Differential dependency of unmyelinated and Aδ epidermal and upper dermal innervation on neurotrophins, trk receptors, and p75LNGFR

Differential effects of TrkC isoforms on sensory axon outgrowth

Neurotrophins are powerful regulators of neuronal morphology. Several lines of evidence are consistent with the idea that characteristic axonal and dendritic morphologies throughout the nervous system may be determined by local patterns of neurotrophin and neurotrophin receptor expression. Neurotrophin receptor tryosine kinases (Trks) exist in both tyrosine-containing (TK+) and tyrosine-lacking (TK-) isoforms, both of which are expressed in many neuronal populations. However, ratios of TK+ to TK- isoforms may vary at different stages of development and may be differentially distributed to cellular compartments. To test whether these isoforms have different functions related to axon outgrowth, full-length or tyrosine kinase-lacking TrkC receptors were overexpressed in embryonic dorsal root ganglion neurons maintained in explant cultures in neurotrophin-3 (NT-3)-containing media. Neurons were transfected with plasmid DNA encoding enhanced yellow fluorescent protein (EYFP) and TrkC receptor isoforms by particle-mediated gene transfer. Control neurons possessed 3.7 +/- 1.3 primary processes and 113.8 +/- 46 branch points. About 80% of the branches were located along the distal part of the axon. Transfection with the trkC TK+ increased the number of primary processes (6.5 +/- 2.8), whereas transfection with trkC TK- reduced the formation of primary processes (3.0 +/- 1.3). Surprisingly, the distribution of branch points was shifted to the proximal region of axons in neurons transfected with trkC TK-. These observations are consistent with the idea that differential expression of Trk isoforms during development may sculpt axonal morphology.

Distinct roles for nerve growth factor and brain-derived neurotrophic factor in controlling the rate of hair follicle morphogenesis

Increasing evidence suggests that neurotrophins play an important part in the control of the development of ectodermal derivatives, such as the hair follicle. Here, we show that, during hair follicle morphogenesis in C57BL/6 mice, nerve growth factor, brain-derived neurotrophic factor and their corresponding high-affinity tyrosine kinase receptors, TrkA and TrkB, show stringently controlled spatiotemporal expression patterns in the follicular epithelium and mesenchyme. Constitutive overexpression of nerve growth factor in mice is associated with a discrete, but significant acceleration of hair follicle morphogenesis, whereas this is not seen in brain-derived neurotrophic factor transgenic mice. In neonatal skin organ culture, nerve growth factor and brain-derived neurotrophic factor differentially influence hair follicle development: nerve growth factor accelerates late stages of hair follicle morphogenesis, whereas brain-derived neurotrophic factor does not show significant effects. This suggests that the morphogenetic properties of locally generated neurotrophins in the skin, similar to their classical neurotrophic functions, are quite distinct and depend on the response patterns of the corresponding neurotrophin target receptor-expressing cells in the developing hair follicle. These data further strengthen the concept that neurotrophin signaling is an important element in controlling the rate of hair follicle morphogenesis, yet also highlight the complexity of this signaling system.

Cysteine-rich domain isoforms of the neuregulin-1 gene are required for maintenance of peripheral synapses

Neuregulin-1 (NRG-1) signaling has been implicated in inductive interactions between pre- and postsynaptic partners during synaptogenesis. We used gene targeting to selectively disrupt cysteine-rich domain-(CRD-) containing NRG-1 isoforms. In CRD-NRG-1-/-mice, peripheral projections defasciculated and displayed aberrant branching patterns within their targets. Motor nerve terminals were transiently associated with broad bands of postsynaptic ACh receptor (AChR) clusters. Initially, Schwann cell precursors accompanied peripheral projections, but later, Schwann cells were absent from axons in the periphery. Following initial stages of synapse formation, sensory and motor nerves withdrew and degenerated. Our data demonstrate the essential role of CRD-NRG-1-mediated signaling for coordinating nerve, target, and Schwann cell interactions in the normal maintenance of peripheral synapses, and ultimately in the survival of CRD-NRG-1-expressing neurons.

Hair cycle-dependent changes in adrenergic skin innervation, and hair growth modulation by adrenergic drugs

Skin nerves may exert "trophic" functions during hair follicle development, growth, and/or cycling. Here, we demonstrate hair cycle-related plasticity in the sympathetic innervation of skin and hair follicle in C57BL/6 mice. Compared with telogen skin, the number of nerve fibers containing norepinephrine or immunoreactive for tyrosine hydroxylase increased during the early growth phase of the hair cycle (anagen) in dermis and subcutis. The number of these fibers declined again during late anagen. beta2-adrenoreceptor-positive keratinocytes were transiently detectable in the noncycling hair follicle epithelium, especially in the isthmus and bulge region, but only during early anagen. In early anagen skin organ culture, the beta2-adrenoreceptor agonist isoproterenol promoted hair cycle progression from anagen III to anagen IV. The observed hair cycle-dependent changes in adrenergic skin innervation on the one hand, and hair growth modulation by isoproterenol, accompanied by changes in beta2-adrenoreceptor expression of selected regions of the hair follicle epithelium on the other, further support the concept that bi-directional interactions between the hair follicle and its innervation play a part in hair growth control. This invites one to systematically explore the neuropharmacologic manipulation of follicular neuroepithelial interactions as a novel therapeutic strategy for managing hair growth disorders.

