1. **NGF/TrkA failure is upstream**
Weak evidence: Most human support is correlational and late-stage. Reduced `NTRK1`/NGF signaling could be a consequence of early tau, endosomal stress, or synapse loss rather than the initiating lesion. “Before severe neuron loss” does not establish before soluble Aβ or seed-competent tau.
Alternative mechanisms: Early tau in entorhinal-limbic circuits, APP/endosomal defects, mitochondrial failure, or vascular hypoperfusion could independently cause both cholinergic dysfunction and apparent NGF signaling failure.
Translational risks: iPSC cholinergic neurons do not reproduce decades of retrograde trophic dependence, long axons, aging, or human basal forebrain circuit architecture. TrkA agonism may improve survival markers without proving disease-ordering relevance.
Falsifying experiment: In longitudinal human cohorts, show that sensitive amyloid/tau biomarkers become abnormal before any cholinergic PET/MRI deficit in people with intact NGF/TrkA proxies. In vitro, if selective `NTRK1` reduction does not increase APP beta-processing or tau seeding susceptibility beyond nonspecific stress, the mechanism weakens substantially.
2. **Amyloid first via alpha7 nicotinic receptor synaptotoxicity**
Weak evidence: The Aβ-`CHRNA7` interaction literature is mixed and heavily model-dependent; binding affinity and functional relevance vary by Aβ preparation. Presynaptic toxicity is plausible, but specificity to cholinergic terminals is not well established in humans.
Alternative mechanisms: Aβ may impair synapses through NMDA receptor dysregulation, membrane pore effects, oxidative stress, microglial signaling, or generic calcium overload rather than a primary `CHRNA7` route.
Translational risks: Patient-derived soluble Aβ fractions are heterogeneous and unstable. `CHRNA7` knockout may alter baseline synapse maturation and excitability, confounding interpretation. Alpha7 antagonism could protect acutely while worsening cognition in vivo.
Falsifying experiment: Use well-characterized human organoid/co-culture systems and demonstrate that alpha7 blockade or `CHRNA7` deletion fails to rescue cholinergic release despite clear Aβ-induced toxicity. Strong falsification would be equal toxicity in `CHRNA7`-null and wild-type conditions.
3. **APOE4-microglial complement selectively hits cholinergic synapses first**
Weak evidence: Complement-mediated pruning is supported broadly, but “selective” vulnerability of cholinergic synapses is mostly inferred from anatomy, not demonstrated. APOE4 effects may amplify many injury pathways, not uniquely cholinergic pruning.
Alternative mechanisms: APOE4 could act primarily through impaired lipid handling, amyloid clearance, BBB dysfunction, or astrocyte-mediated inflammation, with complement as a downstream amplifier rather than the ordering determinant.
Translational risks: Human tri-cultures poorly model region-specific microglial states, complement gradients, and long-range cholinergic projections. Complement blockade may preserve synapses in vitro but fail clinically because it misses upstream tau/amyloid drivers.
Falsifying experiment: In APOE4 human systems and longitudinal human tissue/imaging, test whether complement deposition is not enriched on cholinergic boutons before overt amyloid/tau changes. If `C1q`/`C3` inhibition preserves terminals but does not alter downstream tau spread, the causal claim fails.
4. **Locus coeruleus degeneration gates sequence**
Weak evidence: This is plausible but underspecified. LC tau pathology is early, yet proving it determines whether cholinergic or amyloid/tau changes appear first is much stronger than showing association. LC integrity measures are noisy proxies.
Alternative mechanisms: LC degeneration may simply be another parallel early lesion caused by tau vulnerability, aging, sleep disruption, or vascular disease. The observed ordering could instead reflect subtype biology or measurement sensitivity.
Translational risks: Human stratification studies are vulnerable to confounding by arousal state, antidepressants, vascular burden, and imaging resolution limits. LC-targeted support may have broad neuromodulatory effects without changing core AD progression.
