m6Am methyltransferase PCIF1 negatively regulates ciliation by inhibiting BICD2 expression.

N6, 2'-O-dimethyladenosine (m6Am) is a widespread RNA modification catalyzed by the methyltransferase PCIF1 (phosphorylated CTD interacting factor 1). Despite its prevalence, the biological functions of m6Am in RNA remain largely elusive. Here, we report a critical role of PCIF1-dependent m6Am RNA modification in ciliogenesis in RPE-1 cells. Our findings demonstrate that PCIF1 acts as a negative regulator of ciliation through its m6Am methyltransferase activity. A quantitative proteomic analysis identifies BICD2 as a downstream target of PCIF1, with PCIF1 depletion resulting in a significant increase in BICD2 levels. BICD2 depletion leads to a significant reduction in ciliation. Crucially, the ciliary phenotype in PCIF1-depleted cells is reversed upon BICD2 knockdown. Further investigations reveal that PCIF1 regulates BICD2 protein levels through its m6Am catalytic activity, which reduces the stability and translation efficiency of BICD2 mRNA. Single-base resolution LC-MS analysis identifies the m6Am site on BICD2 mRNA modified by PCIF1. These findings establish the essential involvement of PCIF1-dependent m6Am modification in ciliogenesis.

Corrigendum: NudC L279P mutation destabilizes filamin a by inhibiting the Hsp90 chaperoning pathway and suppresses cell migration.

[This corrects the article DOI: 10.3389/fcell.2021.671233.].

ALKBH3-dependent m1A demethylation of Aurora A mRNA inhibits ciliogenesis.

Primary cilia are antenna-like subcellular structures to act as signaling platforms to regulate many cellular processes and embryonic development. m1A RNA modification plays key roles in RNA metabolism and gene expression; however, the physiological function of m1A modification remains largely unknown. Here we find that the m1A demethylase ALKBH3 significantly inhibits ciliogenesis in mammalian cells by its demethylation activity. Mechanistically, ALKBH3 removes m1A sites on mRNA of Aurora A, a master suppressor of ciliogenesis. Depletion of ALKBH3 enhances Aurora A mRNA decay and inhibits its translation. Moreover, alkbh3 morphants exhibit ciliary defects, including curved body, pericardial edema, abnormal otoliths, and dilation in pronephric ducts in zebrafish embryos, which are significantly rescued by wild-type alkbh3, but not by its catalytically inactive mutant. The ciliary defects caused by ALKBH3 depletion in both vertebrate cells and embryos are also significantly reversed by ectopic expression of Aurora A mRNA. Together, our data indicate that ALKBH3-dependent m1A demethylation has a crucial role in the regulation of Aurora A mRNA, which is essential for ciliogenesis and cilia-associated developmental events in vertebrates.

NudCL2 is an autophagy receptor that mediates selective autophagic degradation of CP110 at mother centrioles to promote ciliogenesis.

Primary cilia extending from mother centrioles are essential for vertebrate development and homeostasis maintenance. Centriolar coiled-coil protein 110 (CP110) has been reported to suppress ciliogenesis initiation by capping the distal ends of mother centrioles. However, the mechanism underlying the specific degradation of mother centriole-capping CP110 to promote cilia initiation remains unknown. Here, we find that autophagy is crucial for CP110 degradation at mother centrioles after serum starvation in MEF cells. We further identify NudC-like protein 2 (NudCL2) as a novel selective autophagy receptor at mother centrioles, which contains an LC3-interacting region (LIR) motif mediating the association of CP110 and the autophagosome marker LC3. Knockout of NudCL2 induces defects in the removal of CP110 from mother centrioles and ciliogenesis, which are rescued by wild-type NudCL2 but not its LIR motif mutant. Knockdown of CP110 significantly attenuates ciliogenesis defects in NudCL2-deficient cells. In addition, NudCL2 morphants exhibit ciliation-related phenotypes in zebrafish, which are reversed by wild-type NudCL2, but not its LIR motif mutant. Importantly, CP110 depletion significantly reverses these ciliary phenotypes in NudCL2 morphants. Taken together, our data suggest that NudCL2 functions as an autophagy receptor mediating the selective degradation of mother centriole-capping CP110 to promote ciliogenesis, which is indispensable for embryo development in vertebrates.

NudC L279P Mutation Destabilizes Filamin A by Inhibiting the Hsp90 Chaperoning Pathway and Suppresses Cell Migration.

Filamin A, the first discovered non-muscle actin filament cross-linking protein, plays a crucial role in regulating cell migration that participates in diverse cellular and developmental processes. However, the regulatory mechanism of filamin A stability remains unclear. Here, we find that nuclear distribution gene C (NudC), a cochaperone of heat shock protein 90 (Hsp90), is required to stabilize filamin A in mammalian cells. Immunoprecipitation-mass spectrometry and western blotting analyses reveal that NudC interacts with filamin A. Overexpression of human NudC-L279P (an evolutionarily conserved mutation in NudC that impairs its chaperone activity) not only decreases the protein level of filamin A but also results in actin disorganization and the suppression of cell migration. Ectopic expression of filamin A is able to reverse these defects induced by the overexpression of NudC-L279P. Furthermore, Hsp90 forms a complex with filamin A. The inhibition of Hsp90 ATPase activity by either geldanamycin or radicicol decreases the protein stability of filamin A. In addition, ectopic expression of Hsp90 efficiently restores NudC-L279P overexpression-induced protein stability and functional defects of filamin A. Taken together, these data suggest NudC L279P mutation destabilizes filamin A by inhibiting the Hsp90 chaperoning pathway and suppresses cell migration.

