Femke Mathot

Chapter 9 150 Pleiotrophin (PTN) PTN expression was measured as it is involved in neurite outgrowth and synaptogenesis after peripheral nerve injury. 37, 38 The expression ratio of PTN was modulated in MSCs by the type of nerve substitute (p<0.001) and by the time after seeding (p=0.001). PTN expression decreased up to 7 days after seeding for both groups, after which particularly the PTN expression of MSCs seeded on NeuraGen®Nerve Guides increased again up to 21 days after seeding. The MSCs on the NeuraGen® Nerve Guide expressed a significant higher PTN ratio than MSCs on Avance® Nerve Grafts after 14 (0.590 ± 0.151 versus 0.174 ± 0.037, p=0.011) and 21 days (0.718 ± 0.29 versus 0.116 ± 0.050, p<0.001). Although the PTN expression of MSCs on NeuraGen® Nerve Guides approximated the baseline gene expression of unseeded MSCs after 21 days, measures at all the other time points demonstrated downregulation of the baseline expression. Growth associated protein 43 (GAP43) GAP43 expression increases after axotomy, eventually leading to enhanced axon density in regenerating nerve fibers. 39 The GAP43 expression of seeded MSCs was significantly affected by the time after seeding (p=0.008) and the type of nerve substitute (p=0.046). The GAP43 expression of MSCs on both nerve substitutes increased over time, of which the expression of MSCs on Avance® Nerve Grafts most evenly increased over time. There were no significant differences in GAP43 expression between both nerve substitutes. All measured expression ratios were below 1, implicating downregulation of the baseline GAP43 expression. Brain-derived Neurotrophic Factor (BDNF) BDNF induces neuronal cell survival and differentiation and accelerates axonal outgrowth 40 , but also is involved in synapse formation. 41, 42 The BDNF expression ratio of seeded MSCs was significantly affected by the time passed after seeding (p<0.001). The BDNF expression in MSCs seeded on Avance® Nerve Grafts (3.846 ± 0.636) was significantly higher after 14 days compared to the expression in MSCs on NeuraGen® Nerve Guides (1.720 ± 0.164)(p=0.020). The expression ratio in both groups approximated 1 directly after seeding, and increased up to 4 (i.e. a fourfold of the baseline BDNF expression in unseeded MSCs) on various time points in later phases (1 day and 21 days after seeding for the NeuraGen® Nerve Guide, 7, 14 and 21 days after seeding for the Avance® Nerve Grafts). Myelination gene expression (Figure 2) Peripheral myelin protein 22 (PMP22) The PMP22 gene is mainly expressed in Schwann cells and encodes a relatively minor but crucial component of the myelin sheeth. 43 Duplication or deletion of PMP22 leads to demyelination and axon loss, resulting in common demyelinating neuropathies. 44, 45 PMP22 expression ratio was significantly affected by the time after seeding (p=0.0.26), but not by the type of nerve substitute (p=0.542) on which the MSCs were seeded. The expression ratio started high for both groups (1.90 ± 0.231 for Avance® Nerve Grafts, 1.78 ± 0.663 for NeuraGen® Nerve Guides) and decreased to a level comparable to unseeded MSCs after 21 days (0.716 ± 0.278 for Avance® Nerve Grafts, 0.928 ± 0.102 for NeuraGen® Nerve Guides). There were no significant differences between groups over time.