Xenopus epidermal and endodermal epithelia as models for mucociliary epithelial evolution, disease, and metaplasia

Interspecific comparisons of anuran embryonic epidermal landscapes and energetic trade-offs in response to changes in salinity

BACKGROUND

Freshwater salinization is an emerging stressor in amphibian populations, and embryonic stages are most vulnerable. To better understand the variation in embryonic osmoregulation, we challenged embryos of two phylogenetically diverse anuran species, Xenopus laevis and Lithobates (Rana) sylvaticus, along a gradient of non-lethal salinities. We hypothesized embryos at higher salinities will display epidermal plasticity as a coping response and increase energy expenditure related to osmoregulation demands, thereby reducing energy for growth and development.

RESULTS

Scanning electron microscopy revealed an extra mucus-secreting cell type and higher ionocyte proportions in the X. laevis epidermis, suggesting more osmoregulatory machinery than L. sylvaticus. Under elevated salinity, X. laevis displayed greater increases in goblet cell proportions, mucus secretion, and reductions in ionocyte apical area compared with L. sylvaticus. Although both species increased oxygen consumption rates and reduced body length with elevated salinity, these effects were proportionally greater in L. sylvaticus at the highest salinity, and only this species slowed developmental rates.

CONCLUSION

These findings support the hypothesis that frog embryos respond to salinity by altering the cellular landscape of their epidermis. We show that epidermal cell types, as well as the magnitude of epidermal plasticity and energetic trade-offs in response to salinity, vary among amphibian species.

Spontaneous Calcium Bursts Organize the Apical Actin Cytoskeleton of Multiciliated Cells

Motile cilia perform crucial functions during embryonic development and in adult tissues. They are anchored by an apical actin network that forms microridge-like structures on the surface of multiciliated cells. Using Xenopus as a model system to investigate the mechanisms underlying the formation of these specialized actin structures, we observed stochastic bursts of intracellular calcium concentration in developing multiciliated cells. Through optogenetic manipulation of calcium signaling, we found that individual calcium bursts triggered the fusion and extension of actin structures by activating non-muscle myosin. Repeated cycles of calcium activation promoted assembly and coherence of the maturing apical actin network. Inhibition of the endogenous inositol triphosphate-calcium pathway disrupted the formation of apical actin/microridge-like structures by reducing local centriolar RhoA signaling. This disruption was rescued by transient expression of constitutively active RhoA in multiciliated cells. Our findings identify repetitive calcium bursts as a driving force that promotes the self-organization of the highly specialized actin cytoskeleton of multiciliated cells.

Convergence of autism proteins at the cilium

Hundreds of high-confidence autism genes have been identified, yet the relevant etiological mechanisms remain unclear. Gene ontology analyses have repeatedly identified enrichment of proteins with annotated functions in gene expression regulation and neuronal communication. However, proteins are often pleiotropic and these annotations are inherently incomplete. Our recent autism functional genetics work has suggested that these genes may share a common mechanism at the cilium, a membrane-bound organelle critical for neurogenesis, brain patterning, and neuronal activity-all processes strongly implicated in autism. Moreover, autism commonly co-occurs with conditions that are known to involve ciliary-related pathologies, including congenital heart disease, hydrocephalus, and blindness. However, the role of autism genes at the cilium has not been systematically investigated. Here we demonstrate that autism proteins spanning disparate functional annotations converge in expression, localization, and function at cilia, and that patients with pathogenic variants in these genes have cilia-related co-occurring conditions and biomarkers of disrupted ciliary function. This degree of convergence among genes spanning diverse functional annotations strongly suggests that cilia are relevant to autism, as well as to commonly co-occurring conditions, and that this organelle should be explored further for therapeutic potential.

Interaction between perfluoro-octanoic sulfonate and common antibiotics induces developmental anomalies and lethality in Xenopus laevis

BACKGROUND

Perfluoroalkyl substances (PFAS) are persistent environmental contaminants previously used for industrial purposes as a non-stick coating and flame retardant. The stability of these molecules prevents their breakdown, which results in ground water contamination across the globe. Perfluoroalkyl substances molecules are known to bioaccumulate in various organisms. However, the health consequences remain unclear due to the large number of molecules in the PFAS family and different effects on various tissues. Here, we use the frog Xenopus laevis to investigate the developmental consequences of exposure to the PFAS molecule perfluoro-octanoic sulfonate (PFOS).

