From the Nate Hellman Unpublished Archive: microRNAs and the Kidney

MicroRNAs: one of the sexiest topics in the science community these days! Pick up a copy of Science, Nature, or Cell and you can usually find something to do with microRNAs within. What do these have to do with the kidney? Well, it's still a little premature, but there are certainly microRNAs present in the kidney, and recent studies have demonstrated that specific microRNAs are upregulated in conditions such as polycystic kidney disease and renal cell carcinomas, for instance.

How microRNAs work: microRNAs are single-stranded RNA molecules of between 21-23 nucleotides in length which are partially complementary to regions in multiple mRNAs. Once they bind to these mRNAs, the microRNAs either inhibit translation or completely degrade their target RNAs. A specific enzymatic machinery--comprised of the proteins Dicer and the RISC complex--is responsible for inhibiting mRNAs via microRNAs.

In essence, the power of microRNAs are that a single microRNA can regulate the expression of multiple genes working in parallel to achieve a similar biologic effect. This technology is of particular use to the pharmaceutical industry: one can envision targeting a particular microRNA which inhibit several pathways to prevent a disease process, such as atherosclerosis, renal fibrosis, or cyst formation to think of a few possibilities. The field is still very new. I wouldn't be surprised if a future Nobel Prize came out of this work. Three scientists (Drs. Ruvkun, Baulcombe, and Ambros) working on microRNAs recently won the 2008 Lasker Prize--considered by many to be the "precursor" prize to the Nobel.

Does nephrology need personalized medicine?

Systems biology is one of science’s growth areas. Sequencing technologies and software tools developed on the back of the human genome project have reduced the cost of, and therefore increased access to, large and complex datasets (ending in -ome) of genome sequences (genomics), gene expression (transcriptomics) and proteins and metabolites (proteomics and metabolomics). Systems biological techniques integrate these datasets and provide insights into how phenotypes may emerge from interacting biological processes rather than isolated genes or proteins.

A recent editorial in the journal Nephrology Dialysis Transplantation examined this field in general and its relevance to nephrology. The authors mention that –omic datasets have been useful in modeling “self-organized highly interconnected networks”, and that such networks have implicated unexpected candidates in disease pathogenesis (see for example, this paper on cardiac hypertrophy). 

The review goes on to suggest that using the tools of systems biology to finely phenotype individuals will usher in an era of truly personalized medicine. However, it is not clear to me that a definite sequel to this type of analysis will be the personalization of treatment or even that the concept of personalized medicine is particularly suited to our current view of what constitutes clinical evidence.

Diseases such as the ANCA-associated vasculitides (AAV) are now known to exhibit genomic variability. Randomised controlled trials (RCTs) in AAV (such as here and here) have been hampered by: 

1. Short follow-up times 
2. Inter-group heterogeneity which may have affected outcomes. These factors have contributed to ongoing debate about the applicability of the results of these trials (see correspondence here). 
3. Additionally a recent trial in membranous nephropathy, likely to represent another disease with distinct –omic subsets, was marked by slow recruitment. 

 

All these points together suggest that it may be difficult to conduct meaningful clinical studies of distinct –omic subtypes in nephrological diseases. Currently, primacy is given to RCTs when evaluating the efficacy of new treatments; and in nephrology the community is finally beginning to produce the RCTs which have been absent historically. 

If the focus is to switch away from RCTs with their large, well-matched study groups and towards splitting groups up by some -omic fingerprint I am able to envisage a time when one has to choose between giving more credence to the results of larger, “non-personalised” trials or smaller studies featuring –omic data but lacking the controlled element of RCTs.  Would this represent progress?

A trap for angiotensin type 1 receptors

The renin-angiotensin system is powerful regulator of blood pressure homeostasis. An article published in the March 24 issue of JASN highlights an emerging area of research into the modulation of AT1 receptors on different tissues.

