Germline Imprinting

Developmental Cell, Vol. 6, 157–165, February, 2004, Copyright 2004 by Cell Press

Previews

Germline Imprinting: Battle of the Sexes or Battle of the X’s?

The X chromosome is largely inactivated in spermatogenesis of heterogametic males, and in multiple phyla it encodes few genes specifically expressed in the male germline. Writing in Nature Genetics, Bean et al. report a parallel between male germline X inactivation in nematodes and a fungal gene-silencing mechanism that alters the way we view the evolution of both phenomena. Dosage compensation, whereby X-linked gene expression is evened out between sexes, intuitively makes good sense, and its implementation in mammals (through random X inactivation) is widely appreciated by fanciers of calico cats. But the purpose of X inactivation in male meiosis seems obscure—why not use that single X? Two explanations have been offered previously. First, conversion of the X and Y to heterochromatin (e.g., the X-Y body in mammalian spermatocytes) prevents recombination between them that could potentially create intersexual progeny (Bull, 1983). In the face of growing repression, genes functioning primarily in male fertility are driven off the X and onto the autosomes or to transcribed regions of the Y. As an alternative, Wu and Xu (2003) recently published what they dub the sexually antagonistic X inactivation (SAXI) hypothesis. Based on the work of William R. Rice, they argue that genes beneficial to male gametogenesis but detrimental to females will tend to be pushed off of the X due to the greater amount of time the X spends in the female environment. The growing dearth of male-related genes and feminization of X function (e.g., Richler et al., 1992; Reinke et al., 2000) eventually selects for wholesale X inactivation, but primarily as a consequence, rather than as the cause, of male gene loss. Both of these ideas have their appeal and are not necessarily mutually exclusive. But neither can address some key issues. The recombination suppression model cannot explain why the process is working in species lacking male recombination or with XO males. In the latter case, perhaps X inactivation is a remnant of earlier times when a Y existed, but the widespread XX/XO state in nematodes implies loss of the putative Y hundreds of millions of years ago. For SAXI, missing is a mechanism to explain how the X is detected and shut down once it is free of spermatogenesis genes. Working in C. elegans, Bean et al. (2004) potentially fill both of these holes. They report that the pattern of histone modifications in the male germline X chromosome are similar to that found in other chromatin lacking a meiotic pairing partner, such as extrachromosomal

transgene arrays and free autosomal duplications. As further confirmation, they find that XO animals transformed into hermaphrodites via mutation of the sexdetermining gene her-1 show similar epigenetic imprints and a correlated loss of X-linked expression. Thus, chromosomal pairing, and not sexual phenotype, is the key determinant of meiotic X inactivation. This is reminiscent of the MSUD (“meiotic silencing by unpaired DNA”) phenomenon of Neurospora (Shiu et al., 2001). Bean et al. suggest that MSUD is an ancient mechanism that exists primarily for “genome surveillance” at a sensitive time, when transposons and other foreign DNA may be lurking. But it also could affect differentiated sex chromosomes the same way. This, then, provides a satisfying explanation for why X inactivation happens in XO animals and males lacking recombination. It further suggests that recombination suppression is not the universal proximate cause of male germline X inactivation, but rather a consequence of heterochromatin formation induced by MSUD. MSUD also provides a mechanism that would dependably inactivate each newly unpaired part of the neo-X as soon as sexual conflict differentiates the chromosomes enough to block meiotic pairing. What about the Y? It is as unpaired as an X, yet predicted to accumulate male genes (Wu and Xu, 2003). The answer appears to be unique for each system examined. In mammals, the Y’s main job is testis determination, and during meiosis is part of the heterochromatic X-Y body. The issue is moot in most nematodes because there is no Y (Bull, 1983). The Drosophila Y is mostly heterochromatic outside of the rDNA repeat region that mediates pairing, and so it also generally fits the MSUD model. However, it is nevertheless essential for spermatogenesis because it harbors a small number of key male-sterile loci, including sperm axoneme dynein. This suggests that the Drosophila Y is a mixed case and that male fertility genes have acquired special mechanisms to prevent silencing. The MSUD model also predicts that in ZZ/ZW systems, in which females are heterogametic, the Z is shut down in oogenesis. Although the existence of somatic dosage compensation in birds is still being debated, there is no equivalent of the XY body in their oocytes. Wu and Xu (2003) suggest that perhaps only in sperm are the chromosomes condensed enough to be inactivated. However, this doesn’t square with the presumably histone-mediated mechanism of inactivation. Alternatively, many basal amniotes use temperature to determine sex, and birds’ sex chromosomes are often less differentiated and probably of more recent origin than those of mammals (Ellegren, 2000). Perhaps they behave as if still paired for the purposes of invoking MSUD. Consistent with this, the chicken ZW pair forms a largely nonhomologous synaptonemal complex that extends the length of the W (Solari, 1989). Paternal X imprinting in C. elegans has come along at an opportune time. A recent study (Prahlad et al., 2003) in Caenorhabditis found that nutritional cues can induce XX L1 larvae to change sex through specific loss

