Bagshaw, M. | Search bioRxiv

bioRxiv2026

Rhodopsin is a tunable capacitor buffering the toxic, desensitizing retinoids of the vertebrate eye

The eye is a marvel of evolution, but visual sensitivity introduces the risk of blinding photodamage. Here we reveal that the locus of sensitivity--the visual pigment rhodopsin --moonlights as a tunable mechanism of retinal photoprotection. Independent of signaling, light activated rhodopsin (R*) serves as an overflow capacitor buffering all-trans retinal (atRAL), a toxic and desensitizing retinoid agonist that accumulates as lipofuscin--a clinical marker of macular degeneration. Across mammals, R* stability reflects binding affinity and sequestration of atRAL (capacitance), thereby mitigating phototoxicity to a degree proportional to species photodamage risk, with human R* uniquely non-protective. A mouse model of defective atRAL clearance treated with a synthetic R* of unnaturally high atRAL capacitance preserved retinal function following light damage. This gene therapy also provided supra-physiological scotopic sensitivity despite being a signal-silent receptor, shielding neighboring dark-state receptors from agonist interference during regeneration, while also enhancing the signaling of endogenous R*. While the consensus human rhodopsin is not photoprotective, during recent evolution, mutations that protect against light damage emerged in high irradiance human environments and are now significantly associated with a 38% reduced risk of blindness and low vision. Together, our findings redefine rhodopsin as a tunable light buffer that can be leveraged to enhance photoreceptor function beyond natural limits.

biochemistry↗

bioRxiv2025

Antibiotic-resistance mutations in penicillin-binding protein 2 from the ceftriaxone-resistant Neisseria gonorrhoeae strain H041 strike a delicate balance between increasing resistance and maintaining transpeptidase activity

The mosaic penA allele (penA41) from H041, the most ceftriaxone-resistant Neisseria gonorrhoeae strain identified to date, encodes a variant of the essential Penicillin-Binding Protein 2 (PBP2) with 60 amino acid mutations compared to PBP2 from the antimicrobial-susceptible strain, FA19. Based on previous work from our lab and others, we identified a minimal set of 10 mutations that, when introduced into the {beta}-lactam antibiotic-susceptible penA allele from FA19 (penA19), confers two-thirds of the ceftriaxone and cefixime resistance compared to the penA41 allele. Three mutations (A311V, I312M, and V316P) are in the 2 helix of PBP2 containing the catalytic serine (Ser310), two (F504L and N512Y) are in the 3-4 loop that is important in binding and acylation, and one (G545S) interacts with conserved amino acids in the active site. The seventh mutation, T483S, confers substantial resistance to ceftriaxone within the minimal mutant set but requires the presence of three epistatic mutations located on the other side of the protein that do not alter resistance on their own yet are necessary to retain essential transpeptidase activity. These epistatic mutations change the backbone dihedral angles at position-447, which may increase flexibility of the enzyme and help restore essential transpeptidation. Our results highlight the complex balance necessary for evolving cephalosporin resistance while also retaining sufficient transpeptidase function in PBP2. Author SummaryIn this study, we set out to understand how Neisseria gonorrhoeae, the bacterium that causes gonorrhea, is able to resist the last remaining recommended antibiotic, ceftriaxone. Gonorrhea is a common sexually transmitted infection worldwide, and rising resistance threatens to make it untreatable. We focused on penicillin-binding protein 2 (PBP2), which is essential for the bacteriums survival and is the lethal target of ceftriaxone. By incorporating a subset of the 60 PBP2 mutations found in a highly resistant strain into PBP2 from an antibiotic-susceptible strain, we discovered that resistance evolved from a combination of mutations that work together to directly reduce the capacity of ceftriaxone to inactivate the protein and others that act as "supporting" mutations to keep the protein functional despite the presence of the resistance mutations. Our study highlights how N. gonorrhoeae successfully negotiates the delicate balance between resistance and function to escape the lethal action of antibiotics.

microbiology↗