The majority of well-known angiotensin II actions are mediated via AT1 receptor stimulation, and angiotensin converting enzyme inhibitors (ACEi) and AT1 receptor blockers (ARBs) have been widely used as antihypertensive agents with cardiovascular protective effects. Elucidating factors that regulate AT1 receptor expression levels in different tissue compartments will hopefully lead to novel agents to treat hypertension and its associated end-organ damage (ESRD, CHF and Stroke). Recent studies have demonstrated the existence of several proteins interacting with AT1 receptors that may modulate AT1 receptor expression level, sensitivity and internalization. A recent review article highlights this emerging field. Atrap (AT1 receptor-associated protein) is the best characterized protein and the focus of the March 24 article by Oppermann et al.

It was previously reported that Atrap (a 19kD protein) interacts specifically with the carboxyl-terminal domain of the A1a receptor and catalyzes its internalization in cultured cells. Prior in vivo studies over-expressing the Atrap protein in the heart, aortae and femoral artery in mice demonstrated a protective effect in these tissues in response to angiotensin II infusion. Overall, the prevailing literature suggested an inhibitory or protective effect of Atrap on AT1 receptor function. This group knocked out the Atrap gene in mice to test this hypothesis in vivo.

They reported that Atrap KO mice have a higher resting blood pressure (by 10 mmHg) as measured by radiotelemety. Suggesting that Atrap is important in regulating basal blood pressure and that its absence leads to systemic hypertension. Interestingly, they found that Atrap is highly expressed in the kidney (then testis=adrenal>heart>lung=liver=aorta=brain). Specifically, they showed that Atrap is highly expressed in the proximal tubule of the kidney. Previous investigations have focused on the distal tubule as potential sites that affect blood pressure as most of the known genetic mutations in sodium transport causing hyper/hypotension are linked to the distal nephron (Bartter, Little, Gitelman syndromes, etc). This group reports that Atrap KO mice have more AT1 receptors in the proximal tubule which allows for increased angII-dependent NHE-3 activity causing volume expansion and hypertension.

This highlights the importance of investigating novel pathways regulating the renin-angiotensin system. Modulation of this system with the use of ACEi, ARBs and now direct renin inhibitors have continued to be the mainstay of therapy for patients with heart failure, diabetes, hypertension and kidney disease. Activation of Atrap may be more specific and more physiological for inhibition of At1 receptor signaling. New drug discovery modulating Atrap protein expression could lead to novel therapy. For now, much more research is needed.

The actin cytoskeleton of the podocyte

Peter Mundel, a well-known researcher of podocyte biology, gave our Renal Grand Rounds today.  Here's what I took away from this morning's talk:

-regulation of the podocyte's actin cytoskeleton is postulated to be the final common pathway in most instances of nephrotic syndrome/proteinuria.

-in some ways it is better to think of the podocyte as a modified smooth muscle cell rather than a modified epithelial cell based on its intricate network of actin & myosin which mediate contractility.

-the medication cyclosporine (used in the treatment of some refractory forms of nephrotic syndrome) acts NOT on modulation of immune cell function as previously assumed, but rather on direct effects on the podocyte actin cytoskeleton, as detailed in a prior post.

-a key event in generating abnormal, protein-leaking podocytes is turning them from a stationary cell to a motile one.  This key transition is mediated by three distinct Rho GTPase proteins, already established to mediate motility in several cell types. In general, Cdc42 and Rac1 promote podocyte motility, whereas RhoA promotes podocyte stabilization. It is possible that targeting these Rho GTPase pathways could lead to novel therapies for podocytopathies, and this is being investigated.  

Check out a good 2009 Kidney International review of the topic by Mundel and Reiser.

How Luminex Beads Work

You may have come across the phrase "Luminex beads" while doing your transplant nephrology rotation, or heard the term while hanging out in the Tissue Typing Lab. This post is intended to briefly describe the Luminex technology and how it works. A good review by Tait et al can be found here.  