RESULTS

We find that exposure to high levels of PFOS results in significant axial shortening of developing tadpoles. Further, we find that PFOS exposure results in a dose-dependent formation of a cellular mass in the dorsal fin. Unexpectedly, we found that these developmental phenotypes are exacerbated upon co-exposure with commonly used antibiotics. Specifically, PFOS and gentamicin co-treatment results in increased apoptosis, loss of cellular integrity, and increased overall lethality.

CONCLUSIONS

Our results suggest a mechanism whereby gentamicin reaches levels that are toxic to mitochondria only in the presence of PFOS. These findings add to our understanding of PFOS exposure to vertebrate development and present an added concern with potential interactions with antibiotics.

Cilia as Wnt signaling organelles

Cilia and Wnt signaling have a complex relationship, wherein Wnt regulates cilia and, conversely, cilia may affect Wnt signaling. Recently, it was shown that Wnt receptors are present in flagella, primary cilia, and multicilia, where they transmit an intraciliary signal that is independent of β-catenin. Intraciliary Wnt signaling promotes ciliogenesis, affecting male fertility, adipogenesis, and mucociliary clearance. Wnt also stimulates the beating of motile cilia, highlighting that these nanomotors, too, are chemosensory. Intraciliary Wnt signaling employs a Wnt-protein phosphatase 1 (PP1) signaling axis, involving the canonical Wnt pathway's inhibition of glycogen synthase kinase 3 (GSK3) to repress PP1 activity. Collectively, these findings support that cilia are Wnt signaling organelles, with implications for ciliopathies and cancer.

The Tmem16a chloride channel is required for mucin maturation after secretion from goblet-like cells in the Xenopus tropicalis tadpole skin

The TMEM16A chloride channel is proposed as a therapeutic target in cystic fibrosis, where activation of this ion channel might restore airway surface hydration and mitigate respiratory symptoms. While TMEM16A is associated with increased mucin production under stimulated or pro-inflammatory conditions, its role in baseline mucin production, secretion and/or maturation is less well understood. Here, we use the Xenopus tadpole skin mucociliary surface as a model of human upper airway epithelium to study Tmem16a function in mucus production. We found that Xenopus tropicalis Tmem16a is present at the apical membrane surface of tadpole skin small secretory cells that express canonical markers of mammalian "goblet cells" such as Foxa1 and spdef. X. tropicalis Tmem16a functions as a voltage-gated, calcium-activated chloride channel when transfected into mammalian cells in culture. Depletion of Tmem16a from the tadpole skin results in dysregulated mucin maturation post-secretion, with secreted mucins having a disrupted molecular size distribution and altered morphology assessed by sucrose gradient centrifugation and electron microscopy, respectively. Our results show that in the Xenopus tadpole skin, Tmem16a is necessary for normal mucus barrier formation and demonstrate the utility of this model system to discover new biology relevant to human mucosal biology in health and disease.

Genetic mechanisms of multiciliated cell development: from fate choice to differentiation in zebrafish and other models

Multiciliated cells (MCCS) form bundles of cilia and their activities are essential for the proper development and physiology of many organ systems. Not surprisingly, defects in MCCs have profound consequences and are associated with numerous disease states. Here, we discuss the current understanding of MCC formation, with a special focus on the genetic and molecular mechanisms of MCC fate choice and differentiation. Furthermore, we cast a spotlight on the use of zebrafish to study MCC ontogeny and several recent advances made in understanding MCCs using this vertebrate model to delineate mechanisms of MCC emergence in the developing kidney.

