Synaptic abnormality is an important pathologic feature of autism spectrum disorders (ASDs) and responsible for various behavioral defects in these neurodevelopmental disorders. Microglia are the major immune cells in the brain and also play an important role in synapse refinement. Although dysregulated synaptic pruning by microglia during the brain development has been associated with ASDs, the underlying mechanism has yet to be fully elucidated. Herein, we observed that expression of Transmembrane protein 59 (TMEM59), a protein recently shown to regulate microglial function, was decreased in autistic patients. Furthermore, we found that both male and female mice with either complete or microglia-specific loss of Tmem59 developed ASD-like behaviors. Microglial TMEM59-deficient mice also exhibited enhanced excitatory synaptic transmission, increased dendritic spine density, and elevated levels of excitatory synaptic proteins in synaptosomes. TMEM59-deficient microglia had impaired capacity for synapse engulfment both in vivo and in vitro. Moreover, we demonstrated that TMEM59 interacted with the C1q receptor CD93 and TMEM59 deficiency promoted CD93 protein degradation in microglia. Downregulation of CD93 in microglia also impaired synapse engulfment. These findings identify a crucial role of TMEM59 in modulating microglial function on synapse refinement during brain development and suggest that TMEM59 deficiency may contribute to ASDs through disrupting phagocytosis of excitatory synapse and thus distorting the excitatory-inhibitory (E/I) neuronal activity balance.

SIGNIFICANCE STATEMENT Microglia play an important role in synapse refinement. Dysregulated synaptic pruning by microglia during brain development has been associated with autism spectrum disorders (ASDs). However, the underlying mechanism has yet to be fully elucidated. Herein, we observe that the expression of Transmembrane protein 59 (TMEM59), an autophagy-related protein, is decreased in autistic patients. Moreover, we find ASD-like behaviors in mice with complete loss and with microglia-specific loss of Tmem59. Mechanistic studies reveal that TMEM59 deficiency in microglia impairs their synapse engulfment ability likely through destabilizing the C1q receptor CD93, thereby leading to enhanced excitatory neurotransmission and increased dendritic spine density. Our findings demonstrate a crucial role of microglial TMEM59 in early neuronal development and provide new insight into the etiology of ASDs.

突触异常是自闭症谱系障碍 (ASD) 的重要病理特征，是这些神经发育障碍中各种行为缺陷的原因。小胶质细胞是大脑中的主要免疫细胞，在突触细化中也起着重要作用。尽管在大脑发育过程中小胶质细胞对突触修剪失调与 ASD 相关，但其潜在机制尚未完全阐明。在此，我们观察到跨膜蛋白 59 (TMEM59) 的表达在自闭症患者中有所降低，这种蛋白最近显示可调节小胶质细胞功能。此外，我们发现雄性和雌性小鼠都完全或小胶质细胞特异性缺失Tmem59发展出类似 ASD 的行为。小胶质细胞 TMEM59 缺陷小鼠还表现出增强的兴奋性突触传递、增加的树突棘密度以及突触体中兴奋性突触蛋白水平升高。TMEM59 缺陷型小胶质细胞在体内和体外的突触吞噬能力均受损。此外，我们证明了 TMEM59 与 C1q 受体 CD93 相互作用，TMEM59 缺陷促进了小胶质细胞中 CD93 蛋白的降解。小胶质细胞中 CD93 的下调也损害了突触吞噬。这些发现确定了 TMEM59 在大脑发育过程中调节小胶质细胞功能对突触细化的关键作用，并表明 TMEM59 缺陷可能通过破坏兴奋性突触的吞噬作用并因此扭曲兴奋性抑制 (E/I) 神经元活动平衡而导致 ASD。

重要性声明小胶质细胞在突触细化中起着重要作用。大脑发育过程中小胶质细胞对突触修剪失调与自闭症谱系障碍 (ASD) 有关。然而，潜在的机制尚未完全阐明。在此，我们观察到自噬相关蛋白 Transmembrane protein 59 (TMEM59) 的表达在自闭症患者中降低。此外，我们在完全缺失和小胶质细胞特异性Tmem59缺失的小鼠中发现了类似 ASD 的行为. 机制研究表明，小胶质细胞中的 TMEM59 缺陷可能通过破坏 C1q 受体 CD93 的稳定性而损害它们的突触吞噬能力，​​从而导致兴奋性神经传递增强和树突棘密度增加。我们的研究结果证明了小胶质细胞 TMEM59 在早期神经元发育中的关键作用，并为 ASD 的病因学提供了新的见解。