LRBA is essential for urinary concentration and body water homeostasis

LRBA, a BEACH protein mutated in human immune deficiency, is widely expressed in epithelia, exocrine and endocrine glands, and neurons

Mutations in LRBA, a BEACH domain protein, cause severe immune deficiency in humans. LRBA is expressed in many tissues and organs according to biochemical analysis, but little is known about its cellular and subcellular localization, and its deficiency phenotype outside the immune system. By LacZ histochemistry of Lrba gene-trap mice, we performed a comprehensive survey of LRBA expression in numerous tissues, detecting it in many if not all epithelia, in exocrine and endocrine cells, and in subpopulations of neurons. Immunofluorescence microscopy of the exocrine and endocrine pancreas, salivary glands, and intestinal segments, confirmed these patterns of cellular expression and provided information on the subcellular localizations of the LRBA protein. Immuno-electron microscopy demonstrated that in neurons and endocrine cells, which co-express LRBA and its closest relative, neurobeachin, both proteins display partial association with endomembranes in complementary, rather than overlapping, subcellular distributions. Prominent manifestations of human LRBA deficiency, such as inflammatory bowel disease or endocrinopathies, are believed to be primarily due to immune dysregulation. However, as essentially all affected tissues also express LRBA, it is possible that LRBA deficiency enhances their vulnerability and contributes to the pathogenesis.

LRBA signalosomes activate vasopressin-induced AQP2 trafficking at recycling endosomes

Aquaporin-2 (AQP2) water channels are proteins that are recycled between intracellular vesicles and the apical plasma membrane in renal collecting ducts. Lipopolysaccharide-responsive beige-like anchor protein (LRBA) is a protein kinase A (PKA) anchoring protein that creates compartmentalized PKA signalling responsible for AQP2 phosphorylation. In response to increased plasma osmolality, vasopressin/cyclic adenosine monophosphate (cAMP)/PKA signalling phosphorylates AQP2, promoting AQP2 trafficking into the apical plasma membrane and increasing water reabsorption from urine. However, the molecular mechanisms by which LRBA mediates vasopressin-induced AQP2 phosphorylation remain unknown. To investigate AQP2 intracellular localization and phosphorylation status in vivo, a density gradient ultracentrifugation technique was combined with an in situ proximity ligation assay, super-resolution structured illumination microscopy and immunoelectron microscopy. Most of the AQP2 was localized on the recycling endosome in the presence of tolvaptan, a vasopressin type 2 receptor (V2R) antagonist. Desmopressin, a V2R agonist, phosphorylated AQP2, translocating it from the recycling endosome to the apical plasma membrane. In contrast, LRBA was constitutively localized at the recycling endosome. Therefore, LRBA and AQP2 were well colocalized in the absence of vasopressin stimulation. The loss of LRBA/PKA signalling by Lrba knockout impaired vasopressin-induced AQP2 phosphorylation, resulting in AQP2 retention at the recycling endosome. Defective AQP2 trafficking caused low urinary concentrating ability in Lrba-/- mice. The LRBA-PKA complex created compartmentalized PKA signalling at the recycling endosome, which facilitated AQP2 phosphorylation in response to vasopressin. KEY POINTS: Membrane proteins are continuously internalized into the endosomal system via endocytosis, after which they are either recycled back to the plasma membrane or degraded at the lysosome. In T cells, lipopolysaccharide-responsive beige-like anchor protein (LRBA) binds directly to the cytotoxic T lymphocyte antigen 4 (CTLA-4), a checkpoint immune molecule, to prevent CTLA-4 lysosomal degradation and promote its vesicle recycling. LRBA has different physiological functions in renal collecting ducts. LRBA and aquaporin-2 (AQP2) water channels were colocalized on the recycling endosome in vivo in the absence of the anti-diuretic hormone vasopressin. LRBA promoted vasopressin-induced AQP2 trafficking, increasing water reabsorption from urine via AQP2. LRBA determined renal responsiveness to vasopressin at recycling endosomes. LRBA is a ubiquitously expressed anchor protein. LRBA signalosomes might regulate membrane trafficking of several constitutively recycled proteins at recycling endosomes.

Methods for studying mammalian aquaporin biology

Aquaporins (AQPs), transmembrane water-conducting channels, have earned a great deal of scrutiny for their critical physiological roles in healthy and disease cell states, especially in the biomedical field. Numerous methods have been implemented to elucidate the involvement of AQP-mediated water transport and downstream signaling activation in eliciting whole cell, tissue, and organ functional responses. To modulate these responses, other methods have been employed to investigate AQP druggability. This review discusses standard in vitro, in vivo, and in silico methods for studying AQPs, especially for biomedical and mammalian cell biology applications. We also propose some new techniques and approaches for future AQP research to address current gaps in methodology.

Role of Protein Kinase A Activation in the Immune System with an Emphasis on Lipopolysaccharide-Responsive and Beige-like Anchor Protein in B Cells

Cyclic AMP-dependent protein kinase A (PKA) is a ubiquitous enzymatic complex that is involved in a broad spectrum of intracellular receptor signaling. The activity of PKA depends on A-kinase anchoring proteins (AKAPs) that attach to PKAs close to their substrates to control signaling. Although the relevance of PKA-AKAP signaling in the immune system is evident in T cells, its relevance in B and other immune cells remains relatively unclear. In the last decade, lipopolysaccharide-responsive and beige-like anchor protein (LRBA) has emerged as an AKAP that is ubiquitously expressed in B and T cells, specifically after activation. A deficiency of LRBA leads to immune dysregulation and immunodeficiency. The cellular mechanisms regulated by LRBA have not yet been investigated. Therefore, this review summarizes the functions of PKA in immunity and provides the most recent information regarding LRBA deficiency to deepen our understanding of immune regulation and immunological diseases.

Proteomics and AQP2 regulation

The advent of modern quantitative protein mass spectrometry techniques around the turn of the 21st century has contributed to a revolution in biology referred to as 'systems biology'. These methods allow identification and quantification of thousands of proteins in a biological specimen, as well as detection and quantification of post-translational protein modifications including phosphorylation. Here, we discuss these methodologies and show how they can be applied to understand the effects of the peptide hormone vasopressin to regulate the molecular water channel aquaporin-2. The emerging picture provides a detailed framework for understanding the molecular mechanisms involved in water balance disorders.