Down-Regulation of Inpp5e Associated With Abnormal Ciliogenesis During Embryonic Neurodevelopment Under Inositol Deficiency

Inpp5e Regulated the Cilium-Related Genes Contributing to the Neural Tube Defects Under 5-Fluorouracil Exposure

Primary cilia are crucial for neurogenesis, and cilium-related genes are involved in the closure of neural tubes. Inositol polyphosphate-5-phosphatase (Inpp5e) was enriched in primary cilia and closely related to the occurrence of neural tube defects (NTDs). However, the role of Inpp5e in the development of NTDs is not well-known. To investigate whether Inpp5e gene is associated with the neural tube closure, we established a mouse model of NTDs by 5-fluorouracil (5-FU) exposure at gestational day 7.5 (GD7.5). The Inpp5e knockdown (Inpp5e-/-) mouse embryonic stem cells (mESCs) were produced by CRISPR/Cas9 system. The expressions of Inpp5e and other cilium-related genes including intraflagellar transport 80 (Ift80), McKusick-Kaufman syndrome (Mkks), and Kirsten rat sarcoma viral oncogene homolog (Kras) were determined, utilizing quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR), western blot, PCR array, and immunofluorescence staining. The result showed that the incidence of NTDs was 37.10% (23 NTDs/62 total embryos) and significantly higher than that in the control group (P < 0.001). The neuroepithelial cells of neural tubes were obviously disarranged in NTD embryos. The mRNA and protein levels of Inpp5e, Ift80, Mkks, and Kras were significantly decreased in NTD embryonic brain tissues, compared to the control (P < 0.05). Knockdown of the Inpp5e (Inpp5e-/-) reduced the expressions of Ift80, Mkks, and Kras in mESCs. Furthermore, the levels of α-tubulin were significantly reduced in NTD embryonic neural tissue and Inpp5e-/- mESCs. These results suggested that maternal 5-FU exposure inhibited the expression of Inpp5e, which resulted in the downregulation of cilium-related genes (Ift80, Mkks, and Kras), leading to the impairment of primary cilium development, and ultimately disrupted the neural tube closure.

BMP/Smad Pathway Is Involved in Lithium Carbonate-Induced Neural-Tube Defects in Mice and Neural Stem Cells

Neural-tube defects (NTDs) are one type of the most serious birth defects. Studies have shown that inositol deficiency is closely related to the occurrence of NTDs. Bone morphogenetic protein (BMP)-mediated Smad signaling pathways have been implicated in neurogenesis and neural-tube closure. However, the role of the BMP/Smad pathway in inositol-deficiency-induced NTDs remains unclear. Inositol-deficiency models in C57 mice and mouse neural stem cells (mNSCs) were induced with Li₂CO₃ treatment or inositol withdrawal. The role of the BMP/Smad pathway in the regulation of cell proliferation and the development of NTDs was determined utilizing qRT-PCR, HE staining, Western blot, immunostaining, MTT assay, EdU staining, and flow cytometry. The intraperitoneal injection of Li₂CO₃ at Embryonic Day 7.5 induced the occurrence of NTDs. The mRNA levels of Bmp2, Bmp4, Smad1, Smad5, Smad8 and Runx2, the phosphorylation of Smad1/5/8, and the nuclear translocation of Runx2 were significantly increased in NTD embryonic brain tissues and mNSCs exposed to Li₂CO₃ or an inositol-free medium, which were suppressed by BMP receptor selective inhibitor LDN-193189. The Li₂CO₃-induced phosphorylation of Smad1/5/8 was inhibited by inositol supplementation. Cell proliferation was significantly promoted by Li₂CO₃ exposure or the absence of inositol in mNSCs, which was reversed by LDN-193189. These results suggest that the activation of the BMP/Smad signaling pathway might play an important role in the development of NTDs induced by maternal Li₂CO₃ exposure via inositol deficiency.

Multiple ciliary localization signals control INPP5E ciliary targeting

Primary cilia are sensory membrane protrusions whose dysfunction causes ciliopathies. INPP5E is a ciliary phosphoinositide phosphatase mutated in ciliopathies like Joubert syndrome. INPP5E regulates numerous ciliary functions, but how it accumulates in cilia remains poorly understood. Herein, we show INPP5E ciliary targeting requires its folded catalytic domain and is controlled by four conserved ciliary localization signals (CLSs): LLxPIR motif (CLS1), W383 (CLS2), FDRxLYL motif (CLS3) and CaaX box (CLS4). We answer two long-standing questions in the field. First, partial CLS1-CLS4 redundancy explains why CLS4 is dispensable for ciliary targeting. Second, the essential need for CLS2 clarifies why CLS3-CLS4 are together insufficient for ciliary accumulation. Furthermore, we reveal that some Joubert syndrome mutations perturb INPP5E ciliary targeting, and clarify how each CLS works: (i) CLS4 recruits PDE6D, RPGR and ARL13B, (ii) CLS2-CLS3 regulate association to TULP3, ARL13B, and CEP164, and (iii) CLS1 and CLS4 cooperate in ATG16L1 binding. Altogether, we shed light on the mechanisms of INPP5E ciliary targeting, revealing a complexity without known parallels among ciliary cargoes.