MHD5 short-fatty acids in health and disease Flashcards
1
Q
Why do we need to consume dietary fibre?
A
To promote regeneration of intestinal cells - keeping the intestinal barrier healthy and reducing the risk of developing colon cancer
2
Q
How do we digest dietary fibre if we do not possess the correct enzymes?
A
Our gut bacteria do
3
Q
What benefits does dietary fibre have?
A
Short-chain fatty acids (SCFAs) that provide fuel for our intestinal cells.
- Anti-inflammatory
- Anti-cancer effects
May also have systemic effects, influencing glucose homeostasis and even the brain
4
Q
What are dietary fibres?
A
Non-digestible carbohydrates and lignin, that are intrinsic and intact in plants
5
Q
Why are fibre-containing foodstuff the major fermentable substrates for gut microbiota?
A
Because almost 100% of it reaches the large intestine intact
6
Q
How are SCFAs formed?
A
The fermentation of dietary fibres
resistant starches
other polyoligosaccharides and oligosaccharides that escape digestion in the stomach and small intestine and which release the large intestine (colon) intact
7
Q
What are the three main SCFAs produced by the gut microbiota in the colon?
A
- acetate
- propionate
- butyrate
8
Q
Describe the structure of:
- acetate
- propionate
- butyrate
A
Conjugate bases of their respective acids
(one oxygen double bonded to a carbon and another oxygen single bonded to the same carbon but that oxygen has a negative charge + a side chain)
-acetate: Two carbons including one bound to oxygens
-propionate: 3 carbons “
-butyrate: four carbons “
9
Q
What condition determines whether SCFAs can be produced or not?
A
Oxygen availability: there must be an absence of oxygen for the fermentation process
10
Q
Describe the varying pH levels and oxygen levels moving through the digestive tract from the mouth to the anus
A
Oral cavity: pH7, aerobic & anaerobic microniches
Stomach: pH 1-4, aerobic, microaerophilic pO2 77mm Hg
Small intestine: pH 5-5.5, facultatively anaerobic pO2 33mm Hg
Colon: pH 5.5-7.5, anaerobic, pO2 <33mm Hg
Anus: pH 5.5-7.5, anaerobic, pO2 <1mm HG
11
Q
how many anaerobic bacteria reside in colon?
A
Largest amount than anywhere else 10^10
12
Q
What other products other than SCFAs are there?
A
- Carbon dioxide
- Methane
- Hydrogen
- Hydrogen Sulphide
- Increased amounts of bacterial cell mass
- Heat
13
Q
What is one of the main processes that contributes to the breakdown of fibres and the generation of SCFAs?
A
Cross-feeding
14
Q
What is cross-feeding?
A
A process where one species of bacteria uses the product(s) of one or more other species to produce a different product
15
Q
What are the two distinct cross-feeding mechanisms in the gastrointestinal tract?
A
1) Due to the consumption of fermentation end products (lactate, succinate, acetate)
2) Due to cross-feeding of partial breakdown products from complex substrates
16
Q
What do both the cross-feeding mechanisms contribute to?
A
The production of butyrate and propionate, causing increasing intestinal levels of both these SCFAs
17
Q
The production of butyrate leads to?
A
The butyrogenic effect (increased intestinal concentrations of butyrate and relative abundance of butyrate-producing bacteria such as Roseburiaspp. and Faecalibacterium prausnitzii).
18
Q
What is the most abundant SCFA in the colon?
A
Acetate - formed by nearly all anaerobic gut bacteria that use organic materials
19
Q
How is acetate formed?
A
By fermentation or reductive acetogenesis
20
Q
What is reductive acetogenesis? What % of acetate is made this way?
A
A process by which acetate is produced directly from carbon dioxide and an electron source. A third occurs in colon.
21
Q
What species forms acetate and how?
A
Blautia productaforms acetate from hydrogen and carbon dioxide
22
Q
How does acetate formation benefit bacteria?
A
Because it results in energy generation by substrate-level phosphorylation of ADP to ATP
23
Q
What are the three main pathways that propionate can be formed?
A
- Succinate pathway
- Acrylate pathway
- Propanediol pathway
Propionate can be formed directly from sugars in a single species or indirectly by cross-feeding from succinate and lactate producers in the human gut
24
Q
How does propionate formation benefit bacteria?
