CLP1-dependent premature transcription termination opposes neurodegeneration.

Usage of alternative mRNA 3' ends has profound functional consequences, particularly in the nervous system. In this issue of Neuron, LaForce et al. (2022) dissect the effect of CLP1 on mRNA 3' end diversity in motor neuron models of neurodegeneration.

Multicolor Laser Scanning Confocal Immunofluorescence Microscopy of DNA Damage Response Biomarkers.

DNA damage through endogenous and environmental toxicants is a constant threat to both a human's ability to pass on intact genetic information to its offspring as well as in somatic cells for its own survival. To counter these threats posed by DNA damage, cells have evolved a series of highly choreographed mechanisms-collectively defined as the DNA-damage response (DDR)-to sense DNA lesions, signal their presence, and mediate their repair. Thus, regular DDR signaling cascades are vital to prevent the initiation and progression of many human diseases including cancer. Consequently, quantitative assessment of DNA damage and response became an important biomarker for assessment of human health and disease risk in biomonitoring studies. However, most quantitative DNA damage biomarker techniques require dissolution of the nuclear architecture and hence loss of spatial information. Laser scanning confocal immunofluorescence microscopy (LSCIM) of three-dimensionally preserved nuclei can be, quantitative and maintain the spatial information. Here we describe the experimental protocols to quantify individual key events of the DDR cascade in three-dimensionally preserved nuclei by LSCIM with high resolution, using the simultaneous detection of Rad50 as well as phosphorylated H2AX and ATM and in somatic and germ cells as an example.

Selective Roles of Vertebrate PCF11 in Premature and Full-Length Transcript Termination.

The pervasive nature of RNA polymerase II (Pol II) transcription requires efficient termination. A key player in this process is the cleavage and polyadenylation (CPA) factor PCF11, which directly binds to the Pol II C-terminal domain and dismantles elongating Pol II from DNA in vitro. We demonstrate that PCF11-mediated termination is essential for vertebrate development. A range of genomic analyses, including mNET-seq, 3' mRNA-seq, chromatin RNA-seq, and ChIP-seq, reveals that PCF11 enhances transcription termination and stimulates early polyadenylation genome-wide. PCF11 binds preferentially between closely spaced genes, where it prevents transcriptional interference and consequent gene downregulation. Notably, PCF11 is sub-stoichiometric to the CPA complex. Low levels of PCF11 are maintained by an auto-regulatory mechanism involving premature termination of its own transcript and are important for normal development. Both in human cell culture and during zebrafish development, PCF11 selectively attenuates the expression of other transcriptional regulators by premature CPA and termination.

The non-canonical SMC protein SmcHD1 antagonises TAD formation and compartmentalisation on the inactive X chromosome.

The inactive X chromosome (Xi) in female mammals adopts an atypical higher-order chromatin structure, manifested as a global loss of local topologically associated domains (TADs), A/B compartments and formation of two mega-domains. Here we demonstrate that the non-canonical SMC family protein, SmcHD1, which is important for gene silencing on Xi, contributes to this unique chromosome architecture. Specifically, allelic mapping of the transcriptome and epigenome in SmcHD1 mutant cells reveals the appearance of sub-megabase domains defined by gene activation, CpG hypermethylation and depletion of Polycomb-mediated H3K27me3. These domains, which correlate with sites of SmcHD1 enrichment on Xi in wild-type cells, additionally adopt features of active X chromosome higher-order chromosome architecture, including A/B compartments and partial restoration of TAD boundaries. Xi chromosome architecture changes also occurred following SmcHD1 knockout in a somatic cell model, but in this case, independent of Xi gene derepression. We conclude that SmcHD1 is a key factor in defining the unique chromosome architecture of Xi.

5C analysis of the Epidermal Differentiation Complex locus reveals distinct chromatin interaction networks between gene-rich and gene-poor TADs in skin epithelial cells.

