Uncovering mechanisms of nuclear degradation in keratinocytes: A paradigm for nuclear degradation in other tissues

Sculpting nuclear envelope identity from the endoplasmic reticulum during the cell cycle

The nuclear envelope (NE) regulates nuclear functions, including transcription, nucleocytoplasmic transport, and protein quality control. While the outer membrane of the NE is directly continuous with the endoplasmic reticulum (ER), the NE has an overall distinct protein composition from the ER, which is crucial for its functions. During open mitosis in higher eukaryotes, the NE disassembles during mitotic entry and then reforms as a functional territory at the end of mitosis to reestablish nucleocytoplasmic compartmentalization. In this review, we examine the known mechanisms by which the functional NE reconstitutes from the mitotic ER in the continuous ER-NE endomembrane system during open mitosis. Furthermore, based on recent findings indicating that the NE possesses unique lipid metabolism and quality control mechanisms distinct from those of the ER, we explore the maintenance of NE identity and homeostasis during interphase. We also highlight the potential significance of membrane junctions between the ER and NE.

Active nuclear positioning and actomyosin contractility maintain leader cell integrity during gonadogenesis

Proper distribution of organelles can play an important role in a moving cell's performance. During C. elegans gonad morphogenesis, the nucleus of the leading distal tip cell (DTC) is always found at the front, yet the significance of this localization is unknown. Here, we identified the molecular mechanism that keeps the nucleus at the front, despite a frictional force that pushes it backward. The Klarsicht/ANC-1/Syne homology (KASH) domain protein UNC-83 links the nucleus to the motor protein kinesin-1 that moves along a polarized acentrosomal microtubule network. Interestingly, disrupting nuclear positioning on its own did not affect gonad morphogenesis. However, reducing actomyosin contractility on top of nuclear mispositioning led to a dramatic phenotype: DTC splitting and gonad bifurcation. Long-term live imaging of the double knockdown revealed that, while the gonad attempted to perform a planned U-turn, the DTC was stretched due to the lagging nucleus until it fragmented into a nucleated cell and an enucleated cytoplast, each leading an independent gonadal arm. Remarkably, the enucleated cytoplast had polarity and invaded, but it could only temporarily support germ cell proliferation. Based on a qualitative biophysical model, we conclude that the leader cell employs two complementary mechanical approaches to preserve its integrity and ensure proper organ morphogenesis while navigating through a complex 3D environment: active nuclear positioning by microtubule motors and actomyosin-driven cortical contractility.

A miRNA-based epigenetic molecular clock for biological skin-age prediction

Skin aging is one of the visible characteristics of the aging process in humans. In recent years, different biological clocks have been generated based on protein or epigenetic markers, but few have focused on biological age in the skin. Arrest the aging process or even being able to restore an organism from an older to a younger stage is one of the main challenges in the last 20 years in biomedical research. We have implemented several machine learning models, including regression and classification algorithms, in order to create an epigenetic molecular clock based on miRNA expression profiles of healthy subjects to predict biological age-related to skin. Our best models are capable of classifying skin samples according to age groups (18-28; 29-39; 40-50; 51-60 or 61-83 years old) with an accuracy of 80% or predict age with a mean absolute error of 10.89 years using the expression levels of 1856 unique miRNAs. Our results suggest that this kind of epigenetic clocks arises as a promising tool with several applications in the pharmaco-cosmetic industry.

Assessing the safety of new germicidal far-UVC technologies

The COVID-19 pandemic underscored the crucial importance of enhanced indoor air quality control measures to mitigate the spread of respiratory pathogens. Far-UVC is a type of germicidal ultraviolet technology, with wavelengths between 200 and 235 nm, that has emerged as a highly promising approach for indoor air disinfection. Due to its enhanced safety compared to conventional 254 nm upper-room germicidal systems, far-UVC allows for whole-room direct exposure of occupied spaces, potentially offering greater efficacy, since the total room air is constantly treated. While current evidence supports using far-UVC systems within existing guidelines, understanding the upper safety limit is critical to maximizing its effectiveness, particularly for the acute phase of a pandemic or epidemic when greater protection may be needed. This review article summarizes the substantial present knowledge on far-UVC safety regarding skin and eye exposure and highlights research priorities to discern the maximum exposure levels that avoid adverse effects. We advocate for comprehensive safety studies that explore potential mechanisms of harm, generate action spectra for crucial biological effects and conduct high-dose, long-term exposure trials. Such rigorous scientific investigation will be key to determining safe and effective levels for far-UVC deployment in indoor environments, contributing significantly to future pandemic preparedness and response.

