Tier 3 — preclinical

SIRT6 is a key regulator of mitochondrial function in the brain

Smirnov D, Eremenko E, Stein D, Kaluski S, Jasinska W, Cosentino C, Martinez-Pastor B, Brotman Y, Mostoslavsky R, Khrameeva E, Toiber D

Cell Death and Disease 2023 Volume 14, issue 1, article 35

Bibliography

DOI: 10.1038/s41419-022-05542-w
PubMed: PMID 36653345
PubMed Central: PMC9849342
Funding: The David and Inez Myers foundation, the Israeli Ministry of Science and Technology, the High-tech, Bio-tech and Negev fellowships of the Kreitman School of Advanced Research of Ben Gurion University. RNA-seq analysis supported by the Russian Science Foundation (grant number 21-74-10102 to E.K.).
Competing interests: The authors declared no competing interests.

Study snapshot

Design	Multi-omics study combining bulk RNA-seq and LC-MS metabolomics in brain-specific SIRT6-knockout mice, with functional validation in human SH-SY5Y neuroblastoma cells and cross-reference against published human brain aging and neurodegenerative disease gene expression datasets.
Model	Brain-specific SIRT6-knockout mice (brSIRT6-KO) and wild-type C57BL/6J littermates; human SH-SY5Y neuroblastoma cell line (wild-type and SIRT6-knockout); mouse embryonic stem cells (wild-type and SIRT6-knockout).
Sample	Mouse brain RNA-seq n = 4 WT and n = 4 brSIRT6-KO. Mouse embryonic stem cell metabolomics n = 3 per group. SH-SY5Y functional assays (mitochondrial membrane potential, ROS, mitochondrial mass) run in 3–5 replicates per condition.
Intervention	Constitutive brain-specific Sirt6 knockout via Cre-lox conditional targeting; CRISPR/Cas9 generated SIRT6 knockout in SH-SY5Y neuroblastoma cells; exogenous SIRT3 or SIRT4 overexpression as rescue; pharmacological inhibitors of individual electron transport chain complexes (FCCP as OXPHOS uncoupler, rotenone for Complex I, hydrogen peroxide for cytochrome c, oligomycin for ATP synthase).
Duration	Terminal endpoint measurements only; no longitudinal component. Cells stained and analysed 48 hours after transfection.
Endpoints	Global brain gene expression profile (differential expression); metabolomic abundance across 235 features; mitochondrial membrane potential (TMRE fluorescence); mitochondrial reactive oxygen species (MitoSox); mitochondrial mass (MitoTracker Green); mtDNA-encoded gene expression; cytochrome c and VDAC1 protein levels; transcriptional overlap with published human brain aging datasets (GSE13120, GSE48911) and neurodegenerative disease KEGG pathways (Parkinson's, Huntington's, Alzheimer's, amyotrophic lateral sclerosis); YY1 ChIP-seq overlap analysis with mitochondrial gene promoters

What the study showed, in plain terms

SIRT6 levels drop in the aging brain and drop even further in Alzheimer's disease. This paper asks what specifically happens to brain cells when SIRT6 is lost, and whether the same changes show up in real aging brains and in neurodegenerative disease.

The authors made mice in which SIRT6 is deleted only in the brain, then compared gene expression and metabolite profiles against wild-type mice of the same background. They also generated a SIRT6 knockout human neuroblastoma cell line to test mitochondrial function directly.

The finding is striking. When SIRT6 is missing from the brain, nearly three thousand genes change their expression significantly, and the strongest signature among the downregulated genes is mitochondrial. Genes encoding electron transport chain subunits, mitochondrial ribosomes, and enzymes of the TCA cycle are broadly reduced. Metabolomics confirmed the same pattern: TCA cycle intermediates such as malate, fumarate, oxoglutarate, and NAD⁺ itself are all reduced without SIRT6.

Functionally, this translates into mitochondrial dysfunction. SIRT6-knockout neuroblastoma cells show reduced mitochondrial membrane potential, elevated reactive oxygen species, and roughly 22% less mitochondrial mass than wild-type cells. The authors then partially rescued the membrane potential by restoring SIRT3 or SIRT4 — two mitochondrial sirtuins that turn out to be transcriptionally regulated by SIRT6.

The mechanism appears to run through the transcription factor YY1: SIRT6 and YY1 together bind the promoters of many mitochondrial genes, including SIRT3.

The paper's clinical hook is the overlap analysis. The gene expression changes seen in SIRT6-deficient mouse brains match a substantial portion of the changes seen in normal aging human brains and in the brains of Alzheimer's, Parkinson's, Huntington's, and ALS patients. This positions SIRT6 decay as one plausible upstream driver of mitochondrial dysfunction in age-related neurodegeneration — although the paper's mouse-to-human comparison is transcriptomic, not causal.

