Sex Differences in Renal Inflammation and Injury in High-Fat Diet-Fed Dahl Salt-Sensitive Rats

Roxanne Fernandes, Hannah Garver, Jack R. Harkema, James J. Galligan, Gregory D. Fink, Hui Xu (Hypertension, 2018)

High fat diets (HFD) are known to lead to obesity, hypertension, and renal dysfunction. A HFD can also lead to an increase in sympathetic outflow, which is the main cause of hypertension. Hypertension is chronically high blood pressure and is one of the major causes of renal disease. Although sex differences in obesity and cardiovascular disease have been researched, it is not known how a HFD causing hypertension and renal injury differ between males and females. The goal of this study is to see if there are differences in blood pressure, renal injury, and fat accumulation between sexes.

This study used HFD Dahl SS rats that exhibit signs of HFD-induced hypertension and Sprague Dawley rats that are also fed HFD but are not hypertensive. Beginning at three weeks old, the rats were either given a control diet that had normal salt levels or a HFD. The Dahl SS rats were given the HFD for either 10, 17, or 24 weeks, while the Sprague Dawley rats were placed on the HFD for 24 weeks.

After 24 weeks HFD males and females exhibited an increase in adipose tissue and body weight than the control rats. Body weights remained similar in all groups through week 8 of the diet, but then the control and the HFD males body weight increased significantly more than the females. Leptin, which is produced by adipose cells, was also shown to increase in males and females that were fed the HFD. Rats given the HFD also exhibited increased blood pressure. After 10 weeks of the diets, there was no significant difference between HFD and control rats. At 15 weeks the blood pressure of HFD rats became significantly higher than the controls rats. From 15 weeks to 24 weeks blood pressure in the HFD rats continued to significantly increase compared to the control rats. The heart rates between the HFD and control rats did not differ throughout the 24 week experiment. 

At 24 weeks, rats were given an injection of hexamethonium, which is a ganglionic blocker for sympathetic nerve activity. The hex injection decreased blood pressure more in the HFD males than the control males. The female rats exhibited a larger blood pressure decrease in the control rats than the HFD rats. When comparing males and females, the HFD males exhibited a larger blood pressure decrease compared to the HFD females. 

Researchers also observed renal histological changes. At 10 weeks, male rats had higher renal injury scores than females in both the HFD and control groups. At 17 weeks, male rats given the HDF exhibited significantly higher renal injury scores than the HFD females or the control groups. At 24 weeks, the HFD males exhibited an increased injury score compared to 17 weeks. This time period was also when blood pressure in HFD males and females was significantly increased.

Researchers then looked at the renal inflammatory responses in HFD rats. At 10 weeks, cortical and medullary macrophage infiltration was higher in males than females. At 17 weeks, macrophage levels increased in females of both diets, while males stayed similar to 10 week males. At 24 weeks, female rats exhibited a decrease in macrophages when compared to 17 week females, but still higher than 10 week females. While macrophage levels in males at 24 weeks was similar to the males at 10 and 17 weeks, levels were overall higher than female macrophage levels. The levels of cortical and medullary T-cells were then measured. At 10 weeks, all male and females had lower levels of T-cells. T-cells increased at 17 weeks through 24 weeks in males but not females, which exhibited similar T-cells levels to 17 weeks. Levels of inflammatory cytokines did not exhibit a significant change between different diets.

In conclusion, the HFD males and females both exhibited an increase in adipose tissue compared to the controls, but males generally have a higher body weight. Male and female rats given the HFD exhibited increase blood pressure, while heart rate shoed no significant difference between groups. The ganglionic blocker hexamethonium lead to a larger decrease in blood pressure in male rats compared to female rats with HFDs. Renal injury increased over the course of 24 weeks in males, while renal injury in females remained lower over the 24 week experiment. Renal T-cell levels increased over 24 in both HFD and control male rats, while females did not. 