Mebendazole preferentially inhibits cilia formation and exerts anticancer activity by synergistically augmenting DNA damage

The cilium is a microtubule-based organelle that plays a pivotal role in embryonic development and maintenance of physiological functions in the human body. In addition to their function as sensors that transduce diverse extracellular signals, including growth factors, fluid flow, and physical forces, cilia are intricately involved in cell cycle regulation and preservation of DNA integrity, as their formation and resorption dynamics are tightly linked to cell cycle progression. Recently, several studies have linked defects in specific ciliary proteins to the DNA damage response. However, it remains unclear whether and how primary cilia contribute to cancer development. Mebendazole (MBZ) is an anthelmintic drug with anticancer properties in some cancer cells. MBZ is continuously being tested for clinical studies, but the precise mechanism of its anticancer activities remains unknown. Here, using Xenopus laevis embryos as a model system, we discovered that MBZ significantly hinders cilia formation and induces DNA damage. Remarkably, primary cilium-bearing cancer cells exhibited heightened vulnerability to combined treatment with MBZ and conventional anticancer drugs. Our findings shed light on the specific influence of MBZ on cilia, rather than cytosolic microtubules, in triggering DNA damage, elucidating a previously unidentified mechanism underlying potential MBZ-mediated cancer therapy.

Crb3 is required to organize the apical domain of multiciliated cells

Cell shape changes mainly rely on the remodeling of the actin cytoskeleton. Multiciliated cells (MCCs) of the mucociliary epidermis of Xenopus laevis embryos, as they mature, dramatically reshape their apical domain to grow cilia, in coordination with the underlying actin cytoskeleton. Crumbs (Crb) proteins are multifaceted transmembrane apical polarity proteins known to recruit actin linkers and promote apical membrane growth. Here, we identify the homeolog Crb3.L as an important player for the migration of centrioles or basal bodies (collectively centrioles/BBs) and apical domain morphogenesis in MCCs. Crb3.L is present in cytoplasmic vesicles close to the ascending centrioles/BBs, where it partially colocalizes with Rab11a. Crb3.L morpholino-mediated depletion in MCCs caused abnormal migration of centrioles/BBs, a reduction of their apical surface, disorganization of their apical actin meshwork and defective ciliogenesis. Rab11a morpholino-mediated depletion phenocopied Crb3.L loss-of-function in MCCs. Thus, the control of centrioles/BBs migration by Crb3.L might be mediated by Rab11a-dependent apical trafficking. Furthermore, we show that both phospho-activated ERM (pERM; Ezrin-Radixin-Moesin) and Crb3.L are recruited to the growing apical domain of MCCs, where Crb3.L likely anchors pERM, allowing actin-dependent expansion of the apical membrane.

β-adrenergic receptor regulates embryonic epithelial extensibility through actomyosin inhibition

During morphogenesis, epithelial tissues reshape and expand to cover the body and organs. The molecular mechanisms of this deformability remain elusive. Here, we investigate the role of the β-adrenergic receptor (ADRB) in orchestrating actomyosin contractility, pivotal for epithelial extensibility. Chemical screens on Xenopus laevis embryos pinpointed ADRB2 as a principal regulator. ADRB2 promotes actomyosin relaxation, facilitating apical cell area expansion during body elongation. In contrast, ADRB2 knockdown results in heightened cell contraction, marked by synchronous oscillation of F-actin and myosin, impeding body elongation. ADRB2 mutants with reduced affinity for ligand binding lack the function to induce cellular relaxation, highlighting the ligand's essential roles even in the developing epidermis. Our findings unveil ADRB2's critical contribution to extensibility of the epidermis and subsequent body elongation during development. This study also offers insights into the physiology of mature epithelial organs deformed by the smooth muscle response to the adrenergic autonomic nervous system.

Expression and cilia associated localization of Histone deacetylases 6 in Xenopus

Histone deacetylases (HDACs) are key posttranslational modulators of the proteome. We show that expression of histone deacetylase 6 ( hdac6 ) is dynamic and appears in a tissue specific manner throughout embryonic development of the frog Xenopus laevis . Interestingly, hdac6 transcripts often associate with ciliated tissues, like the left-right organizer at neurula stage or the pronephros. In the embryonic skin, Hdac6 protein localizes to the cilia base, suggesting a functional link.

