Neuronally differentiated macula densa cells regulate tissue remodeling and regeneration in the kidney.

Tissue regeneration is limited in several organs including the kidney, contributing to the high prevalence of kidney disease globally. However, evolutionary and physiological adaptive responses and the presence of renal progenitor cells suggest existing remodeling capacity. This study uncovered endogenous tissue remodeling mechanisms in the kidney that were activated by the loss of body fluid and salt and regulated by a unique niche of a minority renal cell type called the macula densa (MD). Here we identified neuronal differentiation features of MD cells that sense the local and systemic environment, secrete angiogenic, growth and extracellular matrix remodeling factors, cytokines and chemokines, and control resident progenitor cells. Serial intravital imaging, MD nerve growth factor receptor and Wnt mouse models and transcriptome analysis revealed cellular and molecular mechanisms of these MD functions. Human and therapeutic translation studies illustrated the clinical potential of MD factors including CCN1 as a urinary biomarker and therapeutic target in chronic kidney disease. The concept that a neuronally differentiated key sensory and regulatory cell type responding to organ-specific physiological inputs controls local progenitors to remodel or repair tissues may be applicable to other organs and diverse tissue regenerative therapeutic strategies.

The role of TRPC6 calcium channels and P2 purinergic receptors in podocyte mechanical and metabolic sensing.

Podocyte calcium (Ca2+) signaling plays important roles in the (patho)physiology of the glomerular filtration barrier. Overactivation of podocyte transient receptor potential canonical (TRPC) channels including TRPC6 and purinergic signaling via P2 receptors that are known mechanosensors can increase podocyte intracellular Ca2+ levels ([Ca2+]i) and cause cell injury, proteinuria and glomerular disease including in diabetes. However, important mechanistic details of the trigger and activation of these pathways in vivo in the intact glomerular environment are lacking. Here we show direct visual evidence that podocytes can sense mechanical overload (increased glomerular capillary pressure) and metabolic alterations (increased plasma glucose) via TRPC6 and purinergic receptors including P2Y2. Multiphoton microscopy of podocyte [Ca2+]i was performed in vivo using wild-type and TRPC6 or P2Y2 knockout (KO) mice expressing the calcium reporter GCaMP3/5 only in podocytes and in vitro using freshly dissected microperfused glomeruli. Single-nephron intra-glomerular capillary pressure elevations induced by obstructing the efferent arteriole lumen with laser-induced microthrombus in vivo and by a micropipette in vitro triggered >2-fold increases in podocyte [Ca2+]i. These responses were blocked in TRPC6 and P2Y2 KO mice. Acute elevations of plasma glucose caused >4-fold increases in podocyte [Ca2+]i that were abolished by pharmacological inhibition of TRPC6 or P2 receptors using SAR7334 or suramin treatment, respectively. This study established the role of Ca2+ signaling via TRPC6 channels and P2 receptors in mechanical and metabolic sensing of podocytes in vivo, which are promising therapeutic targets in conditions with high intra-glomerular capillary pressure and plasma glucose, such as diabetic and hypertensive nephropathy.

Correction to: Long-Term Cell Fate Tracking of Individual Renal Cells Using Serial Intravital Microscopy.

The original version of this chapter was inadvertently published without a proper acknowledgement. The authors informed to insert the following acknowledgement in this chapter.

Long-Term Cell Fate Tracking of Individual Renal Cells Using Serial Intravital Microscopy.

Intravital multiphoton microscopy of the kidney is a powerful technique to study alterations in tissue morphology and function simultaneously in the living animal and represents a dynamic and developing research tool in the field. Recent technological advances include serial intravital multiphoton microscopy of the same kidney regions over several weeks and combined with ex vivo histology for cellular biomarker expression of the same cells, which had been subject to serial imaging before. Thus, serial intravital multiphoton microscopy followed by ex vivo histology provides unique tools to perform long-term cell fate tracing of the same renal cells during physiological and pathophysiological conditions, thereby allowing the detection of structural changes of the same renal cells over time. Examples include renal cell migration and proliferation while linking these events to local functional alterations and eventually to the expression of distinct cellular biomarkers. Here, we provide a detailed step-by-step protocol to facilitate serial intravital multiphoton microscopy for long-term in vivo tracking of renal cells and subsequent ex vivo histology for immunohistological staining of the same cells in the fixed tissue.