Key findings

2,870 genes were differentially expressed in brain-specific SIRT6-knockout mouse brains versus wild-type; approximately 85% of these were protein-coding.
Of 1,140 mitochondria-related genes annotated in MitoCarta, 256 were differentially expressed in SIRT6-knockout brains, and over 91% of these were downregulated.
Gene Ontology enrichment of downregulated genes prioritised mitochondrial respiratory chain complex assembly (FDR p = 1.21 × 10⁻²⁰), mitochondrion organisation, oxidative phosphorylation, aerobic respiration, and ATP metabolic process.
Mitochondrial Complex I was the most affected electron transport chain complex (27 of 43 encoding genes downregulated); 57 of 99 electron transport chain subunit genes were differentially expressed overall.
Metabolomic analysis of SIRT6-knockout mouse embryonic stem cells showed 92 of 235 features significantly changed; TCA cycle metabolites (malic acid, fumaric acid, oxoglutaric acid, thiamine pyrophosphate) and OXPHOS-related cofactors (NAD⁺, NADH, ADP) were all reduced.
SIRT6-knockout SH-SY5Y cells showed a 1.21-fold reduction in mitochondrial membrane potential (FDR p = 0.0006); the deficit widened to 1.78-fold with FCCP treatment and up to 1.93-fold when ATP synthase was inhibited.
Mitochondrial mass in SIRT6-knockout SH-SY5Y cells was 21.8% lower than in wild-type cells (t-test p = 0.0087), matching the approximately 19.7% average decrease in mtDNA-encoded gene expression.
Mitochondrial reactive oxygen species (measured by MitoSox) were significantly elevated in SIRT6-knockout cells versus wild-type (p < 0.0001).
SIRT3 (FDR p = 3.60 × 10⁻¹²) and SIRT4 (FDR p = 3.33 × 10⁻⁶) were significantly downregulated in SIRT6-knockout mouse brains; SIRT5 was not. Exogenous SIRT3 or SIRT4 expression significantly rescued mitochondrial membrane potential in SIRT6-knockout cells.
Analysis of published YY1 ChIP-seq data in cortical neurons identified 669 YY1 peaks at promoters of mitochondrial genes; 41 mitochondrial genes had promoter binding sites for both YY1 and SIRT6 (permutation test p = 5.1 × 10⁻⁴), supporting a coordinated SIRT6–YY1 regulatory mechanism.
Gene set enrichment analysis showed significant enrichment of "Parkinson's disease" (FDR p = 0.015), "Huntington's disease" (FDR p = 0.017), "Alzheimer's disease" (FDR p = 0.017), and "Amyotrophic lateral sclerosis" (FDR p = 0.017) KEGG pathways in the SIRT6-knockout transcriptome, all clustering with "Oxidative phosphorylation" in the pathway network.
Ten downregulated mitochondrial genes were shared between SIRT6-knockout mouse brains and two independent published aging brain datasets (mouse aging neocortex and hippocampus), including electron transport chain subunits (Sdhd, Ndufa7, Uqcrq), mitochondrial protein import (Timm10b), and mitochondrial base excision repair (Ung).

What this study can and cannot tell us

The strongest evidence in this paper is preclinical — mouse brain transcriptomes, mouse embryonic stem cell metabolomics, and human neuroblastoma cell functional assays. The link to human disease is drawn primarily by cross-referencing the SIRT6-knockout mouse gene expression signature against publicly available datasets from aging human brains and from patients with Alzheimer's, Parkinson's, Huntington's, and ALS. The overlap is meaningful, but it establishes that similar transcriptional patterns are present, not that SIRT6 decay causes those diseases.

Metabolomics were performed in mouse embryonic stem cells rather than in the brain-specific SIRT6-knockout mice themselves. The authors argue that shared cellular metabolism justifies the extrapolation, but a direct brain metabolomic measurement would have been stronger evidence for the claimed mechanism.

The membrane potential rescue by SIRT3 or SIRT4 overexpression is partial, not complete. This means additional SIRT6-dependent mechanisms beyond SIRT3/SIRT4 downregulation are at work in the mitochondrial phenotype, and the paper does not fully resolve what they are.

The YY1 co-regulation claim rests on overlap analysis of published YY1 ChIP-seq data from cortical neurons compared against SIRT6 ChIP-seq data from mouse embryonic stem cells. These are different cell types studied by different groups, and simultaneous co-occupancy of the same promoter by SIRT6 and YY1 in the same brain cell was not directly demonstrated in this paper.

This is a knockout study. It shows what happens when SIRT6 is absent. It does not test whether increasing or maintaining SIRT6 activity in an already-aged or diseased brain would reverse the observed mitochondrial dysfunction, nor whether any pharmacological SIRT6 activator crosses the blood-brain barrier at meaningful doses. No supplement, dosing, or human clinical outcome can be inferred from this work.