Stem Cell-Associated Marker Expression in Canine Hair Follicles

Basic principles of hair follicle structure, morphogenesis, and regeneration

Diseases affecting the hair follicle are common in domestic animals, but despite the importance of an intact skin barrier and a fully functional hair coat, knowledge about the detailed morphological features and the diversity of these complex mini-organs are often limited, although mandatory to evaluate skin biopsies with a history of alopecia. The factors that regulate the innate hair follicle formation and the postnatal hair cycle are still not completely understood in rodents, only rudimentarily known in humans, and are poorly understood in our companion animals. This review aims to summarize the current knowledge about hair follicle and hair shaft anatomy, the arrangement of hair follicles, hair follicle morphogenesis in the embryo, and the lifelong regeneration during the postnatal hair cycle in domestic animals. The role of follicular stem cells and the need for a multitude of interacting signaling events during hair follicle morphogenesis and regeneration is unquestioned. Because of the lack of state of the art methods that can be applied in rodents but are not feasible in companion animals, most of the information in this review is based on rodent studies. However, the few data from domestic animals that are available will be discussed, and it can be assumed that at least the principal molecular mechanisms are similar in rodents and other species.

Companion animal organoid technology to advance veterinary regenerative medicine

First year medical and veterinary students are made very aware that drugs can have very different effects in various species or even in breeds of one specific species. On the other hand, the “One Medicine” concept implies that therapeutic and technical approaches are exchangeable between man and animals. These opposing views on the (dis)similarities between human and veterinary medicine are magnified in regenerative medicine. Regenerative medicine promises to stimulate the body's own regenerative capacity via activation of stem cells and/or the application of instructive biomaterials. Although the potential is enormous, so are the hurdles that need to be overcome before large scale clinical implementation is realistic. It is in the advancement of regenerative medicine that veterinary regenerative medicine can play an instrumental and crucial role. This review describes the discovery of (adult) stem cells in domesticated animals, mainly cats and dogs. The promise of cell-mediated regenerative veterinary medicine is compared to the actual achievements, and this will lead to a set of unanswered questions (controversies, research gaps, potential developments in relation to fundamental, pre-clinical, and clinical research). For veterinary regenerative medicine to have impact, either for human medicine and/or for domesticated animals, answering these questions is pivotal.

3D skin models in domestic animals

The skin is a passive and active barrier which protects the body from the environment. Its health is essential for the accomplishment of this role. Since several decades, the skin has aroused a strong interest in various fields (for e.g. cell biology, medicine, toxicology, cosmetology, and pharmacology). In contrast to other organs, 3D models were mostly and directly elaborated in humans due to its architectural simplicity and easy accessibility. The development of these models benefited from the societal pressure to reduce animal experiments. In this review, we first describe human and mouse skin structure and the major differences with other mammals and birds. Next, we describe the different 3D human skin models and their main applications. Finally, we review the available models for domestic animals and discuss the current and potential applications.

Transcriptome Profiling and Differential Gene Expression in Canine Microdissected Anagen and Telogen Hair Follicles and Interfollicular Epidermis

The transcriptome profile and differential gene expression in telogen and late anagen microdissected hair follicles and the interfollicular epidermis of healthy dogs was investigated by using RNAseq. The genes with the highest expression levels in each group were identified and genes known from studies in other species to be associated with structure and function of hair follicles and epidermis were evaluated. Transcriptome profiling revealed that late anagen follicles expressed mainly keratins and telogen follicles expressed GSN and KRT15. The interfollicular epidermis expressed predominately genes encoding for proteins associated with differentiation. All sample groups express genes encoding for proteins involved in cellular growth and signal transduction. The expression pattern of skin-associated genes in dogs is similar to humans. Differences in expression compared to mice and humans include BMP2 expression mainly in telogen and high KRT17 expression in the interfollicular epidermis of dogs. Our data provide the basis for the investigation of the structure and function of canine skin or skin disease and support the use of dogs as a model for human cutaneous disease by assigning gene expression to specific tissue states.

Canine Epithelial Skin Tumours: Expression of the Stem Cell Markers Lgr5, Lgr6 and Sox9 in Light of New Cancer Stem Cell Theories

Evidence is accumulating that tumour development is driven by cancer stem cells (CSCs). In order to understand the presence and potential contribution of stem cells (SCs) as tumour-initiating cells in canine cutaneous tumours, we selected three putative SC markers (Lgr5, Lgr6 and Sox9) and investigated their expression pattern, level of protein and mRNA expression, in 43 canine hair follicle (HF) and 18 canine cutaneous epidermal tumours by immunohistochemistry and qRT-PCR, using normal skin samples as controls. Lgr5 protein expression was not detected in epidermal and HF tumours; however, Lgr5 mRNA overexpression was evident in some HF tumours. Sox9 was expressed in several tumour cases, both at the protein and mRNA level. The Lgr6 antibody tested was not suitable for formalin-fixed paraffin-embedded tissue samples, but Lgr6 gene showed higher expression in several samples of both HF and epidermal tumours compared with normal skin. Significantly higher mRNA expression levels of the three SC markers were found in trichoblastomas (TB) compared with basal cell carcinomas (BCC). The present results indicated that canine HF and epidermal tumours might have common tumour-initiating cells. The mRNA expression of the three selected SC markers, especially Lgr5, could be potentially useful in the distinction between canine TB and BCC.

