Performance of one- and two-dimensional models for a slow flow system in a long,permeable tubule

A radially symmetric slow flow system in a long cylinder with moderate wall leakage is considered, with physical parameters taken from the renal proximal tubule. Dimensional analysis yields a simplified system which, under certain assumptions, is well-posed, and possesses a unique solution if a solution exists. A lumped parameter analysis generates a one-dimensional model identical to a typical one-dimensional model in a special case. The one- and two-dimensional models agree well in the situations examined. When the wall fluxes are of a certain class, the concentration to which the solution tends as the fluid proceeds down the tubule can be computed algebraically.     

Neurophysin and the neurosecretory fibres of the sheep infundibulum

Summary A cross-species reactive antiserum was used to study the cellular localization of neurophysin in the sheep hypothalamus and neurohypophysis. The immunofluorescent technique was used to follow the neurosecretory fibres emanating from the magnocellular nuclei to the lower infundibular stem. It was confirmed that neurophysin was present in both the internal and external zone of the infundibulum.This work was supported by a research grant from the New Zealand Medical Research Council.     

Role of C-terminal Membrane-proximal Basic Residues in Cell Surface Trafficking of HIV Coreceptor GPR15 Protein

Cell surface density of G protein-coupled receptors (GPCRs) is controlled by dynamic molecular interactions that often involve recognition of the distinct sequence signals on the cargo receptors. We reported previously that the RXR-type dibasic motif in the distal C-terminal tail of an HIV coreceptor GPR15 negatively regulates the cell surface expression by mediating the coatomer protein I complex-dependent retrograde transport to the endoplasmic reticulum (ER). Here we demonstrate that another pair of basic residues (Arg³¹⁰-Arg³¹¹) in the membrane-proximal region of the C-terminal tail plays a pivotal role in mediating the anterograde trafficking of GPR15. The Ala mutation of the C-terminal membrane-proximal basic residues (MPBRs) (R310/311A) abolished the O-glycosylation and cell surface expression of GPR15. The subcellular fractionation and immunocytochemistry assays indicated that the R310/311A mutant was more localized in the ER but much less in the trans-Golgi when compared with the wild-type GPR15, suggesting the positive role of Arg³¹⁰-Arg³¹¹ in the ER-to-Golgi transport of GPR15. Sequence analysis on human GPCRs showed that the basic residues are frequent in the membrane-proximal region of the C-terminal tail. Similar to GPR15, mutation of the C-terminal MPBRs resulted in a marked reduction of the cell surface expression in multiple different GPCRs. Our results suggest that the C-terminal MPBRs are critically involved in mediating the anterograde trafficking of a broad range of membrane proteins, including GPCRs.     

Alterations in ribosome biogenesis cause specific defects in C. elegans hermaphrodite gonadogenesis

Ribosome biogenesis is a cell-essential process that influences cell growth, proliferation, and differentiation. How ribosome biogenesis impacts development, however, is poorly understood. Here, we establish a link between ribosome biogenesis and gonadogenesis in Caenorhabditis elegans that affects germline proliferation and patterning. Previously, we determined that pro-1(+)activity is required in the soma--specifically, the sheath/spermatheca sublineage--to promote normal proliferation and prevent germline tumor formation. Here, we report that PRO-1, like its yeast ortholog IPI3, influences rRNA processing. pro-1 tumors are suppressed by mutations in ncl-1 or lin-35/Rb, both of which elevate pre-rRNA levels. Thus, in this context, lin-35/Rb acts as a soma-autonomous germline tumor promoter. We further report the characterization of two additional genes identified for their germline tumor phenotype, pro-2 and pro-3, and find that they, too, encode orthologs of proteins involved in ribosome biogenesis in yeast (NOC2 and SDA1, respectively). Finally, we demonstrate that depletion of additional C. elegans orthologs of yeast ribosome biogenesis factors display phenotypes similar to depletion of progenes. We conclude that the C. elegans distal sheath is particularly sensitive to alterations in ribosome biogenesis and that ribosome biogenesis defects in one tissue can non-autonomously influence proliferation in an adjacent tissue.     

