PERIPHERAL MYELIN PROTEIN-22 IS EXPRESSED IN RAT AND MOUSE-BRAIN AND SPINAL-CORD MOTONEURONS

Peripheral myelin protein PMP-22 is expressed by Schwann cells and is a constituent of peripheral nervous system (PNS) myelin. Two PMP-22 tr anscripts, SR13 and CD25, differing in their 5' non-coding sequences h ave been described. SR13 is present both in the PNS and in non-neural tissue, whereas CD25 mRNA is almost exclusively expressed in Schwann c ells. PMP-22 mRNA is also present in the central nervous system (CNS), but at much lower levels than in the PNS. We have investigated the re gional distribution of PMP-22 mRNA in the rat and mouse CNS by the rev erse transcriptase-polymerase chain reaction method, using oligonucleo tide primers specific for the SR13 or CD25 transcripts. SR13 mRNA was detected in all the CNS regions analysed, whereas the CD25 message was present only in the brainstem and the spinal cord. Localization of th e PMP-22 transcripts, determined by in situ hybridization in 21 day-ol d animals, showed selective expression in the motor nuclei. The PMP-22 signal was very weak in the nuclei of the oculomotor and trochlear ne rves and absent in the nucleus of the abducens nerve. A strong PMP-22 signal was observed in the motor nuclei of the trigeminal, facial, amb igus, vagus, hypoglossal and accessory spinal nerves and in the ventra l horn of the spinal cord, The PMP-22-positive cells were identified a s motoneurons on the basis of topographic and morphological criteria, as well as immunolabelling with neuron-specific antibodies. Immunostai ning with specific anti-PMP-22 antibodies demonstrated that neurons ex pressing the PMP-22 message also expressed the corresponding protein i n their cell bodies and axons. In contrast, neurons in the sensory nuc lei of cranial nerves and dorsal root ganglion neurons were PMP-22-neg ative. In the trembler mouse, a PNS dysmyelinating mutant with a misse nse mutation in PMP-22, the motoneuronal expression of PMP-22 mRNA was either normal or increased.