Retinoic acid-mediated increase in TrkA expression is sufficient to elicit NGF-dependent survival of sympathetic neurons

Sympathetic neurons depend on the classical neurotrophin NGF for survival by the time they innervate their targets, but the mechanisms controlling the onset of NGF responsiveness in developing neuroblasts have not been defined. Immature chick sympathetic neurons are unresponsive to NGF, but express low mRNA levels of the high-affinity NGF receptor trkA. Treatment with retinoic acid (RA) leads to increased levels of both trkA mRNA and protein, a response mediated through retinoic acid receptor alpha (RAR alpha). Ectopic expression of trkA in these cells results in the ability to survive with NGF, suggesting that RA-induced trkA expression is sufficient to elicit NGF-dependent survival. Our data establish a mechanism controlling NGF responsiveness and implicate a function for RA at defined late stages of neuron development.

Retinoic acid induces NGF-dependent survival response and high-affinity NGF receptors in immature chick sympathetic neurons

An important step in the development of peripheral sensory and sympathetic neurons is the onset of the survival response and dependence on the presence of nerve growth factor (NGF) or other neurotrophic factors. We have recently observed that immature sympathetic neurons from 7-day-old chick embryos are unable to become NGF-responsive in vitro and we have now used these cells to identify molecules that induce NGF-dependent neuronal survival. We found that retinoic acid (RA) induces the ability of these cells to survive in the presence of NGF. At RA concentrations of 10(−9)-10(−8)M virtually all neurons survived in the presence of NGF. RA was found to also induce the biologically active, high-affinity NGF receptor: high-affinity receptors were undetectable on dissociated E7 sympathetic neurons and were observed in vitro only in RA-treated neurons. These findings suggest that the induction of high-affinity NGF receptors may be sufficient to activate the survival response in sympathetic neurons and imply an important role for RA during neuron differentiation in the peripheral nervous system.

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.

The survival of chick retinal ganglion cells in response to brain-derived neurotrophic factor depends on their embryonic age

The survival effects of brain-derived neurotrophic factor (BDNF) on the ganglion cells of the chick retina were studied in vitro at different embryonic ages. We found these effects to be strongly age-dependent: at E5, when the first ganglion cell axons have crossed the optic chiasm, but not yet reached the tectum, ganglion cells survived on a laminin substrate irrespective of the presence or absence of BDNF. At E6, when the axons of the first-generated ganglion cells reached the rostral pole of the tectum, the ganglion cells began to show a dependency on BDNF for survival, but the majority of them were alive after 2 days in vitro in the absence of BDNF. With increasing age, the BDNF dependency for survival increased, and at E11, the majority of the ganglion cells plated were dependent on BDNF for survival. It is at this age that the maximal number of axons can be found in vivo in the optic nerve, the subsequent elimination of ganglion cells and their axons resulting in the loss of hundreds of thousands of them over the next few days. Taken together, these data indicate that retinal ganglion cells depend on BDNF for survival only when their axons have reached their target in vivo. This situation is reminiscent of that described in the peripheral nervous system for the nerve growth factor responsiveness of mouse trigeminal sensory neurons during the period of innervation of their target.

Binding characteristics of brain-derived neurotrophic factor to its receptors on neurons from the chick embryo

Brain-derived neurotrophic factor (BDNF), a protein known to support the survival of embryonic sensory neurons and retinal ganglion cells, was derivatized with 125I-Bolton-Hunter reagent and obtained in a biologically active, radioactive form (125I-BDNF). Using dorsal root ganglion neurons from chick embryos at 9 d of development, the basic physicochemical parameters of the binding of 125I-BDNF with its receptors were established. Two different classes of receptors were found, with dissociation constants of 1.7 x 10(-11) M (high-affinity receptors) and 1.3 x 10(-9) M (low-affinity receptors). Unlabeled BDNF competed with 125I-BDNF for binding to the high-affinity receptors with an inhibition constant essentially identical to the dissociation constant of the labeled protein: 1.2 x 10(-11) M. The association and dissociation rates from both types of receptors were also determined, and the dissociation constants calculated from these kinetic experiments were found to correspond to the results obtained from steady-state binding. The number of high-affinity receptors (a few hundred per cell soma) was 15 times lower than that of low-affinity receptors. No high-affinity receptors were found on sympathetic neurons, known not to respond to BDNF, although specific binding of 125I-BDNF to these cells was detected at a high concentration of the radioligand. These results are discussed and compared with those obtained with nerve growth factor on the same neuronal populations.