Hox genes and kidney patterning

PRC1-mediated epigenetic programming is required to generate the ovarian reserve

The ovarian reserve defines the female reproductive lifespan, which in humans spans decades due to robust maintenance of meiotic arrest in oocytes residing in primordial follicles. Epigenetic reprogramming, including DNA demethylation, accompanies meiotic entry, but the chromatin changes that underpin the generation and preservation of ovarian reserves are poorly defined. We report that the Polycomb Repressive Complex 1 (PRC1) establishes repressive chromatin states in perinatal mouse oocytes that directly suppress the gene expression program of meiotic prophase-I and thereby enable the transition to dictyate arrest. PRC1 dysfuction causes depletion of the ovarian reserve and leads to premature ovarian failure. Our study demonstrates a fundamental role for PRC1-mediated gene silencing in female reproductive lifespan, and reveals a critical window of epigenetic programming required to establish ovarian reserve.

In humans, the ovarian reserve is maintained over decades by meiotic arrest of oocytes. Here the authors show that Polycomb Repressive Complex 1 (PRC1)-mediated epigenetic programming is essential for formation of ovarian reserve and thus female reproductive lifespan.

Cardiovascular aspects of the (pro)renin receptor: Function and significance

Cardiovascular diseases (CVDs), including all types of disorders related to the heart or blood vessels, are the major public health problems and the leading causes of mortality globally. (Pro)renin receptor (PRR), a single transmembrane protein, is present in cardiomyocytes, vascular smooth muscle cells, and endothelial cells. PRR plays an essential role in cardiovascular homeostasis by regulating the renin-angiotensin system and several intracellular signals such as mitogen-activated protein kinase signaling and wnt/β-catenin signaling in various cardiovascular cells. This review discusses the current evidence for the pathophysiological roles of the cardiac and vascular PRR. Activation of PRR in cardiomyocytes may contribute to myocardial ischemia/reperfusion injury, cardiac hypertrophy, diabetic or alcoholic cardiomyopathy, salt-induced heart damage, and heart failure. Activation of PRR promotes vascular smooth muscle cell proliferation, endothelial cell dysfunction, neovascularization, and the progress of vascular diseases. In addition, phenotypes of animals transgenic for PRR and the hypertensive actions of PRR in the brain and kidney and the soluble PRR are also discussed. Targeting PRR in local tissues may offer benefits for patients with CVDs, including heart injury, atherosclerosis, and hypertension.

An evo-devo perspective of the female reproductive tract

The vertebrate female reproductive tract has undergone considerable diversification over evolution, having become physiologically adapted to different reproductive strategies. This review considers the female reproductive tract from the perspective of evolutionary developmental biology (evo-devo). Very little is known about how the evolution of this organ system has been driven at the molecular level. In most vertebrates, the female reproductive tract develops from paired embryonic tubes, the Müllerian ducts. We propose that formation of the Müllerian duct is a conserved process that has involved co-option of genes and molecular pathways involved in tubulogenesis in the adjacent mesonephric kidney and Wolffian duct. Downstream of this conservation, genetic regulatory divergence has occurred, generating diversity in duct structure. Plasticity of the Hox gene code and wnt signaling, in particular, may underlie morphological variation of the uterus in mammals, and evolution of the vagina. This developmental plasticity in Hox and Wnt activity may also apply to other vertebrates, generating the morphological diversity of female reproductive tracts evident today.

Whole genome sequencing and novel candidate genes for CAKUT and altered nephrogenesis in the HSRA rat

