PROJECT SUMMARY Down syndrome (DS), which is caused by full or partial trisomy of human chromosome 21 (Hsa21), presents various neurological features, including intellectual disability, epilepsy, and Alzheimer’s disease (AD). Although some Hsa21 genes have been identified as key contributors to some of these diseases, the pathogenetic mechanisms that link the triplication of HSA21 genes to most DS-related diseases remain unknown. Particularly, very little is known about whether any of these diseases share common mechanisms. For example, we do not know whether the increased risk for epilepsy and cognitive deficits share common molecular and cellular basis. This is a major gap in our understanding and impedes the development of effective treatments of DS-related diseases. Many of the DS-related neurological conditions are related to dysregulated GABAergic systems. Previous studies by others and ours have shown that excessive functions of GABAergic synapses impair synaptic plasticity in the hippocampus and the cognitive performances in DS mouse models. On the other hand, dysregulated GABAergic systems are also a major contributor to epilepsy. These results indicated that GABAergic dysfunction and altered connectivity might be key to the comorbidity of cognitive deficits and epilepsy in DS. Recently, we found that the triplication of Down syndrome cell adhesion molecule (DSCAM) is a major cause of the increased GABAergic synapses and enhanced GABAergic synaptic transmission in the neocortex of DS mouse models. DSCAM variants are strongly associated with cognitive abilities, raising the possibility that DSCAM contributes to the comorbidity of DS diseases that are caused by dysregulated GABAergic systems. The objective of the proposed study is to identify molecular regulators of GABAergic systems that underlie comorbidities in DS. The central hypothesis is that DSCAM triplication enhances GABAergic synaptic transmission by increasing GABAergic synaptogenesis and contributes to the comorbidity of cognitive deficits and epilepsy in DS. The collaborative team has complementary strengths and is in a unique position for the proposed study. The proposed research is innovative because it will (1) identify a novel molecular mechanism that underlies the comorbidity of cognitive deficits and epilepsy in DS; (2) provide the first complete characterization of the activity dynamics of cell-type specific populations of the DS hippocampus in vivo; (3) apply cutting-edge spatial transcriptomic techniques to provide a unique transcriptomic profile with spatial resolution in a DS mouse model. This research is significant because its successful completion will identify the molecular regulators of the GABAergic system in the DS hippocampus and discover them as common contributors to the comorbidities between epilepsy and intellectual disability in DS. This knowledge will offer insights into the development of therapeutic treatments that target these DS-related conditions....