Physiological roles of AQP7 in the kidney: Lessons from AQP7 knockout mice

The aquaporin7 (AQP7) water channel is known to be a member of the aquaglyceroporins, which allow the rapid transport of glycerol and water. AQP7 is abundantly present at the apical membrane of the proximal straight tubules in the kidney. In this paper, we review the physiological functions of AQP7 in the kidney. To investigate this, we generated AQP7 knockout mice. The water permeability of the proximal straight tubule brush border membrane measured by the stopped flow method was reduced in AQP7 knockout mice compared to wild-type mice (AQP7, 18.0 ± 0.4 × 10⁻³ cm/s vs. wild-type, 20.0 ± 0.3 × 10⁻³ cm/s). Although AQP7 solo knockout mice did not show a urinary concentrating defect, AQP1/AQP7 double knockout mice showed reduced urinary concentrating ability compared to AQP1 solo knockout mice, indicating that the contribution of AQP7 to water reabsorption in the proximal straight tubules is physiologically substantial. On the other hand, AQP7 knockout mice showed marked glycerol in their urine (AQP7, 1.7 ± 0.34 mg/ml vs. wild-type, 0.005 ± 0.002 mg/ml). This finding identified a novel pathway of glycerol reabsorption that occurs in the proximal straight tubules. In two mouse models of proximal straight tubule injury, the cisplatin-induced acute renal failure (ARF) model and the ischemic-reperfusion ARF model, an increase of urine glycerol was observed (pre-treatment, 0.007 ± 0.005 mg/ml; cisplatin, 0.063 ± 0.043 mg/ml; ischemia, 0.076 ± 0.02 mg/ml), suggesting that urine glycerol could be used as a new biomarker for detecting proximal straight tubule injury.     

Modeling Proximal Tubule Cell Homeostasis: Tracking Changes in Luminal Flow

During normal kidney function, there are routinely wide swings in proximal tubule fluid flow and proportional changes in Na⁺ reabsorption across tubule epithelial cells. This “glomerulotubular balance” occurs in the absence of any substantial change in cell volume, and is thus a challenge to coordinate luminal membrane solute entry with peritubular membrane solute exit. In this work, linear optimal control theory is applied to generate a configuration of regulated transporters that could achieve this result. A previously developed model of rat proximal tubule epithelium is linearized about a physiologic reference condition; the approximate linear system is recast as a dynamical system; and a Riccati equation is solved to yield the optimal linear feedback that stabilizes Na⁺ flux, cell volume, and cell pH. The first observation is that optimal feedback control is largely consigned to three physiologic variables, cell volume, cell electrical potential, and lateral intercellular hydrostatic pressure. Parameter modulation by cell volume stabilizes cell volume; parameter modulation by electrical potential or interspace pressure act to stabilize Na⁺ flux and cell pH. This feedback control is utilized in a tracking problem, in which reabsorptive Na⁺ flux varies over a factor of two, in order to represent a substantial excursion of glomerulotubular balance. The resulting control parameters consist of two terms, an autonomous term and a feedback term, and both terms include transporters on both luminal and peritubular cell membranes. Overall, the increase in Na⁺ flux is achieved with upregulation of luminal Na⁺/H⁺ exchange and Na⁺–glucose cotransport, with increased peritubular Na⁺–3HCO₃⁻ and K⁺–Cl⁻ cotransport, and with increased Na⁺, K⁺–ATPase activity. The configuration of activated transporters emerges as a testable hypothesis of the molecular basis for glomerulotubular balance. It is suggested that the autonomous control component at each cell membrane could represent the cytoskeletal effects of luminal flow.     

