下丘脑GAD1表观遗传修饰介导孕期地塞米松暴露子代HPT轴功能活性紊乱的编程机制

Clinical studies have found that the therapeutic dose of dexamethasone during pregnancy can cause testicular dysplasia and reproductive disorders in the offspring. Recent studies in our lab demonstrated that prenatal dexamethasone exposure (PDE) can cause the up-regulated expression of hypothalamic gonadotropin releasing hormone (GnRH) before birth and the dysfunction of the hypothalamic-pituitary-testicular (HPT) axis (enhanced expression of GnRH, decreased testosterone levels in blood and testis, decreased testicular volume, and decreased Leydig cells) after birth in male offspring rats. Recently, we found that PDE can increase the expression of hypothalamic rate-limiting enzyme in the synthesis of -aminobutyric acid (GABA) - Glutamic acid decarboxylase (GAD) 1 in fetal rats and also the adult rats. Based on literatures, we hypothesized that dexamethasone can induce GR activated and translocated to the nucleus, resulting in the epigenetic alteration of GAD1 promoters, and enhance the expression of GAD1. Enhanced expression of GAD1 can increase the transformation of glutamate into GABA in the hypothalamus and activate GnRH neurons. Moreover, the epigenetic programming alteration and enhanced expression of hypothalamic GAD1 can continue to after birth, and excessive GABA contribute to the long-term excitation of GnRH neurons, resulting in pituitary desensitization and dysfunction of HPT axis. This study aims to clarify the intrauterine programming mechanism of the dysfunction of the HPT axis induced by PDE in lab animals and cellular levels, , and confirm the role of epigenetic modification of hypothalamic GAD1 in the dysfunction of the HPT axis, and provide a theoretical basis for the prevention and treatment of male reproductive endocrine system diseases.

临床研究发现，孕期治疗剂量的地塞米松可引起子代睾丸发育异常。近期我们发现，孕期地塞米松暴露(PDE)可致雄性子代大鼠出生前下丘脑促性腺激素释放激素(GnRH)表达上调，出生后下丘脑-垂体-睾丸(HPT)轴功能活性紊乱(GnRH高表达、血和睾丸内睾酮水平降低、睾丸体积减小、睾丸间质细胞减少)；进一步发现，出生前、后下丘脑γ-氨基丁酸(GABA)合成限速酶—谷氨酸脱羧酶(GAD)1表达增加。综合文献，我们推测：PDE可诱导下丘脑GR活化，引起GAD1表观遗传修饰及表达改变，促使谷氨酸向GABA转化增加，GABA对GnRH神经元发挥兴奋作用，这些编程性改变可延续至出生后。过多GABA致GnRH神经元失去脉冲性释放GnRH的生理节律性，致垂体脱敏，进而抑制了睾丸的功能与发育，表现为成年子代HPT轴功能活性紊乱。阐明下丘脑GAD1表观遗传修饰的编程机制，将为男性生殖内分泌系统疾病的防治提供理论依据。

临床研究发现，孕期治疗剂量的地塞米松可引起子代睾丸发育异常，然而其发生机制尚未明了。我们研究发现，孕期地塞米松暴露(PDE)可致PDE组雄性胎下丘脑促性腺激素释放激素(GnRH)1表达上调，出生后子代下丘脑-垂体-睾丸(HPT)轴功能活性紊乱(GnRH高表达、血和睾丸内睾酮水平降低、睾丸体积减小、睾丸间质细胞减少)；进一步发现，出生前、后PDE组下丘脑谷氨酸脱羧酶(GAD)1表达增加，GAD1启动子区甲基化水平降低；地塞米松可直接上调原代下丘脑细胞GnRH1、GAD1、Egr-1而下调DNMT1及DNMT3a的mRNA表达，且可减少与GnRH1共表达的谷氨酸（Glu）能神经元的活性，而增加γ-氨基丁酸（GABA）能神经元的活性，RU486可逆转地塞米松的上述效应。结论：PDE通过活化下丘脑GR，引起GAD1启动子区低甲基化及高表达改变，促使Glu向GABA转化增加，进一步诱导GnRH1高表达，这些编程性改变可延续至出生后，导致HPT轴功能活性紊乱，并最终介导子代睾丸发育不良。