A
- The disposal of reducing equivalents, especially NADH
- ATP synthesis
25
What are reducing equivalents?
Any number of chemical species that transfer the equivalent of one electron in redox reactions
26
What is the most abundant route for propionate production?
Succinate pathway
27
How does the succinate pathway work?
Bacteroides spp. produce acetate and succinate from polysaccharides and peptides.
succinate>propionate: by Bacteroides spp. & cross-feeding bacteria
acetate>butyrate: by cross-feeding bacteria
28
In what bacteria is the acrylate pathway found?
found only in two groups of low-abundance bacteria (Negativicutes, Lachnospiraceae).
29
What bacteria use the succinate pathway?
Negativicutes, Verrucomicrobia, Bacteriodales
30
What bacteria use the propanediol pathway?
Lachnospiraceae
31
How abundant is the propanediol pathway and how does it work?
```
Widespread and relatively abundant.
deoxy sugars (e.g. rhamnose & fucose) > 1,2-propanediol
```
32
What is fucose and where can it be found? What is the significance of this?
Fucose is a major component of host-derived glycans Found on mucin lining the intestinal epithelium. Therefore, gut bacteria can produce SCFAs from host-associated carbohydrates (e.g. glycans) as well as dietary substrates.
33
What genus and species do over 90% of the butyrate-producing bacteria come from?
genus: Roseburia
species: Faecalibacterium prausnitzii
(both phylum Firmicutes)
34
What two factors influence butyrate concentrations in the gut?
- Amount of dietary carbohydrate
- Variations in the dominant metabolic type of butyrate-producer among individuals or variations in diet can influence butyrate supply in the colon
35
What types of bacteria can convert acetate to butyrate in the colon by cross-feeding?
Acetate-utilizing bacteria, such as Roseburia species, Faecalibacterium prausnitzii, Coprococcus sp. strain L2-50 and Anaerostipes caccae
36
Where does much of Faecalibacterium prausnitzii and Roseburia species butyrate carbon come from?
It is generated from acetate via the Coenzyme A (CoA) transferase route.
37
What is required for the formation butyrate from acetate via the Coenzyme A (CoA) transferase route, and why?
An energy source such as glucose, probably to provide a source of reducing power for butyrate synthesis from acetylCoA as well as energy for growth
38
How has it been shown that there is cross-feeding between intestinal bacteria?
- Co-culture experiments
- oligofructose (a prebiotic) used as the energy source, with Roseburia intestinalis and Anaerostipes caccae converting the acetate produced by Bifidobacterium longum to butyrate.
39
Most human-derived gut bacteria that possess genes for __________ synthesis lack genes associated with _________ formation
Most human-derived gut bacteria that possess genes for *butyrate* synthesis lack genes associated with *propionate* formation
40
Butyrate and propionate represent alternative ‘
| ______’ whose pathways have evolved in different groups of bacteria
‘hydrogen sinks’
41
What do hydrogen sinks do? Why is this important?
Reduce intestinal hydrogen gas (H2) concentration. It has to be removed from the intestinal tract to maintain the thermodynamic equilibrium that allows continuous biodegradation of organic compounds.
42
What microbes are the most active H2 sinks?
- Archaea, sulfate-reducing bacteria
| - Acetogens
43
Where are concentrations of SCFA greatest?
Caecum and the ascending colon (131 mmol/kg digesta)
44
Why do SCFA concentrations fall during the passage through the small intestine?
95% of SCFAs are absorbed across the large intestinal wall, before which they pass through the portal blood and liver and then peripheral tissues
45
What is the molar ratio of acetate, propionate and butyrate?
57:22:21
46
What happens to pH levels as the amount of SCFA production increases? why?
They drop: acids and hydrogen ions are generated
47
What is an indirect way that SCFA production is protective? Against what?
Low pH faecal stool samples are correlated to a lower incidence of colorectal cancer, SCFA production does this by maintaining a low pH in the gastrointestinal tract.
48
What are healthy levels of circulating peripheral blood acetate, propionate and butyrate?
a: 22-42uM
p: 0.9-1.2uM
b: 0.3-1.5uM
49
What other organic acids are found in much smaller quantities in the intestine?
```
lactate
succinate
formate
valerate
caproate
branched chain fatty acids generated from amino acids
```
50
What strange product is produced in minor amounts as a by product of carbohydrate fermentation?
