FGFR2 mutations in bent bone dysplasia syndrome activate nucleolar stress and perturb cell fate determination

Fibroblast growth factor 2

Fibroblast Growth Factor 2 (FGF2), also known as basic fibroblast growth factor, is a potent stimulator of growth and differentiation in multiple tissues. Its discovery traces back over 50 years ago when it was first isolated from bovine pituitary extracts due to its ability to stimulate fibroblast proliferation. Subsequent studies investigating the genomic structure of FGF2 identified multiple protein isoforms, categorized as the low molecular weight and high molecular weight FGF2. These isoforms arise from alternative translation initiation events and exhibit unique molecular and cellular functions. In this concise review, we aim to provide an overview of what is currently known about the structure, expression, and functions of the FGF2 isoforms within the contexts of development, homeostasis, and disease.

Knockout of the intellectual disability-linked gene Hs6st2 in mice decreases heparan sulfate 6-O-sulfation, impairs dendritic spines of hippocampal neurons, and affects memory

Heparan sulfate (HS) is a linear polysaccharide that plays a key role in cellular signaling networks. HS functions are regulated by its 6-O-sulfation, which is catalyzed by three HS 6-O-sulfotransferases (HS6STs). Notably, HS6ST2 is mainly expressed in the brain and HS6ST2 mutations are linked to brain disorders, but the underlying mechanisms remain poorly understood. To determine the role of Hs6st2 in the brain, we carried out a series of molecular and behavioral assessments on Hs6st2 knockout mice. We first carried out strong anion exchange-high performance liquid chromatography and found that knockout of Hs6st2 moderately decreases HS 6-O-sulfation levels in the brain. We then assessed body weights and found that Hs6st2 knockout mice exhibit increased body weight, which is associated with abnormal metabolic pathways. We also performed behavioral tests and found that Hs6st2 knockout mice showed memory deficits, which recapitulate patient clinical symptoms. To determine the molecular mechanisms underlying the memory deficits, we used RNA sequencing to examine transcriptomes in two memory-related brain regions, the hippocampus and cerebral cortex. We found that knockout of Hs6st2 impairs transcriptome in the hippocampus, but only mildly in the cerebral cortex. Furthermore, the transcriptome changes in the hippocampus are enriched in dendrite and synapse pathways. We also found that knockout of Hs6st2 decreases HS levels and impairs dendritic spines in hippocampal CA1 pyramidal neurons. Taken together, our study provides novel molecular and behavioral insights into the role of Hs6st2 in the brain, which facilitates a better understanding of HS6ST2 and HS-linked brain disorders.

Dynamic regulation and requirement for ribosomal RNA transcription during mammalian development

Ribosomal RNA (rRNA) transcription by RNA polymerase I (Pol I) is a critical rate-limiting step in ribosome biogenesis, which is essential for cell survival. Despite its global function, disruptions in ribosome biogenesis cause tissue-specific birth defects called ribosomopathies, which frequently affect craniofacial development. Here, we describe a cellular and molecular mechanism underlying the susceptibility of craniofacial development to disruptions in Pol I transcription. We show that Pol I subunits are highly expressed in the neuroepithelium and neural crest cells (NCCs), which generate most of the craniofacial skeleton. High expression of Pol I subunits sustains elevated rRNA transcription in NCC progenitors, which supports their high tissue-specific levels of protein translation, but also makes NCCs particularly sensitive to rRNA synthesis defects. Consistent with this model, NCC-specific deletion of Pol I subunits Polr1a, Polr1c, and associated factor Tcof1 in mice cell-autonomously diminishes rRNA synthesis, which leads to p53 protein accumulation, resulting in NCC apoptosis and craniofacial anomalies. Furthermore, compound mutations in Pol I subunits and associated factors specifically exacerbate the craniofacial anomalies characteristic of the ribosomopathies Treacher Collins syndrome and Acrofacial Dysostosis-Cincinnati type. Mechanistically, we demonstrate that diminished rRNA synthesis causes an imbalance between rRNA and ribosomal proteins. This leads to increased binding of ribosomal proteins Rpl5 and Rpl11 to Mdm2 and concomitantly diminished binding between Mdm2 and p53. Altogether, our results demonstrate a dynamic spatiotemporal requirement for rRNA transcription during mammalian cranial NCC development and corresponding tissue-specific threshold sensitivities to disruptions in rRNA transcription in the pathogenesis of congenital craniofacial disorders.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Nucleolin loss of function leads to aberrant Fibroblast Growth Factor signaling and craniofacial anomalies