Functional adaptation of cancellous bone in human proximal femur predicted by trabecular surface remodeling simulation toward uniform stress state

Two-dimensional simulation of trabecular surface remodeling was conducted for a human proximal femur to investigate the structural change of cancellous bone toward a uniform stress state. Considering that a local mechanical stimulus plays an important role in cellular activities in bone remodeling, local stress nonuniformity was assumed to drive trabecular structural change to seek a uniform stress state. A large-scale pixel-based finite element model was used to simulate structural changes of individual trabeculae over the entire bone. As a result, the initial structure of trabeculae changed from isotropic to anisotropic due to trabecular microstructural changes caused by surface remodeling according to the mechanical environment in the proximal femur. Under a single-loading condition, it was shown that the apparent structural property evaluated by fabric ellipses corresponded to the apparent stress state in cancellous bone. As is observed in the actual bone, a distributed trabecular structure was obtained under a multiple-loading condition. Through these studies, it was concluded that trabecular surface remodeling toward a local uniform stress state at the trabecular level could naturally bring about functional adaptation phenomenon at the apparent tissue level. The proposed simulation model would be capable of providing insight into the hierarchical mechanism of trabecular surface remodeling at the microstructural level up to the apparent tissue level.     

Differential cellular effects in the toxicity of haloalkene and haloalkane cysteine conjugates to rabbit renal proximal tubules

The relationship between the covalent binding, uptake, and toxicity produced by S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC) was investigated in suspensions of rabbit renal proximal tubules (RPT). The DCVC and TFEC at concentrations of 25 μM produced a time-dependent (1–6 hours) loss of RPT viability. The TFEC was bio-transformed rapidly by β-lyase to a reactive metabolite which bound covalently to tubular protein. Approximately 63% of the TFEC-equivalents inside the cell were bound to protein. Covalent binding of TFEC-equivalents was associated with a 30% decrease in tubular basal and state 3 respiration, a sevenfold increase in lipid peroxidation, and, ultimately, cell death. The DCVC was biotransformed rapidly to a reactive metabolite which bound covalently to tubular protein. Approximately 90% of the DCVC-equivalents inside the cell were bound covalently to tubular protein. Following exposure to 25 μM DCVC, the binding of DCVC-equivalents was associated with a 17-fold increase in lipid peroxidation but, in contrast to TFEC, had no effect on tubular respiration. However, exposure of RPT to 100 μM DCVC resulted in a ninefold increase in the binding of DCVC- equivalents and a 30% decrease in tubular state 3 respiration. The β-lyase inhibitor aminooxyacetic acid (AOAA) blocked the covalent binding, mitochondrial dysfunction, lipid peroxidation, and cell death produced by TFEC. The AOAA decreased the covalent binding and the lipid peroxidation produced by DCVC by approximately 60–70% but had no effect on cell death. These results suggest that mitochondria! bioactivation of TFEC by β-lyase is critical for TFEC-induced mitochondrial dysfunction and the resulting cell death. These results also suggest that cytosolic bioactivation and binding, but not mitochondrial bioactivation and dysfunction, are important in the toxicity produced by DCVC to rabbit RPT. The lack of protection against DCVC toxicity by AOAA may be related to incomplete inhibition of DCVC metabolism or bioactivation of DCVC by pathways other than β-lyase.     

Caenorhabditis elegans germline patterning requires coordinated development of the somatic gonadal sheath and the germ line

Interactions between the somatic gonad and the germ line influence the amplification, maintenance, and differentiation of germ cells. In Caenorhabditis elegans, the distal tip cell/germline interaction promotes a mitotic fate and/or inhibits meiosis through GLP-1/Notch signaling. However, GLP-1-mediated signaling alone is not sufficient for a wild-type level of germline proliferation. Here, we provide evidence that specific cells of the somatic gonadal sheath lineage influence amplification, differentiation, and the potential for tumorigenesis of the germ line. First, an interaction between the distal-most pair of sheath cells and the proliferation zone of the germ line is required for larval germline amplification. Second, we show that insufficient larval germline amplification retards gonad elongation and thus delays meiotic entry. Third, a more severe delay in meiotic entry, as is exhibited in certain mutant backgrounds, inappropriately juxtaposes undifferentiated germ cells with cells of the proximal sheath lineage, leading to the formation of a proximal germline tumor derived from undifferentiated germ cells. Tumors derived from dedifferentiated germ cells, however, respond to the proximal interaction differently depending on the mutant background. Our study underscores the importance of strict developmental coordination between neighboring tissues. We discuss these results in the context of mechanisms that may underlie tumorigenesis.