Review Papers Against Their Own Results

Concise critique

The preprint reports compact degron based bioPROTACs (RRRG degron fused to small binders such as SynZip or vhhGFP4) that drive rapid, dose dependent degradation of cytosolic GFP and potent membrane CAR internalization when membrane tethered, and demonstrates an antigen triggered circuit that degrades endogenous ZAP70 to modulate CAR T cell activity; key mechanistic evidence implicates cullin ring ligases (particularly CUL4A/B) and partial dependence on the ubiquitin proteasome system for cytosolic targets while membrane target internalization appears ubiquitination dependent but proteasome independent, with functional suppression of CAR T cytotoxicity in vitro in primary T cells but incomplete ablation of target killing when targeting ZAP70 via SNIPR circuits bioRxiv preprint Degron based bioPROTACs [2024]

Detailed evidence based review and critique

What the paper does (facts from the manuscript)

• Introduces minimal degron RRRG (from Bonger LID) fused to small protein binders (SynZip17/18, SynZip1/2 and nanobody vhhGFP4) to create genetically encoded bioPROTACs that recruit endogenous UPS machinery and drive trans degradation of targets RRRG degron selection and construct design
• Demonstrates dose dependent and rapid degradation kinetics for cytosolic GFP targets (loss within hours; complete by ~12 h under strong induction), and activity across multiple mammalian cell types (Jurkat, primary T cells, HEK293T, mESCs, K562, 3T3) Dose dependent rapid degradation across cell types
• Shows that K to R lysine mutagenesis of binder domains reduces cis ubiquitination and increases trans degradation potency for some binders (notably vhhGFP4 and SynZip18) K to R mutagenesis improves activity
• Membrane tethering of bioPROTACs (DAP10 extracellular/CD8a TM or lyn tag) strongly increases CAR internalization, enabling near complete removal (>99%) of an anti CD19 4-1BBz CAR from primary T cell surface and abrogating CAR cytotoxicity and CD25 upregulation in vitro Membrane tethered bioPROTAC removes CAR from membrane
• Mechanistic probes implicate cullin ring ligases (CUL4A/CUL4B families) as required for degradation, with MLN4924 (cullin NEDDylation inhibitor) showing strong rescue of cytosolic GFP loss; MG132 rescue is partial (and variable) and lysosome inhibitor Bafilomycin A1 has minimal effect for cytosolic targets; for membrane CAR internalization, MLN4924 rescues while MG132 does not, suggesting ubiquitination dependent internalization but proteasome independent fate Cullin ring ligase dependence and inhibitor panel
• Composes a SNIPR triggered genetic circuit that induces vhhGFP4 bioPROTAC expression on recognition of HER2, causing degradation of CRISPR GFP tagged endogenous ZAP70 in primary human T cells and producing slower but measurable reductions in proliferation, CD25 upregulation and ZAP70 levels over multiple tumor challenges; circuit did not fully block killing in short assays and authors note limitations in expression amplitude/timing for degrading abundantly expressed CARs SNIPR activated ZAP70 targeting circuit

Strengths

• Compact genetic payload: minimal 4 aa degron and small binders enable inclusion in single lentiviral cassettes and genetic circuits, solving a practical delivery constraint authors explicitly tested Compactness and delivery rationale
• Demonstrates applicability in primary human T cells, not only cell lines, with functional readouts (cytotoxicity, CD25, proliferation) increasing translational relevance Primary human T cell experiments
• Mechanistic probing with inhibitors and dominant negative cullins gives initial direction for E3 involvement (CUL4 family), guiding future mechanistic work Evidence implicating CUL4 family

Limitations and critical concerns (potential blindspots and experimental caveats)

1. Preprint status and lack of peer review: results have not undergone formal peer review; authors repeatedly note where data are preliminary and where further optimization is needed (e.g., SNIPR expression amplitude and kinetics) Preprint and stated limitations
2. Mechanistic ambiguity for membrane target fate: MG132 (proteasome) had little effect on CAR internalization while MLN4924 rescues it; lysosome inhibitors did not rescue — authors conclude ubiquitination needed but final fate (endocytosis route, ESCRT, autophagy involvement) remains unclear and unproven Unclear mechanism for membrane internalization
3. Quantitative depth and controls: many reported fold changes and rescue effects are based on flow cytometry fluorescence and inhibitors; biochemical validation (western blots for ubiquitination patterns, ubiquitin linkage types, proteasome vs lysosome flux, endosomal markers) and unbiased proteomics are limited or absent in the preprint, reducing mechanistic resolution Limited biochemical mechanistic data
4. Generality across CAR designs and expression levels: authors note higher difficulty degrading CARs expressed from strong constitutive promoters versus endogenous ZAP70; circuit efficacy depends on relative expression stoichiometry which may limit clinical translatability unless expression amplitude and timing are engineered carefully Expression stoichiometry limits circuit efficacy
5. In vivo data absent: all functional assays are in vitro co culture or cell lines; no animal models or safety/toxicity data are presented, limiting claims about therapeutic potential and CRS mitigation timelines suggested by authors No in vivo validation
6. Potential off target ubiquitination / cell stress: compact RRRG degron recruits endogenous E3 machinery—widespread or unintended ubiquitination of off target proteins or perturbation of UPS capacity in T cells (especially if expressed constitutively) is not deeply explored and could produce toxicity or functional impairment Possible off target UPS perturbation not excluded

Overall scientific assessment and numeric scores (critical and skeptical)

Actionable recommendations to strengthen the manuscript and the technology

1. Provide biochemical evidence: western blots for ubiquitination of POI (GFP and CAR), ubiquitin linkage analysis (K48 vs K63), and proteasome flux assays to confirm whether membrane CARs are routed to lysosome, proteasome or recycled endosomes; co localization with endosomal markers (EEA1, Rab5, Rab7) or lysosomal markers (LAMP1) would clarify mechanisms Mechanistic ambiguity noted by authors
2. Identify the specific substrate receptor or DCAF linking RRRG to CUL4A/B (e.g., DDB1 and DCAF subsets) via co immunoprecipitation or CRISPR knockdown of candidate adaptors to establish molecular mechanism
3. Test broader CAR designs and promoter strengths and quantify required bioPROTAC expression thresholds to degrade high abundance membrane proteins; include titration and time course to define design rules for circuits
4. Assess proteome wide off targets and cell fitness with proteomics and long term culture assays to evaluate safety for therapeutic translation
5. Provide plasmid sequences, raw flow cytometry data and code to improve reproducibility and enable independent replication

Concluding synthesis

The work provides a practically useful, compact bioPROTAC design that is genetically encodable and effective in primary human T cells for knocking down CARs (when membrane tethered) and for circuit dependent degradation of endogenous signaling proteins such as ZAP70. The mechanistic signals point to CUL4 family involvement and ubiquitination dependence, but the routing of membrane targets remains unclear. The technology is promising for cell engineering and genetic circuits but requires deeper mechanistic validation, in vivo evaluation, and safety profiling before claiming therapeutic potential. Confidence in the reported in vitro effects is moderate but must be tempered because the manuscript is a preprint and key mechanistic and reproducibility elements are missing Authors conclusions and caveats