SB-BB

Passage 2

The human intestinal tract is inhabited by a diverse assortment of microorganisms known as gut microbiota. These microbial populations are increasingly implicated in the regulation of host metabolism and the development of obesity and diabetes.

The catabolism of dietary fiber by gut microbiota results in the production of short chain fatty acids (SCFAs), such as butyrate and acetate, which are important energy sources for the host. SCFAs also act as signaling molecules, with the ability to bind and activate G protein-coupled receptors (GPCRs).

The GPCR, GPCR43, is expressed in white adipose tissue (WAT) and is not expressed in muscle or liver. Researchers generated Gpcr43-deficient (Gpcr43–/–) mice to analyze the contribution of GPCR43 to energy regulation. Wild-type (WT) and Gpcr43–/– mice were fed a standard diet (Std) or high-fat diet (HFD), and their body weight in grams (g) was measured weekly for 16 weeks (Figure 1).

Figure 1 Body weight of WT and Gpcr43–/– mice fed a Std or HFD for 16 weeks

Next, researchers examined the relationship between GPCR43 function and insulin sensitivity. Adipocytes from WT and Gpcr43–/– WAT were incubated in the presence and absence of both insulin and acetate, and the uptake of radioactive glucose was measured (Figure 2).

Figure 2 Uptake of radioactive glucose in WT and Gpcr43–/– adipocytes in the presence and absence of both insulin and acetate (Note: * indicates p < 0.05.)

Adapted from I. Kimura, K. Ozawa, D. Inoue, T. Imamura, K. Kimura, et al., Nature Communications. ©2013 Nature Publishing Group.

Questions

7. Which table shows the expected body weight of WT and Gpcr43–/– mice fed a HFD while housed for 16 weeks in a conventional (CONV) condition, or a germ-free (GF) condition where the gut does not become colonized?

8. Which amino acid is LEAST likely found in one of the transmembrane domains of GPCR43?

  1. Asp
  2. Gly
  3. Trp
  4. Phe

9. Compared to WT mice, which experimental group of mice is most likely to remain lean when fed a HFD?

  1. Mice treated with antibiotics
  2. Mice treated with a GPCR43 antagonist
  3. Mice in which GPCR43 is overexpressed in WAT
  4. Mice treated with a drug that inhibits the generation of intracellular second messengers

10. Which conclusion about glucose uptake is best supported by the data in Figure 2?

  1. Acetate suppresses insulin-mediated glucose uptake in WT adipocytes, but not Gpcr43–/– adipocytes.
  2. Acetate stimulates insulin-mediated glucose uptake independently of GPCR43 expression in adipocytes.
  3. Insulin suppresses glucose uptake in the presence of acetate in WT adipocytes, but not Gpcr43–/– adipocytes.
  4. D. Insulin stimulates glucose uptake in WT adipocytes, but not Gpcr43–/– adipocytes.

11. Insulin signaling results in the phosphorylation of the downstream target, Akt, which can be quantified by Western blot analysis. Which graphic shows the effect acetate administration will likely have on Akt phosphorylation levels in WAT and muscles of WT mice?

12. Compared to untreated WT mice, antibiotic treatment of WT mice is likely to result in:

  1. increased plasma butyrate levels.
  2. decreased body mass.
  3. increased volume of adipocytes.
  4. decreased insulin sensitivity in adipocytes.

13. Which amino acid contains an unbranched alkyl side chain?

  1. Ala
  2. Ile
  3. Leu
  4. Val

14. The stereochemical designators α and β distinguish between:

  1. enantiomers at an epimeric carbon atom.
  2. enantiomers at an anomeric carbon atom..
  3. epimers at an anomeric carbon atom.
  4. epimers at a non-anomeric carbon atom.

15. Which ideal solution exhibits the greatest osmotic pressure?

  1. 0.1 M MgCl2
  2. 0.2 M NaCl
  3. 0.2 M CaCl2
  4. 0.5 M Glucose

16. Which event is directly mediated by a ligand-gated ion channel?

  1. Release of Ca2+ from the sarcoplasmic reticulum of a muscle fiber to initiate muscle contraction
  2. Influx of Na+ across the axon membrane of a somatic neuron during action potential propagation
  3. Influx of Na+ across the motor end plate resulting in the depolarization of the muscle fiber membrane
  4. Re-entry of Ca2+ back into the sarcoplasmic reticulum of a muscle fiber to end muscle contraction

17. A single point mutation in a gene results in a nonfunctional protein. Individuals heterozygous for this mutation were identified using a Southern blot. Which pair of wild-type (WT) and mutant alleles most likely contains the mutation?

