Comparative Spatial Transcriptomic and Single-Cell Analyses of Human Nail Units and Hair Follicles Show Transcriptional Similarities between the Onychodermis and Follicular Dermal Papilla

Emerging Techniques in Spatial Multiomics: Fundamental Principles and Applications to Dermatology

Molecular pathology, such as high-throughput genomic and proteomic profiling, identifies precise disease targets from biopsies but require tissue dissociation, losing valuable histologic and spatial context. Emerging spatial multi-omic technologies now enable multiplexed visualization of genomic, proteomic, and epigenomic targets within a single tissue slice, eliminating the need for labeling multiple adjacent slices. Although early work focused on RNA (spatial transcriptomics), spatial technologies can now concurrently capture DNA, genome accessibility, histone modifications, and proteins with spatially-resolved single-cell resolution. This review outlines the principles, advantages, limitations, and potential for spatial technologies to advance dermatologic research. By jointly profiling multiple molecular channels, spatial multiomics enables novel studies of copy number variations, clonal heterogeneity, and enhancer dysregulation, replete with spatial context, illuminating the skin's complex heterogeneity.

CD133-positive dermal papilla cells are a major driver in promoting hair follicle formation

A major contributing factor to the failure of cell-based human hair follicle (HF) engineering is our inability to cultivate highly specialized, inductive mesenchymal fibroblasts, which reside in a unique niche at the HF base, called the dermal papilla (DP). We and other groups have discovered a unique DP fibroblast subpopulation that can be identified by the cell surface marker CD133. However, the biological difference between CD133-positive (CD133+) and CD133-negative (CD133-) DP cells remains unknown. Using a newly developed double fluorescent transgenic mouse strain, we isolated CD133 + and CD133- DP cells from mouse anagen HFs. In monolayer culture, both DP populations gradually lost expression of the anagen DP signature gene, versican. When maintained in three-dimensional spheroid culture, versican expression was restored in both CD133 + and CD133- DP cells. Importantly, CD133 + DP spheroids appeared more compact, showed stronger alkaline phosphatase staining (AP), and expressed higher levels of DP signature genes. In in vivo skin reconstitution assays, mice grafted with CD133 + DP spheroids grew more hairs in healed wounds than those grafted with CD133- DP spheroids. The data underscore the importance of CD133 + DP cells as a driver of HF formation, which may present a unique opportunity to improve the use of human DP cells in tissue-engineered skin substitutes (TESS).

Skin Appendage Proteins of Tetrapods: Building Blocks of Claws, Feathers, Hair and Other Cornified Epithelial Structures

Reptiles, birds, mammals and amphibians, together forming the clade tetrapods, have a large diversity of cornified skin appendages, such as scales, feathers, hair and claws. The skin appendages consist of dead epithelial cells that are tightly packed with specific structural proteins. Here, we review the molecular diversity and expression patterns of major types of skin appendage proteins, namely keratin intermediate filament proteins, keratin-associated proteins (KRTAPs) and proteins encoded by genes of the epidermal differentiation complex (EDC), including corneous beta-proteins, also known as beta-keratins. We summarize the current knowledge about the components of skin appendages with a focus on keratins and EDC proteins that have recently been identified in reptiles and birds. We discuss gaps of knowledge and suggest directions of future research.

Convergent Evolution Has Led to the Loss of Claw Proteins in Snakes and Worm Lizards

The evolution of cornified skin appendages, such as hair, feathers, and claws, is closely linked to the evolution of proteins that establish the unique mechanical stability of these epithelial structures. We hypothesized that the evolution of the limbless body anatomy of the Florida worm lizard (Rhineura floridana) and the concomitant loss of claws had led to the degeneration of genes with claw-associated functions. To test this hypothesis, we investigated the evolution of three gene families implicated in epithelial cell architecture, namely type I keratins, type II keratins, and genes of the epidermal differentiation complex in R. floridana in comparison with other squamates. We report that the orthologs of mammalian hair and nail keratins have undergone pseudogenization in R. floridana. Likewise, the epidermal differentiation complex genes tentatively named EDYM1 and EDCCs have been lost in R. floridana. The aforementioned genes are conserved in various lizards with claws, but not in snakes. Proteomic analysis of the cornified claws of the bearded dragon (Pogona vitticeps) confirmed that type I and type II hair keratin homologs, EDYM1 and EDCCs, are protein components of claws in squamates. We conclude that the convergent evolution of a limbless body was associated with the convergent loss of claw keratins and differentiation genes in squamates.