Acute cellular rejection in kidney transplant recipients comes from the presence of recipient anti-HLA antibodies directed against donor antigens. How do we detect such antibodies and prevent donor-recipient mismatches from occurring? Traditionally--and still one of the most practical tests--we use the complement-dependent cytotoxicity (CDC) "cross-match" assay. While useful, however, more recent "solid-phase" technologies have allowed a more sensitive detection of anti-HLA antibodies in recipient serum, and the Luminex system is one of them.  

Briefly, the Luminex technology consists of a series of differently-colored polysterene beads, populations of which contain distinct HLA molecules attached to them. The lab incubates an aliquot of the kidney transplant recipient's serum with an aliquot of beads, and if there are any anti-HLA antibodies present they will bind to the beads. A second phycoerythrin-labelled anti-human IgG antibody (aka, a "secondary antibody") is then added, and after washing off unbound antibody, the beads are passed through a machine which works like a flow cytometer, dropping beads one-by-one through a narrow chamber. As the beads pass through the chamber, they are hit with lasers of a specific wavelength, exciting both the fluorescently-labeled secondary antibody AND the fluorochrome within the bead itself--thus allowing the detection and identification of specific recipient antibodies.  

There are different types of beads:  for example, some beads are coated with many different Class I or Class II molecules, designed to detect a wide range of recipient antibodies. Some beads are designed to be like a single cell, containing HLA antigens encoded by two different alleles from each of the Class I (HLA-A, B, & C) and Class II (HLA-DP, DQ, DR) loci. Finally, there are the "single antigen beads" (SAG), a bead coated with one sole antigen. This can be particularly useful in patients with a high PRA (panel reactive antibody)--you can actually identify the specific HLA antigens to which the individual is sensitized.  

While potentially quite useful, it is important to also realize the limitations of this technology. While much more sensitive than the standard CDC cross-match, it is not always clear what a positive antibody result means--though in most instances, the CDC cross-match is still the primary determinant as to whether a given kidney donor-recipient pairing is deemed acceptable.  

Cool New Cell Paper Demonstrates Molecular Basis for Thyroxic Hypokalemic Periodic Paralysis

The periodic paralysis syndromes are characterized by episodic attacks of acute muscle weakness, typically due to rapid fluxes in the serum potassium concentration, based on an abnormality in intracellular potassium shift. While there have been several instances of inherited mutations that cause periodic paralysis, there is also a subset of individuals who have thyrotoxic periodic paralysis (TPP), in which the presence of hyperthyroidism predisposes to attacks of transient paralysis. An article in this month's Cell by Ryan et al helps determine the molecular basis of TPP in many (but not all) cases, and characterizes this disorder as yet another example of a channelopathy: a disorder of ion channels.

Although thyrotoxicosis is a predisposing factor to this disease, there was also a clue that genetics was involved: Latin American and Asian populations appeared especially susceptible to TPP. The investigators identified a novel inward-rectifying potassium channel, Kir2.6 (interestingly, a gene which had escaped detection in all versions of the human genome thus far!), and sequenced this gene in affected individuals. In 33% of the unrelated patients in their sample, they identified mutations in Kir2.6 which appear to alter the function of this potassium channel and lead to an altered skeletal muscle excitability. Interestingly, the transcription of Kir2.6 was found to be altered by thyroid hormone, providing an explanation as to why the disease manifests itself most commonly during episodes of thyrotoxicosis.

A role for polycystins as blood pressure sensors?

One of the hallmarks of autosomal dominant polycystic kidney disease (ADPKD) is hypertension. True, most patients with advanced kidney disease get hypertension anyways--but often the degree of hypertension in ADPKD patients seems to be especially high. Is there something about the function of the polycystin proteins which explains the high blood pressure?

