MHD6 dietary methylamines Flashcards
1
Q
At any time, how many distinct microbiota-associated metabolites (MAMs) are in the circulation of healthy individuals
A
upwards of 200
2
Q
Where do MAMs come from?
A
produced by gut bacteria from dietary substrates, intestinal secretions and xenobiotics
3
Q
How do MAMs enter the blood?
A
many are taken up from the upper gastrointestinal tract into the bloodstream via the hepatic portal vein
4
Q
What happens between the gut and MAMs?
A
shuttling information between host cells and the gut microbiota. The gut-liver axis
5
Q
Does phenylacetic acid undergo first pass metabolism?
A
No
6
Q
what does phenylacetic acid contribute to?
A
the progression of non-alcoholic fatty liver disease in obesity
7
Q
Where in the liver are MAMs and drugs detoxified?
A
the smooth endoplasmic reticulum of liver cells (hepatocytes)
8
Q
What is first pass metabolism?
A
Metabolism in the liver or gut before it reaches the systemic circulation
9
Q
Where does 75% of the liver’s blood supply come from?
A
The hepatic portal vein, the direct venous outflow from the intestine
10
Q
Why are there differences in the composition and function of the gut microbiota of patients who are obese/have NAFLD and healthy individuals?
A
reduced integrity of the gut barrier - becomes leaky
11
Q
what happens if an individual has a leaky gut?
A
The MAMs healthy/unhealthy individuals are exposed to will differ, because of the changes in microbiota function.
12
Q
Give an example of the microbial byproducts diseased individuals will be exposed to
A
lipopolysaccharide from gram negative bacteria & antigens
13
Q
What does the gut-liver axis refer to?
A
interplay between the gut and liver in health and disease, with the gut microbiota central to this relationship
14
Q
What does the gut-liver axis directly contribute to?
A
host–microbiota co-metabolism
15
Q
What happens in host–microbiota co-metabolism
A
chemical communication between specific hepatic cellular pathways and specific MAMs
16
Q
What is uptake of microbiota-associated metabolites (MAMs) into hepatic circulation is influenced by?
A
health status
17
Q
What does a leaky gut mean?
A
malfunctioning tight-junctions
18
Q
What does NAFLD stand for?
A
Non-alcoholic fatty liver disease - gut microbiotia contributes to it
19
Q
What does steatosis mean?
A
It’s a condition where the liver accumulates lipid, with or without inflammation
20
Q
NAFLD is the most common cause of ______________
A
chronic liver disease
21
Q
What is metabolic syndrome?
A
Having at least three of the five following medical conditions: Abdominal obesity High blood pressure High blood sugar High serum triglycerides Low high-density lipoprotein levels
22
Q
What is metabolic syndrome associated with a higher risk of?
A
Cardiovascular disease and type 2 diabetes.
23
Q
How are you able to tell if someone has NAFLD?
A
Only histological assessment of the degree of steatosis in liver biopsy samples allows accurate diagnosis of NAFLD
24
Q
What’s the problem with diagnosing NAFLD?
A
obtaining biopsy samples is an expensive, invasive approach not without risk, it is not practical for screening large numbers of patients routinely.
25
Where can disruption of the gut–liver axis in steatosis be found?
the gut microbiome, hepatic transcriptome (gene expression profile) and metabolome
26
What is the gut microbiome of patients with NAFLD characterised by?
- low microbial gene richness (i.e. it has a lower number of unique microbial genes)
- small increases in levels of Gram-negative Proteobacteria
- Signalling pathways associated with gram-negative bacteria in the liver are activated
27
Where is PAA found higher in?
Patients with NAFLD
28
What has PAA shown to contribute to?
accumulation of lipids in liver tissue, in cell-line and mice models
29
How would identification of a specific MAM that contributes directly to disease offer?
The potential to develop a low-cost, rapid, blood test for the accurate diagnosis of NAFLD
30
Apart from increased PAA what do patients with steatosis show?
