Uptake of clostridium botulinum C3 exoenzyme into intact HT22 and J774A.1 cells

Active shrinkage protects neurons following axonal transection

Trauma, vascular events, or neurodegenerative processes can lead to axonal injury and eventual transection (axotomy). Neurons can survive axotomy, yet the underlying mechanisms are not fully understood. Excessive water entry into injured neurons poses a particular risk due to swelling and subsequent death. Using in vitro and in vivo neurotrauma model systems based on laser transection and surgical nerve cut, we demonstrated that axotomy triggers actomyosin contraction coupled with calpain activity. As a consequence, neurons shrink acutely to force water out through aquaporin channels preventing swelling and bursting. Inhibiting shrinkage increased the probability of neuronal cell death by about 3-fold. These studies reveal a previously unrecognized cytoprotective response mechanism to neurotrauma and offer a fresh perspective on pathophysiological processes in the nervous system.

Potentiation of Brain-Derived Neurotrophic Factor-Induced Protection of Spiral Ganglion Neurons by C3 Exoenzyme/Rho Inhibitor

Preservation of the excitability of spiral ganglion neurons (SGN) may contribute to an improved speech perception after cochlear implantation. Thus, the application of exogenous neurotrophic factors such as the neurotrophin brain-derived neurotrophic factor (BDNF) to increase SGN survival in vitro and in vivo is a promising pharmacological approach in cochlear implant (CI) research. Due to the difficult pharmacokinetic profile of proteins such as BDNF, there is a quest for small molecules to mediate the survival of SGN or to increase the efficacy of BDNF. The C3 exoenzyme from Clostridium botulinum could be a potential new candidate for the protection and regeneration of SGN. Inhibition of the RhoA GTPase pathway which can be mediated by C3 is described as a promising strategy to enhance axonal regeneration and to exert pro-survival signals in neurons. Nanomolar concentrations of C3, its enzymatically inactive form C3^E174Q, and a 26mer C-terminal peptide fragment covering amino acid 156–181 (C3^156-181) potentiated the neuroprotective effect on SGN mediated by BDNF in vitro. The neuroprotective effect of C3/BDNF was reduced to the neuroprotective effect of BDNF alone after the treatment with wortmannin, an inhibitor of the phosphatidylinositol-3-kinase (PI3K).The exoenzyme C3 (wild-type and enzyme-deficient) and the C3 peptide fragment C3^154–181 present novel biologically active compounds for the protection of the SGN. The exact underlying intracellular mechanisms that mediate the neuroprotective effect are not clarified yet, but the combination of BDNF (TrkB stimulation) and C3 exoenzyme (RhoA inhibition) can be used to protect SGN in vitro.

The Higher Sensitivity of GABAergic Compared to Glutamatergic Neurons to Growth-Promoting C3bot Treatment Is Mediated by Vimentin

The current study investigates the neurotrophic effects of Clostridium botulinum C3 transferase (C3bot) on highly purified, glia-free, GABAergic, and glutamatergic neurons. Incubation with nanomolar concentrations of C3bot promotes dendrite formation as well as dendritic and axonal outgrowth in rat GABAergic neurons. A comparison of C3bot effects on sorted mouse GABAergic and glutamatergic neurons obtained from newly established NexCre;Ai9xVGAT Venus mice revealed a higher sensitivity of GABAergic cells to axonotrophic and dendritic effects of C3bot in terms of process length and branch formation. Protein biochemical analysis of known C3bot binding partners revealed comparable amounts of β1 integrin in both cell types but a higher expression of vimentin in GABAergic neurons. Accordingly, binding of C3bot to GABAergic neurons was stronger than binding to glutamatergic neurons. A combinatory treatment of glutamatergic neurons with C3bot and vimentin raised the amount of bound C3bot to levels comparable to the ones in GABAergic neurons, thereby confirming the specificity of effects. Overall, different surface vimentin levels between GABAergic and glutamatergic neurons exist that mediate neurotrophic C3bot effects.