Developmental Cell 158

of the paternal X (Xp). This predicted the existence of some form of Xp imprinting, and the work of Bean et al. provides one. But reversal of histone-based Xp imprinting occurs by the 20 cell stage, while the capacity for Xp loss remains until after hatching. Whether this is due to persistence of an undetectably small amount of modified histones or to an as-yet-unassayed imprint remains to be seen. Regardless of the details, imprinting of unpaired chromatin will no doubt turn up in other contexts in the future. For example, the discovery of Bean et al. could help address the bane of many C. elegans researchers: poor germline expression of transgenes in extrachromosomal arrays. There is also evidence that Xp imprinting similar to that of nematodes occurs in mammals (Cowell et al., 2002). Eric S. Haag Department of Biology Building #144 University of Maryland College Park, Maryland 20742

When Ras Signaling Reaches the Mediator

Numerous sequence-specific DNA binding proteins that couple extracellular stimuli to transcriptional regulation have been described. Less well understood is how these transcriptional regulators interact with the Mediator complex to initiate transcription and how those interactions are coordinated with the activation of signaling pathways. Recent work has begun to shed light on this important area of research. Mediator complexes consist of 20–30 protein subunits, most of which appear to be conserved from yeast to humans (reviewed in Boube et al., 2002). Much genetic and biochemical data have demonstrated that Mediator complexes interact with sequence-specific DNA binding transcriptional regulators, and in doing so, functionally bridge them to the basal RNA pol II machinery, which can bind to the Mediator complex. Some individual Mediator protein subunits repress transcription, and therefore Mediator complexes can convey both activating and repressive transcriptional instructions to Pol II. There is also variability in the subsets of genes regulated by different subunits of Mediator complexes. Some subunits are required for genome-wide transcriptional regulation, while others appear to regulate a relatively small number of genes. One subunit of the latter class that has garnered attention in recent years is Sur2. Sur2 was first identified in Caenorhabditis elegans as functioning downstream of the Ras/MAPK pathway in vulval induction (Singh and Han, 1995). It was subsequently revealed that homologs of Sur2 (also known by other names such as CRSP130) can be found in Mediator

Selected Reading Bean, C.J., Schaner, C.E., and Kelly, W.G. (2004). Nat. Genet. 36, 100–105. Bull, J.J. (1983). Evolution of Sex Determination Mechanisms (Menlo Park, CA: Benjamin/Cummings). Cowell, I.G., Aucott, R., Mahadevaiah, S.K., Burgoyne, P.S., Huskisson, N., Bogiorni, S., Prantera, G., Fanti, L., Pimpinelli, S., Wu, R., et al. (2002). Chromosoma 111, 22–36. Ellegren, H. (2000). Trends Ecol. Evol. 15, 188–192. Prahlad, V., Pilgrim, D., and Goodwin, E.B. (2003). Science 302, 1046–1049. Reinke, V., Smith, H.E., Nance, J., Wang, J., Van Doren, C., Begley, R., Jones, S.J., Davis, E.B., Scherer, S., Ward, S., and Kim, S.K. (2000). Mol. Cell 6, 605–616. Richler, C., Soreq, H., and Wahrman, J. (1992). Nat. Genet. 2, 192–195. Shiu, P.K.T., Raju, N.B., Zickler, D., and Metzenberg, R.L. (2001). Cell 107, 905–916. Solari, A.J. (1989). In Fertility and Chromosome Pairing: Recent Studies in Plants and Animals, C.B. Gillies, ed. (Boca Raton, FL: CRC Press), pp. 77–107. Wu, C.I., and Xu, E.Y. (2003). Trends Genet. 19, 243–247.

complexes associated with transcriptional activators involved in a wide variety of biological processes. However, Sur2 appears to be required by only a small subset of transcriptional activators for their function. Specifically, it was shown that human Sur2 directly interacted with and promoted transcription by both the viral protein E1A and the Ras/MAPK-activated ETS family member Elk-1 (Boyer et al., 1999). Studies using sur2⫺/⫺ murine embryonic stem cells confirmed these results and demonstrated that more than a dozen other transcriptional activators, including some downstream of MAPKs, had Sur2-independent activity (Stevens et al., 2002). Elk-1 interaction with Sur2 was also shown to be dependent upon its phosphorylation by Erk2. In conjunction with the genetic data from C. elegans, these results suggest that Sur2 evolved to directly coordinate Mediator recruitment and activity with a subset of transcriptional activators functioning downstream of the Ras/MAPK pathway. A new study by Mo et al. (2004) in the January 30 issue of Molecular Cell has revealed another transcriptional activator that recruits the Mediator complex via Sur2. C/EBP␤ is a member of the basic leucine zipper (bZIP) family of transcription factors and is capable of regulating genes involved in proliferation and differentiation. It has previously been shown that phosphorylation of C/EBP␤ by MAPK causes relief of transcriptional inhibition and enhanced transactivation at C/EBP␤ target genes (Nakajima et al., 1993; Kowenz-Leutz et al., 1994). Additionally, C/EBP␤ appears to have a number of E1Alike properties (Spergel et al., 1992). The above observations led Mo et al. to investigate if C/EBP␤ also interacts with the Mediator complex like E1A and Elk-1. Through a series of in vitro and in vivo experiments, the authors show that C/EBP␤ physically interacts with the Sur2 subunit of the Mediator complex to activate transcription. These results strengthen the hypothesis that the