- decrease of hepatic expression of insulin receptor (INSR)
- decrease of short/branched chain acyl-CoA dehydrogenase (ACADSB)
- Hepatic lipoprotein lipase (LPL) expression – directly associated with hepatic lipid (triglyceride; TG) accumulation is increased
- p-Akt (& the normal response to insulin) are decreased
31
What is steatosis positively associated to?
- high circulating levels & urinary levels of the MAMs branched-chain amino acids (BCAAs)
- aromatic amino acids (AAA)
- high serum phenylacetic acid (PAA)
- high hepatic LPL
32
What does decreased phosphorylated Akt (p-Akt) and the normal response to insulin, directly contribute to, in NAFLD patients and is associated with metabolic syndrome?
insulin resistance
33
What is first pass metabolism?
Sometimes referred to as pre-systematic metabolism - the metabolism of a drug or microbiota-associated metabolite before it reaches the systemic circulation, and it can happen in the gut or the liver
34
From what 3 sources are microbiota-associated metabolites derived?
- dietary substrates
- intestinal secretions
- xenobiotics
35
What is the gut-liver axis?
The interplay between the gut and the liver in health and disease, with the gut microbiota central to this relationship because of its production of metabolites that can directly influence or be modified by the liver
36
Why is the liver potentially affected by or involved with metabolism of microbiota-associated metabolites?
The liver is constantly exposed to a range of microbiota-associated metabolites because 75% of its blood supply comes from the hepatic portal vein, the direct venous outflow from the intestine.
37
What is host-microbiota co-metabolism?
Host-metabolism is the communication between host-associated cellular pathway and microbiota-associated metabolites. In the context of the gut-liver axis, this communication is between specific hepatic cellular pathways and specific microbiota-associated metabolites. These metabolites can be subject to first-pass metabolism in some instances, but modified or unmodified microbiota-associated metabolites can influence host-microbiota co-metabolism
38
Give an example of a disease directly influenced by the gut-liver axis
NAFLD or Non-alcoholic fatty liver disease
non-alcoholic fatty liver disease is directly influenced by the gut-liver axis, a condition in which lipids accumulate in the liver of patients. NAFLD patients have low microbial gene richness and increased levels of Gram-negative Proteobacteria compared with healthy patients. In addition, NAFLD patients have signalling pathways associated with Gram-negative bacteria activated in their liver. Phenylacetic acid, a metabolism, is found in higher levels in NAFLD patients compared with healthy individuals. This metabolite contributes to accumulation of lipid in NAFDL.
39
What is Hippurate?
a MAM, a normal component of urine, and metabolomic profiling has shown it to be a distinguishing feature of many physiological and pathological conditions: urinary levels are lower in patients with metabolic disorders such as obesity, type 2 diabetes or NAFLD compared with healthy controls.
40
Where is benzoic acid taken up?
liver and kidneys via the hepatic portal vein
41
What happens to benzoic acid once it has been uptaken via the hepatic portal vein into the liver and kidneys?
It is converted to Hippurate in the mitochondrial matrix of the hepatic and renal cortex
42
Higher amounts of Hippurate in the urine are associated with what?
Diets high in fruit and whole grains: Correlation does not equate to causation, and only with mechanistic data can correlations be shown to be real - there are many speculative routes
43
What are Methylamines?
Organic compounds containing methyl (-CH3) groups attached to a nitrogen (N) atom.
44
What do dietary methylamines comprise?
choline, carnitine, betaine and trimethylamine N-oxide (TMAO): all of these compounds can be used by the gut microbiota to produce trimethylamine (TMA).
45
Where does choline come from
| How does it act in biochemical reactions
Choline is an essential nutrient of the vitamin B complex, acting as a methyl donor in biochemical reactions, involved in homocysteine reduction and as a precursor for the biosynthesis of phospholipids, acetylcholine and lipoproteins in the body.
46
Where does the majority of dietary choline come from? (exogenous)
from exogenous dietary sources such as eggs, meat, fish, soybeans, nuts and whole grains. Choline can also come from phosphatidylcholine (PC) added to a number of foods as an emulsifier, and present in cells of animals, fish and vegetables and in dairy products.
