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Planarian's strong regenerative ability is dependent on stem cells (called neoblasts) that are X-ray-sensitive and proliferative stem cells. In addition to neoblasts, another type of X-ray-sensitive cells was newly identified by recent research. Thus, planarian's X-ray-sensitive cells can be divided into at least two populations, Type 1 and Type 2, the latter corresponding to planarian's classically defined “neoblasts”. Here, we show that Type 1 cells were distributed in the outer region (OR) immediately underneath the muscle layer at all axial levels from head to tail, while the Type 2 cells were distributed in a more internal region (IR) of the mesenchymal space at the axial levels from neck to tail. To elucidate the biological significance of these two regions, we searched for genes expressed in differentiated cells that were locate close to these X-ray-sensitive cell populations in the mesenchymal space, and identified six genes mainly expressed in the OR or IR, named OR1, OR2, OR3, IR1, IR2 and IR3. The predicted amino acid sequences of these genes suggested that differentiated cells expressing OR1, OR3, IR1, or IR2 provide Type 1 and Type 2 cells with specific extracellular matrix (ECM) environments.

The neoblasts are the only somatic stem cells in planarians possessing pluripotency, and can give rise to all types of cells, including germline cells. Recently, accumulated knowledge about the transcriptome and expression dynamics of various pluripotent somatic stem cells has provided important opportunities to understand not only fundamental mechanisms of pluripotency, but also stemness across species at the molecular level. The neoblasts can easily be eliminated by radiation. Also, by using fluorescence activated cell sorting (FACS), we can purify and collect many neoblasts, enabling identification of neoblast-related genes by comparison of the gene expression level among intact and X-ray-irradiated animals, and purified neoblasts. In order to find such genes, here we employed the high coverage expression profiling (HiCEP) method, which enables us to observe and compare genome-wide gene expression levels between different samples without advance sequence information, in the planarian D. japonica as a model organism of pluripotent stem cell research. We compared expression levels of ~17,000 peaks corresponding to independent genes among different samples, and obtained 102 peaks as candidates. Expression analysis of genes identified from those peaks by in situ hybridization revealed that at least 42 genes were expressed in the neoblasts and in neoblast-related cells that had a different distribution pattern in the body than neoblasts. Also, single-cell PCR analysis of those genes revealed heterogeneous expression of some genes in the neoblast population. Thus, using multidimensional gene expression analyses, we were able to obtain a valuable data set of neoblast-related genes and their expression patterns.

Planarians are attractive animals in which various questions related to the central nervous system (CNS) can be addressed, such as its origin and evolution, its degree of functional conservation among different organisms, and the plasticity and regenerative capabilities of neural cells and networks. However, it is first necessary to characterize at the gene expression level how this CNS is organized in intact animals. Previous studies have shown that the planarian brain can be divided into at least three distinct domains based on the expression of otd/Otx-related genes. In order to further characterize the planarian brain, we have recently isolated a large number of planarian neural-specific genes through DNA microarrays and ESTs projects. Here, we describe new molecular domains within the brain of intact planarians by the expression of 16 planarian neural-specific genes, including the putative homologues of protein tyrosine phosphatase receptor, synaptotagmin VII, slit, G protein and glutamate and acetylcholine receptors, by in situ hybridization in both whole-mount and transverse sections. Our results indicate that planarian otd/Otx-positive domains can be further subdivided into distinct molecular regions according to the expression of different neural genes. We found differences at the gene expression level between the dorsal and ventral sides of the brain, along its antero-posterior axis and also between the proximal and distal parts of the brain lateral branches. This high level of regionalization in the planarian brain contrasts with its apparent simplicity at the morphological level.

A novel NADH-dependent glyoxylate reductase has been found in a hyperthermophilic archaeon Thermococcus litoralis DSM 5473. This is the first evidence for glyoxylate metabolism and its corresponding enzyme in hyperthermophilic archaea. NADH-dependent glyoxylate reductase was purified approximately 560-fold from a crude extract of the hyperthermophile by five successive column chromatographies and preparative PAGE. The molecular mass of the purified enzyme was estimated to be 76 kDa, and the enzyme consisted of a homodimer with a subunit molecular mass of approximately 37 kDa. The optimum pH and temperature for enzyme activity were approximately 6.5 and 90 degrees C, respectively. The enzyme was extremely thermostable; the activity was stable up to 90 degrees C. The glyoxylate reductase catalyzed the reduction of glyoxylate and hydroxypyruvate, and the relative activity for hydroxypyruvate was approximately one-quarter that of glyoxylate in the presence of NADH as an electron donor. NADPH exhibited rather low activity as an electron donor compared with NADH. The Km values for glyoxylate, hydroxypyruvate, and NADH were determined to be 0.73, 1.3 and 0.067 mM, respectively. The gene encoding the enzyme was cloned and expressed in Escherichia coli. The nucleotide sequence of the glyoxylate reductase gene was determined and found to encode a peptide of 331 amino acids with a calculated relative molecular mass of 36,807. The amino-acid sequence of the T. litoralis enzyme showed high similarity with those of probable dehydrogenases in Pyrococcus horikoshii and P. abyssi. The purification of the enzyme from recombinant E. coli was much simpler compared with that from T. litoralis; only two steps of heat treatment and dye-affinity chromatography were needed.

The growth in size of freshwater planarians in response to nutrient intake is limited by the eventual separation of tail and body fragments in a process called fission. The resulting tail fragment regenerates the entire body as an artificially amputated tail fragment would do, and the body fragment regenerates a tail, resulting in two whole planarians. This regenerative ability is supported by pluripotent somatic stem cells, called neoblasts, which are distributed throughout almost the entire body of the planarian. Neoblasts are the only planarian cells with the ability to continuously proliferate and give rise to all types of cells during regeneration, asexual reproduction, homeostasis, and growth. In order to investigate the molecular characteristics of neoblasts, we conducted an extensive search for neoblast-specific genes using the High Coverage Expression Profiling (HiCEP) method, and tested the function of the resulting candidates by RNAi. Disruption of the expression of one candidate gene, DjP2X-A (Dugesia japonica membrane protein P2X homologue), resulted in a unique phenotype. DjP2X-A RNAi leads to an increase of fission events upon feeding. We confirmed by im-munohistochemistry that DjP2X-A is a membrane protein, and elucidated its role in regulating neoblast proliferation, thereby explaining its unique phenotype. We found that DjP2X-A decreases the burst of neoblast proliferation that normally occurs after feeding. We also found that DjP2X-A is required for normal proliferation in starved animals. We propose that DjP2X-A modulates stem cell proliferation in response to the nutritional condition.