Pelizaeus Merzbacher disease is an X-linked dysmyelinating disorder of the CNS, resulting from mutations in the proteolipid protein (PLP) gene. An animal model for this disorder, the myelin-deficient (MD) rat, carries a point mutation in the PLP gene and exhibits a phenotype similar to the fatal, connatal disease, including extensive dysmyelination, tremors, ataxia, and death at approximately postnatal day 21 (P21). We postulated that early death might result from disruption of myelinated neural pathways in the caudal brainstem and altered ventilatory response to oxygen deprivation or hypercapnic stimulus. Using barometric plethysmography to measure respiratory function, we found that the MD rat develops lethal hypoxic depression of breathing at P21, but hypercapnic ventilatory response is normal. Histologic examination of the caudal brainstem in the MD rat at this age showed extensive dysmyelination and downregulation of NMDA and to a lesser extent GABA(A) receptors on neurons in the nucleus tractus solitarius, hypoglossal nucleus, and dorsal motor nucleus of the vagus. Unexpectedly, immunoreactive PLP/DM20 was detected in neurons in the caudal brainstem. Not all biosynthetic functions and structural elements were altered in these neurons, because phosphorylated and nonphosphorylated neurofilament and choline acetyltransferase expression were comparable between MD and wild-type rats. These findings suggest that PLP is expressed in neurons in the developing brainstem and that PLP gene mutation can selectively disrupt central processing of afferent neural input from peripheral chemoreceptors, leaving the central chemosensory system for hypercapnia intact.

The interferon-gamma (IFN-gamma) receptor is expressed by all nucleated cells, and binding of its cognate ligand, IFN-gamma, induces a wide variety of biological functions. Transgenic mice expressing a dominant negative IFN-gamma receptor 1 (IFN-gammaR1DeltaIC) on oligodendrocytes under control of the myelin proteolipid protein promoter are described. The mRNA encoding the transgene was only detected in the nervous system and protein expression was confirmed by immunohistochemistry. Transgenic receptor expression does not alter myelination and the mice exhibited no clinically apparent phenotype. Consistent with the restricted nervous system expression of the transgene, no alterations in peripheral immune responses were detected. Flow cytometric analysis demonstrated constitutive expression of both the IFN-gammaR1DeltaIC transgene and the endogenous IFN-gamma receptor 2 at high levels on oligodendrocytes derived from the transgenic mice. These oligodendrocytes also exhibited decreased STAT1 phosphorylation in response to IFN-gamma, confirming dominant negative transgene function. Transgenic mice in which oligodendrocytes have a diminished ability to respond to IFN-gamma showed delayed virus clearance from oligodendroglia compared with wild-type mice. This model will allow evaluation of oligodendrocyte responses to this critical cytokine during CNS inflammation.

The proteolipid protein (PLP) gene (Plp) encodes the major myelin proteins, PLP and DM20. Expression of Plp occurs predominantly in oligodendrocytes, but evidence is accumulating that this gene is also expressed in neurons. In earlier studies, we demonstrated that myelin-deficient (MD) rats, which carry a mutation in the Plp gene, exhibit lethal hypoxic ventilatory depression. Furthermore, we found that, in the MD rat, PLP accumulated in neuronal cell bodies in the medulla oblongata. In the current study, we sought to determine which neurons expressed the Plp gene in the medulla oblongata and whether Plp gene expression changed in neurons with maturation. A transgenic mouse expressing the Plp promoter driving expression of enhanced green fluorescent protein (Plp-EGFP) was used to identify neurons expressing this gene. Plp expression in neurons was confirmed by immunostaining EGFP-positive cells for NeuN and by in situ hybridization for PLP mRNA. The numbers of neurons expressing Plp-EGFP and their distribution increased between P5 and P10 in the medulla. Immunostaining for surface receptors and classes of neurons expressing Plp-EGFP revealed that Plp gene expression in brainstem neurons was restricted to neurons expressing specific ligand-gated channels and biosynthetic enzymes, including glutamatergic NMDA receptors, GABA(A) receptors, and ChAT in defined areas of the medulla. Plp gene expression was rarely found in interneurons expressing GABA and was never found in AMPA receptor- or tyrosine hydroxylase-expressing neurons. Thus, Plp expression in the mouse caudal medulla was found to be developmentally regulated and restricted to specific groups of neurons.

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