A paper1 published in the May 7 issue of The Journal of Neuroscience describes the development of a mouse model that offers a new approach to study narcolepsy and explore potential therapies for the sleep disorder.
“The orexin-tetracycline transactivator (tTA); Tet-Off (TetO) diphtheria toxin (orexin-tTA; TetO DTA or ‘DTA mouse’) model of narcolepsy was genetically engineered to use the ‘Tet-Off’ regulatory system to control specific neurodegeneration of cells that produce hypocretin (also known as orexin),” says Sarah Wurts Black, PhD, a research scientist at SRI International, Menlo Park, Calif, and paper coauthor. “That means that researchers can control the timing and extent of hypocretin cell loss in mice of any age simply by switching the type of mouse chow the DTA mice eat.”
The system works such that if doxycycline is in the mouse chow, the hypocretin cells remain intact. When the doxycycline is removed from the mouse’s diet, neurotoxic degeneration of the hypocretin neurons begins.
For the project, Black and colleague Thomas Kilduff, PhD, who directs the Center for Neuroscience within SRI Biosciences, teamed with colleagues at five institutions in Japan. One of the existing models, called “Ataxin mice,” was used for more than 10 years. Although Ataxin mice have enabled researchers to study narcolepsy, the mice start losing hypocretin neurons from birth. However, this was limiting because the onset of human narcolepsy typically occurs after puberty.
“The beauty of the DTA conditional mouse model is it lets each animal serve as its own control,” Black says. “Assessments can be made on a mouse with intact hypocretin neurons, like a wild type ‘before’ condition, and then neurodegeneration can be induced to make the ‘after’ condition comparisons.”
The new DTA mouse model will enable researchers to study the progressive consequences of hypocretin neurodegeneration, as the cells are dying and as the symptoms of disease emerge over time. “Hypocretin neurons project to many areas of the brain, and we are interested in discerning how these neural connections and downstream networks reorganize after they lose their excitatory hypocretin input,” Black says.
The DTA model also allows the extent of hypocretin neurodegeneration to be controlled. This is done by removing dietary doxycycline for only a few days, and then reinstating normal mouse chow. An absence of doxycline for 1.5 or 3.5 days will cause a loss of about one-third or two-thirds of all hypocretin cells, respectively. “This method will now let us model conditions of partial hypocretin cell loss, such as what we hypothesize might occur in narcolepsy without cataplexy,” she says.
Black says researchers are currently using the DTA mouse model to evaluate several new classes of pharmacotherapeutics for narcolepsy. “We are trying to find alternatives to the classic monoamine-based drugs,” she says. “Hopefully, the new mouse model will help us to get new candidates to patients and help us learn more about the basic neurobiology of hypocretin systems.”
The paper was published with a companion paper that uses the DTA and Ataxin models to demonstrate a new therapeutic candidate (R-baclofen), Black says.
1. Tabuchi S, Tsunematsu T, Black SW, et al. Conditional ablation of orexin/hypocretin neurons: a new mouse model for the study of narcolepsy and orexin system function papers. J Neurosci. 2014;34(19):6495-6509.