A single-cell, time-resolved profiling of Xenopus mucociliary epithelium reveals nonhierarchical model of development

The specialized cell types of the mucociliary epithelium (MCE) lining the respiratory tract enable continuous airway clearing, with its defects leading to chronic respiratory diseases. The molecular mechanisms driving cell fate acquisition and temporal specialization during mucociliary epithelial development remain largely unknown. Here, we profile the developing Xenopus MCE from pluripotent to mature stages by single-cell transcriptomics, identifying multipotent early epithelial progenitors that execute multilineage cues before specializing into late-stage ionocytes and goblet and basal cells. Combining in silico lineage inference, in situ hybridization, and single-cell multiplexed RNA imaging, we capture the initial bifurcation into early epithelial and multiciliated progenitors and chart cell type emergence and fate progression into specialized cell types. Comparative analysis of nine airway atlases reveals an evolutionary conserved transcriptional module in ciliated cells, whereas secretory and basal types execute distinct function-specific programs across vertebrates. We uncover a continuous nonhierarchical model of MCE development alongside a data resource for understanding respiratory biology.

Mucociliary Wnt signaling promotes cilia biogenesis and beating

It is widely thought that Wnt/Lrp6 signaling proceeds through the cytoplasm and that motile cilia are signaling-inert nanomotors. Contrasting both views, we here show in the mucociliary epidermis of X. tropicalis embryos that motile cilia transduce a ciliary Wnt signal that is distinct from canonical β-catenin signaling. Instead, it engages a Wnt-Gsk3-Ppp1r11-Pp1 signaling axis. Mucociliary Wnt signaling is essential for ciliogenesis and it engages Lrp6 co-receptors that localize to cilia via a VxP ciliary targeting sequence. Live-cell imaging using a ciliary Gsk3 biosensor reveals an immediate response of motile cilia to Wnt ligand. Wnt treatment stimulates ciliary beating in X. tropicalis embryos and primary human airway mucociliary epithelia. Moreover, Wnt treatment improves ciliary function in X. tropicalis ciliopathy models of male infertility and primary ciliary dyskinesia (ccdc108, gas2l2). We conclude that X. tropicalis motile cilia are Wnt signaling organelles that transduce a distinct Wnt-Pp1 response.

Facile methods for reusing laboratory plastic in developmental biology experiments

Plastic pollution negatively affects ecosystems and human health globally, with single-use plastic representing the majority of marine litter in some areas. Life science laboratories prefer pristine conditions for experimental reliability and therefore make use of factory standardized single-use plastic products. This contributes to overall plastic waste in the United States and globally. Here, we investigate the potential of reusing plastic culture dishes and subsequently propose methods to mitigate single-use plastic waste in developmental biology research laboratories. We tested the efficacy of bleach and ethyl alcohol in sterilizing used dishes. We then tested the feasibility of washing and reusing plastic to culture Xenopus laevis embryos subjected to various manipulations. Cleaning and reusing laboratory plastic did not affect the development or survival of X. laevis, indicating that these cleaning methods do not adversely affect experimental outcome and can be used to sterilize plastic before reuse or recycling. Lastly, we performed a survey of various life science laboratories to estimate both waste reduction and savings associated with recycling single-use plastics. Standardization of these procedures would allow research laboratories to benefit economically while practicing environmentally conscious consumption.

Temporal Notch signaling regulates mucociliary cell fates through Hes-mediated competitive de-repression

Tissue functions are determined by the types and ratios of cells present, but little is known about self-organizing principles establishing correct cell type compositions. Mucociliary airway clearance relies on the correct balance between secretory and ciliated cells, which is regulated by Notch signaling across mucociliary systems. Using the airway-like Xenopus epidermis, we investigate how cell fates depend on signaling, how signaling levels are controlled, and how Hes transcription factors regulate cell fates. We show that four mucociliary cell types each require different Notch levels and that their specification is initiated sequentially by a temporal Notch gradient. We describe a novel role for Foxi1 in the generation of Delta-expressing multipotent progenitors through Hes7.1. Hes7.1 is a weak repressor of mucociliary genes and overcomes maternal repression by the strong repressor Hes2 to initiate mucociliary development. Increasing Notch signaling then inhibits Hes7.1 and activates first Hes4, then Hes5.10, which selectively repress cell fates. We have uncovered a self-organizing mechanism of mucociliary cell type composition by competitive de-repression of cell fates by a set of differentially acting repressors. Furthermore, we present an in silico model of this process with predictive abilities.