Down-regulation of peptidylarginine deiminase type 1 in reconstructed human epidermis disturbs nucleophagy in the granular layer and affects barrier function

Deimination is a post-translational modification catalyzed by a family of enzymes named peptidylarginine deiminases (PADs). PADs transform arginine residues of protein substrates into citrulline. Deimination has been associated with numerous physiological and pathological processes. In human skin, three PADs are expressed (PAD1-3). While PAD3 is important for hair shape formation, the role of PAD1 is less clear. To decipher the main role(s) of PAD1 in epidermal differentiation, its expression was down-regulated using lentivirus-mediated shRNA interference in primary keratinocytes and in three-dimensional reconstructed human epidermis (RHE). Compared to normal RHEs, down-regulation of PAD1 caused a drastic reduction in deiminated proteins. Whereas proliferation of keratinocytes was not affected, their differentiation was disturbed at molecular, cellular and functional levels. The number of corneocyte layers was significantly reduced, expression of filaggrin and cornified cell envelope components, such as loricrin and transglutaminases, was down-regulated, epidermal permeability increased and trans-epidermal-electric resistance diminished drastically. Keratohyalin granule density decreased and nucleophagy in the granular layer was disturbed. These results demonstrate that PAD1 is the main regulator of protein deimination in RHE. Its deficiency alters epidermal homeostasis, affecting the differentiation of keratinocytes, especially the cornification process, a special kind of programmed cell death.

Extremely small wasps independently lost the nuclei in the brain neurons of at least two lineages

Anucleate animal cells are a peculiar evolutionary phenomenon and a useful model for studying cellular mechanisms. Anucleate neurons were recently found in one genus of miniature parasitic wasps of the family Trichogrammatidae, but it remained unclear how widespread this phenomenon is among other insects or even among different tissues of the same insect species. We studied the anatomy of miniature representatives of another parasitic wasp family (Hymenoptera: Mymaridae) using array tomography and found two more species with nearly anucleate brains at the adult stage. Thus, the lysis of the cell bodies and nuclei of neurons appears to be a more widespread means of saving space during extreme miniaturization, which independently evolved at least twice during miniaturization in different groups of insects. These results are important for understanding the evolution of the brain during miniaturization and open new areas of studying the functioning of anucleate neurons.

Repurposing a Cyclin-Dependent Kinase 1 (CDK1) Mitotic Regulatory Network to Complete Terminal Differentiation in Lens Fiber Cells

Purpose

During lens fiber cell differentiation, organelles are removed in an ordered manner to ensure lens clarity. A critical step in this process is removal of the cell nucleus, but the mechanisms by which this occurs are unclear. In this study, we investigate the role of a cyclin-dependent kinase 1 (CDK1) regulatory loop in controlling lens fiber cell denucleation (LFCD).

Methods

We examined lens differentiation histologically in two different vertebrate models. An embryonic chick lens culture system was used to test the role of CDK1, cell division cycle 25 (CDC25), WEE1, and PP2A in LFCD. Additionally, we used three mouse models that express high levels of the CDK inhibitor p27 to test whether increased p27 levels affect LFCD.

Results

Using chick lens organ cultures, small-molecule inhibitors of CDK1 and CDC25 inhibit LFCD, while inhibiting the CDK1 inhibitory kinase WEE1 potentiates LFCD. Additionally, treatment with an inhibitor of PP2A, which indirectly inhibits CDK1 activity, also increased LFCD. Three different mouse models that express increased levels of p27 through different mechanisms show impaired LFCD.

