Identification of the genes encoding candidate septate junction components expressed during early development of the sea urchin, Strongylocentrotus purpuratus, and evidence of a role for Mesh in the formation of the gut barrier

Septate junctions (SJs) evolved as cell-cell junctions that regulate the paracellular barrier and integrity of epithelia in invertebrates. Multiple morphological variants of SJs exist specific to different epithelia and/or phyla but the biological significance of varied SJ morphology is unclear because the knowledge of the SJ associated proteins and their functions in non-insect invertebrates remains largely unknown. Here we report cell-specific expression of nine candidate SJ genes in the early life stages of the sea urchin Strongylocentrotus purpuratus. By use of in situ RNA hybridization and single cell RNA-seq we found that the expression of selected genes encoding putatively SJ associated transmembrane and cytoplasmic scaffold molecules was dynamically regulated during epithelial development in the embryos and larvae with different epithelia expressing different cohorts of SJ genes. We focused a functional analysis on SpMesh, a homolog of the Drosophila smooth SJ component Mesh, which was highly enriched in the endodermal epithelium of the mid- and hindgut. Functional perturbation of SpMesh by both CRISPR/Cas9 mutagenesis and vivo morpholino-mediated knockdown shows that loss of SpMesh does not disrupt the formation of the gut epithelium during gastrulation. However, loss of SpMesh resulted in a severely reduced gut-paracellular barrier as quantitated by increased permeability to 3-5 ​kDa FITC-dextran. Together, these studies provide a first look at the molecular SJ physiology during the development of a marine organism and suggest a shared role for Mesh-homologous proteins in forming an intestinal barrier in invertebrates. Results have implications for consideration of the traits underlying species-specific sensitivity of marine larvae to climate driven ocean change.

Transcriptional and functional motifs defining renal function revealed by single-nucleus RNA sequencing

Recent advances in single-cell sequencing provide a unique opportunity to gain novel insights into the diversity, lineage, and functions of cell types constituting a tissue/organ. Here, we performed a single-nucleus study of the adult Drosophila renal system, consisting of Malpighian tubules and nephrocytes, which shares similarities with the mammalian kidney. We identified 11 distinct clusters representing renal stem cells, stellate cells, regionally specific principal cells, garland nephrocyte cells, and pericardial nephrocytes. Characterization of the transcription factors specific to each cluster identified fruitless (fru) as playing a role in stem cell regeneration and Hepatocyte nuclear factor 4 (Hnf4) in regulating glycogen and triglyceride metabolism. In addition, we identified a number of genes, including Rho guanine nucleotide exchange factor at 64C (RhoGEF64c), Frequenin 2 (Frq2), Prip, and CG1093 that are involved in regulating the unusual star shape of stellate cells. Importantly, the single-nucleus dataset allows visualization of the expression at the organ level of genes involved in ion transport and junctional permeability, providing a systems-level view of the organization and physiological roles of the tubules. Finally, a cross-species analysis allowed us to match the fly kidney cell types to mouse kidney cell types and planarian protonephridia, knowledge that will help the generation of kidney disease models. Altogether, our study provides a comprehensive resource for studying the fly kidney.

Blending physiology and RNAseq to provide new insights into regulation of epithelial transport: switching between ion secretion and reabsorption

This Review addresses the means by which epithelia change the direction of vectorial ion transport. Recent studies have revealed that insect Malpighian (renal) tubules can switch from secreting to reabsorbing K+. When the gut of larval lepidopterans is empty (during the moult cycle) or when the larvae are reared on K+-deficient diet, the distal ileac plexus segment of the tubule secretes K+ from the haemolymph into the tubule lumen. By contrast, in larvae reared on K+-rich diet, ions and fluid are reabsorbed from the rectal lumen into the perinephric space surrounding the cryptonephridial tubules of the rectal complex. Ions and fluid are then transported from the perinephric space into the lumen of the cryptonephridial tubules, thus supplying the free segments of the tubule downstream. Under these conditions, some of the K+ and water in the tubule lumen is reabsorbed across the cells of the distal ileac plexus, allowing for expansion of haemolymph volume in the rapidly growing larvae, as well as recycling of K+ and base equivalents. RNA sequencing data reveal large-scale changes in gene transcription that are associated with the switch between ion secretion and ion reabsorption by the distal ileac plexus. An unexpected finding is the presence of voltage-gated, ligand-gated and mechanosensitive ion channels, normally seen in excitable cells, in Malpighian tubules. Transcriptomic surveys indicate that these types of channels are also present in multiple other types of vertebrate and invertebrate epithelia, suggesting that they may play novel roles in epithelial cell signalling and regulation of epithelial ion transport.

