With Age, Microglia Pump cGAS, Revving Harmful Inflammation

As people age, chronic inflammation can ensue. What drives this so-called “inflammaging”? A study published August 2 in Nature may have identified a primary cause, at least in the mouse brain. Scientists led by Andrea Ablasser at the Swiss Federal Institute of Technology in Lausanne, report that as mice age, mitochondria within their microglia become misshapen and their membranes weak, seeping DNA into the cytosol. There, the DNA triggers a cytosolic DNA sensor called cyclic GMP-AMP synthase, aka cGAS, which sets off an interferon response via the protein STING. Once the cascade is switched on, the microglia rile other cells as well. Ultimately, these stressful environs spell trouble for neurons, and for memory. Much of this detrimental response was quelled with a STING inhibitor.

  • With age, mitochondrial DNA leaks into cytosol of microglia.
  • The wayward DNA may set off the inflammatory cGAS-STING pathway.
  • This causes neuronal damage and memory loss in mice.
  • STING inhibitor calms microglia, prevents age-related neurodegeneration.

The findings add to mounting evidence that when chronically activated, the cGAS-STING pathway compromises the function, and even survival, of neurons in the brain.

“This study provides key insights into the mechanistic basis of inflammageing in the brain, and a clearer picture of how ageing-associated deterioration of organelle and cell fitness leads to neurodegeneration,” Bart Eggen of the University of Groningen in the Netherlands wrote in an editorial that accompanied the paper in Nature.

One of many sensors of misplaced nucleic acids, cGAS in the cytosol detects DNA that has leaked from damaged nuclei and mitochondria, or has been shed from viruses. Many insults, including aggregated proteins, can cause DNA to wind up in the cytosol. Recently, researchers led by Li Gan, Weill Cornell Medical College in New York, reported that tau fibrils congregate around microglial mitochondria in a mouse model of tauopathy, allowing their DNA to escape and setting off cGAS (Apr 2022 conference news). TDP-43 and parkin/Pink 1 may incite the same cascade (Yu et al., 2020; Sliter et al., 2018).

So might aging. Ablasser’s group previously reported that in senescent cells, which increase in number with age, cGAS-STING promotes secretion of a toxic brew of pro-inflammatory cytokines (Glück et al., 2017). 

“Gascade” STINGs Synapses. Compared to hippocampi from 2- to 3-month-old mice (top), those from 24- to 27-month-olds (middle) have far fewer synapses (red). Treatment with the STING inhibitor H-151 (bottom) prevents synapse loss. [Courtesy of Gulen et al., Nature, 2023.]

To find out if this DNA sensor explains inflammaging, co-first authors Muhammet Gulen and Natasha Samson and colleagues started by testing H-151, a small-molecule STING inhibitor they had developed in different contexts of senescence and age-related inflammation (Haag et al., 2018). In 20- to 26-month-old mice, H-151 counteracted systemic inflammation, bolstered kidney function, boosted strength and endurance, and improved spatial memory. It also thwarted an age-related uptick in immune gene expression in the brain. The findings cast STING as a driver of age-related inflammation and suggested that inhibiting it slows physical and cognitive decline.

Next, the scientists turned their focus to the role of STING in the aging brain. First, they found H-151 counteracted microgliosis, reactive astrocytes, synaptic loss, and even neurodegeneration in the hippocampi in old mice (image at right). So did knocking out STING genetically, implicating this signaling pathway in the process.

An intensive search for the cells responsible identified microglia as the predominant activators of cGAS-STING signaling. Not only did microglia in older mice express type I interferons and other pro-inflammatory cytokines, they also expressed a glut of phosphorylated STING—a marker of STING activation—in their hippocampi.

What triggers this cascade? Suspecting leaky mitochondria, Gulen and colleagues used electron microscopy to examine the organelles. In old, but not young, mice, microglial mitochondria lacked their characteristic shape and internal structure, and clumps of mitochondrial DNA clung to the cytosolic sides of their outer membranes (image below).

Is microglial cGAS the primary driver of inflammaging in the brain? To find out, the researchers generated a transgenic mouse in which a mutant form of cGAS is conditionally triggered in microglia. This variant cannot bind nucleosomes, an interaction that typically suppresses the DNA sensor. Upon induction with tamoxifen in young mice, microglia unleashed a brain interferon response akin to that in the old mice. Single-nucleus RNA sequencing revealed microglia subtypes that have been associated with AD and other neurodegenerative diseases, including microglia that expressed a bevy of interferon signature genes (Jun 2017 news; Sep 2017 news; Roy et al., 2020). Hippocampi had fewer neurons, and the mice performed poorly on tests of spatial memory.