Conclusions

Here we define a conserved nonmitotic role for CDK1 and its upstream regulators in controlling LFCD. We find that CDK1 functionally interacts with WEE1, a nuclear kinase that inhibits CDK1 activity, and CDC25 activating phosphatases in cells where CDK1 activity must be exquisitely regulated to allow for LFCD. We also provide genetic evidence in multiple in vivo models that p27, a CDK1 inhibitor, inhibits lens growth and LFCD.

Stratum corneum as polymer sheet: concept and cornification processes

The skin barrier protects our body from external insults and water loss through a specialized layer, the stratum corneum. The stratum corneum, an accumulation of dead keratinocytes (corneocytes), comprises lipids and supporting cell bodies. We propose a framework of lipid-filled polymer sheet of corneocytes, a unique structure that achieves flexibility and robustness, updating the rigid image of the historical bricks-and-mortar model. The polymerization of polymer sheet (cornification) by cell death of keratinocytes (corneoptosis) is delicately and dynamically controlled by cytoplasmic calcium ion and pH. Understanding the structure and formation of the stratum corneum can lead to better treatments for skin diseases and a better understanding of the evolution of the stratum corneum.

TEX264 at the intersection of autophagy and DNA repair

TEX264 (testes expressed gene 264) is a single-pass transmembrane protein, consisting of an N-terminal hydrophobic region, a gyrase inhibitory (GyrI)-like domain, and a loosely structured C terminus. TEX264 was first identified as an endoplasmic reticulum (ER)-resident Atg8-family-binding protein that mediates the degradation of portions of the ER during starvation (i.e., reticulophagy). More recently, TEX264 was identified as a cofactor of VCP/p97 ATPase that promotes the repair of covalently trapped TOP1 (DNA topoisomerase 1)-DNA crosslinks. This review summarizes the current knowledge of TEX264 as a protein with roles in both autophagy and DNA repair and provides an evolutionary and structural analysis of GyrI proteins. Based on our phylogenetic analysis, we provide evidence that TEX264 is a member of a large superfamily of GyrI-like proteins that evolved in bacteria and are present in metazoans, including invertebrates and chordates.Abbreviations: Atg8: autophagy related 8; Atg39: autophagy related 39; Cdc48: cell division cycle 48; CGAS: cyclic GMP-AMP synthase; DPC: DNA-protein crosslinks; DSB: DNA double-strand break; ER: endoplasmic reticulum; GyrI: gyrase inhibitory domain; LRR: leucine-rich repeat; MAFFT: multiple alignment using fast Fourier transform; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; STUBL: SUMO targeted ubiquitin ligase; SUMO: small ubiquitin-like modifier; TEX264: testis expressed gene 264; TOP1cc: topoisomerase 1-cleavage complex; UBZ: ubiquitin binding Zn finger domain; VCP: valosin containing protein.

Effects of Heat-Killed Lactococcus lactis Strain Plasma on Skin Homeostasis-Related Genes and the Skin Microbiome among Healthy Adults: A Randomized Controlled Double-Blind Study

Lactococcus lactis subsp. lactis strain plasma (LC-plasma) is a bacterial strain that activates plasmacytoid dendritic cells and induces viral resistance genes via the TLR9/MyD88 pathway. We recently showed that oral administration of LC-plasma prevents skin infection by Staphylococcus aureus, possibly by activating skin immunity. In this study, we conducted a double-blind clinical trial to investigate the effect of oral administration of heat-killed LC-plasma on the skin microbiome, gene expression in the skin, and the skin condition of healthy volunteers. Seventy healthy volunteers were randomly assigned to receive either heat-killed LC-plasma or a placebo for eight weeks. Analysis of the skin microbiome by next-generation sequencing suggested that the alpha-diversity of the skin microbiome did not change during the test period in either group. However, the proportion of species that changed significantly during the test period was 10-fold smaller in the LC-plasma group than in the placebo group, suggesting that LC-plasma may maintain the skin microbiome. Quantitative PCR analysis indicated that tight-junction genes, such as CLDN1 and CLDN12, and the antimicrobial peptide gene BD3 were significantly up-regulated in the LC-plasma group but not in the placebo group. Our results suggest that administration of LC-plasma helps to maintain the skin microbiome and that it affects homeostasis-related genes.

