Yves-Alain Barde

Purification of a new neurotrophic factor from mammalian brain.

We report the purification from pig brain of a factor supporting the survival of, and fibre outgrowth from, cultured embryonic chick sensory neurons. The purified factor migrates as one single band, mol. wt. 12 300, on gel electrophoresis in the presence of sodium dodecylsulphate (SDS) and is a basic molecule (pI greater than or equal to 10.1). Approximately 1 microgram factor was isolated from 1.5 kg brain. The final degree of purification was estimated to be 1.4 X 10(6)‐fold, and the specific activity 0.4 ng/ml/unit, which is similar to that of nerve growth factor (NGF) using the same assay system. This factor is the first neurotrophic factor to be purified since NGF, from which it is clearly distinguished because it has different antigenic and functional properties.

Regional distribution of brain-derived neurotrophic factor mRNA in the adult mouse brain.

Brain‐derived neurotrophic factor (BDNF) is a protein that allows the survival of specific neuronal populations. This study reports on the distribution of the BDNF mRNA in the adult mouse brain, where the BDNF gene is strongly expressed, using quantitative Northern blot analysis and in situ hybridization. All brain regions examined were found to contain substantial amounts of BDNF mRNA, the highest levels being found in the hippocampus followed by the cerebral cortex. In the hippocampus, which is also the site of highest nerve growth factor (NGF) gene expression in the central nervous system (CNS), there is approximately 50‐fold more BDNF mRNA than NGF mRNA. In other brain regions, such as the granule cell layer of the cerebellum, the differences between the levels of BDNF and NGF mRNAs are even more pronounced. The BDNF mRNA was localized by in situ hybridization in hippocampal neurons (pyramidal and granule cells). These data suggest that BDNF may play an important role in the CNS for a wide variety of adult neurons.

Brain-derived neurotrophic factor increases survival and differentiated functions of rat septal cholinergic neurons in culture.

Brain-derived neurotrophic factor (BDNF) was found to promote the survival of E17 rat embryo septal cholinergic neurons in culture, as assessed by a histochemical stain for acetylcholinesterase (AChE). A 2.4-fold increase in neuronal survival was achieved with 10 ng/ml BDNF. After initial deprivation of growth factor for 7 days, BDNF failed to bring about this increase, strongly suggesting that BDNF promotes cell survival and not just induction of AChE. BDNF was also found to increase the levels of cholinergic enzymes; choline acetyltransferase (ChAT) and AChE activities were increased by approximately 2-fold in the presence of 50 ng/ml BDNF. BDNF produced a 3-fold increase in the number of cells bearing the NGF receptor, as detected by the monoclonal antibody IgG-192. Although NGF had no additive effect with BDNF in terms of neuronal survival, suggesting that both act on a similar neuronal population, the combination of both produced an additive response, approximately a 6-fold increase, in ChAT activity.

Binding of brain-derived neurotrophic factor to the nerve growth factor receptor

The neurotrophic proteins BDNF and NGF are related in their primary structures, and both have high- and low-affinity receptors on their responsive neurons. In this study, we investigate the extent to which these receptors can discriminate between BDNF and NGF. We found that a 1000-fold excess of the heterologous ligand is needed to reduce binding to the high-affinity receptor by 50%, but that the same concentrations of BDNF and NGF similarly reduce the binding of either ligand to the low-affinity receptor. Results obtained with cells transfected with the low-affinity NGF receptor gene indicate that these cells bind BDNF, in addition to NGF, whereas cells before transfection do not. These data indicate that the low-affinity NGF receptor is also a low-affinity BDNF receptor and that whatever is conferring high-affinity binding and biological response also considerably reinforces the ability of the low-affinity receptor to discriminate between NGF and BDNF.

Selective Activation of NF-κB by Nerve Growth Factor Through the Neurotrophin Receptor p75

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) selectively bind to distinct members of the Trk family of tyrosine kinase receptors, but all three bind with similar affinities to the neurotrophin receptor p75 (p75NTR). The biological significance of neurotrophin binding to p75NTR in cells that also express Trk receptors has been difficult to ascertain. In the absence of TrkA, NGF binding to p75NTR activated the transcription factor nuclear factor kappa B (NF-κB) in rat Schwann cells. This activation was not observed in Schwann cells isolated from mice that lacked p75NTR. The effect was selective for NGF; NF-κB was not activated by BDNF or NT-3.