The HSRA rat is a model of congenital abnormalities of the kidney and urogenital tract (CAKUT). Our laboratory has used this model to investigate the role of nephron number (functional unit of the kidney) in susceptibility to develop kidney disease as 50-75% offspring are born with a single kidney (HSRA-S), while 25-50% are born with two kidneys (HSRA-C). HSRA-S rats develop increased kidney injury and hypertension with age compared with nephrectomized two-kidney animals (HSRA-UNX), suggesting that even slight differences in nephron number can be an important driver in decline in kidney function. The HSRA rat was selected and inbred from a family of outbred heterogeneous stock (NIH-HS) rats that exhibited a high incidence of CAKUT. The HS model was originally developed from eight inbred strains (ACI, BN, BUF, F344, M520, MR, WKY, and WN). The genetic make-up of the HSRA is therefore a mosaic of these eight inbred strains. Interestingly, the ACI progenitor of the HS model exhibits CAKUT in 10-15% of offspring with the genetic cause being attributed to the presence of a long-term repeat (LTR) within exon 1 of the c-Kit gene. Our hypothesis is that the HSRA and ACI share this common genetic cause, but other alleles in the HSRA genome contribute to the increased penetrance of CAKUT (75% HSRA vs. 15% in ACI). To facilitate genetic studies and better characterize the model, we sequenced the whole genome of the HSRA to a depth of ~50×. A genome-wide variant analysis of high-impact variants identified a number of novel genes that could be linked to CAKUT in the HSRA model. In summary, the identification of new genes/modifiers that lead to CAKUT/loss of one kidney in the HSRA model will provide greater insight into association between kidney development and susceptibility to develop cardiovascular disease later in life.

Mouse Models of Congenital Kidney Anomalies

Congenital anomalies of the kidney and urinary tract (CAKUT) are common birth defects, which cause the majority of chronic kidney diseases in children. CAKUT covers a wide range of malformations that derive from deficiencies in embryonic kidney and lower urinary tract development, including renal aplasia, hypodysplasia, hypoplasia, ectopia, and different forms of ureter abnormalities. The majority of the genetic causes of CAKUT remain unknown. Research on mutant mice has identified multiple genes that critically regulate renal differentiation. The data generated from this research have served as an excellent resource to identify the genetic bases of human kidney defects and have led to significantly improved diagnostics. Furthermore, genetic data from human CAKUT studies have also revealed novel genes regulating kidney differentiation.

Collecting duct prorenin receptor knockout reduces renal function, increases sodium excretion, and mitigates renal responses in ANG II-induced hypertensive mice

Augmented intratubular angiotensin (ANG) II is a key determinant of enhanced distal Na⁺ reabsorption via activation of epithelial Na⁺ channels (ENaC) and other transporters, which leads to the development of high blood pressure (BP). In ANG II-induced hypertension, there is increased expression of the prorenin receptor (PRR) in the collecting duct (CD), which has been implicated in the stimulation of the sodium transporters and resultant hypertension. The impact of PRR deletion along the nephron on BP regulation and Na⁺ handling remains controversial. In the present study, we investigate the role of PRR in the regulation of renal function and BP by using a mouse model with specific deletion of PRR in the CD (_CDPRR-KO). At basal conditions, _CDPRR-KO mice had decreased renal function and lower systolic BP associated with higher fractional Na⁺ excretion and lower ANG II levels in urine. After 14 days of ANG II infusion (400 ng·kg^-1·min^-1), the increases in systolic BP and diastolic BP were mitigated in _CDPRR-KO mice. _CDPRR-KO mice had lower abundance of cleaved αENaC and γENaC, as well as lower ANG II and renin content in urine compared with wild-type mice. In isolated CD from _CDPRR-KO mice, patch-clamp studies demonstrated that ANG II-dependent stimulation of ENaC activity was reduced because of fewer active channels and lower open probability. These data indicate that CD PRR contributes to renal function and BP responses during chronic ANG II infusion by enhancing renin activity, increasing ANG II, and activating ENaC in the distal nephron segments.

Genome-wide analysis of gestational gene-environment interactions in the developing kidney

