The channel-forming toxin aerolysin

Electron microscopy as a critical tool in the determination of pore forming mechanisms in proteins

Electron microscopy has consistently played an important role in the description of pore-forming protein systems. The discovery of pore-forming proteins has depended on visualization of the structural pores formed by their oligomeric protein complexes, and as electron microscopy has advanced technologically so has the degree of insight it has been able to give. This review considers a large number of published studies of pore-forming complexes in prepore and pore states determined using single-particle cryo-electron microscopy. Sample isolation and preparation, imaging and image analysis, structure determination and optimization of results are all discussed alongside challenges which pore-forming proteins particularly present. The review also considers the use made of cryo-electron tomography to study pores within their membrane environment and which will prove an increasingly important approach for the future.

Microparticle Encapsulation of a Prostate-targeted Biologic for the Treatment of Liver Metastases in a Preclinical Model of Castration-resistant Prostate Cancer

PRX302 is a highly potent, mutant bacterial pore-forming biologic protoxin engineered for selective activation by PSA, a serine protease expressed by benign and malignant prostate epithelial cells. Although being developed as a local therapy for benign prostatic hyperplasia and localized prostate cancer, PRX302 cannot be administered systemically as a treatment for metastatic disease due to binding to ubiquitously expressed glycosylphosphatidylinositol (GPI)-anchored proteins, which leads to poor accumulation within the tumor microenvironment. To overcome this limitation, poly-lactic-co-glycolic acid (PLGA) microparticles encapsulating the protoxin were developed, which are known to accumulate in the liver, a major site of metastasis for prostate cancer and other solid tumors. A highly sensitive and reproducible sandwich ELISA to quantify PRX302 released from microparticles was developed. Utilizing this assay, PRX302 release from different microparticle formulations was assessed over multiple days. Hemolysis assays documented PSA-dependent pore formation and lytic potential (i.e., function) of the released protoxin. MTT assays demonstrated that conditioned supernatant from PRX302-loaded, but not blank (i.e., unloaded), PLGA microparticles was highly cytotoxic to PC3 and DU145 human prostate cancer cells in the presence of exogenous PSA. Microparticle encapsulation prevented PRX302 from immediately interacting with GPI-anchored proteins as demonstrated in a competition assay, which resulted in an increased therapeutic index and significant antitumor efficacy following a single dose of PRX302-loaded microparticles in a preclinical model of prostate cancer liver metastasis with no obvious toxicity. These results document that PRX302 released from PLGA microparticles demonstrate in vivo antitumor efficacy in a clinically relevant preclinical model of metastatic prostate cancer.

An IL-2 proaerolysin fusion toxin that selectively eliminates regulatory t cells to enhance antitumor immune response

BACKGROUND

Recent success with immune-checkpoint inhibitors in some tumor types has highlighted the power of the immune system to control and eradicate human cancer cells. However, these therapies have demonstrated a limited activity in prostate cancer, which has a more immunosuppressive microenvironment that can be because of the presence of a variety of inhibitory cell types, such as myeloid-derived suppressor cells, mesenchymal stem cells, and regulatory T cells (Tregs). One strategy to improve the efficacy of immune-based therapies for prostate cancer is to selectively eliminate these immunosuppressive cells within the tumor microenvironment.

METHODS

We developed and characterized a chimeric protein consisting of the cytokine IL-2 fused to binding mutant of the highly toxic bacterial toxin proaerolysin (ie IL2-R336A).

RESULTS

The IL2-R336A fusion protein selectively kills immunosuppressive Tregs that express the IL-2 receptor while having little to no effect on cells negative for this target. IL2-R336A depleted Tregs in both tumor bearing and nontumor bearing mice. Tumor bearing mice vaccinated with a GMCSF-expressing CT-26 GVAX vaccine had reduced tumor growth when given IL2-R336A before vaccination. IL2-R336A also enhanced immune response to a model hemagglutinin antigen (HA) in HA-tolerized mice.

CONCLUSION

These results suggest that this IL2-R336A toxin may be a useful in improving the therapeutic efficacy of antitumor vaccines by enhancing the immune response against target tumor antigens.

