Role of diet and gut microbiome in the major depressive disorder

An international group of researchers headed by André Carvalho has published in Psychotherapy and Psychosomatic a paper that provides new data and prospects for the links between the intestinal flora and several disorders, notably depression.

Persistent low-grade immune-inflammatory processes, oxidative and nitrosative stress and hypothalamic-pituitary-adrenal axis activation are integral to the pathophysiology of the major depressive disorder. The microbiome, intestinal compositional changes, and resultant bacterial translocation add a new element to the bidirectional interactions of the gut-brain axis. New evidence implicates these pathways in the onset of the major depressive disorder. In addition, abnormalities in the gut-brain axis are associated with several chronic non-communicable disorders, which frequently co-occur in individuals with depression, including but not limited to irritable bowel syndrome, chronic fatigue syndrome, obesity, and type 2 diabetes mellitus.

The composition of the gut microbiota is influenced by several genetic and environmental factors (e.g. diet). Several lines of evidence indicate that gut-microbiota-diet interactions play a significant pathophysiological role in depression and related medical comorbidities. Gut dysbiosis and the leaky gut may influence several pathways implicated in the biology of major depressive disorder, including but not limited to immune activation, oxidative and nitrosative stress, and neuroplasticity cascades. However, methodological inconsistencies and limitations limit comparisons across studies.

Authors conclude that intestinal dysbiosis and the leaky gut may constitute a key pathophysiological link between depression and its medical comorbidities. This emerging literature opens relevant preventative and therapeutic perspectives.

Citation: Slyepchenko, Anastasiya, Michael Maes, Felice N. Jacka, Cristiano A. Köhler, Tatiana Barichello, Roger S. Mcintyre, Michael Berk, Iria Grande, Jane A. Foster, Eduard Vieta, and André F. Carvalho. “Gut Microbiota, Bacterial Translocation, and Interactions with Diet: Pathophysiological Links between Major Depressive Disorder and Non-Communicable Medical Comorbidities.” Psychotherapy and Psychosomatics 86, no. 1 (2016): 31-46. doi:10.1159/000448957.
Adapted from press release by Karger medical and scientific publishers.

Research unveils structure of crucial bacterial cell wall protein

Duke University researchers have provided the first close-up glimpse of a protein, called MurJ, which is crucial for building the bacterial cell wall and protecting it from outside attack. The research is published in Nature Structural and Molecular Biology.

Researchers at Duke University solved the structure of an enzyme that is crucial for helping bacteria build their cell walls. The molecule, called MurJ (shown in green), must flip cell wall precursors (purple) across the bacteria’s cell membrane before these molecules can be linked together to form the cell wall. This new structure could be important to help develop new broad-spectrum antibiotics. Credit: Alvin Kuk, Duke University

“Until now, MurJ’s mechanisms have been somewhat of a ‘black box’ in the bacterial cell wall synthesis because of technical difficulties studying the protein,” said senior author Seok-Yong Lee, Ph.D., associate professor of biochemistry at Duke University School of Medicine. “Our study could provide insight into the development of broad spectrum antibiotics, because nearly every type of bacteria needs this protein’s action.”

A bacterium’s cell wall is composed of a rigid mesh-like material called peptidoglycan. Molecules to make peptidoglycan are manufactured inside the cell and then need to be transported across the cell membrane to build the outer wall.

In 2014, another group of scientists had discovered that MurJ is the transporter protein located in the cell membrane that is responsible for flipping these wall building blocks across the membrane. Without MurJ, peptidoglycan precursors build up inside the cell and the bacterium falls apart. Many groups have attempted to solve MurJ’s structure without success, partly because membrane proteins are notoriously difficult to work with.

In this study, Lee’s team was able to crystallize MurJ and determine its molecular structure to 2-angstrom resolution by an established method called X-ray crystallography, which is difficult to achieve in a membrane protein. The structure, combined with follow-up experiments in which the scientists mutated specific residues of MurJ, allowed them to propose a model for how it flips peptidoglycan precursors across the membrane.

After determining the first structure of MurJ, Lee’s team is now working to capture MurJ in action, possibly by crystallizing the protein while it is bound to a peptidoglycan precursor. “Getting the structure of MurJ linked to its substrate will be key. It will really help us understand how this transporter works and how to develop an inhibitor targeting this transporter,” Lee said.

Lee’s group is continuing structure and function studies of other key players in bacterial cell wall biosynthesis as well. Last year, they published the structure of another important enzyme, MraY, bound to the antibacterial muraymycin.

Citation: Kuk, Alvin CY, Ellene H. Mashalidis, and Seok-Yong Lee. “Crystal structure of the MOP flippase MurJ in an inward-facing conformation.” Nature Structural & Molecular Biology (2016).
DOI: 10.1038/nsmb.3346
Research funding: Duke University
Adapted from press release by the Duke University.

