Minor collagens of the skin with not so minor functions

Abstract

The structure and function of the skin relies on the complex expression pattern and organisation of extracellular matrix macromolecules, of which collagens are a principal component. The fibrillar collagens, types I and III, constitute over 90% of the collagen content within the skin and are the major determinants of the strength and stiffness of the tissue. However, the minor collagens also play a crucial regulatory role in a variety of processes, including cell anchorage, matrix assembly, and growth factor signalling. In this article, we review the expression patterns, key functions and involvement in disease pathogenesis of the minor collagens found in the skin. While it is clear that the minor collagens are important mediators of normal tissue function, homeostasis and repair, further insight into the molecular level structure and activity of these proteins is required for translation into clinical therapies.

Introduction

The skin is an essential chemical and physical barrier between our bodies and the external environment, and it requires specific mechanical properties to perform this function. It must be strong and stiff enough to resist repeated mechanical stress, yet flexible enough to allow easy movement and bending. Like many connective tissues, the composition and structure of the extracellular matrix (ECM) define the mechanical properties of the skin and are important regulators of cell behaviour. Tissue mechanics also depend on the precise organisation of the ECM, which is mediated by matrix synthesis, assembly and remodelling. Ultimately, these tightly regulated processes maintain tissue homeostasis and facilitate wound repair following injury.

The ECM of the skin contains proteins, such as collagens, elastin and fibronectin, as well as large sulphated proteoglycans. The collagens are the most abundant proteins (70% by dry weight) and consist primarily of the fibrillar type I collagen and type III collagen, which together provide the majority of the strength and stiffness of the tissue. Although the minor collagens make up less than 10% of the total collagen content (Lovell et al. 1987), they play a key role in ECM organisation, such as formation and anchoring of the basement membrane, regulation of fibrillar collagen assembly, and mediating growth factor and cell signalling. In this review, we will explore the diverse functions of each of the minor collagens expressed in the skin and how they contribute to matrix assembly, tissue homeostasis and repair. Finally, we will discuss how these molecules affect various skin diseases and could be targeted in therapeutic interventions.

The collagen superfamily

Collagens are a family of ECM proteins which maintain the architectural integrity of all connective tissues and virtually all organs (Fratzl, 2008). The name collagen itself is derived from the Greek word ‘kolla’, which means glue, and the suffix ‘‐gono’, signifying producer. They are the most abundant mammalian proteins and account for around 30% of the total protein mass in the human body, including more than 70% of the dry weight of the dermis (Bauer & Uitto, 1979). In addition to their essential structural function, collagens are also involved in cell adhesion, chemotaxis and migration. They dynamically interact with cells, growth factors and cytokines to regulate tissue remodelling in the course of cell growth, differentiation, morphogenesis and wound repair (van der Rest & Garrone, 2000; Singer & Clark, 2009; Gelse et al. 2003; Ricard‐Blum, 1991).

All collagens consist of three polypeptide α chains wound in a right‐handed triple helix. The chains can be either identical, a homotrimer, such as type II collagen, or the chains can be different, a heterotrimer as in the case of type IX collagen. The defining characteristic of any collagen molecule is the triple helical domain of repeating Gly‐X‐Y triplet. Glycine, the smallest amino acid, is located at the interior of the helix and is essential for maintaining its structure. Any other amino acid can occupy the X and Y positions, but proline is often at position X and (hydroxy)lysine or (hydroxy)proline is frequently found at position Y. The high prevalence of these amino acids in the repeating triplet indicates their role in facilitating the assembly and stability of the triple helix (Kadler et al. 2007).

The collagen superfamily is an extensive and complex group that to date includes 28 genetically distinct members with 42 different α chains. The collagens are further arranged into nine distinct subclasses depending on their supramolecular assemblies and other features. Twenty additional highly heterogeneous proteins with collagen‐like domains are also considered part of the superfamily. The criteria for a protein to be included in the collagen superfamily are the existence of the characteristic triple helical domain and a structural role in the ECM. The diversity in the collagens highlights the prevalence of the collagenous domain in diverse proteins integral for different ECMs and different structural and biomechanical functions.

