Cell Polarity and Cystic Diseases

There was an interesting article and accompanying editorial in JASN this month about the role of polarity in the development of cystic renal diseases. The maintenance of epithelial cell polarity is vital for the normal functioning of the renal tubules and the usual interpretation of this is the way in which a cell is organized into basolateral and luminal compartments (e.g. in intercalated cells, there is a luminal H+ ATPase and a basolateral anion exchanger and it is this polarity that facilitates acid excretion). In a similar way, mislocalization of the Na-K-ATPase has been noted in patients with ADPKD and is a putative mechanism for cyst formation.

However, there is another facet to polarity which appears to be just as important – planar polarity – this is the correct orientation of cells and specialized structures within cells along the plane of the epithelial sheet. Much research is continuing into the genes responsible for the development and maintenance of polarity. One of the means by which a loss of planar polarity may induce the formation of cysts is through the loss or dysfunction of primary cilia.

The authors in JASN found that both a gain and loss of function mutation in ErbB4 led to a loss of cell polarity. Members of this receptor family have been shown to be responsible for the development of polarity in neurons and in certain cancers.

Genetics of Wilms Tumor

Wilms Tumor--named after the German surgeon/pathologist Max Wilms (pictured at left)--is an embryonal tumor that derives from developing kidney tissue. Wilms was the first to postulate that tumors may arise from precursor cells which arise during development, and indeed study of the molecular pathways active in these "nephroblastoma" shed light on normal kidney development.

There are several genes associated with patients with Wilms Tumor. Here are some of the main ones:

1. WT1 is a transcription factor and considered a tumor suppressor gene. Mutations in WT1 account for between 10-15% of sporadic Wilms tumor. It interacts with p53, a classic tumor suppressor involved in a wide variety of cancers. Denys-Drash Syndrome, a familial and severe form of Wilms tumor, is usually caused by congenital WT1 mutations.

2. beta-catenin is a key component of the canonical Wnt signaling pathway, long known to be a key player in kidney development. Interestingly, most patients with WT1 also have gain-of-function point mutations in the beta-catenin gene which result in increased stability of the beta-catenin protein and subsequent unregulated Wnt signaling.

3. WTX is mutated in a different subset of patients than those with WT1 mutations, and is found on the X-chromosome.

4. BDNF (brain-derived neurotrophic factor): mutations in this growth factor are postulated to result in the WAGR Syndrome--a constellation of symptoms that includes Wilms Tumor along with aniridia, GU abnormalities, and mental retardation.

5. BRCA2: interestingly, mutations in the well-known breast cancer-susceptibility gene can also lead to Wilms tumor.

Some Cool, Geeky Science-Type Kidney Web Resources

Two very interesting web sites for those involved in the study of mouse as a model organism for the study of kidney disease and kidney development:

The first is the Genitourinary Development Molecular Anatomy Project (GUDMAP) site, which contains a useful database of thousands of genes' expression patterns in the developing kidney.   

The second is the EureGene Kidney Atlas, part of the European Renal Genome Project, which contains databases for kidney gene expression, phenotypes of mouse knockout lines, and some nifty images of the developing kidney at different stages.  

Structure & Function of Other Animals' Kidneys

Here's a very brief overview of the kidneys of animals (other than humans). All vertebrates have kidneys and the functional unit of the kidney in all species is the nephron.  Freshwater fish--like the zebrafish--have a kidney which is kind of like a long tube.  In zebrafish embryos (as discussed previously in another post) the kidney starts out as a structure called the pronephric duct, which consists of a single nephron, and as the fish becomes an adult develops into the adult mesonephros which contains multiple nephrons.  The function of the freshwater fish kidney is predominantly to create a dilute urine that osmolar homeostasis. 

The frog kidney must be able to adapt to both freshwater and land conditions--thus the frog kidney must be able to create both a dilute and a concentrated urine, depending on the environment to which the frog is exposed.

Snakes live in a dry environment and therefore their kidneys must retain water.  Snakes metabolize nitrogenous wastes into uric acid, which can be excreted in an insoluble form using very small amounts of water.  

The bird kidney also eliminates waste via uric acid.  The whitish pasty component of birdshit is in fact a uric acid paste that represents the renal waste product, which is usually mixed in with the brown stuff (the feces from the GI tract).  

Mouse kidneys are similar to human kidneys in that they use soluble urea, rather than insoluble uric acid, as the waste product.  

Branching Morphogenesis

If a picture is worth a thousand words, then a movie is worth a thousand pictures, right?

What is this video of exactly?  This elegant work comes from the lab of Frank Costantini and colleagues at Columbia University, who developed a transgenic mouse line which expressed green fluorescent protein (GFP) under the control of the Hoxb7 promoter, leading to expression of GFP in the ureteric bud & developing collecting system, but not the surrounding mesenchyme which will give rise to the glomeruli and proximal nephrons.  They are able to grow the kidney as a tissue explant--that is, they surgically remove the kidney and grow it in a dish so they can closely examine its development. 

