Platelet-derived growth factor is constitutively secreted from neuronal cell bodies but not from axons

Neurons synthesise and secrete many growth and survival factors but it is not usually clear whether they are released locally at the cell body or further afield from axons or axon terminals. Without this information, we cannot predict the site(s) of action or the biological functions of many neuron-derived factors. For example, can neuronal platelet-derived growth factor (PDGF) be secreted from axons and reach glial cells in nerve-fibre (white-matter) tracts? To address this question, we expressed PDGF-A in retinal ganglion neurons in transgenic mice and tested for release of PDGF from cell bodies in the retina and from axons in the optic nerve. In both the retina and optic nerve, there are glial cells that express PDGF receptor alpha (PDGFR alpha) [1] and divide in response to PDGF [2-5], so we could detect functional PDGF indirectly through the mitogenic response of glia at both locations. Expressing PDGF-A in neurons under the control of the neuron-specific enolase promoter (NSE-PDGF-A) resulted in a striking hyperplasia of retinal astrocytes, demonstrating that PDGF is secreted from the cell bodies of neurons in the retina [4]. In contrast, glial proliferation in the optic nerve was unaffected, indicating that PDGF is not released from axons. When PDGF was expressed directly in the optic nerve under the control of an astrocyte-specific promoter (GFAP-PDGF-A), oligodendrocyte progenitors hyperproliferated, resulting in a hypertrophic optic nerve. We conclude that PDGF is constitutively secreted from neuronal cell bodies in vivo, but not from axons in white-matter tracts.