Type I collagen, the most common collagen found in humans, as well as types II, III, V, XI, XXIV and XXVII, all belong in the same subclass. They assemble into fibrils and fibres and are thus categorised as fibril‐forming collagens. Others, such as type IV, VI, VIII and X, are non‐fibrillar collagens and are organised into different types of networks. Type IV collagen can include six different α chains in various trimeric combinations and forms a flexible open network that constitutes the basic architectural framework of the basement membrane, Type VIII and X collagens assemble into hexagonal networks (Myllyharju & Kivirikko, 2004; Shoulders & Raines, 1997). The cell types primarily responsible for collagen production are cells of mesenchymal origin such as fibroblasts and osteoblasts. However, other cell types have been reported to synthesise collagen; for instance, keratinocytes express types IV (Tokimitsu et al. 2016), VII (Chen et al. 1997), XVI (Grässel et al. 1999) and XVII (Schäcke et al. 1998) collagen, melanocytes type XVII collagen (Krenacs et al. 2012), and macrophages express types VI (Schnoor et al. 1998) and VIII (Weitkamp et al. 2008).

There are 20 different types of collagen expressed within the skin. The predominant types are the fibrillar collagens I and III, which make up over 90% of collagens and account for the strength and stiffness of the tissue. The other minor collagens (summarised in Table 1 and Fig. 1) include those associated with the basement membrane (types IV, VII, XV and XVIII), dermal collagens (types V, VI, XII, XIV and XVI) and transmembrane collagens (types XIII and XVII). The transmembrane collagens, also known as MACIT (Membrane Associated Collagens with Interrupted Triple helices), belong to a unique group of collagens that span the lipid bilayer and contain extracellular and intracellular domains. Although these molecules make up a small proportion of the tissue, growing evidence indicates that they regulate a diverse number of processes, including matrix organisation, cell adhesion, and growth factor activation. Thus, the minor collagens are essential for normal skin function.

Table 1.

Summary of minor collagen composition within the skin

Collagen Type	Trimer conformation	Family	Localisation
IV	α1(IV)α1(IV)α2(IV), α3(IV)α4(IV)α5(IV), α5(IV)α5(IV)α6(IV)	Basement membrane associated	Basement membrane
V	α1(V)α1(V)α2(V), α1(V)α2(V)α3(V), α1(V)α1(V)α4(V), α1(V)α1(V)α1(V)	Fibrillar	Papillary dermis, hair follicles, eccrine glands
VI	α1(VI)α2(VI)α3(VI), α1(VI)α2(VI)α5(VI), α1(VI)α2(VI)α6(VI)	Microfibrillar	Papillary dermis, vasculature, hair follicles, nerves
VII	α1(VII)α1(VII)α1(VII)	Anchoring fibril forming	Basement membrane to dermis
XII	α1(XII)α1(XII)α1(XII)	FACIT	Papillary dermis, hair follicles
XIII	α1(XIII)α1(XIII)α1(XIII)	Transmembranous	Dermal–epidermal junction, vasculature, nerves, foetal epidermis and hair follicles
XIV	α1(XIV)α1(XIV)α1(XIV)	FACIT	Reticular dermis
XV	α1(XV)α1(XV)α1(XV)	Multiplexin	Papillary dermis, hair follicles, nerves, adipocytes, vasculature,and pili muscles
XVI	α1(XVI)α1(XVI)α1(XVI)	FACIT	Papillary dermis, dermal–epidermal junction, dermal dendrocytes
XVII	α1(XVII)α1(XVII)α1(XVII)	Transmembranous	Basal keratinocytes
XVIII	α1(XVIII)α1(XVIII)α1(XVIII)	Multiplexin	Basement membrane of epidermis, vasculature, pili muscles, hair follicle bulge cells

Figure 1.

Minor collagens within the skin. Schematic representation of the skin showing where each of the minor collagens preferentially localise in relation to the epidermis, basement membrane, and fibrillar collagens within the papillary and reticular dermis.