[Minimally invasive treatment of benign prostatic hyperplasia]

Novel minimally invasive treatment options for the management of male lower urinary tract symptoms (LUTS) due to bladder outlet obstruction (BOO) aim to provide equal efficacy compared to standard techniques with a more favourable safety profile. The preservation of all aspects of male sexual function including antegrade ejaculation is increasingly important to patients. It should be ideally performed in an outpatient setting under local anesthesia to assure a short recovery time. Novel injectables for intraprostatic application (botulinum neurotoxin A, NX-1207, PRX302) have emerged and first phase III results could not confirm the promising initial data. For mechanical devices like Urolift® the early and mid-term benefits demonstrate a rapid and durable symptom relief without compromising sexual function. Novel innovative procedures like aquablation (AquaBeam®) are entering the scene, but their feasibility, efficacy and safety still need to be addressed in randomized controlled trials.

New intraprostatic injectables and prostatic urethral lift for male LUTS

Treatment modalities for male lower urinary tract symptoms (LUTS) comprise a broad spectrum of medical and surgical options. Interest is growing in minimally invasive treatment options, which should ideally be performed in an outpatient setting and have a short recovery time, durable efficacy and a good safety profile. The preservation of all aspects of sexual function, including antegrade ejaculation, seems to be increasingly important for patients. Initial experimental data on new minimally invasive procedures-such as the intraprostatic injection of novel agents including botulinum neurotoxin A (BoNT-A), NX-1207 and PRX302-were promising, but clinical trials have not confirmed the findings. Trials of the mechanical prostatic urethral lift device-Urolift(®) (Neotract, Inc., USA)-have been positive, but further long-term results are needed to confirm its beneficial effects.

Recombinant prostate-specific antigen proaerolysin shows selective protease sensitivity and cell cytotoxicity

Native proaerolysin is a channel-forming bacterial protoxin that binds to cell-surface receptors and then is activated by furin or furin-like proteases. We genetically engineered proaerolysin by replacing the furin-cleavage sequence with a prostate-specific antigen-selective sequence. The recombinant modified proaerolysin was expressed and purified from Aeromonas salmonicida in good yields and purity. Recombinant modified proaerolysin had no furin sensitivity and markedly increased prostate-specific antigen sensitivity relative to wild-type proaerolysin. Human prostate cancer cells were significantly more sensitive to recombinant modified proaerolysin in the presence of active prostate-specific antigen when compared with the absence of prostate-specific antigen or the presence of potent prostate-specific antigen inhibitors. Most normal human cells with the exception of prostate and renal epithelial cells showed very low sensitivity to recombinant modified proaerolysin. Our results suggest that recombinant modified proaerolysin is a potent prostate-specific antigen-sensitive protoxin that deserves further development for regional therapy of benign and malignant prostate growths.

Pore-forming Pseudomonas aeruginosa cytotoxin

Pseudomonas aeruginosa procytotoxin protein is processed C-terminally during bacterial autolysis to generate the active 29-kDa cytotoxin molecule. Binding to target cell membranes is dependent upon Cys23 and Cys215 and a domain flanked to Cys215. On rabbit erythrocytes, cytotoxin binds to a 28-kDa peptide of a glycoprotein, its N-terminus shows high homology to channel integral membrane protein CHIP28. At concentrations of more than 3 x 10(-9) M, cytotoxin increases plasma membrane permeability of most eucaryotic cells investigated. The role of cytotoxin in the formation of pores with a diameter of 2 nm on mammalian cells is discussed. The cytotoxin effects are coordinated with other pseudomonal products and the resultant concept of pathogenesis is presented.

Structural features of the pore formed by Staphylococcus aureus alpha-toxin inferred from chemical modification and primary structure analysis

Staphylococcus aureus alpha-toxin makes cells and model membranes permeable to ions and uncharged molecules by opening oligomeric pores of uniform size. Its primary sequence reveals peculiar features which give some hints on the structure of the pore. A flexible region separating the toxin into two halves, several amphiphilic beta-strands and two amphiphilic alpha-helices long enough to span the hydrophobic core of the lipid bilayer are predicted. In analogy to bacterial porins, we propose that the inner walls of the pore are, at least in part, built by an amphiphilic beta-barrel. The model is consistent with circular dichroism data and with the electrophysiological properties of the pore. Functional information on this toxin were obtained by chemical modification of its four histidine residues. Specific carbethoxylation suggested they have different roles: one is required for specific receptor binding, one for oligomerisation and two for unspecific lipid binding. A tentative assignment of each histidine to its specific role is done on the basis of the structural predictions. A functionally related hemolysin, Aeromonas hydrophyla aerolysin, reveals remarkably similar features including the presence and location of histidines involved in receptor binding and oligomerisation.