A recent article by Sharif-Naeini et al in this month's issue of Cell claims a pressure-sensing role for the polycystins. ADPKD is caused by mutations in two genes, PKD1 and PKD2, which encode for membrane proteins termed TRPP1 and TRPP2, respectively. It appears these genes function via regulating local calcium fluxes in and out of the cell. Although most efforts to understand TRPP1 and TRPP2 function have focused on renal tubular epithelial cells (since that's where cysts come from), a variety of extra-renal phenotypes are also observed. In this paper, the authors generated mice deficient in TRPP1 or TRPP2 ONLY in smooth muscle cells. Importantly, they found that the balance of TRPP1 and TRPP2 levels was critical for the ability of arterial smooth muscle to maintain appropriate blood pressure. These results imply that function of the polycystins may be different depending on cell type, and suggest a possible mechanism by which ADPKD patients may be especially prone to the development of abnormalities in blood pressure regulation.

Serum Amyloid P prevents renal fibrosis

Congratulations to my colleague at the Brigham, Jeremy Duffield, who made the cover of Science this week for demonstrating human Serum Amyloid P (hSAP) potently inhibits fibrosis in two independent models of renal fibrosis. hSAP is a naturally circulating soluble pattern recognition receptor, and radio-labelled SAP is used clinically to identify sites of amyloid deposition in systemic amyloidosis. In the studies, hSAP was given to mice with either unilateral ureteric obstruction or unilateral ischemia reperfusion mediated kidney injury. In both cases, hSAP potently suppressed fibrotic collagen protein and collagen gene expression in a sustained fashion, preventing the development of interstitial fibrosis. hSAP acts by binding danger molecules at sites of tissue injury, causing the complexes to be cleared by the Fcγ family of receptors on macrophages. This results in suppression of inflammatory and fibrotic gene and protein expression in monocyte-derived cells, via an interleukin-10 dependant mechanism.

These findings raise the possibility of using hSAP as a therapy for kidney diseases with a prominent fibrotic component, such as diabetic nephropathy or chronic allograft nephropathy. A recombinant form of human Serum Amyloid P, PRM-151 (rhSAP), is already in phase 1 trials.

The Uroplakins

We are taught that water is freely diffusable via cell membranes--even in the absence of aquaporin function, there are usually some water molecules which traverse the lipid bilayer.  However, there are tissues within the human body which must be extraordinarily tight:  two that come to mind are the thick ascending limb of the nephron, as well as the bladder epithelium.  How does the epithelium comprising these "water-tight" barriers obtain these characteristics?  

The epithelium lining the bladder (a.k.a. "the urothelium") is one of the tightest epithelia in the body:  the bladder must be capable of holding urine for long periods of time without leakage, and furthermore serves as an important barrier for toxic substances filtered by the kidney.  The urothelium's water-impermeable properties can be explained in large part by a family of proteins called the uroplakins.  The uroplakins form tiny, hexagonal arrays of particles--visualized best by electron microscopy (see figure taken from this excellent recent KI review by Wu et al)--which comprise structures called "urothelial plaques" that overlie the plasma membrane of superficial umbrella cells of the urothelium.  It is thought that these plaques are tethered to the lipid bilayer, limiting the movement of phospholipids and therefore limiting water permeability.  Interestingly, mice deficient in uroplakins show increased water permeability.  

The uroplakins also appear to play a role in urinary tract infections; the uroplakin Ia/Ib glycoprotein is the means by which some strains of E. coli adhere to the urothelium.  

Gordon Syndrome

File "Gordon Syndrome" under "interesting causes of hyperkalemia and metabolic acidosis you may never see."

Also called pseudohypoaldosteronism type II, Gordon Syndrome is relevant less so for the number of patients afflicted but more due to the interesting insights into normal acid-base and electrolyte physiology.