Ribosomal RNA (rRNA) transcription and ribosome biogenesis are global processes required for growth and proliferation of all cells, yet perturbation of these processes in vertebrates leads to tissue-specific defects termed ribosomopathies. Mutations in rRNA transcription and processing proteins often lead to craniofacial anomalies; however, the cellular and molecular reasons for these defects are poorly understood. Therefore, we examined the function of the most abundant nucleolar phosphoprotein, Nucleolin (Ncl), in vertebrate development. ncl mutant (ncl-/-) zebrafish present with craniofacial anomalies such as mandibulofacial hypoplasia. We observed that ncl-/- mutants exhibited decreased rRNA synthesis and p53-dependent apoptosis, consistent with a role in ribosome biogenesis. However, we found that Nucleolin also performs functions not associated with ribosome biogenesis. We discovered that the half-life of fgf8a mRNA was reduced in ncl-/- mutants, which perturbed Fgf signaling, resulting in misregulated Sox9a-mediated chondrogenesis and Runx2-mediated osteogenesis. Consistent with this model, exogenous FGF8 treatment significantly rescued the cranioskeletal phenotype in ncl-/- zebrafish, suggesting that Nucleolin regulates osteochondroprogenitor differentiation. Our work has therefore uncovered tissue-specific functions for Nucleolin in rRNA transcription and post-transcriptional regulation of growth factor signaling during embryonic craniofacial development.

Pulling back the curtain: The hidden functions of receptor tyrosine kinases in development

Receptor tyrosine kinases (RTKs) are a conserved superfamily of transmembrane growth factor receptors that drive numerous cellular processes during development and in the adult. Upon activation, multiple adaptors and signaling effector proteins are recruited to binding site motifs located within the intracellular domain of the RTK. These RTK-effector interactions drive subsequent intracellular signaling cascades involved in canonical RTK signaling. Genetic dissection has revealed that alleles of Fibroblast Growth Factor receptors (FGFRs) that lack all canonical RTK signaling still retain some kinase-dependent biological activity. Here we examine how genetic analysis can be used to understand the mechanism by which RTKs drive multiple developmental processes via canonical signaling while revealing noncanonical activities. Recent data from both FGFRs and other RTKs highlight potential noncanonical roles in cell adhesion and nuclear signaling. The data supporting such functions are discussed as are recent technologies that have the potential to provide valuable insight into the developmental significance of these noncanonical activities.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Integrated Transcriptome and Network Analysis Reveals Spatiotemporal Dynamics of Calvarial Suturogenesis

Craniofacial abnormalities often involve sutures, the growth centers of the skull. To characterize the organization and processes governing their development, we profile the murine frontal suture, a model for sutural growth and fusion, at the tissue- and single-cell level on embryonic days (E)16.5 and E18.5. For the wild-type suture, bulk RNA sequencing (RNA-seq) analysis identifies mesenchyme-, osteogenic front-, and stage-enriched genes and biological processes, as well as alternative splicing events modifying the extracellular matrix. Single-cell RNA-seq analysis distinguishes multiple subpopulations, of which five define a mesenchyme-osteoblast differentiation trajectory and show variation along the anteroposterior axis. Similar analyses of in vivo mouse models of impaired frontal suturogenesis in Saethre-Chotzen and Apert syndromes, Twist1+/- and Fgfr2+/S252W, demonstrate distinct transcriptional changes involving angiogenesis and ribogenesis, respectively. Co-expression network analysis reveals gene expression modules from which we validate key driver genes regulating osteoblast differentiation. Our study provides a global approach to gain insights into suturogenesis.

Fibroblast growth factors signaling in bone metastasis

Many solid tumors metastasize to bone, but only prostate cancer has bone as a single, dominant metastatic site. Recently, the FGF axis has been implicated in cancer progression in some tumors and mounting evidence indicate that it mediates prostate cancer bone metastases. The FGF axis has an important role in bone biology and mediates cell-to-cell communication. Therefore, we discuss here basic concepts of bone biology, FGF signaling axis, and FGF axis function in adult bone, to integrate these concepts in our current understanding of the role of FGF axis in bone metastases.

tp53-dependent and independent signaling underlies the pathogenesis and possible prevention of Acrofacial Dysostosis-Cincinnati type

