Neuronal plasticity of the "brain-skin connection": stress-triggered up-regulation of neuropeptides in dorsal root ganglia and skin via nerve growth factor-dependent pathways.

Emerging research indicates that central-nervous stress perception is translated to peripheral tissues such as the skin not only via classical stress hormones but also via neurotrophins and neuropeptides. This can result in neurogenic inflammation, which is likely to contribute to the triggering and/aggravation of immunodermatoses. Although the existence of such a "brain-skin connection" is supported by steadily increasing experimental evidence, it remains unclear to which extent perceived stress affects the sensory "hardwiring" between skin and its afferent neurons in the corresponding dorsal root ganglia (DRG). In this paper, we provide experimental evidence in a murine model of stress (exposure of C57BL/6 mice to sound stress) that stress exposure, or intracutaneous injection of recombinant nerve growth factor (NGF) to mimic the skin's response to stress, up-regulate the percentage of substance P (SP)+ or calcitonin gene-related peptide (CGRP)+ sensory neurons in skin-innervating DRG. Further, we show that the number of SP+ or CGRP+ sensory nerve fibers in the dermis of stressed C57BL/6 mice is significantly increased. Finally, we document that neutralization of NGF activity abrogates stress-induced effects on the percentage of SP+ and CGRP+ sensory neurons in skin-innervating DRG as well as on dermal sensory nerve fibers. These data suggest that high stress perception results in an intense cross talk between the skin and skin-innervating DRG, which increases the likelihood of NGF-dependent neurogenic skin inflammation by enhancing sensory skin innervation.

Nerve growth factor translates stress response and subsequent murine abortion via adhesion molecule-dependent pathways.

Spontaneous abortion is a frequent threat affecting 10%-25% of human pregnancies. Psychosocial stress has been suggested to be attributable for pregnancy losses by challenging the equilibrium of systems mandatory for pregnancy maintenance, including the nervous, endocrine, and immune system. Strong evidence indicates that stress-triggered abortion is mediated by adhesion molecules, i.e., intercellular adhesion molecule 1 (ICAM1) and leukocyte function associated molecule 1, now being referred to as integrin alpha L (ITGAL), which facilitate recruitment of inflammatory cells to the feto-maternal interface. The neurotrophin beta-nerve growth factor (NGFB), which has been shown to be upregulated in response to stress in multiple experimental settings including in the uterine lining (decidua) during pregnancy, increases ICAM1 expression on endothelial cells. Here, we investigated whether and how NGFB neutralization has a preventive effect on stress-triggered abortion in the murine CBA/J x DBA/2J model. We provide experimental evidence that stress exposure upregulates the frequency of abortion and the expression of uterine NGFB. Further, adhesion molecules ICAM1 and selectin platelet (SELP, formerly P-Selectin) and their ligands ITGAL and SELP ligand (SELPL, formerly P selectin glycoprotein ligand 1) respectively increase in murine deciduas in response to stress. Subsequently, decidual cytokines are biased toward a proinflammatory and abortogenic cytokine profile. Additionally, a decrease of pregnancy protective CD8alpha(+) decidual cells is present. Strikingly, all such uterine stress responses are abrogated by NGFB neutralization. Hence, NGFB acts as a proximal mediator in the hierarchical network of immune rejection by mediating an abortogenic environment comprised of classical signs of neurogenic inflammation.

Mast cell deficient and neurokinin-1 receptor knockout mice are protected from stress-induced hair growth inhibition.

Despite the lack of insight on distinct mediators in the skin orchestrating the pathophysiological response to stress, hair loss has often been reported to be caused by stress. Recently we revealed the existence of a "brain-hair follicle axis" by characterizing the neurokinin (NK) substance P (SP) as a central element in the stress-induced threat to the hair follicle, resulting in premature onset of catagen accompanied by mast cell activation in the skin. However, our understanding of possible SP-mast cell interactions in the skin in response to stress was limited since the receptor by which SP activates skin mast cells and the extent of mast cell mediated aggravation of SP remained to be elucidated. We now employed NK-1 receptor knockout mice (NK-1R(-/-)) and mast cell deficient W/W(v) mice and observed that stress-triggered premature induction of catagen and hair follicle apoptosis does not occur in NK1(-/-) and W/W(v) mice. Furthermore, the activation status of mast cells was less in stressed NK1(-/-) mice than in wild-type control. Additionally, stress-induced upregulation of SP positive nerve fibers was absent in both NK-1R and W/W(v) mice. These results indicate that the cross-talk between SP and mast cell activation via NK-1R appears to be the most important pathway in the regulation of hair follicle cycling upon stress response.

Stress exposure modulates peptidergic innervation and degranulates mast cells in murine skin.

Stress is said to induce itchiness of the skin, exacerbate inflammatory skin diseases, and inhibit wound healing. Neuropeptides such as substance P (SP) may play a role in these processes. Recently, we were able to show that both stress or SP are associated with neurogenic inflammation and increased apoptosis in the murine hair follicle. Moreover, peptidergic cutaneous innervation is subject to lifelong plasticity due to its association with the cyclic growth of hair follicles. However, peripheral neuronal plasticity has never been reported in altered interactions between the nervous and immune systems under perceived stress. Here, we show for the first time plasticity of the cutaneous peptidergic innervation in response to stress. After exposure to sonic stress, the number of SP+ nerve fibers in the back skin of C57BL/6 mice with their hair follicles in the resting phase of the hair cycle (telogen-low numbers of nerve fibers) increased significantly. Such nerve fibers contacted mast cells more frequently. At the same time, the percentage of degranulated mast cells increased significantly associated with a rise in apoptotic cells in the skin. Increased numbers of peptidergic nerve fibers correlated with increased numbers of growth-associated protein 43 (Gap-43)+ nerve fibers, which is a marker for growing nerves. Thus, neuronal plasticity and increased neuro-immune interaction occur under stress and may alter inflammatory skin diseases and trophic functions in the skin where neurogenic inflammation plays a part.

Neurogenic inflammation in stress-induced termination of murine hair growth is promoted by nerve growth factor.

Recently, we have revealed the existence of a "brain-hair follicle axis" in murine skin and have identified the neuropeptide substance P (SP) as a key mediator of stress-induced hair growth inhibition in vivo. Published evidence suggests that increased numbers of SP-immunoreactive sensory fibers, as seen in the dermis of stressed mice in anagen-catagen transition, are a result of transient high levels of nerve growth factor (NGF). Thus, we now aimed at dissecting the role of NGF in stress-triggered hair growth termination in our murine model. By real time PCR and immunohistochemistry, stress-exposed mice showed an up-regulation of NGF and its low-affinity receptor p75NTR; the NGF high-affinity receptor TrkA was moderately down-regulated. On neutralization of NGF, premature onset of catagen, apoptosis, and increased number/activation of perifollicular mast cells and antigen-presenting cells, which reflects the skin response to stress, was significantly abrogated. Stress or subcutaneous injection of recombinant NGF (to mimic stress) resulted in an increased percentage of SP(+) neurons in dorsal root ganglia, as measured by retrograde tracing. Taken together, these data suggest that NGF is a central element in the perifollicular neurogenic inflammation that develops during the murine skin response to stress and antagonizing NGF may be a promising therapeutic approach to counter the negative effect of stress on hair growth.