The synaptic vesicle protein SV31 assembles into a dimer and transports Zn2.

The integral synaptic vesicle protein SV31 has been shown to bind divalent cations. Here, we demonstrate that SV31 protein synthesized within a cell-free system binds Zn2+ and to a lower extent Ni2+ and Cu2+ ions. Expression with Zn2+ stabilized the protein and increased solubility. SV31 was preferentially monomeric in detergent and revealed specific binding of Zn2+ . When co-translationally inserted into defined nanodisc bilayers, SV31 assembled into dimeric complexes, resulting in increased binding of Zn2+ . Putative Zn2+ -binding motifs within SV31 comprise aspartic acid and histidine residues. Site-directed mutagenesis of two conserved aspartic acid residues leads to a potent decrease in Zn2+ binding but did not affect dimerization. Chemical modification of histidine residues abolished some of the Zn2+ -binding capacity. We demonstrate proton-dependent transport of Zn2+ as by accumulation of fluorescent FluoZin-1 inside of SV31-containing proteoliposomes. Transport activity has a Km value of 44.3 µM and required external Zn2+ and internal acidic pH. Our results demonstrate that the synaptic vesicle-integral protein SV31 functions as a proton-dependent Zn2+ transporter. SV31 may attribute specific and yet undiscovered functions to subsets of synapses.

Pembrolizumab and maraviroc in refractory mismatch repair proficient/microsatellite-stable metastatic colorectal cancer - The PICCASSO phase I trial.

BACKGROUND: PD-1/PD-L1 inhibitors do not show activity in mismatch repair proficient (MMRp) colorectal cancer. Inhibition of C-C motif chemokine receptor 5 (CCR5) leads to an antitumoral activation of macrophages, affecting immune cell infiltrates. PICCASSO is a phase I trial exploring safety and efficacy of pembrolizumab and maraviroc in refractory MMRp CRC. METHODS: Twenty patients received pembrolizumab and maraviroc (core period, eight cycles), followed by pembrolizumab monotherapy. Primary endpoint was the feasibility rate (patients without treatment-related grade ≥3 immune-related adverse events, treatment-related grade ≥4 adverse events, or any toxicity-related premature withdrawal of treatment). Secondary endpoints included safety/toxicity, overall response rate (ORR), progression-free survival (PFS), and overall survival (OS). Optional biopsies of liver metastases were performed for analyses of the micromilieu. RESULTS: The feasibility rate was 94.7% [90% CI 77.4-99.7%], with one grade 4 hyperglycemia and no additional ≥ grade 3 treatment-related toxicities. ORR according to RECIST was 5.3%. Median PFS according to RECIST was 2.10 months [95%CI 1.68-2.30], median OS 9.83 months [95% CI, 5.59-20.02]. Disease control rate of poststudy salvage treatment was >70%. Translational analyses showed an increase of antitumoral chemokines during treatment; eotaxin, a chemokine involved in chemotaxis, was identified as a biomarker linked to OS. CONCLUSIONS: Therapy with pembrolizumab and maraviroc was feasible and showed a beneficial toxicity pattern. Clinical activity in MMRp CRC patients was limited with prolonged disease stabilizations observed in single patients. Efficacy of poststudy salvage treatment and OS was higher than expected in this heavily pretreated population. THIS TRIAL IS REGISTERED AT CLINICALTRIALS.GOV: NCT03274804.

PD-L1 targeting and subclonal immune escape mediated by PD-L1 mutations in metastatic colorectal cancer.

BACKGROUND: In patients with microsatellite stable (MSS) metastatic colorectal cancer (mCRC), immune checkpoint blockade is ineffective, and combinatorial approaches enhancing immunogenicity need exploration. METHODS: We treated 43 patients with predominantly microsatellite stable RAS/BRAF wild-type mCRC on a phase II trial combining chemotherapy with the epidermal growth factor receptor antibody cetuximab and the programmed cell death ligand 1 (PD-L1) antibody avelumab. We performed next-generation gene panel sequencing for mutational typing of tumors and liquid biopsy monitoring as well as digital droplet PCR to confirm individual mutations. Translational analyses included tissue immunohistochemistry, multispectral imaging and repertoire sequencing of tumor-infiltrating T cells. Detected PD-L1 mutations were mechanistically validated in CRISPR/Cas9-generated cell models using qRT-PCR, immunoblotting, flow cytometry, complement-dependent cytotoxicity assay, antibody-dependent cytotoxicity by natural killer cell degranulation assay and LDH release assay as well as live cell imaging of T cell mediated tumor cell killing. RESULTS: Circulating tumor DNA showed rapid clearance in the majority of patients mirroring a high rate of early tumor shrinkage. In 3 of 13 patients expressing the high-affinity Fcγ receptor 3a (FcγR3a), tumor subclones with PD-L1 mutations were selected that led to loss of tumor PD-L1 by nonsense-mediated RNA decay in PD-L1 K162fs and protein degradation in PD-L1 L88S. As a consequence, avelumab binding and antibody-dependent cytotoxicity were impaired, while T cell killing of these variant clones was increased. Interestingly, PD-L1 mutant subclones showed slow selection dynamics reversing on avelumab withdrawal and patients with such subclones had above-average treatment benefit. This suggested that the PD-L1 mutations mediated resistance to direct antitumor effects of avelumab, while at the same time loss of PD-L1 reduced biological fitness by enhanced T cell killing limiting subclonal expansion. CONCLUSION: The addition of avelumab to standard treatment appeared feasible and safe. PD-L1 mutations mediate subclonal immune escape to avelumab in some patients with mCRC expressing high-affinity FcγR3a, which may be a subset experiencing most selective pressure. Future trials evaluating the addition of avelumab to standard treatment in MSS mCRC are warranted especially in this patient subpopulation. TRIAL REGISTRATION NUMBER: NCT03174405.