Xmab21l3 mediates dorsoventral patterning in Xenopus laevis

Missense Mutations in MAB21L1: Causation of Novel Autosomal Dominant Ocular BAMD Syndrome

Purpose

Biallelic MAB21L1 variants have been reported to cause autosomal recessive cerebellar, ocular, craniofacial, and genital syndrome (COFG), whereas only five heterozygous pathogenic variants have been suspected to cause autosomal dominant (AD) microphthalmia and aniridia in eight families. This study aimed to report an AD ocular syndrome (blepharophimosis plus anterior segment and macular dysgenesis [BAMD]) syndrome based on clinical and genetic findings from patients with monoallelic MAB21L1 pathogenic variants in our cohort and reported cases.

Methods

Potential pathogenic variants in MAB21L1 were detected from a large in-house exome sequencing dataset. Ocular phenotypes of the patients with potential pathogenic variants in MAB21L1 were summarized, and the genotype-phenotype correlation was analyzed through a comprehensive literature review.

Results

Three heterozygous missense variants in MAB21L1, predicted to be damaging, were detected in 5 unrelated families, including c.152G>T in 2, c.152G>A in 2, and c.155T>G in one. All were absent from gnomAD. The variants were de novo in two families, transmitted from affected parents to offspring in two families, and with an unknown origin in the other family, demonstrating strong evidence of AD inheritance. All patients revealed similar BAMD phenotypes, including blepharophimosis, anterior segment dysgenesis, and macular dysgenesis. Genotype-phenotype analysis suggested that patients with monoallelic MAB21L1 missense variants had only ocular anomalies (BAMD), whereas patients with biallelic variants presented both ocular and extraocular symptoms.

Conclusions

Heterozygous pathogenic variants in MAB21L1 account for a new AD BAMD syndrome, which is completely different from COFG caused by homozygous variants in MAB21L1. Nucleotide c.152 is likely a mutation hot spot, and the encoded residue of p.Arg51 might be critical for MAB21L1.

MAB21L1 promotes survival of lens epithelial cells through control of αB-crystallin and ATR/CHK1/p53 pathway

The male abnormal gene family 21 (mab21), was initially identified in C. elegans. Since its identification, studies from different groups have shown that it regulates development of ocular tissues, brain, heart and liver. However, its functional mechanism remains largely unknown. Here, we demonstrate that Mab21L1 promotes survival of lens epithelial cells. Mechanistically, Mab21L1 upregulates expression of αB-crystallin. Moreover, our results show that αB-crystallin prevents stress-induced phosphorylation of p53 at S-20 and S-37 through abrogating the activation of the upstream kinases, ATR and CHK1. As a result of suppressing p53 activity by αB-crystallin, Mab21L1 downregulates expression of Bak but upregulates Mcl-1 during stress insult. Taken together, our results demonstrate that Mab21L1 promotes survival of lens epithelial cells through upregulation of αB-crystallin to suppress ATR/CHK1/p53 pathway.

Grass carp cGASL negatively regulates interferon activation through autophagic degradation of MAVS

In mammals, cyclic GMP-AMP synthase (cGAS) is a crucial cytosolic DNA sensor responsible for activating the interferon (IFN) response. A cGAS-like (cGASL) gene was previously identified from grass carp Ctenopharyngodon idellus, which is evolutionarily closest to cGAS but not a true ortholog of cGAS. Here, we found that grass carp cGASL targets mitochondrial antiviral signaling protein (MAVS) for autophagic degradation to negatively regulate fish IFN response. Firstly, the transcriptional level of cellular cgasl was upregulated by poly I:C stimulation, and overexpression of cGASL significantly decreased poly I:C- and MAVS-induced promoter activities and transcriptional levels of IFN and IFN-stimulated genes (ISGs). In addition, cGASL associated with MAVS and prompted autophagic degradation of MAVS in a dose-dependent manner. Finally, overexpression of cGASL attenuated MAVS-mediated cellular antiviral response. These results collectively indicate that cGASL negatively regulates fish IFN response by triggering autophagic degradation of MAVS.

Tbx2 mediates dorsal patterning and germ layer suppression through inhibition of BMP/GDF and Activin/Nodal signaling

BACKGROUND

Members of the T-box family of DNA-binding proteins play a prominent role in the differentiation of the three primary germ layers. VegT, Brachyury, and Eomesodermin function as transcriptional activators and, in addition to directly activating the transcription of endoderm- and mesoderm-specific genes, serve as regulators of growth factor signaling during induction of these germ layers. In contrast, the T-box gene, tbx2, is expressed in the embryonic ectoderm, where Tbx2 functions as a transcriptional repressor and inhibits mesendodermal differentiation by the TGFβ ligand Activin. Tbx2 misexpression also promotes dorsal ectodermal fate via inhibition of the BMP branch of the TGFβ signaling network.

RESULTS

Here, we report a physical association between Tbx2 and both Smad1 and Smad2, mediators of BMP and Activin/Nodal signaling, respectively. We perform structure/function analysis of Tbx2 to elucidate the roles of both Tbx2-Smad interaction and Tbx2 DNA-binding in germ layer suppression.