Voltage-gated calcium channels regulate K+ transport in the Malpighian tubules of the larval cabbage looper, Trichoplusia ni

Transporting epithelia are tissues that specialize in the directional movements of ions and water and are typically either secretory or reabsorptive. Recent work on the Malpighian tubule of larval lepidopterans (caterpillars) demonstrated that the distal ileac plexus segment of this epithelium is capable of rapidly switching between ion secretion and reabsorption. Subsequent transcriptomic studies suggested expression of voltage-gated ion channels in the lepidopteran MTs (which are not contractile and not innervated). The present study shows that isolated MTs of larval Trichoplusia ni express α1, β2, and α2δ4 subunits of voltage-gated Ca²⁺ channel Ca_V1 and that pan-Ca_Vα immunoreactivity is present in the apical and basolateral membranes of the principal cells. Basolateral membrane potential (V_bl) in isolated MTs of larval Trichoplusia ni was influenced by Ca_V1 functioning; pharmacological inhibition of Ca_V1 reversed V_bl from inside-negative to inside-positive, and also reduced transepithelial potential (V_te), lowered [Ca²⁺]_i and reversed the direction of K⁺ transport from secretion to reabsorption. Thus, our findings indicate that a functional Ca_V1 channel is necessary for constitutive K⁺ secretion observed in isolated preparations of lepidopteran MTs. Lastly, V_te and V_bl of isolated MTs were influenced by changes in bathing saline [K⁺]. Our findings suggest that epithelia may rely on Ca_V channels to enable robust ion secretion and downregulation of Ca_V channels, together with other transcriptional changes, enables ion reabsorption.

Help, there are ‘omics’ in my comparative physiology!

‘Omics’ methods, such as transcriptomics, proteomics, lipidomics or metabolomics, yield simultaneous measurements of many related molecules in a sample. These approaches have opened new opportunities to generate and test hypotheses about the mechanisms underlying biochemical and physiological phenotypes. In this Commentary, we discuss general approaches and considerations for successfully integrating omics into comparative physiology. The choice of omics approach will be guided by the availability of existing resources and the time scale of the process being studied. We discuss the use of whole-organism extracts (common in omics experiments on small invertebrates) because such an approach may mask underlying physiological mechanisms, and we consider the advantages and disadvantages of pooling samples within biological replicates. These methods can bring analytical challenges, so we describe the most easily analyzed omics experimental designs. We address the propensity of omics studies to digress into ‘fishing expeditions’ and show how omics can be used within the hypothetico-deductive framework. With this Commentary, we hope to provide a roadmap that will help newcomers approach omics in comparative physiology while avoiding some of the potential pitfalls, which include ambiguous experiments, long lists of candidate molecules and vague conclusions.

The septate junction protein Mesh is required for epithelial morphogenesis, ion transport, and paracellular permeability in the Drosophila Malpighian tubule

Septate junctions (SJs) are occluding cell-cell junctions that have roles in paracellular permeability and barrier function in the epithelia of invertebrates. Arthropods have two types of SJs, pleated SJs and smooth SJs (sSJs). In Drosophila melanogaster, sSJs are found in the midgut and Malpighian tubules, but the functions of sSJs and their protein components in the tubule epithelium are unknown. Here we examined the role of the previously identified integral sSJ component, Mesh, in the Malpighian tubule. We genetically manipulated mesh specifically in the principal cells of the tubule at different life stages. Tubules of flies with developmental mesh knockdown revealed defects in epithelial architecture, sSJ molecular and structural organization, and lack of urine production in basal and kinin-stimulated conditions, resulting in edema and early adult lethality. Knockdown of mesh during adulthood did not disrupt tubule epithelial and sSJ integrity but decreased the transepithelial potential, diminished transepithelial fluid and ion transport, and decreased paracellular permeability to 4-kDa dextran. Drosophila kinin decreased transepithelial potential and increased chloride permeability, and it stimulated fluid secretion in both control and adult mesh knockdown tubules but had no effect on 4-kDa dextran flux. Together, these data indicate roles for Mesh in the developmental maturation of the Drosophila Malpighian tubule and in ion and macromolecular transport in the adult tubule.