One of the major limitations of this study that researchers discussed is how sex hormones effect hypertension and organ damage. They discuss that future studies could observe how sex hormones and receptors play a role in sex differences in hypertension. Another limitation discussed by researchers was how the HFD caused fat accumulation. Future studies could show that other high calorie diets can dramatically change renal inflammation or hypertension. I chose this paper because Dr. Fink recently spoke to the department and although his research is different than what we are doing in lab, it is also very closely related. Dr. Fink’s lab is mostly focusing on the sedentary side of our research and is now beginning to look towards how sex hormones are affecting hypertension.

-Paul M.

Microglia in the RVLM of SHR have reduced P2Y12R and CX3CR1 expression, shorter processes, and lower cell density

E. Myfanwy Cohen, Suja Mohammed, Mary Kavurma, Polina E. Nedoboy, Sian Cartland, Melissa M.J. Farnham, Paul M. Pilowsky. Autonomic Neuroscience: Basic and Clinical (2019).

Glial cells are cells of the central and peripheral nervous system that are not neurons. One type of glial cell, called microglia, act as a clean-up crew for the central nervous system. Microglia are macrophages that maintain the brain by looking for damage to the neurons or infectious chemicals. Since microglia look for damaged neurons, they also regulate inflammation in the brain. This study focuses on the microglia within a part of the brainstem called the rostral ventrolateral medulla (RVLM), which contributes to the control of blood pressure. The goal of the study was to determine if chronic high blood pressure is associated with a decrease in microglia function in the RVLM.

Fifteen week old spontaneously hypertensive rats (SHRs) and wild type rats were used in this experiment. Blood pressure was taken using a tail cuff to determine if the rats were hypertensive. The brainstems were removed and then placed in the freezer. The frozen brainstems were sectioned and the RVLM and the facial nucleus were punched out. Anti-Iba1 and donkey anti-rabbit AlexaFluor488 were used in the fluorescent tests. At the end of the experiment, rats were euthanized with sodium pentobarbitone.

A significant difference in systolic blood pressure was measured to demonstrate the two distinct groups. The SHRs exhibited a significant higher blood pressure (195 ±8 mmHg) than the wild type rats (144 ±8 mmHg). Two G-coupled protein receptors involved in the normal function of microglia, P2Y12R and CX3CR1, were measured and compared between the two groups of rats.  In the RVLM, P2Y12R expression was significantly lower in SHRs by about 37% when compared to the wild type rats. Expression of P2Y12R was also measured in the facial nucleus and exhibited no significant difference between SHRs and the wild type rats. Expression of CX3CR1 was then measured in the RVLM and the facial nucleus. In the RVLM, CX3CR1 expression was shown to be 30.9% lower in the SHRs when compared to the wild type. Expression of CX3CR1 in the facial nucleus was not significantly different between SHRs and the wild type.

The enzyme phenylethanolamine N-methyltransferase (PNMT) is found in the adrenal medulla and plays a role in converting noradrenaline to adrenaline. This allows PNMT to be a marker for adrenergic neurons, which are found in the RVLM but not the facial nucleus. The expression of PNMT was then compared between SHRs and wild type rats. The RVLM exhibited a significant amount of PNMT mRNA, while the facial nucleus showed a very small amount, which verifies the tissue punches have the correct sites.

Researches then observed the differences in microglia cell density between SHRs and wild types rats. The SHRs exhibited 22.9% lower cell density than the wild type. To further observe how active the microglia were, branch length, endpoints, and branch number were also examined. While the number of endpoints and branch numbers did not show a significant difference, branch length was significantly lower in the SHRs when compared to the wild type.

In conclusion, the G-coupled receptors CX3CR1 and P2Y12R play a major role in the normal function of microglia. Spontaneously hypertensive rats exhibit decreased expression of P2Y12R which may lead to the decreased microglia cell density in the RVLM. The researchers do state in the article that more research needs to be done to determine if the change in microglia are the cause or effect of over-activation of the RVLM and exactly how the microglia are being affected. I found this article interesting to our lab work because we constantly discuss the increased activation of RVLM but may not think about the exact repair mechanisms involved that may also not be working properly.