With more than 60 million Americans currently classified as obese we now know that increased body mass confers a host of secondary medical problems, including high blood pressure, type 2 diabetes, liver damage and cardiovascular disease. It also can lead to elevated levels of cholesterol and triglycerides in the blood. This constellation of abnormalities is referred to as metabolic syndrome. Common sense tells us that the development of obesity and, by extension, metabolic syndrome, is a fairly straightforward process; if people consume more calories than they expend over the long term, they should not be surprised if they end up obese.
Within each of us, the thousands of species of bacteria residing in the intestines, collectively referred to as the microbiota, outnumber human cells 10:1 and are responsible for such indispensible functions as the degradation of toxic compounds, vitamin synthesis, protection from harmful microbes and the digestion of dietary fiber. These roughly 10 trillion bacteria, separated from their human host by a single layer of intestinal epithelial cells, exert a significant influence over metabolism and health in general. In healthy people, the microbiota exists peacefully with and provides benefits to the host in exchange for a warm, reasonably safe and nutrient-rich environment. This equilibrium is predicated upon a “good fences make good neighbors” policy whereby the host limits direct contact with the microbiota by maintaining an intervening layer of antibacterial mucus, passively detects the signs of bacterial invasion and actively clears those invasive bacteria from intestinal tissues.
Research has shown that removing specific genes can have negative consequences for the microbiota as a whole and, consequently, its host. The work of our group, led by Drs. Andrew Gewirtz and Matam Vijay-Kumar at Emory University, deals largely with Toll-Like Receptor 5 (TLR5), a protein found on intestinal epithelial cells that detects the presence of bacterial flagellin, required for the assembly of the locomotive structure known as the flagellum. Previous work by our group had shown that roughly 10 percent of the mice engineered to lack this protein, which we can refer to as mutant mice, developed unchecked inflammation of the colon, a faulty immune response to the presence of normal microbiota.
After working with these mutant mice, it became apparent that those mutants which did not develop severe inflammation tended to weigh 15-20 percent more than normal mice of the same age. Further analysis confirmed that the extra weight was fat and that the mutants also exhibited higher levels of triglycerides and cholesterol as well as increased blood pressure when compared with normal mice. Interestingly, the fat of mutant mice produced more pro-inflammatory cytokines than did the fat of normal mice. These mutants, like humans with Type 2 diabetes, were also found to be resistant to the effects of insulin, the hormone responsible for regulating the amount of glucose in the blood, and to have generally higher levels of glucose in circulation.
We mimicked a Western diet by feeding both normal and mutant mice a diet high in saturated fats and, as anticipated, observed an increase in weight. In contrast to normal mice, mutants fed a high-fat diet became so insulin-resistant as to be considered truly diabetic, suffered destruction of the pancreatic islets by immune cells and exhibited increased levels of fat in the liver—all conditions which mark severe metabolic syndrome.
Having established that mice lacking TLR5 spontaneously develop metabolic syndrome and suffer exacerbated symptoms when fed a Western diet, we sought to learn why this was the case. We found that the mutant mice consumed, on average, 10 percent more food than did normal mice, a condition referred to as hyperphagia. By restricting the amount of food available to the mutants to the amount consumed by normal mice of the same age, we were able to reduce their weight as well as levels of glucose, triglycerides and cholesterol in circulation. However, mild insulin resistance persisted, indicating to us that this facet of their metabolic syndrome was not a result of overeating, per se.
We next administered antibiotics capable of reducing the total number of intestinal bacteria by 90 percent and found that this corrected insulin resistance, hyperphagia and obesity, making the mutants metabolically indistinguishable from similarly treated normal mice. Knowing that metabolic syndrome could be prevented by removing the microbiota led us to hypothesize that the microbiota itself was the cause. With this in mind, we sought to determine the extent to which the mutant’s microbiota was altered when compared with the microbiota of normal mice.
By looking at ribosomal RNA, part of the cell’s machinery to produce proteins, we identified 116 known bacterial species whose numbers were significantly increased or reduced in the absence of host TLR5. Having determined that the microbiota of a mutant mouse was, in fact, quantitatively different from a normal microbiota, we decided to transplant a mutant’s microbiota into normal germ-free mice engineered and housed so as to have no intestinal bacteria of their own. Surprisingly, once transplanted, the mutant’s microbiota induced metabolic syndrome in these new hosts. We observed increases in food intake, fat mass and body weight accompanied by insulin resistance and increases in pro-inflammatory cytokine secretion.
These findings reinforce the idea that the microbiota itself is capable of effecting lasting harmful changes to the metabolism of its host. Our research, as well as the work of others, suggests that the nature and degree of those changes are determined by the composition of the microbiota. This concept has the potential to redefine the way human obesity is diagnosed, studied, managed and, possibly, prevented.
Our group is working to determine if and to what extent manipulation of the microbiota can reduce the severity of metabolic syndrome in mice lacking TLR5 as well as the mechanism by which the microbiota acquired at birth are able to exert a lifetime influence over host-microbiota interactions. By answering these questions, we hope to move closer to a microbiota-based therapy to treat existing obesity as well as uncovering potential means of preventing obesity via treatment at birth.
Vijay-Kumar, M., Aitken, J.D., Gewirtz, A.T. Toll-Like Receptor 5: Protecting the Gut from Enteric Microbes. Semin Immunopathol. 2008 Feb 8; (1): 11-21.
Vijay-Kumar, M., Sanders, C.J., Taylor, R.T., Kumar, A., Aitken, J.D., Sitaraman, S.V., Neish, A.S., Uematsu, S., Akira, S., Williams, I.R., Gewirtz, A.T. Deletion of TLR5 Results in Spontaneous Colitis in Mice. J Clin Invest. 2007 Dec; 117 (12): 3909-21.
“Metabolic Syndrome and Altered Gut Microbiota in Mice Lacking Toll-Like Receptor-5.”
Matam Vijay-Kumar,1 Jesse D. Aitken,1 Frederic A. Carvalho,1 Tyler C. Cullender,2 Simon Mwangi,3 Shanthi Srinivasan,3 Shanthi V. Sitaraman,3 Rob Knight,4 Ruth E. Ley,2 Andrew T. Gewirtz1,*
1 Department of Pathology, Emory University, Atlanta, GA 30322, USA.
2 Department of Microbiology, Cornell University, Ithaca, NY 14853, USA.
3 Department of Medicine, Emory University, Atlanta, GA 30322, USA.
4 Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
(This is a summary by one of the study’s co-authors. Full text appears in Science, Apr. 9, 2010, Vol. 328. no. 5975, pp. 228-231.)