CONCLUSION

Our studies demonstrate that Tbx2 associates with intracellular mediators of the Activin/Nodal and BMP/GDF pathways. We identify a novel repressor domain within Tbx2, and have determined that Tbx2 DNA-binding activity is required for repression of TGFβ signaling. Finally, our data also point to overlapping yet distinct mechanisms for Tbx2-mediated repression of Activin/Nodal and BMP/GDF signaling.

Tbx2 is required for the suppression of mesendoderm during early Xenopus development

BACKGROUND

T-box family proteins are DNA-binding transcriptional regulators that play crucial roles during germ layer formation in the early vertebrate embryo. Well-characterized members of this family, including the transcriptional activators Brachyury and VegT, are essential for the proper formation of mesoderm and endoderm, respectively. To date, T-box proteins have not been shown to play a role in the promotion of the third primary germ layer, ectoderm.

RESULTS

Here, we report that the T-box factor Tbx2 is both sufficient and necessary for ectodermal differentiation in the frog Xenopus laevis. Tbx2 is expressed zygotically in the presumptive ectoderm, during blastula and gastrula stages. Ectopic expression of Tbx2 represses mesoderm and endoderm, while loss of Tbx2 leads to inappropriate expression of mesoderm- and endoderm-specific genes in the region fated to give rise to ectoderm. Misexpression of Tbx2 also promotes neural tissue in animal cap explants, suggesting that Tbx2 plays a role in both the establishment of ectodermal fate and its dorsoventral patterning.

CONCLUSIONS

Our studies demonstrate that Tbx2 functions as a transcriptional repressor during germ layer formation, and suggest that this activity is mediated in part through repression of target genes that are stimulated, in the mesendoderm, by transactivating T-box proteins. Taken together, our results point to a critical role for Tbx2 in limiting the potency of blastula-stage progenitor cells during vertebrate germ layer differentiation. Developmental Dynamics 247:903-913, 2018. © 2018 Wiley Periodicals, Inc.

Transcriptional profiling of Toll-like receptor 2-deficient primary murine brain cells during Toxoplasma gondii infection

BACKGROUND

Toxoplasma gondii is capable of persisting in the brain, although it is efficiently eliminated by cellular immune responses in most other sites. While Toll-like receptor 2 (TLR2) reportedly plays important roles in protective immunity against the parasite, the relationship between neurological disorders induced by T. gondii infection and TLR2 function in the brain remains controversial with many unknowns. In this study, primary cultured astrocytes, microglia, neurons, and peritoneal macrophages obtained from wild-type and TLR2-deficient mice were exposed to T. gondii tachyzoites. To characterize TLR2-dependent functional pathways activated in response to T. gondii infection, gene expression of different cell types was profiled by RNA sequencing.

RESULTS

During T. gondii infection, a total of 611, 777, 385, and 1105 genes were upregulated in astrocytes, microglia, neurons, and macrophages, respectively, while 163, 1207, 158, and 1274 genes were downregulated, respectively, in a TLR2-dependent manner. Overrepresented Gene Ontology (GO) terms for TLR2-dependently upregulated genes were associated with immune and stress responses in astrocytes, immune responses and developmental processes in microglia, metabolic processes and immune responses in neurons, and metabolic processes and gene expression in macrophages. Overrepresented GO terms for downregulated genes included ion transport and behavior in astrocytes, cell cycle and cell division in microglia, metabolic processes in neurons, and response to stimulus, signaling and cell motility in macrophages.

CONCLUSIONS

To our knowledge, this is the first transcriptomic study of TLR2 function across different cell types during T. gondii infection. Results of RNA-sequencing demonstrated roles for TLR2 varied by cell type during T. gondii infection. Our findings facilitate understanding of the detailed relationship between TLR2 and T. gondii infection, and elucidate mechanisms underlying neurological changes during infection.

Structural and biochemical characterization of the cell fate determining nucleotidyltransferase fold protein MAB21L1

The exceptionally conserved metazoan MAB21 proteins are implicated in cell fate decisions and share considerable sequence homology with the cyclic GMP-AMP synthase. cGAS is the major innate immune sensor for cytosolic DNA and produces the second messenger 2′-5′, 3′-5′ cyclic GMP-AMP. Little is known about the structure and biochemical function of other proteins of the cGAS-MAB21 subfamily, such as MAB21L1, MAB21L2 and MAB21L3. We have determined the crystal structure of human full-length MAB21L1. Our analysis reveals high structural conservation between MAB21L1 and cGAS but also uncovers important differences. Although monomeric in solution, MAB21L1 forms a highly symmetric double-pentameric oligomer in the crystal, raising the possibility that oligomerization could be a feature of MAB21L1. In the crystal, MAB21L1 is in an inactive conformation requiring a conformational change - similar to cGAS - to develop any nucleotidyltransferase activity. Co-crystallization with NTP identified a putative ligand binding site of MAB21 proteins that corresponds to the DNA binding site of cGAS. Finally, we offer a structure-based explanation for the effects of MAB21L2 mutations in patients with eye malformations. The underlying residues participate in fold-stabilizing interaction networks and mutations destabilize the protein. In summary, we provide a first structural framework for MAB21 proteins.