Gary R. Lewin

Severe neuropathies in mice with targeted mutations in the ErbB3 receptor.

Neuregulins and their specific receptors, members of the ErbB family of tyrosine kinases, have been implicated in the control of growth and development of Schwann cells, specialized cells that wrap around nerve axons to provide electrical insulation. Here we use gene targeting to generate mice that lack ErbB3, a high-affinity neuregulin receptor. Homozygous erbB3 mutant embryos lack Schwann-cell precursors and Schwann cells that accompany peripheral axons of sensory and motor neurons. The initial development of motor neurons and sensory neurons of dorsal root ganglia occurs as it should, but at later stages most motor neurons (79%) and sensory neurons in dorsal root ganglia (82%) undergo cell death in erbB3 mutant embryos. Degeneration of the peripheral nervous system in erbB3 mutant pups is thus much more severe than the cell death in mice that lack neurotrophins or neurotrophin receptors,. We also show that ErbB3 functions in a cell-autonomous way during the development of Schwann cells, but not in the survival of sensory or motor neurons. Our results indicate that sensory and motor neurons require factors for their survival that are provided by developing Schwann cells.

The DRASIC Cation Channel Contributes to the Detection of Cutaneous Touch and Acid Stimuli in Mice

Cation channels in the DEG/ENaC family are proposed to detect cutaneous stimuli in mammals. We localized one such channel, DRASIC, in several different specialized sensory nerve endings of skin, suggesting it might participate in mechanosensation and/or acid-evoked nociception. Disrupting the mouse DRASIC gene altered sensory transduction in specific and distinct ways. Loss of DRASIC increased the sensitivity of mechanoreceptors detecting light touch, but it reduced the sensitivity of a mechanoreceptor responding to noxious pinch and decreased the response of acid- and noxious heat-sensitive nociceptors. The data suggest that DRASIC subunits participate in heteromultimeric channel complexes in sensory neurons. Moreover, in different cellular contexts, DRASIC may respond to mechanical stimuli or to low pH to mediate normal touch and pain sensation.

The Homeodomain Factor Lbx1 Distinguishes Two Major Programs of Neuronal Differentiation in the Dorsal Spinal Cord

Dorsal horn neurons in the spinal cord integrate and relay sensory information. Here, we show that the expression of the homeobox gene Lbx1 distinguishes two major neuronal classes generated in the dorsal spinal cord. The Lbx1(-) (class A) and Lbx1(+) (class B) neurons differ in their dependence on roof plate BMP signals for specification and settle in the deep and superficial dorsal horn, respectively. Lbx1 misexpression blocks the differentiation of class A neurons. Conversely, in Lbx1 mutant mice, class B neurons assume the identity of class A neurons. As a consequence, the morphology and neuronal circuitry of the dorsal horn are aberrant. We conclude that Lbx1 distinguishes two major neuronal classes in the dorsal spinal cord and is an important determinant of their distinct differentiation programs.

Roles for the pro-neurotrophin receptor sortilin in neuronal development, aging and brain injury

Neurotrophins are essential for development and maintenance of the vertebrate nervous system. Paradoxically, although mature neurotrophins promote neuronal survival by binding to tropomyosin receptor kinases and p75 neurotrophin receptor (p75NTR), pro-neurotrophins induce apoptosis in cultured neurons by engaging sortilin and p75NTR in a death-signaling receptor complex. Substantial amounts of neurotrophins are secreted in pro-form in vivo, yet their physiological significance remains unclear. We generated a sortilin-deficient mouse to examine the contribution of the p75NTR/sortilin receptor complex to neuronal viability. In the developing retina, Sortilin 1 (Sort1)−/− mice showed reduced neuronal apoptosis that was indistinguishable from that observed in p75NTR-deficient (Ngfr−/−) mice. To our surprise, although sortilin deficiency did not affect developmentally regulated apoptosis of sympathetic neurons, it did prevent their age-dependent degeneration. Furthermore, in an injury protocol, lesioned corticospinal neurons in Sort1−/− mice were protected from death. Thus, the sortilin pathway has distinct roles in pro-neurotrophin–induced apoptotic signaling in pathological conditions, but also in specific stages of neuronal development and aging.

Neurotrophins Live or Let Die: Does p75NTR Decide?

However, enhancing neurotrophin binding is not the Cornell University Medical College only role for p75. A number of reports suggest that it New York, New York 10021 also modulates trk signaling. While it is clear that trk †Growth Factor and Regeneration Group signaling can occur in the absence of p75, the reDepartment of Neuroscience sponses are increased in its presence. For example, Max Delbrück Institute for Molecular Medicine sympathoadrenal cells (MAH cells) expressing only trkA Berlin-Buch D-13122 extended neurites and survived in the presence of NGF Federal Republic of Germany but when coexpressing p75 showed an 8-fold higher

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.

