January 2004, Issue 1

Manipulating Mouse Genetics to Answer Intriguing Questions

Maribel Rios, PhD (Tufts 1997), trained with Suzanne Roffler-Tarlov in the Department of Anatomy (Program in Cell, Molecular, and Developmental Biology). During her time at Tufts, Rios' collaboration with Dona Chikaraishi in the Department of Neuroscience was critical to her future interests and success. Rios then spent four and a half years with Rudolf Jaenisch at the Whitehead Institute for Biomedical Research, and returned to Tufts in May 2002 as an assistant professor of neuroscience. The Rios Lab is located on the third floor of the Stearns Building, and their main focus is brain-derived neurotrophic factor (BDNF), an important regulator of neuronal survival, differentiation and plasticity. Although a significant body of research exists on the role of BDNF in fetal development, Rios is using her expertise in mouse genetics to look at the action of BDNF postnatally.

Rios generates lines of conditional BDNF mutant mice by using the Cre-loxP recombination system to control (both temporally and regionally) BDNF expression in the brain. The mutant mice are generated by crossing mice carrying loxP sites that flank an essential sequence in a gene of interest with mice that express the Cre recombinase, which eliminates this essential part of the gene in the offspring by recombining the loxP sites in the genome of the new mutants. "The pattern of removal of your gene of interest is determined by the promoter you use to drive expression of Cre recombinase. For example, to generate mutants without BDNF in the brain, we use the Cam kinase II promoter to direct expression of Cre recombinase. In our mice, that promoter is active during the postnatal period in postmitotic neurons across the brain, with the exception of the cerebellum. The beauty of this system is the versatility — you make your line of 'floxed' BDNF mice that you can cross to any line of Cre mice to generate another brand new mutant." "Floxed" mice contain loxP sites in a particular locus of their genome, such as the BDNF locus.

Rios' research is driven not only by her own interests and the questions that must be answered but also by the surprises her mutant mice throw at her. "Often, when using mouse genetics to address the role of a factor in a specific process, you end up with three other phenotypes that you totally didn't expect but that are very interesting. So you can't help yourself from looking into that." She owes her current interest in the role of BDNF in body weight regulation and psychiatric disorders to her mice. "When we made these mutants we found that they were dramatically obese, hyperaggressive, and anxiety prone. And that pushed me towards those two lines of research."

Delving into the molecular action of BDNF, Rios wants to answer the not-so-simple question: "BDNF where is important for what?" In the brain BDNF is expressed in the hypothalamus, a center that integrates information important for body weight regulation through modulation of food intake and metabolic function. In the periphery, BDNF is expressed at significant levels in metabolic tissues like the liver, the gut, and the endocrine pancreas. "We're trying to access what it's doing there. BDNF could be regulating peripheral metabolic function through activity in the brain that modulates the autonomic nervous system innervation of metabolic organs. Alternatively, BDNF could be regulating the functions of those organs through local action." The fact that BDNF homology between a number of species, including humans and mice, is nearly 100% adds to the potential usefulness of Rios' research.

To investigate the hyperaggressive behavior of the conditional BDNF mutant mice, Rios decided to look, in collaboration with electrophysiologist George Aghajanian of Yale University, at the serotonergic system in the brain. Aghajanian recorded both from cells that make serotonin and from those that receive innervation by these serotonergic neurons. He identified severe deficits in serotonergic transmission at both levels. Rios is following up with the biochemistry. Preliminary data suggest that there are two independent defects through which serotonergic transmission is compromised — one that enhances signaling through presynaptic serotonin autoreceptors and another that interferes with expression of postsynaptic serotonin receptors.

Rios acknowledges that "you get farther more quickly when other people help you, so I'm all for collaborations. When you're doing mouse genetics, you find yourself reinventing yourself all the time — especially now that I'm venturing outside the brain. All of a sudden I'm trying to be an endocrinologist. I'm learning liver physiology, pancreas physiology. That kind of expertise would be great. With the body-weight-regulation research, the HNRCA [Tufts Human Nutrition Research Center on Aging] might be a potential source of collaboration." Rios currently has two graduate students and one technician working in her laboratory, and she hopes to add a postdoc and another grad student by next summer. "I feel really fortunate that I have students in the lab. It's always helpful to get a fresh perspective because one can become really focused on one aspect and lose sight of the big picture."

For more information, go to http://www.neurosci.tufts.edu/rios/



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