The G protein-coupled bradykinin B2 receptor (Bdkrb2) plays an important role in regulation of blood pressure under conditions of excess salt intake. Our previous work has shown that Bdkrb2 also plays a developmental role since Bdkrb2(-/-) embryos, but not their wild-type or heterozygous littermates, are prone to renal dysgenesis in response to gestational high salt intake. Although impaired terminal differentiation and apoptosis are consistent findings in the Bdkrb2(-/-) mutant kidneys, the developmental pathways downstream of gene-environment interactions leading to the renal phenotype remain unknown. Here, we performed genome-wide transcriptional profiling on embryonic kidneys from salt-stressed Bdkrb2(+/+) and Bdkrb2(-/-) embryos. The results reveal significant alterations in key pathways regulating Wnt signaling, apoptosis, embryonic development, and cell-matrix interactions. In silico analysis reveal that nearly 12% of differentially regulated genes harbor one or more Pax2 DNA-binding sites in their promoter region. Further analysis shows that metanephric kidneys of salt-stressed Bdkrb2(-/-) have a significant downregulation of Pax2 gene expression. This was corroborated in Bdkrb2(-/-);Pax2(GFP+/tg) mice, demonstrating that Pax2 transcriptional activity is significantly repressed by gestational salt-Bdkrb2 interactions. We conclude that gestational gene (Bdkrb2) and environment (salt) interactions cooperate to impact gene expression programs in the developing kidney. Suppression of Pax2 likely contributes to the defects in epithelial survival, growth, and differentiation in salt-stressed BdkrB2(-/-) mice.

Analysis of nephric duct specification in the avian embryo

Vertebrate kidney tissue exhibits variable morphology that in general increases in complexity when moving from anterior to posterior along the body axis. The nephric duct, a simple unbranched epithelial tube, is derived in the avian embryo from a rudiment located in the anterior intermediate mesoderm (IM) adjacent to somites 8 to 10. Using quail-chick chimeric embryos, the current study finds that competence to form nephric duct is fixed when IM precursor cells are still located in the primitive streak, significantly before the onset of duct differentiation. In the primitive streak, expression of the gene HoxB4 is associated with prospective duct IM, whereas expression of the more posterior Hox gene HoxA6 is associated with more posterior, non-duct-forming IM. Misexpression of HoxA6, but not of HoxB4, in prospective duct-forming regions of the IM resulted in repression of duct formation, suggesting a mechanism for the restriction of duct formation to the anterior-most IM. The results are discussed with respect to their implications for anterior-posterior patterning of kidney tissue and of mesoderm in general, and for the loss of duct-forming ability in more posterior regions of the IM that has occurred during vertebrate evolution.

Comparative spatiotemporal analysis of Hox gene expression in early stages of intermediate mesoderm formation

BACKGROUND

Hox genes are key players in AP patterning of the vertebrate body plan and are necessary for organogenesis. Several studies provide evidence for the role Hox genes play during kidney development and especially regarding metanephros initiation and formation. However, the role Hox genes play during early stages of kidney development is largely unknown. A recent study in our lab revealed the role Hoxb4 plays in conferring the competence of intermediate mesodermal cells to respond to kidney inductive signals and express early kidney regulators.

RESULTS

As a first step in understanding the role Hox genes play in setting the formation of the pronephros morphogenetic field and the expression of early regulators of kidney development, we studied in detail the expression pattern of 10 Hox genes in relation to the 6th somite axial level, the anterior sharp border of the kidney field. Despite the idea of spatial co-linearity as exemplified in the Hox gene expression pattern in late developmental stages, a very dynamic spatio-temporal expression of these genes was found in early stages.

CONCLUSIONS

Since mesodermal patterning occurs at gastrula stages, the relevance of a "Hox code" at early stages is questioned in this study.

A comprehensive analysis of gene expression profiles in distal parts of the mouse renal tubule

The distal parts of the renal tubule play a critical role in maintaining homeostasis of extracellular fluids. In this review, we present an in-depth analysis of microarray-based gene expression profiles available for microdissected mouse distal nephron segments, i.e., the distal convoluted tubule (DCT) and the connecting tubule (CNT), and for the cortical portion of the collecting duct (CCD; Zuber et al., Proc Natl Acad Sci USA 106:16523-16528, 2009). Classification of expressed transcripts in 14 major functional gene categories demonstrated that all principal proteins involved in maintaining the salt and water balance are represented by highly abundant transcripts. However, a significant number of transcripts belonging, for instance, to categories of G-protein-coupled receptors or serine/threonine kinases exhibit high expression levels but remain unassigned to a specific renal function. We also established a list of genes differentially expressed between the DCT/CNT and the CCD. This list is enriched by genes related to segment-specific transport functions and by transcription factors directing the development of the distal nephron or collecting ducts. Collectively, this in silico analysis provides comprehensive information about relative abundance and tissue specificity of the DCT/CNT and the CCD expressed transcripts and identifies new candidate genes for renal homeostasis.

