The Germline Erez Raz ZMBE Mnster Lecture slides
The Germline Erez Raz, ZMBE Münster Lecture slides courtesy of Anthony Wood, modified by Nana Kamei 2017
Germ cells are a distinct lineage
Germ plasm theory and the Weissman Barrier Inheritance info lost Inheritance info soma Cannot reproduce the organism Restricted potency germline Reproduces the organism Latent totipotency
Germ plasm theory and the Weissman Barrier Inheritance info lost Inheritance info soma Cannot reproduce the organism Restricted potency germline Reproduces the organism Latent totipotency
Germ plasm theory and the Weissman Barrier Inheritance info lost Inheritance info soma Weissman Barrier Cannot reproduce the organism Restricted potency germline Reproduces the organism Latent totipotency Soma contributes nothing to germline, therefore Darwin is right and Lamarck is wrong
August Weismann: Germ Plasm Theory Where was he right? Where was he wrong? What was the compelling proof?
Soma Germline Totipotent Pluripotent PGC Ectoderm Mesoderm Endoderm f. GSC Egg SSC + Sperm
Challenges of the germline. . . Totipotent zygote/morula …switching between differentiated and totipotent…
Challenges of the germline. . . and avoiding inappropriate expressions of totipotency
Key events in germline development Sasaki and Matsui, 2008
(1) Germ cell specification
Two modes of germline specification Preformation (Determinate) Drosophila C. elegans Frogs Zebrafish Chick/quail Epigenesis (Inductive) Humans Mice Rats Salamanders “Germ plasm” BMP 4 A BMP 2 P E 6. 25 Extavour & Akam, 2005
Determinate Anterior Posterior Germ plasm is deposited by mother into oocyte Anterior Posterior Ectopic germ cells A P 4% mwh e Extavour & Akam, 2005 Mutant germ cells were induced by wt germ plasm (Illmensee and Mahowald 1974)
Inductive Cells are competent to become germ cells in response to signals Tang et al. , 2016
Inductive Most model organisms are determinate! Determinate Extavour & Akam, 2005
(2) Repression of somatic differentiation
Derailing the somatic program: Blimp 1 (Prdm 1) Hayashi and Surani 2009
“Mesoderm” origins of the mouse germline Hox Sox 2 Blimp 1
Transcriptional repression Blimp 1 is a txn repressor Prmt 5 dimethylates histones to suppress txn Blimp 1 Prmt 5 Histone dimethylation Somatic transcription factors (Hox etc. ) DNA methylation (Dnmts) RNApol. II
(3) Germline-specific transcription
Many markers of germ cell identity… Oct 4 Sox 2 Nanog Nanos Stella Dead end Vasa Dazl fragilis …are pluripotency markers
Determinate vs. inductive BMP 4 “Germ plasm” BMP 2 A V vasa deadend oskar Xcat dazl tudor gurken
Transcription of “germline” genes Saitou 2009
Generating PGCs in vitro: cytokine induction Hayashi et al, 2011
Generating PGCs in vitro: txn factor induction Blimp 1 (Prdm 1) Prdm 14 AP 2 gamma (TFAP 2 c) Nakaki et al, 2013
Generating PGCs in vitro: txn factor induction Nanog Blimp 1 (Prdm 1) Prdm 14 AP 2 gamma (TFAP 2 c) Murakami et al, 2016
Generating PGCs in vitro: female germline reconstitution Hikabe et al, 2016
Generating PGCs in vitro: female germline reconstitution Hikabe et al, 2016
CBFA 2 T 2 acts as a scaffold Tu et al, 2016
(4) Epigenetic reprogramming
Epigenetic Memory DNA methylation Gene silencing
Histone Modifications Acetylation Euchromatin De/acetylation Methylation Heterochromatin
Reprogramming events Tang et al. , 2016
What memory do germ cells erase? (1) Gametogenic signatures (at fertilization) Sperm and egg specific programs (2) Zygotic/embryonic epigenetic memory PGCs come from epiblast (mesoderm origins) (3) Parental imprints
(5) Imprint erasure
Imprinting and Epigenetic memory
Dietary influence on the epigenome Schagdarsurengin and Steger, 2016
Incomplete imprint erasure in oocytes can be affected by nutritional supplementation: Avy mouse Transposon activated expression in agouti promoter leads to excess agouti gene (yellow) Silencing the transposon results in a normal agouti mouse (brown) Brown moms have more brown pups because imprint marks accumulate Excess methylation leads to more silencing and more brown grandpups Morgan et al. 1999, Cropley et al. 2006
Re-establish sex-specific imprints Highly methylated
in utero undernourishment perturbs sperm methylome and intergenerational metabolism Radford et al. , 2014
(6) X-inactivation/reactivation
(6) X-inactivation ~2000 genes on human X chromosome Female – XX Male - XY
Cycle of X-in/reactivation in females sperm ? ? zygote oocyte Xp inactivation Xm/Xp X-reactivation Xm/Xp or Xm/Xp Xist m. RNA TE Xm/Xp Xp reactivation Soma X inactivation Xm/Xp or Xm/Xp ICM Xm/Xp
Cycle of X-in/reactivation in females sperm ? ? zygote oocyte Xp inactivation Xm/Xp X-reactivation Xm/Xp or Xm/Xp Xist m. RNA TE Xm/Xp Xp reactivation Soma X inactivation Xm/Xp or Xm/Xp If you cloned a calico cat, would Molly the clone still be calico? ICM Xm/Xp
Papers Discussion Fertile offspring from sterile sex chromosome trisomic mice. Hirota T, Ohta H, Powell BE, Mahadevaiah SK, Ojarikre OA, Saitou M, Turner JMA. Science. 2017 Sep 1; 357(6354): 932 -5. doi: 10. 1126/science. aam 9046. Epub 2017 Aug 17. PMID: 28818972 Review Specification and epigenetic programming of the human germ line. Tang WW, Kobayashi T, Irie N, Dietmann S, Surani MA. Nat Rev Genet. 2016 Oct; 17(10): 585 -600. doi: 10. 1038/nrg. 2016. 88. Epub 2016 Aug 30. PMID: 27573372 Optional Just For Fun The Story of I. Claire Ainsworth, Nature 2006. www. stembook. org
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