The positive transcriptional elongation factor (P-TEFb) is required for neural crest specification

Skeletal progenitor LRP1 deficiency causes severe and persistent skeletal defects with Wnt pathway dysregulation

Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional endocytic receptor whose dysfunction is linked to developmental dysplasia of the hip, osteoporosis and osteoarthritis. Our work addresses the critical question of how these skeletal pathologies emerge. Here, we show the abundant expression of LRP1 in skeletal progenitor cells at mouse embryonic stage E10.5 and onwards, especially in the perichondrium, the stem cell layer surrounding developing limbs essential for bone formation. Lrp1 deficiency in these stem cells causes joint fusion, malformation of cartilage/bone template and markedly delayed or lack of primary ossification. These abnormalities, which resemble phenotypes associated with Wnt signalling pathways, result in severe and persistent skeletal defects including a severe deficit in hip joint and patella, and markedly deformed and low-density long bones leading to dwarfism and impaired mobility. Mechanistically, we show that LRP1 regulates core non-canonical Wnt/planar cell polarity (PCP) components that may explain the malformation of long bones. LRP1 directly binds to Wnt5a, facilitates its cell-association and endocytic degradation and recycling. In the developing limbs, LRP1 partially colocalises with Wnt5a and its deficiency alters abundance and distribution of Wnt5a and Vangl2. Finally, using Xenopus as a model system, we show the regulatory role for LRP1 in Wnt/PCP signalling. We propose that in skeletal progenitors, LRP1 plays a critical role in formation and maturity of multiple bones and joints by regulating Wnt signalling, providing novel insights into the fundamental processes of morphogenesis and the emergence of skeletal pathologies.

Dinaciclib inhibits the stemness of two subtypes of human breast cancer cells by targeting the FoxM1 and Hedgehog signaling pathway

Cyclin‑dependent kinase (CDK)4/6 inhibitors in combination with endocrine therapy are the current standard of care used in the first‑line treatment of hormone receptor‑positive/HER2‑negative metastatic breast cancer (BC). Although CDK4/6 inhibitors mainly target the cell cycle, emerging evidence has indicated further potential roles of CDKs other than regulating cell cycle progression. The G₁ and G₂/M transition regulators, including cyclins D and E, as well as their catalytic partners, CDK2, CDK4 and CDK6, have been reported to play crucial roles in pluripotency maintenance and cell fate decisions of human pluripotent stem cells by controlling transcription factors, signaling pathways and epigenetic regulators. Dinaciclib, a CDK1/2/5/9 inhibitor, is currently being evaluated in clinical trials against various cancer types, including BC. However, the underlying molecular mechanisms of CDK1/2/5/9 inhibitors in regulating BC stemness remain poorly understood. The present study aimed to examine the stemness‑inhibitory effects of dinaciclib in MCF‑7 (luminal) and HCC‑1806 (triple‑negative) BC cells. We found that this drug not only effectively reduced the self‑renewal abilities and other malignant properties, but also dose‑dependently decreased the protein expression levels of three BC stem cell markers, CD44, aldehyde dehydrogenase 1 family member A1 (ALDH1A1) and BMI1 proto‑oncogene, polycomb ring finger (Bmi1), as well as three embryonic stem cell markers, Oct4, Nanog and Sox2. Moreover, the dinaciclib‑induced decrease of Oct4 and Nanog protein expression was able to be restored by co‑treatment with MG‑132, a proteasome inhibitor. Forkhead box M1 (FoxM1), both a stemness‑stimulating transcription factor and a cell cycle regulator, along with the Hedgehog signaling pathway, were identified as the therapeutic targets of dinaciclib. Collectively, the present results demonstrated a novel role of dinaciclib in suppressing BC stemness and indicated its potential use for future cancer treatments.