A New Pathway Promotes Adaptation of Human Glioblastoma Cells to Glucose Starvation

Adaptation of glioblastoma to caloric restriction induces compensatory changes in tumor metabolism that are incompletely known. Here we show that in human glioblastoma cells maintained in exhausted medium, SHC adaptor protein 3 (SHC3) increases due to down-regulation of SHC3 protein degradation. This effect is reversed by glucose addition and is not present in normal astrocytes. Increased SHC3 levels are associated to increased glucose uptake mediated by changes in membrane trafficking of glucose transporters of the solute carrier 2A superfamily (GLUT/SLC2A). We found that the effects on vesicle trafficking are mediated by SHC3 interactions with adaptor protein complex 1 and 2 (AP), BMP-2-inducible protein kinase and a fraction of poly ADP-ribose polymerase 1 (PARP1) associated to vesicles containing GLUT/SLC2As. In glioblastoma cells, PARP1 inhibitor veliparib mimics glucose starvation in enhancing glucose uptake. Furthermore, cytosol extracted from glioblastoma cells inhibits PARP1 enzymatic activity in vitro while immunodepletion of SHC3 from the cytosol significantly relieves this inhibition. The identification of a new pathway controlling glucose uptake in high grade gliomas represents an opportunity for repositioning existing drugs and designing new ones.

Supramolecular Toxin Complexes for Targeted Pharmacological Modulation of Polymorphonuclear Leukocyte Functions

The targeted pharmacological modulation of polymorphonuclear leukocytes (PMNs) is of major medical interest. These innate immune cells play a central role in the defense against pathogenic microorganisms. However, their excessive chemotactic recruitment into tissues after traumatic injury is detrimental due to local and systemic inflammation. Rho-GTPases, being the master regulators of the actin cytoskeleton, regulate migration and chemotaxis of PMNs, are attractive pharmacological targets. Herein, supramolecular protein complexes are assembled in a "mix-and-match" approach containing the specific Rho-inhibiting clostridial C3 enzyme and three PMN-binding peptides using an avidin platform. Selective delivery of the C3 Rho-inhibitor with these complexes into the cytosol of human neutrophil-like NB-4 cells and primary human PMNs ex vivo is demonstrated, where they catalyze the adenosine diphosphate (ADP) ribosylation of Rho and induce a characteristic change in cell morphology. Notably, the complexes do not deliver C3 enzyme into human lung epithelial cells, A549 lung cancer cells, and immortalized human alveolar epithelial cells (hAELVi), demonstrating their cell type-selectivity. The supramolecular complexes represent attractive molecular tools to decipher the role of PMNs in infection and inflammation or for the development of novel therapeutic approaches for diseases that are associated with hyperactivity and reactivity of PMNs such as post-traumatic injury.

Release of astroglial vimentin by extracellular vesicles: Modulation of binding and internalization of C3 transferase in astrocytes and neurons

Clostridium botulinum C3 transferase (C3bot) ADP-ribosylates rho proteins to change cellular functions in a variety of cell types including astrocytes and neurons. The intermediate filament protein vimentin as well as transmembrane integrins are involved in internalization of C3bot into cells. The exact contribution, however, of these proteins to binding of C3bot to the cell surface and subsequent cellular uptake remains to be unraveled. By comparing primary astrocyte cultures derived from wild-type with Vim^-/- mice, we demonstrate that astrocytes lacking vimentin exhibited a delayed ADP-ribosylation of rhoA concurrent with a blunted morphological response. This functional impairment was rescued by the extracellular excess of recombinant vimentin. Binding assays using C3bot harboring a mutated integrin-binding RGD motif (C3bot-G89I) revealed the involvement of integrins in astrocyte binding of C3bot. Axonotrophic effects of C3bot are vimentin dependent and postulate an underlying mechanism entertaining a molecular cross-talk between astrocytes and neurons. We present functional evidence for astrocytic release of vimentin by exosomes using an in vitro scratch wound model. Exosomal vimentin+ particles released from wild-type astrocytes promote the interaction of C3bot with neuronal membranes. This effect vanished when culturing Vim^-/- astrocytes. Specificity of these findings was confirmed by recombinant vimentin propagating enhanced binding of C3bot to synaptosomes from rat spinal cord and mouse brain. We hypothesize that vimentin+ exosomes released by reactive astrocytes provide a novel molecular mechanism constituting axonotrophic (neuroprotective) and plasticity augmenting effects of C3bot after spinal cord injury.