Falsifying experiment: In a longitudinal prodromal cohort, test whether LC metrics fail to predict subsequent ordering once age, APOE, baseline amyloid/tau burden, vascular disease, and sleep variables are controlled. If predictive value disappears, the gating hypothesis is weak.
5. **Endosomal trafficking defects are the common upstream lesion**
Weak evidence: This is the strongest mechanistically because AD genetics converges on trafficking, but evidence for basal-forebrain selectivity remains incomplete. Parallel outputs from one lesion are plausible, yet direct proof that cholinergic dysfunction and amyloid arise from the same earliest defect in humans is lacking.
Alternative mechanisms: Trafficking defects may be one branch of a broader aging network that includes lysosomal failure, mitochondrial stress, and proteostasis collapse. Amyloid and cholinergic dysfunction may still be partially sequential, not merely parallel outputs.
Translational risks: Editing `SORL1`/`BIN1` in culture can generate artificial large-effect phenotypes unlike human heterozygous aging. Cell-autonomous models miss glia, circuit activity, and decades-long compensation.
Falsifying experiment: In isogenic human neurons, if AD-risk trafficking variants change APP processing similarly in cortical and cholinergic neurons without a cholinergic-selective trophic transport deficit, the proposed selectivity is undermined. Human falsification would be normal early trafficking markers in individuals who still develop cholinergic-first trajectories.
6. **Reactive astrocytes and cholinesterases create a low-ACh niche that accelerates tau**
Weak evidence: This is the weakest of the set. Increased `BCHE`/astrocyte reactivity in AD tissue is real, but causality and timing are unclear, and the link to tau-first progression is indirect. Astrocyte cholinesterase changes may be compensatory or late-stage.
Alternative mechanisms: Reactive astrocytes may primarily reflect response to existing amyloid/tau, cytokines, or neuronal injury. Tau acceleration could come from inflammatory kinases, lipid dysregulation, or impaired glutamate handling independent of cholinergic tone.
Translational risks: Organoids poorly model extracellular acetylcholine dynamics and mature astrocyte states. Cholinesterase inhibition may alter network activity broadly, creating false-positive “rescue” effects unrelated to disease mechanism.
Falsifying experiment: Induce reactive astrocytes or `BCHE` overexpression in human cortical systems and show no increase in tau phosphorylation/propagation after controlling for general inflammation and excitability. If selective `BCHE` inhibition does not rescue tau phenotypes, the hypothesis largely collapses.
7. **Temporal order is subtype-specific**
Weak evidence: Heterogeneity is highly plausible, but this formulation risks becoming unfalsifiable because any ordering can be assigned to a subtype after the fact. Existing human trajectory studies are sensitive to biomarker choice and clustering method.
Alternative mechanisms: Apparent subtypes may reflect staging, measurement thresholds, ascertainment bias, co-pathologies, or continuous variation rather than discrete disease classes.
Translational risks: Longitudinal clustering often yields unstable classes across cohorts. Over-stratification can generate nonreproducible therapeutic niches and underpowered trials.
Falsifying experiment: Pre-register clustering features and replication criteria across independent cohorts. If the same two or more trajectory classes do not reproduce with stable assignments and distinct outcomes, the subtype-ordering model is not robust.
**Cross-cutting skeptical points**
- The main weakness across all seven is temporal inference from cross-sectional or reductionist systems.
- Measurement asymmetry matters: amyloid/tau biomarkers are currently more sensitive and standardized than cholinergic ones, so “amyloid-first” may partly reflect assay availability.
- Co-pathologies, vascular injury, sleep dysfunction, medications, and age-related neuromodulatory decline are major confounds that can mimic cholinergic-first trajectories.
- A strong discriminating program would require longitudinal multimodal human data with repeated amyloid, tau, cholinergic, LC, inflammatory, and vascular measures, plus prespecified causal models rather than post hoc narrative fitting.