An efficient miRNA knockout approach using CRISPR-Cas9 in Xenopus

In recent years CRISPR-Cas9 knockouts (KO) have become increasingly ultilised to study gene function. MicroRNAs (miRNAs) are short non-coding RNAs, 20-22 nucleotides long, which affect gene expression through post-transcriptional repression. We previously identified miRNAs-196a and -219 as implicated in the development of Xenopus neural crest (NC). The NC is a multipotent stem-cell population, specified during early neurulation. Following EMT, NC cells migrate to various points in the developing embryo where they give rise to a number of tissues including parts of the peripheral nervous system, pigment cells and craniofacial skeleton. Dysregulation of NC development results in many diseases grouped under the term neurocristopathies. As miRNAs are so small, it is difficult to design CRISPR sgRNAs that reproducibly lead to a KO. We have therefore designed a novel approach using two guide RNAs to effectively 'drop out' a miRNA. We have knocked out miR-196a and miR-219 and compared the results to morpholino knockdowns (KD) of the same miRNAs. Validation of efficient CRISPR miRNA KO and phenotype analysis included use of whole-mount in situ hybridization of key NC and neural plate border markers such as Pax3, Xhe2, Sox10 and Snail2, q-RT-PCR and Sanger sequencing. To show specificity we have also rescued the knockout phenotype using miRNA mimics. MiRNA-219 and miR-196a KO's both show loss of NC, altered neural plate and hatching gland phenotypes. Tadpoles show gross craniofacial and pigment phenotypes.

Neural is Fundamental: Neural Stemness as the Ground State of Cell Tumorigenicity and Differentiation Potential

Tumorigenic cells are similar to neural stem cells or embryonic neural cells in regulatory networks, tumorigenicity and pluripotent differentiation potential. By integrating the evidence from developmental biology, tumor biology and evolution, I will make a detailed discussion on the observations and propose that neural stemness underlies two coupled cell properties, tumorigenicity and pluripotent differentiation potential. Neural stemness property of tumorigenic cells can hopefully integrate different observations/concepts underlying tumorigenesis. Neural stem cells and tumorigenic cells share regulatory networks; both exhibit neural stemness, tumorigenicity and pluripotent differentiation potential; both depend on expression or activation of ancestral genes; both rely primarily on aerobic glycolytic metabolism; both can differentiate into various cells/tissues that are derived from three germ layers, leading to tumor formation resembling severely disorganized or more degenerated process of embryonic tissue differentiation; both are enriched in long genes with more splice variants that provide more plastic scaffolds for cell differentiation, etc. Neural regulatory networks, which include higher levels of basic machineries of cell physiological functions and developmental programs, work concertedly to define a basic state with fast cell cycle and proliferation. This is predestined by the evolutionary advantage of neural state, the ground or initial state for multicellularity with adaptation to an ancient environment. Tumorigenesis might represent a process of restoration of neural ground state, thereby restoring a state with fast proliferation and pluripotent differentiation potential in somatic cells. Tumorigenesis and pluripotent differentiation potential might be better understood from understanding neural stemness, and cancer therapy should benefit more from targeting neural stemness.

Using an aquatic model, Xenopus laevis, to uncover the role of chromodomain 1 in craniofacial disorders

The chromodomain family member chromodomain 1 (CHD1) has been shown to have numerous critical molecular functions including transcriptional regulation, splicing, and DNA repair. Complete loss of function of this gene is not compatible with life. On the other hand, missense and copy number variants of CHD1 can result in intellectual disabilities and craniofacial malformations in human patients including cleft palate and Pilarowski-Bjornsson Syndrome. We have used the aquatic developmental model organism Xenopus laevis, to determine a specific role for Chd1 in such cranioafcial disorders. Protein and gene knockdown techniques in Xenopus, including antisense oligos and mosaic Crispr/Cas9-mediated mutagenesis, recapitulated the craniofacial defects observed in humans. Further analysis indicated that embryos deficient in Chd1 had defects in cranial neural crest development and jaw cartilage morphology. Additionally, flow cytometry and immunohistochemistry revealed that decreased Chd1 resulted in increased in apoptosis in the developing head. Together, these experiments demonstrate that Chd1 is critical for fundamental processes and cell survival in craniofacial development. We also presented evidence that Chd1 is regulated by retinoic acid signaling during craniofacial development. Expression levels of chd1 mRNA, specifically in the head, were increased by RAR agonist exposure and decreased upon antagonist treatment. Subphenotypic levels of an RAR antagonist and Chd1 morpholinos synergized to result in orofacial defects. Further, RAR DNA binding sequences (RAREs) were detected in chd1 regulatory regions by bioinformatic analysis. In summary, by combining human genetics and experiments in an aquatic model we now have a better understanding of the role of CHD1 in craniofacial disorders.

