17.1. INTRODUCTION

Itch (or pruritus) has been defined as an unpleasant sensation that provokes the desire to scratch. Itch is also believed to signal danger from various environmental factors or physiological abnormalities. Therefore, it frequently accompanies a variety of inflammatory skin conditions and systemic diseases.

Histamine is the best-known pruritogen in humans and also acts as an experimental itch-causing substance. Clinically, antihistamines, i.e., histamine H₁-receptor blockers, are commonly used to treat all types of itching resulting from renal and liver diseases, as well as from serious skin diseases such as atopic dermatitis. However, antihistamines often lack efficacy in patients with chronic itch that may involve other agonists, including proteases, neuropeptides, cytokines, and opioids, and their cognate receptors, such as thermoreceptors, PARs, Mrgprs, and opioid receptors. Such pruritogenic mediators and modulators released in the periphery may directly activate itch-sensitive fibers, especially C-fibers, by binding to specific receptors on the nerve terminals (Ikoma et al. 2006; Paus et al. 2006; Xiao and Patapoutian 2011). Nerve fibers are also activated by exogenous mechanical, chemical, and biological stimuli, resulting in itch responses (Akiyama et al. 2010; Tominaga and Takamori 2010).

Histological analyses have shown that epidermal nerve densities are increased in patients with atopic dermatitis and xerosis, suggesting that the higher density is partly responsible for itch sensitization in the periphery. Such hyperinnervation is probably caused by an imbalance of nerve elongation factors, such as nerve growth factor (NGF), and nerve repulsion factors, such as semaphorin 3A (Sema3A), produced by keratinocytes (Tominaga and Takamori 2010). These axonal guidance molecules may also act on keratinocytes, immune cells and vascular endothelial cells, and be indirectly involved in the modulation of itching. This chapter presents recent knowledge regarding itch sensitization associated with epidermal nerve density controlled by NGF and Sema3A, especially in atopic dermatitis.

17.2. SKIN DISEASES WITH ITCH INVOLVING EPIDERMAL NERVE FIBERS

Sensory nerve fibers are acceptors of itch sensation as well as pain in the skin. The neuronal mechanisms underlying intractable itch in the periphery have been partly identified. Histological investigations have shown that the density of epidermal nerve fibers is higher in the skin of patients with atopic dermatitis, contact dermatitis, and xerosis than in healthy controls (Figure 17.1) (Ikoma et al. 2006; Tominaga and Takamori 2010), although the nerve density in patients with pruigo nodularis and psoriasis remains debatable (Schuhknecht et al. 2011; Taneda et al. 2011; Kou et al. 2012). Similar findings have been observed in animal models, such as NC/Nga mice, an atopic dermatitis model (Tominaga et al. 2007a, 2009a), and dry skin model mice (Miyamoto et al. 2002; Tominaga et al. 2007b). These findings are indicative of increases in sensory nerve fibers responsive to exogenous trigger factors and to various endogenous pruritogens from immune cells and keratinocytes, suggesting that hyperinnervation is partly responsible for itch sensitization.

FIGURE 17.1

Distribution of epidermal nerve fibers in healthy and pruritic skin samples. Immunohistochemical staining of the skin of healthy controls (HC), patients with atopic dermatitis (AD), and xerotic patients with antibody to protein gene product 9.5 (anti-PGP9.5). (more...)

Several recent studies have found that epidermal innervation density is reduced in various pruritic skin conditions, such as lichen amyloidosis (Maddison et al. 2008), prurigo nodularis (Schuhknecht et al. 2011), nummular dermatitis (Maddison et al. 2011) and keloids (Tey et al. 2012). Chronic stimulation of itch-transmitting nerve fibers may result in a self-regulated hypoplasia that modulates the intensity and persistence of sensory input. This hypothesis is supported by the results of one study, which showed diminished skin innervation in the skin of patients with neuropathic itch (Wallengren et al. 2002).

17.3. ROLES OF NGF AND Sema3A IN EPIDERMAL NERVE FIBERS

17.3.1. NGF

NGF is a neurotrophin that affects neurite outgrowth and neuronal survival (Lewin and Mendell 1993). Keratinocyte-derived NGF is a major mediator of cutaneous innervation density, in that local NGF concentrations are higher in the lesional skin of patients with prurigo nodularis, atopic dermatitis, psoriasis, contact dermatitis, and xerosis than in normal skin (Figure 17.2a) (Ikoma et al. 2006). In adult rat primary sensory neurons, NGF has been shown to upregulate neuropeptides, especially substance P (SP) and calcitonin gene-related peptide (CGRP) (Verge et al. 1995), both of which are involved in the hypersensitivity of itch sensation and neurogenic inflammation (Steinhoff et al. 2003). A more recent study demonstrated that human atopic keratinocytes produced elevated levels of NGF and mediated an increased outgrowth of CGRP-immunoreactive sensory fibers, whereas human atopic fibroblasts did not mediate this outgrowth (Roggenkamp et al. 2012). This result indicates that keratinocytes are key factors in hyperinnervation in individuals with atopic dermatitis. Intradermal injection of NGF was shown to sensitize nociceptors for cowhage- but not histamine-induced itch in human skin (Rukwied et al. 2012). Thus, increased NGF in the skin may also sensitize primary afferents, thereby contributing to chronic itch such as atopic dermatitis.