I have included it to illustrate the principle of branching morphogenesis:  a developmental program that is responsible for the highly branched collecting system of the mammalian kidney, which in the human kidney can ultimately will contain hundreds of thousands of nephrons.  How does the ureteric bud know when to start branching?  How does it know when to STOP branching?  What decisions control which cells get to branch and which one stay behind to form, say, the calices as opposed to the collecting ducts?  This model enables these researchers to ask cool developmental biology questions such as this.  This work has broad relevance to developmental biologists, and not just those interested in the kidney--other epithelial tissues such as the lung, breast, and salivary glands for instance appear to use a similar program of branching morphogenesis.  

EMT = epithelial-to-mesenchymal transition

The three letter acronym EMT:  what should it mean to nephrologists?  Not just "emergency medical technician"...but also "epithelial-to-mesenchymal transition."  

As we all know the nephron is comprised of a long tube of highly-differentiated epithelial cells.  These cells have clearly defined apical and basolateral membranes, tight junctions, and cilia.  Under normal conditions, these cells should remain epithelial cells for the duration of their lifespan.  However, under various pathologic conditions they may become mesenchymal cells, which are less differentiated, lack such an obvious apical-basolateral polarity, and have an absence of tight junctions.  One current hypothesis is that the renal parenchymal fibrosis that characterizes CKD is due to a TGF-beta-mediated epithelial-to-mesenchymal transition of tubular cells, turning them into more of a fibroblast-like cell type which secretes extracellular matrix.  

The same type of cellular re-programming is seen in cancers which derive from well-differentiated epithelium (e.g., colon cancer, breast cancer) but develop into metastatic, poorly-differentiated cells.  

During development, the EMT process essentially works in reverse:  you start with mesenchymal cells, and via cross-talk between the metanephric mesenchyme and the ureteric bud there is a gradual commitment of cells to an epithelial morphology.  

Ret & Kidney Development

The Ret gene plays an integral role in kidney development. As is covered in any basic embryology class, the cross-talk between the ureteric bud (destined to become the collecting system of the kidney) and the metanephrogenic mesenchyme (destined to become the renal parencyma and tubules) is a prime example of how reciprocal signals from two distinct tissue types can influence the development of the other. In the case of the kidney, GDNF (glial-derived neurotrophic factor) secreted by the mesenchyme interacts with the Ret receptor, a membrane tyrosine kinase, on the ureteric bud. This interaction is critical for branching morphogenesis, the process by which the kidney serially reiterates in order to create an organ with a million nephrons, as demonstrated by the fact that mice deficient in the Ret gene have a congenital absence of the kidney.

Ret is important other human diseases as well. It is a proto-oncogene, as gain-of-function mutations lead to various forms of multiple endocrine neoplasia (MEN). Loss-of-function mutations in contrast can lead to either Hirschprung's disease (a congenital absence of the enteric nerves) or renal hypoplasia.

UPJ Obstruction

UPJ obstruction is another common congenital abnormality which under some circumstances can lead to significant renal scarring and chronic kidney diseases. About 50% of ultrasounds during pregnancy which detect antenatal hydronephrosis turn out to have UPJ obstruction.

UPJ obstruction is defined as any blockage of urine flow from the renal pelvis to the proximal ureter. The subsequent back pressure within the pelvis can lead to dilatation of the renal collecting system and, if left untreated, can lead to permanent renal damage.

Children with UPJ obstruction typically present with pain, hematuria, recurrent UTIs, failure to thrive, and palpable mass--however, with the increased use of ultrasound, it is not uncommon now to make the diagnosis with routine ultrasound.

UPJ obstruction is a relatively non-specific diagnosis: an anatomical obstruction here can occur due to several reasons: scarring of ureteral valves due to recurrent UTIs, ureteral hypoplasia or abnormal insertion of the ureter into the renal pelvis which might occur as a result of congenital problems of kidney development, and fibrosis following surgery for stone disease are all possible etiologies for UPJ obstruction.

Horseshoe Kidney versus Crossed Fused Ectopia

Horseshoe kidney is a relatively common (1 in 400 live births) congenital malformation of the kidney in which the kidneys are fused together. 90% of the time the fusion occurs at the lower poles (upper left photo) and the isthmus (the tissue connecting the two poles) is comprised of either renal parencyhmal tissue or fibrous tissue. In humans, renal development begins during the 4th gestational week, and presumably errors during this process can lead to horseshoe kidney. In the majority of cases, patients are asymptomatic; however, the anatomic abnormalities may be associated with a more complex developmental syndrome OR may result in urologic abnormalities (e.g. reflux nephropathy) that causes CKD. In addition, there is some evidence that some renal tumors (e.g., Wilms' Tumor) may occur with increased frequency in patients with these congenital abnormalities.

In contrast, crossed fused ectopia involves both fused kidneys which lie on one side of the spine, and the ureter of the crossed kidney crosses the midline in order to enter the bladder (lower left photo).