Type IV collagen

Type IV collagen is a major component of the basement membrane in nearly every tissue, including the skin (reviewed specifically elsewhere). It forms a woven network of fibrils that provide structure for the membrane and scaffolding for the other constituents, such as laminin 332 and nidogen. Type IV collagen is expressed from six different genes (COL4A1, COL4A2, COL4A3, COL4A4, COL4A5, and COL4A6; Zhou et al. 1994), and the translated polypeptides assemble into three distinct heterotrimers: α1(IV)α1(IV)α2(IV), α3(IV)α4(IV)α5(IV) and α5(IV)α5(IV)α6(IV) (Borza et al. 2001). Only the α1(IV)α1(IV)α2(IV) and α5(IV)α5(IV)α6(IV) forms are expressed within the skin (Tanaka et al. 2001), whereas all three forms can be found in other tissues, such as the kidney. Type IV collagen heterotrimers each contain a long, central collagenous domain flanked by non‐collagenous domains at the N‐ and C‐termini (Khoshnoodi et al. 2008). The molecule assembles into a polygonal polymer network through homotypic, end‐to‐end interactions between the C‐terminal domains in conjunction with lateral interactions between four N‐terminal domains (Khoshnoodi et al. 2006). Interruptions of the G‐X‐Y motifs in the collagenous domains confer a high degree of flexibility to individual molecules and the network as a whole (Hofmann et al. 1984).

Although type IV collagen plays an important role in the structure of the basement membrane, only a few diseases are associated with mutations in the collagen IV genes. Kidney diseases, such as Alport syndrome or Goodpasture syndrome, caused by type IV collagen dysfunction most often affect the glomerulus basement membrane and are a result of mutations in the COL4A3, 4 or 5 genes (Barker et al. 1990; Mochizuki et al. 1994) or through an auto‐immune reaction to the α3(IV) chain of the protein (Kalluri et al. 1996). There are surprisingly few examples of diseases associated with type IV collagen within the skin; however, a few rare cases of epidermal blistering in patients with auto‐immune responses to the α5(IV) chain have been reported (Ghohestani et al. 2000, 2003). The lack of type IV collagen‐associated phenotypes in the skin may be due to compensation from the different molecular forms or potentially through additional support from the type VII collagen anchoring fibrils, which are not present in the kidney. Still, the importance of type IV collagen in skin function is underscored by the embryonic lethality of Col4a1 and Col4a2 double knockouts in mice (Pöschl et al. 2007) and by the failure to form an intact basement membrane in mice lacking lysyl hydroxylase 3, which catalyses hydroxylation of lysine residues and is required for type IV collagen cross‐linking (Rautavuoma et al. 2005; Ruotsalainen et al. 2002).

Type V collagen

Several isoforms of type V collagen exist. In its principal molecular form, heterotrimer α1(V) α1(V)α2(V), it co‐assembles with type I and type III collagens, forming heterotypic fibrils in tissues including skin (Birk et al. 1990; Linsenmayer et al. 1993). Other isoforms such as heterotrimers α1(V)α2(V)α3(V) and α1(V) α1(V)α4(V) and homotrimer α1(V)α1(V)α1(V) exhibit a much more restricted expression pattern (Ricard‐Blum & Ruggiero, 2011). In normal skin, type V collagen is present throughout the dermis with a higher expression in the papillary dermis and around the hair follicles and eccrine glands (Chanoki et al. 1988).

Mutations in COL5A1 or COL5A2 result in Ehlers–Danlos syndrome (EDS) classic type, whose typical clinical manifestations include fragile, hyperextensive skin and abnormal wound healing (Malfait et al. 1993). The role of type V collagen in fibrillogenesis has been well established, mainly with the use of mouse models of EDS (Andrikopoulos et al. 1995; Chanut‐Delalande et al. 2004; Wenstrup et al. 1999; Sun et al. 2012). The minor homotrimer form of type V collagen has been recently reported to cause ultrastructural modifications and mechanical alterations in the skin of transgenic mice that overexpress it in the epidermis. Moreover, it interacts with collagens type IV and VI and laminin‐111, suggesting that it functions as a bridging molecule across the dermal–epidermal interface (Bonod‐Bidaud et al. 2012). Type V collagen has also been implicated in the pathogenesis of systemic sclerosis, where overexpression of abnormal type V collagen correlates with disease stage and activity (Martin et al. 2012). Finally, elevated type V collagen content in the dermis is associated with mouse skin tumour promotion. Following treatment with tumour promoter 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), the content of type V collagen is significantly enhanced, potentially reflecting a role of this collagen in tumour development, even though the exact link remains to be elucidated further (Marian & Danner, 1987).