Progenitor cells expressing nestin, a neural crest stem cell marker, differentiate into outer root sheath keratinocytes

BACKGROUND

Nestin, which was originally described as a neural crest stem cell marker, is known to be expressed in bulge follicle cells of human, canine and murine anagen hairs. However, the capacity of nestin-expressing cells to differentiate into the components of the hair follicle or the epidermis has been insufficiently investigated.

HYPOTHESIS/OBJECTIVES

To determine whether nestin-expressing cells are capable of differentiating into keratinocytes.

ANIMALS/MATERIALS

A double-transgenic mouse line Nes-Cre/CAG-CAT-EGFP, in which enhanced green fluorescent protein (EGFP) is expressed upon Cre-based recombination driven by the nestin promoter.

METHODS AND MATERIALS

The tissue distribution of EGFP⁺ and nestin⁺ cells in the skin of the mouse line was analysed by immunofluorescence and immunohistochemical analyses.

RESULTS

EGFP⁺ cells were recognized in the outer epithelial cell layers of anagen and telogen hair follicles, but rarely seen in the interfollicular epidermis. The EGFP⁺ cells in the outer layers of the hair follicles coexpressed keratin 14, a marker of the outer root sheath (ORS) keratinocytes, but not trichohyalin granules, an inner root sheath keratinocyte cell marker. Immunostaining for nestin failed to detect its expression in the majority of hair follicle epithelial cells, suggesting that the EGFP⁺ cells in the ORS were derived from nestin-expressing progenitor cells that had become further committed along the epithelial cell lineage, where nestin is no longer expressed.

CONCLUSIONS AND CLINICAL IMPORTANCE

These results suggest that progenitor cells that differentiate into ORS keratinocytes are distinct from those for other hair follicle or epidermal components and provide implications for regenerative medicine and the molecular classification of hair follicle tumours.

CD200 Expression Marks a Population of Quiescent Limbal Epithelial Stem Cells with Holoclone Forming Ability

One of the main challenges in limbal stem cell (LSC) biology and transplantation is the lack of definitive cell surface markers which can be used to identify and enrich viable LSCs. In this study, expression of 361 cell surface proteins was assessed in ex vivo expanded limbal epithelial cells. One marker, CD200 was selected for further characterization based on expression in a small subset of limbal epithelial cells (2.25% ± 0.69%) and reduced expression through consecutive passaging and calcium induced differentiation. CD200 was localized to a small population of cells at the basal layer of the human and mouse limbal epithelium. CD200⁺ cells were slow cycling and contained the majority of side population (SP) and all the holoclone forming progenitors. CD200⁺ cells displayed higher expression of LSCs markers including PAX6, WNT7A, CDH3, CK14, CK15, and ABCB5 and lower expression of Ki67 when compared to CD200^- . Downregulation of CD200 abrogated the ability of limbal epithelial cells to form holoclones, suggesting an important function for CD200 in the maintenance and/or self-renewal of LSCs. A second marker, CD109, which was expressed in 56.29% ± 13.96% of limbal epithelial cells, was also found to co-localize with ΔNp63 in both human and mouse cornea, albeit more abundantly than CD200. CD109 expression decreased slowly through calcium induced cell differentiation and CD109⁺ cells were characterized by higher expression of Ki67, when compared to CD109^- subpopulation. Together our data suggest that CD200 expression marks a quiescent population of LSCs with holoclone forming potential, while CD109 expression is associated with a proliferative progenitor phenotype. Stem Cells 2018;36:1723-1735.