Piezo2 is the major transducer of mechanical forces for touch sensation in mice

The sense of touch provides critical information about our physical environment by transforming mechanical energy into electrical signals. It is postulated that mechanically activated cation channels initiate touch sensation, but the identity of these molecules in mammals has been elusive. Piezo2 is a rapidly adapting, mechanically activated ion channel expressed in a subset of sensory neurons of the dorsal root ganglion and in cutaneous mechanoreceptors known as Merkel-cell–neurite complexes. It has been demonstrated that Merkel cells have a role in vertebrate mechanosensation using Piezo2, particularly in shaping the type of current sent by the innervating sensory neuron; however, major aspects of touch sensation remain intact without Merkel cell activity. Here we show that mice lacking Piezo2 in both adult sensory neurons and Merkel cells exhibit a profound loss of touch sensation. We precisely localize Piezo2 to the peripheral endings of a broad range of low-threshold mechanoreceptors that innervate both hairy and glabrous skin. Most rapidly adapting, mechanically activated currents in dorsal root ganglion neuronal cultures are absent in Piezo2 conditional knockout mice, and ex vivo skin nerve preparation studies show that the mechanosensitivity of low-threshold mechanoreceptors strongly depends on Piezo2. This cellular phenotype correlates with an unprecedented behavioural phenotype: an almost complete deficit in light-touch sensation in multiple behavioural assays, without affecting other somatosensory functions. Our results highlight that a single ion channel that displays rapidly adapting, mechanically activated currents in vitro is responsible for the mechanosensitivity of most low-threshold mechanoreceptor subtypes involved in innocuous touch sensation. Notably, we find that touch and pain sensation are separable, suggesting that as-yet-unknown mechanically activated ion channel(s) must account for noxious (painful) mechanosensation.

POINT MUTATION IN TRKB CAUSES LOSS OF NT4-DEPENDENT NEURONS WITHOUT MAJOR EFFECTS ON DIVERSE BDNF RESPONSES

Neurotrophins are a family of soluble ligands that promote the survival and differentiation of peripheral and central neurons and regulate synaptic function. The two neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4), bind and activate a single high-affinity receptor, TrkB. Experiments in cell culture have revealed that an intact Shc adaptor binding site on TrkB and subsequent activation of the Ras/MAPK pathway are important for neuronal survival and neurite outgrowth. To elucidate the intracellular signaling pathways that mediate the diverse effects of BDNF and NT4 in vivo, we have mutated in the mouse germline the Shc binding site in the trkB gene. This trkB(shc) mutation revealed distinctive responses to BDNF and NT4. While nearly all NT4-dependent sensory neurons were lost in trkB(shc/shc) mutant mice, BDNF-dependent neurons were only modestly affected. Activation of MAP kinases and in vitro survival of cultured trkB(shc/shc) neurons were reduced in response to both neurotrophins, with NT4 being less potent than BDNF, suggesting differential activation of TrkB by the two ligands. Moreover, while the Ras/MAPK pathway is required for in vitro differentiation of neuronal cells, trkB(shc/shc) mutant mice do not show any defects in BDNF-dependent differentiation of CNS neurons or in the function of sensory neurons that mediate innocuous touch.

The AMPA Receptor Subunits GluR-A and GluR-B Reciprocally Modulate Spinal Synaptic Plasticity and Inflammatory Pain

Ca(2+)-permeable AMPA receptors are densely expressed in the spinal dorsal horn, but their functional significance in pain processing is not understood. By disrupting the genes encoding GluR-A or GluR-B, we generated mice exhibiting increased or decreased numbers of Ca(2+)-permeable AMPA receptors, respectively. Here, we demonstrate that AMPA receptors are critical determinants of nociceptive plasticity and inflammatory pain. A reduction in the number of Ca(2+)-permeable AMPA receptors and density of AMPA channel currents in spinal neurons of GluR-A-deficient mice is accompanied by a loss of nociceptive plasticity in vitro and a reduction in acute inflammatory hyperalgesia in vivo. In contrast, an increase in spinal Ca(2+)-permeable AMPA receptors in GluR-B-deficient mice facilitated nociceptive plasticity and enhanced long-lasting inflammatory hyperalgesia. Thus, AMPA receptors are not mere determinants of fast synaptic transmission underlying basal pain sensitivity as previously thought, but are critically involved in activity-dependent changes in synaptic processing of nociceptive inputs.

Selective activation of nociceptors by P2X receptor agonists in normal and inflamed rat skin

1 ATP can elicit pain in humans and, together with other P2X channel agonists, can produce nocifensive responses in rodents. We used the rat in vitro skin‐nerve preparation to quantify primary afferent responses to ATP and its stable analogue α,β‐methylene ATP in normal and carrageenan‐inflamed skin. 2 Both ATP and α,β‐methylene ATP were found to specifically activate the peripheral terminals of Aδ and C‐fibre nociceptors in the skin. Thirty‐nine per cent of the nociceptors tested responded to the maximal dose of α,β‐methylene ATP (5 mm). In contrast, non‐nociceptive, low‐threshold mechano‐sensitive fibres were never activated by the same agonist concentrations. 3 Amongst the nociceptor population, C‐mechanoheat fibres (C‐MH or polymodal nociceptors) were markedly more responsive to P2X agonists than mechanonociceptors (C‐M nociceptors) with Aδ‐ or C‐fibre axons. Both C‐mechanoheat and C‐mechanonociceptors were activated by α,β‐methylene ATP doses as low as 50 μm. 4 In skin inflamed with carrageenan 3‐4 h before recording both the number of responsive C‐fibre nociceptors and their response magnitude increased. The increased neural response under inflammatory conditions was largely observed in C‐mechanoheat or polymodal nociceptors. After low doses of P2X agonists C‐MH fibres but not C‐M fibres developed elevated ongoing activity and this effect was only seen after carrageenan inflammation. The time course of α,β‐methylene ATP‐evoked discharges in nociceptors was found to correlate well with the time course of behavioural nocifensive responses in rats to the same agonist described in a previous study ( Hamilton et al. 1999 ). 5 We conclude that the rapid increase in the number of α,β‐methylene ATP responsive nociceptors and the increased magnitude of the neural response following carrageenan inflammation explains why very low concentrations of such agonists can cause pain in inflammatory states.