Natalie M. Niemi, Ph.D. (Year 1)
ABSTRACT
Sarcopenia – the age-linked, involuntary decline of skeletal muscle mass and function – substantially contributes to frailty and diminished quality of life in the elderly. Decreased mitochondrial content and compromised mitochondrial function have long been linked to the development and progression of sarcopenia. Despite this strong link, few pharmacological strategies exist to bolster skeletal muscle mitochondria, creating a need for novel therapeutic approaches to maintain mitochondrial content and function in aging patients.
Our data suggest that modulating mitochondrial protein phosphorylation may be a powerful way to control mitochondrial content. Our recent data suggest that phosphorylation is a key regulator of mitophagy – the autophagy-dependent turnover of mitochondria. We found that genetic disruption of a mitochondrial phosphatase, Pptc7, causes the accumulation of phosphorylation on the mitophagy receptor Bnip3, which promotes Bnip3 overexpression. This Bnip3 overexpression leads to fewer mitochondria and substantially compromises mitochondrial function. Importantly, Bnip3 overexpression is strongly linked to the onset of skeletal muscle atrophy in both mice and humans, with Bnip3 recently classified as a bona fide atrogene. Our data suggest that dysregulated protein phosphorylation promotes Bnip3 overexpression, leading to skeletal muscle atrophy in mice and humans.
The goal of this proposal is to characterize the extent to which Bnip3 overexpression drives excessive mitophagy and skeletal muscle pathology, as well as how its phosphorylation at two specific serines modulates these phenotypes. Importantly, our data suggest that inhibiting specific kinases can promote Bnip3 turnover through disruption of this phosphorylation-based regulation. As kinases are pharmacologically tractable and have had significant clinical impact, these studies may reveal a novel therapeutic strategy to thwart the development and progression of sarcopenia.
Progress Report — Final (Year 2 of 2)
Sarcopenia — the age-linked, involuntary decline of skeletal muscle mass and function — substantially contributes to frailty in the elderly. Decreased mitochondrial content and function is strongly linked to the onset and progression of sarcopenia, yet mitochondria remain largely refractory to current pharmaceutical approaches. Our work investigates the mitochondrial phosphatase PPTC7 as a potential mediator of mitochondrial content through its role in suppressing mitophagy — the autophagy-dependent clearance of damaged or excess mitochondria.
PPTC7 promotes the turnover of the mitophagy receptor BNIP3. In the absence of PPTC7, BNIP3 becomes hyperphosphorylated and highly overexpressed. Since BNIP3 is a bona fide atrogene found to be overexpressed in over ten independent studies of skeletal muscle atrophy and sarcopenia in both mice and humans, we hypothesized that phosphorylation-driven BNIP3 stabilization increases mitophagy and drives atrophy.
Establishing New Models and Experimental Platforms
With LLF support, we established new mouse models and experimental platforms to determine the extent to which elevated BNIP3 induces excessive mitophagy and skeletal muscle pathology. Key findings:
- Pptc7 knockout (KO) animals manifested a robust loss of lean mass, with BNIP3 protein upregulation confirmed across all tested skeletal muscle types.
- While BNIP3 was upregulated across all muscle types, only some showed signs of atrophy by mass — suggesting selectivity in the physiological response to dysregulated mitophagy.
- We generated a Pptc7/Bnip3 double knockout model that restores lean mass in Pptc7 KO animals, demonstrating the critical importance of BNIP3 in mediating body composition and positioning it as a promising therapeutic target for muscle atrophy.
Age-Dependent Phenotype
Of particular relevance to the Longer Life Foundation, these phenotypes appear to be age-dependent: animals do not manifest skeletal muscle atrophy at early timepoints, but the phenotype emerges with age. The factors contributing to this delay are of high interest and will continue to be investigated in mouse models and in cellular models of skeletal muscle biology.
Future Directions
The results from this award have laid the foundation for a manuscript describing these findings and will directly support future studies on the role of properly regulated mitophagy in skeletal muscle biology. The long-term goal of this project is to evaluate the therapeutic potential of limiting BNIP3 expression to attenuate excessive mitophagy and skeletal muscle atrophy — first in mice, and ultimately in human patients with sarcopenia.