Expression of phosphacan and neurocan during early development of mouse retinofugal pathway

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We have investigated whether the two major brain chondroitin sulfate (CS) proteoglycans (PGs), phosphacan and neurocan, are expressed in patterns that correlate to the axon order changes in the mouse retinofugal pathway. Expression of these proteoglycans was examined by polyclonal antibodies against phosphacan and N- and C-terminal fragments of neurocan. In E13–E15 mouse embryos, when most optic axons grow in the chiasm and the optic tract, phosphacan and neurocan were observed in the inner regions of the retina. In the chiasm and the tract, phosphacan but not neurocan was expressed prominently at the midline and in the deep parts of the tract. Both proteoglycans were observed on the chiasmatic neurons, which have been shown to regulate axon divergence at the chiasmatic midline and the chronotopic fiber ordering in the tract, but phosphacan appeared to be the predominant form that persists to later developmental stages. Intense staining of both proteoglycans was also observed in a strip of glial-like elements in lateral regions of the chiasm, partitioning axons in the stalk from those in the tract. We conclude that phosphacan but not neurocan is likely the major carrier of the CS glycosaminoglycans that play crucial functions in axon divergence and age-related axon ordering in the mouse optic pathway. Furthermore, localization of these carrier proteins in the optic pathway raises a possibility that these two proteoglycans regulate axon growth and patterning not only through the sulfated sugars but also by interactions of the protein parts with guidance molecules on the optic axons.