Hypothesis

Circadian fluctuations of the microbial metabolite indole‑3‑propionic acid (IPA) dictate a sequential autophagy program in hippocampal astrocytes: IPA first activates mitophagy via the GPR109A‑AMPK‑ULK1 axis, then, as its concentration wanes, permits aggrephagy through TFEB nuclear translocation. Disruption of this ordered sequence—by either prematurely blocking mitophagy or sustaining aggrephagy—uncouples astrocytic metabolic support from neuronal GABAergic tone, producing anxiety‑like phenotypes.

Mechanistic Rationale

1. Metabolite‑triggered hierarchy – IPA, a tryptophan catabolite produced by Clostridium sporogenes, binds the astrocytic receptor GPR109A, raising intracellular cAMP and activating AMPK. AMPK phosphorylates ULK1 (Ser555) to initiate mitophagy, prioritizing removal of damaged mitochondria to limit ROS‑driven NF‑κB signaling. This step is rapid and transient, matching the peak of IPA levels observed during the dark phase in mice.
2. Metabolite‑dependent switch – As IPA declines, reduced GPR109A signaling lowers AMPK activity, allowing mTORC1 re‑activation. mTORC1 inhibition of TFEB is relieved, driving TFEB nuclear translocation and lysosomal biogenesis, which enables aggrephagy of ubiquitinated protein aggregates. The switch ensures that astrocytes first protect energy homeostasis then clear proteostatic stress.
3. Consequences of mistimed autophagy – Genetic astrocyte‑specific knockout of Gpr109a or pharmacological AMPK inhibition blocks the mitophagy phase while leaving aggrephagy intact. Resultant mitochondrial ROS elevates extracellular glutamate via reversed EAAT2 transport, overstimulating neuronal NMDA receptors and triggering a compensatory increase in GABA synthesis that paradoxically reduces GABA release due to vesicular exhaustion. Behavioral assays show heightened anxiety‑like activity in the elevated plus maze without overt depressive‑like changes. Conversely, constitutive activation of TFEB (lysosomal overexpression) forces premature aggrephagy, depleting mitochondrial reserves before mitophagy can compensate, leading to ATP deficiency, impaired GABA uptake, and increased anxiety.

Testable Predictions

• Prediction 1: In wild‑type mice, hippocampal astrocyte IPA levels will peak at ZT14, correlating with maximal phospho‑ULK1 (Ser555) and minimal LC3‑II accumulation under aggrephagy markers; at ZT2, TFEB nuclear localization and p62‑positive aggregates will rise.
• Prediction 2: Astrocyte‑specific Gpr109a KO will abolish the ZT14 mitophagy peak, elevate mitochondrial ROS (MitoSOX), increase extracellular glutamate (microdialysis), and elevate anxiety‑like behavior; rescuing mitophagy with a mitochondria‑targeted antioxidant (MitoQ) will normalize glutamate and anxiety without affecting aggrephagy flux.
• Prediction 3: Pharmacological activation of TFEB (e.g., with trehalose) during the IPA peak will shift the autophagy hierarchy toward aggrephagy, decrease astrocytic ATP (luciferase assay), and increase anxiety; inhibiting TFEB nuclear export (with XPO1 antagonist) will exacerbate the effect.
• Prediction 4: Administering a timed‑release IPA formulation that restores the natural circadian metabolite rhythm will re‑establish the mitophagy‑first sequence and attenuate anxiety in Gpr109a KO mice.

Experimental Approach

• Use Gpr109a floxed mice crossed with Aldh1l1‑CreERT2 for astrocyte‑specific deletion; administer tamoxifen at 8 weeks.
• Measure IPA concentrations in hippocampal tissue via LC‑MS across 4‑hour intervals.
• Assess mitophagy (mt‑Keima assay) and aggrephagy (GFP‑LC3‑p62 reporter) in isolated astrocytes via flow cytometry.
• Record extracellular glutamate with enzyme‑based biosensors; evaluate GABA release using synaptosomal uptake assays.
• Conduct behavioral testing (elevated plus maze, light‑dark box) and correlate with biochemical readouts.

Significance

This hypothesis translates the metaphor of autophagy as a "cannibalism ritual with hierarchy" into a precise, metabolite‑driven timetable that links gut ecology to astrocyte function and emotional behavior. It shifts the focus from merely restoring autophagy flux to rescuing the correct temporal order of selective autophagy subtypes, offering a falsifiable framework for microbiome‑based interventions in anxiety disorders.