Regulated exocytosis: Renal Aquaporin-2 3D Vesicular Network Organization and Association with F-actin

Many kinases for controlling the water channel aquaporin-2

Aquaporin-2 (AQP2) is a member of the aquaporin water channel family. In the kidney, AQP2 is expressed in collecting duct principal cells where it facilitates water reabsorption in response to antidiuretic hormone (arginine vasopressin, AVP). AVP induces the redistribution of AQP2 from intracellular vesicles and its incorporation into the plasma membrane. The plasma membrane insertion of AQP2 represents the crucial step in AVP-mediated water reabsorption. Dysregulation of the system preventing the AQP2 plasma membrane insertion causes diabetes insipidus (DI), a disease characterised by an impaired urine concentrating ability and polydipsia. There is no satisfactory treatment of DI available. This review discusses kinases that control the localisation of AQP2 and points out potential kinase-directed targets for the treatment of DI.

Following the cellular itinerary of renal aquaporin-2 shuttling with 4.5x expansion microscopy

The shuttling of renal collecting duct aquaporin-2 (AQP2) between intracellular vesicles and the apical plasma membrane is paramount for regulation of renal water reabsorption. The binding of the circulating antidiuretic hormone arginine vasopressin (AVP) to the basolateral AVP receptor increases intracellular cAMP, which ultimately leads to AQP2 plasma membrane accumulation via a dual effect on AQP2 vesicle fusion with the apical plasma membrane and reduced AQP2 endocytosis. This AQP2 plasma membrane accumulation increases water reabsorption and consequently urine concentration. Conventional fluorescent microscopy provides a lateral resolution of ∼250 nm, which is insufficient to resolve the AQP2-containing endosomes/vesicles. Therefore, detailed information regarding the AQP2 vesicular population is still lacking. Newly established 4.5x Expansion Microscopy (ExM) can increase resolution to 60-70 nm. Using 4.5x ExM, we detected AQP2 vesicles/endosomes as small as 79 nm considering an average expansion factor of 4.3 for endosomes. Using different markers of the endosomal system provided detailed information of the cellular AQP2 itinerary upon changes in endogenous cAMP levels. Before cAMP elevation, AQP2 colocalized with early and recycling, but not late endosomes. Forskolin-induced cAMP increase was characterized by AQP2 insertion into the plasma membrane and AQP2 withdrawal from large perinuclear endosomes as well as some localization to lysosomal compartments. Forskolin washout promoted AQP2 endocytosis where AQP2 localized to not only early and recycling endosomes but also late endosomes and lysosomes indicating increased AQP2 degradation. Thus, our results show that 4.5 ExM is an attractive approach to obtain detailed information regarding AQP2 shuttling.NEW & NOTEWORTHY Renal aquaporin-2 (AQP2) imaged by expansion microscopy provides unprecedented 3-D information regarding the AQP2 itinerary in response to changes in cellular cAMP.

Glial Cell Adhesion Molecule (GlialCAM) Determines Proliferative versus Invasive Cell States in Glioblastoma

The malignant brain cancer glioblastoma (GBM) contains groups of highly invasive cells that drive tumor progression as well as recurrence after surgery and chemotherapy. The molecular mechanisms that enable these GBM cells to exit the primary mass and disperse throughout the brain remain largely unknown. Here we report using human tumor specimens and primary spheroids from male and female patients that glial cell adhesion molecule (GlialCAM), which has normal roles in brain astrocytes and is mutated in the developmental brain disorder megalencephalic leukoencephalopathy with subcortical cysts (MLC), is differentially expressed in subpopulations of GBM cells. High levels of GlialCAM promote cell-cell adhesion and a proliferative GBM cell state in the tumor core. In contrast, GBM cells with low levels of GlialCAM display diminished proliferation and enhanced invasion into the surrounding brain parenchyma. RNAi-mediated inhibition of GlialCAM expression leads to activation of proinvasive extracellular matrix adhesion and signaling pathways. Profiling GlialCAM-regulated genes combined with cross-referencing to single-cell transcriptomic datasets validates functional links among GlialCAM, Mlc1, and aquaporin-4 in the invasive cell state. Collectively, these results reveal an important adhesion and signaling axis comprised of GlialCAM and associated proteins including Mlc1 and aquaporin-4 that is critical for control of GBM cell proliferation and invasion status in the brain cancer microenvironment.SIGNIFICANCE STATEMENT Glioblastoma (GBM) contains heterogeneous populations of cells that coordinately drive proliferation and invasion. We have discovered that glial cell adhesion molecule (GlialCAM)/hepatocyte cell adhesion molecule (HepaCAM) is highly expressed in proliferative GBM cells within the tumor core. In contrast, GBM cells with low levels of GlialCAM robustly invade into surrounding brain tissue along blood vessels and white matter. Quantitative RNA sequencing identifies various GlialCAM-regulated genes with functions in cell-cell adhesion and signaling. These data reveal that GlialCAM and associated signaling partners, including Mlc1 and aquaporin-4, are key factors that determine proliferative and invasive cell states in GBM.

