Mark Bothwell

Expression and structure of the human NGF receptor

The nucleotide sequence for the human nerve growth factor (NGF) receptor has been determined. The 3.8 kb receptor mRNA encodes a 427 amino acid protein containing a 28 amino acid signal peptide, an extracellular domain containing four 40 amino acid repeats with six cysteine residues at conserved positions followed by a serine/threonine-rich region, a single transmembrane domain, and a 155 amino acid cytoplasmic domain. The sequence of the extracellular domain of the NGF receptor predicts a highly ordered structure containing a negatively charged region that may serve as the ligand-binding site. This domain is conserved through evolution. Transfection of a full-length cDNA in mouse fibroblasts results in stable expression of NGF receptors that are recognized by monoclonal antibodies to the human NGF receptor and that bind [125I]NGF.

Neurotrophin expression in rat hippocampal slices: A stimulus paradigm inducing LTP in CA1 evokes increases in BDNF and NT-3 mRNAs

We report that stimulation inducing long-term potentiation (LTP) in the CA1 pyramidal cell layer of the hippocampus evokes significant increases in both BDNF and NT-3 mRNAs in CA1 neurons. No changes in BDNF or NT-3 mRNA levels were seen in the nonstimulated regions of the pyramidal cell layer or the dentate. No change was seen in the levels of NGF mRNA at the time point examined. These results suggest that relatively normal levels of activity may regulate region-specific neurotrophin levels in the hippocampus. Given that known effects of NGF (and presumably of BDNF and NT-3) include elevation of neurotransmitter levels, elevation of sodium channels, and promotion of axonal terminal sprouting, activity-associated changes in neurotrophin levels may play a role in regulating neural connections in the adult as well as the developing nervous system.

Novel roles for neurotrophins are suggested by BDNF and NT-3 mRNA expression in developing neurons

The results of our in situ hybridization experiments demonstrate that sensory neurons, sympathetic neurons, and motoneurons express brain-derived neurotrophic factor and/or neurotrophin-3 mRNAs during development in mouse. In accordance with previous data, we also find neurotrophins in the targets of sensory neurons (skin) and motoneurons (muscle) and the neurotrophin receptors p75, trkA, and trkB in sensory and sympathetic ganglia. These results suggest that neurotrophins have roles other than being target-derived factors that support neuron survival during developmental cell death (neurotrophic hypothesis), but may be transported in an orthograde fashion in neurons and released from axon terminals. We discuss several novel roles for neurotrophins, including autocrine/paracrine regulation of neuron survival, regulation of Schwann cell activity, and neuron to target signaling.

A p75 NTR and Nogo receptor complex mediates repulsive signaling by myelin-associated glycoprotein

Myelin-associated glycoprotein (MAG), an inhibitor of axon regeneration, binds with high affinity to the Nogo-66 receptor (NgR). Here we report that the p75 neurotrophin receptor (p75NTR) is a co-receptor of NgR for MAG signaling. In cultured human embryonic kidney (HEK) cells expressing NgR, p75NTR was required for MAG-induced intracellular Ca2+ elevation. Co-immunoprecipitation showed an association of NgR with p75NTR that can be disrupted by an antibody against p75NTR (NGFR5), and extensive coexpression was observed in the developing rat nervous system. Furthermore, NGFR5 abolished MAG-induced repulsive turning of Xenopus axonal growth cones and Ca2+ elevation, both in neurons and in NgR/p75NTR-expressing HEK cells. Thus we conclude that p75NTR is a co-receptor of NgR for MAG signaling and a potential therapeutic target for promoting nerve regeneration.

