Mudhar, Hardeep Singh; (1994) Platelet-derived growth factor and its receptors in retinal development. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Many polypeptide growth factors are expressed in the mammalian central nervous system (CNS) during development and in the adult. In order to understand the functions of these factors we need to identify their cellular sources and targets. I have concentrated on the peripheral visual system, the optic nerve and retina, which is one of the simplest and best-characterized regions of the CNS. Platelet-derived growth factor (PDGF) and its receptors are expressed in the developing and mature CNS. PDGF is a disulphide-linked dimer of A and B chains, with the structure AA, BB or AB. There are also two types of PDGF receptor with different ligand specificities. PDGF-A only activates the alpha receptor whereas PDGF-B activates both alpha and beta receptors. We have used in situ hybridization to visualize cells in the developing rat retina and optic nerve that express mRNAs encoding the A and B chains of PDGF, and the alpha and beta subunits of the receptors. We have also visualized PDGF-A protein in these tissues by immunohistochemistry. In the retina, PDGF-A mRNA is present in retinal ganglion cells (RGCs) and a subset of amacrine neurons. Transcripts accumulate in RGCs during target innervation and in amacrine neurons around the time of eye opening, suggesting that PDGF-A expression in these cells may be regulated by target derived signals or electrical activity. In the mouse retina, PDGF-A immunoreactivity is present in the cell bodies, dendrites and proximal axons of RGCs, and throughout the inner nuclear layer (INL). PDGF-alpha receptor mRNA is expressed in the retina by astrocytes in the optic fibre layer and by a subset of cells in the INL. Taken together, our data suggest short-range paracrine interactions between PDGF-A and PDGF-alpha receptor, the ligand and its receptor expressed in neighbouring layers of cells in the retina. What is the biological significance of these local interactions. There are never any retinal neuroepithelial cell divisions in the INL, so PDGF-A cannot be a mitogen for retinal cells. We tested the possibility that PDGF-A may act as a survival factor, by injecting recombinant PDGF-AA into P-7 rat eyes and counting the pyknotic cell nuclei at P-10, which is the time of maximal naturally-occurring cell death in the INL, during normal development. PDGF- AA reduced the number of pyknotic cell nuclei by approximately 50[percent] as compared to controls. This implies that PDGF-A may be a survival factor in the retina and furthermore that PDGF-A may be present in limiting amounts in the developing inner nuclear layer (INL). Secondly, what is the role of PDGF in astrocyte development. This question was addressed by neutralizing endogenous retinal PDGF, by using a soluble extracellular portion of the rat PDGF- receptor. Cos cells transfected with expression vector pR[alpha]17, encoding the mutant receptor were injected into the vitreous, behind the lens. As compared to controls, the presence of the soluble extracellular alpha receptor significantly reduced the migration of the astrocytes across the retina and caused their processes to fasiculate. In the optic nerve, PDGF-A immunoreactivity is present in astrocytes but apparently not in the retinal ganglion cell axons. PDGF-alpha receptor cells in the optic nerve first appear near the optic chiasm and subsequently spread to the retinal end of the nerve: these PDGF-alpha receptor positive cells are probably oligodendrocyte precursors. RNA transcripts encoding the PDGF-B and PDGF-beta receptor are expressed by cells of the hyaloid and mature vascular systems in the eye and optic nerve.

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