The concept of onychodermis containing onychofibroblasts has histological (microanatomical), immunohistochemical as well as molecular basis

The terms "onychofibroblast" (nail-specific fibroblast) and onychodermis (nail-specific dermis) were first introduced in 2006 and 2012, respectively, based on distinctive histologic and immunohistochemical features from the dermis of the surrounding skin and have been demonstrated in multiple studies. Recently, based on molecular research, the definition of onychodermis containing onychofibroblasts has been expanded to encompass the area located between the nail matrix and bed epithelium and periosteum. Single-cell RNA sequencing and in situ hybridization demonstrated that onychofibroblasts within the onychodermis express the genes including RSPO4, MSX1, WIF-1, and BMP5, which are implicated in nail formation and/or in disorders with nail phenotype. A mutation in RSPO4, a component of the Wnt signaling pathway, causes anonychia congenita. Nail matrix onychodermis and nail bed onychodermis share many similar characteristics which differ from the surrounding normal dermis of the skin. Comparative spatial transcriptomic and single-cell analyses of human nail units and hair follicles suggest that onychodermis is the counterpart of follicular dermal papilla, which plays a key role in hair follicle growth and morphogenesis. Onychomatricoma, as a nail-specific tumor, has been demonstrated to be a mesenchymal tumor that originates from onychofibroblasts and is associated with the upregulation of Wnt signaling. Collectively, the onychodermis and onychofibroblasts play crucial roles in nail development and these specialized nail mesenchymal elements are key components in the pathogenesis of onychomatricoma. The concept of onychodermis containing onychofibroblasts is very important for nail biology and pathology.

Cell differentiation in the embryonic periderm and in scaffolding epithelia of skin appendages

Terminal differentiation of epithelial cells is critical for the barrier function of the skin, the growth of skin appendages, such as hair and nails, and the development of the skin of amniotes. Here, we present the hypothesis that the differentiation of cells in the embryonic periderm shares characteristic features with the differentiation of epithelial cells that support the morphogenesis of cornified skin appendages during postnatal life. The periderm prevents aberrant fusion of adjacent epithelial sites during early skin development. It is shed off when keratinocytes of the epidermis form the cornified layer, the stratum corneum. A similar role is played by epithelia that ensheath cornifying skin appendages until they disintegrate to allow the separation of the mature part of the skin appendage from the adjacent tissue. These epithelia, exemplified by the inner root sheath of hair follicles and the epithelia close to the free edge of nails or claws, are referred to as scaffolding epithelia. The periderm and scaffolding epithelia are similar with regard to their transient functions in separating tissues and the conserved expression of trichohyalin and trichohyalin-like genes in mammals and birds. Thus, we propose that parts of the peridermal differentiation program were coopted to a new postnatal function during the evolution of cornified skin appendages in amniotes.

Differential requirement of the transcription factor HOXC13 for the stable maintenance of human papillomavirus genome among high-risk genotypes

The viral genome of the high-risk human papillomavirus (HPV), the causative agent of cervical cancer, is stably maintained as extrachromosomal episomes that establish persistent infection. We previously identified homeobox-transcription factor HOXC13 as an important host protein mediating the short-term retention of the HPV16 and HPV18 genomes in normal human immortalized keratinocytes (NIKS). Here, we used CRISPR-Cas9 technology to construct HOXC13 knockout (KO) NIKS cells to determine whether HOXC13 is required for the long-term maintenance of high-risk HPV genomes. HPV16, HPV18, HPV52, and HPV58 whole genomes were transfected into HOXC13 KO cells, and the copy number of viral genomes per cell was monitored over cell passages. Copy numbers of HPV16, HPV52, and HPV58 genomes decreased continuously in HOXC13 KO cells, whereas HPV18 genomes remained stable throughout passages. Thus, HOXC13 is critical for the stable maintenance of the viral genomes of HPV16, HPV52, and HPV58, but not HPV18.

The development of hair follicles and nail

The hair follicle and nail unit develop and regenerate through epithelial-mesenchymal interactions. Here, we review some of the key signals and molecular interactions that regulate mammalian hair follicle and nail formation during embryonic development and how these interactions are reutilized to promote their regeneration during adult homeostasis and in response to skin wounding. Finally, we highlight the role of some of these signals in mediating human hair follicle and nail conditions.