47
What are the endogenous sources of choline?
In the form of phosphatidylcholine (PC) from biliary lipid: shedded villus cells and gut bacteria, and the body can synthesise choline.
48
What can choline deficiency lead to?
Hepatic and muscle damage. Infertility, cognitive impairment, renal haemorrhage and hypertension are also associated with choline deficiency.
49
What is carnitine not degraded by?
Human-associated enzymes
50
What is carnitine's primary role in the human body?
transfer long-chain fatty acids, as acylcarnitine esters, across the inner mitochondrial membrane.
51
Exogenously, where is carnitine found?
In many foodstuffs, and is particularly abundant in meat and dairy products.
52
How is carnitine synthesised in humans?
From methionine and lysine with choline serving as a methyl donor
53
How does carnitine act in the human body?
may act (indirectly) to reduce the accumulation of liver lipid by providing carnitine to enhance hepatic fatty acid oxidation
54
Where is carnitine produced in the body?
Throughout the body, but primarily in the kidneys, and is excreted in urine.
55
What is important to consider whenever measuring carnitine in the blood and urine?
Whether reduced levels are due to microbial use of dietary carnitine, or impaired host-associated biosynthetic pathways.
56
How does Betaine act in biochemical reactions?
methyl donor, similar to choline.
| It is also involved in the detoxification of homocysteine.
57
Where was betaine originally discovered?
Sugar beet, but is present in a wide range of foodstuffs (e.g. grains, vegetables, fish)
58
Where is TMAO found in high abundance? Why?
fish, where it acts as an osmoprotectant and contributes to proper protein folding.
Human body cannot produce trimethylamine (TMA)
59
What do osmoprotectants do?
Protect from the effects of osmosis, maintaining cell volume and fluid balance and helping organisms survive extreme osmotic stress (such as the salt water habitat of many fish)
60
What does TMA smell like?
Rotting fish (rotting fish smell comes from TMA)
61
How do you demonstrate TMA is produced by gut microbiota?
Administering broad-spectrum antibiotics to an individual or mouse will suppress the gut microbiota and its activities, stopping it from converting dietary methylamines to TMA.
62
What is TMA produced from?
PC -> choline
TMAO
Betaine
Carnitine
63
What happens to TMA produced by microbiota using dietary substrates?
It is transported from the gastrointestinal tract by the hepatic portal vein to liver cells, where it is converted to TMAO by hepatic flavin mono-oxygenases (FMOs) or excreted in urine
64
How can TMA impair body functions? How is this counteracted?
It impairs proper formation of cells at the blood–brain barrier at a physiologically normal and relevant concentration (~0.4 μM), while TMAO (~40 μM) protects the blood–brain barrier from these effects and actually contributes to proper cell formation.
65
Under normal circumstances, urinary concentrations of TMA is at what concentration?
Around 5 μmol/mmol creatinine
66
Under normal circumstances, urinary concentrations of TMAO is at what concentration?
Around 100 μmol/mmol creatinine
67
How much TMAO is excreted in faeces?
nanomolar concentrations
68
What happens to individuals who have a mutation in the FMO3 gene?
They are unable to convert TMA to TMAO, and have fish-odour syndrome (also known as trimethylaminuria) caused by the high amounts of TMA circulating in their body.
69
Where is TMA converted to TMAO?
liver (by hepatic FMOs)
70
Where is microbial use of dietary methylamine concentrated?
Upper gastrointestinal tract
71
How is it known that microbial use of dietary methylamine takes place in the upper gastrointestinal tract
Circulating levels of TMA and TMAO increase within 2 hours of an oral dose of any of the dietary methylamines.
72
Why is PC given to Alzheimer's patients?
PC is more resistant to microbial attack than the other methylamines: in an effort to boost choline bioavailability (which is associated with improved cognitive function).
73
Computer-based predictions suggest several members of the human gut microbiota can produce TMA from choline: what are they?
