STRC Research

STRC ZP Domain Prion-Like Seeding

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STRC ZP Domain Prion-Like Seeding

Stereocilin contains a ZP-like module. ZP-domain proteins self-assemble into linear fibrils via template-driven polymerization (the same mechanism zona pellucida uses to form oocyte coats). A small therapeutic dose of wild-type STRC delivered to Misha’s cochlea could serve as a polymerization seed, recruiting Misha’s own folded-but-weak-binding E1659A monomers into mixed fibrils where wild-type contacts stabilize the mutant contribution. This is therapeutic prion-like seeding — using controlled templating as medicine, not as disease.

The hidden assumption this breaks

Prion-like propagation is treated in the literature as exclusively pathological (tau, α-synuclein, TDP-43 in neurodegeneration). But templated self-assembly is also the normal mechanism for many structural extracellular proteins (ZP matrix, collagen, elastin). The same physics that underlies disease can be inverted into therapy when the seeding agent is correctly folded.

Why it might work specifically for E1659A

From STRC Electrostatic Analysis E1659A:

  • pLDDT at 1659 = 95.69 → the mutant protein is folded.
  • ΔΔG_folding = +0.9 kcal/mol → folding barely affected.
  • The defect is at the interface: salt bridge + H-bonds lost, cavity formed.

If the protein folds, it can participate in ZP-fibril assembly. If inserted into a fibril where neighboring monomers are WT, the mutant’s solvent-exposed interface is at least partially shielded, and its contribution to bundle mechanics depends on the fibril as a whole, not on a single monomer’s binding affinity.

Analogy: one weak brick in a wall is held in place by mortar and neighboring bricks. The wall is load-bearing even with weak bricks, as long as weak bricks are dilute.

Ratio and dose

  • Misha: one null allele (deletion) + one E1659A allele → all stereocilin he makes is E1659A.
  • Seed with ~10% WT : 90% mutant mixed fibril. Bundle stiffness target: 40–60% of WT per STRC Stereocilia Bundle Mechanics Model.
  • WT protein dose: ~0.1× of full replacement dose → an order of magnitude lower delivery burden than pure replacement therapy. Feasible with either AAV (mini-STRC low-expression construct) or ear-drop protein (see STRC Protein Replacement Therapy).

Computational proof path

  1. AF3-Multimer of ZP-domain oligomer (4–8 mer) mixed composition — one WT : seven mutants, three WT : five mutants, five WT : three mutants. Score pTM of full oligomer and ipTM at each WT–mutant interface. Accept if mixed oligomers stabilize to pTM ≥ 0.75.
  2. Domain boundary confirmation — which residues form the ZP-polymerization interface? Residue-level contact analysis on AF3 CIF of STRC homo-oligomer.
  3. Coarse-grained MD of mixed fibril under mechanical load — stretch the fibril at bundle-physiological forces (10–100 pN). Compare force response of 100% WT vs 10/90 mixed vs 100% mutant. Target: 10/90 mixed ≥ 40% of WT response.
  4. Bundle-level integration — re-derive f (HTC coupling fraction) as a function of mixed-fibril content. Feed into STRC Stereocilia Bundle Mechanics Model. Expect: small fraction of WT monomers yields disproportionate fibril-level stability.
  5. Seed dose / kinetics — ODE of fibril nucleation and elongation (classical nucleation theory: nucleation rate ∝ concentration^n, elongation linear). Steady-state seed concentration needed in perilymph.

Why seeding beats replacement

Replacement therapy must raise WT stereocilin to therapeutic plateau (~15,000 proteins/OHC, per existing models). That requires AAV with strong promoter or large LNP doses.

Seeding therapy only needs initiation. Once WT seeds are present and incorporating mutant monomers, the mutant’s own baseline production keeps feeding the growing fibrils. The mutant becomes the raw material. Much lower WT dose needed.

Precedent

  • Classical ZP polymerization: egg coat assembly from ZP1/ZP2/ZP3/ZP4 monomers (Jovine et al., dozens of structural papers).
  • Controlled amyloid: Alzheimer’s AAV-β-amyloid clearance + hypothetical seeded rescue (Tomlinson 2021) — the wrong direction conceptually but same mechanism.
  • Not applied in cochlea, not applied for structural-ECM gene therapy, not applied for any human disease where mutant protein is folded-but-non-binding.

Risks and open questions

  1. Does STRC actually polymerize via its ZP module into fibrils in vivo? The Derstroff 2026 data suggests TMEM145 acts as a scaffold rather than STRC polymerizing freely — needs direct test.
  2. Could WT seeds template MUTANT seed-like behavior that is worse than either alone? (Prion-disease risk inverted.) Need AF3 on large oligomers to check for misfolded populations.
  3. Off-target seeding of OTOA or TECTA (if ZP domains are cross-reactive) — sequence + structural specificity analysis required.
  4. E1659A fibril monomer may have subtly altered ZP-ZP interface from the same charge change that broke the TM interface. Need positional check: does the ZP-polymerization surface overlap with the TM-binding surface?

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