Membrane-bound CSPG mediates growth cone outgrowth and substrate specificity by Schwann cell contact with the DRG neuron cell body and not via growth cone contact

The central nervous system and peripheral nervous system (CNS/PNS) contain factors that inhibit axon regeneration, including myelin-associated glycoprotein (MAG), the Nogo protein, and chondroitin sulfate proteoglycan (CSPG). They also contain factors that promote axon regeneration, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Axon regeneration into and within the CNS fails because the balance of factor favors inhibiting regeneration, while in the PNS, the balance of factor favors promoting regeneration. The balance of influences in the CNS can be shifted toward promoting axon regeneration by eliminating the regeneration-inhibiting factors, overwhelming them with regeneration-promoting factors, or making axon growth cones non-receptive to regeneration-inhibiting factors. The present in vitro experiments, using adult rat dorsal root ganglion (DRG) neurons, were designed to determine whether the regeneration-inhibiting influences of Schwann cell CSPG are mediated via Schwann cell membrane contact with the DRG neuron cell body or their growth cones. The average longest neurite of neurons in cell body contact with Schwann cells was 7.4-fold shorter than those of neurons without Schwann cell-neuron cell body contact (naked neurons), and the neurites showed substrate specificity, growing only on the Schwann cell membranes and not extending onto the laminin substrate. The neurites of naked neurons showed no substrate specificity and extended over the laminin substrate, as well as onto and off the Schwann cells. After digesting the Schwann cell CSPG with the enzyme C-ABC, neurons in cell body contact with Schwann cells extended neurites the same length as those of naked neurons, and their neurites showed no substrate selectivity. Further, the neurites of naked neurons were not longer than those of naked neurons not exposed to C-ABC. These data indicate that the extent of neurite outgrowth from adult rat DRG neurons and substrate specificity of their growth cone is mediated via contact between the Schwann cell membrane-bound CSPG and the DRG neuron cell body and not with their growth cones. Further, there was no apparent influence of diffusible or substrate-bound CSPG on neurite outgrowth. These results show that eliminating the CSPG of Schwann cells in contact with the cell body of DRG neurons eliminates the sensitivity of their growth cones to the CSPG-induced outgrowth inhibition. This may in turn allow the axons of these neurons to regenerate through the dorsal roots and into the spinal cord.