A kinetic analysis of substance P trafficking

The potential for administering substance P (SP) nocitoxins for the treatment of chronic pain has been identified. To characterize treatment protocols for the spinal cord or elsewhere, binding/internalization of these compounds at the cellular targets must be understood quantitatively. Thus, a kinetic model of SP binding and intracellular trafficking has been developed from data. The eight differential equation model describes surface binding between SP and neurokinin 1 receptor, clathrin-mediated endocytosis followed by spatial translation to a perinuclear endosome where SP is sorted from its receptor, SP degradation in late endosomes/early lysosomes, and return of sorted receptor to plasma membrane via recycling endosomes. With suitably optimized parameters, the model accounts for the kinetics of total, membrane-associated, and internalized SP in cells continuously exposed to SP, as well as the fractions of internalized SP remaining intact at 30 and 60 min. Simultaneously, the model accounts for the kinetics of internalization and receptor recycling after SP preloading of membrane and subsequent exposure to SP-free media. Rate constants (min(-1)) are: 0.034 +/- 0.004 (receptor off-rate), 0.15 +/- 0.03 (internalization), 0.048 +/- 0.003 (exit from sorting endosome), 0.062 +/- 0.008 (exit of labeled SP amino acids from prelysosome), and 0.029 +/- 0.004 (receptor return from recycling endosome to plasma membrane). The SP kinetics resemble those of transferrin and its receptor at the internalization step, but are several-fold slower in the sorting and recycling steps.