Reversal of high-glucose-induced transcriptional and epigenetic memories through NRF2 pathway activation

Diabetes complications such as nephropathy, retinopathy, or cardiovascular disease arise from vascular dysfunction. In this context, it has been observed that past hyperglycemic events can induce long-lasting alterations, a phenomenon termed "metabolic memory." In this study, we evaluated the genome-wide gene expression and chromatin accessibility alterations caused by transient high-glucose exposure in human endothelial cells (ECs) in vitro. We found that cells exposed to high glucose exhibited substantial gene expression changes in pathways known to be impaired in diabetes, many of which persist after glucose normalization. Chromatin accessibility analysis also revealed that transient hyperglycemia induces persistent alterations, mainly in non-promoter regions identified as enhancers with neighboring genes showing lasting alterations. Notably, activation of the NRF2 pathway through NRF2 overexpression or supplementation with the plant-derived compound sulforaphane, effectively reverses the glucose-induced transcriptional and chromatin accessibility memories in ECs. These findings underscore the enduring impact of transient hyperglycemia on ECs' transcriptomic and chromatin accessibility profiles, emphasizing the potential utility of pharmacological NRF2 pathway activation in mitigating and reversing the high-glucose-induced transcriptional and epigenetic alterations.

Polycomb complexes redundantly maintain epidermal stem cell identity during development

In this study, Cohen et al. sought to understand the functional contribution of PRC1 and PRC2, which largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3, to gene repression. By using the developing murine epidermis as a paradigm, they uncovered a previously unappreciated functional redundancy between Polycomb complexes, and their findings show how PRC1 and PRC2 function as two independent counterparts, providing a repressive safety net that protects and preserves lineage identity.

Polycomb repressive complex 1 (PRC1) and PRC2 are critical epigenetic developmental regulators. PRC1 and PRC2 largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3. However, the functional contribution of each complex to gene repression has been a subject of debate, and understanding of its physiological significance requires further studies. Here, using the developing murine epidermis as a paradigm, we uncovered a previously unappreciated functional redundancy between Polycomb complexes. Coablation of PRC1 and PRC2 in embryonic epidermal progenitors resulted in severe defects in epidermal stratification, a phenotype not observed in the single PRC1-null or PRC2-null epidermis. Molecular dissection indicated a loss of epidermal identity that was coupled to a strong derepression of nonlineage transcription factors, otherwise repressed by either PRC1 or PRC2 in the absence of its counterpart. Ectopic expression of subsets of PRC1/2-repressed nonepidermal transcription factors in wild-type epidermal stem cells was sufficient to suppress epidermal identity genes, highlighting the importance of functional redundancy between PRC1 and PRC2. Altogether, our studies show how PRC1 and PRC2 function as two independent counterparts, thereby providing a repressive safety net that protects and preserves lineage identity.

PRC1 Fine-tunes Gene Repression and Activation to Safeguard Skin Development and Stem Cell Specification

Polycomb repressive complexes (PRCs) 1 and 2 are essential chromatin regulators of cell identity. PRC1, a dominant executer of Polycomb-mediated control, functions as multiple sub-complexes that possess catalytic-dependent H2AK119 mono-ubiquitination (H2AK119ub) and catalytic-independent activities. Here, we show that, despite its well-established repressor functions, PRC1 binds to both silent and active genes. Through in vivo loss-of-function studies, we show that global PRC1 function is essential for skin development and stem cell (SC) specification, whereas PRC1 catalytic activity is dispensable. Further dissection demonstrated that both canonical and non-canonical PRC1 complexes bind to repressed genes, marked by H2AK119ub and PRC2-mediated H3K27me3. Interestingly, loss of canonical PRC1, PRC1 catalytic activity, or PRC2 leads to expansion of mechanosensitive Merkel cells in neonatal skin. Non-canonical PRC1 complexes, however, also bind to and promote expression of genes critical for skin development and SC formation. Together, our findings highlight PRC1's diverse roles in executing a precise developmental program.

Dissecting the Roles of Polycomb Repressive Complex 2 Subunits in the Control of Skin Development

Polycomb repressive complex 2 (PRC2) is an essential regulator of cell physiology. Although there have been numerous studies on PRC2 function in somatic tissue development and stem cell control, these have focused on the loss of a single PRC2 subunit. Recent studies, however, have shown that PRC2 subunits may function independently of the PRC2 complex. To investigate the function of PRC2 in the control of skin development, we generated and analyzed three conditional knockout mouse lines, in which the essential PRC2 subunits embryonic ectoderm development (EED), suppressor of zeste 12 homolog (Suz12), and enhancer of zeste homologs 1 and 2 (Ezh1/2) are conditionally ablated in the embryonic epidermal progenitors that give rise to the epidermis, hair follicles, and Merkel cells. Our studies showed that the observed loss-of-function phenotypes are shared between the three knockouts, indicating that in the skin epithelium, EED, Suz12, and Ezh1/2 function largely as subunits of the PRC2 complex. Interestingly, the absence of PRC2 results in dramatically different phenotypes across the different skin lineages: premature acquisition of a functional epidermal barrier, formation of ectopic Merkel cells, and defective postnatal development of hair follicles. The strikingly different roles of PRC2 in the formation of three lineages exemplify the complex outcomes that the lack of PRC2 can have in a somatic stem cell system.