Stray mtDNA. In microglia from old mice, mitochondrial DNA is spotted on the inside (green) and outside (red) of the mitochondrial membrane (purple). Nucleus in blue. [Courtesy of Gulen et al., Nature, 2023.]

In cell culture, microglia expressing mutant cGAS thwarted the survival of co-cultured, wild-type neurons. Conditioned media from these microglial cultures had the same effect, invoking soluble factors. Because TNF-α is a potent pro-inflammatory cytokine released downstream of cGAS-STING activation, the scientists wondered if it might be involved. When they added antibodies to the co-cultures to neutralize this cytokine, the microglia no longer harmed neurons.

“This new study is a rigorous exploration into the cGAS-STING pathway as a source of neuroinflammation in the aging brain,” wrote Bess Frost and Morgan Lambert of the University of Texas in San Antonio. They asked if other sources of cytosolic DNA, beyond leaky mitochondria, might contribute to cGAS activation. For example, retrotransposons, which generate new DNA copies that can loiter in the cytoplasm, increase in the aging brain and in neurodegenerative disease. “It will be interesting and important to determine the full repertoire of cytoplasmic DNA in the context of aging and age-related neurodegenerative disorders,” they wrote (comment below).

Eggen made a similar point in the editorial. He also wondered if stray DNA from other damaged brain cells might be capable of switching on the pathway within microglia. As with all mouse studies, time will tell if the findings translate to the human brain.—Jessica Shugart

Comments

    • User Profile Image Bess Frost
      Barshop Institute for Longevity and Aging Studies/University of Texas Health Science Center
    • User Profile Image Morgan Lambert
      UT Health San Antonio
    • Posted:

    This is a rigorous exploration into the cGAS-STING pathway as a source of neuroinflammation in the aging brain. The findings are likely relevant to age-related neurodegenerative disorders since they complement work from Li Gan’s group, who recently reported that mitochondrial DNA leakage activates cGAS-STING in microglia in the context of tauopathy.

    While this manuscript specifically investigates mitochondrial-derived cytoplasmic DNA as a cGAS-STING activator in the aging brain, other potential sources of cytosolic DNA should be considered. Retrotransposons, for example, can generate new DNA copies that can exist in a cytosolic episomal state. Such elements are increased in the aging brain and in many neurodegenerative contexts. It will be interesting and important to determine the full repertoire of cytoplasmic DNA in the context of aging and age-related neurodegenerative disorders. 

    View all comments by Bess Frost View all comments by Morgan Lambert

  2. This work from the Ablasser team reveals how cGAS–STING, a ubiquitous senescence pathway sensing exogenous insults, can be activated in the liver by irradiation, in the brain by physiological aging, and in microglia by exposure to mtDNA. Thereby, it contributes to the process of inflammaging and neurodegeneration.

    The functional activation mechanisms of the cGAS–STING complex are convincing and demonstrated by loss of function, via targeted pharmacological inhibition, gain of function, and by ectopic activation of cGAS–STING via a microglial-inducible transgenic mouse line. Using the latter, the authors identify two activated microglia types—DAM-2-MG and the IFN-MG—with distinct inflammatory signatures, which recapitulate AD fingerprints.

    The cGAS-STING pathway is evolutionarily conserved. It is well known for sensing exogenous microbial and cytosolic DNA traces and for triggering immune-protective response mechanism against infectious/pathogenic agents—also called the danger-associated molecular pattern (DAMP) (Cheng et al., 2020; Zheng et al., 2023). It had been shown previously that the cGAS-STING pathway is essential for inducing cellular senescence and an inflammatory phenotype (Yang et al., 2017). 

    Extending the effort of the Ablasser group in identifying small inhibitory molecules of cGAS-STING (Haag et al., 2018), the valuable addition of this in vivo work demonstrates that cGAS-STING pathway activation occurs in microglial cells adopting an inflammatory phenotype similar to that of AD.

    For researchers investigating the infectious etiology of sporadic neurodegenerative diseases, the question remains whether the priming of the cGAS-STING pathway via early infection, or recurrent infections, can sustain chronic low-grade inflammation over a lifetime, which when reactivated by senescence signals can spiral the neuroinflammatory progression of AD.