Briefly, patients with Gordon Syndrome, a genetically-inherited condition, exhibit salt-sensitive hypertension, hyperkalemia, and a non-anion gap metabolic acidosis in association with a normal GFR. These metabolic derangements tend to be highly responsive to thiazide diuretics, correctly implying the disease is due to a constitutive activation of thiazide-sensitive Na channels in the distal convoluted tubule. In fact, Gordon Syndrome can be thought of as a mirror image of Gitelman's Syndrome, in which there is inactivation of the thiazide-sensitive Na channels causing the exact opposite metabolic abnormalities (hypokalemia and metabolic alkalosis).

It turns out that Gordon Syndrome is caused by mutations in two different, related genes which encode for a type of kinase: either gain-of-function mutations in WNK1, or loss-of-function mutations in WNK4 ("WNK kinase" stands for "with no lysine kinase"). WNK4 is responsible for tonic inhibition of the thiazide-sensitive Na; its loss-of-function therefore results in unregulated Na reabsorption in the distal tubule. This leads to decreased Na delivery to the collecting duct, resulting in reduced tubular lumen electronegativity, the driving force for aldosterone-mediated potassium and H+ secretion. WNK1 is a negative regulator of WNK4 and this explains why gain-of-function in WNK1 can cause the same phenotype as loss-of-function in WNK4. Part of the clinical phenotype seen in these patients may also have to do with WNK effects on the potassium channel ROMK, illustrating the complex molecular biology of this pathway.

Telomeres, Renal Cell Carcinoma, and the 2009 Nobel Prize in Medicine

The winners of the 2009 Nobel Prize in Medicine were revealed today, shared by three Americans: Carol Greider, Elisabeth Blackburn, and Jack Szostak, the latter of whom works at my home institution of Massachusetts General Hospital. The award was given for their work on telomeres, the regions of repetitive DNA which form protective "caps" on the ends of chromosomal DNA, necessary for preventing the degradation of DNA ends. The study of telomeres has implications not only for basic molecular biology, but also for cancer biology: many tumor lines express an enzyme called telomerase, which catalyzes the addition of telomeres onto DNA ends by virtue of a reverse transcriptase-based mechanism, allowing cancer cells to sustain their high rate of cell division.

For instance, in a 1999 KI study by Dahse et al, increased telomerase activity was detected in 55 out of 60 different primary renal cell carcinoma lines.

Aquaporins and the Kidney

The discovery of aquaporins--the protein family of water channels which regulate water transport in the kidney and many other tissues--was considered significant enough to win the Nobel Prize in Chemistry for Peter Agre in 2003.  

The aquaporins are a large family--there are at least 12 aquaporin genes in humans and possibly more--but which ones are important in the kidney?  It turns out that different aquaporins are expressed in discrete segments along the nephron.  The one which gets the most attention is likely aquaporin-2 (AQP2), which is expressed in the apical membrane and intracellular vesicles of principal cells of the collecting duct.  AQP2 is the predominant vasopressin-responsive aquaporin, and is therefore responsible for the majority of regulation of water reabsorption in the collecting duct.  According to the current model, vasopressin binds to its receptor on the basolateral surface of collecting duct cells, resulting in an increase in cyclic AMP, protein kinase A activity, and the increased trafficking of intracellular AQP2-containing vesicles to the apical membrane, where they fuse with the plasma membrane and allow water reabsorption to take place.  Mutations in the AQP2 gene account for some cases of nephrogenic diabetes insipidus. 

 

The other aquaporins also play specific roles in water transport.  For instance, aquaporins-3 and -4 are expressed at the basolateral surface of cortical collecting duct cells and are responsible for movement of water taken in by AQP2.  Aquaporin-1 is extremely abundant in the proximal convoluted tubule and descending thin limb, but is notably absent in highly water-impermeable segments such as the ascending thin limb, thick ascending limb, and distal tubule.  

How the Chloride Sweat Test in Cystic Fibrosis Works

Mutations in the CFTR gene are responsible for cystic fibrosis.  CFTR encodes a chloride transporter which in addition to its being expressed in the lungs and pancreas, is also found in the kidney.  As I have blogged about previously, the CFTR chloride channel is responsible for chloride secretion in the human kidney, and may play a role in cyst expansion in individuals with polycystic kidney disease.  

