Genetics, epigenetics and gene silencing in differentiating mammalian embryos

Epigenetic control of embryonic renal cell differentiation by L1 retrotransposon

BACKGROUND

L1 retroelements may play a central role in morphogenesis through epigenetic mechanisms involving recruitment of chromatin modifying protein complexes. Retroelements are repressed in terminally differentiated cells, and highly active in embryonic, undifferentiated, and transformed cells. It is not clear if the modulation of differentiation by L1 is a "cause" or "effect". The purpose of this study was to determine if murine embryonic kidney cells of clonal origin (mK4 cells) harbor retrotransposition events upon ectopic expression of L1, and the impact of L1 on embryonic kidney cell differentiation. Given that L1 is reactivated by aryl hydrocarbon receptor (AHR) ligands, we also sought to investigate the effects of benzo(a)pyrene (BaP) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the genetic network of mK4 cells.

METHODS

The mK4 cells overexpressing human L1(RP) were assessed for changes in proliferation and expression of molecular markers of cellular differentiation.

RESULTS

L1(RP) increased proliferation rates and markedly downregulated differentiation programming in mK4 cells. These genetic alterations were recapitulated by exogenous activation of L1 by AHR ligands.

CONCLUSION

L1 regulates nephrogenesis in vitro via both insertional and non-insertional mechanisms that disrupt mesenchymal to epithelial transition. Thus, a feedback loop involving L1, WT1, and AHR may play a role in regulation of kidney morphogenesis. Birth Defects Research (Part A), 2011. © 2011 Wiley-Liss, Inc.

Abnormal expression of TIMP-2, SOD, vimentin and PAI proteins in cloned bovine placentae

Cloned mammals suffer from high rates of placental abnormality and foetal loss during pregnancy. We previously used 2-D gel electrophoresis and mass spectrometry for global proteomic analysis of cloned and normal bovine placentae to identify differential protein expression patterns. Here, we used Western blot analysis to confirm the expression levels of several pregnancy-related proteins putatively identified as being differentially expressed in somatic cell nuclear transfer (SCNT) vs normal bovine placentae. The expression levels of tissue inhibitor of metalloproteinase-2 (TIMP-2), its downstream protein, matrix metalloproteinase-2 (MMP-2), superoxide dismutase (SOD), vimentin and plasminogen activator inhibitor-1 (PAI) were analysed in the placentae of SCNT cloned Korean native cattle that died immediately after birth and in normal placentae obtained by AI. Our results revealed that TIMP-2 and SOD were up-regulated in SCNT placenta compared with normal placenta, whereas MMP-2 levels were comparable in cloned and normal placentae, and vimentin and PAI were significantly down-regulated in SCNT compared with normal placentae. Our results suggest that key proteins of placental development are abnormally expressed in SCNT cloned bovine placentae, probably resulting in abnormal placental function and clonal mortality.

From embryonic stem cells to blastema and MRL mice

New scientific knowledge offers fresh opportunities for regenerative medicine and tissue repair. Among various clinical options, multipotent embryonic stem cells (ESC) prepared from inner cell masses of rabbit blastocysts have been tested over many years. More recently, stem cells have been isolated from individual tissues and from umbilical cord blood. These methods seemingly offer similar rates of repair and avoid ethical complexities arising from the need for human embryos to prepare ESC. Different methods of regenerating tissues have now emerged, based on the well-known forms of organ regeneration in urodeles such as salamanders. These methods depend on the formation of a blastema, and recent studies on MRL mice have revealed that they possess similar methods of repair as in salamanders. There is also some evidence showing that this form of repair is also active in human fetuses but not in adults. Detailed knowledge of these various forms of tissue repair is now urgently needed in order to assess the benefits of each form of treatment. These matters are discussed at the end of this review where various investigations clarify the benefits and drawbacks of these varied approaches to tissue repair.

Elucidating nuclear reprogramming mechanisms: taking a synergistic approach

Nuclear reprogramming is the process by which a differentiated somatic nucleus has developmental potential restored to it. It involves heritable changes in gene expression as well as structural and functional changes to chromatin structure. This process is naturally induced immediately after fertilization, but can also be artificially induced by nuclear transfer, cell fusion and also now by viral transduction with four stem cell genes. However, the frequency of successful reprogramming is low in each system. The highest success rates, those using nuclear transfer, are only of the order of 2-5%. This article briefly reviews these three methods and proposes a synergistic approach where conditions that facilitate reprogramming in one system are transposed to the others. This might increase the incidence of successful reprogramming and identify common steps necessary for the reacquisition of developmental potential.

Extended embryo culture to increase implantation rate

Multiple pregnancies are a serious adverse event of IVF treatment. Several strategies have been developed to improve embryo selection and embryo developmental potential to increase pregnancy rates without increasing the risk of multiple pregnancies. The only way to effectively solve this problem is to reduce the number of embryos transferred to one. The introduction of extended embryo culture has provided IVF programmes with a valuable tool to select more accurately those embryos with a higher implantation potential. The concept of single embryo transfer presents a clinical dilemma with seemingly opposing sides: one seeking to maintain acceptable pregnancy rates and the other seeking to reduce the number of twin births or high order multiple pregnancies. The present review addresses the potential benefits and drawbacks of blastocyst transfer.