Institute for Physiology and Pathophysiology, University of Erlangen-Nuremberg, Universitaetsstrasse 17, 91054 Erlangen, Germany.
Department of Anesthesiology, University of Colorado Denver, Anschutz Medical Campus, Aurora, CO 80045, USA.
Exp Neurol. 2021 Dec;346:113838. doi: 10.1016/j.expneurol.2021.113838. Epub 2021 Aug 25.
Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the Ca3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy Ca3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific Ca3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic Ca3.2 KOs. These data suggest that diabetes abrogates all Ca3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of Ca3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of Ca3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the Ca3.2 distribution. The results may strengthen the validation of Ca3.2 channel as a therapeutic target of treating painful diabetic neuropathy.
疼痛性糖尿病周围神经病变发生在约 20%的糖尿病患者中，其潜在的病理机制尚未完全阐明。我们评估了 T 型钙通道 Ca3.2 同工型在高血糖诱导的皮肤感觉 C 纤维功能和神经肽释放变化中的作用，使用链脲佐菌素 (STZ) 糖尿病模型，包括同源小鼠品系中的全局敲除 (KO)。在雄性 C57BL/6J 小鼠中建立 3-5 周的高血糖导致周围 C 纤维功能的重大重组。在分离的后爪皮肤的机械敏感单纤维中进行无偏电生理筛选，发现 (多模态) 热感觉相对丧失，有利于冷感觉。在健康的 Ca3.2 KO 小鼠中，C 纤维中的热和冷敏感性似乎都不足，有利于机械敏感性，特别是低阈值，在糖尿病 KO 中这种缺陷变得显著。糖尿病还导致野生型小鼠 C 纤维自发性放电活动的发生率显著增加，这种增加被特异性 Ca3.2 阻断剂 TTA-P2 降低，在 KO 中则基本不存在。将评估仅限于肽能类神经纤维 - 测量 KCl 刺激的 CGRP 释放 - 发现 TTA-P2 在健康但非糖尿病野生型小鼠的坐骨神经中显著减少，后者的 CGRP 释放与健康和糖尿病的 Ca3.2 KO 一样减少。这些数据表明，糖尿病消除了周围神经轴突中所有的 Ca3.2 功能。相比之下，糖尿病显著增加了野生型而非 KO 小鼠分离后爪皮肤的 KCl 刺激的 CGRP 释放，TTA-P2 逆转了这种增加，强烈表明在肽能性皮肤神经末梢中存在 Ca3.2 的新表达，这可能有助于增强自发性活动。神经氨酸酶的去糖基化作用显示出明显的脱敏作用，无论是自发性活动还是刺激的 CGRP 释放，但包括与 Ca3.2 无关的作用。然而，由于糖尿病增强的糖基化对轴内运输是决定性的，它可能解释了 Ca3.2 分布的实质性重组。这些结果可能会增强 Ca3.2 通道作为治疗疼痛性糖尿病周围神经病变的治疗靶点的验证。