Type V collagen may also be involved in wound repair. It first appears in wounded human skin after 3 days and is still detected after 2.5 months (Betz et al. 1993a), and it is present in post‐burn granulation tissue (Latha et al. 1999). It has also been reported as a component of the connective tissue matrix in hypertrophic scars along with type I and III collagen (Ehrlich & White, 1981). In a mouse model of wound healing the pro‐α3(V) chain co‐localises with heparan sulphate via binding to an acidic portion of the pro‐a3(V) chain as shown with immunoelectron microscopy and may be an essential component of the matrix required for the initiation of wound repair (Sumiyoshi et al. 1997).

Type VI collagen

Type VI collagen is a microfibrillar collagen with a characteristic beaded filament structure and is expressed in many connective tissues. It was first isolated in 1979, following pepsin digestion of human aortic intima and was initially called ‘intima collagen’ or ‘short chain collagen’ (Chung et al. 2010; Furthmayr et al. 1983). The main monomeric unit is a heterotrimer comprising α1(VI), α2(VI) and α3(VI). Recently, three new α3(VI)‐like chains were identified: α4(VI), α5(VI) and α6(VI). These chains are believed to substitute the α3(VI) chain and form triple helices with the α1(VI) and α2(VI). However, the overall distribution of the newly discovered chains is limited to a few select locations within the skin, skeletal muscle and reproductive organs (Gara et al. 2011; Sabatelli et al. 2001, 2011). Type VI collagen is expressed throughout the papillary, reticular and hypodermis in both neonatal and adult skin. It is also found around nerve fibres and blood vessels, and the levels are enriched just below the basement membrane at the dermal–epidermal junction (Keene et al. 1988; Gara et al. 2011; Theocharidis et al. 1998). As opposed to the broad distribution of the three main chains of type VI collagen, α5(VI) only localises to the papillary dermis, whereas α6(VI) is found around the blood vessels (Sabatelli et al. 2001). Type VI collagen expression is developmentally regulated and is deposited in the embryonic ECM at a later developmental stage than is type I collagen, and this expression correlates with the expression of the COL6A3 gene (Dziadek et al. 1996).

Type VI collagen serves as a repository for platelet‐derived growth factor (Somasundaram & Schuppan, 1999), keratinocyte growth factor (Ruehl et al. 2005), matrix metalloproteinases −1, −2, −3, −8 and −9 (Freise et al. 2009), interleukin 2 (Somasundaram et al. 2002) and cytokine oncostatin M (Somasundaram et al. 2000), thus mediating their activity and availability in the course of normal tissue turnover, fibrosis and wound healing. Type VI collagen is also an important organiser of ECM structures within the dermis. Sabatelli et al. (1998) first observed abnormal fibronectin organisation in the ECM from type VI collagen‐deficient patient fibroblasts and also from Col6a1 null mice. Similarly, our own studies have shown that knockdown of COL6A1 in human fibroblasts leads to the formation of cell‐derived matrices with thicker, more widely spaced fibronectin fibres, and reduced total collagen and proteoglycan content (Theocharidis et al. 1998). Type VI collagen modulates type I collagen fibrillogenesis (Minamitani et al. 2004), and type I collagen fibrils appear irregular in the skin of type VI collagen‐deficient patients (Kirschner et al. 2005). Thus, type VI collagen is a key regulator of matrix assembly within the dermis.