Integrating scRNA-seq and bulk RNA-seq to explore the differentiation mechanism of human nail stem cells mediated by onychofibroblasts

Introduction: Nail stem cell (NSC) differentiation plays a vital role in maintaining nail homeostasis and facilitating digit regeneration. Recently, onychofibroblasts (OFs), specialized mesenchymal cells beneath the nail matrix, have emerged as potential regulators of NSC differentiation. However, limited understanding of OFs' cellular properties and transcriptomic profiles hinders our comprehension of their role. This study aims to characterize human OFs and investigate their involvement in NSC differentiation. Methods: Human OFs were isolated and characterized for their mesenchymal stem cell (MSC)-like phenotype through flow cytometry and multilineage differentiation assays. Bulk RNA-seq analysis was conducted on three samples of OFs and control fibroblasts from human nail units to delineate their molecular features. Integrated analysis with scRNA-seq data was performed to identify key signaling pathways involved in OF-induced NSC differentiation. Co-culture experiments, siRNA transfection, RT-qPCR, and immunocytochemistry were employed to investigate the effect of OF-derived soluble proteins on NSC differentiation. Drug treatments, RT-qPCR, western blotting, and immunocytochemistry were used to verify the regulation of candidate signaling pathways on NSC differentiation in vitro. Results: Human OFs exhibited slow cell cycle kinetics, expressed typical MSC markers, and demonstrated multilineage differentiation potential. Bulk RNA-seq analysis revealed differential gene expression in OFs compared to control fibroblasts, highlighting their role in coordinating nail development. Integrated analysis identified BMP4 as a pivotal signal for OFs to participate in NSC differentiation through mesenchymal-epithelial interactions, with the TGF-beta pathway possibly mediating this signal. OFs synthesized and secreted more BMP4 than control fibroblasts, and BMP4 derived from OFs induced NSC differentiation in a co-culture model. Recombinant human BMP4 activated the TGF-beta pathway in NSCs, leading to cell differentiation, while the BMP type I receptor inhibitor LDN193189 attenuated this effect. Discussion: This study characterizes the cellular and molecular features of human OFs, demonstrating their ability to regulate NSC differentiation via the TGF-beta signaling pathway. These findings establish a connection between the dermal microenvironment and NSC differentiation, suggesting the potential of OFs, in conjunction with NSCs, for developing novel therapies targeting nail and digit defects, even severe limb amputation.

Nail Telocytes: Identification, Potential Physiological Function, and Role in Pathology: A Reappraisal of the So-Called Onychofibroblasts/Onychodermis

ABSTRACT

Some authors have suggested that the fibroblasts of the nail mesenchyme (onychofibroblasts) can be distinguished from skin fibroblasts by their high expression of CD10. My 2015 study documented the presence of a relatively sparse CD34 + /CD10 + dendritic subpopulation in the dermis and hypodermis of the matrix. For some time now, my hypothesis has been that these interstitial dendritic mesenchymal cells of the matrix correspond to telocytes. Telocytes have been described as peculiar interstitial dendritic cells present in the mesenchymal tissue of numerous organs, including the skin, but their presence and characteristics in the nail unit have not been explored. This study was undertaken to more comprehensively investigate the existence and characteristics of nail telocytes. A series of 20 normal adult nail units were examined with a combination of morphological and immunohistochemical analyses. The matrix dermis contained a sparse subpopulation of CD34 + /CD10 + elongated telocytes with a higher density in the lunular region and, at this distal level, a change in their immunohistochemical profile, resulting in a progressive loss of CD34 expression. The matrix hypodermis showed CD34 + /CD10 + telocytes in their classical elongated aspect, which acquired, especially in the distal fibromyxoid area of the thumb, an oval to round morphology with multiple intracytoplasmic vacuoles. The characteristic dynamic immunophenotypic profile of the dermal telocytes with a progressive distal loss of the defining molecule CD34 was equally observed in the distal hypodermis. The nail bed dermis was thick with a dense fibrous connective tissue. A reticular network of CD34 - /CD10 + telocytes was present in the superficial dermis of the proximal nail bed. The mesenchymal cells of the deep part of the proximal nail bed dermis and the entire distal nail bed dermis were CD34 - /CD10 - . The adult nail mesenchyme is composed of 3 microanatomically distinct regions. Only the thumb has a distal hypodermis rich in mucinous material. The population of telocytes is relatively sparse compared with the fibroblastic population of the entire nail mesenchyme. The concept of onychodermis/onychofibroblasts is not valid. Nail telocytes have a dynamic immunohistochemical profile depending on whether they are located proximally or distally. The CD34 + /CD10 + profile correlates with the onychogenic epithelial region, while the CD34 - /CD10 + profile correlates with a spatial rearrangement of the nail epidermal bed.

