by University of California, Los Angeles

Cardiac-specific ATG3 deletion blocks autophagy and precipitates cardiac contractile dysfunction(A) ATG3, LC3 (MAP1LC3A), SQSTM1, and beta-actin (ACTB) protein levels in WT and ciAtg3 KO mouse hearts 1 week after last tamoxifen injection. Mice, at 6 weeks of age, were intraperitoneally injected with tamoxifen at a dose of 100 µg/g body weight/day for 5 consecutive days. One week after last tamoxifen injection, mice were euthanized and hearts were harvested for experiments. Age-matched ATG3loxP/loxP mice without Cre were injected with the same amount of tamoxifen as WT control. Representative blots are shown. n = 3 per group. (B) Heart weight of WT and ciAtg3 KO mice, 1 week after last tamoxifen injection. n = 7–8. Data are mean ± SEM. An unpaired t test was used to determine statistical significance between two groups. *P < 0.05. (C) Ejection fraction in WT and ciAtg3 KO mice 1, 4, 14, and 30 weeks after last tamoxifen injection. Measurements were performed under light sedation with midazolam. n = 7–12 per group. Data are mean ± SEM. Unpaired t tests were used to determine statistical significance between two groups at corresponding time points. **P < 0.01. (D) Survival curve of WT and ciAtg3 KO mice after the last tamoxifen injection. n = 12 per group. (E) ATG3, SQSTM1, and GAPDH protein levels in cardiomyocytes. Cardiomyocytes were enzymatically isolated from WT and cAtg3-KO mouse hearts. Representative blots are shown. This experiment was repeated twice independently. (F) LC3, SQSTM1, and GAPDH protein levels in mouse hearts. Mice were either randomly fed or fasted for 48 h, n = 5 per group. Data are mean ± SEM. One-way ANOVA followed by Bonferroni’s multiple comparison tests was used to determine statistical significance. *P < 0.05. (G) Heart weight of WT and cAtg3-KO mice at 16 weeks of age. n = 13 per group. Data are mean ± SEM. An unpaired t test was used to determine statistical significance between two groups. **P < 0.01. (H) NPPA, NPPB, MYH7, and PLN mRNA expression levels in WT and ATG3 KO mouse hearts at 16 weeks of age, n = 5 per group. Unpaired t tests were used to determine statistical significance between two groups. Data are mean ± SEM. **P < 0.01. (I) Ejection fraction in WT and cAtg3-KO mice at ages as indicated. Measurements were performed under light sedation with midazolam. n = 4–12 per group. Unpaired t tests were used to determine statistical significance between two groups at corresponding time points. Data are mean ± SEM. *P < 0.05. (J) Survival curve of WT and cAtg3-KO mice. n = 18. Data information: Cardiomyocytes for (E) were enzymatically isolated from 16-week-old WT and cAtg3-KO mice at a time when cardiac function was preserved in cAtg3-KO (I). After being enzymatically isolated, cardiomyocytes were immediately lysed in ice-cold homogenization buffer without any pre-treatments or culture. This experiment was repeated twice independently. Source data are available online for this figure. Credit: The EMBO Journal (2024). DOI: 10.1038/s44318-023-00009-w

A new study sheds light on how autophagy, the body's process for removing damaged cell parts, when impaired, can play a role in causing heart failure.

The research team led by Dr. E. Dale Abel, chair of the Department of Medicine at UCLA, and Dr. Quanjiang Zhang, adjunct assistant professor of medicine at UCLA, identified a signaling pathway that links autophagy to the control of cellular levels of a key coenzyme known as NAD+, which is found in all living cells and is central to how our metabolism works. Researchers say these findings may have implications for heart failure treatment.

"Heart failure remains a leading cause of mortality, and there is a strong need to develop new therapies that can improve cardiac function and increase survival. Identifying a new pathway linking impaired autophagy, a characteristic of heart failure, and increased NAD+ breakdown could open new avenues for therapeutic intervention," Abel said.

Autophagy is a natural, self-serving mechanism in which the body removes damaged or dysfunctional cell parts and recycles their components for cellular repair or energy production. Abel describes it as our body's cellular recycling system that allows cells to break down bad parts of themselves and salvage some of those parts to repurpose into new, usable parts.

Given the important role of autophagy in cell breakdown and repair, impaired autophagy is known to cause a range of disorders, including cancer, neurodegeneration, and heart failure.

Until now, however, scientists weren't sure what mechanisms underlie mitochondrial and cardiac dysfunction in the setting of impaired autophagy. One proposed mechanism has been the build-up of dysfunctional mitochondria that takes place when autophagy is impaired, as this build-up can activate inflammatory and other responses that can lead to cell death or dysfunction.

Another proposed mechanism is the degradation of specific proteins involved in metabolism that reduce the functioning of pathways that regulate heart muscle contraction. The present study identified a new mechanism, namely NAD+ depletion, that leads to heart muscle cell dysfunction in the context of autophagy.

Using a mouse model with autophagy dysfunction in heart cells, the research team found that autophagy regulates the enzyme NNMT, which increases levels in this heart failure model. Inhibiting the activity of this enzyme with a small molecule led to an improvement in heart failure even though autophagy failure persisted.

The study elucidated a chain event that explains how and why impaired autophagy can lead to cardiac dysfunction. The first step of this chain is accumulating a protein known as SQSTM1. Increased SQSTM1 activates a signaling protein called NF-κB (also known as RELA). NF-κB enters the nucleus and causes increased gene activity that encodes the nicotinamide N-methyltransferase enzyme (NNMT). NNMT ultimately leads to a breakdown of NAD+ precursors, ultimately lowering NAD+ levels. Low NAD+ levels then cause mitochondrial and cardiac dysfunction.

Unraveling this pathway has linked NAD+ metabolism and autophagy. These findings indicate a new potential therapy for reversing mitochondrial dysfunction and improving heart failure by preventing NAD+ loss and boosting its level in cardiac muscle.

This research is published in The EMBO Journal.

More information: Quanjiang Zhang et al, Control of NAD+ homeostasis by autophagic flux modulates mitochondrial and cardiac function, The EMBO Journal (2024). DOI: 10.1038/s44318-023-00009-w

Journal information: EMBO Journal 

Provided by University of California, Los Angeles