Investigation of lipid metabolism in peripheral nerve regeneration in human vs. mouse nerves

雪旺细胞(SCs)是周围神经系统的髓鞘细胞。损伤后，损伤部位远端的干细胞去分化，脱髓鞘，获得一种新的表型，称为修复SC。修复后的干细胞经历基因表达谱的巨大变化，改变其形态，进入增殖状态，产生信号分子和炎症因子，促进轴突生长，确保适当的神经再生。从髓鞘形成到修复SC的转变称为SC重编程。参与SC重编程的过程主要是在啮齿动物身上进行的研究。在人类中，对SC对损伤的反应知之甚少，但神经再生的效率远远低于啮齿动物。因此，研究人类干细胞的损伤反应有助于了解小鼠和人类之间不同的再生能力。在我的工作中，我利用人类神经外植体建立了一种新的体外损伤模型，该模型允许在与体内情况非常相似的环境中比较人和小鼠干细胞的损伤反应。目的是找出差异，这可以解释人类神经再生潜力有限的原因。有趣的是，我发现了脂肪代谢的主要调节因子PPARg和几个致脂基因在损伤后在小鼠干细胞中的表达发生了巨大的变化，而人类干细胞在这方面几乎没有任何反应。此前已经证明，脂质合成对小鼠的髓鞘形成至关重要，因为髓鞘膜含有大量的脂类。考虑到修复SCs必须停止髓鞘的产生，它们通过下调造脂基因来适应脂代谢是一致的。有趣的是，对受损神经中PPARg活性的药物调节不仅改变了造脂基因的表达，还影响了SC的重新编程。为了验证这一假设，我将从以下几个方面分析PPARg在坐骨神经损伤小鼠模型中对SC重新编程和随后的重新分化的影响：i)吡格列酮治疗的药理学方法；ii)PPARg在SCs中PPARg缺失的遗传学方法。我认为抑制造脂基因会加速SC的重新编程，并影响重新髓鞘形成。相比之下，脂类代谢的激活很可能会产生相反的效果。此外，我还将研究人神经和神经瘤组织中干细胞在损伤后和药物治疗后的形态变化，以调节脂质代谢。该项目可以确定脂代谢作为促进人类神经再生的可能治疗靶点。

Schwann cells (SCs) are myelinating cells of the peripheral nervous system. After injury, SCs distal to the injury site dedifferentiate, shed their myelin and obtain a new phenotype described as repair SC. Repair SCs undergo a massive change in their gene expression profile, alter their morphology, enter a proliferative state and produce signalling molecules and inflammatory factors, which promote axonal outgrowth ensuring a proper nerve regeneration. The transition from a myelinating to a repair SC is termed SC reprogramming. Processes involved in SC reprogramming have been mainly studied in rodents. In humans, only little is known about SC reaction to injury, yet nerve regeneration is far less efficient than in rodents. Therefore, investigation of the injury response in human SCs could prove useful in understanding the different regeneration capacities between mice and humans.In my work, I established a novel ex vivo injury model using human nerve explants, which allows for the comparison of injury response in human and murine SCs in an environment closely resembling the in vivo situation. The aim was to identify differences, which could explain the limited regeneration potential of human nerves. Intriguingly, I identified the expression of the major regulator of lipid metabolism PPARg and several lipogenic genes to be drastically altered in murine SCs after injury, while human SCs showed hardly any response in this regard. Lipid synthesis has been previously shown to be crucial for myelination in mice, since myelin membranes contain large amounts of lipids. Considering that repair SCs have to stop myelin production, it is consistent that they adapt lipid metabolism by downregulating lipogenic genes. Interestingly, pharmacological modulation of PPARg activity in injured nerves not only altered the expression of lipogenic genes, but also influenced SC reprogramming. Hence, I hypothesize that adaptation of lipid metabolism is an essential process for the induction of SC reprogramming during peripheral nerve injury and could account for differences between mice and humans.To test this hypothesis, I will analyze the influence of PPARg on SC reprogramming and subsequent redifferentiation in a mouse model of sciatic nerve injury in vivo in the following approaches: i) a pharmacological approach with pioglitazone treatment, ii) a genetic approach with PPARg depletion in SCs. I expect that inhibition of lipogenic genes accelerates SC reprogramming and affects remyelination. In contrast, activation of lipid metabolism will most probably have the opposite effects. Further, I will study morphological changes of human SCs in nerves and neuroma tissue after injury and after pharmacological treatment to modulate lipid metabolism. This project could identify lipid metabolism as a possible therapeutic target for promoting nerve regeneration in humans.