Elucidation of time course and molecular mechanism of intraepidermal nerve fiber loss in diabetic polyneuropathy

皮肤活检分析表皮内神经纤维（IENFs）是最近引入的一种评估疑似疼痛感觉神经病变患者的技术。最近使用皮肤活检标本的研究表明，糖尿病前期的糖耐量受损（IGT）状态与疼痛性感觉神经病变有关，与早期糖尿病神经病变的典型表型难以区分，表现为IENFs的丧失。IGT是代谢综合征的一个组成部分，代谢综合征由一系列与胰岛素抵抗（IR）相关的特定代谢紊乱组成。据报道，疼痛性感觉神经病变患者血清胰岛素水平明显高于对照组，且IR较大，提示胰岛素代谢异常与小纤维功能障碍之间存在联系。然而，与胰岛素抵抗型2型糖尿病相关的神经病变不同严重程度的IENF丧失的时间过程和分子机制仍有待探索。在本研究中，我们描述了2型糖尿病Zucker糖尿病脂肪（ZDF）大鼠神经病变特定阶段IENF密度的纵向趋势和周围神经胰岛素信号的改变。8-10周龄的ZDF大鼠出现代偿性高胰岛素血症，并在明显高血糖发作前出现热痛觉过敏。这些动物也表现出从热痛觉过敏到痛觉减退的进展，并在18周龄后出现机械性痛觉过敏，此时高血糖不再伴有代偿性高胰岛素血症。尽管有异常的伤害性行为，但在23周龄和39周龄时IENF密度没有变化，而在10周龄时，ZDF大鼠的IENF密度与年龄匹配的瘦大鼠相比增加了26%。免疫荧光研究显示，与年龄匹配的瘦鼠相比，23周龄的ZDF大鼠背根神经节神经元的胰岛素受体和磷酸化胰岛素受体免疫反应活性显著降低。western blot结果显示，糖尿病动物坐骨神经中胰岛素受体和Akt蛋白水平分别降低了70%和25%，而磷酸/总p44和p42 MAP激酶比值分别升高了33%和52%。综上所述，本研究表明ZDF大鼠在39周龄时存在进行性伤害性功能障碍和外周神经胰岛素信号异常，并保留皮肤神经支配。ZDF大鼠可能不是研究2型糖尿病相关IENF损失的合适动物模型。少

Skin biopsies for analysis of intraepidermal nerve fibers (IENFs) represent a recently introduced technique for assessment of patients with suspected painful sensory neuropathy. Recent studies using skin biopsy specimen have indicated that the pre-diabetic state of impaired glucose tolerance (IGT) is associated with painful sensory neuropathy that is indistinguishable from the typical phenotype of early diabetic neuropathy, showing loss of IENFs. IGT is one component of metabolic syndrome, which consists of a constellation of specific metabolic derangements associated with insulin resistance (IR). It is reported that patients with painful sensory neuropathy have significantly higher serum insulin with greater IR than control subjects, indicating a linkage between insulin dysmetabolism and small fiber dysfunction. However, time course and molecular mechanism of IENF loss at different stages of severity of neuropathy associated with insulin resistant type 2 diabetes remain to be explored … More . In the present study, we characterized longitudinal trends in IENF density and alterations in peripheral nerve insulin signaling at the specific stages of neuropathy in type 2 diabetic Zucker diabetic fatty (ZDF) rats. ZDF rats at 8-10 weeks of age showed compensatory hyperinsulinemia and developed thermal hyperalgesia prior to the onset of overt hyperglycemia. These animals also exhibited progression from thermal hyperalgesia to hypoalgesia and developed mechanical hyperalgesia after 18 weeks of age when hyperglycemia was no longer accompanied by compensatory hyperinsulinemia. Despite the abnormal nociceptive behaviors, the IENF density was unaltered at 23 and 39 weeks of age, whereas it insignificantly increased by 26% at 10 weeks of age in ZDF rats, compared with age-matched lean rats. Immunofluorescence studies revealed significant decreases in insulin receptor and phosphorylated insulin receptor immunoreactivities in dorsal root ganglion neurons in 23 week-old ZDF rats compared with age-matched lean rats. In western blot analyses, insulin receptor and Akt protein levels were reduced by 70 % and 25%, respectively, whereas the phospho/total p44 and p42 MAP kinase ratios were increased by 33% and 52%, respectively, in the sciatic nerves of diabetic animals. In summary, the present study demonstrated progressive nociceptive dysfunction and aberrant peripheral nerve insulin signaling with preserved skin innervation up to 39 weeks of age in ZDF rats. ZDF rats may not be an appropriate animal model for studying IENF loss associated with type 2 diabetes. Less