Peroxisome development in the metanephric kidney of mouse.

The relationship of enzymatic activity to organelle development and organelle number during differentiation of the metanephric kidney in the mouse was approached from several experimental directions. Biochemical analyses of marker enzymes for peroxisomes (catalase and D-amino acid oxidase), mitochondria (cytochrome oxidase) and lysosomes (acid phosphatase) were performed on kidneys at ages from 17 days prenatal to adult. These data were correlated with a morphometric analysis of populations of peroxisomes and mitochondria in differentiating cells of the proximal tubule. Postnatal development of the metanephric kidney was found to be accompanied by a rapid increase in both the specific activity of catalase and the number of peroxisomes per 100 mu2 in the proximal tubule during the first 4 weeks of postnatal growth. Elaboration of the endoplasmic reticulum (ER) was seen to parallel the increase in number of peroxisomes to which segments of ER were often in close apposition. Extensive interactions between segments of ER and peroxisomes were readily visible in 0.5-mu sections viewed in the high voltage electron microscope. In contrast to peroxisomes, neither mitochondria nor lysosomes followed a similar pattern of net organelle increase, suggesting that a defined population density of mitochondria and lysosomes may exist in the proximal tubule at birth, prior to complete development of the kidney.     

New mouse genetic models for human contraceptive development

Genetic strategies for the post-genomic sequence age will be designed to provide information about gene function in a myriad of physiological processes. Here an ENU mutagenesis program (http://reprogenomics.jax.org) is described that is generating a large resource of mutant mouse models of infertility; male and female mutants with defects in a wide range of reproductive processes are being recovered. Identification of the genes responsible for these defects, and the pathways in which these genes function, will advance the fields of reproduction research and medicine. Importantly, this program has potential to reveal novel human contraceptive targets.     

The role of Smad signaling in vascular and hematopoietic development revealed by studies using genetic mouse models

Smads are intracellular mediators of transforming growth factor β (TGF-β) superfamily signaling. In this review, we focus on the genetic mouse models for Smad pathways, which have provided functional evidence regarding the complex circuitry in angiogenesis and hematopoiesis during development. In the early stages of vascular development, TGF-β signaling is a contri buting factor in angiogenesis and vascular maturation. Whereas in the later embryogenesis, selected molecules of Smad pathways, such as TGF-β type II receptor (TbRII), ALK5, and Smad5, seem to be dispensable for vessel morphogenesis and integrity. TGF-β signaling is not required in the induction of hematopoietic precursors from mesoderm, but inhibits the subsequent expansion of committed hematopoietic precursors. By contrast, bone morphogenetic protein 4 (BMP4) has long been acknowledged pivotal in mesoderm induction and hematopoietic commitment during development. However, recent genetic evidence shows the BMP4-ALK3 axis is not crucial for the formation of hematopoietic cells from FLK1+ mesoderm. Because of the highly redundant mechanisms within the Smad pathways, the precise role of the Smad signaling involved in vascular and hematopoietic development remains nebulous. The generation of novel cell lineage restricted Cre transgenes would shed new light on the future relevant investigations.     

Immunocytochemical localization of gamma-glutamyltranspeptidase during fetal development of mouse kidney

In the fully developed kidney, gamma-glutamyltranspeptidase is localized predominantly to the apical plasma membrane of the proximal tubules. The appearance of this activity during murine fetal nephrogenesis was quantitated using a sensitive fluorometric assay, and development of membrane polarity was assessed by immunocytochemistry. Specific activity of the transpeptidase in 13-day fetal kidney was approximately 1 mU/mg protein. Between 13-21 days of gestation, total transpeptidase activity increased 7500-fold, whereas specific activity increased 50-fold. At 13 days of gestation, gamma-glutamyltranspeptidase immunoreactivity is localized to the apical surfaces of developing renal vesicles and the proximal segment of the S-shaped tubules. The organized cell structures have tight tubular junctions but lack a well-defined brush-border membrane. By 15 days of gestation, immunostaining of the apical surface of developing proximal segments is more prominent, and slight reactivity of the basolateral membrane is evident. By 17 days of gestation, the kidney is organized into discrete zones. The large increase in gamma-glutamyltranspeptidase activity correlates with the appearance of increased immunostaining of the developing brush-border membranes of the proximal tubules contained in the inner cortex. A very similar although somewhat delayed pattern of appearance of transpeptidase activity and immunostaining was observed in metanephric organ culture. Induction of proximal tubular cyst formation had no effect on the increase in transpeptidase activity that occurred during organotypic nephrogenesis.     

Trb2, a mouse homolog of tribbles, is dispensable for kidney and mouse development

Glomeruli comprise an important filtering apparatus in the kidney and are derived from the metanephric mesenchyme. A nuclear protein, Sall1, is expressed in this mesenchyme, and we previously reported that Trb2, a mouse homolog of Drosophila tribbles, is expressed in the mesenchyme-derived tissues of the kidney by microarray analyses using Sall1-GFP knock-in mice. In the present report, we detected Trb2 expression in a variety of organs during gestation, including the kidneys, mesonephros, testes, heart, eyes, thymus, blood vessels, muscle, bones, tongue, spinal cord, and ganglions. In the developing kidney, Trb2 signals were detected in podocytes and the prospective mesangium of the glomeruli, as well as in ureteric bud tips. However, Trb2 mutant mice did not display any apparent phenotypes and no proteinuria was observed, indicating normal glomerular functions. These results suggest that Trb2 plays minimal roles during kidney and mouse development.     