Hierarchical Cluster Analysis of Cytokeratins and Stem Cell Expression Profiles of Canine Cutaneous Epithelial Tumors

The diagnosis of cutaneous epithelial tumors (CETs) in dogs is based on predominant histological differentiation patterns. However, the expression of a broad panel of antigens has not been comprehensively examined with immunohistochemistry. The present study aims to establish a comprehensive expression profile and identify useful diagnostic markers for each CET type. Cytokeratin (CK), stem cell, and other associated markers were immunohistochemically examined in 110 canine CETs. Among these, CK16 was useful for differentiating between basal and squamous cell carcinomas. Acantholytic squamous cell carcinomas were positive for CK8, CK18, and CK19, suggesting their close association with the apocrine duct. Unlike their benign counterparts, sebaceous carcinomas coexpressed CK5/6 and adipophilin. Smooth muscle actin (SMA) and p63 immunostaining were useful for accurately distinguishing between glandular and ductal differentiation in apocrine tumors. A case of apocrine carcinoma and malignant myoepithelioma was identified using anti-SMA antibodies. Stem cell expression profiles (CK8, CK15, CK19, and CD34) of hair follicle tumors were discrete and indicative of their anatomic origins. The effectiveness of immunohistochemistry for tumor diagnosis was further confirmed by hierarchical cluster analysis, through which selected markers were able to sort CETs into specific groups: CK5/6, CK8, CK14, CK16, CK18, CK19, p63, adipophilin, and SMA sorted tumors of epidermal, apocrine, or sebaceous origin; while CK8, CK14, CK15, CK16, CK19, CD34, and p63 sorted hair follicle tumors in agreement with their histological differentiation. In conclusion, the present study provides comprehensive immunohistochemical information, which could complement histomorphological features for the future classification of canine CETs.

Establishment and characterization of a canine keratinocyte organoid culture system

BACKGROUND

Perturbations of epidermal and follicular homeostasis have been attributed to a variety of skin diseases affecting dogs. The availability of an in vitro system to investigate these diseases is important to understand underlying pathomechanisms.

OBJECTIVES

To establish an accurate and reliable in vitro 3D system of canine keratinocyte organoids to lay the basis for studying functional defects in interfollicular epidermis (IFE) and hair follicle (HF) morphogenesis, reconstitution and differentiation that lead to alopecic and epidermal diseases.

ANIMALS

Skin biopsies were obtained from freshly euthanized dogs of different breeds with no skin abnormalities.

METHODS

Cells derived from microdissected IFE and HFs were seeded in Matrigel and keratinocyte organoids were grown and characterized using immunohistochemistry, RT-qPCR and RNA sequencing.

RESULTS

Both organoid lines develop into a basal IFE-like cell type. Gene and protein expression analysis revealed high mRNA and protein levels of keratins 5 and 14, IFE differentiation markers and intercellular molecules. Key markers of HF stem cells were lacking. Withdrawal of growth factors resulted in upregulation of markers such as KRT16, Involucrin, KRT17 and SOX9, showing the potential of the organoids to develop towards more differentiated tissue.

CONCLUSION AND CLINICAL IMPORTANCE

Our 3D in vitro culture system provides the basis to explore epidermal function, to investigate the culture conditions necessary for the development of organoids with a HF signature and to address cutaneous disorders in dogs. However, for induction of HF signatures or hair growth, addition of different growth factors or co-culture with dermal papilla will be required.

In vitro identification of a stem cell population from canine hair follicle bulge region

Skin is an extensive and easily accessible organ possessing various cell types that are constantly renewed. Previous studies have suggested the presence of a stem cell niche at the bulge region of the hair follicle, which contains cells positive for CD200 and CD34. Thus, this study sought to identify these cell populations in canine skin cells using the following methods 1- collecting samples of adult and fetal skin and isolating and culturing these cells using a method of simple enzymatic digestion and 2- testing the cell cultures for CD200 and CD34 in vitro and comparing them with skin tissue samples (in situ). Immunofluorescence results were negative for both CD200 and CD34 in frozen and paraffin embedded tissue, whereas the analysis showed that cultured cells positive for CD34, CD200 and double positive cells could be visualized in different percentages. Additionally, the pluripotency marker OCT4 was positive in the isolated cells. Analysis of CD34, CD200 and OCT4 by RT-qPCR showed that there is expression in fetal and adult cells, although no difference was observed between groups. Our results suggest that bulge stem cells from both fetuses and adult dogs were reported with the use of CD34 and CD200 markers in this study, and further techniques for cell isolation and in vitro cultivation are needed in order to obtain enriched populations of skin stem cells in dogs.

A comparison of transcriptomic patterns measured in the skin of Chinese fine and coarse wool sheep breeds

We characterised wool traits, and skin gene expression profiles of fine wool Super Merino (SM) and coarse wool Small Tail Han (STH) sheep. SM sheep had a significantly higher total density of wool follicles, heavier fleeces, finer fibre diameter, and increased crimp frequency, staple length and wool grease (lanolin) production. We found 435 genes were expressed at significantly different levels in the skin of the two breeds (127 genes more highly in SM and 308 genes more highly in STH sheep). Classification of the genes more highly expressed in SM sheep revealed numerous lipid metabolic genes as well as genes encoding keratins, keratin-associated proteins, and wool follicle stem cell markers. In contrast, mammalian epidermal development complex genes and other genes associated with skin cornification and muscle function were more highly expressed in STH sheep. Genes identified in this study may be further evaluated for inclusion in breeding programs, or as targets for therapeutic or genetic interventions, aimed at altering wool quality or yield. Expression of the lipid metabolic genes in the skin of sheep may be used as a novel trait with the potential to alter the content or properties of lanolin or the fleece.