These are mostly Clostridium species (obligately anaerobes) and Enterobacteriaceae (e.g. Klebsiella, Escherichia, Providencia, Yokenella and Proteus spp.; facultatively anaerobes)
74
in vitro, with only which bacteria has consistently found to produce TMA from choline?
Clostridium sporogenes
75
Computer-based predictions have suggested a range of gut bacteria can convert carnitine and _______ to TMA, but so far only the Enterobacteriaceae have been shown to carry out this process.
betaine
76
TMAO is used by Enterobacteriaceae as an alternative for what? Leading to what?
electron acceptor for anaerobic respiration, leading to the increased growth of the bacteria and production of TMA.
77
It has recently been shown in mice that TMAO is subject to what?
Metabolic retroconversion
| i.e. it is subject to breakdown by gut microbiota contrary to previous expectations
78
What is retroconversion?
The process by which a dietary substrate (e.g. TMAO) can be broken down by the gut microbiota to a metabolite that is converted back to the starting substrate by liver enzymes.
79
In the case of TMAO, what does retroconversion involve?
A cycle of reductive followed by oxidative reactions to regenerate TMA. This TMA is then taken up via the portal vein to the liver, where it is converted back to TMAO by hepatic FMOs.
80
High circulating levels of TMAO is associated with what?
cardiovascular disease and its co-morbidities in human populations
81
What was TMAO shown to do in vitro and in pancreatic islets in vivo?
educe endoplasmic reticulum stress and formation of fats (lipogenesis) in adipocytes (cells specialised for fat storage) in vitro, and improved glucose homeostasis by stimulating insulin production by pancreatic islets in vivo and in vitro. It remains to be seen whether these effects are replicated in humans.
82
Plasma levels of TMAO are significantly lower in which type of individuals?
Vegans/vegetarians compared with omnivores
83
What evidence is there to suggest that onivores' gut microbiota is better adapted to using this meat-associated methylamine compared to vegans/vegetarians?
Omnivores have significantly more plasma TMAO after ingesting carnitine than vegans/vegetarians
84
Long-term feeding (15 weeks) of high amounts of dietary carnitine to mice causes their gut microbiota to produce increased amounts of what?
What did this lead to an increase in?
TMA, with the increased levels of TMAO detected in their plasma accelerating atherosclerosis (a disease of the arteries characterised by the deposition of fatty material on their inner walls) via modulation of cholesterol and sterol metabolism.
85
What contributes to the formation of plaques lining the arteries and also contributes to cardiovascular diseases?
Butyrobetaine, produced from aerobic breakdown of carnitine by gut bacteria
86
What do high plasma levels of TMAO predict?
Incidence thrombosis (heart attack and stroke) risk in humans
87
What does TMAO directly contribute to?
Platelet aggregation and development of thrombosis in animal models of disease
88
Mice fed a high-choline diet exhibit inflammation of what?
Aortic endothelial cells, with TMAO promoting recruitment of activated leukocytes to these endothelial cells and contributing to endothelial dysfunction. This effect is likely to contribute to enhancement of atherosclerosis and cardiovascular risks.
89
Increased circulating levels of TMAO causes what?
Vascular inflammation and oxidative stress, likely to contribute to impaired endothelial function. This effect is likely to contribute to enhancement of atherosclerosis and cardiovascular risks.
90
Where is Methanomassiliicoccus luminyensis found?
Human gut
| It is a methylotrophic archaeon
91
What can Methanomassilicoccus luminyensis used as an electron acceptor to produce methane?
TMA
92
Why was it proposed for archaea to be used as probiotics? (archaebiotics)
Its ability to remove TMA from the gut has potential health benefits to humans
93
Under what circumstance would archaea be used for probiotics?
For individuals with fish-odour syndrome and those at risk of cardiovascular disease.
94
What is the current research to reduce circulating TMAO levels?
Mice to knock out microbial genes associated with generating TMA from choline in an effort to reduce circulating levels of TMAO in animal models of disease. Their hope is that these therapeutics will one day be used in patients at risk of thrombosis to reduce TMAO loads and systemic effects of the metabolite, thereby offering protection from disease.