It turns out the CFTR chloride channel also has some interesting interactions with ENaC:  the epithelial sodium transporter expressed in the collecting ducts of the kidney we all know and love.  Like CFTR, ENaC is expressed in a variety of tissues other than just the kidney.  It is also expressed in the skin and lungs, for example.  

In the skin, coupled ionic transport via CFTR and ENaC are responsible for normal sweat gland function; CFTR secretes chloride and the ENaC reabsorbs sodium from the skin.  In the sweat gland, CFTR has a stimulatory effect on ENaC.  Thus, in cystic fibrosis, when CFTR isn't working, ENaC activity is also depressed, and sodium reabsorption from the skin is suboptimal.  This explains why patients with cystic fibrosis have sweat which tastes extremely salty, and provides the rationale for the chloride sweat test used in the diagnosis of cystic fibrosis.  

The CFTR/ENaC relationship is just the opposite in tissues such as the lung:   CFTR exerts an inhibitory effect on ENaC, and therefore in patients with cystic fibrosis, ENaC activity is significantly upregulated.  The abnormally high sodium uptake from airway mucus results in a thick, viscous, dehydrated substance which is retained in the airways and predisposes to pulmonary infections, the hallmark of cystic fibrosis.  

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.  

Zebrafish Model of Cystic Kidney Disease

For the research component of my Nephrology fellowship, I've chosen an unorthodox route:  I am using the zebrafish as a model organism for the study of kidney disease.  

The zebrafish system has a number of advantages.  First, a single male/female pair can produce hundreds of eggs overnight, thus opening the door for great genetic studies.  It's akin to the fruit fly in this respect, but because the zebrafish is a vertebrate and the fruit fly is not, the zebrafish genome is much more similar to the human genome.  Zebrafish embryos are completely transparent, and thus you can watch their kidneys develop in real-time.  The early zebrafish kidney (called the pronephros) first appears within 24-48 hours, so experiments can be done with a rapidity unmatched in mice.  Furthermore, the zebrafish pronephros is much more simple than the mammalian kidney:  in fact, it consists of a single nephron!!  Despite this simplicity, the nephron contains discrete proximal, distal, and collecting duct domains which appear to be very similar to their mammalian counterparts.  

I'm presently using the zebrafish system as a model of cystic kidney disease.  For example, knocking down the genes for PKD1 and PKD2 (mutations in which result in ADPKD in humans) result in glomerular cyst formation within the single nephron of the zebrafish.  I am presently using this system in order to identify new genes which may be part of the PKD1/PKD2 pathway.  For instance,  here is an example of a gene I knocked down which interestingly results in dilatation of the pronephric ducts (shown by arrows) and cysts which form in the single glomerulus (arrowheads, looks kind of like a big bubble). 

alpha & beta intercalated cells of the collecting duct

Heard a great talk today from the 2007 recipient of the Homer Smith Award & editor of Kidney International: Qais Al-Awqati of Columbia. The full details of much of the talk can be found in this JASN article describing his acceptance talk at the ASN, but I will attempt to paraphrase.

The collecting duct has two types of cells: principal cells (the aldosterone-responsive, ENac-expressing cells which also mediate water reabsorption via ADH) and the intercalated cells. The intercalated cells come in two different varieties: alpha-intercalated cells (which secrete acid) and beta-intercalated cells (which secrete base). The alpha-intercalated cells (on the left in the figure) are tall, columnar epithelial cells which contain apical H+ ATPase, explaining its ability to secrete protons. The beta-intercalated cells (on the right) in contrast are shorter, flatter cells which contain an apical chloride-bicarbonate exchanger (called pendrin) which enables it to secrete base.

