Regulation and function of novel brain-specific PGC-1alpha isoforms

Perturbations of PGC-1alpha expression and function have been implicated in the pathogenesis of several neurodegenerative disorders. Our recent investigation to clarify the function of an SNP (rs7665116) in intron 2 of the reference PPARGC1A gene, that showed associations with Huntington disease (HD) age-of-onset, led to the discovery of multiple transcripts that initiated from a common novel promoter located 587 kb upstream of exon 2. Using real-time PCR, RNase protection assays and Northern blotting, we showed that the majority of these transcripts were brain-specific and perhaps more abundant than the reference sequence PPARGC1A transcripts in whole brain. Two main transcripts containing independent methionine start codons were shown to encode full length brain-specific PGC-1alpha proteins that differed only at their N-termini from reference PGC-1alpha. Additional truncated isoforms containing these N-termini exist that are similar to N-terminal (NT)-PGC-1alpha. Other transcripts that lack the second LXXLL motif that serves as an interaction site for several nuclear receptors encode dominant negative forms. Furthermore, we showed that the new promoter is active in neuronal cell lines and described haplotypes encompassing this region that are associated with HD disease age-of-onset. The discovery of such a large PPARGC1A locus and multiple isoforms in brain warrant further studies on their basic biology. The primary objective this proposal is to compare the function and regulation of the new brain-specific isoforms with PGC-1alpha, encoded by the reference gene, in mouse and human brain and neuronal cell models. A secondary objective is to determine how potential differences in their regulation and function may affect the pathogenesis of HD and Parkinson disease (PD). These objectives will be approached by the following specific aims: i) to quantify the levels of the brain-specific and reference gene transcripts and their respective proteins in control and HD-transgenic mouse models subjected to neuroprotective regimens, in post-mortem human brain specimens and in cellular models of HD and PD; ii) to characterize the function of the brain-specific isoforms in relation to PGC-1alpha in control and HD and PD cell culture models with respect to their coactivation properties, their bioenergetic effects and their role in the defense against reactive oxygen species (ROS); iii) To further characterize the new brain-specific PPARGC1A promoter in comparison to the reference gene promoter and delineate the signaling pathways that converge at the two distinct promoter regions. These studies should provide essential data on the recently discovered brain-specific isoforms and their potential role in neurodegenerative disorders. Hence, results of this project may identify new tissue-specific targets for treating neurodegenerative diseases.

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