Interplay between activin and Hox genes determines the formation of the kidney morphogenetic field

The kidney develops in a specific position along the anterior-posterior axis. All vertebrate kidney tissues are derived from the intermediate mesoderm (IM), and early kidney genes such as Lim1 and Pax2 are expressed in amniotes posterior to the sixth somite axial level. IM cells anterior to this level do not express kidney genes owing to changes in their competence to respond to kidney-inductive signals present along the entire axis. We aimed to understand the molecular mechanisms governing the loss of competence of anterior IM cells and the formation of the anterior border of the kidney morphogenetic field. We identified the dorsal neural tube as the potential kidney-inductive tissue and showed that activin, a secreted morphogen, is necessary but insufficient for Lim1 induction and establishment of the kidney field. Activin or activin-like and BMP signaling cascades are activated along the entire axis, including in anterior non-kidney IM, suggesting that competence to respond to these signals involves downstream or other components. Detailed expression pattern analysis of Hox genes during early chick development revealed that paralogous group four genes share the same anterior border as the kidney genes. Ectopic expression of Hoxb4 in anterior non-kidney IM, either by retinoic acid (RA) administration or plasmid-mediated overexpression, resulted in ectopic kidney gene expression. The anterior expansion of Lim1 expression was restrained when Hoxb4 was co-expressed with a truncated form of activin receptor. We suggest a model in which the competence of IM cells to respond to TGFbeta signaling and express kidney genes is driven by RA and mediated by Hoxb4.

Role of HOXA7 to HOXA13 and PBX1 genes in various forms of MRKH syndrome (congenital absence of uterus and vagina)

The Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome refers to the congenital absence or severe hypoplasia of the female genital tract, often described as uterovaginal aplasia which is the prime feature of the syndrome. It is the second cause of primary amenorrhea after gonadal dysgenesis and occurs in approximately 1 in 4500 women. Aetiology of this syndrome remains poorly understood. Frequent association of other malformations with the MRKH syndrome, involving kidneys, skeleton and ears, suggests the involvement of major developmental genes such as those of the HOX family. Indeed mammalian HOX genes are well known for their crucial role during embryogenesis, particularly in axial skeleton, hindbrain and limb development. More recently, their involvement in organogenesis has been demonstrated notably during urogenital differentiation. Although null mutations of HOX genes in animal models do not lead to MRKH-like phenotypes, dominant mutations in their coding sequences or aberrant expression due to mutated regulatory regions could well account for it. Sequence analysis of coding regions of HOX candidate genes and of PBX1, a likely HOX cofactor during Müllerian duct differentiation and kidney morphogenesis, did not reveal any mutation in patients showing various forms of MRKH syndrome. This tends to show that HOX genes are not involved in MRKH syndrome. However it does not exclude that other mechanisms leading to HOX dysfunction may account for the syndrome.

The Mayer-Rokitansky-Küster-Hauser syndrome (congenital absence of uterus and vagina)--phenotypic manifestations and genetic approaches

The Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome affects at least 1 out of 4500 women and has for a long time been considered as a sporadic anomaly. Congenital absence of upper vagina and uterus is the prime feature of the disease which, in addition, is often found associated with unilateral renal agenesis or adysplasia as well as skeletal malformations (MURCS association). The phenotypic manifestations of MRKH overlap various other syndromes or associations and thus require accurate delineation. Since MRKH manifests itself in males, the term GRES syndrome (Genital, Renal, Ear, Skeletal) might be more appropriate when applied to both sexes. The MRKH syndrome, when described in familial aggregates, seems to be transmitted as an autosomal dominant trait with an incomplete degree of penetrance and variable expressivity. This suggests the involvement of either mutations in a major developmental gene or a limited chromosomal deletion. Until recently progress in understanding the genetics of MRKH syndrome has been slow, however, now HOX genes have been shown to play key roles in body patterning and organogenesis, and in particular during genital tract development. Expression and/or function defects of one or several HOX genes may account for this syndrome.

