Promoting remyelination in multiple sclerosis by simultaneously modulating myelin debris clearance and myelin lipid synthesis

摘要/Abstract 多发性硬化症（MS）是最常见的脱髓鞘疾病，影响美国约40万人。 美国和全世界250万人。目前尚不清楚是什么原因导致MS，但许多人认为这是因为 我们自身的免疫系统会攻击产生髓磷脂的少突胶质细胞。然而，目前的治疗， 我们的免疫系统只能缓解症状，而不能治愈疾病本身。因此，寻找小说迫在眉睫 可以治愈疾病的治疗方法，例如通过促进髓鞘再生。中枢神经 然而，系统具有再生少突胶质细胞和响应髓鞘损伤重新髓鞘化的巨大潜力， 在MS损伤中髓鞘再生的能力大大降低。两个主要原因是众所周知的，以防止有效的 MS损伤中的髓鞘再生：1）受损的髓鞘不能被有效清除，从而阻止新的髓鞘形成。 少突胶质细胞，和2）新产生的和/或现有的少突胶质细胞已经失去形成新髓磷脂的能力。 我们已经确定了一个关键的调节器- Quaking（蛋白质名称：Qki;基因名称：Qk）-它可以有效地克服这两个问题 障碍.首先，我们发现Qki是小胶质细胞吞噬作用的关键调节因子。小胶质细胞中Qki的消耗 大大降低了小胶质细胞的吞噬活性，这对清除髓鞘碎片至关重要， 髓鞘再生其次，我们发现Qki是少突胶质细胞分化和髓磷脂的主要调节因子， 通过调节新形成的少突胶质细胞和存在的少突胶质细胞的脂质代谢来维持体内平衡， 脱髓鞘病变几十年来，成熟的髓磷脂一直被认为是一种惰性物质。然而，我们的研究 表明成熟的髓鞘实际上是一种非常动态的材料，通过消耗我们的遗传系统， 成年小鼠成熟髓鞘少突胶质细胞中的Qki。比较脂质组学和转录组学分析 通过控制脂质代谢的转录，将Qki鉴定为髓磷脂脂质生物合成的必需因子 基因，特别是那些脂肪酸去饱和和延长，通过共激活的过氧化物酶体增殖物， 活化受体β（PPARβ）-类维生素A X受体α（RXRα）复合物。这些发现得到了以下方面的证实： 使用脑渗透性PPARβ/RXRα激动剂、KD 3010和贝沙罗汀进行功能性拯救实验。我们 假设通过激活PPARβ/RXRα/Qki功能恢复脂质代谢有助于MS髓鞘再生 通过两种途径：1）激活小胶质细胞清除髓鞘碎片的功能，从而促进少突胶质细胞 再生，和2）增强现有的和新产生的少突胶质细胞的脂质产生。为了验证这一 根据这一假设，我们提出以下三个具体目标。为了验证这一假设，我们提出以下三点： 具体目的：1）研究Qki/PPARβ在小胶质细胞吞噬髓鞘碎片和吞噬神经元中的作用， 2）研究Qki/PPARβ在髓鞘脂质代谢和髓鞘再生中的作用， 3)为了阐明Qki作为共激活因子增强PPARβ转录活性的机制， 小胶质细胞和少突胶质细胞。我们的研究不仅将提供深入了解的病因机制， 但更重要的是，帮助MS患者通过靶向髓鞘再生途径找到治愈方法。

Summary Statement/Abstract Multiple sclerosis (MS) is the most common demyelinating disease, affecting approximately 400,000 people in the United States and 2.5 million people worldwide. It is not clear what causes MS, but many believe that it is because our own immune system attacks oligodendrocytes that generate myelin. However, the current therapies that dampen our immune system can only relieve the symptoms but not cure the disease itself. Therefore, it is urgent to find novel therapeutic approaches that can cure the disease, for instance by promoting remyelination. The central nervous system has great potential to regenerate oligodendrocytes and remyelinate in response to myelin damage, however the ability of remyelination is greatly diminished in the MS lesions. Two major reasons are known to prevent efficient remyelination in MS lesions: 1) damaged myelin cannot be efficiently cleared, thereby preventing formation of new oligodendrocytes, and 2) newly generated and/or existing oligodendrocytes have lost the ability to form new myelin. We have identified a key regulator – Quaking (protein name: Qki; gene name: Qk) – that is potent to overcome both obstacles. Firstly, we discovered that Qki is a key regulator of phagocytosis of microglia. Depletion of Qki in microglia greatly reduced the phagocytic activity of microglia, which is critical for clearance of myelin debris and consequently remyelination. Secondly, we discovered that Qki is a major regulator of oligodendrocyte differentiation and myelin homeostasis by regulating lipid metabolism of both newly formed oligodendrocytes and existing oligodendrocytes in the demyelinating lesions. Mature myelin has been considered an inert material for decades. However, our study showed that mature myelin is in fact a very dynamic material through exploiting our genetic systems by depleting Qki in mature myelinating oligodendrocytes of adult mice. The comparative lipidomic and transcriptomic analyses identified Qki as an essential factor for myelin lipid biosynthesis by controlling the transcription of the lipid metabolism genes, particularly those for fatty acid desaturation and elongation, via coactivation of the peroxisome proliferator- activated receptor beta (PPARβ)-retinoid X receptor alpha (RXRα) complex. These findings were corroborated by functional rescue experiments with brain penetrant PPARβ/RXRα agonists, KD3010 and bexarotene. We hypothesize that restoring lipid metabolism by activating PPARβ/RXRα/Qki function will help remyelination in MS through two ways: 1) activating microglia’s function to clear myelin debris, consequently promoting oligodendrocyte regeneration, and 2) enhancing lipid generation of existing and newly generated oligodendrocytes. To test this hypothesis, we propose the following three specific aims. To test this hypothesis, we propose the following three specific aims: 1) To investigate the role of Qki/PPARβ in microglial phagocytosis in clearing myelin debris and promoting remyelination, 2) to investigate the role of Qki/PPARβ in myelin lipid metabolism and remyelination, and 3) to elucidate the mechanism by which Qki functions as a coactivator to enhance PPARβ transcription activity in both microglia and oligodendrocytes. Our studies will not only provide insights into the etiological mechanism for MS, but more importantly, help MS patients find a cure through targeting remyelinating pathway.