Ribosome biogenesis is a global process required for growth and proliferation in all cells, but disruptions in this process surprisingly lead to tissue-specific phenotypic disorders termed ribosomopathies. Pathogenic variants in the RNA Polymerase (Pol) I subunit POLR1A cause Acrofacial Dysostosis-Cincinnati type, which is characterized by craniofacial and limb anomalies. In a zebrafish model of Acrofacial Dysostosis-Cincinnati type, we demonstrate that polr1a-/- mutants exhibit deficient 47S rRNA transcription, reduced monosomes and polysomes and, consequently, defects in protein translation. This results in Tp53-dependent neuroepithelial apoptosis, diminished neural crest cell proliferation and cranioskeletal anomalies. This indicates that POLR1A is critical for rRNA transcription, which is considered a rate limiting step in ribosome biogenesis, underpinning its requirement for neuroepithelial cell and neural crest cell proliferation and survival. To understand the contribution of the Tp53 pathway to the pathogenesis of Acrofacial Dysostosis-Cincinnati type, we genetically inhibited tp53 in polr1a-/- mutant embryos. Tp53 inhibition suppresses neuroepithelial apoptosis and partially ameliorates the polr1a mutant phenotype. However, complete rescue of cartilage development is not observed due to the failure to improve rDNA transcription and neural crest cell proliferation. Altogether, these data reveal specific functions for both Tp53-dependent and independent signaling downstream of polr1a in ribosome biogenesis during neural crest cell and craniofacial development, in the pathogenesis of Acrofacial Dysostosis-Cincinnati type. Furthermore, our work sets the stage for identifying Tp53-independent therapies to potentially prevent Acrofacial dysostosis-Cincinnati type and other similar ribosomopathies.

Nuclear Fibroblast Growth Factor Receptor Signaling in Skeletal Development and Disease

PURPOSE OF REVIEW

Fibroblast growth factor receptor (FGFR) signaling regulates proliferation and differentiation during development and homeostasis. While membrane-bound FGFRs play a central role in these processes, the function of nuclear FGFRs is also critical. Here, we highlight mechanisms for nuclear FGFR translocation and the effects of nuclear FGFRs on skeletal development and disease.

RECENT FINDINGS

Full-length FGFRs, internalized by endocytosis, enter the nucleus through β-importin-dependent mechanisms that recognize the nuclear localization signal within FGFs. Alternatively, soluble FGFR intracellular fragments undergo nuclear translocation following their proteolytic release from the membrane. FGFRs enter the nucleus during the cellular transition between proliferation and differentiation. Once nuclear, FGFRs interact with chromatin remodelers to alter the epigenetic state and transcription of their target genes. Dysregulation of nuclear FGFR is linked to the etiology of congenital skeletal disorders and neoplastic transformation. Revealing the activities of nuclear FGFR will advance our understanding of 20 congenital skeletal disorders caused by FGFR mutations, as well as FGFR-related cancers.

Nuclear FGFR2 regulates musculoskeletal integration within the developing limb

BACKGROUND

Bent bone dysplasia syndrome (BBDS), a congenital skeletal disorder caused by dominant mutations in fibroblast growth factor receptor 2 (FGFR2), is characterized by bowed long bones within the limbs. We previously showed that the FGFR2 mutations in BBDS enhance nuclear and nucleolar localization of the receptor; however, exactly how shifts in subcellular distribution of FGFR2 affect limb development remained unknown.

RESULTS

Targeted expression of the BBDS mutations in the lateral plate mesoderm of the developing chick induced angulated hindlimbs, a hallmark feature of the disease. Whole-mount analysis of the underlying skeleton revealed bent long bones with shortened bone collars and, in severe cases, dysmorphic epiphyses. Epiphyseal changes were also correlated with joint dislocations and contractures. Histological analysis revealed that bent long bones and joint defects were closely associated with irregularities in skeletal muscle patterning and tendon-to-bone attachment. The spectrum of limb phenotypes induced by the BBDS mutations were recapitulated by targeted expression of wild-type FGFR2 appended with nuclear and nucleolar localization signals.

CONCLUSIONS

Our results indicate that the bent long bones in BBDS arise from disruptions in musculoskeletal integration and that increased nuclear and nucleolar localization of FGFR2 plays a mechanistic role in the disease phenotype. 248:233-246, 2019. © 2018 Wiley Periodicals, Inc.

Fibroblast growth factors in skeletal development

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are expressed throughout all stages of skeletal development. In the limb bud and in cranial mesenchyme, FGF signaling is important for formation of mesenchymal condensations that give rise to bone. Once skeletal elements are initiated and patterned, FGFs regulate both endochondral and intramembranous ossification programs. In this chapter, we review functions of the FGF signaling pathway during these critical stages of skeletogenesis, and explore skeletal malformations in humans that are caused by mutations in FGF signaling molecules.