Summary

Much has been made about the link between amyloid beta (Aß) plaques and Alzheimer's Disease (AD), yet attempts to treat AD by targeting Aß have not been tremendously successful. In addition to Aß, tau protein abnormalities have also been linked to AD, and both remain active topics of Alzheimers research. This paper from Genentech examines a third angle: the involvement of mitochondrial dysfunction in AD. They examined over 4500 AD cases and over 3000 age-matched controls and found variations in the gene for the mitochondrial protein PTCD1 were enriched in AD cases. To test this genetic link functionally, they used Neuro9™ made using the NanoAssemblr® Spark™ to encapsulate siRNA and knockdown the PTCD1 in primary rat neuronal cultures. They found reduced PTCD1 decreases ATP content leaving neurons with less energy available and hence these neurons fire fewer action potential bursts. They also observed reduced network activity in these cultures. 

Using bioinformatic techniques, they have found genetic variations in human AD cases that point to a new avenues for potential treatments. Using Neuro9 and NanoAssemblr technology, they were able to elucidate some of the molecular mechanisms through functional models in vitro. Primary neurons are known to be difficult to transfect and hence to manipulate genetically, due to their sensitivity. Neuro9 transfection kits are well tolerated by sensitive primary cell cultures while offering high encapsulation efficiency and highly potent siRNA mediated knockdown in primary rat neurons, thus enabling functional genomic studies in relevant models. These results have opened the door to developing new therapeutic approaches to AD which could potentially include a PTCD1 gene replacement therapy to rescue function, which could be realized by using LNPs to deliver mRNA encoding PTCD1 .

Abstract

Besides amyloid-beta plaques and tau tangles, mitochondrial dysfunction is implicated in the pathology of Alzheimer's disease (AD). Neurons heavily rely on mitochondrial function, and deficits in brain energy metabolism are detected early in AD; however, direct human genetic evidence for mitochondrial involvement in AD pathogenesis is limited. We analyzed whole exome sequencing data of 4549 AD cases and 3332 age-matched controls and discovered that rare protein altering variants in the gene pentatricopeptide repeat containing protein (PTCD1) show a trend for enrichment in cases compared to controls. We show here that PTCD1 is required for normal mitochondrial rRNA levels, proper assembly of the mitochondrial ribosome and hence for mitochondrial translation and assembly of the electron transport chain. Loss of PTCD1 function impairs oxidative phosphorylation and forces cells to rely on glycolysis for energy production. Cells expressing the AD-linked variant of PTCD1 fail to sustain energy production under increased metabolic stress. In neurons, reduced PTCD1 expression leads to lower ATP levels and impacts spontaneous synaptic activity. Thus, our study uncovers a possible link between a protein required for mitochondrial function and energy metabolism and AD risk.

Significance Statement 

Mitochondria are the main source of cellular energy and mitochondrial dysfunction is implicated in the pathology of Alzheimer's disease (AD) and other neurodegenerative disorders. Here, we identify a variant in the gene PTCD1 that is enriched in AD patients and demonstrate that PTCD1 is required for ATP generation through oxidative phosphorylation. PTCD1 regulates the level of 16S rRNA, the backbone of the mitoribosome, and is essential for mitochondrial translation and assembly of the electron transport chain. Cells expressing the AD-associated variant fail to maintain adequate ATP production during metabolic stress, and reduced PTCD1 activity disrupts neuronal energy homeostasis and dampens spontaneous transmission. Our work provides a mechanistic link between a protein required for mitochondrial function and genetic AD risk.