    As much as the data reported are clean and solid, there is probably more than one mechanism at play, and understanding both the additive and the resilience mechanisms that push or prevent this cascade with age will be crucial.  

    References:

    Cheng Z, Dai T, He X, Zhang Z, Xie F, Wang S, Zhang L, Zhou F. The interactions between cGAS-STING pathway and pathogens. Signal Transduct Target Ther. 2020 Jun 10;5(1):91. PubMed.

    Zheng W, Feng D, Xiong X, Liao X, Wang S, Xu H, Le W, Wei Q, Yang L. The Role of cGAS-STING in Age-Related Diseases from Mechanisms to Therapies. Aging Dis. 2023 Aug 1;14(4):1145-1165. PubMed.

    Yang H, Wang H, Ren J, Chen Q, Chen ZJ. cGAS is essential for cellular senescence. Proc Natl Acad Sci U S A. 2017 Jun 6;114(23):E4612-E4620. Epub 2017 May 22 PubMed.

    Haag SM, Gulen MF, Reymond L, Gibelin A, Abrami L, Decout A, Heymann M, van der Goot FG, Turcatti G, Behrendt R, Ablasser A. Targeting STING with covalent small-molecule inhibitors. Nature. 2018 Jul;559(7713):269-273. Epub 2018 Jul 4 PubMed.

    View all comments by Lavinia Alberi
    • User Profile Image Li Gan
      Weill Cornell Medicine
    • Posted:

    This report fits really well with our findings that activation of cGAS-STING, accelerated by tau pathology, represents a converging pathway and a promising therapeutic target for aging-associated neurodegeneration beyond AD (Udeochu et al., 2023). Besides tau, other mitochondrial stresses that accumulate during aging and which may be accelerated by other pathologies, such as TDP-43 dysfunction, could also result in release of mitochondrial DNA into the cytosol. Although direct evidence has been lacking, cytosolic DNA could also result from nuclear DNA damage and genomic instability in aging cells.

    That normal aging is sufficient to induce sufficient cGAS-STING activation to cause cognitive deficits in mice is somewhat surprising, but nevertheless highlights a critical role in aging-associated cognitive decline.

    How does microglia cGAS-STING activation drive cognitive decline? Our study identified loss of neuronal MEF2C, and an associated transcriptional network, as the major mechanism in tauopathy. Whether similar or distinct neuronal mechanisms are involved in aging is an intriguing question, and an exciting one to address next.

    References:

    Udeochu JC, Amin S, Huang Y, Fan L, Torres ER, Carling GK, Liu B, McGurran H, Coronas-Samano G, Kauwe G, Mousa GA, Wong MY, Ye P, Nagiri RK, Lo I, Holtzman J, Corona C, Yarahmady A, Gill MT, Raju RM, Mok SA, Gong S, Luo W, Zhao M, Tracy TE, Ratan RR, Tsai LH, Sinha SC, Gan L. Tau activation of microglial cGAS-IFN reduces MEF2C-mediated cognitive resilience. Nat Neurosci. 2023 May;26(5):737-750. Epub 2023 Apr 24 PubMed.

    View all comments by Li Gan

  4. Aging is associated with loss of cellular resilience and damage to molecules, both of which accumulate over time. This leads to activation of the innate immune system to generate detrimental inflammatory responses that are now considered as critical determinants of neurodegenerative diseases.

    Leveraging earlier established work from the same laboratory that showed the role of the cGAS/STING system of cytosolic DNA recognition in promoting cellular senescence (Glück et al., 2017), Gulen et al. extend its impact and provide a key insight into the link between senescence and neurodegeneration in vivo. They demonstrate that activation of the microglial cGAS/STING pathway is the culprit for low-grade neuroinflammation, neuronal loss, and cognitive deficit, resulting from cytosolic accumulation of DNA derived from perturbed or damaged mitochondria due to aging.

    Interestingly, in mice, autoactivation of the cGAS variant R241E, corresponding to human cGAS R255E, was sufficient to trigger the same neuropathological phenotypes. Since a recent paper on tauopathy reported that microglial mtDNA release and cGAS/STING activity modulate neuroinflammation, the pathway represents a promising therapeutic target to halt neurodegenerative processes (Udeochu et al., 2023). Furthermore, one of the major highlights in the current study is that the authors measured brain penetrance of the STING inhibitor H-151, confirming its ability to reach the brain in mice. Together with other indications of the benefit of H-151 across preclinical models (Haag et al., 2018; Yu et al., 2020), the new data will be foundational to advance this small molecule through to a Phase 1 clinical trial.