Mammalian genomes contain several dozens of large (>0.5 Mbp) lineage-specific gene loci harbouring functionally related genes. However, spatial chromatin folding, organization of the enhancer-promoter networks and their relevance to Topologically Associating Domains (TADs) in these loci remain poorly understood. TADs are principle units of the genome folding and represents the DNA regions within which DNA interacts more frequently and less frequently across the TAD boundary. Here, we used Chromatin Conformation Capture Carbon Copy (5C) technology to characterize spatial chromatin interaction network in the 3.1 Mb Epidermal Differentiation Complex (EDC) locus harbouring 61 functionally related genes that show lineage-specific activation during terminal keratinocyte differentiation in the epidermis. 5C data validated by 3D-FISH demonstrate that the EDC locus is organized into several TADs showing distinct lineage-specific chromatin interaction networks based on their transcription activity and the gene-rich or gene-poor status. Correlation of the 5C results with genome-wide studies for enhancer-specific histone modifications (H3K4me1 and H3K27ac) revealed that the majority of spatial chromatin interactions that involves the gene-rich TADs at the EDC locus in keratinocytes include both intra- and inter-TAD interaction networks, connecting gene promoters and enhancers. Compared to thymocytes in which the EDC locus is mostly transcriptionally inactive, these interactions were found to be keratinocyte-specific. In keratinocytes, the promoter-enhancer anchoring regions in the gene-rich transcriptionally active TADs are enriched for the binding of chromatin architectural proteins CTCF, Rad21 and chromatin remodeler Brg1. In contrast to gene-rich TADs, gene-poor TADs show preferential spatial contacts with each other, do not contain active enhancers and show decreased binding of CTCF, Rad21 and Brg1 in keratinocytes. Thus, spatial interactions between gene promoters and enhancers at the multi-TAD EDC locus in skin epithelial cells are cell type-specific and involve extensive contacts within TADs as well as between different gene-rich TADs, forming the framework for lineage-specific transcription.

p63 and Brg1 control developmentally regulated higher-order chromatin remodelling at the epidermal differentiation complex locus in epidermal progenitor cells.

Chromatin structural states and their remodelling, including higher-order chromatin folding and three-dimensional (3D) genome organisation, play an important role in the control of gene expression. The role of 3D genome organisation in the control and execution of lineage-specific transcription programmes during the development and differentiation of multipotent stem cells into specialised cell types remains poorly understood. Here, we show that substantial remodelling of the higher-order chromatin structure of the epidermal differentiation complex (EDC), a keratinocyte lineage-specific gene locus on mouse chromosome 3, occurs during epidermal morphogenesis. During epidermal development, the locus relocates away from the nuclear periphery towards the nuclear interior into a compartment enriched in SC35-positive nuclear speckles. Relocation of the EDC locus occurs prior to the full activation of EDC genes involved in controlling terminal keratinocyte differentiation and is a lineage-specific, developmentally regulated event controlled by transcription factor p63, a master regulator of epidermal development. We also show that, in epidermal progenitor cells, p63 directly regulates the expression of the ATP-dependent chromatin remodeller Brg1, which binds to distinct domains within the EDC and is required for relocation of the EDC towards the nuclear interior. Furthermore, Brg1 also regulates gene expression within the EDC locus during epidermal morphogenesis. Thus, p63 and its direct target Brg1 play an essential role in remodelling the higher-order chromatin structure of the EDC and in the specific positioning of this locus within the landscape of the 3D nuclear space, as required for the efficient expression of EDC genes in epidermal progenitor cells during skin development.

Multicolor laser scanning confocal immunofluorescence microscopy of DNA damage response biomarkers.

DNA damage through endogenous and environmental toxicants is a constant threat to both a human's ability to pass on intact genetic information to its offspring as well as somatic cells for their own survival. To counter these threats posed by DNA damage, cells have evolved a series of highly choreographed mechanisms--collectively defined as the DNA damage response (DDR)--to sense DNA lesions, signal their presence, and mediate their repair. Thus, regular DDR signalling cascades are vital to prevent the initiation and progression of many human diseases including cancer. Consequently, quantitative assessment of DNA damage and response became an important biomarker for assessment of human health and disease risk in biomonitoring studies. However, most quantitative DNA damage biomarker techniques require dissolution of the nuclear architecture and hence loss of spatial information. Laser scanning confocal immunofluorescence microscopy (LSCIM) of three-dimensionally preserved nuclei can be quantitative and maintain the spatial information. Here we describe the experimental protocols to quantify individual key events of the DDR cascade in three-dimensionally preserved nuclei by LSCIM with high resolution, using the simultaneous detection of Rad50 as well as phosphorylated H2AX and ATM and in somatic and germ cells as an example.

Remodeling of three-dimensional organization of the nucleus during terminal keratinocyte differentiation in the epidermis.

