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Sickling Hemoglobinopathy and the Kidney #ExpBio

HbSS from Wikipedia

This year a number of abstracts about sickle cell and the kidney caught my attention, having just initiated dialysis on a patient with HbSS. Since the science forces of the universe seemed determined to focus my attention on this disorder, I gave into their wishes.

Hemoglobin (hb), the molecule that carries oxygen to tissues in our body, is composed of two alpha protein chains and two beta chains. In the HbS mutation, a single change in the beta chain changes the structure of the protein so that when oxygen levels drop, it becomes straighter instead of round, stretching red blood cells into a crescent or sickle shape. Abnormally shaped red blood cells are prone to damage (hemolysis) and may clog the smallest vessels in the body, resulting in organ damage and pain.

Actual Sickle

If a child gets one copy of the HbS gene, then they are a carrier; this condition is also called sickle cell trait and occurs in 1 in 13 African American babies. Most people with trait have no symptoms, although under conditions of low oxygen their disorder may be unmasked. For example, the central part of the kidney (the medulla) has much lower oxygen tension normally. Cells often sickle there and cause damage, so individuals with otherwise asymptomatic sickle trait may not be able to fully concentrate their urine.

If a child gets a copy of HbS from both parents, then they have HbSS, the full-blown sickle cell anemia disease. This happens to about 1 in 365 African American children. These individuals have anemia because the sickling cells don’t last as long as those with normal Hb (3 weeks vs 3 months). The abnormally shaped cells may also impair blood flow to organs cause acute pain episodes, what most people think about with sickle cell disease. In addition, other organs may be damaged by these events, including the brain, heart, lungs, and kidneys.

For more general information about sickle cell disease, including other associated hemoglobin disorders, the NIH has an excellent resource here.

Kasztan M, Fox BM, Speed JS, Townes TM, and Pollock DM:  KIM-1 as a new biomarker for glomerular hyperfiltration and chronic kidney disease in humanized sickle cell disease mice

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The first poster on my list dealt with KIM-1 (Kidney Injury Molecule 1) as a new biomarker for kidney damage in a humanized mouse model.They followed glomerular filtration rate (GFR) and urinary biomarkers in HbSS mice and genetic controls every 4 weeks for 24 weeks starting at 8 weeks of age. At that starting point, no differences in GFR or proteinuria were demonstrated. By 12 weeks, the HbSS mice had a significant rise in GFR and proteinuria. By 32 weeks of age, GFR was lower in HbSS than Hb AA mice, even as proteinuria climbed higher. Urine biomarkers demonstrated early KIM-1 as a potential predictor of loss of GFR later in the course. Not included in the abstract was a cohort of patients with HbSS, some of whom have developed elevated KIM-1 excretion. This has the potential to drive important translational studies in the future.

Taylor CM, Kasztan M, Yoder B, Pollock JS, and Pollock D:  Reduced Renal Primary Cilia Expression in Humanized Sickle Cell Mice

The next poster looked at renal cilia in this same mouse model. Ciliary disorders often result in cysts in the kidneys, and patients with HbSS have an increased risk of cyst formation. Kidneys from HbSS mice have reduced ciliary proteins in their kidneys, suggesting reduced numbers or size of cilia in this model. Additional studies applied hypoxic stress to the mice, a maneuver that increased expression of the proteins under study. These experiments are still preliminary, but I would never have guessed that cilia would be a problem in sickle cell kidneys.

Eshback ML, Kaur A, Rbaibi Y, Agarwal Y, Zhang Q, Nolin TD, Tejero J, and Weisz OA:  Hemoglobin Inhibits Uptake of Filtered Proteins by Proximal Tubule Cells: Implications for Sickle Cell Disease and Vitamin D Status

The final poster examined the role of Hb uptake in tubular cells in nickels cell disease. Much of this study occurred in vitro, examining what happened to tubular transport when free Hb gets filtered and the tubules reabsorb it. It competes for transport with a number of other proteins, such as albumin and the vitamin D binding protein. Patients with HbSS have reduced red blood cell lifespan and episodic hemolysis, so they do filter higher amounts of free Hb. They also seem to be much harder to make replete with Vitamin D. These basic science observations may help explain some interesting clinical observations, even though these phenomena would seem unrelated at first glance.

Sometimes an abstract just grabs me; this time, three of them jumped me and made me write about them. All were really good science with excellent presenters, and a lot more information than I’ve included here (click the links, people). Best of luck helping us understand this difficult consequence of a fairly common disorder.

