Permissive role of Cdc2 activity induced from astrocytes in neurite outgrowth.

Following spinal cord injury, glial cells are recognized as major environmental factors hampering axon's regenerative responses. However, recent studies suggested that, in certain circumstances, reactive astrocytes may have a permissive role for axonal regeneration and functional recovery. Here, we report that Cdc2 activation in astrocytes is positively linked to axon growth. Cdc2 was strongly, but transiently, induced from reactive astrocytes within and around the injury cavity. Cdc2 levels in primary, non-neuronal cells prepared from injured spinal cord were up-regulated by extending the pre-injury period. Cdc2-mediated vimentin phosphorylation was strongly induced in astrocytes after long-term culture (7 days, LTC) as compared with short-term culture (3 days, STC). Induction levels of phospho-vimentin in LTC astrocytes were positively associated with increased neurite outgrowth in co-cultured dorsal root ganglion neurons. ß3 integrin mRNA was induced in LTC astrocytes and activation of ß3 integrin was regulated by Cdc2 activity. Furthermore, genetic depletion and pharmacological blockade experiments demonstrate that activation of Cdc2 and ß3 integrin in LTC astrocytes is required for neurite outgrowth. Our data suggest that the Cdc2 pathway may play an important role in determining phenotypic expression of astrocytes such that astrocytes provide permissive environments for axonal regeneration following spinal cord injury.

Shengmai-san-mediated enhancement of regenerative responses of spinal cord axons after injury in rats.

Shengmai-san (SMS) is a traditional Chinese medicine used to treat diverse symptoms including cardiovascular and neurological disorders. Here we investigated the effects of SMS on regenerative responses of spinal cord axons in rats that were given contusion injury at the lower thoracic level. The injury cavity was confined to a restricted area by SMS treatment, and the signals of glial scar protein chondroitin sulphate proteoglycan (CSPG) and inflammatory cell marker protein CD11beta were heavily observed within the injury cavity in SMS-treated animals. Anterograde tracing of DiI-labeled corticospinal tract (CST) axons revealed increases in collateral arborization around and within the injury cavity and caudal elongation by SMS treatment. Furthermore, SMS treatment facilitated neurite elongation of dorsal root ganglion (DRG) sensory neurons that were co-cultured with non-neuronal cells prepared from injured spinal cord. Phospho-Erk1/2 was strongly induced in both spinal cord and motor cortical areas after spinal cord injury (SCI), and it was further unregulated in the motor cortex by SMS treatment. In contrast, upregulation of cell division cycle 2 (Cdc2) production by SMS treatment was limited to a local, SCI area. These data suggest that SMS may play an active role in regenerative responses and facilitate axonal regrowth after SCI.

Axonal outgrowth and Erk1/2 activation by training after spinal cord injury in rats.

Abstract Physical training in experimental animals can improve locomotor activity via the regulation of spinal neural circuitry or peripheral nerve regeneration. Here we investigated the effects of treadmill training (TMT) on regenerative responses of the corticospinal tract (CST) after contusive spinal cord injury (SCI). One week after injury of the low thoracic spinal cord, rats were given TMT or sedentary treatment for 1-4 weeks. Anterograde tracing of descending CST axons revealed that TMT enhanced collateral arborization of CST axons surrounding the injury cavity and promoted extension into the caudal spinal cord. The number of oligodendrocytes in the vicinity of the injury cavity was significantly increased at 2 or 4 weeks after TMT compared to sedentary controls. The data further showed that TMT increased phosphorylation of Erk1/2 in the motor cortex as well as the spinal cord injury area, and inhibition of Erk1/2 activity by administration of the MEK1 inhibitors PD98059 and U0126 reduced collateral outgrowth of descending CST axons in TMT animals. TMT for 2-4 weeks significantly improved behavioral scores as assessed by the Basso-Beattie-Bresnahan scale, as well as on motor function and gridwalk testing. Our data imply that Erk1/2 may be an important mediator for transmitting signals from the injury site to the cell body, and further suggest that activation of the Erk1/2 signaling pathway may be involved in enhanced outgrowth of CST axons after TMT.

ERK1/2-mediated Schwann cell proliferation in the regenerating sciatic nerve by treadmill training.

Proliferation of Schwann cells in the injured peripheral nerve supports axonal regeneration, and physical training in experimental animals has been shown to promote nerve regeneration. Extracellular signal-regulated kinase 1/2 (ERK1/2) activity can mediate neuronal responses to lesion signals, but its role in non-neuronal cells in the injured area is largely unknown. Here we report that treadmill training (TMT) facilitates axonal regeneration via the upregulation of phospho-ERK1/2 protein levels in Schwann cells in the injured sciatic nerve. Low-intensity, but not high-intensity, TMT increased neurite outgrowth of dorsal root ganglion (DRG) sensory neurons and potentiated Schwann cell proliferation. TMT elevated levels of GAP-43 mRNA and protein, and phospho-ERK1/2 protein in the injured sciatic nerves. TMT also enhanced phospho-c-Jun protein levels in the injured nerve. In-vivo administration of the ERK1/2 inhibitor PD98059 eliminated phospho-c-Jun, suggesting ERK1/2 phosphorylation of the c-Jun protein. PD98059 treatment decreased levels of BrdU-labeled proliferating Schwann cells in the distal portion of the injured nerve, and delayed the axonal regrowth that was promoted by TMT. The present data suggest that increased ERK1/2 activity in Schwann cells may play an important role in TMT-mediated enhancement of axonal regeneration in the injured peripheral nerve.