    Interesting questions for future studies include if this inflammatory pathway is activated in a similar fashion in other cGAS/STING-expressing cells in aged brain, including neurons and astrocytes. It is possible that molecular events underlying the cGAS/STING-mediated neurodegeneration vary between cell types and conditions. Indeed, while cytokine production is downstream of cGAS/STING activation, the authors reported that TNF-α is the neurotoxic driver in senescent cells, whereas others showed that type I interferons cause tau-associated neurodegeneration (Udeochu et al., 2023).

    STING can be activated independently of cGAS. Could this partly contribute to age-related neurodegeneration if mtDNA release is not the sole trigger? In addition, can low-grade inflammation trigger protein aggregation (or proteinopathy), a pathological hallmark across many neurodegenerative diseases, given that a recent study reported α-synuclein pathology in a STING gain-of-function mouse model (Szego et al., 2022)?

    This study substantially highlights the relationship between cGAS/STING neuroinflammation and age-related neurodegeneration.

    References:

    Glück S, Guey B, Gulen MF, Wolter K, Kang TW, Schmacke NA, Bridgeman A, Rehwinkel J, Zender L, Ablasser A. Innate immune sensing of cytosolic chromatin fragments through cGAS promotes senescence. Nat Cell Biol. 2017 Sep;19(9):1061-1070. Epub 2017 Jul 31 PubMed.

    Udeochu JC, Amin S, Huang Y, Fan L, Torres ER, Carling GK, Liu B, McGurran H, Coronas-Samano G, Kauwe G, Mousa GA, Wong MY, Ye P, Nagiri RK, Lo I, Holtzman J, Corona C, Yarahmady A, Gill MT, Raju RM, Mok SA, Gong S, Luo W, Zhao M, Tracy TE, Ratan RR, Tsai LH, Sinha SC, Gan L. Tau activation of microglial cGAS-IFN reduces MEF2C-mediated cognitive resilience. Nat Neurosci. 2023 May;26(5):737-750. Epub 2023 Apr 24 PubMed.

    Haag SM, Gulen MF, Reymond L, Gibelin A, Abrami L, Decout A, Heymann M, van der Goot FG, Turcatti G, Behrendt R, Ablasser A. Targeting STING with covalent small-molecule inhibitors. Nature. 2018 Jul;559(7713):269-273. Epub 2018 Jul 4 PubMed.

    Yu CH, Davidson S, Harapas CR, Hilton JB, Mlodzianoski MJ, Laohamonthonkul P, Louis C, Low RR, Moecking J, De Nardo D, Balka KR, Calleja DJ, Moghaddas F, Ni E, McLean CA, Samson AL, Tyebji S, Tonkin CJ, Bye CR, Turner BJ, Pepin G, Gantier MP, Rogers KL, McArthur K, Crouch PJ, Masters SL. TDP-43 Triggers Mitochondrial DNA Release via mPTP to Activate cGAS/STING in ALS. Cell. 2020 Oct 29;183(3):636-649.e18. Epub 2020 Oct 7 PubMed.

    Szego EM, Malz L, Bernhardt N, Rösen-Wolff A, Falkenburger BH, Luksch H. Constitutively active STING causes neuroinflammation and degeneration of dopaminergic neurons in mice. Elife. 2022 Oct 31;11 PubMed.

    View all comments by Chien-Hsiung (Alan) Yu
    • User Profile Image Zoë Van Acker
      VIB-KU Leuven Center for Brain & Disease Research
    • Posted:

    This research of the Ablasser lab nicely explores the role of the cGAS-STING signaling pathway in age-related inflammation with subsequent neuronal damage and memory loss. Interestingly, there is a striking accordance with our recently published paper: Van Acker et al., 2023.

    Where Gulen and colleagues showed mitochondria to leak into the cytosol of aged microglia, we showed a similar cGAS-STING activation upon leakage of mtDNA out of dysfunctional lysosomes in a PLD3-linked neuronal model of late-onset Alzheimer’s disease. In line with our results, the Ablasser lab also detected less expression of the lysosomal marker MAC3 when STING was inhibited; pointing to a lysosomal involvement even when lysosomes would not be the primary etiopathology. Also in our neuronal model, we were able to detect a near-restoration of the lysosomal pathology upon STING inhibition, attributed to a lowered mitophagy/autophagy induction. Hence, our paper adds to the data that boosting autophagy alone will unlikely be beneficial in an advanced disease stage, strengthening the idea to target lysosomal catabolism in general.