Abundant production of brain-derived neurotrophic factor by adult visceral epithelia. Implications for paracrine and target-derived Neurotrophic functions

Brain-derived neurotrophic factor (BDNF) plays a crucial role for the survival of visceral sensory neurons during development. However, the physiological sources and the function of BDNF in the adult viscera are poorly described. We have investigated the cellular sources and the potential role of BDNF in adult murine viscera. We found markedly different amounts of BDNF protein in different organs. Surprisingly, BDNF levels in the urinary bladder, lung, and colon were higher than those found in the brain or skin. In situ hybridization experiments revealed that BDNF mRNA was made by visceral epithelial cells, several types of smooth muscle, and neurons of the myenteric plexus. Epithelia that expressed BDNF lacked both the high- and low-affinity receptors for BDNF, trkB and p75(NTR). In contrast, both receptors were present on neurons of the peripheral nervous system. Studies with BDNF-/-mice demonstrated that epithelial and smooth muscle cells developed normally in the absence of BDNF. These data provide evidence that visceral epithelia are a major source, but not a target, of BDNF in the adult viscera. The abundance of BDNF protein in certain internal organs suggests that this neurotrophin may regulate the function of adult visceral sensory and motor neurons.

Neurotrophins, nociceptors, and pain

It is now well established that neurotrophins play a crucial role in the development of the nervous system. However, there is increasing evidence that the function of neurotrophins persists throughout adulthood. The broad scope of neurotrophin action is well documented in the case of nerve growth factor (NGF) and its effect on nociceptors and nociception. Here, we review the evidence for these multiple roles for NGF. Two manipulations influencing NGF levels are discussed in detail. The first involves the use of transgenic mice that overexpress or underexpress neurotrophins. A second strategy involves administration of NGF or its antibody in vivo to increase or decrease its level. During prenatal development, NGF is required for survival of nociceptors. In the early postnatal period, NGF is required for expression of the appropriate nociceptor phenotype. In adults, NGF acts as an important intermediate in inflammatory pain, contributing to both peripheral and central sensitization. The sensitization of peripheral nociceptors can be very rapid and can involve non-neural cells such as mast cells, neutrophils, fibroblasts, and macrophages. Recent evidence indicates that other neurotrophins also play key supporting roles in the development of nociceptors (e.g., NT-3) and in inflammatory pain (e.g., BDNF, NT-4/5). Furthermore, molecules from other superfamilies (e.g., GDNF) also are required to assure survival of certain classes of nociceptors. The diverse effects of neurotrophins on nociceptive processing emphasize their broad importance in the development and function of the nervous system.

A new role for neurotrophins: involvement of brain-derived neurotrophic factor and neurotrophin-4 in hair cycle control

Neurotrophins exert many biological effects not directly targeted at neurons, including modulation of keratinocyte proliferation and apoptosis in vitro. Here we exploit the cyclic growth and regression activity of the murine hair follicle to explore potential nonneuronal functions of neurotrophins in the skin, and analyze the follicular expression and hair growth-modulatory function of BDNF, NT-4, and their high-affinity receptor, TrkB. The cutaneous expression of BDNF and NT-4 mRNA was strikingly hair cycle dependent and peaked during the spontaneous, apoptosis-driven hair follicle regression (catagen). During catagen, BDNF mRNA and immunoreactivity, as well as NT-4-immunoreactivity, were expressed in the regressing hair follicle compartments, whereas TrkB mRNA and immunoreactivity were seen in dermal papilla fibroblasts, epithelial strand, and hair germ. BDNF or NT-4 knockout mice showed significant catagen retardation, whereas BDNF-overexpressing mice displayed acceleration of catagen and significant shortening of hair length. Finally, BDNF and NT-4 accelerated catagen development in murine skin organ culture. Together, our data suggest that BDNF and NT-4 play a previously unrecognized role in skin physiology as agents of hair growth control. Thus, TrkB agonists and antagonists deserve exploration as novel hair growth-modulatory drugs for the management of common hair growth disorders.

BDNF overexpression induces differential increases among subsets of sympathetic innervation in murine back skin

Besides their recognized dependence on nerve growth factor (NGF) during development, the dependence of mature sympathetic ganglion neurons on other neurotrophins is still unclear. Here, we have investigated the sympathetic innervation of back skin in mice overexpressing brain-derived neurotrophic factor (BDNF) under the alpha-myosin heavy-chain promoter, as well as in BDNF knockout (-/-) mice. Compared with wild-type controls, the dorsal skin of BDNF overexpressing mice displayed a significantly enhanced number of adrenergic, tyrosine hydroxylase-immunoreactive (IR) nerve fibres, while cholinergic or peptidergic sensory nerve fibres appeared unaltered. The adrenergic hyperinnervation in dorsal skin of BDNF overexpressing mice was most pronounced in the arrector pili muscle of hair follicles, while no increase of tyrosine hydroxylase-or neuropeptide Y-IR fibres associated with subcutaneous blood vessels was found. Instead, back skin of BDNF knockout (-/-) mice contained significantly fewer tyrosine hydroxylase-IR dermal nerve fibres than wild-type animals. This suggests that BDNF plays an important role in the control of different subsets of adrenergic innervation in murine back skin, and indicates that paravertebral sympathetic ganglia display a previously unrecognized differential BDNF-dependence in vivo.