Advances in Renal Cell Imaging.

A great variety of cell imaging technologies are used routinely every day for the investigation of kidney cell types in applications ranging from basic science research to drug development and pharmacology, clinical nephrology, and pathology. Quantitative visualization of the identity, density, and fate of both resident and nonresident cells in the kidney, and imaging-based analysis of their altered function, (patho)biology, metabolism, and signaling in disease conditions, can help to better define pathomechanism-based disease subgroups, identify critical cells and structures that play a role in the pathogenesis, critically needed biomarkers of disease progression, and cell and molecular pathways as targets for novel therapies. Overall, renal cell imaging has great potential for improving the precision of diagnostic and treatment paradigms for individual acute kidney injury or chronic kidney disease patients or patient populations. This review highlights and provides examples for some of the recently developed renal cell optical imaging approaches, mainly intravital multiphoton fluorescence microscopy, and the new knowledge they provide for our better understanding of renal pathologies.

Combined use of electron microscopy and intravital imaging captures morphological and functional features of podocyte detachment.

The development of podocyte injury and albuminuria in various glomerular pathologies is still incompletely understood due to technical limitations in studying the glomerular filtration barrier (GFB) in real-time. We aimed to directly visualize the early morphological and functional changes of the GFB during the development of focal segmental glomerulosclerosis (FSGS) using a combination of transmission electron microscopy (TEM) and in vivo multiphoton microscopy (MPM) in the rat puromycin aminonucleoside (PAN) model. We hypothesized that this combined TEM + MPM experimental approach would provide a major technical improvement that would benefit our mechanistic understanding of podocyte detachment. Male Sprague-Dawley (for TEM) or Munich-Wistar-Frömter (for MPM) rats were given a single dose of 100-150 mg/kg body weight PAN i.p. and were either sacrificed and the kidneys processed for TEM or surgically instrumented for in vivo MPM imaging at various times 2-14 days after PAN administration. Both techniques demonstrated hypertrophy and cystic dilatations of the subpodocyte space that developed as early as 2-3 days after PAN. Adhesions of the visceral epithelium to the parietal Bowman's capsule (synechiae) appeared at days 8-10. TEM provided unmatched resolution of podocyte foot process remodeling, while MPM revealed the rapid dynamics of pseudocyst filling, emptying, and rupture, as well as endothelial and podocyte injury, misdirected filtration, and podocyte shedding. Due to the complementary advantages of TEM and MPM, this combined approach can provide an unusally comprehensive and dynamic portrayal of the alterations in podocyte morphology and function during FSGS development. The results advance our understanding of the role and importance of the various cell types, hemodynamics, and mechanical forces in the development of glomerular pathology.

Prox1 expression in the endolymphatic sac revealed by whole-mount fluorescent imaging of Prox1-GFP transgenic mice.

This study describes a technical breakthrough in endolymphatic sac research, made possible by the use of the recently generated Prox1-GFP transgenic mouse model. Whole-mount imaging techniques through the decalcified temporal bone and three-dimensional observations of Prox1-GFP mouse tissue revealed the positive labeling of the endolymphatic sac in adult stage, and allowed, for the first time, the GFP-based identification of endolymphatic sac epithelial cells. Prox1 expression was observed in all parts of the endolymphatic sac epithelia. In intermediate portion of the endolymphatic sac, mitochondria-rich cells did not express Prox1, although ribosome-rich cells showed strong GFP labeling. The anatomical relationship between the endolymphatic sac and the surrounding vasculature was directly observed. In the endolymphatic sac, expression of Prox1 may suggest progenitor cell-like pluripotency or developmental similarity to systemic lymphatic vessels in other organs. This whole-mount imaging technique of the endolymphatic sac can be combined with other conventional histological, sectioning, and labeling techniques and will be very useful for future endolymphatic sac research.

Local pH domains regulate NHE3-mediated Na⁺ reabsorption in the renal proximal tubule.

