation of the genes involved the obtained results will also be very relevant for other organisms and for humans.

When stem cells divide they have the potential to remain stem cells or differentiate into various cell types of an organism. The question how stem cells maintain their undifferentiated state and how they are directed to differentiate into a certain lineage is of central interest in biomedical research. Flatworms (Platyhelminthes) have been shown to be useful model organisms to study stem cell biology. They possess totipotent stem cells - called neoblasts - which are able to differentiate into all cell types of the animals and can also give rise to the germ line. These stem cells are responsible for tissue maintenance in adult animals and account for the enormous capacity to regenerate a whole animal from a tiny piece of tissue. We established two flatworm species - Macrostomum lignano and Isodiametra pulchra as model systems to address specific questions related with stem cell biology, germ cell development, reproduction, aging, and evolution. We identified several genes which regulate stem cells and germ cells in flatworms. We analyzed the expression and function of those genes in adults, during development, regeneration, and starvation. We further developed a method to specifically eliminate stem cells using irradiation. Our results contribute to a better understanding how stem cells and germ cells are regulated in flatworms and in higher organisms including humans. Defects in the corresponding human stem cell genes lead to cancer or sterility when germ cell genes are affected. Furthermore, the large phylum of the Platyhelminthes includes also parasitic species such as Schistosoma mansoni - the cause of Bilharzia - and various other animal and human pathogenic tapeworms. Our findings might help in the development of antihelminthic drugs. In addition, we were able to address the question of the origin of all higher organisms. One of the species we analysed holds a central position in animal evolution and is supposed to be the grand-grandfather of higher life on earth.