Given its role in ECM organisation, type VI collagen may also be involved in wound repair. Our group and others have shown that type VI collagen is deposited in granulation tissue of acute wounds in mice (Lettmann et al. 2014; Chen et al. 1994; Theocharidis et al. 1998), and mRNA expression of type VI collagen genes during normal human wound healing is regulated in a time‐dependent manner. It is deposited during the early stages of wound healing, being first detected by day 3, and its synthesis increases in parallel with type I collagen, reaching a peak approximately 14 days post‐injury (Betz et al. 1993b; Oono et al. 1993). Although no overt effect of type VI collagen deficiency on wound closure time was described in a recent study, the wounds of Col6a1 null mice exhibited disrupted collagen fibril architecture, and their unwounded skin shows reduced tensile strength (Lettmann et al. 2014). Interestingly, the levels of type VI collagen only moderately increase in hypertrophic scars but are abundant in keloids, especially at the expanding edge of the scar (Peltonen et al. 1981; Zhang et al. 2003; Theocharidis et al. 1998). Together, these studies suggest that type VI collagen can be both a positive and negative regulator of wound healing, and effective wound repair may require a specific level of expression.

Determining the precise function of type VI collagen is complicated by the fact that individuals with mutations in the COL6A1, COL6A2 and COL6A3 genes exhibit a wide range of different cutaneous abnormalities such as hypertrophic and keloid scars, striae rubra, follicular hyperkeratosis and dry skin (Pepe et al. 1999; Briñas et al. 2010; Collins et al. 2012; Saroja et al. 2014). A link between type VI collagen and the rare hereditary multisystem disorder trichothidystrophy (TTD) has also been reported. In primary dermal fibroblasts from TTD patients, whose skin is frequently very dry and scaly (ichthyotic), a considerable decrease in expression of COL6A1 is noted and associated with reduced activity of the COL6A1 promoter (Orioli et al. 2013). In the skin of fetuses with trisomy 21, the distribution of type VI collagen is much broader, extending from the basement membrane to the subcutis (von Kaisenberg et al. 1998), and it contributes to the fetal nuchal oedema and altered ECM that are frequently present in these patients (Brand‐Saberi et al. 1994). Interestingly, a proteomics analysis of skin samples from patients with complete loss of type VII collagen, which is the principal component of the anchoring fibrils that emanate from the basement membrane into the dermis, revealed an upregulation of all three main type VI collagen genes. As type VI collagen is already known to be highly expressed at the dermal–epidermal junction, it could compensate for the loss of type VII collagen and play a role in the related pathology, recessive dystrophic epidermolysis bullosa (Küttner et al. 2013). Finally, work in the Col6a1 null mouse has recently identified a few interesting skin phenotypes. One study showed that absence of type VI collagen retards normal hair growth, but wound‐induced hair regrowth is greatly accelerated in Col6a1 knockout mice. This process is triggered by the activation of the Wnt/β‐catenin signalling pathway and is abolished following pathway inhibition (Chen et al. 1994).

Type VII collagen

Type VII collagen has a well established role in anchoring the epidermis to the dermis and the pathogenesis of the dystrophic epidermolysis bullosa (DEB) family of blistering skin diseases (reviewed specifically elsewhere; Chung & Uitto, 1976). In normal skin, the protein localises to the basement membrane zone and assembles into fibrils, which loop down from the basement membrane and hook into the dermal collagen fibres (Sakai et al. 2012). Type VII collagen is a homotrimer of three collagen α1(VII) chains, and it contains a central triple helical domain flanked by two long, non‐collagenous domains (Christiano et al. 1994a,b). The molecule assembles into anchoring fibrils by first forming anti‐parallel dimers, which then aggregate laterally to create a larger fibril visible by electron microscopy (Sakai et al. 2012). The non‐collagenous domains of the N‐terminus (NC‐1) present at either end of the fibril, bind directly to type IV collagen and laminin‐332 within the basement membrane with high specificity (Chen et al. 2015; Shimizu et al. 2008; Brittingham et al. 2006). Type VII collagen fibrils thereby form loops, which are thought to intertwine with large fibrillar collagens and anchor the basement membrane to the dermis (Sakai et al. 2012; Villone et al. 2011).