GDNF-inducible zinc finger protein 1 is a sequence-specific transcriptional repressor that binds to the HOXA10 gene regulatory region

The RET tyrosine kinase receptor and its ligand, glial cell line-derived neurotrophic factor (GDNF) are critical regulators of renal and neural development. It has been demonstrated that RET activates a variety of downstream signaling cascades, including the RAS/mitogen-activated protein kinase and phosphatidylinositol-3-kinase(PI3-K)/AKT pathways. However, nuclear targets specific to RET-triggered signaling still remain elusive. We have previously identified a novel zinc finger protein, GZF1, whose expression is induced during GDNF/RET signaling and may play a role in renal branching morphogenesis. Here, we report the DNA binding property of GZF1 and its potential target gene. Using the cyclic amplification and selection of targets technique, the consensus DNA sequence to which GZF1 binds was determined. This sequence was found in the 5' regulatory region of the HOXA10 gene. Electrophoretic mobility shift assay revealed that GZF1 specifically binds to the determined consensus sequence and suppresses transcription of the luciferase gene from the HOXA10 gene regulatory element. These findings thus suggest that GZF1 may regulate the spatial and temporal expression of the HOXA10 gene which plays a role in morphogenesis.

Ectopic expression of Hoxd10 in thoracic spinal segments induces motoneurons with a lumbosacral molecular profile and axon projections to the limb

Hox genes encode anterior-posterior identity during central nervous system development. Few studies have examined Hox gene function at lumbosacral (LS) levels of the spinal cord, where there is extensive information on normal development. Hoxd10 is expressed at high levels in the embryonic LS cord but not the thoracic cord. To test the hypothesis that restricted expression of Hoxd10 contributes to the attainment of an LS identity, and specifically an LS motoneuron identity, Hoxd10 was ectopically expressed in thoracic segments in chick embryos by means of in ovo electroporation. Regional motoneuron identity was assessed after the normal period of motoneuron differentiation. Subsets of motoneurons in transfected thoracic segments developed a molecular profile normally shown by LS motoneurons, including Lim 1 and RALDH2 expression. In addition, motoneurons in posterior thoracic segments showed novel axon projections to two muscles in the anterodorsal limb, the sartorius and anterior iliotibialis muscles. At thoracic levels, we also found a decrease in motoneuron numbers and a reduction in gonad size. These last findings suggest that early and high levels of Hox expression impeded motoneuron development and neural-mesodermal interactions. Despite these adverse effects, our data indicate that Hoxd10 expression is sufficient to induce LS motoneuron identity and axon trajectories characteristic of motoneurons in the LS region.

Contribution of Hox genes to the diversity of the hindbrain sensory system

The perception of environmental stimuli is mediated through a diverse group of first-order sensory relay interneurons located in stereotypic positions along the dorsoventral (DV) axis of the neural tube. These interneurons form contiguous columns along the anteroposterior (AP) axis. Like neural crest cells and motoneurons, first-order sensory relay interneurons also require specification along the AP axis. Hox genes are prime candidates for providing this information. In support of this hypothesis, we show that distinct combinations of Hox genes in rhombomeres (r) 4 and 5 of the hindbrain are required for the generation of precursors for visceral sensory interneurons. As Hoxa2 is the only Hox gene expressed in the anterior hindbrain(r2), disruption of this gene allowed us to also demonstrate that the precursors for somatic sensory interneurons are under the control of Hox genes. Surprisingly, the Hox genes examined are not required for the generation of proprioceptive sensory interneurons. Furthermore, the persistence of some normal rhombomere characteristics in Hox mutant embryos suggests that the loss of visceral and somatic sensory interneurons cannot be explained solely by changes in rhombomere identity. Hox genes may thus directly regulate the specification of distinct first-order sensory relay interneurons within individual rhombomeres. More generally, these findings contribute to our understanding of how Hox genes specifically control cellular diversity in the developing organism