Inositol Diphosphate Signaling Regulates Telomere Length

Abstract

Activation of phospholipase C-dependent inositol polyphosphate signaling pathways generates distinct messengers derived from inositol 1,4,5-trisphosphate that control gene expression and mRNA export. Here we report the regulation of telomere length by production of a diphosphorylinositol tetrakisphosphate, PP-IP 4 , synthesized by the KCS1 gene product. Loss of PP-IP 4 production results in lengthening of telomeres, whereas overproduction leads to their shortening. This effect requires the presence of Tel1, the yeast homologue of ATM, the protein mutated in the human disease ataxia telangiectasia. Our data provide in vivo evidence of a regulatory link between inositol polyphosphate signaling and the checkpoint kinase family and describe a third nuclear process modulated by phospholipase C activation.Appropriate cellular responses to environmental changes involve intracellular second messenger systems that transduce the signals from cytoplasm to nucleus, thereby initiating adaptive genetic programs. One well described intracellular messenger system works through receptor-coupled activation of phospholipase C (PLC), 1 which hydrolyzes phosphatidylinositol 4,5-bisphosphate to yield inositol 1,4,5-trisphosphate (IP 3 ) (reviewed in Refs. 1 and 2). In metazoans, cellular production of IP 3 functions as a signal for calcium release from intracellular stores through allosteric activation of an IP 3 receptor channel. Recent studies indicate that IP 3 also plays an important role as precursor to multiple inositol polyphosphates (IPs), each with potentially unique signaling capability (reviewed in Ref. 3). This signaling potential is not restricted to the cytoplasm, because IPs are known to function in nuclear processes in budding yeast (reviewed in Ref. 4). Activation of yeast phospholipase C (Plc1) produces IP 3 , which is rapidly phosphorylated by the dual specificity 6-/3-kinase Ipk2 (initially cloned as ArgRIII/Arg82 (5)) to yield IP 4 and IP 5 (6 -8). This phosphorylation step is coupled to transcriptional regulation, possibly through chromatin remodeling (7, 9 -11). IP 5 is then phosphorylated by the 2-kinase Ipk1, generating IP 6 , which is required for efficient mRNA export from the nucleus (12-14). Both IP 5 and IP 6 can be converted to the diphosphoryl inositols PP-IP 4 and PP-IP 5 through the action of Kcs1, a kinase required for normal vacuolar morphology (15-18). PP-IP 5 has recently been suggested to play a role in chemotaxis in Dictyostelium (19). A nuclear role for Kcs1 is implied by its initial cloning as a regulator of mitotic DNA recombination, and inositol kinase activity is required for this regulation, but further understanding of its nuclear activity is lacking (20,21).Recent work has provided further linking of IP production and nuclear function through an important family of protein serine/threonine kinases known as phosphatidylinositol 3-kinase related kinases (PIKKs). IP 6 stimulates DNA repair by non-homologous end joining with mammalian proteins in vitro (22). Non-homologous end joining can b...

Inositol pyrophosphates modulate cell cycle independently of alteration in telomere length

“…Deletion strains ipk1::KANMX, tel1::ura3, as well as strains in which Kcs1 was overexpressed were generated as described previously (York et al, 2005). For labeling experiments, cells were inoculated in synthetic medium without inositol with addition of 5.7 mg/ml ammonium sulphate, 0.82 mg/ml amino acids, 2% glucose, and 5 µCi/ml 3 [H]inositol (labeling medium) and grown to achieve isotopic equilibrium (8-9 divisions) in a shaker incubator at 30 o C and 200 r.p.m.…”

“…The fragments were transferred to Hybond N+ nylon membranes (Amersham Biosciences) and hybridized at 65 o C to a probe derived from pYT14 containing telomeric DNA and a portion of the Y ' subtelomeric repeat. The probe was labeled with of [-32 P]dCTP using ready-to-go labeling beads (Amersham Biosciences) as described previously (York et al, 2005).…”

“…Furthermore, as ipk1 cells are unable to synthesize IP 6 , IP 7 or IP 8, but will accumulate alternative inositol pyrophosphates, PP-IP 4 and (PP) 2 -IP 3 and have telomeres slightly shorter than wild type (Saiardi et al, 2005;York et al, 2005), therefore herein we have investigated how overexpression of Kcs1 affects the level of alternative inositol pyrophosphates, telomere length and progression of cells through the cell cycle.…”

“…(PP) 2 -IP 4 (IP 8 ) is generated by either Kcs1-mediated phosphorylation of 1-PP-IP 5 or Vip1-mediated phosphorylation of 5-PP-IP 5 (Tsui and York, 2010). Kcs1 has been reported to modulate telomere length by generation of 5-PP-IP 4 (Saiardi et al, 2005;York et al, 2005), and Vip1-generated IP 7 binds to the cyclin-CDK-CDK inhibitor complex to regulate transcription of the yeast phosphate (Pi)-responsive (PHO) genes (Lee et al, 2007).…”

HSP90 regulates cell survival via inositol hexakisphosphate kinase-2

“…IP6K-deficient yeast manifest major abnormalities in vesicular endocytosis (8)(9)(10) and telomere length (11,12). Abundant evidence implicates PP-IPs in vesicular trafficking.…”