Identification and functional characterization of the RGM family in mouse

1.) Identification and Functional Characterization of the Mouse RGM Family
The establishment of topographic projections in the developing visual system depends on spatially and temporally controlled expression of axon guidance molecules. In the developing chick tectum, the graded expression of Repulsive Guidance Molecule (RGM) has been proposed to be involved in controlling topography of retinal ganglion cell (RGC) axon termination zones along the anterior-posterior axis of the tectum. We show that there are three mouse proteins homologous to chick RGM, displaying similar proteolytic processing but exhibiting differential cell surface targeting by GPI anchor addition. Two members of this gene family (mRGMa and mRGMb) are expressed in complementary patterns in the nervous system, with mRGMa prominently expressed in the superior colliculus at the time of anterior-posterior targeting of RGC axons. The third member of the family (mRGMc) is expressed most strongly in skeletal muscles, but also in heart and liver.
Surprisingly, mice lacking mRGMa or mRGMb do not exhibit defects in anterior-posterior targeting of RGC axons to their stereotypic termination zones in the superior colliculus. Instead, mRGMa mutant mice show a defect in cephalic neural tube closure. The in vivo function of mRGMb still remains to be elucidated.
Mice lacking mRGMc mimic the phenotype observed in patients suffering from juvenile hereditary hemochromatosis, an iron overload disease caused by disruption of HFE2, the human ortholog of mRGMc. Moreover, mRGMc mutant mice exhibit a dramatic decrease in hepatic Hamp, a negative regulator of iron absorption, expression, yet retain Hamp inducibility via the inflammatory pathway. Our findings define a key role for mRGMc in the normal iron-sensing pathway and also reveal how this homeostatic pathway is uncoupled during inflammation, through the coordinate extinction of mRGMc and activation of Hamp expression.
2.) Peripheral NT3 Signaling Is Required for ETS Protein Expression and Central Patterning of Proprioceptive Sensory Afferents
To study the role of NT3 in directing axonal projections of proprioceptive dorsal root ganglion (DRG) neurons, NT3−/− mice were crossed with mice carrying a targeted deletion of the proapoptotic gene Bax. In Bax−/−/NT3−/− mice, NT3-dependent neurons survived and expressed the proprioceptive neuronal marker parvalbumin. Initial extension and collateralization of proprioceptive axons into the spinal cord occurred normally, but proprioceptive axons extended only as far as the intermediate spinal cord. This projection defect is similar to the defect in mice lacking the ETS transcription factor ER81. Few if any DRG neurons from Bax−/−/NT3−/− mice expressed ER81 protein. Expression of an NT3 transgene in muscle restored ER81 expression in DRGs of NT3−/− mice. Finally, addition of NT3 to DRG explant cultures resulted in induction of ER81 protein. Our data indicate that NT3 mediates the formation of proprioceptive afferent-motor neuron connections via regulation of ER81.