Tuberous Sclerosis (TSC) is an autosomal dominant genetic disorder characterized by benign tumors of many organs. Recently, the genetic basis of TSC has been identified, and it points to unregulated cell growth and division as a primary cause of the tumors. However, children with TSC also frequently have mental retardation and epilepsy, and the causes of these neurological symptoms are not understood. Two main possibilities exist. The first is that the presence of tumors within the brain leads to a generally disorganized brain architecture. The second is that the function of each neuron is perturbed and that, because of these defects in each brain cell, the brain as whole does not function properly. Our proposal is designed to examine this second possibility by analyzing neuronal function in a mouse model of TSC. We will focus our efforts on understanding primary perturbations of neuronal function by introducing a TSC-causing genetic mutation into a few neurons located within otherwise normal brain tissue. These perturbed cells will be analyzed, and their ability to develop proper neuronal structures and connections will be examined. Since, with our approach, only a vanishing small percentage of the cells harbor the TSC-causing mutation, any functional perturbation found in a mutated cell must result from disrupted signaling within that cell.

Our analysis will use high-resolution, fluorescence-based microscopy to determine the fine structure of each neuron. Particular attention will be paid to the morphology of a peculiar structure that is exclusive to neurons and is called a dendritic spine. Spines are the sites at which contacts between neurons are made as well as where information is passed from neuron to neuron. Spines are highly regulated and their morphology is affected by activity patterns that are thought to be important for the formation of memories. Therefore, altered spine morphology is often an indication of perturbed neuronal function. Furthermore, connections between cells will be directly examined by recording the electrical activity triggered in one cell by activity in neighboring cells. Throughout our analysis, the properties of normal cells will be compared to those of cells in which the TSC-causing mutation has been induced.

Our study will shed light on the causes of the neurological symptoms seen in TSC. Identification of the perturbations of neuronal function in TSC is needed for two reasons. First, it is necessary to understand the neuronal perturbations in order to design therapies that address the neurological symptoms. Second, with knowledge of the exact neuronal perturbations, it will be possible to test if proposed therapies are likely to ameliorate the neurological symptoms.