Department of Pharmacology, Biocenter Niederursel, University of Frankfurt, Goethe-University, Max-von-Laue-St. 9, 60438, Frankfurt, Germany.
Department of Pharmacology, Geriatric Research, Education and Clinical Center, University of Minnesota School of Medicine, VAMC, Minneapolis, MN, 55417, USA.
Neuromolecular Med. 2021 Mar;23(1):130-139. doi: 10.1007/s12017-020-08640-0. Epub 2020 Dec 30.
Synaptic impairment may be the main cause of cognitive dysfunction in brain aging that is probably due to a reduction in synaptic contact between the axonal buttons and dendritic spines. Rho proteins including the small GTPase Rac1 have become key regulators of neuronal morphogenesis that supports synaptic plasticity. Small Rho- and Ras-GTPases are post-translationally modified by the isoprenoids geranylgeranyl pyrophosphate (GGPP) and farnesyl pyrophosphate (FPP), respectively. For all GTPases, anchoring in the plasma membrane is essential for their activation by guanine nucleotide exchange factors (GEFs). Rac1-specific GEFs include the protein T lymphoma invasion and metastasis 1 (Tiam1). Tiam1 interacts with the TrkB receptor to mediate the brain-derived neurotrophic factor (BDNF)-induced activation of Rac1, resulting in cytoskeletal rearrangement and changes in cellular morphology. The flavonoid 7,8-dihydroxyflavone (7,8-DHF) acts as a highly affine-selective TrkB receptor agonist and causes the dimerization and autophosphorylation of the TrkB receptor and thus the activation of downstream signaling pathways. In the current study, we investigated the effects of 7,8-DHF on cerebral lipid isoprenoid and Rho protein levels in male C57BL/6 mice aged 3 and 23 months. Aged mice were daily treated with 100 mg/kg b.w. 7,8-DHF by oral gavage for 21 days. FPP, GGPP, and cholesterol levels were determined in brain tissue. In the same tissue, the protein content of Tiam1 and TrkB in was measured. The cellular localization of the small Rho-GTPase Rac1 and small Rab-GTPase Rab3A was studied in total brain homogenates and membrane preparations. We report the novel finding that 7,8-DHF restored levels of the Rho proteins Rac1 and Rab3A in membrane preparations isolated from brains of treated aged mice. The selective TrkB agonist 7,8-DHF did not affect BDNF and TrkB levels, but restored Tiam1 levels that were found to be reduced in brains of aged mice. FPP, GGPP, and cholesterol levels were significantly elevated in brains of aged mice but not changed by 7,8-DHF treatment. Hence, 7,8-DHF may be useful as pharmacological tool to treat age-related cognitive dysfunction although the underlying mechanisms need to be elucidated in detail.
突触损伤可能是脑老化导致认知功能障碍的主要原因，这可能是由于轴突末梢和树突棘之间的突触接触减少所致。Rho 蛋白包括小分子 GTP 酶 Rac1，已成为支持突触可塑性的神经元形态发生的关键调节剂。小分子 Rho 和 Ras-GTP 酶分别被异戊烯焦磷酸（GGPP）和法呢基焦磷酸（FPP）的异戊二烯基化修饰。对于所有 GTP 酶，锚定在质膜中对于它们被鸟嘌呤核苷酸交换因子（GEFs）的激活是必不可少的。Rac1 特异性 GEF 包括蛋白 T 淋巴瘤侵袭和转移 1（Tiam1）。Tiam1 与 TrkB 受体相互作用，介导脑源性神经营养因子（BDNF）诱导的 Rac1 激活，导致细胞骨架重排和细胞形态变化。类黄酮 7,8-二羟基黄酮（7,8-DHF）作为一种高亲和力选择性的 TrkB 受体激动剂，引起 TrkB 受体的二聚化和自身磷酸化，从而激活下游信号通路。在本研究中，我们研究了 7,8-DHF 对 3 个月和 23 个月龄雄性 C57BL/6 小鼠大脑脂质异戊烯和 Rho 蛋白水平的影响。老年小鼠每天通过口服灌胃给予 100mg/kg b.w.7,8-DHF21 天。在脑组织中测定 FPP、GGPP 和胆固醇水平。在同一组织中，测量 Tiam1 和 TrkB 的蛋白质含量。在全脑匀浆和膜制剂中研究了小分子 Rho-GTP 酶 Rac1 和小分子 Rab-GTP 酶 Rab3A 的细胞定位。我们报告了一个新发现，即 7,8-DHF 恢复了来自处理过的老年小鼠大脑膜制剂中 Rac1 和 Rab3A 等 Rho 蛋白的水平。选择性 TrkB 激动剂 7,8-DHF 不影响 BDNF 和 TrkB 水平，但恢复了在老年小鼠大脑中发现的降低的 Tiam1 水平。FPP、GGPP 和胆固醇水平在老年小鼠的大脑中显著升高，但 7,8-DHF 治疗并未改变这些水平。因此，7,8-DHF 可能是治疗与年龄相关的认知功能障碍的有用药物工具，尽管需要详细阐明其潜在机制。