Answers

Question 7 Solution: The correct answer is B. This is a Biology question that falls under the content category “Structure and integrative functions of the main organ system.” The answer to this question is B because as shown in Figure 1, Gpcr43–/– mice gain more weight than wild-type mice when fed a HFD. This suggests that this receptor functions to suppress fat accumulation. Since this receptor is activated by SCFAs produced by the gut microbiota, when mice are grown under germ-free conditions they do not activate GPCR43 and therefore would gain more weight than under normal conditions. Gpcr43–/– mice would gain more weight than WT regardless of the presence or absence of SCFAs (row 1 to row 3 – basal). This is a Data-based and Statistical Reasoning question as it requires evaluating representations, body weight in this case, for particular scientific conditions.
Question 8 Solution: The correct answer is A. This is a Biochemistry question that falls under the content category “Assemblies of molecules, cells, and groups of cells within single cellular and multicellular organisms.” The answer to this question is A because aspartate (Asp) has a negatively charged R group and is very hydrophilic. This is a Knowledge of Scientific Concepts and Principles question because it requires assumed knowledge of the different amino acids.
Question 9 Solution: The correct answer is C. This is a Biology question that falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is C because increased GPCR43 activity would be expected to compensate for the increased fat intake and prevent fat accumulation. This is a Reasoning about the Design and Execution of Research question as it requires you to match a particular experimental approach with a defined outcome.
Question 10 Solution: The correct answer is A. This is a Biochemistry question that falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is A because Figure 2 shows that insulin stimulates glucose uptake in adipocytes from WT and Gpcr43–/– mice. Acetate suppresses the glucose uptake in WT, but not Gpcr43–/– mice. This is a Data-based and Statistical Reasoning question as it requires the analysis of data presented in Figure 2.
Question 11 Solution: The correct answer is B. This is a Biochemistry question that falls under the content category “Structure and functions of the nervous and endocrine systems and ways in which these systems coordinate the organ systems.” The answer to this question is B because as shown in the passage, insulin signaling is inhibited by activation of GPCR43 by acetate. This effect would only expect to be seen in WAT and not muscle cells, where GPCR43 is not expressed. This is a Scientific Reasoning and Problem Solving question because it asks you to make a scientific prediction about the outcome of an experiment.
Question 12 Solution: The correct answer is C. This is a Biology question that falls under the content category “The structure, growth, physiology, and genetics of prokaryotes and viruses.” The answer to this question is C because if SCFA producing microbes were removed by antibiotic treatment, then GPCR43 would not be activated to reduce glucose uptake into adipocytes. This is a Scientific Reasoning and Problem Solving question because it asks you to evaluate an argument about causes and consequences.
Question 13 Solution: The correct answer is A. This is an Organic Chemistry question that falls under the content category “Structure and function of proteins and their constituent amino acids.” The answer to this item is A since alanine contains a methyl side chain, which is not considered a branched alkane. This is a Knowledge of Scientific Concepts and Principles question since you must recall the distinction between branched and unbranched alkanes and identify the choice that features an unbranched alkyl side chain.
Question 14 Solution: The correct answer is C. This is an Organic Chemistry question that falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this item is C since the α versus β designation distinguishes between molecules with multiple chiral centers, but differ only in the configuration of the site known as the anomeric carbon atom. This is a Knowledge of Scientific Concepts and Principles question since you must recall and identify the distinction between α and β designations for molecules.
Question 15 Solution: The correct answer is C. This is a General Chemistry question that falls under the content category “Assemblies of molecules, cells, and groups of cells within single cellular and multicellular organisms.” The answer to this question is C because the solution listed has the greatest concentration of solute particles. CaCl2 dissociates into three ions, making the solution listed 0.6 M in solute particles. It is a Scientific Reasoning and Problem Solving question because you are asked to determine which formula applies to the stated scenario and apply it correctly to make a prediction.
Question 16 Solution: The correct answer is C. This is a Biology question that falls under the content category “Assemblies of molecules, cells, and groups of cells within single cellular and multicellular organisms.” The answer to this question is C because the influx of Na+ across the motor end plate occurs when Na+ ion channels bind the ligand acetylcholine. It is a Knowledge of Scientific Concepts and Principles question because you are asked to recall the mechanism by which action potentials are triggered in muscle fibers.
Question 17 Solution: The correct answer is A. This is a Biology question that falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is A because a Southern blot uses a restriction digest to differentiate between mutant and wild-type alleles. In order for a Southern blot to be useful, the mutation should either create or eliminate a restriction site, most of which are palindromes and 4 to 6 base pairs long. The mutation shown in this option is the only one that disrupts a palindromic sequence, AAGCTT. This sequence is the recognition sequence for HindIII. It is a Scientific Reasoning and Problem Solving question because you are asked to demonstrate an understanding of the appropriate use of specific DNA-based biotechnology techniques.