The nucleus of epidermal keratinocytes (KCs) is a complex and highly compartmentalized organelle, whose structure is markedly changed during terminal differentiation and transition of the genome from a transcriptionally active state seen in the basal and spinous epidermal cells to a fully inactive state in the keratinized cells of the cornified layer. Here, using multicolor confocal microscopy, followed by computational image analysis and mathematical modeling, we demonstrate that in normal mouse footpad epidermis, transition of KCs from basal epidermal layer to the granular layer is accompanied by marked differences in nuclear architecture and microenvironment including the following: (i) decrease in the nuclear volume; (ii) decrease in expression of the markers of transcriptionally active chromatin; (iii) internalization and decrease in the number of nucleoli; (iv) increase in the number of pericentromeric heterochromatic clusters; and (v) increase in the frequency of associations between the pericentromeric clusters, chromosomal territory 3, and nucleoli. These data suggest a role for nucleoli and pericentromeric heterochromatin clusters as organizers of nuclear microenvironment required for proper execution of gene expression programs in differentiating KCs, and provide important background information for further analyses of alterations in the topological genome organization seen in pathological skin conditions, including disorders of epidermal differentiation and epidermal tumors.

Epigenetic regulation of gene expression in keratinocytes.

The nucleus is a complex and highly compartmentalized organelle, which undergoes major organization changes during cell differentiation, allowing cells to become specialized and fulfill their functions. During terminal differentiation of the epidermal keratinocytes, the nucleus undergoes a programmed transformation from active status, associated with execution of the genetic programs of cornification and epidermal barrier formation, to a fully inactive condition and becomes a part of the keratinized cells of the cornified layer. Tremendous progress achieved within the past two decades in understanding the biology of the nucleus and epigenetic mechanisms controlling gene expression allowed defining several levels in the regulation of cell differentiation-associated gene expression programs, including an accessibility of the gene regulatory regions to DNA-protein interactions, covalent DNA and histone modifications, and ATP-dependent chromatin remodeling, as well as higher-order chromatin remodeling and nuclear compartmentalization of the genes and transcription machinery. Here, we integrate our current knowledge of the mechanisms controlling gene expression during terminal keratinocyte differentiation with distinct levels of chromatin organization and remodeling. We also propose directions to further explore the role of epigenetic mechanisms and their interactions with other regulatory systems in the control of keratinocyte differentiation in normal and diseased skin.

Cigarette smoke-induced transgenerational alterations in genome stability in cord blood of human F1 offspring.

The relevance of preconceptional and prenatal toxicant exposures for genomic stability in offspring is difficult to analyze in human populations, because gestational exposures usually cannot be separated from preconceptional exposures. To analyze the roles of exposures during gestation and conception on genomic stability in the offspring, stability was assessed via the Comet assay and highly sensitive, semiautomated confocal laser scans of γH2AX foci in cord, maternal, and paternal blood as well as spermatozoa from 39 families in Crete, Greece, and the United Kingdom. With use of multivariate linear regression analysis with backward selection, preconceptional paternal smoking (% tail DNA: P>0.032; γH2AX foci: P>0.018) and gestational maternal (% tail DNA: P>0.033) smoking were found to statistically significantly predict DNA damage in the cord blood of F1 offspring. Maternal passive smoke exposure was not identified as a predictor of DNA damage in cord blood, indicating that the effect of paternal smoking may be transmitted via the spermatozoal genome. Taken together, these studies reveal a role for cigarette smoke in the induction of DNA alterations in human F1 offspring via exposures of the fetus in utero or the paternal germline. Moreover, the identification of transgenerational DNA alterations in the unexposed F1 offspring of smoking-exposed fathers supports the claim that cigarette smoke is a human germ cell mutagen.

p63 regulates Satb1 to control tissue-specific chromatin remodeling during development of the epidermis.

During development, multipotent progenitor cells establish tissue-specific programs of gene expression. In this paper, we show that p63 transcription factor, a master regulator of epidermal morphogenesis, executes its function in part by directly regulating expression of the genome organizer Satb1 in progenitor cells. p63 binds to a proximal regulatory region of the Satb1 gene, and p63 ablation results in marked reduction in the Satb1 expression levels in the epidermis. Satb1(-/-) mice show impaired epidermal morphology. In Satb1-null epidermis, chromatin architecture of the epidermal differentiation complex locus containing genes associated with epidermal differentiation is altered primarily at its central domain, where Satb1 binding was confirmed by chromatin immunoprecipitation-on-chip analysis. Furthermore, genes within this domain fail to be properly activated upon terminal differentiation. Satb1 expression in p63(+/-) skin explants treated with p63 small interfering ribonucleic acid partially restored the epidermal phenotype of p63-deficient mice. These data provide a novel mechanism by which Satb1, a direct downstream target of p63, contributes in epidermal morphogenesis via establishing tissue-specific chromatin organization and gene expression in epidermal progenitor cells.