Localization of nerve growth factor (NGF) and low‐affinity NGF receptors in touch domes and quantification of NGF mRNA in keratinocytes of adult rats

Abstract

Touch domes are clearly delineated mechanoreceptors that are visible on the depilated skin of mammals. These structures consist of a sharply circumscribed disk of thickened epithelium surmounting a group of Merkel cells that are innervated by type I sensory neurons. These characteristic cutaneous structures provide an ideal opportunity for investigating whether the localization of nerve growth factor (NGF) in the skin is related to sites of sensory axon termination. For these reasons, we have used immunocytochemistry to study the distribution of NGF and the low-affinity NGF receptor (p75NGFR) in the touch domes of adult rat skin. Intense NGF-like immunoreactivity was sharply restricted to keratinocytes (excluding the stratum corneum) of the thickened epidermis of touch domes. The epidermis immediately surrounding touch domes and the epidermis of the tylotrich hair follicle associated with touch domes were not stained by anti-NGF antiserum. Merkel cells of the basal epidermis of touch domes were immunonegative for NGF but were immunopositive for p75NGFR as were the type I nerve endings innervating these cells. Quantitative Northern blotting revealed that the level of NGF mRNA was substantially higher in keratinocytes isolated from the stratum granulosum and stratum spinosum than in keratinocytes isolated from the stratum germinativum. These findings indicate that NGF synthesis in mature skin has a highly restricted regional distribution that is primarily associated with the innervation of a specialized touch receptor.

Neurotrophin‐3 signaling in mammalian Merkel cell development

“…Conversely, over-expression of NT-3 in the epidermis led to a 42% increase in the number of dorsal root ganglion neurons and a concommitant increase in Merkel cell numbers in touch domes by 4 -8 months of age (Albers et al, 1996). These data and denervation studies (Burgess et al, 1974;English et al, 1980English et al, , 1994 suggest that a trophic factor, such as NT-3, that is released by nerve terminals supports postnatal Merkel cell survival (reviewed by Saxod, 1996). We can exclude NT-3 as a neuronderived trophic factor, as we find high levels of NT-3 in Merkel cells themselves and in neighboring epithelial cells in the outer root sheath.…”

“…Conversely, NT-3 over-expression in the epidermis leads to a significant increase in the number of dorsal root ganglion sensory neurons and an increase in Merkel cells per touch domes in the periphery, indicating that NT-3 is involved in the regulation of the final number of sensory neurons and Merkel cells (Albers et al, 1996). Immunohistochemical studies of p75NTR immunoreactivity in the skin of adult rats (Ribeiro-da-Silva et al, 1991;English et al, 1994) and the fetal human footpad (Narisawa et al, 1992) showed p75NTR immunoreactivity in Merkel cells in touch domes of the basal epidermis. p75NTR immunoreactivity is seen in the outer root sheath of the hair follicle at postnatal age day 3-6 of murine back skin (Botchkareva et al, 1999).…”

Mice lacking the p75 receptor fail to acquire a normal complement of taste buds and geniculate ganglion neurons by adulthood

“…It is possible that neurotrophins produced in or around taste buds bind directly to p75 receptors and influence taste buds via autocrine or paracrine mechanisms. This type of nerve-independent mechanism functions to maintain adult Merkel cells (English et al, 1994;Kinkelin et al, 1999;SieberBlum et al, 2004). However, it is unlikely that neurotrophins act directly on taste buds via p75, since unlike previous reports (Fan et al, 2004;Yee et al, 2005), no p75-like immunoreactivity was observed in taste cells.…”

“…Mice lacking p75 (p75 À/À ) have sensory neuron and receptor deficits and develop periodic lesions on their footpads (Lee et al, 1992). A specific receptor complex, comprised of specialized p75 receptor expressing cells of the epidermis known as Merkel cells (English et al, 1994;Szeder et al, 2003;Sieber-Blum et al, 2004) and slowly adapting type 1 (SA1) mechanosensory neurons, requires the p75 receptor for its maintenance during late postnatal development (Fundin et al, 1997;Stucky and Koltzenburg, 1997;Rice et al, 1998;Kinkelin et al, 1999). In p75 À/À animals, Merkel cells are lost between 2 weeks and 2 months of age (Fundin et al, 1997;Kinkelin et al, 1999;Cronk et al, 2002).…”

“…In p75 À/À animals, Merkel cells are lost between 2 weeks and 2 months of age (Fundin et al, 1997;Kinkelin et al, 1999;Cronk et al, 2002). Merkel cells express the p75 receptor (English et al, 1994;Szeder et al, 2003;Sieber-Blum et al, 2004) and in some instances Merkel cell loss appears to be a direct effect of p75 signaling in Merkel cells and is not due to loss of innervation (Kinkelin et al, 1999). Thus, the p75 receptor is important for late postnatal development and/or adult maintenance of some sensory receptor cells and their associated innervation.…”