Mechanisms of organelle elimination for lens development and differentiation

A hallmark feature of lens development and differentiation is the complete elimination of organelles from the center of the eye lens. A long unanswered question in lens biology is what are the mechanisms that control the elimination of organelles during the terminal remodeling program to form mature lens fiber cells? Recent advances have expanded our understanding of these mechanisms including newly discovered signaling pathways, proteasomal regulators, autophagy proteins, transcription factors and the hypoxic environment of the lens itself. These recent discoveries suggest that distinct mechanisms coordinate the elimination of the nucleus, mitochondria, endoplasmic reticulum and Golgi apparatus during lens fiber cell differentiation. Since regulation of organelle number and distribution is also a feature of the terminal remodeling programs of more complex cell-types and tissues, these advances are likely to impact a wide-variety of fields.

Faces of cellular senescence in skin aging

The skin is comprised of different cell types with different proliferative capacities. Skin aging occurs with chronological age and upon exposure to extrinsic factors such as photodamage. During aging, senescent cells accumulate in different compartments of the human skin, leading to impaired skin physiology. Diverse skin cell types may respond differently to senescence-inducing stimuli and it is not clear how this results in aging-associated skin phenotypes and pathologies. This review aims to examine and provide an overview of current evidence of cellular senescence in the skin. We will focus on cellular characteristics and behaviour of different skin cell types undergoing senescence in the epidermis and dermis, with a particular focus on the complex interplay between mitochondrial dysfunction, autophagy and DNA damage pathways. We will also examine how the dermis and epidermis cope with the accumulation of DNA damage during aging.

Akt1-associated actomyosin remodelling is required for nuclear lamina dispersal and nuclear shrinkage in epidermal terminal differentiation

Keratinocyte cornification and epidermal barrier formation are tightly controlled processes, which require complete degradation of intracellular organelles, including removal of keratinocyte nuclei. Keratinocyte nuclear destruction requires Akt1-dependent phosphorylation and degradation of the nuclear lamina protein, Lamin A/C, essential for nuclear integrity. However, the molecular mechanisms that result in complete nuclear removal and their regulation are not well defined. Post-confluent cultures of rat epidermal keratinocytes (REKs) undergo spontaneous and complete differentiation, allowing visualisation and perturbation of the differentiation process in vitro. We demonstrate that there is dispersal of phosphorylated Lamin A/C to structures throughout the cytoplasm in differentiating keratinocytes. We show that the dispersal of phosphorylated Lamin A/C is Akt1-dependent and these structures are specific for the removal of Lamin A/C from the nuclear lamina; nuclear contents and Lamin B were not present in these structures. Immunoprecipitation identified a group of functionally related Akt1 target proteins involved in Lamin A/C dispersal, including actin, which forms cytoskeletal microfilaments, Arp3, required for actin filament nucleation, and Myh9, a component of myosin IIa, a molecular motor that can translocate along actin filaments. Disruption of actin filament polymerisation, nucleation or myosin IIa activity prevented formation and dispersal of cytoplasmic Lamin A/C structures. Live imaging of keratinocytes expressing fluorescently tagged nuclear proteins showed a nuclear volume reduction step taking less than 40 min precedes final nuclear destruction. Preventing Akt1-dependent Lamin A/C phosphorylation and disrupting cytoskeletal Akt1-associated proteins prevented nuclear volume reduction. We propose keratinocyte nuclear destruction and differentiation requires myosin II activity and the actin cytoskeleton for two intermediate processes: Lamin A/C dispersal and rapid nuclear volume reduction.

