Regeneration is a process that renews damaged or lost cells, tissues, or even of entire body structures, and is a phenomenon which is widespread in the animal kingdom. Urodeles such as newts and salamanders have a remarkable regeneration ability. They can regenerate organs such as gills, lower jaws, retina, appendages like fore- and hind limbs, and also the tail including the spinal cord.
The regeneration process requires the use of resident stem cells or somatic cells, which have to be reprogrammed. In both cases the reprogrammed cells are less differentiated, meaning the cell would have the ability to form any kind of fetal or adult cell which rose from the three different germ layers, the ectoderm, mesoderm and endoderm. Artificial reprogramming of differentiated mammalian somatic cell had been reported previously. It was shown that four pluripotency factors, OCT4 (also called POU5f1), SOX2, c-MYC and KLF4 are sufficient to generate an induced pluripotent stem (iPS) cell. It has been shown that some of these factors are also involved in regenerating processes. In newt limb and lens tissue, Sox2, c-Myc and Klf4 mRNA levels were upregulated in the beginning of blastema formation when compared to non-amputated tissue. Oct4 mRNA however, was not detected. During xenopus tail regeneration, Sox2 and c-Myc were expressed, while the xenopus Pou homologs Pou25, Pou60, Pou79, Pou91 were not detected. In regenerating zebrafish fin tissue, Sox2, Pou2, c-Myc and Klf4 mRNA were not upregulated.
The mammalian transcription factor OCT4, a class V POU protein, is responsible in maintaining pluripotency in gastrula stage embryos. It was reported that mouse OCT4 is also expressed in the caudal node of embryos having 16 somites. It is further known that progenitors exist in mouse tailbud, which give rise to neural and mesodermal cell lineage. This suggests that the OCT4 expressing cells in caudal node might be a stem cell reservoir. Oct4 was detected in axolotl during embryonic development, and prior to my work we found Oct4 when screening the axolotl blastema cDNA library. In addition, we also identified Pou2, another class V POU gene. Phylogenetic analysis showed a clear distinction of both genes in the axolotl. We determined the mRNA pattern of Pou2 during embryogenesis and compared it to Oct4 mRNA and protein. Both genes are expressed in the primordial germ cells and the pluripotent animal cap region of the embryo. Apart from this similarity, both genes have a different expression pattern in the embryo. We are interested in the involvement of OCT4, POU2, as well as the transcription factor SOX2 in regenerating axolotl spinal cord. We asked whether the cellular pluripotent character conferred by POU factors is limited to mammals or if it is an ancient characteristic of lower vertebrates. To answer the question we performed in vitro and in vivo studies. Hence this thesis is separated into two chapter.
By in vitro studies we investigated the pluripotent PouV orthologs from different species. Therefore, we performed reprogramming experiments using mouse or human fibroblasts and transduced them with axolotl Oct4 or Pou2, in combination with human or axolotl Sox2, c-Myc and/or Klf4. The generated iPS cells with the different sets of factors had similar endogenous pluripotency gene expression profiles to embryonic stem cells. Further, iPS cells expressed the pluripotency markers like OCT4, NANOG, SSEA4, TRA1-60 and TRA1-81. Another evaluation of the iPS cells was the formation of embryoid bodies. Immunouorescence staining showed that tissue from all three germ layers was formed after induction. We observed a positive staining for the endoderm marker !-FEROPROTEIN, the mesoderm marker !-SMOOTH MUSCLE ACTIN and the ectoderm marker \"III TUBULIN in the generated cells. This indicated that the iPS cells generated using axolotl Oct4 and Sox2 in combination with mammalian Klf4 and with or without c-Myc, as well as iPS cell generated with axolotl Pou2 and mammalian Sox2 and Klf4 and with or without c-Myc have a pluripotent potential. In addition, the axolotl factors are able to form heterodimers with the mammalian proteins.
Furthermore, we compared the reprogramming ability with POU factors from mouse, human, zebrash, medaka and xenopus. We showed that xenopus Pou91, as the only non-mammalian example, is nearly as efficient as mouse and human Oct4 cDNAs in inducing GFP expressing cells. Also axolotl Pou2, axolotl Oct4 and medaka Pou2 showed reprogramming character however at a much lower efficiency. In contrast, zebrash Pou2 is not able to establish iPS cells. This indicates that a reprogramming ability to a pluripotent cell state is an ancient trait of Pou2 and Oct4 homologs.
By in vivo studies we investigated the role of Oct4, Pou2 and Sox2 gene expression in regenerating spinal cord tissue. Performed in situ hybridizations and antibody staining studies in the regenerating spinal cord showed that Oct4, Pou2 and Sox2 were expressed during spinal cord regeneration. Knockdown experiments in regenerating spinal cord using morpholino showed that Pou2-morpholino does not have an effect. In contrast, SOX2 was required for spinal cord regeneration but to a lesser extent, than OCT4, which decreased the regenerated length signicantly compared to control. Even though, with Sox2-morpholino we did not observe the phenotype as a significantly shorter regenerated spinal cord, about 45% of SOX2 knocked down cells were not cycling and proliferating anymore. This indicates that axolotl SOX2 has an effect in regeneration.
Therefore we wanted to know whether spinal cord cells would also have a pluripotent character in vivo and form other tissue types. Regenerating cells of the spinal cord are only able to form the same cell type and thus they keep their cell memory. However, when we performed transplantations of OCT4/SOX2 expressing spinal cord cells into somite stage embryos, we could show the formation of muscle cells. This shows that the spinal cord cells have the potential to change their fate in an embryonic context, where the normal environment of spinal cord has changed. However, our data do not indicate whether muscle is formed directly from the spinal cord or whether spinal cord cells fuse to developmental myoblasts, a cell type of embryonic progenitors, which give rise to muscle cells. To clearly state whether regenerating OCT4/SOX2 expressing spinal cord cells are pluripotent we have to perform OCT4 knock down in spinal cord and transplant these less proliferating cells into embryos, observing their cell fate.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:28061 |
| Date | 25 June 2013 |
| Creators | Duemmler, Annett |
| Contributors | Tanaka, Elly M., Ferretti, Patrizia, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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