Mechanisms and regulation of chloride transport in the Malpighian tubules of the larval cabbage looper Trichoplusia ni

Malpighian tubules (MTs) and the hindgut together constitute the excretory system of insects. Larvae of lepidopterans (butterflies and moths) demonstrate the so-called cryptonephric arrangement, where the distal blind end of each MT is embedded into the rectal complex. The rest of the free tubule is modified into several distinct regions that differ greatly in the transport of cations and water. However, relatively little is known about the transport of counter-anions (e.g., Cl^- and HCO₃^-) by the MTs of lepidopteran larvae. In the current study we used ion-selective microelectrodes to characterize Cl^- transport in the distinct regions of the free MT of the larval Trichoplusia ni. Firstly, we note that Cl^- transport in the MTs is sensitive to the Cl^- concentration of the bathing saline, and several regions of the MTs are capable of either secreting or reabsorbing Cl^-. In the distal ileac plexus (DIP), a region previously characterized by cellular heterogeneity and its ability to switch between cation secretion and reabsorption, principal cells (PCs) toggled between Cl^- reabsorption (in high-Cl^- saline) and Cl^- secretion (in low-Cl^- saline). In contrast, secondary cells (SCs) in the DIP secreted Cl^- regardless of saline Cl^- concentration. Mechanistically, we have detected a number of 'leak' and ligand-gated Cl^- channels (ClC) and demonstrated that Cl^- channels are involved in Cl^- secretion. Additionally, we demonstrated that the lumen-positive transepithelial potential increased in response to glycine. Using the scanning ion-selective electrode technique we demonstrated that glycine stimulated Cl^- secretion by SCs, but not by PCs. In contrast, when MTs were deprived of glycine, a decrease in Cl^- secretion, coupled with a decrease in the TEP, was observed. In contrast to the effects of glycine, an active dose of helicokinin reduced Cl^- secretion by PCs, but not by SCs. Lastly, we detected expression of chloride-bicarbonate exchangers (CBE) in all regions of the free tubule. Scans of H⁺ transport across the tubule indicated that base equivalents are likely reabsorbed across the ileac plexus. Blocking ClC or CBE led to secretion of a more basic fluid, indicating lack of base reabsorption. We suggest that the transport of Cl^- in the MTs of larval lepidopterans (i) may be correlated with the reabsorption of base, (ii) may be sensitive to Cl^- concentration in the haemolymph, and (iii) could be regulated by helicokinin and glycine.

Malpighian tubules of caterpillars: blending RNAseq and physiology to reveal regional functional diversity and novel epithelial ion transport control mechanisms

Malpighian tubules (MTs) and hindgut constitute the functional kidney of insects. MTs are outpouches of the gut and in most insects demonstrate proximodistal heterogeneity in function. In most insects, such heterogeneity is confined to ion/fluid secretion in the distal portion and ion/fluid reabsorption in the proximal portion. In contrast, MTs of larval Lepidoptera (caterpillars of butterflies and moths), are comprised of five regions that differ in their association with the gut, their structure, and ion/fluid transport function. Recent studies have shown that several regions can rapidly and reversibly switch between ion secretion and reabsorption. The current study employed RNAseq, pharmacology and electrophysiology to characterize four distinct regions of the MT in larval Trichoplusia ni. Luminal microelectrode measurements indicate changes in [K+], [Na+] and pH as fluid passes through different regions of the tubule. In addition, the regions examined differ in gene ontology enrichment, and demonstrate robust gradients in expression of ion transporters and endocrine ligand receptors. Lastly, the study provides evidence for direct involvement of voltage-gated and ligand-gated ion channels in epithelial ion transport of insect MTs.