The COL7A1 gene exists within a 32‐kb region in 3P21.1 of chromosome 3 and contains 118 exons (Christiano et al. 1994a). This unusually large number of exons, the second largest of any gene described, is unique to the COL7A1 gene among all collagens (Christiano et al. 1994a; Lin et al. 2012). So far, at least 730 mutations in COL7A1 have been described, including insertions deletions, and missense mutations, that lead to a range of DEB diseases with varying severity (Bruckner‐Tuderman et al. 1989; Hovnanian et al. 1992; Fine et al. 2014). Autosomal recessive forms of DEB (RDEB), in which premature stop codons prevent expression of the protein, are the most severe and cause extensive blistering and scarring of the skin and mucosa (Hilal et al. 1993; Christiano et al. 1994c). Other recessive and dominant forms of the disease where type VII collagen is reduced or dysfunctional have milder blistering phenotypes but are still painful and debilitating (McGrath et al. 1995; Fine et al. 2014). DEB patients suffer from increased susceptibility to infection, poor nutrition, and increased risk of squamous cell carcinoma (SCC), leading to shortened life expectancy (Murat‐Sušić et al. 2011). Recent studies have also identified an important role for type VII collagen in basement membrane formation and re‐epithelialisation during wound healing (Nyström et al. 2013). In addition, depletion of COL7A1 contributes to SCC formation via enhanced transforming growth factor (TGF)‐β signalling and angiogenesis. Both number of blood vessels and vessel size are increased in a murine xenograft SCC model with loss of type VII collagen and in RDEB SCC patient samples and the effect is rescued by exogenous addition of type VII collagen (Martins et al. 2016). Therapeutic strategies currently being investigated for the treatment of DEB include direct injection of recombinant type VII protein, cell‐based therapies, gene therapies, and exon‐skipping drugs. These different approaches ultimately aim to restore functional anchoring fibrils at the dermal–epidermal junction (Woodley & Chen, 2004).

Type XII and XIV collagens

Type XII and XIV collagen are members of the FACIT (fibril‐associated collagens with interrupted triple helices) family of collagens and are similar in structure, containing a thin triple‐helical tail and three non‐triple‐helical sections (Aubert‐Foucher et al. 1992). Type XII collagen comprises homotrimers of α1(XII) and can be found in two distinct forms as a result of alternative splicing: a short and a long form (Mayne & Brewton, 1993). In skin, type XII collagen is expressed throughout the dermis, predominantly in the short form, and it localises most prominently in the papillary dermis (Lunstrum et al. 1991; Agarwal et al. 1994c). It is also observed around hair follicles (Sasaki et al. 2013). A mutation of type XII collagen resulted in disrupted matrix architecture in the papillary dermis (Reichenberger et al. 2004), suggesting a role in dermal matrix organisation.

Type XIV collagen is also composed of homotrimers of α1(XIV) and is weakly expressed in newborn dermis. Its expression, however, intensifies in children and adults (Castagnola et al. 1992; Berthod et al. 1997). The skin of mature Col14a1 null mice is significantly weaker than wild‐type animals, while tendons have altered biomechanical properties and ultrastructure at P4 and P7 compared with P60. These findings suggest that type XIV collagen regulates the early stages of collagen fibrillogenesis and tissue mechanics (Ansorge et al. 2009).

Type XIII collagen

Type XIII collagen is a transmembrane collagen that is expressed in the epidermis, at both cell–cell contact sites and the dermal epidermal junction. It co‐localises with E‐cadherin, thus suggesting an association with adherens junctions. It is also present in blood vessel walls and nerves of the dermis (Peltonen et al. 1991). In human fetal skin, type XIII collagen is found in the epidermis and scalp hair follicles (Sandberg et al. 1986). At the early stages of mouse development it is expressed at the thin ectodermal layer covering the fetus but is limited to the basal layers of the epidermis later in life (Sund et al. 2011). These results suggest that type XIII collagen may play a role in epidermal homeostasis.