Genetic linkage of von Recklinghausen neurofibromatosis to the nerve growth factor receptor gene

von Recklinghausen neurofibromatosis (VRNF) is one of the most common inherited disorders affecting the human nervous system. VRNF is transmitted as an autosomal dominant defect with high penetrance but variable expressivity. The disorder is characterized clinically by hyperpigmented patches of skin (café au lait macules, axillary freckles) and by multiple tumors of peripheral nerve, spinal nerve roots, and brain (neurofibromas, optic gliomas). These tumors can cause disfigurement, paralysis, blindness, and death. We have determined the chromosomal location of the VRNF gene by genetic linkage analysis using DNA markers. The VRNF gene is genetically linked to the locus encoding nerve growth factor receptor, located on the long arm of chromosome 17 in the region 17q12----17q22. However, crossovers with the VRNF locus suggest that a mutation in the nerve growth factor receptor gene itself is unlikely to be the fundamental defect responsible for the VRNF phenotype.

Keeping track of neurotrophin receptors.

Mark Bothwell Department of Physiology and Biophysics University of Washington Seattle, Washington 98195 In the manner of many other growth factors, the prototypi- cal neurotrophic factor, nerve growth factor (NGF), has recently been found to be a member of a small gene family encoding structurally and functionally related proteins, collectively referred to as neurotrophins. Individual neuro- trophins exert similar functional effects, but on different neuronal populations. Each of the neurotrophins de- scribed to date-NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin3 (NT-3)-interacts with cell- surface receptors that are heterogeneous with regard to binding affinity. For NGF and BDNF, analysis of equilib- rium binding data indicates the existence of two receptor populations with dissociation constants of about IO-” M (“high affinity”) and 10e9 M (“low affinity”) (Sutter et al., 1979; Rodriguez-TBbar and Barde, 1988). The low affinity receptor binds NGF, BDNF, and NT-3 with similar affinity, while high affinity receptors exist that bind either NGF or BDNF selectively (Rodriguez-TBbar et al., 1990; Squint0 et al., 1991). Functional response to aneurotrophin appar- ently is mediated specifically by the high affinity receptors. Rodriguez-TBbar et al. have proposed that there is a shared component of neurotrophin receptors, known as ~75, with the specificity of each receptor determined by a second component. A variety of studies have shown that low and high affinity NGF receptor forms are intercon- vertible. The interrelationshipsof thevarious neurotrophin recep- tors and of their high and low affinity forms have now been described. The emerging story has profound implications for neurobiology and quite probably for the biology of non- neural tissues also. The first neurotrophin receptor molecular clones iso- lated (Johnson et al., 1986; Radeke et al., 1987) encode a 75-80 kd intrinsic membrane protein (~75) that binds NGF, BDNF, and NT-3 with comparable low affinity (Kd of 10d9 M) when expressed in fibroblastic cells (Rodriguez- Tb Squintoet al., 1991). ~75 has a relatively small cytoplasmic domain containing none of the struc- tural motifs known to function in signal transduction in other receptors. Yet, transfection of ~75 cDNA clones into appropriate neuronal cell lines generates both high and low affinity NGF binding and renders these cells func- tionally responsive to NGF with regard to gene induction and enhanced neurite outgrowth (e.g., Hempstead et al., 1989). The response of such cell lines is weak in comparison with the response of primary neuronal cultures and the rat PC12 neuronal cell line, which expresses endogenous high affinity NGF receptors. The meager cellular response of transfected neuronal cell lines expressing ~7.5 may be

Loss of nerve growth factor receptor-containing neurons in Alzheimer's disease: A quantitative analysis across subregions of the basal forebrain

