Phenotypic Screening Using High-Content Imaging to Identify Lysosomal pH Modulators in a Neuronal Cell Model.

Lysosomes are intracellular organelles responsible for the degradation of diverse macromolecules in a cell. A highly acidic pH is required for the optimal functioning of lysosomal enzymes. Loss of lysosomal intralumenal acidity can disrupt cellular protein homeostasis and is linked to age-related diseases such as neurodegeneration. Using a new robust lysosomal pH biosensor (FIRE-pHLy), we developed a cell-based fluorescence assay for high-throughput screening (HTS) and applied it to differentiated SH-SY5Y neuroblastoma cells. The goal of this study was twofold: (1) to screen for small molecules that acidify lysosomal pH and (2) to identify molecular targets and pathways that regulate lysosomal pH. We conducted a screen of 1835 bioactive compounds with annotated target information to identify lysosomal pH modulators (both acidifiers and alkalinizers). Forty-five compounds passed the initial hit selection criteria, using a combined analysis approach of population-based and object-based data. Twenty-three compounds were retested in dose-response assays and two compounds, OSI-027 and PP242, were identified as top acidifying hits. Overall, data from this phenotypic HTS screen may be used to explore novel regulatory pathways of lysosomal pH regulation. Additionally, OSI-027 and PP242 may serve as useful tool compounds to enable mechanistic studies of autophagy activation and lysosomal acidification as potential therapeutic pathways for neurodegenerative diseases.

Improved Electrophile Design for Exquisite Covalent Molecule Selectivity.

Covalent inhibitors are viable therapeutics. However, off-target reactivity challenges the field. Chemists have attempted to solve this issue by varying the reactivity attributes of electrophilic warheads. Here, we report the development of an approach to increase the selectivity of covalent molecules that is independent of warhead reactivity features and can be used in concert with existing methods. Using the scaffold of the Bruton's tyrosine kinase (BTK) inhibitor Ibrutinib for our proof-of-concept, we reasoned that increasing the steric bulk of fumarate-based electrophiles on Ibrutinib should improve selectivity via the steric exclusion of off-targets but retain rates of cysteine reactivity comparable to that of an acrylamide. Using chemical proteomic techniques, we demonstrate that elaboration of the electrophile to a tert-butyl (t-Bu) fumarate ester decreases time-dependent off-target reactivity and abolishes time-independent off-target reactivity. While an alkyne-bearing probe analogue of Ibrutinib has 247 protein targets, our t-Bu fumarate probe analogue has only 7. Of these 7 targets, BTK is the only time-independent target. The t-Bu inhibitor itself is also more selective for BTK, reducing off-targets by 70%. We investigated the consequences of treatment with Ibrutinib and our t-Bu analogue and discovered that only 8 proteins are downregulated in response to treatment with the t-Bu analogue compared to 107 with Ibrutinib. Of these 8 proteins, 7 are also downregulated by Ibrutinib and a majority of these targets are associated with BTK biology. Taken together, these findings reveal an opportunity to increase cysteine-reactive covalent inhibitor selectivity through electrophilic structure optimization.

Programmable assembly of 2D crystalline protein arrays into covalently stacked 3D bionanomaterials.

Rational embellishment of self-assembling two-dimensional (2D) proteins make it possible to build 3D nanomaterials with novel catalytic, optoelectronic and mechanical properties. However, introducing multiple sites of embellishment into 2D protein arrays without affecting the self-assembly is challenging, limiting the ability to program in additional functionality and new 3D configurations. Here we introduce two orthogonal covalent linkages at multiple sites in a 2D crystalline-forming protein without affecting its self-assembly. We first engineered the surface-layer protein SbsB from Geobacillus stearothermophilus pV72/p2 to display isopeptide bond-forming protein conjugation pairs, SpyTag or SnoopTag, at four positions spaced 5.7-10.5 nm apart laterally and 3 nm axially. The C-terminal and two newly-identified locations within SbsB monomer accommodated the short SpyTag or SnoopTag peptide tags without affecting the 2D lattice structure. Introducing tags at distinct locations enabled orthogonal and covalent binding of SpyCatcher- or SnoopCatcher-protein fusions to micron-sized 2D nanosheets. By introducing different types of bifunctional cross-linkers, the dual-functionalized nanosheets were programmed to self-assemble into different 3D stacks, all of which retain their nanoscale order. Thus, our work creates a modular protein platform that is easy to program to create dual-functionalized 2D and lamellar 3D nanomaterials with new catalytic, optoelectronic, and mechanical properties.