Capicua regulates the survival of Cajal-Retzius cells in the postnatal hippocampus

Programmed cell death is crucial for organ morphogenesis and tissue homeostasis. Understanding programmed cell death in the developing brain is essential for comprehending both normal brain development and neurological disorders. In this study, we utilize Cajal-Retzius (CR) cells, transient neurons that populate the embryonic cortex and are predominantly eliminated in early postnatal stages, as a model to investigate the regulation of programmed cell death. While many CR cells typically undergo postnatal cell death, some persist into adulthood in the hippocampus, influencing local circuits and behaviors. Here, we show that the loss of capicua (CIC), a transcriptional repressor implicated in a rare neurodevelopmental syndrome and multiple cancers, results in aberrant survival of CR cells in the adult hippocampus. Altered cell survival is mediated by the cell-autonomous function of CIC in hippocampal CR cells. Surprisingly, the atypical persistence of CR cells following CIC loss does not impact hippocampal-dependent behaviors or susceptibility to kainic acid-induced seizures. Single-cell transcriptomic analysis unveils previously unrecognized heterogeneity among hippocampal CR cells and suggests a role of CIC in repressing Fgf1 expression. Additionally, we reveal that FGF1 and BCL2 serve as pivotal regulators enhancing CR cell survival in the postnatal hippocampus. Our findings shed light on a previously unacknowledged role of CIC upstream of FGF signaling and elucidate the apoptosis mechanism governing developmental programmed CR cell death. Four Np73-Cre; Cicflox/+; tdT and four Np73-Cre; Cicflox/flox; tdT mice (littermates) at P14 were perfused with artificial cerebrospinal fluid (aCSF; 87 mM NaCl; 2.5 mM KCl; 1.25 mM NaH2PO4; 26 mM NaHCO3; 75 mM sucrose; 20 mM glucose; 2 mM CaCl2; 2 mM MgSO4) equilibrated in carbogen (95% O2, 5% CO2) via a gas stone. The brain was promptly removed and kept in aCSF as the hippocampus was dissected. Dissected tissue was kept submerged in aCSF saturated with carbogen on ice.程序性细胞死亡（Programmed cell death）对于器官形态发生与组织稳态至关重要。深入理解发育过程中的程序性细胞死亡，对于理解正常脑发育及神经疾病均不可或缺。 本研究以Cajal-Retzius（CR）细胞作为模型，探究程序性细胞死亡的调控机制。这类细胞为分布于胚胎大脑皮层的瞬时神经元，且主要在出生早期阶段被清除。尽管多数CR细胞通常会在出生后发生细胞死亡，但仍有部分CR细胞可存活至成年海马体中，对局部神经环路及行为产生影响。 本研究发现，capicua（CIC）——一种与罕见神经发育综合征及多种癌症相关的转录阻遏物——的缺失，会导致成年海马体中CR细胞出现异常存活。这种存活异常由CIC在海马CR细胞中的细胞自主性功能所介导。令人意外的是，CIC缺失后CR细胞的异常存留并未影响海马依赖性行为，亦未改变红藻氨酸诱导的癫痫发作易感性。 单细胞转录组分析揭示了海马CR细胞群体中此前未被认知的异质性，并表明CIC可抑制Fgf1（成纤维细胞生长因子1）的表达。此外，本研究还发现FGF1与BCL2（B细胞淋巴瘤因子2）可作为关键调控因子，增强出生后海马体中CR细胞的存活能力。 本研究结果阐明了CIC在FGF信号通路上游的此前未被认知的作用，并解析了调控发育性程序性CR细胞死亡的凋亡机制。 本研究共使用4只Np73-Cre; Cicflox/+; tdT与4只Np73-Cre; Cicflox/flox; tdT小鼠（均为同窝幼鼠），于出生后第14天（P14）时，通过气石（gas stone）将经95% O₂、5% CO₂混合气（carbogen）平衡的人工脑脊液（artificial cerebrospinal fluid, aCSF；87 mM NaCl、2.5 mM KCl、1.25 mM NaH₂PO₄、26 mM NaHCO₃、75 mM蔗糖、20 mM葡萄糖、2 mM CaCl₂、2 mM MgSO₄）进行灌流。随后迅速取出脑组织并置于人工脑脊液中，分离出海马体。分离得到的脑组织需全程浸没于经混合气饱和的人工脑脊液中，并置于冰上保存。