Department of Neuroscience and Cell Biology, University of Texas Medical Branch, Galveston, Texas 77555, and.
Department of Physiology and Neurobiology, U-3156, University of Connecticut, Storrs, Connecticut 06269.
J Neurosci. 2019 Oct 16;39(42):8408-8421. doi: 10.1523/JNEUROSCI.2851-18.2019. Epub 2019 Aug 30.
HIV-1 infection of the nervous system causes various neurological diseases, and synaptic degeneration is likely a critical step in the neuropathogenesis. Our prior studies revealed a significant decrease of synaptic protein, specifically in the spinal dorsal horn of patients with HIV-1 in whom pain developed, suggesting a potential contribution of synaptic degeneration to the pathogenesis of HIV-associated pain. However, the mechanism by which HIV-1 causes the spinal synaptic degeneration is unclear. Here, we identified a critical role of microglia in the synaptic degeneration. In primary cortical cultures (day 14) and spinal cords of 3- to 5-month-old mice (both sexes), microglial ablation inhibited gp120-induced synapse decrease. Fractalkine (FKN), a microglia activation chemokine specifically expressed in neurons, was upregulated by gp120, and knockout of the FKN receptor CX3CR1, which is predominantly expressed in microglia, protected synapses from gp120-induced toxicity. These results indicate that the neuron-to-microglia intercellular FKN/CX3CR1 signaling plays a role in gp120-induced synaptic degeneration. To elucidate the mechanism controlling this intercellular signaling, we tested the role of the Wnt/β-catenin pathway in regulating FKN expression. Inhibition of Wnt/β-catenin signaling blocked both gp120-induced FKN upregulation and synaptic degeneration, and gp120 stimulated Wnt/β-catenin-regulated FKN expression via NMDA receptors (NMDARs). Furthermore, NMDAR antagonist APV, Wnt/β-catenin signaling suppressor DKK1, or knockout of CX3CR1 alleviated gp120-induced mechanical allodynia in mice, suggesting a critical contribution of the Wnt/β-catenin/FKN/CX3R1 pathway to gp120-induced pain. These findings collectively suggest that HIV-1 gp120 induces synaptic degeneration in the spinal pain neural circuit by activating microglia via Wnt3a/β-catenin-regulated FKN expression in neurons. Synaptic degeneration develops in the spinal cord dorsal horn of HIV patients with chronic pain, but the patients without the pain disorder do not show this neuropathology, indicating a pathogenic contribution of the synaptic degeneration to the development of HIV-associated pain. However, the mechanism underlying the synaptic degeneration is unclear. We report here that HIV-1 gp120, a neurotoxic protein that is specifically associated with the manifestation of pain in HIV patients, induces synapse loss via microglia. Further studies elucidate that gp120 activates microglia by stimulating Wnt/β-catenin-regulated fractalkine in neuron. The results demonstrate a critical role of microglia in the pathogenesis of HIV-associated synaptic degeneration in the spinal pain neural circuit.
HIV-1 感染神经系统会导致各种神经疾病，而突触退化很可能是神经发病机制中的一个关键步骤。我们之前的研究表明，HIV-1 患者的脊髓背角中突触蛋白显著减少，而这些患者出现了疼痛，这表明突触退化可能对 HIV 相关疼痛的发病机制有潜在贡献。然而，HIV-1 导致脊髓突触退化的机制尚不清楚。在这里，我们确定了小胶质细胞在突触退化中的关键作用。在原代皮质培养物（第 14 天）和 3-5 个月大的小鼠脊髓中（雌雄两性），小胶质细胞消融抑制了 gp120 诱导的突触减少。趋化因子 fractalkine（FKN）是一种特异性表达于神经元的小胶质细胞激活趋化因子，被 gp120 上调，而 CX3CR1 的缺失，其主要在小胶质细胞中表达，可保护突触免受 gp120 诱导的毒性。这些结果表明，神经元-小胶质细胞间的 FKN/CX3CR1 信号在 gp120 诱导的突触退化中起作用。为了阐明控制这种细胞间信号的机制，我们测试了 Wnt/β-catenin 途径在调节 FKN 表达中的作用。Wnt/β-catenin 信号的抑制阻断了 gp120 诱导的 FKN 上调和突触退化，并且 gp120 通过 NMDA 受体（NMDARs）刺激 Wnt/β-catenin 调节的 FKN 表达。此外，NMDAR 拮抗剂 APV、Wnt/β-catenin 信号抑制剂 DKK1 或 CX3CR1 的缺失减轻了小鼠 gp120 诱导的机械性痛觉过敏，表明 Wnt/β-catenin/FKN/CX3R1 途径对 gp120 诱导的疼痛有重要贡献。这些发现共同表明，HIV-1 gp120 通过 Wnt3a/β-catenin 调节的神经元中 FKN 的表达激活小胶质细胞，从而在脊髓疼痛神经回路中诱导突触退化。在慢性疼痛的 HIV 患者的脊髓背角中会发生突触退化，但没有疼痛障碍的患者不会出现这种神经病理学改变，这表明突触退化对 HIV 相关疼痛的发展有致病作用。然而，突触退化的机制尚不清楚。我们在这里报告，HIV-1 gp120 是一种神经毒性蛋白，与 HIV 患者疼痛的表现特别相关，它通过小胶质细胞诱导突触丧失。进一步的研究表明，gp120 通过刺激神经元中的 Wnt/β-catenin 调节的 fractalkine 来激活小胶质细胞。研究结果表明，小胶质细胞在 HIV 相关突触退化的脊髓疼痛神经回路发病机制中起关键作用。
