Telomeric repeat silencing in germ cells is essential for early development in Drosophila

The germline-specific role of telomeres consists of chromosome end elongation and proper chromosome segregation during early developmental stages. Despite the crucial role of telomeres in germ cells, little is known about telomere biology in the germline. We analyzed telomere homeostasis in the Drosophila female germline and early embryos. A novel germline-specific function of deadenylase complex Ccr4-Not in the telomeric transcript surveillance mechanism is reported. Depletion of Ccr4-Not complex components causes strong derepression of the telomeric retroelement HeT-A in the germ cells, accompanied by elongation of the HeT-A poly(A) tail. Dysfunction of transcription factors Woc and Trf2, as well as RNA-binding protein Ars2, also results in the accumulation of excessively polyadenylated HeT-A transcripts in ovaries. Germline knockdowns of Ccr4-Not components, Woc, Trf2 and Ars2, lead to abnormal mitosis in early embryos, characterized by chromosome missegregation, centrosome dysfunction and spindle multipolarity. Moreover, the observed phenotype is accompanied by the accumulation of HeT-A transcripts around the centrosomes in early embryos, suggesting the putative relationship between overexpression of telomeric transcripts and mitotic defects. Our data demonstrate that Ccr4-Not, Woc, Trf2 and Ars2, components of different regulatory pathways, are required for telomere protection in the germline in order to guarantee normal development. INTRODUCTION Telomeres are nucleoprotein complexes at the ends of linear chromosomes that protect chromosome ends from fusion and degradation owing to the end under-replication process (1). Protection of chromosome ends is essential in all cell types to prevent chromosome end-to-end fusion. At the same time, loss of the terminal DNA is repaired only in germ and stem cells by the synthesis of tandem telomeric repeats (2,3). Elongation of chromosome ends in proliferating somatic cells is believed to lead to cancer (4). In germ cells, a special mechanism of telomere length maintenance is present, providing replicative potential for the shortened telomeres in normal terminally differentiated somatic cells (5). Telomere length control is an essential germline-specific function implying that telomere organization in the germ cells has distinctive characteristics. However, the basic aspects of telomere biology in the germline and in early development are far from being understood. In most eukaryotres, telomeric DNA is maintained through telomerase action, which is responsible for the synthesis of 6–9 nucleotide repeats using its RNA component as a template (6). The telomeres of Drosophila are maintained as a result of the retrotransposition of specialized telomeric non-long terminal repeat retrotransposons (7– 11). In all cases, telomere elongation utilizes reverse transcription, i.e. synthesis of DNA on the RNA template. Also, telomeric DNA serves as a platform for the binding of telomeric proteins, required for the protection of chromosome termini and the maintenance of specific chromatin structure. In particular, heterochromatic protein 1 (HP1) and the DNA repair complex provide protection against telomere fusions in both Drosophila and humans (12,13), demonstrating strong similarities in the telomere protection pathways in different organisms. Recent data show that the transcription of telomeric DNA and the requirement for RNA-interference (RNAi) *To whom correspondence should be addressed. Tel: +7 499 1960019; Fax: +7 499 1960221; Email: allakalm@img.ras.ru †These authors contributed equally to the paper as first authors.