Protective skin bacteria

There are more and more examples of the ways in which we can benefit from our bacteria. According to researcher Rolf Lood from Lund University in Sweden, this is true for the skin as well. He has shown that the most common bacteria on human skin secrete a protein which protects us from the reactive oxygen species thought to contribute to several skin diseases. The skin bacterium is called Propionibacterium acnes.

Propionibacterium acnes. Credit: Matthias Mörgelin, Lund University

He has discovered that the “acne bacterium” secretes a protein called RoxP. This protein protects against what is known as oxidative stress, a condition in which reactive oxygen species damage cells. A common cause of oxidative stress on the skin is UV radiation from the sun.

“This protein is important for the bacterium’s very survival on our skin. The bacterium improves its living environment by secreting RoxP, but in doing so it also benefits us”, explains Rolf Lood.
Oxidative stress is considered to be a contributing factor in several skin diseases, including atopic dermatitis, psoriasis and skin cancer.

Since Propionibacterium acnes is so common, it is present in both healthy individuals and people with skin diseases. According to Rolf Lood, however, people have different amounts of the bacterium on their skin, and it can also produce more or less of the protective protein RoxP.

This will now be further investigated in both patients and laboratory animals by Lood and his team. The human study will compare patients with basal cell carcinoma, a pre-cancerous condition called actinic keratosis and a healthy control group. The study will be able to show whether there is any connection between the degree of illness and the amount of RoxP on the patient’s skin.

The study on laboratory animals will also examine whether RoxP also f
unctions as protection. Here, mice who have been given RoxP and others who have not will be exposed to UV radiation. The researchers will then observe whether the RoxP mice have a better outcome than those who were not given the protective protein.

“If the study results are positive, they could lead to the inclusion of RoxP in sunscreens and its use in the treatment of psoriasis and atopic dermatitis”, hopes Rolf Lood. His research findings have recently been published in an article in the Nature journal Scientific Reports.

Citation: Allhorn, Maria, Sabine Arve, Holger Brüggemann, and Rolf Lood. “A novel enzyme with antioxidant capacity produced by the ubiquitous skin colonizer Propionibacterium acnes.” Scientific Reports 6 (2016): 36412.
DOI: http://dx.doi.org/10.1038/srep36412
Adapted from press release by Lund University.

Modified bacterial genes used to boost electrical conductivity in human cells

(Durham) Duke University biomedical engineers have harvested genes for ion channels from bacteria that, with a few tweaks, can create and enhance electrical signaling in human cells, making the cells more electrically excitable. The technique could one day be used to treat cardiac arrhythmia or to restore electrical functions to scarred heart or nervous system tissues. It might also prove useful for treating a variety of genetic diseases involving poor conductivity in human sodium and calcium channels. The study appears online in Nature Communications on October 18.

In mammals, the genes controlling the sodium ion channels responsible for a cell’s electrical activity are surprisingly large. Too large, unfortunately, to be readily delivered to cells through a virus — standard procedure in modern gene therapy techniques. To skirt this size issue, the Duke team delivered smaller ion channels engineered from bacterial genes to primary human cells in a laboratory setting. With the replacement channels, cells that don’t normally produce electrical signals became electrically active, and cells that normally produce signals did so more strongly.

While bacterial genes encoding sodium channels are different than their human counterparts, evolution has conserved many similarities of ion channel design since multi-celled animals diverged from bacteria hundreds of millions of years ago. Hung Nguyen, a doctoral student in Bursac’s laboratory, mutated these bacterial genes so that channels they encode could become active in human cells.

In one experiment, the researchers placed cultured cells in several parallel lines, alternating between electrically active and inactive cells. When stimulated at one end, the electrical signal traveled across the lines very slowly. The researchers then delivered three genes to the electrically inactive cells: one bacterial gene for a sodium ion channel and two supporting genes encoding a potassium channel and connexin-43, a protein that helps shuttle electrical signals between cells.

When delivered to unexcitable cells taken from the skin, heart and brain, the trio of genes caused the cells to become electrically active, speeding up the electrical signals as they raced across the lines.
“You could imagine using this to alter electrically dead cardiac scar tissue after a heart attack to bridge gaps between healthy cells,” said Nguyen, who also points out that all three genes are small enough to be delivered simultaneously by a single virus.

Nguyen and Bursac also showed that the gene encoding the bacterial sodium channel could, by itself, enhance the excitability of cells that are already electrically active. In a second experiment, they delivered the sodium channel gene to cardiomyocytes — electrically active heart cells — in conditions mimicking various diseases or stressful situations, such as a heart attack.

Nguyen adds that this work contributes to a growing body of research that is looking to so-called “primitive” organisms for help with our own health.

Citation: Engineering prokaryotic channels for control of mammalian tissue excitability.
Authors: Hung X. Nguyen, Robert D. Kirkton & Nenad Bursac
DOI: http://dx.doi.org/10.1038/NCOMMS13132
Journal: Nature communications
Research funding: American Heart Association Predoctoral Fellowship, National Institute of Health
 Adapted from press release by Duke University