Embryonic maturation of epidermal Merkel cells is controlled by a redundant transcription factor network

Merkel cell-neurite complexes are located in touch-sensitive areas of the mammalian skin and are involved in recognition of the texture and shape of objects. Merkel cells are essential for these tactile discriminations, as they generate action potentials in response to touch stimuli and induce the firing of innervating afferent nerves. It has been shown that Merkel cells originate from epidermal stem cells, but the cellular and molecular mechanisms of their development are largely unknown. In this study, we analyzed Merkel cell differentiation during development and found that it is a temporally regulated maturation process characterized by a sequential activation of Merkel cell-specific genes. We uncovered key transcription factors controlling this process and showed that the transcription factor Atoh1 is required for initial Merkel cell specification. The subsequent maturation steps of Merkel cell differentiation are controlled by cooperative function of the transcription factors Sox2 and Isl1, which physically interact and work to sustain Atoh1 expression. These findings reveal the presence of a robust transcriptional network required to produce functional Merkel cells that are required for tactile discrimination.

Epigenetic regulation of epidermal differentiation

In a cell, the chromatin state is controlled by the highly regulated interplay of epigenetic mechanisms ranging from DNA methylation and incorporation of different histone variants to posttranslational modification of histones and ATP-dependent chromatin remodeling. These changes alter the structure of the chromatin to either facilitate or restrict the access of transcription machinery to DNA. These epigenetic modifications function to exquisitely orchestrate the expression of different genes, and together constitute the epigenome of a cell. In the skin, different epigenetic regulators form a regulatory network that operates to guarantee skin stem cell maintenance while controlling differentiation to multiple skin structures. In this review, we will discuss recent findings on epigenetic mechanisms of skin control and their relationship to skin pathologies.

Polycomb subunits Ezh1 and Ezh2 regulate the Merkel cell differentiation program in skin stem cells

While the Polycomb complex is known to regulate cell identity in ES cells, its role in controlling tissue-specific stem cells is not well understood. Here we show that removal of Ezh1 and Ezh2, key Polycomb subunits, from mouse skin results in a marked change in fate determination in epidermal progenitor cells, leading to an increase in the number of lineage-committed Merkel cells, a specialized subtype of skin cells involved in mechanotransduction. By dissecting the genetic mechanism, we showed that the Polycomb complex restricts differentiation of epidermal progenitor cells by repressing the transcription factor Sox2. Ablation of Sox2 results in a dramatic loss of Merkel cells, indicating that Sox2 is a critical regulator of Merkel cell specification. We show that Sox2 directly activates Atoh1, the obligate regulator of Merkel cell differentiation. Concordantly, ablation of Sox2 attenuated the Ezh1/2-null phenotype, confirming the importance of Polycomb-mediated repression of Sox2 in maintaining the epidermal progenitor cell state. Together, these findings define a novel regulatory network by which the Polycomb complex maintains the progenitor cell state and governs differentiation in vivo.

CUP-1 is a novel protein involved in dietary cholesterol uptake in Caenorhabditis elegans

Sterols transport and distribution are essential processes in all multicellular organisms. Survival of the nematode Caenorhabditis elegans depends on dietary absorption of sterols present in the environment. However the general mechanisms associated to sterol uptake in nematodes are poorly understood. In the present work we provide evidence showing that a previously uncharacterized transmembrane protein, designated Cholesterol Uptake Protein-1 (ChUP-1), [corrected] is involved in dietary cholesterol uptake in C. elegans. Animals lacking ChUP-1 [corrected] showed hypersensitivity to cholesterol limitation and were unable to uptake cholesterol. A ChUP-1-GFP [corrected] fusion protein colocalized with cholesterol-rich vesicles, endosomes and lysosomes as well as the plasma membrane. Additionally, by FRET imaging, a direct interaction was found between the cholesterol analog DHE and the transmembrane "cholesterol recognition/interaction amino acid consensus" (CRAC) motif present in C. elegans ChUP-1. [corrected]. In-silico analysis identified two mammalian homologues of ChUP-1. [corrected]. Most interestingly, CRAC motifs are conserved in mammalian ChUP-1 [corrected] homologous. Our results suggest a role of ChUP-1 [corrected] in cholesterol uptake in C. elegans and open up the possibility for the existence of a new class of proteins involved in sterol absorption in mammals.