Epithelial immune response in Drosophila malpighian tubules: interplay between Diap2 and ion channels

Compromised junctional integrity phenocopies age-dependent renal dysfunction in Drosophila Snakeskin mutants

Transporting epithelia provide a protective barrier against pathogenic insults while allowing the controlled exchange of ions, solutes and water with the external environment. In invertebrates, these functions depend on formation and maintenance of 'tight' septate junctions (SJs). However, the mechanism by which SJs affect transport competence and tissue homeostasis, and how these are modulated by ageing, remain incompletely understood. Here, we demonstrate that the Drosophila renal (Malpighian) tubules undergo an age-dependent decline in secretory capacity, which correlates with mislocalisation of SJ proteins and progressive degeneration in cellular morphology and tissue homeostasis. Acute loss of the SJ protein Snakeskin in adult tubules induced progressive changes in cellular and tissue architecture, including altered expression and localisation of junctional proteins with concomitant loss of cell polarity and barrier integrity, demonstrating that compromised junctional integrity is sufficient to replicate these ageing-related phenotypes. Taken together, our work demonstrates a crucial link between epithelial barrier integrity, tubule transport competence, renal homeostasis and organismal viability, as well as providing novel insights into the mechanisms underpinning ageing and renal disease.

Aedes aegypti Malpighian tubules are immunologically activated following systemic Toll activation

BACKGROUND

Canine heartworm is a widespread and potentially fatal mosquito-borne disease caused by infections with the parasitic nematode, Dirofilaria immitis. We have previously shown that systemic activation of the Toll immune pathway via silencing of the negative regulator Cactus in Aedes aegypti blocks parasite development in the Malpighian tubules (MT), the mosquito renal organ. However, it was not established whether the MT were directly responding to Toll activation or were alternatively responding to upregulated proteins or other changes to the hemolymph driven by other tissues. Distinguishing these possibilities is crucial for developing more precise strategies to block D. immitis while potentially avoiding the fitness cost to the mosquito associated with Cactus silencing.

METHODS

This study defines the transcriptional response of the MT and changes to the hemolymph proteome of Ae. aegypti after systemic Toll activation via intra-thoracic injection of double-stranded Cactus (dsCactus) RNA.

RESULTS

Malpighian tubules significantly increased expression of the Toll pathway target genes that significantly overlapped expression changes occurring in whole mosquitoes. A significant overlap between the transcriptional response of the MT and proteins upregulated in the hemolymph was also observed.

CONCLUSIONS

Our data show that MT are capable of RNA interference-mediated gene silencing and directly respond to dsCactus treatment by upregulating targets of the canonical Toll pathway. Although not definitive, the strong correspondence between the MT transcriptional response and the hemolymph proteomic responses provides evidence that the MT may contribute to mosquito humoral immunity.

Multiple Functions of Malpighian Tubules in Insects: A Review

The Malpighian Tubules (MTs) are the main excretory organs in most insects. They play a key role in the production of primary urine and osmoregulation, selectively reabsorbing water, ions, and solutes. Besides these functions conserved in most insects, MTs can serve some specialized tasks at different stages of some species' development. The specialized functions include the synthesis of mucopolysaccharides and proteins for the building of foam nests, mucofibrils for the construction of dwelling tubes, adhesive secretions to help the locomotion, and brochosomes for protection as well as the usage of inorganic salts to harden the puparia, eggs chorion, and pupal cells' closing lids. MTs are also the organs responsible for the astonishing bioluminescence of some Diptera glowworms and can go through some drastic histological changes to produce a silk-like fiber utilized to spin cocoons. The specialized functions are associated with modifications of cells within the entire tubules, in specific segments, or, more rarely, modified secretory cells scattered along the MTs. In this review, we attempted to summarize the observations and experiments made over more than a century concerning the non-excretive functions of insects' MTs, underlying the need for new investigations supported by the current, advanced technologies available to validate outdated theories and clarify some dubious aspects.

Modeling Renal Disease "On the Fly"

Detoxification is a fundamental function for all living organisms that need to excrete catabolites and toxins to maintain homeostasis. Kidneys are major organs of detoxification that maintain water and electrolyte balance to preserve physiological functions of vertebrates. In insects, the renal function is carried out by Malpighian tubules and nephrocytes. Due to differences in their circulation, the renal systems of mammalians and insects differ in their functional modalities, yet carry out similar biochemical and physiological functions and share extensive genetic and molecular similarities. Evolutionary conservation can be leveraged to model specific aspects of the complex mammalian kidney function in the genetic powerhouse Drosophila melanogaster to study how genes interact in diseased states. Here, we compare the human and Drosophila renal systems and present selected fly disease models.

Drosophila Malpighian Tubules: A Model for Understanding Kidney Development, Function, and Disease

The Malpighian tubules of insects are structurally simple but functionally important organs, and their integrity is important for the normal excretory process. They are functional analogs of human kidneys which are important physiological organs as they maintain water and electrolyte balance in the blood and simultaneously help the body to get rid of waste and toxic products after various metabolic activities. In addition, it receives early indications of insults to the body such as immune challenge and other toxic components and is essential for sustaining life. According to National Vital Statistics Reports 2016, renal dysfunction has been ranked as the ninth most abundant cause of death in the USA. This chapter provides detailed descriptions of Drosophila Malpighian tubule development, physiology, immune function and also presents evidences that Malpighian tubules can be used as a model organ system to address the fundamental questions in developmental and functional disorders of the kidney.

Ecdysone regulates morphogenesis and function of Malpighian tubules in Drosophila melanogaster through EcR-B2 isoform

Malpighian tubules are the osmoregulatory and detoxifying organs of Drosophila and its proper development is critical for the survival of the organism. They are made up of two major cell types, the ectodermal principal cells and mesodermal stellate cells. The principal and stellate cells are structurally and physiologically distinct from each other, but coordinate together for production of isotonic fluid. Proper integration of these cells during the course of development is an important pre-requisite for the proper functioning of the tubules. We have conclusively determined an essential role of ecdysone hormone in the development and function of Malpighian tubules. Disruption of ecdysone signaling interferes with the organization of principal and stellate cells resulting in malformed tubules and early larval lethality. Abnormalities include reduction in the number of cells and the clustering of cells rather than their arrangement in characteristic wild type pattern. Organization of F-actin and β-tubulin also show aberrant distribution pattern. Malformed tubules show reduced uric acid deposition and altered expression of Na(+)/K(+)-ATPase pump. B2 isoform of ecdysone receptor is critical for the development of Malpighian tubules and is expressed from early stages of its development.