Research in the Brennan Laboratory
Humans and most other animals rely on phagocytic immune blood cells to protect them from infection. Such white blood cells are able to engulf microbial invaders such as bacteria, viruses, and fungi into vesicles known as phagosomes. Within minutes of engulfment, the internal environment of the newly formed phagosome begins to change, becoming lethal to microbes. Such changes include acidification, the production of toxic reactive oxygen and nitrogen species, and eventually delivery of degradative enzymes from another organelle, the lysosome.
Although the molecular events that control engulfment are well-characterized, our understanding of the mechanisms that govern the maturation of the phagosome towards the degradative state and towards fusion with the lysosome is much murkier. Understanding these processes is important! Some of the most serious bacterial infectious diseases of humans work by blocking phagosome maturation. The bacteria that cause tuberculosis are a classic example: they allow themselves to be phagocytosed, but then stall phagosome maturation, and proliferate in the resulting protected environment. With over a third of the world’s population infected with TB, and a growing crisis in antibiotic resistance in TB bacteria, understanding the cellular regulation of phagosome maturation would go a long way towards the development of alternatives to antibiotics for the medical treatment of tuberculosis.
Although flies and humans look very different, our cells actually function in much the same way. In fact, we share 60% of our genes with flies, likely including most of the genes that encode proteins that control phagosome maturation. The advantages to working with Drosophila include that we can breed flies lacking the function of individual genes (mutants), and by studying the defects that arise, we can infer the normal function of the genes and proteins.
Dr. Brennan discovered one of the first phagosome maturation mutants in any organism. The phagocytic cells of flies mutant for (lacking the function of) the psidin gene are able to engulf bacteria, but not kill them. This tells us that the Psidin protein normally makes an important contribution to phagosome maturation. A major focus of our laboratory is determining what this molecular contribution of Psidin to phagosome maturation is. The phagosomes of mice (and so probably humans, too) also have the Psidin protein on them, suggesting that our work is likely to lead to important insights into how our own white blood cells manage the killing of phagocytosed microbes. We have also discovered other genes required for phagosome maturation, and are investigating their specific functions.
Our research has been funded by CSUF, CSUPERB, and we are currently seeking funding from the NIH. We collaborate with laboratories around the world in our analyses of phagosome function.
We are always looking for undergraduate and master’s students with a passion for research! Techniques that students in the Brennan laboratory master include live fluorescent microscopy, Western blotting, protein purification, qPCR, standard DNA molecular biology, Gibson assembly, CRISPR, and Drosophila genetics. Students are able to present their research at regional and national scientific conferences, and all of our research is geared towards publication in high-profile journals.
This is how we infect the larvae: a pulled capillary needle filled with fluorescent bacteria is inserted into the larva, and about 50 nanoliters injected using air pressure from a Picospritzer microinjector. Image credit: Mobina Roshandell.
In this image we see a fly blood cell that has phagocytosed a bacterium. The bacterium is expressing the red fluorescent protein mCherry, which allows us to follow its degradation in the phagosome. The green glow of the blood cell comes from expression of a GFP-Rab5 fusion protein. The intense ring of green around the bacterium represents the recruitment of the Rab5 protein to the phagosome, an early transient step in phagosome maturation. Image credit: Mobina Roshandell.