FIGURE 17.2

Epidermal NGF and Sema3A levels in pruritic skin samples. (a) Skin biopsies from HC and patients with AD were stained with anti-NGF antibody. Epidermal NGF levels (green) were higher in AD patients than in HC. Nuclei were counterstained with DAPI (blue). (more...)

Tumor necrosis factor (TNF)-α has also been found to enhance NGF production by human keratinocytes (Takaoka et al. 2009). TNF-α is a pivotal proinflammatory cytokine in the innate immune response and a key molecule for skin inflammation. Mast cells have been identified as important potential sources of TNF-α (Steinhoff et al. 2003). Plasma TNF-α concentration is higher in individuals with atopic dermatitis than without atopic dermatitis (Sumimoto et al. 1992), and both TNF-α and its receptors are upregulated in the dermal blood vessels of patients with psoriasis (Kristensen et al. 1993). In addition, skin barrier disruption induces the upregulation of TNF-α in the epidermis of acetone-treated mice, an acute dry skin model (Wood et al. 1992). These findings suggest that TNF-α is an important upregulator of NGF in the epidermis, a finding supported by results showing that TNF produced by mast cells promotes the elongation of cutaneous nerve fibers in a mouse model of contact dermatitis (Kakurai et al. 2006).

17.3.3. Mechanisms by Which Nerve Fibers Penetrate Basement Membranes

NGF stimulation of nerve fiber elongation in pruritic skin may result in the accumulation at the growth cone of integrins, which interact with a variety of extracellular matrix (ECM) components (Grabham and Goldberg 1997; Gardiner 2011). During this process, matrix metalloproteinases (MMPs) would be required for the growth cone to abrogate the three-dimensional ECM barriers. Using an in vitro model of basement membrane, consisting of Boyden chambers and Matrigel, and constituting a unique culture system of rat dorsal root ganglion (DRG) neurons, we recently showed that MMP-2 localized on the growth cone was involved in the penetration mechanism (Figure 17.3) (Tominaga et al. 2009b). In Boyden chamber, cultures using type I collagen gels and cultured DRG neurons, MMP-8 secreted by the nerve fibers was shown to be involved in nerve growth within the gels (Figure 17.3) (Tominaga et al. 2011). The expression of MMP-2 and MMP-8 was upregulated by NGF and downregulated by Sema3A. Interestingly, MMP2 expression was induced by its enzymatic substrates, including type IV, collagen, laminin, and fibronectin, as well as by MG. Moreover, MMP-8 expression was upregulated by its substrates types I and III collagens. The expression of both MMP genes was not altered by nonsubstrate molecules. The selection and upregulation of MMPs corresponding to the ECM components surrounding the growing nerve fibers are required for efficient nerve fiber penetration, suggesting that the coordinated activation of neurotrophin and ECM-integrin signaling is necessary for efficient and long-distance axon extension (Grabham and Goldberg 1997; Gardiner et al. 2011). Because class 3 semaphorin signaling inhibits integrin-mediated adhesion signaling, Sema3A stimulation of the growing nerve fibers may provide a reverse signaling pathway for these events (Zhou et al. 2008). Thus, although the integrin-mediated regulatory system remains unclear in our in vitro studies, this mechanism might be applicable to pruritic skin diseases involving skin hyperinnervation.

FIGURE 17.3

Possible models of nerve fiber growth into basement membrane or within the dermis. (a) NGF, which is produced by cutaneous cells such as epidermal keratinocytes, immune cells, and fibroblasts, promotes the production of MMP-2 and MMP-8 in sensory nerve (more...)

17.4. NGF AND Sema3A AS ANTIPRURITIC TARGETS

17.5. CONCLUSION

Nerve density in the epidermis may be involved in itch sensitization in pruritic skin diseases. Epidermal innervation is thought to be regulated by a fine balance between nerve elongation factors and nerve repulsion factors through the regulation of expression of MMPs. Treatment with anti-NGF agents, Sema3A replacement and other treatments such as UV-based therapies may normalize epidermal nerve fiber density. These findings may expand the knowledge of potential therapeutic strategies for ameliorating pruritus associated with epidermal nerve density, including patients with atopic dermatitis and xerosis.

ACKNOWLEDGMENTS

This work was supported by a Health Labor Sciences Research Grant for Research on Allergic Disease and Immunology from the Japanese Ministry of Health, Labor and Welfare, by a KAKENHI (20591354 and 2079081) and a “High-Tech Research Center” Project for Private Universities. Matching fund subsidy from MEXT and by a JSPS Research Fellow.

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