PIWI Homologs Mediate Histone H4 mRNA Localization to Planarian Chromatoid Bodies

The well-known regenerative abilities of planarian flatworms are attributed to a population of adult stem cells called neoblasts that proliferate and differentiate to produce all cell types. A characteristic feature of neoblasts is the presence of large cytoplasmic ribonucleoprotein granules named chromatoid bodies, the function of which has remained largely elusive. This study shows that histone mRNAs are a common component of chromatoid bodies. Our experiments also demonstrate that accumulation of histone mRNAs, which is typically restricted to the S phase of eukaryotic cells, is extended during the cell cycle of neoblasts. The planarian PIWI homologs SMEDWI-1 and SMEDWI-3 are required for proper localization of germinal histone H4 (gH4) mRNA to chromatoid bodies. The association between histone mRNA and chromatoid body components extends beyond gH4 mRNA, since transcripts of other core histone genes were also found in these structures. Additionally, piRNAs corresponding to loci of every core histone type have been identified. Altogether, this work provides evidence that links PIWI proteins and chromatoid bodies to histone mRNA regulation in planarian stem cells. The molecular similarities between neoblasts and undifferentiated cells of other organisms raise the possibility that PIWI proteins might also regulate histone mRNAs in stem cells and germ cells of other metazoans

PRMT5 and the Role of Symmetrical Dimethylarginine in Chromatoid Bodies of Planarian Stem Cells

Planarian flatworms contain a population of adult stem cells (neoblasts) that proliferate and generate cells of all tissues during growth, regeneration and tissue homeostasis. A characteristic feature of neoblasts is the presence of chromatoid bodies, large cytoplasmic ribonucleoprotein (RNP) granules morphologically similar to structures present in the germline of many organisms. This study aims to reveal the function, and identify additional components, of planarian chromatoid bodies. We uncover the presence of symmetrical dimethylarginine (sDMA) on chromatoid body components and identify the ortholog of protein arginine methyltransferase PRMT5 as the enzyme responsible for sDMA modification in these proteins. RNA interference-mediated depletion of planarian PRMT5 results in defects in homeostasis and regeneration, reduced animal size, reduced number of neoblasts, fewer chromatoid bodies and increased levels of transposon and repetitive-element transcripts. Our results suggest that PIWI family member SMEDWI-3 is one sDMA-containing chromatoid body protein for which methylation depends on PRMT5. Additionally, we discover an RNA localized to chromatoid bodies, germinal histone H4. Our results reveal new components of chromatoid bodies and their function in planarian stem cells, and also support emerging studies indicative of sDMA function in stabilization of RNP granules and the Piwi-interacting RNA pathway

A Primer on Regneration

The ability to restore lost tissues and body regions, a process known as regeneration, is broadly represented in both plant and animal kingdoms (Birnbaum and Sanchez Alvarado, 2008). Despite a wide phylogenetic distribution of regeneration abilities among metazoans, cumulative studies have identified a conserved series of events that take place during regeneration of complex animal structures (King and Newmark, 2012). Immediately following amputation, an organism recognizes damage and initiates wound healing, which is followed by programmed cell death in the vicinity of tissue damage and subsequent proliferation and migration of cells that foster the development of new tissue. Finally, rearrangement of pre-existing tissue and integration with newly differentiated cells restore the function and proportionality displayed prior to damage. While these conserved processes suggest that the ability to regenerate is ancestrally common (Bely, 2010), heterogeneity exists in some basic mechanisms displayed during regeneration in different animal species. Perhaps one of the most noticeable differences is the cellular source contributing to the formation of new tissue during regeneration. Organisms such as planarians and Hydra rely on active reservoirs of somatic pluripotent stem cells abundantly distributed throughout their bodies while vertebrates rely primarily on progenitor cell activation and dedifferentiation to generate cells with limited potential that then develop specific structures. However, not all regenerative events rely on cellular replacement. Recent research has identified autonomous repair mechanisms and functional regeneration of single cells – be it neurons or ciliated protozoa. The fact that organisms can achieve regeneration through diverse cellular sources is remarkable, but can these processes and conserved molecular pathways be activated to achieve regeneration in species lacking such abilities? Analysis of these pathways will contribute to better understanding of human development and provide potential avenues for regenerative medicine

A Primer on Regneration

The ability to restore lost tissues and body regions, a process known as regeneration, is broadly represented in both plant and animal kingdoms (Birnbaum and Sanchez Alvarado, 2008). Despite a wide phylogenetic distribution of regeneration abilities among metazoans, cumulative studies have identified a conserved series of events that take place during regeneration of complex animal structures (King and Newmark, 2012). Immediately following amputation, an organism recognizes damage and initiates wound healing, which is followed by programmed cell death in the vicinity of tissue damage and subsequent proliferation and migration of cells that foster the development of new tissue. Finally, rearrangement of pre-existing tissue and integration with newly differentiated cells restore the function and proportionality displayed prior to damage. While these conserved processes suggest that the ability to regenerate is ancestrally common (Bely, 2010), heterogeneity exists in some basic mechanisms displayed during regeneration in different animal species. Perhaps one of the most noticeable differences is the cellular source contributing to the formation of new tissue during regeneration. Organisms such as planarians and Hydra rely on active reservoirs of somatic pluripotent stem cells abundantly distributed throughout their bodies while vertebrates rely primarily on progenitor cell activation and dedifferentiation to generate cells with limited potential that then develop specific structures. However, not all regenerative events rely on cellular replacement. Recent research has identified autonomous repair mechanisms and functional regeneration of single cells – be it neurons or ciliated protozoa. The fact that organisms can achieve regeneration through diverse cellular sources is remarkable, but can these processes and conserved molecular pathways be activated to achieve regeneration in species lacking such abilities? Analysis of these pathways will contribute to better understanding of human development and provide potential avenues for regenerative medicine

Annotated \u3cem\u3eGirardia dorotocephala\u3c/em\u3e MA-C2 Transcriptome Sequences

Girardia dorotocephala MA-C2 transcriptome sequences encoding for products with homology to annotated metazoan proteins

PRMT5 and the role of symmetric dimethylarginine in chromatoid bodies of planarian stem cells

Reference transcriptome for Schmidtea mediterranea. File used and referred to by Rouhana et al., (2012), in work published in the journal Development.This work was supported by NIH (R01 HD043403) and NSF (IOS-0744689) awards to P.A.N., and an NSF Minority Postdoctoral Fellowship (Award #0804021) to L.R. P.A.N. is a Howard Hughes Medical Institute Investigator.published or submitted for publicationis peer reviewe

BLAST2GO Homolog, E-value, and GO Annotation for \u3cem\u3eGirardia dorotocephala\u3c/em\u3e MA-C2 Transcriptome Assembly

Excel file listing Top BLAST2GO homolog, E-value, and GO annotation for Girardia dorotocephala MA-C2 transcriptome assembly