👤 Arkadi Manukyan

Mammals have three HP1 protein isotypes HP1 beta (CBX1), HP1 alpha (CBX3) and HP1 alpha (CBX5) that are encoded by the corresponding genes Cbx1, Cbx3 and Cbx5. Recent work has shown that reduction of CBX3 protein in homozygotes for a hypomorphic allele (Cbx3hypo) causes a severe postnatal mortality with around 99 percent of the homozygotes dying before weaning. It is not known what the causes of the postnatal mortality are. Here we show that Cbx3hypo/hypo conceptuses are significantly reduced in size and the placentas exhibit a haplo-insufficiency. Late gestation Cbx3hypo/hypo placentas have reduced mRNA transcripts for genes involved in growth regulation, amino acid and glucose transport. Blood vessels within the Cbx3hypo/hypo placental labyrinth are narrower than wild-type. Newborn Cbx3hypo/hypo pups are hypoglycemic, the livers are depleted of glycogen reserves and there is almost complete loss of stored lipid in brown adipose tissue (BAT). There is a 10-fold reduction in expression of the BAT-specific Ucp1 gene, whose product is responsible for nonshivering themogenesis. We suggest that it is the small size of the Cbx3hypo/hypo neonates, a likely consequence of placental growth and transport defects, combined with a possible inability to thermoregulate that causes the severe postnatal mortality. Show less

We measured the dynamics of an essential epigenetic modifier, HP1β, in human cells at different stages of differentiation using Fluorescence Recovery After Photobleaching (FRAP). We found that HP1β mobility is similar in human embryonic stem cells (hES) and iPS cells where it is more mobile compared to fibroblasts; HP1β is less mobile in senescent fibroblasts than in young (dividing) fibroblasts. Introduction of "reprogramming factors", Oct4, Sox2, Klf4, cMyc and Lin28, into senescent fibroblasts and measuring the changes in HP1β mobility as reprogramming proceeds shows that the mobility of HP1β in senescent cells increases and by day 9 is the same as that found in young fibroblasts. Thus the dynamics of a key epigenetic modifier can be rejuvenated without de-differentiation through an embryonic stage. Future work will test whether other aspects of cellular physiology that age can be so rejuvenated without de-differentiation. Show less

Large, multisubunit Ccr4-Not complexes are evolutionarily conserved global regulators of gene expression. Deletion of CCR4 or several components of Ccr4-Not complexes results in abnormally large cells. Since yeast must attain a critical cell size at Start to commit to division, the large size of ccr4 delta cells implies that they may have a size-specific proliferation defect. Overexpression of CLN1, CLN2, CLN3, and SWI4 reduces the size of ccr4 delta cells, suggesting that ccr4 delta cells have a G(1)-phase cyclin deficiency. In support of this, we find that CLN1 and CLN2 expression and budding are delayed in ccr4 delta cells. Moreover, overexpression of CCR4 advances the timing of CLN1 expression, promotes premature budding, and reduces cell size. Genetic analyses suggest that Ccr4 functions independently of Cln3 and downstream of Bck2. Thus, like cln3 delta bck2 delta double deletions, cln3 delta ccr4 delta cells are also inviable. However, deletion of Whi5, a transcriptional repressor of CLN1 and CLN2, restores viability. We find that Ccr4 negatively regulates the half-life of WHI5 mRNAs, and we conclude that, by modulating the stability of WHI5 mRNAs, Ccr4 influences the size-dependent timing of G1-phase cyclin transcription. Show less