Novel insights into the pathways regulating the canine hair cycle and their deregulation in alopecia X

Alopecia X is a hair cycle arrest disorder in Pomeranians. Histologically, kenogen and telogen hair follicles predominate, whereas anagen follicles are sparse. The induction of anagen relies on the activation of hair follicle stem cells and their subsequent proliferation and differentiation. Stem cell function depends on finely tuned interactions of signaling molecules and transcription factors, which are not well defined in dogs. We performed transcriptome profiling on skin biopsies to analyze altered molecular pathways in alopecia X. Biopsies from five affected and four non-affected Pomeranians were investigated. Differential gene expression revealed a downregulation of key regulator genes of the Wnt (CTNNB1, LEF1, TCF3, WNT10B) and Shh (SHH, GLI1, SMO, PTCH2) pathways. In mice it has been shown that Wnt and Shh signaling results in stem cell activation and differentiation Thus our findings are in line with the lack of anagen hair follicles in dogs with Alopecia X. We also observed a significant downregulation of the stem cell markers SOX9, LHX2, LGR5, TCF7L1 and GLI1 whereas NFATc1, a quiescence marker, was upregulated in alopecia X. Moreover, genes coding for enzymes directly involved in the sex hormone metabolism (CYP1A1, CYP1B1, HSD17B14) were differentially regulated in alopecia X. These findings are in agreement with the so far proposed but not yet proven deregulation of the sex hormone metabolism in this disease.

Desmoplastic tricholemmoma in a dog

A 10-year-old mixed breed dog was presented with a 0.8 cm diameter mass below the left eye region. The mass was surgically removed and processed for histopathological examination. Microscopically, tumor cells proliferated in small lobules, nests and cords, and the tumor parenchyma was separated by desmoplastic stroma. Majority of the tumor cells were periodic acid-Schiff (PAS)-positive, and the desmoplastic stroma was densely collagenous and mucinous. Immunohistochemical results showed that the tumor cells were diffusely positive for cytokeratin 15, cytokeratin 19 and CD 34, while cytokeratin 8 reactivity was limited to the tumor cells proliferating in cords. Few tumor cells were positive for nestin. Based on the histopathological findings, the tumor was diagnosed as desmoplastic tricholemmoma.

The Hair Follicle: A Comparative Review of Canine Hair Follicle Anatomy and Physiology

The hair follicle (HF) has a wide range of functions including thermoregulation, physical and immunological protection against external insults, sensory perception, social interactions, and camouflage. One of the most characteristic features of HFs is that they self-renew during hair cycle (HC) throughout the entire life of an individual to continuously produce new hair. HC disturbances are common in humans and comparable to some alopecic disorders in dogs. A normal HC is maintained by follicular stem cells (SCs), which are predominately found in an area known as the bulge. Due to similar morphological characteristics of the human and canine bulge area, the particularity of compound HFs in humans and dogs as well as similarities in follicular biomarker expression, the dog might be a promising model to study human HC and SC disorders. In this review, we give an overview of normal follicular anatomy, the HC, and follicular SCs and discuss the possible pathogenetic mechanisms of noninflammatory alopecia.

Spatial Distribution of Stem Cell-Like Keratinocytes in Dissected Compound Hair Follicles of the Dog

Hair cycle disturbances are common in dogs and comparable to some alopecic disorders in humans. A normal hair cycle is maintained by follicular stem cells which are predominately found in an area known as the bulge. Due to similar morphological characteristics of the bulge area in humans and dogs, the shared particularity of compound hair follicles as well as similarities in follicular biomarker expression, the dog is a promising model to study human hair cycle and stem cell disorders. To gain insight into the spatial distribution of follicular keratinocytes with stem cell potential in canine compound follicles, we microdissected hair follicles in anagen and telogen from skin samples of freshly euthanized dogs. The keratinocytes isolated from different locations were investigated for their colony forming efficiency, growth and differentiation potential as well as clonal growth. Our results indicate that i) compound and single hair follicles exhibit a comparable spatial distribution pattern with respect to cells with high growth potential and stem cell-like characteristics, ii) the lower isthmus (comprising the bulge) harbors most cells with high growth potential in both, the anagen and the telogen hair cycle stage, iii) unlike in other species, colonies with highest growth potential are rather small with an irregular perimeter and iv) the keratinocytes derived from the bulbar region exhibit characteristics of actively dividing transit amplifying cells. Our results now provide the basis to conduct comparative studies of normal dogs and those with hair cycle disorders with the possibility to extend relevant findings to human patients.