Type XV and XVIII collagens

Type XV and XVIII collagens are homotrimers that constitute the distinct family of multiplexins (multiple triple‐helix domains with interruptions; Oh et al. 1994). As they contain glycosaminoglycan chains, chondroitin sulphate chains in type XV and heparan sulphate chains in type XVIII, they are also considered proteoglycans (Halfter et al. 1998; Li et al. 2000; Kawashima et al. 2003). In the skin, high levels of type XVIII collagen are found in the epidermal basement membrane, where type XV is weakly expressed. Type XVIII also localises to arterial smooth muscle in the dermis, where again type XV has a small presence. Both types are, however, found at similar levels in the basement membranes of vasculature and pili muscles in the dermis. Moreover, basement membranes in neurons and surrounding adipocytes only stain positive for type XV collagen (Saarela et al. 2006; Tomono et al. 1989). Type XV is also observed around the hair follicles and in the thin fibrils of the papillary dermis (Hägg et al. 1997). The expression of type XV collagen in seborrheic keratosis, melanocytic nevi and malignant melanomas suggests a potential role in keratinocyte and melanocyte tumorigenesis (Fukushige et al. 2005). In Col18a1 ^−/− mice, the epidermal basement membrane is significantly thicker (Utriainen et al. 2004), and the skin exhibits accelerated wound healing and vascularisation (Seppinen et al. 1997). Importantly, type XVIII is expressed by epidermal cells of the hair follicle bulge (Blanpain et al. 2004; Morris et al. 2004; Tumbar et al. 2002). Generation of double knockout mice for types XV and XVIII collagen resulted in viable animals without any new major defects, thus indicating that the functions of these two collagens are distinct (Ylikärppä et al. 2015).

Type XVI collagen

Type XVI collagen, a member of the FACIT family, is expressed in many connective tissues (reviewed specifically by Grässel & Bauer, 2013). In skin it is expressed in the papillary dermis and localises to the dermal–epidermal junction, close to collagen type VII, and is produced by both fibroblasts and keratinocytes. It is therefore thought to contribute to the structural integrity of the dermal–epidermal junction (Pan et al. 1992; Grässel et al. 1999). In addition, dermal dendrocytes produce type XVI collagen, indicating a unique function of this collagen in dermal immunological processes (Akagi et al. 2002). Increased levels of type XVI collagen have been reported in patients with localised and systemic scleroderma, but not in keloid scars (Akagi et al. 1999). Notably, COL16A1 is upregulated in all three epidermolysis bullosa subtypes (Knaup et al. 2012).

Type XVII collagen

Similar to type VII collagen, type XVII collagen has been studied in depth and is associated with the junctional form of epidermolysis bullosa (JEB), as well as the auto‐immune disease bullus pemphigoid (BP; Powell et al. 2004). Type XVII collagen is a homotrimeric, transmembrane collagen specifically expressed by basal keratinocytes and comprises a globular intracellular domain, a rod‐like transmembrane domain, and a large extracellular domain (ECD) with 15 triple helical collagenous domains (Schäcke et al. 1998; Areida et al. 2001). The ECD spans the lamina lucida and interacts with laminin 332 in the lamina densa (Masunaga et al. 1997). The intracellular domain interacts with the β4 integrin and plectin and is therefore an important component of hemidesmosomes (Schaapveld et al. 1996; Koster et al. 2003). Loss of type XVII collagen leads to destabilisation of hemidesmosomes and impaired adhesion of keratinocytes to the basement membrane (Borradori et al. 1997; Löffek et al. 2014). Mutations in the COL17A1 gene resulting in decreased protein expression or function cause several different forms of JEB, with symptoms including epidermal fragility, non‐scarring blistering, and skin atrophy (McGrath et al. 1993; Schumann et al. 2008). In addition to JEB, type XVII collagen is an auto‐antigen for bullus pemphigoid and is often referred to as bullus pemphigoid 180 (BP180) or bullus pemphigoid auto‐antigen 2 (BPAG2). The inflammatory auto‐immune response generated by type XVII collagen results in non‐scarring blister formation at the dermal epidermal junction (Diaz et al. 1990; Giudice et al. 1993). Thus, type XVII collagen plays an important role in both inherited and acquired skin diseases. An association of COL17A1 with hair follicle stem cell (HFSC) ageing was demonstrated recently. Type XVII collagen proteolysis leads to HFSC elimination and hair follicle ageing and miniaturisation, indicating a critical involvement of this collagen in hair loss (Matsumura et al. 2016).

