Early Loss of Xist RNA Expression and Inactive X Chromosome Associated Chromatin Modification in Developing Primordial Germ Cells

The inactive X chromosome characteristic of female somatic lineages is reactivated during development of the female germ cell lineage. In mouse, analysis of protein products of X-linked genes and/or transgenes located on the X chromosome has indicated that reactivation occurs after primordial germ cells reach the genital ridges.We present evidence that the epigenetic reprogramming of the inactive X-chromosome is initiated earlier than was previously thought, around the time that primordial germ cells (PGCs) migrate through the hindgut. Specifically, we find that Xist RNA expression, the primary signal for establishment of chromosome silencing, is extinguished in migrating PGCs. This is accompanied by displacement of Polycomb-group repressor proteins Eed and Suz(12), and loss of the inactive X associated histone modification, methylation of histone H3 lysine 27.We conclude that X reactivation in primordial germ cells occurs progressively, initiated by extinction of Xist RNA around the time that germ cells migrate through the hindgut to the genital ridges. The events that we observe are reminiscent of X reactivation of the paternal X chromosome in inner cell mass cells of mouse pre-implantation embryos and suggest a unified model in which execution of the pluripotency program represses Xist RNA thereby triggering progressive reversal of epigenetic silencing of the X chromosome

Effect of peptide chain length on absorption of egg protein hydrolysates in the normal human jejunum.

The influence of the peptide chain length of partial enzymic hydrolysates of protein on nitrogen and amino acid absorption was studied in 12 subjects using a jejunal perfusion technique. Three hydrolysates of egg white and an equivalent amino acid mixture were perfused at 30 mmol/L and 100 mmol/L in two separate experiments. Two hydrolysates (OH1 and OH2) contained mainly dipeptides and tripeptides, whereas the third (OH3) comprised tripeptide to pentapeptides as judged chromatographically. Nitrogen absorption was significantly slower from the higher chain length mixture, OH3, than from the short chain mixtures, OH1 and OH2, at both concentrations. Similarly, several amino acid residues were absorbed less well from OH3 than from OH1 and OH2. These data demonstrate that the chain length of heterogeneous mixtures of peptides affects absorption of nitrogen and individual amino acid residues, and suggest that brush border hydrolysis of tetrapeptides and pentapeptides limits absorption from enzymic hydrolysates of protein which simulate the composition of the postprandial luminal contents