Peripheral Myelin Protein‐22 is Expressed in Rat and Mouse Brain and Spinal Cord Motoneurons

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Abstract
Peripheral myelin protein PMP‐22 is expressed by Schwann cells and is a constituent of peripheral nervous system (PNS) myelin. Two PMP‐22 transcripts, SR73 and CD25, differing in their 5’non‐coding sequences have been described. SR73 is present both in the PNS and in non‐neural tissue, whereas CD25 mRNA is almost exclusively expressed in Schwann cells. 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 regional distribution of PMP‐22 mRNA in the rat and mouse CNS by the reverse transcriptase‐polymerase chain reaction method, using oligonucleotide primers specific for the SR73 or CD25 transcripts. SR73 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 the PMP‐22 transcripts, determined by in situ hybridization in 21 day‐old animals, showed selective expression in the motor nuclei. The PMP‐22 signal was very weak in the nuclei of the oculomotor and trochlear nerves and absent in the nucleus of the abducens nerve. A strong PMP‐22 signal was observed in the motor nuclei of the trigeminal, facial, ambigus, vagus, hypoglossal and accessory spinal nerves and in the ventral horn of the spinal cord. The PMP‐22‐positive cells were identified as motoneurons on the basis of topographic and morphological criteria, as well as immunolabelling with neuron‐specific antibodies. Immunostaining with specific anti‐PMP‐22 antibodies demonstrated that neurons expressing the PMP‐22 message also expressed the corresponding protein in their cell bodies and axons. In contrast, neurons in the sensory nuclei of cranial nerves and dorsal root ganglion neurons were PMP‐22‐negative. In the trembler mouse, a PNS dysmyelinating mutant with a missense mutation in PMP‐22, the motoneuronal expression of PMP‐22 mRNA was either normal or increased.