    How is the autophagic system impacted in the old microglia? Where Gulen and colleagues focus on the inflammatory cascade downstream of STING, STING activation also does promote autophagy. This is of interest given that the authors detect an increase in the ARM/DAM microglia activation state, generally detected around plaques in Alzheimer’s disease. It would be exciting to find out how the autophagic/lysosomal system of these aged microglia are impacted by STING.

    References:

    Van Acker ZP, Perdok A, Hellemans R, North K, Vorsters I, Cappel C, Dehairs J, Swinnen JV, Sannerud R, Bretou M, Damme M, Annaert W. Phospholipase D3 degrades mitochondrial DNA to regulate nucleotide signaling and APP metabolism. Nat Commun. 2023 May 24;14(1):2847. PubMed.

    View all comments by Zoë Van Acker

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References

News Citations

  1. Just Like Viruses, Tau Can Unleash Interferons
  2. Hot DAM: Specific Microglia Engulf Plaques
  3. ApoE and Trem2 Flip a Microglial Switch in Neurodegenerative Disease

Paper Citations

  1. Yu CH, Davidson S, Harapas CR, Hilton JB, Mlodzianoski MJ, Laohamonthonkul P, Louis C, Low RR, Moecking J, De Nardo D, Balka KR, Calleja DJ, Moghaddas F, Ni E, McLean CA, Samson AL, Tyebji S, Tonkin CJ, Bye CR, Turner BJ, Pepin G, Gantier MP, Rogers KL, McArthur K, Crouch PJ, Masters SL. TDP-43 Triggers Mitochondrial DNA Release via mPTP to Activate cGAS/STING in ALS. Cell. 2020 Oct 29;183(3):636-649.e18. Epub 2020 Oct 7 PubMed.
  2. Sliter DA, Martinez J, Hao L, Chen X, Sun N, Fischer TD, Burman JL, Li Y, Zhang Z, Narendra DP, Cai H, Borsche M, Klein C, Youle RJ. Parkin and PINK1 mitigate STING-induced inflammation. Nature. 2018 Sep;561(7722):258-262. Epub 2018 Aug 22 PubMed.
  3. Glück S, Guey B, Gulen MF, Wolter K, Kang TW, Schmacke NA, Bridgeman A, Rehwinkel J, Zender L, Ablasser A. Innate immune sensing of cytosolic chromatin fragments through cGAS promotes senescence. Nat Cell Biol. 2017 Sep;19(9):1061-1070. Epub 2017 Jul 31 PubMed.
  4. Haag SM, Gulen MF, Reymond L, Gibelin A, Abrami L, Decout A, Heymann M, van der Goot FG, Turcatti G, Behrendt R, Ablasser A. Targeting STING with covalent small-molecule inhibitors. Nature. 2018 Jul;559(7713):269-273. Epub 2018 Jul 4 PubMed.
  5. Roy ER, Wang B, Wan YW, Chiu G, Cole A, Yin Z, Propson NE, Xu Y, Jankowsky JL, Liu Z, Lee VM, Trojanowski JQ, Ginsberg SD, Butovsky O, Zheng H, Cao W. Type I interferon response drives neuroinflammation and synapse loss in Alzheimer disease. J Clin Invest. 2020 Apr 1;130(4):1912-1930. PubMed.

Further Reading

Papers

  1. Roy ER, Wang B, Wan YW, Chiu G, Cole A, Yin Z, Propson NE, Xu Y, Jankowsky JL, Liu Z, Lee VM, Trojanowski JQ, Ginsberg SD, Butovsky O, Zheng H, Cao W. Type I interferon response drives neuroinflammation and synapse loss in Alzheimer disease. J Clin Invest. 2020 Apr 1;130(4):1912-1930. PubMed.

Primary Papers

  1. Gulen MF, Samson N, Keller A, Schwabenland M, Liu C, Glück S, Thacker VV, Favre L, Mangeat B, Kroese LJ, Krimpenfort P, Prinz M, Ablasser A. cGAS-STING drives ageing-related inflammation and neurodegeneration. Nature. 2023 Aug;620(7973):374-380. Epub 2023 Aug 2 PubMed.

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