Cognitive impairment, mental retardation, and autistic disorders are devastating neurological components in a large number of patients with tuberous sclerosis (TS). Imaging studies and neuropsychological testing trace these phenotypes to specific brain regions including the cerebral cortex and thalamus. In addition, morphological, histological, and electrophysiological analyses reveal that some neurons in TS exhibit abnormal morphology and function. TS is also a developmental disorder that may be manifest at birth. Neural circuits established during brain development are central features of normal complex brain function and when perturbed contribute to insidious forms of neuropathology. However, whether anatomical correlates of cognitive dysfunction include altered neuron-to-neuron interactions and whether neural circuits are affected in TS have not been thoroughly investigated. In addition, when during development neural circuits might be impaired has not been ascertained. My lab uses sophisticated mouse genetic approaches to systematically and reproducibly label and mutate cells from distinct lineages at discrete stages of development in vivo in mouse. Specifically, we are utilizing a novel approach, genetic inducible fate mapping, to mark small populations of neurons in vivo with fine spatial and temporal control. We have exploited our marking strategy to gather morphological and circuitry information of neurons derived from defined genetic lineages. Importantly, we are combining cell marking and conditional gene deletion using a modified allele of Tsc1. Subsequent to Tsc1 deletion/marking, mutant cells specifically in the thalamus use axonal tracing methods to follow control versus mutant thalamic projections that connect to the cerebral cortex. Thus, our first aim is to "mark and mutate" specific populations of neurons in the thalamus during embryogenesis to ascertain how Tsc1 loss-of-function impacts on the formation of neural circuits. Preliminary data from our lab indicate that indeed thalamic neurons are sensitive to Tsc1 deletion, which results in increased pS6 levels, thalamic neuron doubling in size, and aberrant thalamocortical projections. Our second aim augments these findings and uses electrophysiology to determine the physiological consequences of Tsc1 deletion and altered thalamic morphology and projections. Collectively, these experiments will test the hypothesis that genetically defined neural circuits underlying sensory integration, perception, and cognition mediated by the cortex, thalamus, and cerebellum are incorrectly established during early brain development resulting in broad neural network dysfunction in TS. These genetic tools will provide a valuable framework for understanding mechanisms involved in altered brain function associated with TS and will provide insight into developmental changes that may underpin TS. Importantly, because our genetic lines of mice can investigate morphology and circuitry relevant to specific brain regions, they can serve as framework to evaluate physiological and behavioral correlates of Tsc1 mutations in selective contexts and may be valuable substrates upon which to test therapeutic approaches relevant to ameliorating TS-related abnormalities in neurodevelopment and brain function.
Ultimately, this research is applicable to understanding the neurological component of TS, the cellular and molecular basis for TS, and may have direct clinical applications in terms of screening and testing therapeutic approaches in vivo. The patients who may benefit most from the genetics-based approach are children who may exhibit early developmental involvement and patients with neurological deficits including cognitive impairment and associated autism-spectrum like disorders. The reason that these patients are most likely to benefit is that we are specifically and methodically testing hypotheses related to aberrant neural circuit formation during early brain development while at the same time establishing an animal model system that can then be used to assay therapeutic compounds in a cell-type specific manner. The additional clinical benefit is that we will ascertain the stage of brain development in which Tsc1 loss of function impacts neural circuits and neuron morphology, which may provide insight into when patients should be screened for TS. By carefully delineating the cellular and physiological components of TS that are relevant to specific populations, we can uncover the logic of whether a common thematic pathology or a cell-type specific pathology underlies brain disorders associated with TS. An additional benefit of this research is that animal models will be established that may prove to be fruitful for screening compounds that affect specific populations of neurons. Finally, unraveling the brain regions, neuronal cell types, and neural circuits that cause cognitive abnormalities in many patients with TS will likely be a significant contribution to advancing TS research/knowledge.