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

Urinary extracellular vesicles and tubular transport

Tubular transport is a key function of the kidney to maintain electrolyte and acid-base homeostasis. Urinary extracellular vesicles (uEVs) harbor water, electrolyte, and acid-base transporters expressed at the apical plasma membrane of tubular epithelial cells. Within the uEV proteome, the correlations between kidney and uEV protein abundances are strongest for tubular transporters. Therefore, uEVs offer a non-invasive approach to probe tubular transport in health and disease. Here, we will review how kidney tubular physiology is reflected in uEVs and, conversely, how uEVs may modify tubular transport. Clinically, uEV tubular transporter profiling has been applied to rare diseases such as inherited tubulopathies, but also to more common conditions such as hypertension and kidney disease. Although uEVs hold the promise to advance the diagnosis of kidney disease to the molecular level, several biological and technical complexities still need to be addressed. The future will tell if uEV analysis will mainly be a powerful tool to study tubular physiology in humans or if it will move forward to become a diagnostic bedside test.

High-throughput muscle fiber typing from RNA sequencing data

Background

Skeletal muscle fiber type distribution has implications for human health, muscle function, and performance. This knowledge has been gathered using labor-intensive and costly methodology that limited these studies. Here, we present a method based on muscle tissue RNA sequencing data (totRNAseq) to estimate the distribution of skeletal muscle fiber types from frozen human samples, allowing for a larger number of individuals to be tested.

Methods

By using single-nuclei RNA sequencing (snRNAseq) data as a reference, cluster expression signatures were produced by averaging gene expression of cluster gene markers and then applying these to totRNAseq data and inferring muscle fiber nuclei type via linear matrix decomposition. This estimate was then compared with fiber type distribution measured by ATPase staining or myosin heavy chain protein isoform distribution of 62 muscle samples in two independent cohorts (n = 39 and 22).

Results

The correlation between the sequencing-based method and the other two were r_ATPas = 0.44 [0.13–0.67], [95% CI], and r_myosin = 0.83 [0.61–0.93], with p = 5.70 × 10^–3 and 2.00 × 10^–6, respectively. The deconvolution inference of fiber type composition was accurate even for very low totRNAseq sequencing depths, i.e., down to an average of ~ 10,000 paired-end reads.

Conclusions

This new method (https://github.com/OlaHanssonLab/PredictFiberType) consequently allows for measurement of fiber type distribution of a larger number of samples using totRNAseq in a cost and labor-efficient way. It is now feasible to study the association between fiber type distribution and e.g. health outcomes in large well-powered studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00299-4.

Updates and Perspectives on Aquaporin-2 and Water Balance Disorders

Ensuring the proper amount of water inside the body is essential for survival. One of the key factors in the maintenance of body water balance is water reabsorption in the collecting ducts of the kidney, a process that is regulated by aquaporin-2 (AQP2). AQP2 is a channel that is exclusively selective for water molecules and impermeable to ions or other small molecules. Impairments of AQP2 result in various water balance disorders, including nephrogenic diabetes insipidus (NDI), which is a disease characterized by a massive loss of water through the kidney and consequent severe dehydration. Dysregulation of AQP2 is also a cause of water retention with hyponatremia in heart failure, hepatic cirrhosis, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone vasopressin is an upstream regulator of AQP2. Its binding to the vasopressin V2 receptor promotes AQP2 targeting to the apical membrane and thus enables water reabsorption. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for water retention with hyponatremia. However, there are no studies showing improvement in hard outcomes or long-term prognosis. A possible reason is that vasopressin receptors have many downstream effects other than AQP2 function. It is expected that the development of drugs that directly target AQP2 may result in increased treatment specificity and effectiveness for water balance disorders. This review summarizes recent progress in studies of AQP2 and drug development challenges for water balance disorders.