Ethanol
51
What is exogenous acetate production defined as?
colonic production (produced by gut microbiota) after accounting for intestinal retention
52
What percentage of SCFA from carbohydrate fermentation does exogenous acetate production make up?
60-75%
53
What is the principal way by which energy from carbohydrates, not absorbed by small intestine, is obtained?
Exogenous acetate is taken up by the intestinal epithelium, appears in hepatic portal blood and eventually passes through the liver to peripheral tissues, where it is metabolised
54
How is acetate used in the liver?
Acetate is used as a substrate for the synthesis of cholesterol and long-chain fatty acids and as a co-substrate for glutamine and glutamate synthesis
55
What tissues is acetate metabolised by?
heart, adipose tissue, kidney, and muscles.
56
What impact does acetate have on health, as shown through human and mice studies?
- Causes significant weight loss in humans after drinking 1.5g a day for 12 weeks
- In mice it can cross blood brain barrier where it is metabolised and promotes appetite suppression & increased production of lactate and gamma-aminobutyric acid.
- It is unknown if acetate has a hypothalamic effect in humans
57
What is endogenous acetate?
Acetate produced by host cells
58
50% of propionate acts as a precursor for _____ _____
hepatic gluconeogenesis
59
What does microbiota-produced propionate reduce?
cancer cell proliferation in the intestine and liver and induces apoptosis.
60
What are the levels of propionate in the blood and intestinal circulation?
20–26 μM in intestinal contents to 1 μM in systemic circulation
61
Once in the circulation, what does propionate contribute to?
Improving glucose tolerance and insulin sensitivity
Reduction of high-density lipoprotein
Increase in serum triglyceride concentrations
More stable metabolic homeostasis
62
How does propionate play a role in lipid metabolism?
By reducing hepatic use of acetate for fatty acid and cholesterol synthesis
63
How does propionate directly influence satiety?
Stimulates leptin, GLP-1 and PYY production.
Propionate increases production of gut-derived satiety hormones, making you feel less hungry and reducing your desire to eat.
64
What does propionate protect the blood brain barrier from?
Inflammatory stimuli, suggesting it plays an important role in the microbiome–gut–brain axis
65
Name the disease that is an inborn error of metabolism associated with propionyl-CoA decarboxylase, is the most frequent (but rare) disorder of organic acid metabolism in humans.
Propionic acidaemia
66
What are individuals affected by Propionic acidaemia unable to do?
unable to convert propionate to methylmalonyl CoA through propionyl-CoA.
67
When do symptoms of propionate acidaemia occur?
Immediately after birth, and include poor feeding, lethargy, vomiting, dehydration, acidosis, seizures and poor muscle tone. Left untreated, the condition is fatal.
68
What does treatment for propionic acidaemia involve?
placing an affected individual on a low-protein diet free of methionine, threonine, valine, and isoleucine, and providing carnitine supplements.
69
What are patients with propionic acidaemia given in order to suppress gut microbial activity associated with propionate production?
antibiotics 10 days per month
70
What are intestinal epithelial cells called?
colonocytes
71
What is the principal source of energy for colonocytes?
Butyrate
72
What does butyrate do?
- essential in maintaining tissue homeostasis in the colonic epithelium
- plays roles in suppressing intestinal inflammation and in lipid metabolism
- exhibits a range of anti-tumorigenic effects on many cancer cell lines
73
What percentage of energy does butyrate provide to the colonic mucosa?
40-70%
74
Where is butyrate taken up from? via which transporter?
Butyrate is taken up from the colon via monocarboxylate transporter 1 (MCT1) and used to produce energy for intestinal cells.
75
In the progression from normal to malignant cells in the human colon, what happens to the expression of MCT1 and intracellular concentrations of butyrate?
Significantly reduces
76
A reduction in MCT1-mediated butyrate uptake has also been linked to what?
Reduced levels of butyrate oxidation observed in intestinal inflammation, relevant to conditions such as inflammatory bowel disease in which a significant reduction in butyrate-producing bacteria is seen in patients.
77
In patients with inflammatory bowel disease, explain why only a small amount (< 1 μM) of butyrate is detected in the blood
Remaining butyrate not used as fuel by intestinal cells is mostly oxidised in the liver, to prevent toxic systemic concentrations.
78
What do the SCFAs (Acetate, Propionate and Butyrate) activate?