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Cool Imaging #ExpBio

Each kidney contains a bunch of discrete units or nephrons that include a filtering unit, the glomerulus, and a tubule that reclaims all the useful stuff from the filtrate. When we discuss how the kidneys work, we often treat the kidney like it’s a single nephron, but it’s a whole bunch of units that need to be on the same page regarding the body’s requirements.

Prior studies have demonstrated that nephrons can signal each other over short distances, but these have been hampered by the inability to look at more than a handful of nephrons at once. Enter a possible solution for imaging of circulation:

Postnov D, Marsh DJ, Holstein-Rathlou N-H, Cupples W, and Sosnovtseva O:  High-resolution optical imaging of synchronization in the renal circulation.

I have a difficult time doing this poster justice, since I do not have images to post. Suffice it to say that they have a laser speckle imaging set up that provides spatial resolution to 0.8 μm per pixel. They were able to analyze clustering of flow in ~1.5 x 1.5 mm^2 areas of renal cortex and gauge how well synchronized flow was.

This is a cool toy I would love to play with, and I can imagine some interesting studies coming from this new technology. If you missed the poster, check out the abstract above.

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SGLT2 in the Diabetic Kidney #ExpBio

Empagliflozin (original image https://commons.wikimedia.org/wiki/File:Empagliflozin.png )

Glucose in the glomerular filtrate gets reabsorbed in the proximal tubule by two transporters, known as sodium-glucose cotransporters (SGLT). The bulk of the glucose gets removed by SGLT2 with a smaller amount retrieved farther down the tubule by SGLT1. With normal blood sugar levels, these molecules can reclaim all the filtered load of glucose, leaving none of this sugar in the urine.

In diabetes, glycosuria occurs when blood sugars exceed the limits of SGLTs. Agents have been around for a while that inhibit these transporters, but only recently have inhibitors of SGLT2 been shown to reduce blood glucose in diabetes in clinical settings. Large trials over the past couple of years have shown additional benefits of this class of drug beyond their ability to reduce hyperglycemia. They seem to reduce cardiovascular events and to have beneficial effects on progression of diabetic kidney disease.

Liu Z, Hall E, and Singh P:  SGLT2 inhibition decreases oxygen consumption and increases oxygen tension in diabetic rats

Liu et al present a possible mechanism for these beneficial effects at this meeting. They made rats diabetic with streptozocin (a model of type 1 diabetes, not the condition these drugs are used for clinically) and examined oxygen consumption and tension in their kidneys.

Sodium reabsorption drives metabolic demand for oxygen consumption in the kidney. In addition to allowing glucose to escape, SGLT inhibitors prevent sodium reclamation and reduce this demand. They found that diabetes increased renal oxygen consumption as previously demonstrated and the SGLT inhibitor Empagliflozin (EMPA in figure) prevented this change. Oxygen tension in the renal cortex was reduced by diabetes, with SGLT inhibition once again preventing this change. Medullary oxygen tension was not affected by these states.

Click link to abstract to see other specific data

SGLT inhibitors may have important effects in organs aside from lowering blood glucose. Studies like these may help us understand these interesting new agents, and point toward other therapeutic targets.

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Earnest Starling Lecture 2018 #ExpBio

On Sunday David Mattson presented the Starling Distinguished Lecture of the American Physiological Society Water and Electrolyte Homeostasis Section. His talk addressed the role of inflammation in salt-sensitive hypertension, using the Dahl salt-sensitive rat and some supporting human data.

About half of human adults have hypertension, and about half of those patients are salt-sensitive like this rat. Feeding this rodent a high salt diet leads to rising blood pressure and albuminuria, with enlarged glomeruli, tubular damage, and inflammation. Other types of rats given the same level of sodium intake do not develop these findings. Studies from people with hypertension also show increased lymphocytes in their kidneys, suggesting that there are parallels with human disease.

In a series of experiments, Dr. Mattson showed that there were increased B and T cells in these rat kidneys and that the immune cells were activated and producing a number of cytokines. Inhibiting the presence and activity of these immune cells attenuated both the hypertensive and renal damages induced by salt loading. They then took genes identified in human association studies of hypertension and mutated the analogous genes in rats. They transplanted bone marrow from these mutant animals into Dahl rats without the mutation, so only hematopoietic cells carried the mutation in question. When salt loaded, these rats showed less hypertension than intact Dahl rats, suggesting that the immune response provided major input for hypertension.

