The researchers affiliated with the Down Syndrome Research Center have many ongoing projects. We will profile selected individual projects here, and report on recent progress.
A major focus of our lab has been the roles of sleep and circadian rhythms in learning and memory. We began to direct these interests to understanding the cognitive challenges of individuals with Down Syndrome (DS) with the discovery that over-inhibition in the brain involving the major inhibitory neurotransmitter gamma aminobutyric acid (GABA) is a critical factor and that application of GABA antagonists improved learning and memory in the DS model Ts65Dn mouse. GABA plays roles in both sleep and circadian rhythms, so the work we were doing on these mice indicated possible involvement of abnormal sleep or circadian mechanisms as a cause of the learning difficulties of individuals with DS. Our suspicions were strengthened when we discovered that the GABA antagonists that improved learning and memory in the Ts65Dn mice were only active when given to the animals during the daily light phase, which for mice is when they sleep the most.
Our subsequent studies of sleep and circadian rhythms in the Ts65Dn mice did not reveal any obvious differences from littermate mice that did not have triplication of genes. In fact, in some ways the Ts65Dn mice had stronger circadian rhythms.
Circadian rhythms are generated in a tiny brain region called the suprachiasmatic nucleus (SCN), and this brain region has lots of neurons that use GABA as their neurotransmitter. A classic and simple way of investigating the function of a particular brain area is to damage it. When we damaged the SCN of the Ts65Dn mice, they lost their circadian rhythms (became arrhythmic) as did the non-trisomic mice, but their performance on mouse learning and memory tasks became as good as that of the non-trisomic mice that were either rhythmic or arrhythmic. These results led us to hypothesize that a normal function of the circadian system is to reduce neuroplasticity during while memories of wakeful experiences are being consolidated during sleep.
Dr. Elsa Pittaras in our lab is now using more sophisticated methods called DREADDS to reversibly activate and inactivate the SCN as a means of investigating its role in learning and memory. DREADDS stands for “designer receptors exclusively activated by designer drugs. These techniques are also called chemo-genetics because they use genetic engineering methods to insert non-natural neurotransmitter receptors into certain types of neurons so that administration of non-natural drugs can reversibly activate or inactivate those neurons in behaving animals. Thus Dr. Pittaras is able to increase the activity of the SCN at times of day when it would not be active (the dark phase). Or, she can decrease the activity of the SCN at times of day when it would normally be most active. Doing these manipulations in the context of learning and memory tasks should produce new insights into the role of the SCN in memory consolidation and indicate whether we should explore ways of modulating circadian rhythms in individuals with DS as a means of improving their cognitive performance.
Clarke and Heller Laboratories
These two laboratories have collaborated on several research projects involving the role of a gene – USP16 – that plays a role in controlling stem cell division. Low expression of USP16 favors stem cell renewal, meaning that when the stem cell divides, both daughter cells remain stem cells. If USP16 expression is high, the daughter cells tend to differentiate thus depleting the stem cell pool. Maintenance of stem cells is important for normal brain development and function. Stem cells are also important for support of many tissues in the body such as bone and blood. Individuals with DS have three copies of the USP16 gene. To discover what aspects of DS might be due to elevated levels of USP16, Dr. Adorno in the Clarke lab produced a strain of Ts65Dn mice that had normal copy number of the USP16 gene. Dr. Adorno showed that normalizing USP16 copy number protected neural stem cells. She also showed that USP16 is involved in many signaling pathways that influence the maintenance and repair of tissues other than neuronal. For example Ts65Dn mice with only two copies of USP16 had improved bone density. Of great interest was the fact that normalizing copy number of USP16 improved learning and memory in Ts65Dn mice and also improved their gait and balance indicating an effect on brain development. That study of brain development, in particular the development of the cerebellum, continues. The cerebellum is critical for coordination, gait, and balance, continues.
The Clarke and Heller labs have also collaborated in studies of how normalization of copy number of USP16 influences the progression of symptomatology in mouse models of Alzheimer’s disease (AD). This research was conducted by doctoral students Felicia Reinitz and Elizabeth Chen from the Clarke lab and Dr. Bayarsaikhan Chuluun from the Heller lab assisted both in conducting learning and memory trials. A gene (APP) involved in the development of AD is triplicated in individuals with DS contributing to an earlier onset of AD symptoms than in the overall population. Thus, understanding the interactions of these two genes in mouse models of AD and DS is important. Translational opportunities are likely to arise out of elucidation of the signaling pathways involved in these effects and interactions, and one is being pursued by Dr. Adorno through the formation of a start-up called Dorian.