The interesting thing is that inducing metabolic acidosis results in the conversion of beta-intercalated cells to alpha-intercalated cells--giving the kidney a greater ability to secrete protons and return pH to the normal range. This conversion event is regulated by a secreted extracellular matrix molecule called hensin. Since these cells evidently retain the ability to switch between markedly different cellular phenotypes, it has become of interest to the ever-evolving field of stem cell biology.

The Erythropoietin Receptor

As we all know, erythropoietin is secreted predominantly by the kidney (85% from the kidney, 15% from the liver) in response to hypoxia and its function is to stimulate erythropoiesis in the bone marrow.  What  is the receptor on which erythropoietin works?  

The Epo receptor is a member of the cytokine receptor family expressed on the cell surface of erythroid precursors.  Although the Epo receptor itself does not have kinase activity, it is bound by the tyrosine kinase Jak2, also called Janus kinase 2.  The activation of downstream transcription factors by Jak2 (e.g., the Stat family of transcription factors) results in accelerated erythrocyte maturation.

Interestingly, a common mutation in the Jak2 gene (V617F) accounts for the majority of cases of polycythemia vera--the hematologic condition in which there is a primary elevation in the hematocrit which can result in thrombotic complications.  These patients have a low circulating EPO level, but as the EPO receptor is constitutively active, there is always accelerated erythropoeisis.  

SGK1 Polymorphisms

The most recent issue of NephSAP ("Fluids, Electrolytes, and Acid-Base Disturbances") features a mini-review describing the role of the kinase SGK1 in the regulation of various nephrology-related disorders, such as salt-sensitive hypertension, metabolic syndrome, and renal fibrosis. How does it work?

SGK1 stands for "serum and glucocorticoid-inducible kinase 1", and it appears to regulate a number of key ion transport mechanisms within the nephron. Of particular relevance is the role of SGK1 in regulating sodium uptake in the collecting duct, where SGK1 enhances the activity of ENac by several mechanisms, including direct phosphorylation of the ENac channel while simultaneously inhibiting the kinase Nedd 4-2, which in the absence of SGK1 ubiquitinates and degrades ENac.

How does this relate to human disease? It turns out that there is a common polymorphism within the SGK1 gene, present in 2-3% of the Caucasian population and about 10% of the African-American population, which predisposes to the development of hypertension in the setting of hyperinsulinemia. Carriers of this polymorphism are also prone to develop an elevated BMI and thus this appears to be a key gene regulating metabolic syndrome. Perhaps drugs targeting SGK1 will be someday useful in the treatment of metabolic syndrome or regulating distal salt handling.

microRNAs and the Kidney

MicroRNAs: one of the sexiest topics in the science community these days! Pick up a copy of Science, Nature, or Cell and you can usually find something to do with microRNAs within. What do these have to do with the kidney? Well, it's still a little premature, but there are certainly microRNAs present in the kidney, and recent studies have demonstrated that specific microRNAs are upregulated in conditions such as polycystic kidney disease and renal cell carcinomas, for instance.

How microRNAs work: microRNAs are single-stranded RNA molecules of between 21-23 nucleotides in length which are partially complementary to regions in multiple mRNAs. Once they bind to these mRNAs, the microRNAs either inhibit translation or completely degrade their target RNAs. A specific enzymatic machinery--comprised of the proteins Dicer and the RISC complex--is responsible for inhibiting mRNAs via microRNAs.

In essence, the power of microRNAs are that a single microRNA can regulate the expression of multiple genes working in parallel to achieve a similar biologic effect. This technology is of particular use to the pharmaceutical industry: one can envision targeting a particular microRNA which inhibit several pathways to prevent a disease process, such as atherosclerosis, renal fibrosis, or cyst formation to think of a few possibilities. The field is still very new. I wouldn't be surprised if a future Nobel Prize came out of this work. Three scientists (Drs. Ruvkun, Baulcombe, and Ambros) working on microRNAs recently won the 2008 Lasker Prize--considered by many to be the "precursor" prize to the Nobel.