Adult stem cells constitute an important reservoir of self-renewing progenitor cells and are crucial for maintaining tissue and organ homeostasis. The capacity of stem cells to self-renew or differentiate can be attributed to distinct metabolic states, and it is now becoming apparent that metabolism plays instructive roles in stem cell fate decisions. Lipids are an extremely vast class of biomolecules, with essential roles in energy homeostasis, membrane structure and signaling. Imbalances in lipid homeostasis can result in lipotoxicity, cell death and diseases, such as cardiovascular disease, insulin resistance and diabetes, autoimmune disorders and cancer. Therefore, understanding how lipid metabolism affects stem cell behavior offers promising perspectives for the development of novel approaches to control stem cell behavior either in vitro or in patients, by modulating lipid metabolic pathways pharmacologically or through diet. In this review, we will first address how recent progress in lipidomics has created new opportunities to uncover stem-cell specific lipidomes. In addition, genetic and/or pharmacological modulation of lipid metabolism have shown the involvement of specific pathways, such as fatty acid oxidation (FAO), in regulating adult stem cell behavior. We will describe and compare findings obtained in multiple stem cell models in order to provide an assessment on whether unique lipid metabolic pathways may commonly regulate stem cell behavior. We will then review characterized and potential molecular mechanisms through which lipids can affect stem cell-specific properties, including self-renewal, differentiation potential or interaction with the niche. Finally, we aim to summarize the current knowledge of how alterations in lipid homeostasis that occur as a consequence of changes in diet, aging or disease can impact stem cells and, consequently, tissue homeostasis and repair.

The regulation of transcription is a fundamental process underlying the determination of cell identity and its maintenance during development. In the last decades, most of the transcription factors, which have to be expressed at the right place and at the right time for the proper development of the fly embryo, have been identified. However, mostly because of the lack of methods to visualize transcription as the embryo develops, their coordinated spatiotemporal dynamics remains largely unexplored. Efforts have been made to decipher the transcription process with single molecule resolution at the single cell level. Recently, the fluorescent labeling of nascent RNA in developing fly embryos allowed the direct visualization of ongoing transcription at single loci within each nucleus. Together with powerful imaging and quantitative data analysis, these new methods provide unprecedented insights into the temporal dynamics of the transcription process and its intrinsic noise. Focusing on the Drosophila embryo, we discuss how the detection of single RNA molecules enhanced our comprehension of the transcription process and we outline the potential next steps made possible by these new imaging tools. In combination with genetics and theoretical analysis, these new imaging methods will aid the search for the mechanisms responsible for the robustness of development. WIREs Dev Biol 2016, 5:296–310. doi: 10.1002/wdev.221For further resources related to this article, please visit the WIREs website.

In eukaryotic cells, the organization of genomic DNA into chromatin regulates many biological processes, from the control of gene expression to the regulation of chromosome segregation. The proper maintenance of this structure upon cell division is therefore of prime importance during development for the maintenance of cell identity and genome stability. The chromatin assembly factor 1 (CAF-1) is involved in the assembly of H3-H4 histone dimers on newly synthesized DNA and in the maintenance of a higher order structure, the heterochromatin, through an interaction of its large subunit with the heterochromatin protein HP1a. We identify here a conserved domain in the large subunit of the CAF-1 complex required for its interaction with HP1a in the Drosophila fruit fly. Functional analysis reveals that this domain is dispensable for viability but participates in two processes involving heterochromatin: position-effect variegation and long range chromosomal interactions during meiotic prophase. Importantly, the identification in the large subunit of CAF-1 of a domain required for its interaction with HP1 allows the separation of its functions in heterochromatinrelated processes from its function in the assembly of H3-H4 dimers onto newly synthesized DNA. KEYWORDS heterochromatin; variegation; HP1; CAF-1; Drosophila I N eukaryotic cells, the chromatin is partitioned into two cytologically and functionally distinct structures: heterochromatin and euchromatin. Heterochromatin was initially defined as the part of the genome that remains condensed during the whole cell cycle and stains intensively with DNA dyes. Heterochromatin is generally gene poor; rich in repeated sequences and transposable elements (Hoskins et al. 2007). Initially considered to correspond to "junk DNA," heterochromatin contains essential protein-coding genes whose expression depends on the neighboring heterochromatin structure (Schulze et al. 2005). It encodes essential chromosomal structures such as centromeres (Sun et al. 1997) or telomeres (Mason et al. 2008) and is required for essential chromosomal functions such as homolog pairing during meiosis (Dernburg et al. 1996;Karpen et al. 1996) . While essential for the biology of the genome, many of these structures are not directly encoded in the sequence of these regions and epigenetic mechanisms are likely required for their maintenance through generations.The chromatin assembly factor-1 (CAF-1) is a heterotrimeric complex first isolated as a histone chaperone able to deposit H3-H4 dimers onto newly synthesized DNA during replication or repair (Smith and Stillman 1989;Gaillard et al. 1996). Its large subunit interacts directly with PCNA (Shibahara and Stillman 1999;Moggs et al. 2000) and the CAF-1 complex is found in vivo at the replication foci (Krude 1995;Taddei et al. 1999). The large subunit of CAF-1 has also been associated to the maintenance of heterochromatin: it was shown to be essential for the stable inheritance of gene silencing in et al. 1997); its absence in fission yeast led ...

Chromatin packaging and modifications are important to define the identity of stem cells. How chromatin properties are retained over multiple cycles of stem cell replication, while generating differentiating progeny at the same time, remains a challenging question. The chromatin assembly factor CAF1 is a conserved histone chaperone, which assembles histones H3 and H4 onto newly synthesized DNA during replication and repair. Here, we have investigated the role of CAF1 in the maintenance of germline stem cells (GSCs) in ovaries. We depleted P180, the large subunit of CAF1, in germ cells and found that it was required in GSCs to maintain their identity. In the absence of P180, GSCs still harbor stem cell properties but concomitantly express markers of differentiation. In addition, P180-depleted germ cells exhibit elevated levels of DNA damage and de-repression of the transposable I element. These DNA damages activate p53- and Chk2-dependent checkpoints pathways, leading to cell death and female sterility. Altogether, our work demonstrates that chromatin dynamics mediated by CAF1 play an important role in both the regulation of stem cell identity and genome integrity.

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Metabolism participates in the control of stem cell function and subsequent maintenance of tissue homeostasis. How this is achieved in the context of adult stem cell niches in coordination with other local and intrinsic signaling cues is not completely understood. The Target of Rapamycin (TOR) pathway is a master regulator of metabolism and plays essential roles in stem cell maintenance and differentiation. We observe differential expression of the Tor kinase in the Drosophila male germline, which correlates with restriction of mTORC1 activity to germline stem cells (GSCs) and early germ cells. Targeted RNAi-mediated downregulation of Tor in early germ cells causes a block and/or a delay in differentiation, resulting in an accumulation of germ cells with GSC-like features. These early germ cells also contain unusually large and dysfunctional autolysosomes. In addition, downregulation of Tor in adult male GSCs and early germ cells causes non-autonomous activation of mTORC1 in neighboring cyst cells, which correlates with a disruption in the coordination of germline and somatic differentiation. Our study identifies a previously uncharacterized role of the TOR pathway in regulating male germline differentiation