Phillip Darwin Bell

Macula densa cell signaling involves ATP release through a maxi anion channel

Macula densa cells are unique renal biosensor cells that detect changes in luminal NaCl concentration ([NaCl]L) and transmit signals to the mesangial cell/afferent arteriolar complex. They are the critical link between renal salt and water excretion and glomerular hemodynamics, thus playing a key role in regulation of body fluid volume. Since identification of these cells in the early 1900s, the nature of the signaling process from macula densa cells to the glomerular contractile elements has remained unknown. In patch–clamp studies of macula densa cells, we identified an [NaCl]L-sensitive ATP-permeable large-conductance (380 pS) anion channel. Also, we directly demonstrated the release of ATP (up to 10 μM) at the basolateral membrane of macula densa cells, in a manner dependent on [NaCl]L, by using an ATP bioassay technique. Furthermore, we found that glomerular mesangial cells respond with elevations in cytosolic Ca2+ concentration to extracellular application of ATP (EC50 0.8 μM). Importantly, we also found increases in cytosolic Ca2+ concentration with elevations in [NaCl]L, when fura-2-loaded mesangial cells were placed close to the basolateral membrane of macula densa cells. Thus, cell-to-cell communication between macula densa cells and mesangial cells, which express P2Y2 receptors, involves the release of ATP from macula densa cells via maxi anion channels at the basolateral membrane. This mechanism may represent a new paradigm in cell-to-cell signal transduction mediated by ATP.

Dysfunctional cilia lead to altered ependyma and choroid plexus function, and result in the formation of hydrocephalus

Cilia are complex organelles involved in sensory perception and fluid or cell movement. They are constructed through a highly conserved process called intraflagellar transport (IFT). Mutations in IFT genes, such as Tg737, result in severe developmental defects and disease. In the case of the Tg737orpk mutants, these pathological alterations include cystic kidney disease, biliary and pancreatic duct abnormalities, skeletal patterning defects, and hydrocephalus. Here, we explore the connection between cilia dysfunction and the development of hydrocephalus by using the Tg737orpk mutants. Our analysis indicates that cilia on cells of the brain ventricles of Tg737orpk mutant mice are severely malformed. On the ependymal cells, these defects lead to disorganized beating and impaired cerebrospinal fluid (CSF) movement. However, the loss of the cilia beat and CSF flow is not the initiating factor, as the pathology is present prior to the development of motile cilia on these cells and CSF flow is not impaired at early stages of the disease. Rather, our results suggest that loss of cilia leads to altered function of the choroid plexus epithelum, as evidenced by elevated intracellular cAMP levels and increased chloride concentration in the CSF. These data suggest that cilia function is necessary for regulating ion transport and CSF production, as well as for CSF flow through the ventricles.

Angiotensin I Conversion to Angiotensin II Stimulates Cortical Collecting Duct Sodium Transport

Abstract—Angiotensin (Ang) II directly stimulates epithelial sodium channel activity in the rabbit cortical collecting duct. Because Ang I and converting enzyme analogues might be present in the distal nephron, this raises the possibility of intraluminal generation of Ang II. Conversion of Ang I to Ang II was monitored by Ang II–dependent changes in intracellular sodium concentration as a reflection of sodium transport across the apical membrane. This involved imaging-based fluorescence microscopy with sodium-binding benzofuran isophthalate in isolated, perfused, cortical collecting-duct segments from rabbit kidney. Principal and intercalated cells were differentiated by rhodamine-conjugated peanut lectin. Control principal cell intracellular sodium concentration, during perfusion with 25 mmol/L NaCl and zero sodium in the bath plus monensin (10−5 mol/L) averaged 5.8±0.14 mmol/L (n=156). The increase in intracellular sodium concentration, when luminal NaCl was increased from 25 to 150 mmol/L, was elevated by 3.5-fold in the presence of intraluminal Ang I (10−6 mol/L). Also, the effects of Ang I on sodium transport were not significantly different from the effects of Ang II (10−9 mol/L). Ang I was used in micromolar concentrations to ensure that there was sufficient substrate available for conversion to Ang II. Inhibition of the angiotensin-converting enzyme with captopril reduced the stimulatory effect of Ang I. These results suggest that intraluminal conversion of Ang I to Ang II can occur in the cortical collecting duct, resulting in enhanced apical sodium entry.

Tubuloglomerular feedback mechanisms in nephron segments beyond the macula densa.

Purpose of reviewTo summarize recent evidence regarding the role of distal nephron segments other than the macula densa in sensing the tubular environment and transmitting this signal to the adjacent vasculature. Recent findingsIn addition to the classical contact site between the macula densa plaque and the afferent arteriole, there is accumulating evidence suggesting a functional association between the distal nephron and the vasculature at three distinct additional sites: at the terminal cortical thick ascending limb, at the early distal tubule and also at the connecting tubule segment. The epithelial cells around the macula densa also sense and respond to changes in tubular flow and salt content and may transmit this signal to the adjacent afferent arteriole. SummaryThere are multiple sites of anatomical and functional contact between the distal nephron and the vasculature supplying the glomerulus, and these may contribute to the regulation of glomerular filtration rate and renal hemodynamics.