P300 Acetyltransferase Mediates Stiffness-Induced Activation of Hepatic Stellate Cells Into Tumor-Promoting Myofibroblasts

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) contribute to desmoplasia and stiffness of liver metastases by differentiating into matrix-producing myofibroblasts. We investigated whether stiffness due to the presence of tumors increases activation of HSCs into myofibroblasts and their tumor-promoting effects, as well as the role of E1A binding protein p300, a histone acetyltransferase that regulates transcription, in these processes.

METHODS

HSCs were isolated from liver tissues of patients, mice in which the p300 gene was flanked by 2 loxP sites (p300F/F mice), and p300+/+ mice (controls). The HSCs were placed on polyacrylamide gels with precisely defined stiffness, and their activation (differentiation into myofibroblasts) was assessed by immunofluorescence and immunoblot analyses for alpha-smooth muscle actin. In HSCs from mice, the p300 gene was disrupted by cre recombinase. In human HSCs, levels of p300 were knocked down with small hairpin RNAs or a mutant form of p300 that is not phosphorylated by AKT (p300S1834A) was overexpressed. Human HSCs were also cultured with inhibitors of p300 (C646), PI3K signaling to AKT (LY294002), or RHOA (C3 transferase) and effects on stiffness-induced activation were measured. RNA sequencing and chromatin immunoprecipitation-quantitative polymerase chain reaction were used to identify HSC genes that changed expression levels in response to stiffness. We measured effects of HSC-conditioned media on proliferation of HT29 colon cancer cells and growth of tumors following subcutaneous injection of these cells into mice. MC38 colon cancer cells were injected into portal veins of p300F/Fcre and control mice, and liver metastases were measured. p300F/Fcre and control mice were given intraperitoneal injections of CCl₄ to induce liver fibrosis. Liver tissues were collected and analyzed by immunofluorescence, immunoblot, and histology.

RESULTS

Substrate stiffness was sufficient to activate HSCs, leading to nuclear accumulation of p300. Disrupting p300 level or activity blocked stiffness-induced activation of HSCs. In HSCs, substrate stiffness activated AKT signaling via RHOA to induce phosphorylation of p300 at serine 1834; this caused p300 to translocate to the nucleus, where it up-regulated transcription of genes that increase activation of HSCs and metastasis, including CXCL12. MC38 cells, injected into portal veins, formed fewer metastases in livers of p300F/Fcre mice than control mice. Expression of p300 was increased in livers of mice following injection of CCl4; HSC activation and collagen deposition were reduced in livers of p300F/Fcre mice compared with control mice.

CONCLUSIONS

In studies of mice, we found liver stiffness to activate HSC differentiation into myofibroblasts, which required nuclear accumulation of p300. p300 increases HSC expression of genes that promote metastasis.

The Binary Toxin CDT of Clostridium difficile as a Tool for Intracellular Delivery of Bacterial Glucosyltransferase Domains

Binary toxins are produced by several pathogenic bacteria. Examples are the C2 toxin from Clostridium botulinum, the iota toxin from Clostridium perfringens, and the CDT from Clostridium difficile. All these binary toxins have ADP-ribosyltransferases (ADPRT) as their enzymatically active component that modify monomeric actin in their target cells. The binary C2 toxin was intensively described as a tool for intracellular delivery of allogenic ADPRTs. Here, we firstly describe the binary toxin CDT from C. difficile as an effective tool for heterologous intracellular delivery. Even 60 kDa glucosyltransferase domains of large clostridial glucosyltransferases can be delivered into cells. The glucosyltransferase domains of five tested large clostridial glucosyltransferases were successfully introduced into cells as chimeric fusions to the CDTa adapter domain (CDTaN). Cell uptake was demonstrated by the analysis of cell morphology, cytoskeleton staining, and intracellular substrate glucosylation. The fusion toxins were functional only when the adapter domain of CDTa was N-terminally located, according to its native orientation. Thus, like other binary toxins, the CDTaN/b system can be used for standardized delivery systems not only for bacterial ADPRTs but also for a variety of bacterial glucosyltransferase domains.

