LINKER HISTONE TRANSITIONS DURING MAMMALIAN OOGENESIS AND EMBRYOGENESIS

A unique characteristic of ihs oocyte is that, although it is a differ entiated cell, it can to give rise io a population of undifferentiated embryonic cells, This transition from a differentiated to a totipoten tial condition is thought to be mediated in pari by changes in chromat in composition or configuration. in many nonmammalian organisms, oocyt es contain unique subtypes of the linker histone H1, which ore replace d in early embryos by the so-coiled somatic histone H1 subtypes. We re view evidence that such histone Hi subtype switches also occur in mamm als. immunologically detectable somatic H1 is present in mitotically p roliferating oogonia but gradually becomes undetectable after the oocy tes enter meiosis. Immunoreactive somatic Hi remains undetectable thro ughout oogenesis and the early cell cycles after fertilization. Follow ing activation of the embryonic genome, ii is assembled onto chromatin . In contrast io the absence of immunoreactive protein, mRNAs encoding each of the five mammalian somatic Hi subtypes are present in growing oocytes and newly fertilized embryos, indicating that post-transcript ional mechanisms regulate expression of these genes. This maternal mRN A is degraded al the late 2-cell stage, and embryonically encoded mRNA s accumulate after embryos reach the 4-cell stage. During the period w hen somatic H1 is not detectable, oocytes and embryos contain mRNA enc oding a sixth subtype, histone H1 degrees, which accumulates in differ entiated somatic cells, and the nuclei can be stained with an H1 degre es-specific antibody. We propose that the linker histone composition o f the oocyte lineage resembles that of other mammalian cells, namely, that the somatic Hi subtypes predominate in mitotically active oogonia , that histone H1 degrees becomes prominent in differentiated oocytes, and following fertilization and transcriptional activation of the emb ryonic somatic H1 genes, the somatic H1 subtypes are reassembled onto chromatin of the embryonic cells. Potential functions of these linker histone subtype switches are discussed, including stabilization by H1 degrees of the differeniiaied stale of ihe oocyies, protection of the oocyte chromatin from factors that remodel sperm chromatin after ferti lization, and restoration by the incorporation of the somatic Hi subty pes of the totipotential stale of embryonic nuclei. (C) 1998 Wiiey-Lis s, Inc.