Evolutionary origin of Hoxc13-dependent skin appendages in amphibians

Cornified skin appendages, such as hair and nails, are major evolutionary innovations of terrestrial vertebrates. Human hair and nails consist largely of special intermediate filament proteins, known as hair keratins, which are expressed under the control of the transcription factor Hoxc13. Here, we show that the cornified claws of Xenopus frogs contain homologs of hair keratins and the genes encoding these keratins are flanked by promoters in which binding sites of Hoxc13 are conserved. Furthermore, these keratins and Hoxc13 are co-expressed in the claw-forming epithelium of frog toe tips. Upon deletion of hoxc13, the expression of hair keratin homologs is abolished and the development of cornified claws is abrogated in X. tropicalis. These results indicate that Hoxc13-dependent expression of hair keratin homologs evolved already in stem tetrapods, presumably as a mechanism for protecting toe tips, and that this ancestral genetic program was coopted to the growth of hair in mammals.

Spatial transcriptomics reveals heterogeneity of histological subtypes between lepidic and acinar lung adenocarcinoma

BACKGROUND

Patients who possess various histological subtypes of early-stage lung adenocarcinoma (LUAD) have considerably diverse prognoses. The simultaneous existence of several histological subtypes reduces the clinical accuracy of the diagnosis and prognosis of early-stage LUAD due to intratumour intricacy.

METHODS

We included 11 postoperative LUAD patients pathologically confirmed to be stage IA. Single-cell RNA sequencing (scRNA-seq) was carried out on matched tumour and normal tissue. Three formalin-fixed and paraffin-embedded cases were randomly selected for 10× Genomics Visium analysis, one of which was analysed by digital spatial profiler (DSP).

RESULTS

Using DSP and 10× Genomics Visium analysis, signature gene profiles for lepidic and acinar histological subtypes were acquired. The percentage of histological subtypes predicted for the patients from samples of 11 LUAD fresh tissues by scRNA-seq showed a degree of concordance with the clinicopathologic findings assessed by visual examination. DSP proteomics and 10× Genomics Visium transcriptomics analyses revealed that a negative correlation (Spearman correlation analysis: r = -.886; p = .033) between the expression levels of CD8 and the expression trend of programmed cell death 1(PD-L1) on tumour endothelial cells. The percentage of CD8+ T cells in the acinar region was lower than in the lepidic region.

CONCLUSIONS

These findings illustrate that assessing patient histological subtypes at the single-cell level is feasible. Additionally, tumour endothelial cells that express PD-L1 in stage IA LUAD suppress immune-responsive CD8+ T cells.

Molecular Genetic Characteristics of the Hoxc13 Gene and Association Analysis of Wool Traits

Homobox C13 (Hoxc13) is an important transcription factor in hair follicle cycle development, and its deletion had been found in a variety of animals leading to abnormal hair growth and disruption of the hair follicle system. In this study, we used immunofluorescence, immunohistochemistry, real-time fluorescence quantitative PCR (RT-qPCR), and Kompetitive Allele-Specific PCR (KASP) genotyping to investigate molecular genetic characteristics of the Hoxc13 gene in Gansu alpine fine-wool sheep. The results revealed that Hoxc13 was significantly expressed during both the anagen and catagen phases (p < 0.05). It was found to be highly expressed predominantly in the dermal papillae and the inner and outer root sheaths, showing a distinct spatiotemporal expression pattern. Two single nucleotide polymorphisms (SNPs) in the exon 1 of Hoxc13, both the individual locus genotypes and the combined haplotypes were found to be correlated with wool length (p < 0.05). It was determined the mutations led to changes in mRNA expression, in which higher expression of this gene was related with longer wool length. In summary, this unique spatiotemporal expression pattern of the Hoxc13 gene may regulate the wool length of Gansu alpine fine-wool sheep, which can be used as a molecular genetic marker for wool traits and thus improve the breed.

RB1 loss in the mesenchymal component of onychomatricoma

BACKGROUND

Onychomatricoma is a nail neoplasm that usually presents as longitudinal nail plate thickening, involving either the partial or whole nail. Histopathologically, it is characterized by deep invaginations of the proliferating nail matrix and proliferation of CD34+ and CD10+ spindle cells with collagenous to myxoid stroma. Onychomatricoma has been considered a fibroepithelial neoplasm. Recently, RB1 loss has been verified using array comparative genomic hybridization.

METHODS

This study investigated the RB1 status in onychomatricoma with morphological methods.

RESULTS

Six patients with onychomatricoma were included in the study. RB1 status was assessed using immunohistochemical staining and fluorescence in situ hybridization. Immunohistochemical staining showed that all six cases experienced RB1 loss in the mesenchymal component of onychomatricoma but not in the proliferated nail matrix. Fluorescence in situ hybridization in five cases showed a monoallelic deletion of the RB1 locus in the mesenchymal component but not in the proliferated nail matrix.