The proximal tubule Na(+)/H(+) exchanger 3 (NHE3), located in the apical dense microvilli (brush border), plays a major role in the reabsorption of NaCl and water in the renal proximal tubule. In response to a rise in blood pressure NHE3 redistributes in the plane of the plasma membrane to the base of the brush border, where NHE3 activity is reduced. This NHE3 redistribution is assumed to provoke pressure natriuresis; however, it is unclear how NHE3 redistribution per se reduces NHE3 activity. To investigate if the distribution of NHE3 in the brush border can change the reabsorption rate, we constructed a spatiotemporal mathematical model of NHE3-mediated Na(+) reabsorption across a proximal tubule cell and compared the model results with in vivo experiments in rats. The model predicts that when NHE3 is localized exclusively at the base of the brush border, it creates local pH microdomains that reduce NHE3 activity by >30%. We tested the model's prediction experimentally: the rat kidney cortex was loaded with the pH-sensitive fluorescent dye BCECF, and cells of the proximal tubule were imaged in vivo using confocal fluorescence microscopy before and after an increase of blood pressure by ∼50 mmHg. The experimental results supported the model by demonstrating that a rise of blood pressure induces the development of pH microdomains near the bottom of the brush border. These local changes in pH reduce NHE3 activity, which may explain the pressure natriuresis response to NHE3 redistribution.

Can kidney regeneration be visualized?

BACKGROUND: Various cell types, including podocytes and parietal epithelial cells, play important roles in the development and progression of glomerular kidney diseases, albuminuria, and glomerulosclerosis. Besides their role in renal pathologies, glomerular cells have emerging new functions in endogenous repair mechanisms. A better understanding of the dynamics of the glomerular environment and cellular composition in an intact living kidney is critically important for the development of new regenerative therapeutic strategies for kidney diseases. However, progress in this field has been hampered by the lack of in vivo research tools. SUMMARY: This review summarizes the current state-of-the-art in the application of the unique intravital imaging technology of multiphoton fluorescence microscopy for the dynamic visualization of glomerular structure and function over time in the intact, living kidney. Recently, this imaging approach in combination with transgenic mouse models allowed tracking of the fate of individual glomerular cells in vivo over several days and depicted the highly dynamic nature of the glomerular environment, particularly in disease conditions. KEY MESSAGES: The technology is ready and available for future intravital imaging studies investigating new glomerular regenerative approaches in animal models.

Intravital imaging of podocyte calcium in glomerular injury and disease.

Intracellular calcium ([Ca²âº]i) signaling mediates physiological and pathological processes in multiple organs, including the renal podocyte; however, in vivo podocyte [Ca²âº]i dynamics are not fully understood. Here we developed an imaging approach that uses multiphoton microscopy (MPM) to directly visualize podocyte [Ca²âº]i dynamics within the intact kidneys of live mice expressing a fluorescent calcium indicator only in these cells. [Ca²âº]i was at a low steady-state level in control podocytes, while Ang II infusion caused a minor elevation. Experimental focal podocyte injury triggered a robust and sustained elevation of podocyte [Ca²âº]i around the injury site and promoted cell-to-cell propagating podocyte [Ca²âº]i waves along capillary loops. [Ca²âº]i wave propagation was ameliorated by inhibitors of purinergic [Ca²âº]i signaling as well as in animals lacking the P2Y2 purinergic receptor. Increased podocyte [Ca²âº]i resulted in contraction of the glomerular tuft and increased capillary albumin permeability. In preclinical models of renal fibrosis and glomerulosclerosis, high podocyte [Ca²âº]i correlated with increased cell motility. Our findings provide a visual demonstration of the in vivo importance of podocyte [Ca²âº]i in glomerular pathology and suggest that purinergic [Ca²âº]i signaling is a robust and key pathogenic mechanism in podocyte injury. This in vivo imaging approach will allow future detailed investigation of the molecular and cellular mechanisms of glomerular disease in the intact living kidney.

Tracking the fate of glomerular epithelial cells in vivo using serial multiphoton imaging in new mouse models with fluorescent lineage tags.

Podocytes are critical in the maintenance of a healthy glomerular filter; however, they have been difficult to study in the intact kidney because of technical limitations. Here we report the development of serial multiphoton microscopy (MPM) of the same glomeruli over several days to visualize the motility of podocytes and parietal epithelial cells (PECs) in vivo. In podocin-GFP mice, podocytes formed sporadic multicellular clusters after unilateral ureteral ligation and migrated into the parietal Bowman's capsule. The tracking of single cells in podocin-confetti mice featuring cell-specific expression of CFP, GFP, YFP or RFP revealed the simultaneous migration of multiple podocytes. In phosphoenolpyruvate carboxykinase (PEPCK)-GFP mice, serial MPM found PEC-to-podocyte migration and nanotubule connections. Our data support a highly dynamic rather than a static nature of the glomerular environment and cellular composition. Future application of this new approach should advance our understanding of the mechanisms of glomerular injury and regeneration.