Crystallin gene expression: Insights from studies of transcriptional bursting

Cellular differentiation is marked by temporally and spatially regulated gene expression. The ocular lens is one of the most powerful mammalian model system since it is composed from only two cell subtypes, called lens epithelial and fiber cells. Lens epithelial cells differentiate into fiber cells through a series of spatially and temporally orchestrated processes, including massive production of crystallins, cellular elongation and the coordinated degradation of nuclei and other organelles. Studies of transcriptional and posttranscriptional gene regulatory mechanisms in lens provide a wide range of opportunities to understand global molecular mechanisms of gene expression as steady-state levels of crystallin mRNAs reach very high levels comparable to globin genes in erythrocytes. Importantly, dysregulation of crystallin gene expression results in lens structural abnormalities and cataracts. The mRNA life cycle is comprised of multiple stages, including transcription, splicing, nuclear export into cytoplasm, stabilization, localization, translation and ultimate decay. In recent years, development of modern mRNA detection methods with single molecule and single cell resolution enabled transformative studies to visualize the mRNA life cycle to generate novel insights into the sequential regulatory mechanisms of gene expression during embryogenesis. This review is focused on recent major advancements in studies of transcriptional bursting in differentiating lens fiber cells, analysis of nascent mRNA expression from bi-directional promoters, transient nuclear accumulation of specific mRNAs, condensation of chromatin prior lens fiber cell denucleation, and outlines future studies to probe the interactions of individual mRNAs with specific RNA-binding proteins (RBPs) in the cytoplasm and regulation of translation and mRNA decay.

Perspective: Controlling Epidermal Terminal Differentiation with Transcriptional Bursting and RNA Bodies

The outer layer of the skin, the epidermis, is the principal barrier to the external environment: post-mitotic cells terminally differentiate to form a tough outer cornified layer of enucleate and flattened cells that confer the majority of skin barrier function. Nuclear degradation is required for correct cornified envelope formation. This process requires mRNA translation during the process of nuclear destruction. In this review and perspective, we address the biology of transcriptional bursting and the formation of ribonuclear particles in model organisms including mammals, and then examine the evidence that these phenomena occur as part of epidermal terminal differentiation.

Addressing Differentiation in Live Human Keratinocytes by Assessment of Membrane Packing Order

Differentiation of keratinocytes is critical for epidermal stratification and formation of a protective stratum corneum. It involves a series of complex processes leading through gradual changes in characteristics and functions of keratinocytes up to their programmed cell death via cornification. The stratum corneum is a relatively impermeable barrier, comprised of dead cell remnants (corneocytes) embedded in lipid matrix. Corneocyte membranes are comprised of specialized lipids linked to late differentiation proteins, contributing to the formation of a stiff and mechanically strengthened layer. To date, the assessment of the progression of keratinocyte differentiation is only possible through determination of specific differentiation markers, e.g., by using proteomics-based approaches. Unfortunately, this requires fixation or cell lysis, and currently there is no robust methodology available to study keratinocyte differentiation in living cells in real-time. Here, we explore new live-cell based approaches for screening differentiation advancement in keratinocytes, in a "calcium switch" model. We employ a polarity-sensitive dye, Laurdan, and Laurdan general polarization function (GP) as a reporter of the degree of membrane lateral packing order or condensation, as an adequate marker of differentiation. We show that the assay is straightforward and can be conducted either on a single cell level using confocal spectral imaging or on the ensemble level using a fluorescence plate reader. Such systematic quantification may become useful for understanding mechanisms of keratinocyte differentiation, such as the role of membrane in homogeneities in stiffness, and for future therapeutic development.

Knockout of DNase1l1l abrogates lens denucleation process and causes cataract in zebrafish

Removal of nuclei in lens fiber cells is required for organelle-free zone (OFZ) formation during lens development. Defect in degradation of nuclear DNA leads to cataract formation. DNase2β degrades nuclear DNA of lens fiber cells during lens differentiation in mouse. Hsf4 is the principal heat shock transcription factor in lens and facilitates the lens differentiation. Knockout of Hsf4 in mouse and zebrafish resulted in lens developmental defect that was characterized by retaining of nuclei in lens fiber cells. In previous in vitro studies, we found that Hsf4 promoted DNase2β expression in human and mouse lens epithelial cells. In this study, it was found that, instead of DNase2β, DNase1l1l is uniquely expressed in zebrafish lens and was absent in Hsf4^-/- zebrafish lens. Using CRISPR-Cas9 technology, a DNase1l1l knockout zebrafish line was constructed, which developed cataract. Deletion of DNase1l1l totally abrogated lens primary and secondary fiber cell denucleation process, whereas had little effect on the clearance of other organelles. The transcriptional regulation of DNase1l1l was dramatically impaired in Hsf4^-/- zebrafish lens. Rescue of DNase1l1l mRNA into Hsf4^-/- zebrafish embryos alleviated its defect in lens fiber cell denucleation. Our results in vivo demonstrated that DNase1l1l is the primary DNase responsible for nuclear DNA degradation in lens fiber cells, and Hsf4 can transcriptionally activate DNase1l1l expression in zebrafish.

