Functional difference of receptor-type protein tyrosine phosphatase ζ/β isoforms in neurogenesis of hippocampal neurons

Receptor-type protein tyrosine phosphatase ζ/β (RPTPζ) is a transmembrane chondroitin sulfate proteoglycan (CSPG) and has been shown to play crucial roles in controlling axonal growth and neuronal migration. The RPTPζ has two transmembranous isoforms, shorter receptor form of RPTPζ (sRPTPζ) and full-length receptor form of RPTPζ (fRPTPζ), but no studies have been reported about functional difference of these two isoforms. In the present study, therefore, we examined whether or not two RPTPζ isoforms have different role in controlling dendritic morphology and synaptic number in cultured hippocampal neurons using the quantitative morphometrical analysis. Confocal microscopic observation showed that the immunoreactivity of RPTPζ was observed throughout cells such as axons, growth cones, and dendrites at the early stages of neuronal culture, while it was seen predominantly on dendrites at the late stages. Western blotting analysis revealed that fRPTPζ was mainly expressed at the early stages of culture and both RPTPζ isoforms were expressed at late stages of culture. The overexpression of sRPTPζ in hippocampal neurons increased the dendritic arborization without altering the average length of dendritic branches, whereas that of fRPTPζ decreased the dendritic arborization and increased the average length of dendritic branches. The RNA interference of fRPTPζ expression increased the dendritic arborization without altering the average length of dendritic branches. The overexpression of fRPTPζ decreased the density of hippocampal dendritic synapses, but that of sRPTPζ had no effects. Pleiotrophin, a ligand for RPTPζ to interfere the phosphatase activity, increased the density of hippocampal dendritic synapses. Thus, the present study demonstrates that two transmembranous RPTPζ isoforms have different functions for regulating dendritogenesis and synaptogenesis.