Members of the free fatty acid receptor (FFAR) family of G protein-coupled receptors;
A,P&B -> activate FFAR2 (formally known as GPR43), at millimolar to micromolar concentrations
P&B -> activate FFAR3 (formerly known as GPR41) at micromolar concentrations.
79
Where are both FFAR2 and FFAR3 expressed? What does this suggest?
Human colonic tissue, white adipose tissue, skeletal muscle and liver
-> suggesting SCFAs influence substrate and energy metabolism directly in peripheral tissues
80
Where is FFAR2 expressed and not FFAR3?
Immune cells such as monocytes, B-lymphocytes, and polymorphonuclear cells
81
FFAR3 has recently been shown to be expressed where?
Human brain endothelial cells, where it is likely activated by propionate and butyrate
82
Where is hydroxycarboxylic receptor 2 (HCAR2, GPR109a) expressed?
gut epithelial cells
adipocytes
immune cells
83
What is hydrocarboxylic receptor 2 activated by?
Butyrate
84
What do acetate and propionate activate, expressed in kidney and vascular smooth muscle cells, to contribute to modulation of blood pressure?
Olfactory receptor 51E2 (OR51E2),
85
In the large intestine, what do SCFAs produced from dietary substrates by the gut microbiota, bind to?
FFAR3 (GPR41) and FFAR2 (GPR43)
86
What does SCFAs produced from dietary substrates by the gut microbiota binding to FFAR2 and 3 lead to?
The production of the gut-derived satiety hormones PYY and GLP-1 and (GLP-2?), affecting satiety and glucose homeostasis.
87
Propionate and butyrate might also induce intestinal gluconeogenesis and sympathetic activity, what would this do?
Improving glucose and energy homeostasis.
88
When some SCFAs taken up from the gut get into systemic circulation what else may they affect?
May also directly affect peripheral adipose tissue, liver and muscle substrate metabolism and function.
89
How might circulating acetate influence the microbiome–gut–brain axis?
It might be taken up by the brain and regulate satiety via a central homeostatic mechanism
90
Are metabolic effects mainly explained by direct effects of SCFAs or indirectly via gut-derived signalling molecules?
It remains unclear.
91
What does PYY and GLP-1 and GLP-2 increase leads to a feeling of what?
reduced appetite
improved gut barrier function
improved glucose homeostasis (e.g. insulin sensitivity)
reduction in inflammation
92
Where did the first suggestion that the gut microbiota influenced brain chemistry come from?
In 1986; demonstrated significant differences in hypothalamic histamine levels between germ-free and conventionally raised animals.
93
What are the four major pathways by which the gut microbiota and the brain interact?
1) Direct modification of vagal or sympathetic sensorimotor function.
2) Inflammatory/immune activity.
3) Neuroendocrine crosstalk.
4) Gut-derived microbial metabolites interact directly with the blood–brain barrier.
94
How do SCFAs contribute to the gut brain axis?
SCFAs are recognised by FFAR2 &3 expressed at the surface of enteroendocrine L-cells in the intestinal wall, producing GLP-1, GLP-2, and PYY. These peptides send signals to the brain that lead to reduced appetite, improved gut barrier function, improved glucose homeostasis (e.g. insulin sensitivity), and a reduction in inflammation.
95
Propionate produced by gut microbes in the large intestine induces intestinal gluconeogenesis via FFAR3 where?
in the periportal afferent neural system (a gut–brain neural circuit)
96
What does inducing intestinal gluconeogenesis via this gut–brain neural circuit improve?
various features of energy metabolism (e.g. insulin sensitivity)
97
Increased intestinal propionate has also been associated with what?
reduced stress behaviours and reward pathway activity in mice and humans, respectively.
98
Where is FFAR3 present? What does this mean?
Brain endothelial cells; SCFAs in the blood can interact with this receptor. Propionate is a highly potent FFAR3 agonist - having close to optimal ligand efficiency for this receptor.
99
In vitro, what has propionate has been shown to protect?
The human blood–brain barrier from inflammation caused by lipopolysaccharide (LPS) and against oxidative stress
100
What is exogenous acetate production defined as?
Colonic production after accounting for intestinal retention and is responsible for the majority of circulating SCFAs in human blood
101
What is the gut-brain axis?
Complex multidimensional, bidirectional communication network of gut microbiome, the gut and the enteroendocrine system, linking emotional and cognitive centres of the brain with peripheral intestinal functions