CONCLUSIONS

RB1 loss was observed only in the mesenchymal component of onychomatricoma. Our findings suggest that the proliferated nail matrix in onychomatricoma represents reactive hyperplasia of various degrees secondary to neoplastic mesenchymal proliferation. This indicates that onychomatricoma should be recognized as an RB1-deleted soft tissue neoplasm rather than a fibroepithelial neoplasm.

Male pattern hair loss: Can developmental origins explain the pattern?

Male pattern hair loss (MPHL), also referred to as male androgenetic alopecia (AGA) is the most common type of non-scarring progressive hair loss, with 80% of men suffering from this condition in their lifetime. In MPHL, the hair line recedes to a specific part of the scalp which cannot be accurately predicted. Hair is lost from the front, vertex, and the crown, yet temporal and occipital follicles remain. The visual effect of hair loss is due to hair follicle miniaturisation, where terminal hair follicles become dimensionally smaller. Miniaturisation is also characterised by a shortening of the growth phase of the hair cycle (anagen), and a prolongation of the dormant phase (kenogen). Together, these changes result in the production of thinner and shorter hair fibres, referred to as miniaturised or vellus hairs. It remains unclear why miniaturisation occurs in this specific pattern, with frontal follicles being susceptible while occipital follicles remain in a terminal state. One main factor we believe to be at play, which will be discussed in this viewpoint, is the developmental origin of the skin and hair follicle dermis on different regions of the scalp.

Defining ovine dermal papilla cell markers and identifying key signaling pathways regulating its intrinsic properties

Dermal papilla cell (DPC), one of the key cell types during hair follicle development and regeneration, specifies hair size, shape and cycling. It is also an important in vitro screening model for hair growth. Although some characteristics of DPCs, such as agglutinative growth and marker genes, have been studied in mice and humans, the intrinsic properties of ovine DPCs and the regulatory mechanism of the intrinsic properties during continued culture in vitro remained unknown. In this study, based on our previous single-cell transcriptome sequencing on sheep lambskin, we verified SOX18 and PDGFRA as the novel marker genes of ovine DPCs through immunofluorescence staining on skin sections and cultured DPCs. Using continued cell culture and alkaline phosphatase staining, we found that different from mice and humans, ovine DPCs exhibit particularly robust and stable aggregation with unbated alkaline phosphatase activity till 30 passages during continued culture in vitro. Also, we found that the expression of some marker genes and the activity of Wnt/β-catenin signaling differ between early passaged DPCs and multiple passaged DPCs. Further, using Wnt/β-catenin agonist and antagonist, we demonstrated that Wnt/β-catenin signaling could regulate cell aggregation and alkaline phosphatase activity of ovine DPCs through regulating FGF and IGF signaling. This study provides the basis for isolating ovine DPCs and defines their intrinsic properties, which contribute to improving wool performance and medicine of hair regeneration.

Molecular characterization of onychomatricoma: Spatial profiling reveals the role of onychofibroblasts in its pathogenesis

Onychomatricoma (OM) is a rare nail unit tumour with a characteristic presentation of finger-like projections arising from the nail matrix. Due to the lack of transcriptome information, the mechanisms underlying its development are largely unknown. To characterize molecular features involved in the disease pathogenesis, we used digital spatial profiling (DSP) in 2 cases of OM and normal control nail units. Based on the histological evaluation, we selectively profiled 69 regions of interest covering epithelial and stromal compartments of each tissue section. Dermoscopic and histopathologic findings were reviewed in 6 cases. Single-cell RNA sequencing of nail units and DSP were combined to define cell type contributions of OM. We identified 173 genes upregulated in stromal compartments of OM compared to onychodermis, specialized nail mesenchyme. Gene ontology analysis of the upregulated genes suggested the role of Wnt pathway activation in OM pathogenesis. We also found PLA2G2A, a known modulator of Wnt signalling, is strongly and specifically expressed in the OM stroma. The potential role of Wnt pathway was further supported by strong nuclear localization of β-catenin in OM. Compared to the nail matrix epithelium, only a few genes were increased in OM epithelium. Deconvolution of nail unit cell types showed that onychofibroblasts are the dominant cell type in OM stroma. Altogether, integrated spatial and single-cell multi-omics concluded that OM is a tumour that derives a significant proportion of its origin from onychofibroblasts and is associated with upregulation of Wnt signals, which play a key role in the disease pathogenesis.