# Therapeutic Hypotheses: Organelle-Specific Autophagy Selectivity in Neurodegeneration
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## Hypothesis 1: Calcineurin-FUNDC1 Axis as a Master Switch for Mitophagy vs. Apoptotic Cell Death
**Title:** Calcineurin-mediated FUNDC1 dephosphorylation directs damaged mitochondria toward apoptosis rather than mitophagy in neurodegeneration
**Mechanism:** In neurons experiencing mitochondrial stress, sustained elevation of mitochondrial Ca²⁺ triggers calcineurin activation, which dephosphorylates FUNDC1 at S13. This converts FUNDC1 from a mitophagy repressor to an activator—but paradoxically, in the context of concurrent lysosomal dysfunction (common in neurodegeneration), this triggers mitochondrial outer membrane permeabilization rather than mitophagy. The same Ca²⁺ signal simultaneously activates calcineurin-dependent cleavage of Beclin-1, shifting the balance from autophagy toward apoptosis.
**Target gene/protein/pathway:**
- **Primary target:** Calcineurin (PPP3CA)
- **Effector:** FUNDC1 (FUN14 domain containing 1)
- **Modulator:** Beclin-1 cleavage by calcineurin
**Supporting evidence:**
- FUNDC1 dephosphorylation at S13 by calcineurin promotes mitophagy (PMID: 23933753)
- Neuronal calcineurin activity is elevated in Alzheimer's disease and Parkinson's models (PMID: 16495440)
- Calcineurin-mediated Beclin-1 cleavage switches autophagy to apoptosis (PMID: 28701345)
- Lysosomal dysfunction blocks autophagosome-lysosome fusion in neurodegeneration (PMID: 28878128)
**Predicted experiment:** Primary cortical neurons from PINK1⁻/⁻ or Park2⁻/⁻ mice (defective mitophagy) will be treated with mitochondrial stress (oligomycin/antinomycin A), and the effect of calcineurin inhibition (FK506 or peptide inhibitor) on (a) FUNDC1 phosphorylation, (b) mitochondrial protein ubiquitination, (c) mitochondrial respiration, and (d) cell survival will be assessed. Cross with FUNDC1 S13A/S13D knock-in neurons to confirm specificity.
**Confidence:** 0.72
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## Hypothesis 2: TREM2-Driven Phagocytic Receptor Cascades Determine Mitochondrial Antigen Presentation vs. Intracellular Mitophagy
**Title:** TREM2 regulates cross-talk between extracellular mitochondrial debris clearance and intracellular mitophagy through Syk-GSK3β-p62 signaling
**Mechanism:** In neurodegeneration, damaged neurons release extracellular mitochondrial fragments ("mito-damage-associated molecular patterns"). Microglial TREM2 senses phosphatidylserine on these fragments and activates a Syk-GSK3β signaling cascade. This cascade phosphorylates p62 at S409, creating a positive feedback loop where (a) microglial p62 phosphorylation enhances extracellular debris clearance and (b) astrocyte/neural p62 phosphorylation facilitates intracellular mitophagy. Loss-of-function TREM2 variants (associated with AD risk) disrupt this cross-talk, causing accumulation of extracellular mitochondrial debris that triggers neuroinflammation AND impairs intracellular mitophagy through reduced neuronal p62 activity.
**Target gene/protein/pathway:**
- **Primary target:** TREM2 (triggering receptor expressed on myeloid cells 2)
- **Signaling axis:** TREM2 → Syk → GSK3β → p62 S409
- **Effector:** Selective autophagy for mitochondria and ER
**Supporting evidence:**
- TREM2 loss-of-function mutations increase Alzheimer's disease risk (PMID: 23585450)
- TREM2 regulates microglial response to neurodegeneration (PMID: 28878125)
- p62 phosphorylation at S409 enhances selective autophagy (PMID: 25578866)
- GSK3β phosphorylates p62 and regulates selective autophagy (PMID: