Neurofibromatosis: the role of guanosine triphosphatase activating proteins in sensory neuron function.
Neurofibromatosis type 1 (NF1) is a common autosomal dominant disease characterized by formation of multiple benign and malignant tumors. People with this disorder also experience chronic pain, which can be disabling. Neurofibromin, the protein product of the Nf1 gene, is a guanosine triphosphatase activating protein (GAP) for p21Ras (Ras). Loss of Nf1 results in an increase in activity of the Ras transduction cascade. Because of the growing evidence suggesting involvement of downstream components of the Ras transduction cascade in the sensitization of nociceptive sensory neurons, we examined the stimulus-evoked release of the neuropeptides, substance P (SP) and calcitonin gene-related peptide (CGRP), from primary sensory neurons of mice with a mutation of the Nf1 gene (Nf1+/-). Measuring the levels of SP and CGRP by radioimmunoassay, we demonstrated that capsaicin-stimulated release of neuropeptides is 3-5 folds higher in spinal cord slices from Nf1+/- mice than that from wildtype mouse tissue. In addition, the potassium- and capsaicin-stimulated release of CGRP from the culture of sensory neurons isolated from Nf1+/- mice was more than double that from the culture of wildtype neurons. Using patch-clamp electrophysiological techniques, we also examined the excitability of capsaicin-sensitive sensory neurons. It was found that the number of action potentials generated by the neurons from Nf1+/- mice, responding to a ramp of depolarizing current, was more than three times of that generated by wildtype neurons. Consistent with that observation, neurons from Nf1+/- mice had lower firing thresholds, lower rheobase currents and shorter firing latencies compared with wildtype neurons. These data clearly demonstrate that GAPs, such as neurofibromin, can alter the excitability of nociceptive sensory neurons. The augmented response of sensory neurons with altered Ras signaling may explain the abnormal pain sensations experienced by people with NF1 and suggests an important role of GAPs in the mechanism of sensory neuron sensitization.