To Eat or to Die: Deciphering Selective Forms of Autophagy

Autophagy is an evolutionarily conserved process whereby damaged and redundant components of the cell are degraded in structures called autophagolysosomes. Currently, three main types of autophagy are recognized: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). However, we still know little about some specific types of autophagy that are linked to various intracellular compartments and their roles in the physiology of the whole organism and connections to various diseases. Here, we aim to shed light on the latest insights on and mechanisms of several selective forms of autophagy.

Transcriptomic analysis and novel insights into lens fibre cell differentiation regulated by Gata3

Gata3 is a DNA-binding transcription factor involved in cellular differentiation in a variety of tissues including inner ear, hair follicle, kidney, mammary gland and T-cells. In a previous study in 2009, Maeda et al. (Dev. Dyn.238, 2280–2291; doi:10.1002/dvdy.22035) found that Gata3 mutants could be rescued from midgestational lethality by the expression of a Gata3 transgene in sympathoadrenal neuroendocrine cells. The rescued embryos clearly showed multiple defects in lens fibre cell differentiation. To determine whether these defects were truly due to the loss of Gata3 expression in the lens, we generated a lens-specific Gata3 loss-of-function model. Analogous to the previous findings, our Gata3 null embryos showed abnormal regulation of cell cycle exit during lens fibre cell differentiation, marked by reduction in the expression of the cyclin-dependent kinase inhibitors Cdkn1b/p27 and Cdkn1c/p57, and the retention of nuclei accompanied by downregulation of Dnase IIβ. Comparisons of transcriptomes between control and mutated lenses by RNA-Seq revealed dysregulation of lens-specific crystallin genes and intermediate filament protein Bfsp2. Both Cdkn1b/p27 and Cdkn1c/p57 loci are occupied in vivo by Gata3, as well as Prox1 and c-Jun, in lens chromatin. Collectively, our studies suggest that Gata3 regulates lens differentiation through the direct regulation of the Cdkn1b/p27and Cdkn1c/p57 expression, and the direct/or indirect transcriptional control of Bfsp2 and Dnase IIβ.

Protein kinases involved in epidermal barrier formation: The AKT family and other animals

Formation of a stratified epidermis is required for the performance of the essential functions of the skin; to act as an outside-in barrier against the access of microorganisms and other external factors, to prevent loss of water and solutes via inside-out barrier functions and to withstand mechanical stresses. Epidermal barrier function is initiated during embryonic development and is then maintained throughout life and restored after injury. A variety of interrelated processes are required for the formation of a stratified epidermis, and how these processes are both temporally and spatially regulated has long been an aspect of dermatological research. In this review, we describe the roles of multiple protein kinases in the regulation of processes required for epidermal barrier formation.

Generation of two-cell cloned embryos from mouse faecal cell

Cloning animals using nuclear transfer (NT) provides the opportunity to preserve endangered species. However, there are risks associated with the collection of donor cells from a body, which may cause accidental death of the animal. Here, we tried to collect faeces-derived cells and examined the usability of those nuclei as a donor for NT. A relatively large number of cells could be collected from GFP-Tg mouse faeces by this method. After NT, only 4.2% of the reconstructed oocytes formed pseudo-pronucleus. This rate increased up to 25% when GFP and Hoechst were used as a marker to select better cells. However, the reconstructed oocytes/embryos showed several abnormalities, such as shrunken nuclear membranes and abnormal distribution of tubulin, and none of them developed beyond one-cell stage embryos. These developmental failures were caused by not only toxic substances derived from faeces but also intrinsic DNA damage of donor cell nuclei. However, when the serial NT was performed, some of the cloned embryos could develop to the two-cell stage. This method may remove toxic substances and enhance DNA repair in the oocyte cytoplasm. Thus, these results indicate that faeces cells might be useful for the conservation of endangered species when technical improvements are achieved.