The Paf1 complex and P-TEFb have reciprocal and antagonist roles in maintaining multipotent neural crest progenitors

Multipotent progenitor populations are necessary for generating diverse tissue types during embryogenesis. We show the RNA polymerase-associated factor 1 complex (Paf1C) is required to maintain multipotent progenitors of the neural crest (NC) lineage in zebrafish. Mutations affecting each Paf1C component result in near-identical NC phenotypes; alyron mutant embryos carrying a null mutation in paf1 were analyzed in detail. In the absence of zygotic paf1 function, definitive premigratory NC progenitors arise but fail to maintain expression of the sox10 specification gene. The mutant NC progenitors migrate aberrantly and fail to differentiate appropriately. Blood and germ cell progenitor development is affected similarly. Development of mutant NC could be rescued by additional loss of positive transcription elongation factor b (P-TEFb) activity, a key factor in promoting transcription elongation. Consistent with the interpretation that inhibiting/delaying expression of some genes is essential for maintaining progenitors, mutant embryos lacking the CDK9 kinase component of P-TEFb exhibit a surfeit of NC progenitors and their derivatives. We propose Paf1C and P-TEFb act antagonistically to regulate the timing of the expression of genes needed for NC development.

Specifying neural crest cells: From chromatin to morphogens and factors in between

Neural crest (NC) cells are a stem-like multipotent population of progenitor cells that are present in vertebrate embryos, traveling to various regions in the developing organism. Known as the "fourth germ layer," these cells originate in the ectoderm between the neural plate (NP), which will become the brain and spinal cord, and nonneural tissues that will become the skin and the sensory organs. NC cells can differentiate into more than 30 different derivatives in response to the appropriate signals including, but not limited to, craniofacial bone and cartilage, sensory nerves and ganglia, pigment cells, and connective tissue. The molecular and cellular mechanisms that control the induction and specification of NC cells include epigenetic control, multiple interactive and redundant transcriptional pathways, secreted signaling molecules, and adhesion molecules. NC cells are important not only because they transform into a wide variety of tissue types, but also because their ability to detach from their epithelial neighbors and migrate throughout developing embryos utilizes mechanisms similar to those used by metastatic cancer cells. In this review, we discuss the mechanisms required for the induction and specification of NC cells in various vertebrate species, focusing on the roles of early morphogenesis, cell adhesion, signaling from adjacent tissues, and the massive transcriptional network that controls the formation of these amazing cells. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Signaling Pathways > Cell Fate Signaling.

ADAMTS9, a member of the ADAMTS family, in Xenopus development

Extracellular matrix (ECM) remodeling by metalloproteinases is crucial during development. The ADAMTS (A Disintegrin and Metalloproteinase with Thrombospondin type I motifs) enzymes are secreted, multi-domain matrix-associated zinc metalloendopeptidases that have diverse roles in tissue morphogenesis and patho-physiological remodeling. The human family includes 19 members. In this study we identified the 19 members of the ADAMTS family in Xenopus laevis and Xenopus tropicalis. Gene identification and a phylogenetic study revealed strong conservation of the ADAMTS family and contributed to a better annotation of the Xenopus genomes. Expression of the entire ADAMTS family was studied from early stages to tadpole stages of Xenopus, and detailed analysis of ADAMTS9 revealed expression in many structures during organogenesis such as neural crest (NC) derivative tissues, the pronephros and the pancreas. Versican, a matrix component substrate of ADAMTS9 shows a similar expression pattern suggesting a role of ADAMTS9 in the remodeling of the ECM in these structures by degradation of versican.

microRNAs associated with early neural crest development in Xenopus laevis

BACKGROUND

The neural crest (NC) is a class of transitory stem cell-like cells unique to vertebrate embryos. NC cells arise within the dorsal neural tube where they undergo an epithelial to mesenchymal transition in order to migrate and differentiate throughout the developing embryo. The derivative cell types give rise to multiple tissues, including the craniofacial skeleton, peripheral nervous system and skin pigment cells. Several well-studied gene regulatory networks underpin NC development, which when disrupted can lead to various neurocristopathies such as craniofrontonasal dysplasia, DiGeorge syndrome and some forms of cancer. Small RNAs, such as microRNAs (miRNAs) are non-coding RNA molecules important in post-transcriptional gene silencing and critical for cellular regulation of gene expression.

