Analysis of Sir2E in the cellular slime moldDictyostelium discoideum: Cellular localization, spatial expression and overexpression

Sir2D, a Sirtuin family protein, regulates adenylate cyclase A expression through interaction with the MybB transcription factor early in Dictyostelium development upon starvation

Sirtuin interacts with many regulatory proteins involved in energy homeostasis, DNA repair, cell survival, and lifespan extension. We investigated the functional roles of Sir2D during early Dictyostelium development upon starvation. We found that ectopic expression of Sir2D accelerated development among three Sirtuins containing highly homologous catalytic domain sequences to mouse Sirt1. Sir2D expression upregulated adenylate cyclase A (aca) mRNA expression 2, 4 and 6 h after starvation. We have previously reported that nicotinamide, a Sirt1 inhibitor, treatment delayed the development and decreased the expression of aca at 4 h after starvation. Sir2D expressing cells showed resistance against the nicotinamide effect. RNAi-mediated Sir2D knockdown cells were generated, and their development was also delayed. Aca expression was decreased 4 h after starvation. Sir2D expression restored the developmental impairment of Sir2D knockdown cells. The induction of aca upon starvation starts with transcriptional activation of MybB. The ectopic expression of MybB accelerated the development and increased the expression of aca 2 and 4 h after starvation but did not restore the phenotype of Sir2D knockdown cells. Sir2D expression had no effects on MybB-null mutant cells during early development. Thus, MybB is necessary for the upregulation of aca by Sir2D, and Sir2D is necessary for the full induction of aca after 4 h by MybB. MybB was coimmunoprecipitated with Sir2D, suggesting an interaction between MybB and Sir2D. These results suggest that Sir2D regulates aca expression through interaction with the MybB transcription factor early in Dictyostelium development upon starvation.

ABC transporters in Dictyostelium discoideum development

ATP-binding cassette (ABC) transporters can translocate a broad spectrum of molecules across the cell membrane including physiological cargo and toxins. ABC transporters are known for the role they play in resistance towards anticancer agents in chemotherapy of cancer patients. There are 68 ABC transporters annotated in the genome of the social amoeba Dictyostelium discoideum. We have characterized more than half of these ABC transporters through a systematic study of mutations in their genes. We have analyzed morphological and transcriptional phenotypes for these mutants during growth and development and found that most of the mutants exhibited rather subtle phenotypes. A few of the genes may share physiological functions, as reflected in their transcriptional phenotypes. Since most of the abc-transporter mutants showed subtle morphological phenotypes, we utilized these transcriptional phenotypes to identify genes that are important for development by looking for transcripts whose abundance was unperturbed in most of the mutants. We found a set of 668 genes that includes many validated D. discoideum developmental genes. We have also found that abcG6 and abcG18 may have potential roles in intercellular signaling during terminal differentiation of spores and stalks.

The histone methyltransferase Dot1 is required for DNA damage repair and proper development in Dictyostelium

Posttranslational histone modifications play an important role in modulating gene expression and chromatin structure. Here we report the identification of histone H3K79 dimethylation in the simple eukaryote Dictyostelium discoideum. We have deleted the D. discoideum Dot1/KMT4 homologue and demonstrate that it is the sole enzyme responsible for histone H3K79me2. Cells lacking Dot1 are reduced in growth and delayed in development, but do not show apparent changes in cell cycle regulation. Furthermore, our results indicate that Dot1 contributes to UV damage resistance and DNA repair in D. discoideum. In summary, the data support the view that the machinery controlling the setting of histone marks is evolutionary highly conserved and provide evidence that D. discoideum is a suitable model system to analyze these modifications and their functions during development and differentiation.

Silent information regulator 2 proteins encoded by Cryptosporidium parasites

Screening in a database has revealed that Cryptosporidium hominis encodes a silent information regulator 2 (Sir2), a nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase. Cellular localization of the protein, ChSir2, was analyzed by the use of the social amoeba Dictyostelium discoideum as a model system. Fluorescent microscopic analysis showed that ChSir2 fused with green fluorescent protein was localized in the D. discoideum nucleus. D. discoideum expressing ChSir2 grew faster and reached higher cell density than did D. discoideum harboring a control vector. These results suggest that ChSir2 is a nucleus-localizing protein that plays an important role in the growth of C. hominis. We cloned and sequenced the genes for Sir2 orthologs encoded by three isolates of C. hominis, two isolates of Cryptosporidium parvum and one isolate of Cryptosporidium meleagridis. The orthologs conserve critical catalytic or NAD-binding residues but do not have similarity with human Sir2 proteins (SIRTs). Cryptosporidium Sir2 orthologs would therefore be attractive therapeutic targets. The Cryptosporidium orthologs were classified into four variants based on their nucleotide sequences. Each of the four variants produces its own unique restriction fragment length polymorphism pattern by digestion with TfiI.