Microglia population is primarily determined by a finely-regulated proliferation process during early development of the central nervous system (CNS). Nitric oxide (NO) is known to inhibit proliferation in numerous cell types. However, how NO signaling regulates microglia proliferation remains elusive. Using wildtype (WT) and inducible nitric oxide synthase knockout (iNOS-/-) mice, this study investigated the role and underlying mechanisms of iNOS/NO signaling in microglia proliferation. Here we reported that iNOS-/- mice displayed significantly more BrdU-labeled proliferating microglia in the cortex than that in WT mice at postnatal day 10. Compared to microglia isolated from WT mouse cortex, significantly more iNOS-/- microglia displayed the specific cell-cycle markers Ki67 and phospho-histone H3 (pH3) in their nuclei. In addition, treating WT microglia with the NOS inhibitor LNAME drastically increased the percentage of cells expressing Ki67 and pH3, whereas treating iNOS-/- microglia with NOC18, a slow-release NO-donor, significantly decreased the percentage of microglia expressing the two cell-cycle markers. Moreover, inhibition of protein kinase-G (PKG) in WT microglia increased the proportion of microglia expressing Ki67 and pH3, whereas activation of PKG signaling using 8Br-cGMP in iNOS-/- microglia significantly decreased the fraction of microglia displaying Ki67 and pH3. Interestingly, in the presence of a PKG inhibitor, NOC18 increased the quantity of iNOS-/- microglia expressing Ki67 and pH3. Together, these results indicate that basal activity of iNOS/NO signaling impedes microglial cell-cycle progression and attenuates proliferation through activation of the cGMP-PKG pathway. However, NO increases microglia cell-cycle progression in the absence of cGMP-PKG signaling.

中文翻译：

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一氧化氮信号传导通过激活蛋白激酶-G抑制小胶质细胞的增殖。

小胶质细胞的数量主要由中枢神经系统（CNS）早期发育过程中精细调节的增殖过程决定。一氧化氮（NO）可以抑制多种细胞类型的增殖。但是，NO信号调节小胶质细胞增殖的方式仍然不清楚。使用野生型（WT）和诱导型一氧化氮合酶敲除（iNOS-/-）小鼠，本研究调查了iNOS / NO信号在小胶质细胞增殖中的作用和潜在机制。在这里，我们报道了iNOS-/-小鼠在出生后第10天在皮质中显示出的BrdU标记的增殖性小胶质细胞比WT小鼠中的显着多。与从WT小鼠皮质分离的小胶质细胞相比，iNOS-/-小胶质细胞中显示出特定的细胞核中的循环标记Ki67和磷酸组蛋白H3（pH3）。此外，用NOS抑制剂LNAME处理野生小胶质细胞显着增加了表达Ki67和pH3的细胞的百分比，而用缓慢释放的NO供体NOC18处理iNOS-/-小胶质细胞则显着降低了表达两种细胞周期标记的小胶质细胞的百分比。 。此外，WT小胶质细胞中蛋白激酶-G（PKG）的抑制增加了表达Ki67和pH3的小胶质细胞的比例，而在iNOS-/-小胶质细胞中使用8Br-cGMP激活PKG信号显着降低了显示Ki67和pH3的小胶质细胞的比例。有趣的是，在存在PKG抑制剂的情况下，NOC18增加了表达Ki67和pH3的iNOS //-小胶质细胞的数量。一起，这些结果表明，iNOS / NO信号的基础活性可通过激活cGMP-PKG途径来阻止小胶质细胞周期进程并减弱增殖。但是，在没有cGMP-PKG信号传导的情况下，NO会增加小胶质细胞的细胞周期进程。