Biochemistry In complex diseases that are systemic throughout the body, there is
ID: 210924 • Letter: B
Question
Biochemistry
In complex diseases that are systemic throughout the body, there is often no way to model the disease process without the use of in vivo animal model systems. The study of lipid metabolism and atherosclerosis in the body relies on animal research to test hypotheses and develop new treatments to help improve human health.
You are working in a research team using several mouse strains that are gene knockouts for key proteins in cholesterol and apolipoprotein metabolism. Consider the following mouse strains, which have had genes turned off, referred to as “knockouts”:
- Apolipoprotein E knockout mice (Apoe/) that cannot make ApoE protein
- LDL receptor -/- mice( Ldlr/ ) that do not make LDL receptor.
- Apob100/100 mice expressing only ApoB-100 and not ApoB-48
- Apob48/48 mice expressing only ApoB-48 and not ApoB-100.
- Apob+/+ normal mice that express both forms of ApoB.
The two hypercholesterolemic mouse models, the apo E–deficient mouse (Apoe/) and the LDL receptor–deficient mouse (Ldlr/) have been used to study atherogenesis, or the development of fatty plaques at the start of atherosclerosis. On a normal mouse chow diet, Apoe/ mice have the highest cholesterol levels, with total cholesterol levels of ~400 mg/dL. Chow-fed Ldlr/ mice have mildly increased plasma cholesterol levels (175 to 225 mg/dL) from an accumulation of LDL and develop only minimal atherosclerotic lesions. Apo B-48 is the predominant apolipoprotein in the VLDL remnants of Apoe/ mice, whereas apo B-100 predominates in the LDL of Ldlr/ mice. The following table summarizes the characteristics observed in these two hypercholesterolemic mouse models.
These two knockout strains of mice have allowed scientists to investigate the genetic influence on atherosclerosis. Moreover, these mice can be fed controlled diets with different fat profiles and their health status can be closely monitored, which cannot be done in a human population. However, there are drawbacks to these models. In the case of the Ldlr-/- mice, there is a minimal development of the atherosclerotic plaques, even after 9-12 months, which is very late in a mouse life span. Conversely, Apoe-/-mice have incredibly high plasma cholesterol levels, far higher than normally observed in people, and it is mostly ApoB-48 associated cholesterol. Humans with high cholesterol normally have high ApoB-100, which is different from these mice. To understand the implications, mice expressing only ApoB-100 or ApoB-48 were also developed. These two strains have similar total and HDL cholesterol to Apob+/+ normal mice, but the Apob48/48 mice have significantly lower plasma TAG, while the Apob100/100 have significantly higher plasma TAG.
Based on your knowledge of Apolipoproteins in the different types of cholesterol, explain why you think there are these differences in lipoprotein and apolipoprotein composition between the four mouse strains of Apoe-/-, Ldlr-/-, Apob48/48, and Apob100/100 mice:
1. How would the differences in ApoB protein expression between human and the Ldlr-/- mice be a potential problem to using this mouse model to describe human disease?
2. Does the composition of the Apob48/48 and Apob100/100 cholesterol make sense based on what we know about ApoB-48 and ApoB-100?
Apoe Mice 400 mg/dL, 5 times above controls Greatly increased Modestly increased Decreased 3 months streaks, 8 months plaques Many large plaques after 14 weeks Ldir Mice 200 mg/dL, 2-3 times higher than controls Modestly increased Greatly increased Modestly increased Phenotype Hypercholesterolemia (cholesterol in the blood) VLDL IDL/LDLe HDL Spontaneous fatty plaques Medium plaques after 12 weeks on high cholesterol diet High fat diet induced plaquesExplanation / Answer
1) LDL receptor is a membrane receptor having 160kDa molecular weight, that perform the cholesterol-rich LDL endocytosis and hence balance the LDL plasma level. It also supports the cellular uptake of apoB and E carrying lipoproteins. Lack of LDL receptor beside with mutations in the the LDL receptor encoding gene count for the phenotypic case explained in familial hypercholesterolaemia. As Comparing with wild-type, LDLr-/- mice show modestly increased cholesterol levels of plasma and evolve no or only mild atherosclerosis after normal diet. In case of lipoprotein particles, the expansion is larger among IDL and LDL sized particles, while HDL and triglycerides stay unaffected.
2) Apob48/48 mice had reduced LDL level cholesterol and lower plasma triglyceride levels compared to wild-type mice, while the Apob100/100 mice had elevated levels as well as significantly increased triglyceride levels. Due to the presence of apoB-48 in all mammal's intestines, it appeared similarly that apoB-100 intestinal synthesis in the apoB-100-only mice might be suboptimal for the arrangement of chylomicron and absorption of fat. The protein found in 2 isoforms ApoB48 and ApoB100, ApoB48 produced by small intestine and ApoB100 produced by the liver.
As a outcome of the RNA editing, ApoB48 as well as ApoB100 have a typical N-terminal sequence, but ApoB48 deficient C-terminal of ApoB100's LDL receptor binding region.
ApoB 48 is a specific protein to chylomicrons from the small intestine. When majority of the chylomicron lipids have been absorbed, then ApoB48 move back to the liver as remnant, there it is endocytosed and degraded.
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