Branching morphogenesis: Rac signaling "PIX" tubulogenesis. Focus on "Pak1 regulates branching morphogenesis in 3D MDCK cell culture by a PIX and beta1-integrin-dependent mechanism"

Genetic Variants in ARHGEF6 Cause Congenital Anomalies of the Kidneys and Urinary Tract in Humans, Mice, and Frogs

BACKGROUND

About 40 disease genes have been described to date for isolated CAKUT, the most common cause of childhood CKD. However, these genes account for only 20% of cases. ARHGEF6, a guanine nucleotide exchange factor that is implicated in biologic processes such as cell migration and focal adhesion, acts downstream of integrin-linked kinase (ILK) and parvin proteins. A genetic variant of ILK that causes murine renal agenesis abrogates the interaction of ILK with a murine focal adhesion protein encoded by Parva , leading to CAKUT in mice with this variant.

METHODS

To identify novel genes that, when mutated, result in CAKUT, we performed exome sequencing in an international cohort of 1265 families with CAKUT. We also assessed the effects in vitro of wild-type and mutant ARHGEF6 proteins, and the effects of Arhgef6 deficiency in mouse and frog models.

RESULTS

We detected six different hemizygous variants in the gene ARHGEF6 (which is located on the X chromosome in humans) in eight individuals from six families with CAKUT. In kidney cells, overexpression of wild-type ARHGEF6 -but not proband-derived mutant ARHGEF6 -increased active levels of CDC42/RAC1, induced lamellipodia formation, and stimulated PARVA-dependent cell spreading. ARHGEF6-mutant proteins showed loss of interaction with PARVA. Three-dimensional Madin-Darby canine kidney cell cultures expressing ARHGEF6-mutant proteins exhibited reduced lumen formation and polarity defects. Arhgef6 deficiency in mouse and frog models recapitulated features of human CAKUT.

CONCLUSIONS

Deleterious variants in ARHGEF6 may cause dysregulation of integrin-parvin-RAC1/CDC42 signaling, thereby leading to X-linked CAKUT.

An mTOR anti-sense oligonucleotide decreases polycystic kidney disease in mice with a targeted mutation in Pkd2

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening hereditary disease in the USA. In human ADPKD studies, sirolimus, a mammalian target of rapamycin complex 1 (mTORC1) inhibitor, had little therapeutic effect. While sirolimus robustly inhibits mTORC1, it has a minimal effect on mTOR complex 2 (mTORC2). Polycystic kidneys of Pkd2WS25/- mice, an orthologous model of human ADPKD caused by a mutation in the Pkd2 gene, had an early increase in pS6 (marker of mTORC1) and pAktSer(473) (marker of mTORC2). To investigate the effect of combined mTORC1 and 2 inhibition, Pkd2WS25/- mice were treated with an mTOR anti-sense oligonucleotide (ASO) that blocks mTOR expression thus inhibiting both mTORC1 and 2. The mTOR ASO resulted in a significant decrease in mTOR protein, pS6 and pAktSer(473). Pkd2WS25/- mice treated with the ASO had a normalization of kidney weights and kidney function and a marked decrease in cyst volume. The mTOR ASO resulted in a significant decrease in proliferation and apoptosis of tubular epithelial cells. To demonstrate the role of mTORC2 on cyst growth, Rictor, the functional component of mTORC2, was silenced in Madin-Darby canine kidney cell cysts grown in 3D cultures. Silencing Rictor significantly decreased cyst volume and expression of pAktSer(473). The decreased cyst size in the Rictor silenced cells was reversed by introduction of a constitutively active Akt1. In vitro, combined mTORC1 and 2 inhibition reduced cyst growth more than inhibition of mTORC1 or 2 alone. In conclusion, combined mTORC1 and 2 inhibition has therapeutic potential in ADPKD.

3D in vitro cell culture models of tube formation

Building the complex architecture of tubular organs is a highly dynamic process that involves cell migration, polarization, shape changes, adhesion to neighboring cells and the extracellular matrix, physicochemical characteristics of the extracellular matrix and reciprocal signaling with the mesenchyme. Understanding these processes in vivo has been challenging as they take place over extended time periods deep within the developing organism. Here, I will discuss 3D in vitro models that have been crucial to understand many of the molecular and cellular mechanisms and key concepts underlying branching morphogenesis in vivo.