ATP releasing connexin 30 hemichannels mediate flow-induced calcium signaling in the collecting duct.

ATP in the renal tubular fluid is an important regulator of salt and water reabsorption via purinergic calcium signaling that involves the P2Y2 receptor, ENaC, and AQP2. Recently, we have shown that connexin (Cx) 30 hemichannels are localized to the non-junctional apical membrane of cells in the distal nephron-collecting duct (CD) and release ATP into the tubular fluid upon mechanical stimuli, leading to reduced salt and water reabsorption. Cx30(-/-) mice show salt-dependent elevations in BP and impaired pressure-natriuresis. Thus, we hypothesized that increased tubular flow rate leads to Cx30-dependent purinergic intracellular calcium ([Ca(2+)]i) signaling in the CD. Cortical CDs (CCDs) from wild type and Cx30(-/-) mice were freshly dissected and microperfused in vitro. Using confocal fluorescence imaging and the calcium-sensitive fluorophore pair Fluo-4 and Fura Red, we found that increasing tubular flow rate from 2 to 20 nl/min caused a significant 2.1-fold elevation in [Ca(2+)]i in wild type CCDs. This response was blunted in Cx30(-/-) CCDs ([Ca(2+)]i increased only 1.2-fold, p < 0.0001 vs. WT, n = 6 each). To further test our hypothesis we performed CD [Ca(2+)]i imaging in intact mouse kidneys in vivo using multiphoton microscopy and micropuncture delivery of the calcium-sensitive fluorophore Rhod-2. We found intrinsic, spontaneous [Ca(2+)]i oscillations in free-flowing CDs of wild type but not Cx30(-/-) mice. The [Ca(2+)]i oscillations were sensitive also to P2-receptor inhibition by suramin. Taken together, these data confirm that mechanosensitive Cx30 hemichannels mediate tubular ATP release and purinergic calcium signaling in the CD which mechanism plays an important role in the regulation of CD salt and water reabsorption.

Localization and proliferation of lymphatic vessels in the tympanic membrane in normal state and regeneration.

We clarified the localization of lymphatic vessels in the tympanic membrane and proliferation of lymphatic vessels during regeneration after perforation of the tympanic membrane by using whole-mount imaging of the tympanic membrane of Prox1 GFP mice. In the pars tensa, lymphatic vessel loops surrounded the malleus handle and annulus tympanicus. Apart from these locations, lymphatic vessel loops were not observed in the pars tensa in the normal tympanic membrane. Lymphatic vessel loops surrounding the malleus handle were connected to the lymphatic vessel loops in the pars flaccida and around the tensor tympani muscle. Many lymphatic vessel loops were detected in the pars flaccida. After perforation of the tympanic membrane, abundant lymphatic regeneration was observed in the pars tensa, and these regenerated lymphatic vessels extended from the lymphatic vessels surrounding the malleus at day 7. These results suggest that site-specific lymphatic vessels play an important role in the tympanic membrane.

Multiphoton imaging of the glomerular permeability of angiotensinogen.

Patients and animals with renal injury exhibit increased urinary excretion of angiotensinogen. Although increased tubular synthesis of angiotensinogen contributes to the increased excretion, we do not know to what degree glomerular filtration of systemic angiotensinogen, especially through an abnormal glomerular filtration barrier, contributes to the increase in urinary levels. Here, we used multiphoton microscopy to visualize and quantify the glomerular permeability of angiotensinogen in the intact mouse and rat kidney. In healthy mice and Munich-Wistar-Frömter rats at the early stage of glomerulosclerosis, the glomerular sieving coefficient of systemically infused Atto565-labeled human angiotensinogen (Atto565-hAGT), which rodent renin cannot cleave, was only 25% of the glomerular sieving coefficient of albumin, and its urinary excretion was undetectable. In a more advanced phase of kidney disease, the glomerular permeability of Atto565-hAGT was slightly higher but still very low. Furthermore, unlike urinary albumin, the significantly higher urinary excretion of endogenous rat angiotensinogen did not correlate with either the Atto565-hAGT or Atto565-albumin glomerular sieving coefficients. These results strongly suggest that the vast majority of urinary angiotensinogen originates from the tubules rather than glomerular filtration.

