Single-cell RNA sequencing of human nail unit defines RSPO4 onychofibroblasts and SPINK6 nail epithelium

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.

Revealing the pathogenesis of keloids based on the status: Active vs inactive

Recently, the pathomechanisms of keloids have been extensively researched using transcriptomic analysis, but most studies did not consider the activity of keloids. We aimed to profile the transcriptomics of keloids according to their clinical activity and location within the keloid lesion, compared with normal and mature scars. Tissue samples were collected (keloid based on its activity (active and inactive), mature scar from keloid patients and normal scar (NS) from non-keloid patients). To reduce possible bias, all keloids assessed in this study had no treatment history and their location was limited to the upper chest or back. Multiomics assessment was performed by using single-cell RNA sequencing and multiplex immunofluorescence. Increased mesenchymal fibroblasts (FBs) was the main feature in keloid patients. Noticeably, the proportion of pro-inflammatory FBs was significantly increased in active keloids compared to inactive ones. To explore the nature of proinflammatory FBs, trajectory analysis was conducted and CCN family associated with mechanical stretch exhibited higher expression in active keloids. For vascular endothelial cells (VECs), the proportion of tip and immature cells increased in keloids compared to NS, especially at the periphery of active keloids. Also, keloid VECs highly expressed genes with characteristics of mesenchymal activation compared to NS, especially those from the active keloid center. Multiomics analysis demonstrated the distinct expression profile of active keloids. Clinically, these findings may provide the future appropriate directions for development of treatment modalities of keloids. Prevention of keloids could be possible by the suppression of mesenchymal activation between FBs and VECs and modulation of proinflammatory FBs may be the key to the control of active keloids.

Onychomatricoma: a clinicopathological, immunohistochemical, and molecular study of 10 cases highlighting recurrent RB1 deletion and the potential diagnostic value of LEF-1

AIMS

Onychomatricoma (OM), an uncommon benign fibroepithelial neoplasm of the nail unit, is sometimes diagnostically challenging for clinicians and pathologists. OM consistently expresses CD34, but no specific immunohistohemical markers or recurrent genetic alterations have been identified to date. Recent studies have suggested that Wnt signalling is a key molecular characteristic of OM.

METHODS AND RESULTS

Ten cases were analysed: four classical OM including two with pleomorphic cells; two superficial acral fibromyxoma-like variants of OM; three micropapilliferum variants of OM including one with pleomorphic cells; and one proliferating variant of OM. Immunohistochemically, the spindle cells were positive with CD34 (n = 10) and CD99 (n = 1), with focal reactivity for CD10 (n = 5). The epithelial component of the tumours expressed immunopositivity for LEF-1. Using array comparative genomic hybridization (aCGH), we demonstrated that all OM, including its variants that were tested (n = 8), harboured a few copy number alterations with losses of whole or part of chromosome 13 including the RB1 gene (n = 8) and chromosome 16 (n = 6).

CONCLUSION

We report a recurrent loss of RB1 (13q) as a possible driver molecular event in OM indicating a relationship between OM and other lesions of the spectrum of the so-called '13q/RB1' family of tumours. We did not identify a role for the Wnt/beta-catenin signalling pathway, as has been proposed in a recent study. LEF-1 could be a potential sensitive and specific marker of OM and should be used in the differential diagnosis between OM, superficial acral fibromyxoma, and the CD34-positive fibrosing family of tumours.

Failure of digit tip regeneration in the absence of Lmx1b suggests Lmx1b functions disparate from dorsoventral polarity

Mammalian digit tip regeneration is linked to the presence of nail tissue, but a nail-explicit model is missing. Here, we report that nail-less double-ventral digits of ΔLARM1/2 mutants that lack limb-specific Lmx1b enhancers fail to regenerate. To separate the nail's effect from the lack of dorsoventral (DV) polarity, we also interrogate double-dorsal double-nail digits and show that they regenerate. Thus, DV polarity is not a prerequisite for regeneration, and the nail requirement is supported. Transcriptomic comparison between wild-type and non-regenerative ΔLARM1/2 mutant blastemas reveals differential upregulation of vascularization and connective tissue functional signatures in wild type versus upregulation of inflammation in the mutant. These results, together with the finding of Lmx1b expression in the postnatal dorsal dermis underneath the nail and uniformly in the regenerative blastema, open the possibility of additional Lmx1b roles in digit tip regeneration, in addition to the indirect effect of mediating the formation of the nail.

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.

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

The nail unit and hair follicle are both hard keratin-producing organs that share various biological features. In this study, we used digital spatial profiling and single-cell RNA sequencing to define a spatially resolved expression profile of the human nail unit and hair follicle. Our approach showed the presence of a nail-specific mesenchymal population called onychofibroblasts within the onychodermis. Onychodermis and follicular dermal papilla both expressed Wnt and bone morphogenetic protein signaling molecules. In addition, nail matrix epithelium and hair matrix showed very similar expressions profile, including the expression of hard keratins and HOXC13, a transcriptional regulator of the hair shaft. Integration of single-cell RNA sequencing and digital spatial profiling data through computational deconvolution methods estimated epithelial and mesenchymal cell abundance in the nail- and hair-specific regions of interest and revealed close transcriptional similarity between these major skin appendages. To analyze the function of bone morphogenetic proteins in nail differentiation, we treated cultured human nail matrix keratinocytes with BMP5, which are highly expressed by onychofibroblasts. We observed increased expressions of hard keratin and its regulator genes such as HOXC13. Collectively, our data suggest that onychodermis is the counterpart of dermal papilla and that BMP5 in onychofibroblasts plays a key role in the differentiation of nail matrix keratinocytes.

Challenges and Opportunities for the Translation of Single-Cell RNA Sequencing Technologies to Dermatology

Skin is a complex and heterogeneous organ at the cellular level. This complexity is beginning to be understood through the application of single-cell genomics and computational tools. A large number of datasets that shed light on how the different human skin cell types interact in homeostasis-and what ceases to work in diverse dermatological diseases-have been generated and are publicly available. However, translation of these novel aspects to the clinic is lacking. This review aims to summarize the state-of-the-art of skin biology using single-cell technologies, with a special focus on skin pathologies and the translation of mechanistic findings to the clinic. The main implications of this review are to summarize the benefits and limitations of single-cell analysis and thus help translate the emerging insights from these novel techniques to the bedside.