The goal of the proposed research is to achieve in-depth understanding of the molecular mechanisms underlying the CNS pathogenesis in neurodegenerative GM1-gangliosidosis (GM1). This lysosomal storage disease (LSD) is caused by deficiency of lysosomal 2-galactosidase (2-gal) that results in impaired lysosomal degradation and storage of GM1-ganglioside (GM1). Gangliosides are basic components of cell membranes, and GM1 is the primary ganglioside in the vertebrate brain. The accumulation of GM1 contributes to disease pathogenesis, but the molecular pathways involved remain largely unexplored. Our successful generation of 2- gal-/- mice, an animal model that closely resembles GM1 in humans, has facilitated studies that would be difficult or impossible to undertake in children. During the last granting period, we initiated a comprehensive analysis of neuronal cell death in the 2-gal-/- mouse model. Findings from those studies have contributed insight about the role of GM1 in normal cell metabolism and enabled us to develop the following 2 Specific Aims for this proposal.
In Aim 1, we will elucidate the pathologic consequences of GM1 accumulation at the level of the ER membrane that leads to Ca2+release and, in turn, activation of the UPR and neuronal cell death in 2-gal-/- mice. We will determine whether GM1 directly influences the biochemical properties of the ER-specific Ca2+ pump and channels that control intracellular Ca2+ levels. We will apply sophisticated in vitro and in vivo assays to verify a potential physical interaction of GM1 with these ER membrane proteins.
In Aim 2 we propose to investigate whether a cross-talk between the ER and the mitochondria occurs during the neuronal apoptosis mediated by the disruption of intracellular Ca2+ homeostasis in the 2-gal-/- mice. These studies are based on the hypothesis that excessive release of Ca2+ from the ER provokes Ca2+ imbalance in the cytosol, which, in turn, may impact on other organelles, in particular the mitochondria. Analyses of mitochondrial morphology and function will be paralleled by direct measurement of intracellular Ca2+ trafficking. We will also examine whether GM1 directly affects mitochondrial membrane permeability by testing if this ganglioside is incorporated in the mitochondrial membranes and, hence, perturbs mitochondrial integrity. Considering the pivotal role assigned to the Bcl-2 family of proteins in both ER- and mitochondria-mediated apoptosis, and their interplay with both ER and mitochondrial membrane components we will also determine whether GM1 may act upstream of ER Ca2+ release by activating these apoptogenic factors.
GM1 is a catastrophic neurodegenerative disease that affects infants and children. We are in the position to gain full understanding of the events in GM1 that cause cell death in the brain. The proposed studies may also reveal basic biological processes controlled by the molecules that are accumulated in GM1. This knowledge is essential for designing new therapies for children with GM1 and possibly those with other LSDs.
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