LRIG1 controls proliferation of adult neural stem cells by facilitating Tgfß and BMP signalling pathways

Adult Neural Stem Cells (aNSCs) in the ventricular-subventricular zone (V-SVZ) are largely quiescent. Here, we characterize the mechanism underlying the functional role of a cell-signalling inhibitory protein, LRIG1, in the control of aNSCs proliferation. Using constitutive Lrig1 knockout models, we show that Lrig1 ablation results in increased aNSCs proliferation with no change in neuronal progeny and that this hyperproliferation likely does not result from activation of the epidermal growth factor receptor (EGFR). Loss of LRIG1, however, does lead to impaired activation of canonical transforming growth factor beta (TGFß) and bone morphogenic protein (BMP) signalling pathways. Biochemically, we show in vitro that LRIG1 is able to bind TGFß/BMP receptors and the TGFß1 ligand. Finally, we show that the consequences of these interactions are to facilitate SMAD phosphorylation and pathway activation. Collectively, these data suggest that, unlike in embryonic neural stem cells where EGFR is the primary mechanism of action, LRIG1 and TGFß pathways positively work together to fulfill their inhibitory roles in aNSCs. Overall design: To identify putative signalling pathways disrupted by loss of LRIG1 we compated whole transcriptomes of Lrig1 knockout (KO) and wild-type (WT) mice. We performed mRNA-seq on mRNA isolated from V-SVZ tissue dissected from 8-week old Lrig1 KO and WT mice.

室管膜下区（ventricular-subventricular zone，V-SVZ）内的成体神经干细胞（Adult Neural Stem Cells，aNSCs）大多处于静息状态。本研究针对细胞信号抑制蛋白LRIG1在调控成体神经干细胞增殖中的功能机制展开系统解析。我们通过构建组成型Lrig1敲除模型，发现敲除Lrig1可导致成体神经干细胞增殖水平升高，但神经元子代细胞数量未发生改变；且该过度增殖大概率并非由表皮生长因子受体（epidermal growth factor receptor，EGFR）的激活所介导。
然而，LRIG1的缺失确实会削弱经典转化生长因子β（transforming growth factor beta，TGFβ）与骨形态发生蛋白（bone morphogenic protein，BMP）信号通路的激活。
生化实验层面，本研究在体外证实LRIG1可结合TGFβ/BMP受体以及TGFβ1配体。
最终，本研究证实上述蛋白互作可促进SMAD磷酸化与信号通路激活。
综合来看，本研究数据表明：与胚胎神经干细胞中EGFR作为核心作用机制不同，LRIG1与TGFβ通路可协同发挥正向调控作用，共同在成体神经干细胞中实现其抑制性功能。
实验整体设计：为鉴定LRIG1缺失所干扰的潜在信号通路，本研究对Lrig1敲除（knockout，KO）与野生型（wild-type，WT）小鼠的全转录组进行了比较分析。我们对从8周龄Lrig1敲除与野生型小鼠的室管膜下区（V-SVZ）组织中分离提取的mRNA开展了mRNA测序（mRNA-seq）。