What about the role of hypertension beating up on these kidneys? Dahl rats have poor autoregulation, so systemic hypertension gets transmitted to the kidneys. They placed an aortic cuff just above the left renal artery in rats and used it to maintain a normal blood pressure into that kidney. The right kidney saw and felt the higher pressure. This maneuver alleviated the inflammatory response in the cuffed kidney.

In the Dahl model of salt-sensitive hypertension, both inflammation and barotrauma appear to contribute to kidney damage. The story does not end here; the Mattson lab is presenting more fascinating work on this topic at this year’s meeting.

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Smells and Tastes #ExpBio

This year the Renal Section Young Investigator Award goes to a favorite of mine, and not just because her work has let me create a great image of the kidney. Jennifer Pluznick, now an Associate Professor at Johns Hopkins, began her journey in physiology as a doctoral student at University of Nebraska while I was there. While a post-doctoral fellow at Yale, she began exploring G-protein-coupled-receptors (GCPRs) in the kidney, especially those previously involved with smell, and her lab now explores their role in kidney function. Her lab logo is pretty cool; I’ll wait while you take a look.

 

Olfactory receptors constitute the largest gene family with 350 members in humans and approximately 1000 members in rodents. So far, 18 of these olfactory receptors have been located in the kidney with an additional 11 taste-associated GCPRs and 76 without known ligands. She first studied olfactory receptor 78 (Olfr78), an interesting chemosensor that localizes to the afferent arteriole, a major site of control for blood pressure and kidney function. It’s ligands (activators) include short-chain fatty acids such as acetate and proprionate, molecules derived primarily from our gut microbiota. Proprionate has been shown to cause the release of renin via its interactions with Olfr78.

Changes in gut microbiota have been associated with changes in blood pressure, so her lab wanted to explore changes in the metabolites that might occur with hypertension. They implanted micro pumps into conventional mice and into germ-free mice (mice in a plastic bubble) and infused them with angiotensin II, an established model of hypertension. Germ-free mice showed no changes (no microbiome, no metabolites), but of more than 800 substances studied, 13 were unregulated with angiotensin II infusion. Some are uremic toxins and many are known to lower blood pressure; perhaps our microbiome is trying to help us out when our kidneys fail or our blood pressures otherwise get elevated. After all, our death would not be particularly helpful for the bacteria residing in our gut. Also interesting were some sex-specific changes identified. The meaning of these differences should provide fertile ground for further study.

As expected, Dr. Pluznick delivered an excellent presentation of her impressive work. If you aren’t here, you missed a great talk. You can still catch her on the TED stage:

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A Gathering Crowd #ExpBio

Saturday of Experimental Biology can be a bit slow. Some folks are still traveling, while others are already exhausted by committee work. My flights yesterday proved uneventful, and I even got moved from 24F (coach window seat) to 3F (first class window) on the longer segment from Houston to San Diego (after I was settling into my narrow chair in the mid plane). Thanks to free red wine, I took cab to my hotel, unpacked, and hit the feather bed early.

Morning View

This morning a gentle sunrise woke me to a lovely view of distant mountains and the bridge to Coronado. Eventually I collected my meeting materials and attended two sessions.

The first one dealt with considering sex and age as biological variables in applications to the National Institutes of Health. Popular perception suggests that this is a mandate to look for sex differences, but really you just have to consider them.

  • Sometimes it’s inappropriate since a condition is sex-limited: prostate cancer, pregnancy, etc.
  • Sometimes a lack of sex differences have already been demonstrated
  • Sometimes you need to do a pilot experiment to show sex differences are not in play and then justify a single-sex study

Now the NIH will be adding consideration of age as a variable in the same manner. I see this as a bigger problem, because while there are two sexes, there could be infinite divisions of age!

I then moved on to a session on Social Media for Science, presented by the American Physiological Society Communications Committee. It provided basic introductions to Facebook, Twitter, and blogging, with brief shout-outs to LinkedIn, Instagram, and Research Gate.

#ISpyMarty

I am posting this blog before the Opening Receptions start. One fun activity this year is #ISpyMarty. The long-standing Executive Director of APS, Marty Frank, is retiring. To mark the occasion, Flat Marty is making the rounds for selfies that can be tagged with #ISpyMarty on Instagram, Facebook, and Twitter. Be on the lookout for everyone’s favorite cardboard cutout!

Now, let the science (and shenanigans) begin!