Hypoxia Therapy Dramatically Extends Lifespan in Mouse Model of HTRA2-Linked Mitochondrial Disease
核心洞察
Hypoxia (11% oxygen) extended median lifespan nearly threefold in mice with a defective HTRA2 (搜索) protease, a model of severe neurodegenerative mitochondrial disease.
HTRA2 (搜索) and CLPB (搜索) cooperate as a mitochondrial proteostasis network; loss of either protein destabilizes Complex 1, leading to toxic tissue hyperoxia.
Hypoxia rescued Complex 1 subunit levels, reduced striatal inflammation, and improved motor function, though Complex 1 enzymatic activity was not fully restored.
Scientists at Gladstone Institutes (搜索) have demonstrated that breathing air with reduced oxygen levels can dramatically extend lifespan and improve neurological function in mice carrying a mutation that disables the mitochondrial protease HTRA2 (搜索). The study, published in Nature Metabolism, represents one of the most striking in vivo rescues of mitochondrial dysfunction observed to date and substantially broadens the therapeutic scope of hypoxia therapy.
The research, led by Gladstone Investigator Isha Jain, PhD, in collaboration with James Shorter, PhD, of the University of Pennsylvania and Daniel Southworth, PhD, of UC San Francisco, shows that when HTRA2 (搜索) malfunctions, a dangerous buildup of excess oxygen occurs in tissues. Lowering ambient oxygen reverses this toxicity.
"By showing these mice could be successfully treated with low oxygen, our study expands the potential of hypoxia therapy to a wide range of conditions that affect mitochondrial Complex 1, either directly or indirectly as in the case of HTRA2 (搜索) deficiency," said Ankur Garg, PhD, postdoctoral fellow in Jain's lab and first author of the study.
A genome-wide screen identifies hypoxia-responsive disease genes
The team reanalyzed a prior genome-wide CRISPR screen in K562 cells cultured under normoxia or hypoxia to identify mitochondrial disease genes amenable to hypoxia therapy. By intersecting screen hits with the Online Mendelian Inheritance in Man database, the researchers nominated 75 candidate disease genes. HTRA2 (搜索) emerged as a top hit: all four HTRA2-targeting guide RNAs ranked among the top hypoxia-rescued hits, comparable to validated Complex 1 (C1) subunits.
HTRA2 (搜索) is linked to multiple neurological syndromes, and its disease relevance motivated deeper investigation. The researchers leveraged a mouse model carrying a naturally occurring point mutation (S276C) in Htra2—commonly known as the mnd2 mutation—that abolishes the protease activity of HtrA2. Homozygous mutants display severe neurodegenerative phenotypes, including tremors, muscle atrophy, gait instability, and early mortality within weeks after birth.
Hypoxia nearly triples median lifespan and improves motor function
Mice were exposed to 11% oxygen starting at postnatal day 11, the time point that produced the most pronounced survival benefit. Hypoxia led to a nearly threefold extension in median lifespan in both males and females across two independent cohorts. In addition to survival, hypoxia mildly rescued body weight and significantly improved motor function as assessed by the inverted grid test and rotarod test.
Brain region-specific analysis revealed that striatal inflammation—measured by staining for the inflammatory markers Gfap and Iba1—was rescued by hypoxia, while inflammation in the thalamus was not. "These results point to region-specific vulnerability and rescue, motivating future studies into the biochemical basis of this differential response," the authors note.
HTRA2 (搜索) deficiency causes secondary Complex 1 instability
Proteomic analysis of HTRA2 (搜索) knockout K562 cells revealed reduced abundance of electron transport chain C1 subunits, particularly within the NADH-oxidizing N-module, the most labile region of C1. Hypoxia largely restored C1 subunit levels. These findings were confirmed in vivo: proteomic analysis of microdissected striata from Htra2 mutant mice showed markedly reduced C1 subunit levels, which were partially rescued by hypoxia exposure.