Loss of the endothelial glycocalyx links albuminuria and vascular dysfunction.

Patients with albuminuria and CKD frequently have vascular dysfunction but the underlying mechanisms remain unclear. Because the endothelial surface layer, a meshwork of surface-bound and loosely adherent glycosaminoglycans and proteoglycans, modulates vascular function, its loss could contribute to both renal and systemic vascular dysfunction in proteinuric CKD. Using Munich-Wistar-Fromter (MWF) rats as a model of spontaneous albuminuric CKD, multiphoton fluorescence imaging and single-vessel physiology measurements revealed that old MWF rats exhibited widespread loss of the endothelial surface layer in parallel with defects in microvascular permeability to both water and albumin, in both continuous mesenteric microvessels and fenestrated glomerular microvessels. In contrast to young MWF rats, enzymatic disruption of the endothelial surface layer in old MWF rats resulted in neither additional loss of the layer nor additional changes in permeability. Intravenous injection of wheat germ agglutinin lectin and its adsorption onto the endothelial surface layer significantly improved glomerular albumin permeability. Taken together, these results suggest that widespread loss of the endothelial surface layer links albuminuric kidney disease with systemic vascular dysfunction, providing a potential therapeutic target for proteinuric kidney disease.

The first decade of using multiphoton microscopy for high-power kidney imaging.

In this review, we highlight the major scientific breakthroughs in kidney research achieved using multiphoton microscopy (MPM) and summarize the milestones in the technological development of kidney MPM during the past 10 years. Since more and more renal laboratories invest in MPM worldwide, we discuss future directions and provide practical, useful tips and examples for the application of this still-emerging optical sectioning technology. Advantages of using MPM in various kidney preparations that range from freshly dissected individual glomeruli or the whole kidney in vitro to MPM of the intact mouse and rat kidney in vivo are reviewed. Potential combinations of MPM with micromanipulation techniques including microperfusion and micropuncture are also included. However, we emphasize the most advanced and complex, quantitative in vivo imaging applications as the ultimate use of MPM since the true mandate of this technology is to look inside intact organs in live animals and humans.

Postischemic inflammatory syndrome: a critical mechanism of progression in diabetic nephropathy.

Diabetes is a major epidemic, and diabetic nephropathy is the most common cause of end-stage renal disease. Two critical components of diabetic nephropathy are persistent inflammation and chronic renal ischemia from widespread vasculopathy. Moreover, acute ischemic renal injury is common in diabetes, potentially causing chronic kidney disease or end-stage renal disease. Accordingly, we tested the hypothesis that acute renal ischemia accelerates nephropathy in diabetes by activating proinflammatory pathways. Lean and obese-diabetic ZS rats (F(1) hybrids of spontaneously hypertensive heart failure and Zucker fatty diabetic rats) were subjected to bilateral renal ischemia or sham surgery before the onset of proteinuria. The postischemic state in rats with obesity-diabetes was characterized by progressive chronic renal failure, increased proteinuria, and renal expression of proinflammatory mediators. Leukocyte number in obese-diabetic rat kidney was markedly increased for months after ischemia. Intrarenal blood flow velocity was decreased after ischemia in lean control and obese-diabetic rats, although it recovered in lean rats. At 2 mo after ischemia, blood flow velocity decreased further in sham-surgery and postischemia obese-diabetic rats, so that RBC flow velocity was only 39% of control in the obese-diabetic rats after ischemia. In addition, microvascular density remained depressed at 2 mo in kidneys of obese-diabetic rats after ischemia. Abnormal microvascular permeability and increases in interstitial fibrosis and apoptotic renal cell death were also more pronounced after ischemia in obese-diabetic rats. These data support the hypothesis that acute renal ischemia in obesity-diabetes severely aggravates chronic inflammation and vasculopathy, creating a self-perpetuating postischemia inflammatory syndrome, which accelerates renal failure.