You have created transgenic mice that carry the gene encoding green fluorescent protein (GFP) under the control of a ubiquitous promoter. The GFP-encoding gene, however, is inactive unless recombination by Cre removes a blocking sequence between the promoter and the gene. These mice have been mated with those harboring the Cre recombinase under the control of the insulin promoter. The Cre recombinase is fused to the estrogen receptor, which keeps the fusion in the cytosol unless it binds to tamoxifen, in which case it allows nuclear entry. After mating, you select adult mice harboring both of these transgenes and inject them with either tamoxifen or water. Which of the following would you expect to observe later as a result when you examine brain and pancreas tissue samples under a fluorescence microscope?
If adult mice harboring both the transgenes GFP under the control of a ubiquitous promoter and Cre recombinase under the control of the insulin promoter are injected with tamoxifen, the fusion of Cre recombinase and estrogen receptor will enable nuclear entry, leading to recombination of the GFP gene. This will remove the blocking sequence between the promoter and the gene, and activate the GFP gene. As a result, under a fluorescence microscope, you would expect to observe fluorescence only in the brain tissue, as the insulin promoter controls the expression of Cre recombinase in the pancreas cells, not in the brain cells.
On the other hand, if the mice are injected with water instead of tamoxifen, the Cre recombinase will remain in the cytosol and will be unable to enter the nucleus. Therefore, the blocking sequence will remain intact, and the GFP gene will remain inactive. Consequently, under a fluorescence microscope, you would not observe any fluorescence in either the brain or the pancreas tissue.
If adult mice harboring both transgenes are injected with tamoxifen, the estrogen receptor-Cre recombinase fusion will enter the nucleus. This will result in the removal of the blocking sequence between the promoter and the gene encoding green fluorescent protein (GFP). As a result, the GFP gene will become active and the mice will express green fluorescence only in the cells where the Cre recombinase is active.
In this case, since the Cre recombinase is under the control of the insulin promoter, it will be active in the pancreatic cells. Therefore, the pancreas tissue samples from these mice will show green fluorescence under a fluorescence microscope.
However, in the brain tissue samples, there will be no or minimal green fluorescence observed. This is because the Cre recombinase is not under the control of a promoter that is active in the brain cells. Therefore, the blocking sequence will not be removed and the GFP gene will remain inactive in the brain cells.
In summary, after examining brain and pancreas tissue samples under a fluorescence microscope, you would expect to observe green fluorescence specifically in the pancreatic cells, while the brain cells will not show significant green fluorescence.
To answer this question, we need to understand the experimental setup and how the different transgenes work in the mice.
The first transgene carries the gene encoding green fluorescent protein (GFP) under the control of a ubiquitous promoter. However, the GFP gene is inactive unless recombination by Cre removes a blocking sequence between the promoter and the gene. This means that unless Cre removes the blocking sequence, GFP expression will not occur.
The second transgene harbors the Cre recombinase under the control of the insulin promoter. The Cre recombinase is fused to the estrogen receptor, which keeps the fusion protein in the cytosol unless it binds to tamoxifen. When tamoxifen binds to the estrogen receptor, it allows the fusion protein to enter the cell nucleus.
Now, let's consider the possible outcomes after mating these two transgenic mice and injecting them with either tamoxifen or water.
1. If an adult mouse harbors both transgenes and is injected with water (no tamoxifen), nothing significant is expected to happen. The Cre recombinase fusion protein will remain in the cytosol and not enter the nucleus. Without Cre activity, the blocking sequence will not be removed, and the GFP gene will remain inactive. Therefore, no GFP expression will be observed, neither in the brain nor in the pancreas tissue samples.
2. If an adult mouse harbors both transgenes and is injected with tamoxifen, significant changes are expected. Tamoxifen binding to the estrogen receptor allows the Cre recombinase fusion protein to enter the nucleus. Once inside the nucleus, Cre can catalyze recombination and remove the blocking sequence from the GFP gene. This will activate the GFP expression under the control of the ubiquitous promoter.
In brain and pancreas tissue samples examined under a fluorescence microscope after tamoxifen administration, GFP expression would be observed since the blocking sequence has been removed. Thus, both the brain and pancreas tissues would exhibit green fluorescence due to the activation of the GFP gene.
In summary, injecting adult mice harboring both transgenes with tamoxifen would result in GFP expression, observed as green fluorescence in brain and pancreas tissue samples under a fluorescence microscope. However, without tamoxifen, no GFP expression would occur.