However, blue native PAGE and C1 activity assays revealed that while hypoxia rescued C1 subunit stability, it did not substantially improve fully assembled C1 or enzymatic activity. "These observations suggest that while hypoxia rescues C1 subunit stability, it may not restore its forward electron transfer capacity in this disease model," the researchers wrote.
Importantly, the team found that ISC-containing proteins were depleted in the Htra2 (搜索) mutant striatum under normoxia and this was reversed by hypoxia, supporting the hypothesis that tissue hyperoxia contributes to disease and that hypoxia mitigates this stress. RNA-sequencing analysis further revealed that canonical HIF target genes were downregulated in Htra2 mutant striatum under normoxia—consistent with a relatively hyperoxic state—and these targets were largely normalized following hypoxia exposure.
HTRA2 (搜索) and CLPB (搜索) form a cooperative proteostasis network
Using the Dependency Map (DepMap) dataset, the researchers identified CLPB (搜索), a metazoan mitochondrial protein disaggregase localized to the intermembrane space (IMS), as a strong bidirectional co-essential partner of HTRA2 (搜索). Both CLPB and HAX1 (搜索) were found in HTRA2 BioID interactomes, and CLPB is also a known 3-methylglutaconic aciduria (搜索) (3-MGA) disease gene.
Biochemical assays showed that HTRA2 (搜索) and CLPB (搜索) compete for overlapping aggregated substrates—HTRA2 acting as a protease to degrade them, CLPB as a disaggregase to restore their function. Proteomics of CLPB knockout cells revealed widespread depletion of C1 subunits, mirroring the defects seen in HTRA2 knockout cells.
Critically, the C1 subunit NDUFA13 (搜索) aggregated in both HTRA2 (搜索) and CLPB (搜索) knockout cells, providing a direct mechanistic link between IMS proteostasis and C1 stability. In vitro cleavage assays confirmed that HTRA2 can directly recognize and cleave aggregated NDUFA13.
"Together, these two proteins act like a clean-up crew inside mitochondria, preventing the machinery from becoming clogged with clumps of misfolded proteins," said Jain.
CLPB (搜索) and HAX1 (搜索) also emerge as hypoxia-responsive hits
Revisiting the original CRISPR screen, the researchers found that CLPB (搜索) and HAX1 (搜索) were also among the top hits whose fitness defects in normoxia were rescued under hypoxia. This suggests that, like HTRA2 (搜索) deficiency, CLPB and HAX1 deficiency may also be amenable to hypoxia therapy, opening the door to extending hypoxia-based treatments to additional forms of 3-MGA and other disorders of mitochondrial protein quality control.
Therapeutic implications and next steps
The findings extend the concept of hypoxia therapy from the limited set of disease-associated mutations in C1 subunit-encoding genes to the much broader class of indirect regulators of C1 stability. Mitochondrial protein quality control is disrupted in a wide range of diseases, including rare inborn errors such as 3-MGA (due to mutations in HTRA2 (搜索), CLPB (搜索), DNAJC19, or TIMM50), as well as common neurodegenerative disorders such as Parkinson's disease (搜索), Alzheimer's disease, and amyotrophic lateral sclerosis.
"Right now, no treatments are uniformly available for mitochondrial diseases, and hypoxia therapy offers the hope that we could treat not just one, but many of these genetic conditions," Jain said. "We're working hard to make it a practical treatment for human patients in the clinic."
Toward this goal, Jain and colleagues have developed a small molecule called HypoxyStat (搜索) that mimics the effects of hypoxia by increasing the binding affinity of oxygen to hemoglobin, thereby reducing oxygen delivery to tissues. HypoxyStat has already shown efficacy in the Ndufs4 knockout mouse model of primary C1 disease. Next-generation derivatives are being developed to allow for earlier intervention, which will be critical since effective rescue of Htra2 (搜索) mutant mice requires treatment initiation at an early postnatal window.