Cyclic GMP-AMP synthase promotes the inflammatory and autophagy responses in Huntington disease

Manish Sharma,Sumitha Rajendrarao,Neelam Shahani,Uri Nimrod Ramírez-Jarquín,Srinivasa Subramaniam

doi:10.1073/pnas.2002144117

Manish Sharma, Sumitha Rajendrarao + Show 3 more

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https://doi.org/10.1073/pnas.2002144117

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Abstract

Huntington disease (HD) is caused by an expansion mutation of the N-terminal polyglutamine of huntingtin (mHTT). mHTT is ubiquitously present, but it induces noticeable damage to the brain's striatum, thereby affecting motor, psychiatric, and cognitive functions. The striatal damage and progression of HD are associated with the inflammatory response; however, the underlying molecular mechanisms remain unclear. Here, we report that cGMP-AMP synthase (cGAS), a DNA sensor, is a critical regulator of inflammatory and autophagy responses in HD. Ribosome profiling revealed that the cGAS mRNA has high ribosome occupancy at exon 1 and codon-specific pauses at positions 171 (CCG) and 172 (CGT) in HD striatal cells. Moreover, the protein levels and activity of cGAS (based on the phosphorylated STING and phosphorylated TBK1 levels), and the expression and ribosome occupancy of cGAS-dependent inflammatory genes (Ccl5 and Cxcl10) are increased in HD striatum. Depletion of cGAS diminishes cGAS activity and decreases the expression of inflammatory genes while suppressing the up-regulation of autophagy in HD cells. In contrast, reinstating cGAS in cGAS-depleted HD cells activates cGAS activity and promotes inflammatory and autophagy responses. Ribosome profiling also revealed that LC3A and LC3B, the two major autophagy initiators, show altered ribosome occupancy in HD cells. We also detected the presence of numerous micronuclei, which are known to induce cGAS, in the cytoplasm of neurons derived from human HD embryonic stem cells. Collectively, our results indicate that cGAS is up-regulated in HD and mediates inflammatory and autophagy responses. Thus, targeting the cGAS pathway may offer therapeutic benefits in HD.

Highlights

Huntington disease (HD) is a fatal neurodegenerative disorder that is caused by CAG expansion mutation of the Huntingtin gene (HTT), which codes for polyglutamine
CGMP-AMP synthase is an enzyme that produces cyclic guanosine monophosphate–adenosine monophosphate, a second messenger that is activated upon binding of cGMP-AMP synthase (cGAS) to DNA or RNA:DNA hybrids in the cytoplasm [22, 23]. cGAS can induce signaling that is known to promote the up-regulation of inflammatory genes and play a critical role in age-related macular degeneration and cellular senescence [24,25,26]. cGAS-induced cGAMP binds to the endoplasmic reticulum (ER)-associated transmembrane protein STING
We examined whether the ribosome profile of two major cGAS downstream targets, namely Sting and Tbk1, is altered in HD cells

Summary

Introduction

Huntington disease (HD) is a fatal neurodegenerative disorder that is caused by CAG expansion mutation of the Huntingtin gene (HTT), which codes for polyglutamine. Inflammation is closely linked to autophagy, a catabolic process that is dysregulated in HD [20, 21] These findings indicate that inflammation is a prominent cellular response in HD patients and across various HD models, the mechanisms are not entirely clear. CGAS can induce signaling that is known to promote the up-regulation of inflammatory genes and play a critical role in age-related macular degeneration and cellular senescence [24,25,26]. CGAS plays a major role in the regulation of autophagy; this indicates that there is a close molecular and signaling link between inflammatory response and autophagy [20, 29,30,31]. MHTT is known to induce DNA damage and affect autophagy, both associated with inflammatory responses, but what mediates all these were unknown.

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