Glial cells generate neurons: the role of the transcription factor Pax6

Radial glial cells, ubiquitous throughout the developing CNS, guide radially migrating neurons and are the precursors of astrocytes. Recent evidence indicates that radial glial cells also generate neurons in the developing cerebral cortex. Here we investigated the role of the transcription factor Pax6 expressed in cortical radial glia. We showed that radial glial cells isolated from the cortex of Pax6 mutant mice have a reduced neurogenic potential, whereas the neurogenic potential of non-radial glial precursors is not affected. Consistent with defects in only one neurogenic lineage, the number of neurons in the Pax6 mutant cortex in vivo is reduced by half. Conversely, retrovirally mediated Pax6 expression instructs neurogenesis even in astrocytes from postnatal cortex in vitro. These results demonstrated an important role of Pax6 as intrinsic fate determinant of the neurogenic potential of glial cells.

Brain-derived neurotrophic factor supports the survival of cultured rat retinal ganglion cells

Brain-derived neurotrophic factor (BDNF) is a small, basic protein purified from the mammalian brain that has been shown previously to support the survival of cultured spinal sensory neurons (Barde et al., 1982). In current studies, BDNF was tested for its ability to support the survival of cultured CNS cells isolated from the perinatal rat retina. Both immunofluorescent labeling of Thy-1 and prior retrograde labeling with HRP were used as retinal ganglion cell markers in vitro. With embryonic day (E) 17 retinas, it was found that BDNF allowed the survival of a small subpopulation of neurons (about 7% of the cells plated at this age) identified by the immunofluorescent labeling of Thy- 1. No detectable effects were seen when either the total number of cells or the number of tetanus toxin-positive neurons was measured. BDNF also had an effect on cultured neurons retrogradely labeled after HRP injections in the superior colliculi of neonatal rats. The BDNF- responsive population was therefore detected only in retinal cultures with specific markers and identified as consisting of retinal ganglion cells. These cells could be enriched about 80-fold by density gradient centrifugation, and purified ganglion cell cultures were shown to be responsive to BDNF. Whereas with E17 retinas, the number of surviving Thy-1 positive neurons could be kept constant for at least 4 d, the survival of postnatal neurons was only transiently increased by BDNF. We conclude that in the retina, BDNF affects only the survival of ganglion cells in vitro by a direct action on these cells. The results are discussed in terms of target-derived neurotrophic support during development.

The neurotrophin receptor p75(NTR) : novel functions and implications for diseases of the nervous system

Neurotrophins have long been known to promote the survival and differentiation of vertebrate neurons. However, these growth factors can also induce cell death through the p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor receptor superfamily. Consistent with a function in controlling the survival and process formation of neurons, p75NTR is mainly expressed during early neuronal development. In the adult, p75NTR is re-expressed in various pathological conditions, including epilepsy, axotomy and neurodegeneration. Potentially toxic peptides, including the amyloid β- (Aβ-) peptide that accumulates in Alzheimers disease, are ligands for p75NTR. Recent work also implicates p75NTR in the regulation of both synaptic transmission and axonal elongation. It associates with the Nogo receptor, a binding protein for axonal growth inhibitors, and appears to be the transducing subunit of this receptor complex.

Neurotrophin binding to the p75 receptor modulates Rho activity and axonal outgrowth

While the neurotrophin receptor p75NTR is expressed by many developing neurons, its function in cells escaping elimination by programmed cell death remains unclear. The lack of intrinsic enzymatic activity of p75NTR prompted a search for protein interactors expressed in the developing retina, which resulted in the identification of the GTPase RhoA. In transfected cells, p75NTR activated RhoA, and neurotrophin binding abolished RhoA activation. In cultured neurons, inactivation of Rho proteins mimicked the effect of neurotrophins by increasing the rate of neurite elongation. In vivo, axonal outgrowth was retarded in mice carrying a mutation in the p75NTR gene. These results indicate that p75NTR modulates in a ligand-dependent fashion the activity of intracellular proteins known to regulate actin assembly.

Hyperphosphorylation and aggregation of tau in mice expressing normal human tau isoforms

Neurofibrillary tangles are composed of insoluble aggregates of the microtubule‐associated protein tau. In Alzheimers disease the accumulation of neurofibrillary tangles occurs in the absence of tau mutations. Here we present mice that develop pathology from non‐mutant human tau, in the absence of other exogenous factors, including β‐amyloid. The pathology in these mice is Alzheimer‐like, with hyperphosphorylated tau accumulating as aggregated paired helical filaments. This pathologic tau accumulates in the cell bodies and dendrites of neurons in a spatiotemporally relevant distribution.