Magnocellular neurons comprising the Ch1-Ch4 regions of the basal forebrain provide topographic cholinergic innervation to the cerebral cortex, thalamus, and basolateral nucleus of the amygdala. Most quantitative studies analyzing the status of these neurons in Alzheimers disease (AD) have employed Nissl-stained preparations. These studies principally analyzed large neurons of a prespecified cell diameter. Since basal forebrain neurons atrophy in Alzheimers disease, an immunocytochemical marker for these neurons would appear to be a better alternative for determining whether there is regionally specific degeneration of cholinergic neurons across subregions of the basal forebrain. Brain sections from seven AD and five aged-matched control patients were immunocytochemically stained with a monoclonal antibody raised against the receptor for nerve growth factor (NGF), a probe which has previously been demonstrated to extensively and exclusively colocalize with cholinergic basal forebrain neurons in humans (17, 25, 35). NGF receptor-immunoreactive neurons within the hippocampal projecting nuclei of the medial septum (Ch1) and vertical limb of the diagonal band (Ch2) were minimally affected in AD as compared to control cases. In contrast, the Ch4 region demonstrated a significant loss of NGF receptor-immunoreactive neurons in AD that inversely correlated (-0.786) with the duration of the disease process. All four subregions of Ch4 were affected in the AD cases with the anterolateral (76.4%), intermediate (62.1%) and posterior divisions (76.5%) demonstrating the greatest reduction in NGF receptor-immunoreactive neurons. Nissl-counterstained sections failed to reveal magnocellular neurons which were not immunoreactive for the NGF receptor, suggesting that reductions in immunocytochemically stained neurons reflects neuron loss and not the failure of viable neurons to synthesize NGF receptors. These data indicate that cholinergic basal forebrain neurons which project to the amygdala, as well as to the temporal, frontobasal, and frontodorsal cortices, are most affected in AD.

Neurotrophins: To Cleave or Not to Cleave

The family of neurotrophic factors known as neurotrophins has yielded a series of surprises, both with regard to the broad extent of their functional roles and the remarkable complexity of their signaling mechanisms. The recent discovery that a neurotrophin precursor protein and its proteolytically processed products may differentially activate pro- and antiapoptotic cellular responses, through preferential activation of Trk or p75 receptors, promises to unveil yet another level of regulatory complexity.

Alternating phases of FGF receptor and NGF receptor expression in the developing chicken nervous system

Patterns of expression of transcripts encoding receptors for fibroblast growth factor and nerve growth factor (FGF-R and NGF-R) in the developing chick nervous system are compared using in situ hybridization histochemistry. FGF-R transcripts are expressed abundantly in the germinal neuroepithelial layer. Expression ceases as cells migrate into the mantle layer and returns during late maturation of neuronal populations, including cholinergic nuclei of the basal forebrain, brainstem reticular and motor nuclei, and cerebellar Purkinje and granule neurons. The pattern of NGF-R expression is generally reciprocal to that of FGF-R in the CNS and in some phases of development of the PNS. These results suggest that FGF and NGF may act sequentially rather than in concert during neuronal development.

Retrograde Transport of Neurotrophins from the Eye to the Brain in Chick Embryos: Roles of the p75NTR and trkB Receptors

The receptors involved in retrograde transport of neurotrophins from the retina to the isthmo-optic nucleus (ION) of chick embryos were characterized using antibodies to the p75 neurotrophin receptor and trkB receptors. Survival of neurons in the ION has been shown previously to be regulated by target-derived trophic factors with survival promoted or inhibited by ocular injection of brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF), respectively. In the present paper, we show that during the period of target dependence, these neurons express trkB and p75 neurotrophin receptor but not trkA or trkC mRNAs. We also show that BDNF and NT-3 were transported efficiently at low doses, whereas NGF was transported significantly only at higher doses. The transport of BDNF and NT-3 was reduced by high concentrations of NGF or by antibodies to either trkB or the p75 neurotrophin receptor. Thus both receptors help mediate retrograde transport of these neurotrophins. Ocular injection of the comparatively specific trk inhibitor K252a did not reduce transport of exogenous BDNF, but did induce significant neuronal death in the ION, which could not be prevented by co-injection of BDNF. Thus, transport of BDNF alone does not generate a trophic signal at the cell body when axonal trkB is inactivated. In summary, our results indicate that both p75 neurotrophin and trkB receptors can mediate internalization and retrograde transport of BDNF, but activation of trkB seems to be essential for the survival-promoting actions of this neurotrophin.