Wnt/beta-catenin signaling regulates nephron induction during mouse kidney development

Mammalian nephrons form as a result of a complex morphogenesis and patterning of a simple epithelial precursor, the renal vesicle. Renal vesicles are established from a mesenchymal progenitor population in response to inductive signals. Several lines of evidence support the sequential roles of two Wnt family members, Wnt9b and Wnt4, in renal vesicle induction. Using genetic approaches to specifically manipulate the activity of beta-catenin within the mesenchymal progenitor pool in mice, we investigated the potential role of the canonical Wnt pathway in these inductive events. Progenitor-cell-specific removal of beta-catenin activity completely blocked both the formation of renal vesicles and the expected molecular signature of an earlier inductive response. By contrast, activation of stabilized beta-catenin in the same cell population causes ectopic expression of mesenchymal induction markers in vitro and functionally replaces the requirement for Wnt9b and Wnt4 in their inductive roles in vivo. Thus, canonical Wnt signaling is both necessary and sufficient for initiating and maintaining inductive pathways mediated by Wnt9b and Wnt4. However, the failure of induced mesenchyme with high levels of beta-catenin activity to form epithelial structures suggests that modulating canonical signaling may be crucial for the cellular transition to the renal vesicle.     

Regulation of water permeability of collecting ducts in mouse kidney during postnatal development

Water permeability of epithelial cells and response to vasopressin was studied on isolated fragments of collecting ducts in the kidney of C57BL/6J mice of three age groups (9, 18, and 60–90 days). The coefficient of osmotic water permeability P _f was evaluated from the rate of cell swelling after medium osmolality was changed from 300 to 200 mOsm/l. The P _f value proved to be significantly lower in mice at the age of 9 days than at the age of 18 days, i.e., after the transition to mixed feeding; although P _f at the age of 18 days does not reach the level of adult animals (58.6 ± 7.7, 94.5 ± 8.8, and 168.4 ± 11.8 μM/s, respectively). The antagonist of vasopressin V₂ receptors desmopressin at 1 nM significantly increased P _f in both 18-day-old and adult mice but induced no changes in 9-day-old animals. The inhibitor of protein kinase C Ro-31-8220 in the concentration of 100 nM inhibited the desmopressin effect on P _f in 18 day-old and adult mice but did not inhibit the effect of the analog of the vasopressin secondary messenger cAMP, N⁶,O²-dibutyryl cyclic monophosphate, on P _f of the plasma membrane in collecting duct cells. Thus, the response of collecting duct cells to vasopressin appears at the end of wining and correlates with the increase in unstimulated osmotic water permeability of the plasma membrane in collecting duct cells. The vasopressin signal transduction via V₂ receptors is proposed to require the activity of protein kinase C and calcium-dependent systems of intercellular mediators apart from the cAMP-mediated mechanism.     

Expression of Bcl-2 and Bax in mouse renal tubules during kidney development

Bcl-2 and Bax play an important role in apoptosis regulation, as well as in cell adhesion and migration during kidney morphogenesis, which is structurally and functionally related to mitochondria. In order to elucidate the role of Bcl-2 and Bax during kidney development, it is essential to establish the exact location of their expression in the kidney. The present study localized their expression during kidney development. Kidneys from embryonic (E) 16-, 17-, 18-day-old mouse fetuses, and postnatal (P) 1-, 3-, 5-, 7-, 14-, 21-day-old pups were embedded in Epon. Semi-thin serial sections from two E17 kidneys underwent computer assisted 3D tubule tracing. The tracing was combined with a newly developed immunohistochemical technique, which enables immunohistochemistry on glutaraldehyde fixated plastic embedded sections. Thereby, the microstructure could be described in detail, and the immunochemistry can be performed using exactly the same sections. The study showed that Bcl-2 and Bax were strongly expressed in mature proximal convoluted tubules at all time points, less strongly expressed in proximal straight tubules, and only weakly in immature proximal tubules and distal tubules. No expression was detected in ureteric bud and other earlier developing structures, such as comma bodies, S shaped bodies, glomeruli, etc. Tubules expressing Bcl-2 only were occasionally observed. The present study showed that, during kidney development, Bcl-2 and Bax are expressed differently in the proximal and distal tubules, although these two tubule segments are almost equally equipped with mitochondria. The functional significance of the different expression of Bcl-2 and Bax in proximal and distal tubules is unknown. However, the findings of the present study suggest that the mitochondrial function differs between mature proximal tubules and in the rest of the tubules. The function of Bcl-2 and Bax during tubulogenesis still needs to be investigated.     

Towards a comprehensive integration of morphological and genetic studies of floral development

Floral developmental genetics has exploded as a discipline. In addition to the rich genetic database for well-established models (Arabidopsis, Antirrhinum, Zea), numerous species have become the focus of floral genomic and genetic initiatives. Extensive documentation of the developmental morphology of Arabidopsis flowers has facilitated the linkage of genes and morphology. Complete developmental series also need to be assembled for emerging systems of molecular studies of floral genes. We address issues of homology assessment in floral structures, emphasize the need for assembling a complete floral developmental series for each species studied molecularly and genetically, stress the importance of a common set of terminology, and suggest a set of 'landmarks' to designate major events in floral development. We compare the floral developmental series of three species with different floral morphologies and propose a consensus of developmental stages to facilitate comparison of gene expression patterns. Taxa occupying key phylogenetic positions offer great potential for generating hypotheses about the regulation of floral development.