The role of the MAP- and SAP- kinase pathways in the survival, proliferation and death of Schwann cells of the injured sciatic nerve

Sammanfattning: This thesis concerns alterations in signal transduction in Schwann cells as a response to nerve injury as well as their survival, proliferation and death. Enzymes and transcription factors belonging to the mitogen activated protein kinase (MAPK) and the stress activated protein kinase (SAPK) pathways were studied. This included the extracellular-signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), c-Jun and the activating transcription factor 3 (ATF-3). Also the activity of the Ca2+ dependent enzyme calpain M was investigated. Experiments were performed both in vivo after transection of the rat sciatic nerve and on organ cultured sciatic nerve pieces. Using imunohistochemistry we could show that a peripheral nerve injury induced an increase in p-ERK, JNK, p-JNK, c-Jun, pc-Jun and ATF-3 in the Schwann cells and calpain M in the neurites. In diabetic rats the activation of ERK1/2 as a response to injury was attenuated, suggesting deficiencies in the regenerative response of the Schwann cells. Both activation of ERK1/2 and up-regulation of c-Jun in the Schwann cells were reduced by the ERK1/2 inhibitor U0126 and therefore we suggest an interaction between the MAPK and SAPK pathways. The JNK inhibitor SP600125 on the other hand did not inhibit any of the signal transduction elements in our studies implicating a JNK independent activation of c-Jun and ATF-3 in the Schwann cells. This interestingly differs from the neurons where the JNK inhibitor blocked the activation of c-jun and the up-regulation of ATF-3 in the nerve cell nucleus. By bromodeoxy uridine (BrdU) incorporation we demonstrated that Schwann cell proliferation increases after injury. The ERK1/2 inhibitor U0126 reduced proliferation, suggesting that proliferation is mediated via the activation of ERK1/2. We also managed to inhibited Schwann cell proliferation via the JNK inhibitor SP600125. Dye exclusion (propidium iodide) used to label dying and dead cells with PI staining showed an increase in Schwann cell death as a response to nerve injury. Calpain M was activated in the neurites of the organ cultured nerve pieces. This activation of calpain M in the neurites was reduced when deprived of extracellular Ca2+. In contrast, p-ERK1/2 in the Schwann cells increased in response to the same treatment suggesting that Ca2+ has an inhibitory effect on the activation of ERK1/2. Taken together the present research demonstrates that activation of ERK1/2 is an important early response to nerve injury and that this activation seems to be related to survival. I could also show a difference in the activation of c-Jun between Schwann cells and neurons. If the latter response is related to the way neurons and Schwann cells survive after a nerve injury remains to be determined.