Other collagens

Type XIX, XX, XXII, XXIII, XXIV, XXVII and XXVIII collagens have been reported to exist in the skin, but little is known about their exact function. Col19a1 is expressed in the skin during mouse development, only to be significantly downregulated in the adult tissue. The same expression pattern occurs in human neonatal and adult fibroblasts (Sumiyoshi et al. 1996). Small quantities of type XX collagen have been detected in chick embryonic skin (Koch et al. 2001). Type XXII collagen exhibits a very distinctive pattern and localises in tissue junctions. In the dermis it is found around sebaceous glands and the lower portion of hair follicles (Koch et al. 2004). Type XXIII collagen belongs to the transmembranous family of collagens and is expressed in all the layers of the epidermis and around the hair follicles, but not in the dermis (Koch et al. 2006). Detailed studies have shown that it localises at the non‐desmosomal membrane of keratinocytes, directly interacts with integrin α₂β₁, and is largely absent in migrating keratinocytes at the wound margin (Veit et al. 2006). Col24a1 mRNA is expressed at very low levels in the skin of developing mice (Koch et al. 2003). Type XXVII collagen is only transiently present in the dermis of the developing embryo. It is expressed in upper dermal regions at E14.5 days but not at E18.5 (Plumb et al. 2002), and it is found in the epidermis and hair follicles (Boot‐Handford et al. 2003). Finally, the most recently discovered member of the collagen superfamily, type XXVIII collagen, has been detected in low quantities in zebrafish and mouse skin (Gebauer et al. 2016; Veit et al. 2004). It is also specifically present around terminally differentiated Schwann cells in both glabrous and hairy human skin, is associated with Merkel cells and is part of the Ruffini corpuscle matrix (Grimal et al. 2010). These findings suggest a unique role for this collagen in the peripheral nervous system.

Conclusion and outlook

In summary, the minor collagens regulate a diverse, yet crucial, set of functions within the skin. Basement membrane collagens, such as types IV, VII, and XVII are essential for the structural integrity of the basement membrane zone and anchorage of the epidermis to the dermis. Interestingly, these membrane collagens are all associated with autoimmune responses, possibly pointing to a link of collagen tissue localisation with auto‐immunity (Hershko et al. 2007). In the dermis, types V, VI and XII mediate assembly of larger macromolecules and influence tissue mechanics, as well as processes such as wound healing and hair growth. Growth factors bind to a wide range of collagens, and the specificity and affinity for different collagen types has yet to be established. Insights in this area could help explain how different collagens regulate specific cell and tissue functions. Furthermore, altered expression or function of many of these collagens leads to a variety of skin disorders ranging from blistering skin diseases, such as DEB and JEB, to auto‐immune reactions, scarring and increased cancer susceptibility.

A large number of the findings on the functions of the minor collagens in skin are based on knockout mouse models where there is a likelihood of compensation from other collagens or systemic rather than tissue‐specific effects (Barbaric et al. 2007). These results should therefore be interpreted cautiously and further studies with tissue‐specific knockouts, for instance with the Cre/LoxP system, could be used to confirm them (Kos, 2004).

Current therapeutic approaches for treating these collagen‐based skin conditions, most notably RDEB, include gene therapy, exon skipping drugs, recombinant protein delivery, and cell therapy (Hsu et al. 2014). Although there has been excellent progress in these areas in recent years, still more work is needed to develop effective and reliable treatments. A potential breakthrough for gene therapy has recently come from the development of CRISPR/Cas9 gene editing technology, which could allow for mutated genes from a patient's own cells to be corrected directly (Sander & Joung, 1989). Nevertheless, much work will be required to translate this approach safely to the clinic.

Beyond gene therapy, insights into minor collagen functions in matrix assembly could have important implications for regenerative medicine. As these molecules have the ability to direct the organisation of collagen and fibronectin fibres, it may be possible to adopt a bio‐mimetic or bio‐inspired approach to design molecules or materials with similar capabilities. Such technologies could have applications in encouraging new matrix deposition within healing tissues, as well as controlling matrix structure and reducing scar formation. However, additional studies and a molecular level understanding of minor collagen functions are necessary before further progress can be made in this area.

Author contributions

G.T. and J.T.C. co‐wrote the manuscript.