Centrosomal protein FOR20 knockout mice display embryonic lethality and left-right patterning defects.

Centrosomal protein FOR20 has been reported to be crucial for essential cellular processes, including ciliogenesis, cell migration, and cell cycle in vertebrates. However, the function of FOR20 during mammalian embryonic development remains unknown. To investigate the in vivo function of the For20 gene in mammals, we generated For20 homozygous knockout mice by gene targeting. Our data reveal that homozygous knockout of For20 results in significant embryonic growth arrest and lethality during gestation, while the heterozygotes show no obvious defects. The absence of For20 leads to impaired left-right patterning of embryos and reduced cilia in the embryonic node. Deletion of For20 also disrupts angiogenesis in yolk sacs and embryos. These results highlight a critical role of For20 in early mammalian embryogenesis.

KLF9 suppresses gastric cancer cell invasion and metastasis through transcriptional inhibition of MMP28.

Gastric cancer (GC) is among the most lethal human malignancies, and the leading cause of GC mortality is metastasis. However, the precise mechanism of GC metastasis remains unclear. To screen key transcriptional factors (TFs) involved in GC metastasis, we performed bioinformatics analysis of The Cancer Genome Atlas database and found that Krüppel-like factor 9 (KLF9) is a GC metastasis-associated TF. KLF9 is significantly decreased in patients with GC with distant metastasis compared with those patients without distant metastasis. Ectopic expression of KLF9 evidently inhibited the migration and invasion capabilities of GC cells. Conversely, knockdown of KLF9 endowed GC cells with stronger invasive capacity. Moreover, tail intravenous injection confirmed that KLF9 strongly inhibits the lung metastasis process of GC in vivo. Mechanistically, chromatin immunoprecipitation coupled with high-throughput sequencing data from Encyclopedia of DNA Elements revealed that KLF9 specifically binds to the promoter region of matrix metalloproteinase (MMP)28. Further quantitative real-time PCR and dual-luciferase assay indicated that KLF9 directly inhibited MMP28 transcription. Importantly, decreased invasion and metastasis capability of GC cells caused by ectopic KLF9 expression could be rescued via reinforcing MMP28 expression in vivo. Collectively, our study indicates that KLF9 significantly suppresses GC cell invasion and metastasis through inhibiting MMP28 transcription.-Li, Y., Sun, Q., Jiang, M., Li, S., Zhang, J., Xu, Z., Guo, D., Gu, T., Wang, B., Xiao, L., Zhou, T., Zhuo, W. KLF9 suppresses gastric cancer cell invasion and metastasis through transcriptional inhibition of MMP28.

Centrosomal protein FOR20 is essential for cilia-dependent development in zebrafish embryos.

Centrosomal proteins play critical roles in ciliogenesis. Mutations in many centrosomal proteins have been documented to contribute to developmental defects and cilium-related diseases. Centrosomal protein fibroblast growth factor receptor 1 oncogene partner-related protein of 20 kDa (FOR20) is crucial for ciliogenesis in mammalian cells and the unicellular eukaryote Paramecium; however, the biologic significance of FOR20 in vertebrate development remains unclear. We cloned the zebrafish homolog of the for20 gene and found that for20 mRNA is enriched in ciliated tissues during early zebrafish development. Knockdown of for20 by morpholino oligonucleotides in zebrafish results in multiple ciliary phenotypes, including curved body, hydrocephaly, pericardial edema, kidney cysts, and left-right asymmetry defects. for20 morphants show reduced number and length of cilia in Kupffer's vesicle and pronephric ducts. High-speed video microscopy reveals that cilia in most for20 morphants are consistently paralyzed or beat arrhythmically. To confirm the ciliary phenotypes of for20 morphants, we used the CRISPR/Cas9 system to disrupt for20 gene in zebrafish. for20 mutants exhibit multiple ciliary phenotypes resembling the defects in for20 morphants. All of these phenotypes in for20 morphants and mutants are significantly reversed by exogenous expression of for20 mRNA. Taken together, these data suggest that FOR20 is required for cilium-mediated processes during zebrafish embryogenesis.-Xie, S., Jin, J., Xu, Z., Huang, Y., Zhang, W., Zhao, L., Lo, L. J., Peng, J., Liu, W., Wang, F., Shu, Q., Zhou, T. Centrosomal protein FOR20 is essential for cilia-dependent development in zebrafish embryos.

Twa1/Gid8 is a ß-catenin nuclear retention factor in Wnt signaling and colorectal tumorigenesis.

Hyperactivation of Wnt/ß-catenin signaling is one of the major causes of human colorectal cancer (CRC). A hallmark of Wnt signaling is the nuclear accumulation of ß-catenin. Although ß-catenin nuclear import and export have been widely investigated, the underlying mechanism of ß-catenin's nuclear retention remains largely unknown. Here, we report that Twa1/Gid8 is a key nuclear retention factor for ß-catenin during Wnt signaling and colorectal carcinogenesis. In the absence of Wnt, Twa1 exists together with ß-catenin in the Axin complex and undergoes ubiquitination and degradation. Upon Wnt signaling, Twa1 translocates into the nucleus, where it binds and retains ß-catenin. Depletion of Twa1 attenuates Wnt-stimulated gene expression, dorsal development of zebrafish embryos and xenograft tumor growth of CRC cells. Moreover, nuclear Twa1 is significantly upregulated in human CRC tissues, correlating with the nuclear accumulation of ß-catenin and poor prognosis. Thus, our results identify Twa1 as a previously undescribed regulator of the Wnt pathway for promoting colorectal tumorigenesis by facilitating ß-catenin nuclear retention.