IF a dual beta-cell-targeted gene therapy consisting of AAV8-RIP-hIDE (3×10¹¹ vg, to enzymatically degrade soluble and oligomeric hIAPP species) co-delivered intraductally with AAV8-RIP-TFEB (1×10¹¹ vg, to transcriptionally activate beta-cell-intrinsic autophagy for fibril-phase amyloid clearance) is administered as a single retrograde pancreatic duct injection to 6–8-month-old female FVB/N-Tg(Ins2-IAPP)RHFSoel/J hemizygous mice with established islet amyloidosis (≥5.5% Congo red-positive islets),

THEN ≥40% reduction in thioflavin-S-positive amyloid area per islet (µm²/islet, ≥50 islets/mouse) and ≥60% restoration of glucose-stimulated insulin secretion (GSIS fold-change at 16.7 mM glucose vs. 2.8 mM glucose, perifusion assay) compared to AAV8-RIP-GFP controls will be observed at 24 weeks post-injection, exceeding the outcome achieved by AAV8-RIP-hIDE alone (predicted to show ≤15% fibril reduction due to IDE's intrinsic limitation against mature cross-beta amyloid cores),

BECAUSE the following mechanistic chain operates:

1. IDE is enzymatically active against soluble and oligomeric IAPP but not against mature amyloid fibrils. Microglia secreting endogenous IDE partially degrade soluble Aβ peptides in the surrounding medium but fail to clear pre-formed fibrillar deposits, demonstrating that IDE's active-site chamber (~16,000 ų enclosed catalytic cavity) physically excludes large fibrillar substrates from productive cleavage (IDE degrades soluble but not fibrillar Aβ)[https://doi.org/10.1016/j.neuroscience.2020.07.020]. This same steric limitation applies to hIAPP mature fibrils in beta-cell islets, meaning AAV8-RIP-hIDE monotherapy will clear the soluble/oligomeric pool but leave the fibril core intact. [SPECULATIVE for direct IAPP fibrils, extrapolated from Aβ fibril data]

2. Soluble hIAPP oligomers are the primary cytotoxic species and the seeding reservoir for continuous fibril growth. Stabilized hIAPP oligomers induce beta-cell apoptosis via membrane disruption, and this oligomer-driven toxicity is the proximate cause of GSIS impairment rather than fibril mass per se (hIAPP oligomers induce apoptosis neutralized by diabetes-specific antibodies)[https://doi.org/10.1038/srep04267]. IDE clearance of the oligomeric pool thus directly addresses beta-cell functional impairment, but ongoing hIAPP transgene expression in surviving beta cells continuously reseeds the fibril core, explaining why IDE alone is predicted insufficient for sustained amyloid burden reduction.

3. Autophagy-competent cells can degrade extracellular amyloid fibrils that enzymatic clearance cannot access. Microglia activate LC3-dependent autophagy to internalize and degrade extracellular Aβ fibrils — a substrate class explicitly refractory to secreted IDE — demonstrating that the autophagy–lysosome axis represents the mechanistically orthogonal route for fibril-phase amyloid clearance (autophagy in microglia degrades extracellular Aβ fibrils and regulates NLRP3)...

SENS category: LysoSENS

Key references: • doi.org/10.1016/j.neuroscience.2020.07.020]. • doi.org/10.1038/srep04267]. • doi.org/10.4161/auto.29647]. • doi.org/10.1186/s13024-022-00518-y], • doi.org/10.3109/13506129.2016.1160882]