Decrease in the number of lower affinity (type II) nerve growth factor receptors on embryonic sensory neurons does not affect fiber outgrowth

Nerve growth factor binds to two different specific receptors on responsive cells. The relationship of these two receptors is not fully understood at this time. We have studied the binding of labeled NGF to a different strain of white leghorn chicken embryo dorsal root ganglionic cells. The equilibrium dissociation constants for the two sites (KdI = 4.1 +/- 1.8 x 10(-11) M, KdII = 1.0 +/- 0.8 x 10(-9) M) are identical to those obtained previously. Also, the number of type I sites per cell (3.8 +/- 1.3 x 10(3)) is the same as that previously determined. However, the number of type II sites per cell (1.9 +/- 1.3 x 10(4)) is significantly different than that previously determined. This 2.5-fold decrease in the number of type II sites does not affect the concentration of NGF needed to obtain maximal fiber outgrowth from explanted sensory ganglia. The rate of association (1.2 +/- 0.2 x 10(7) M-1 sec-1 at 22 degrees C) of labeled NGF with receptors on sensory neurons from this different strain of chickens is identical to that previously obtained. The rate of association of NGF with its receptors on sensory neurons was also determined at 4 degrees C. This rate constant (2.1 +/- 1.1 x 10(6) M-1 sec-1) along with the rate constants obtained at 22 degrees and 37 degrees C were used to determine an activation energy for the binding of NGF to its receptors. The activation energy obtained (16.2 kcal/mole) suggests that binding is not a diffusion-controlled process.