Hypothesis

Microbiota‑derived succinate acts as a circulating metabolite that stabilizes HIF‑1α in nucleus accumbens neurons, triggering ER stress/UPR and reducing dopaminergic signaling, which manifests as anhedonia‑like behavior.

Mechanistic Rationale

Gut bacteria belonging to the family Succinivibrionaceae and certain Proteobacteria produce succinate during carbohydrate fermentation[https://www.frontiersin.org/journals/aging-neuroscience/articles/10.3389/fnagi.2025.1667448/full]. Succinate can cross the intestinal epithelium via monocarboxylate transporters, enter the portal circulation and, after escaping hepatic uptake, reach the brain[https://pmc.ncbi.nlm.nih.gov/articles/PMC12883760/]. In the brain, succinate binds to its G‑protein coupled receptor SUCNR1 (GPR91) on neurons and glia, inhibiting prolyl hydroxylases and thereby stabilizing HIF‑1α even under normoxic conditions[https://pmc.ncbi.nlm.nih.gov/articles/PMC12366197/]. Stabilized HIF‑1α drives transcription of genes involved in the unfolded protein response (UPR), such as BiP/GRP78 and CHOP, amplifying ER stress[https://doi.org/10.1101/2025.06.18.660280]. ER stress in nucleus accumbens medium spiny neurons impairs dopamine release and synaptic plasticity, a core neurobiological substrate of anhedonia[https://pmc.ncbi.nlm.nih.gov/articles/PMC12909529/].

Testable Predictions

1. Individuals with major depressive disorder and elevated anhedonia scores will show higher plasma succinate levels compared with healthy controls.
2. Plasma succinate concentration will positively correlate with HIF‑1α target gene expression in peripheral mononuclear cells and with ER stress markers (e.g., phospho‑PERK, ATF4) measured in cerebrospinal fluid or proxy neuronal exosomes.
3. Fermentation of a high‑fiber, low‑protein diet in germ‑free mice colonized with a defined Succinivibrionaceae consortium will raise plasma succinate, increase HIF‑1α stabilization in nucleus accumbens, elevate ER stress markers, and reduce sucrose preference relative to mice colonized with a succinate‑low consortium.
4. Pharmacologic inhibition of SUCNR1 or genetic knock‑down of HIF‑1α in nucleus accumbens will blunt the succinate‑induced ER stress and restore sucrose preference, despite high plasma succinate.
5. Administration of a probiotic strain that competitively excludes succinate producers (e.g., a Bifidobacterium longum variant) will lower plasma succinate, decrease HIF‑1α activity in nucleus accumbens, and improve anhedonia‑like behavior in the murine model.

Experimental Approach

• Human cohort: recruit 60 MDD patients and 60 controls, collect fasting plasma, perform targeted metabolomics for succinate, quantify HIF‑1α target transcripts in PBMCs, and obtain CSF via lumbar puncture for ER stress markers; correlate with MADRS and Snaith‑Hamilton Pleasure Scale scores.
• Mouse model: germ‑free C57BL/6J mice receive fecal transplants from human donors stratified by high vs. low plasma succinate; after 2 weeks, measure plasma succinate, immunoblot for HIF‑1α and p‑PERK in nucleus accumbens extracts, conduct sucrose preference test and forced swim test.
• Intervention arms: treat succinate‑high mice with SUCNR1 antagonist (e.g., RS‑43485), HIF‑1α siRNA delivered via AAV to nucleus accumbens, or the probiotic B. longum; assess behavioral and molecular outcomes.

Falsifiability

If plasma succinate does not differ between depressed and control groups, or if manipulating succinate signaling fails to alter nucleus accumbens ER stress or anhedonia‑related behavior, the hypothesis would be refuted. Conversely, consistent support across the predicted links would substantiate a mechanistic conduit from gut microbial metabolism to central ER stress in depression.