RESULTS

To uncover novel small RNAs in NC development we used high definition adapters and next generation sequencing of libraries derived from ectodermal explants of Xenopus laevis embryos induced to form neural and NC tissue. Ectodermal and blastula animal pole (blastula) stage tissues were also sequenced. We show that miR-427 is highly abundant in all four tissue types though in an isoform specific manner and we define a set of 11 miRNAs that are enriched in the NC. In addition, we show miR-301a and miR-338 are highly expressed in both the NC and blastula suggesting a role for these miRNAs in maintaining the stem cell-like phenotype of NC cells.

CONCLUSION

We have characterised the miRNAs expressed in Xenopus embryonic explants treated to form ectoderm, neural or NC tissue. This has identified novel tissue specific miRNAs and highlighted differential expression of miR-427 isoforms.

The anti-rheumatic drug, leflunomide, synergizes with MEK inhibition to suppress melanoma growth

Cutaneous melanoma, which develops from the pigment producing cells called melanocytes, is the most deadly form of skin cancer. Unlike the majority of other cancers, the incidence rates of melanoma are still on the rise and the treatment options currently available are being hindered by resistance, limited response rates and adverse toxicity. We have previously shown that an FDA approved drug leflunomide, used for rheumatoid arthritis (RA), also holds potential therapeutic value in treating melanoma especially if used in combination with the mutant BRAF inhibitor, vemurafenib. We have further characterized the function of leflunomide and show that the drug reduces the number of viable cells in both wild-type and BRAF^V600E mutant melanoma cell lines. Further experiments have revealed leflunomide reduces cell proliferation and causes cells to arrest in G1 of the cell cycle. Cell death assays show leflunomide causes apoptosis at treatment concentrations of 25 and 50 µM. To determine if leflunomide could be used combinatorialy with other anti-melanoma drugs, it was tested in combination with the MEK inhibitor, selumetinib. This combination showed a synergistic effect in the cell lines tested. This drug combination led to an enhanced decrease in tumor size when tested in vivo compared to either drug alone, demonstrating its potential as a novel combinatorial therapy for melanoma.

Frizzled-7 is required for Xenopus heart development

Wnt signalling regulates cardiogenesis during specification of heart tissue and the morphogenetic movements necessary to form the linear heart. Wnt11-mediated non-canonical signalling promotes early cardiac development whilst Wnt11-R, which is expressed later, also signals through the non-canonical pathway to promote heart development. It is unclear which Frizzled proteins mediate these interactions. Frizzled-7 (fzd7) is expressed during gastrulation in the mesodermal cells fated to become heart, and then in the primary heart field. This expression is complementary to the expression of wnt11 and wnt11-R. We further show co-localisation of fzd7 with other early- and late-heart-specific markers using double in situ hybridisation. We have used loss of function analysis to determine the role of fzd7 during heart development. Morpholino antisense oligonucleotide-mediated knockdown of Fzd7 results in effects on heart development, similar to that caused by Wnt11 loss of function. Surprisingly, overexpression of dominant-negative Fzd7 cysteine rich domain (Fzd7 CRD) results in a cardia bifida phenotype, similar to the loss of wnt11-R phenotype. Overexpression of Fzd7 and activation of non-canonical wnt signalling can rescue the effect of Fzd7 CRD. We propose that Fzd7 has an important role during Xenopus heart development.

Summary: Wnt signalling has been shown to be important in heart development. Here, we demonstrate that the wnt receptor fzd7 is required in mediating these Wnt signals.

CDK9: A key player in cancer and other diseases

Cyclin-Dependent Kinase 9 (CDK9) is part of a functional diverse group of enzymes responsible for cell cycle control and progression. It associates mainly with Cyclin T1 and forms the Positive Transcription Elongation Factor b (p-TEFb) complex responsible for regulation of transcription elongation and mRNA maturation. Recent studies have highlighted the importance of CDK9 in many relevant pathologic processes, like cancer, cardiovascular diseases, and viral replication. Herein we provide an overview of the different pathways in which CDK9 is directly and indirectly involved.