Gp78 E3 Ubiquitin Ligase: Essential Functions and Contributions in Proteostasis
The peptide sequence of autocrine motility factor (AMF), a tumor secreted cytokine that induces cell motility, corresponds to that of the previously identified cytokine/enzyme, neuroleukin/glucose-6-phosphate isomerase. Neuroleukin is a neurotrophic factor that promotes neuronal survival and sprouting at the neuromuscular junction. The AMF receptor (AMF-R) has been identified and shown to be highly expressed in malignant tumors with minimal expression in adjacent normal tissue. Neuroleukin mRNA is highly expressed in the cerebellum and we therefore undertook a developmental study of AMF-R expression in rat cerebellum. As determined by immunoblot, AMF-R is expressed at equivalent high levels in brain and cerebellum of postnatal day 5 (P5) and 12 (P12) rats and at significantly reduced levels in the adult. Coimmunofluorescence studies with MAP-2 and gamma-actin revealed that at P12, AMF-R was mainly localized to Purkinje and granule cells. Moreover, the premigratory cells of the external granular layer were also immunoreactive for AMF-R suggesting a role for AMF-R in granule cell migration during cerebellar development in the first two weeks after birth. In the adult, AMF-R distribution was similar to P12, although weaker, and was localized to Purkinje and granule cells. AMF-R labeling of GFAP positive glial processes could not be detected in cerebellar sections although in cerebellar primary cultures, both neurons and glial cells were labeled for AMF-R. In neurons, AMF-R labeling was present in the cell body, neurites and growth cones. These data indicate that regulation of the neurotrophic function of neuroleukin might be regulated spatially and temporally by expression of its receptor, AMF-R, in developing and adult cerebellum.