Rho-inhibiting C2IN-C3 fusion toxin inhibits chemotactic recruitment of human monocytes ex vivo and in mice in vivo

Bacterial protein toxins became valuable molecular tools for the targeted modulation of cell functions in experimental pharmacology and attractive therapeutics because of their potent and specific mode of action in human cells. C2IN-C3lim, a recombinant fusion toxin (~50 kDa) of the Rho-inhibiting C3lim from Clostridium (C.) limosum and a non-toxic portion of the C. botulinum C2 toxin (C2IN), is selectively internalized into the cytosol of monocytic cells where C3lim specifically ADP-ribosylates Rho A and -B, thereby inhibiting Rho-mediated signaling. Thus, we hypothesized that these unique features make C2IN-C3lim an attractive molecule for the targeted pharmacological down-regulation of Rho-mediated functions in monocytes. The analysis of the actin structure and the Rho ADP-ribosylation status implied that C2IN-C3lim entered the cytosol of primary human monocytes from healthy donors ex vivo within 1 h. Moreover, it inhibited the fMLP-induced chemotaxis of human monocytes in a Boyden chamber model ex vivo. Similarly, in a 3-dimensional ex vivo model of extravasation, single cell analysis revealed that C2IN-C3lim-treated cells were not able to move. In a clinically relevant mouse model of blunt chest trauma, the local application of C2IN-C3lim into the lungs after thorax trauma prevented the trauma-induced recruitment of monocytes into the lungs in vivo. Thus, C2IN-C3lim might be an attractive lead compound for novel pharmacological strategies to avoid the cellular damage response caused by monocytes in damaged tissue after trauma and during systemic inflammation. The results suggest that the pathophysiological role of clostridial C3 toxins might be a down-modulation of the innate immune system.

The Rho ADP-ribosylating C3 exoenzyme binds cells via an Arg-Gly-Asp motif

The Rho ADP-ribosylating C3 exoenzyme (C3bot) is a bacterial protein toxin devoid of a cell-binding or -translocation domain. Nevertheless, C3 can efficiently enter intact cells, including neurons, but the mechanism of C3 binding and uptake is not yet understood. Previously, we identified the intermediate filament vimentin as an extracellular membranous interaction partner of C3. However, uptake of C3 into cells still occurs (although reduced) in the absence of vimentin, indicating involvement of an additional host cell receptor. C3 harbors an Arg–Gly–Asp (RGD) motif, which is the major integrin-binding site, present in a variety of integrin ligands. To check whether the RGD motif of C3 is involved in binding to cells, we performed a competition assay with C3 and RGD peptide or with a monoclonal antibody binding to β1-integrin subunit and binding assays in different cell lines, primary neurons, and synaptosomes with C3-RGD mutants. Here, we report that preincubation of cells with the GRGDNP peptide strongly reduced C3 binding to cells. Moreover, mutation of the RGD motif reduced C3 binding to intact cells and also to recombinant vimentin. Anti-integrin antibodies also lowered the C3 binding to cells. Our results indicate that the RGD motif of C3 is at least one essential C3 motif for binding to host cells and that integrin is an additional receptor for C3 besides vimentin.

The intermediate filament protein vimentin is essential for axonotrophic effects of Clostridium botulinum C3 exoenzyme

The type III intermediate filament protein vimentin was recently identified to mediate binding and uptake of Clostridium botulinum C3 exoenzyme (C3bot) in two cell lines. Here, we used primary neuronal cultures from vimentin knockout (Vim^-/- ) mice to study the impact of vimentin on axonal growth and internalization of C3bot. In contrast to wild type, vimentin knockout neurons were insensitive to C3bot. Application of extracellular vimentin to Vim^-/- neurons completely restored the growth-promoting effects of C3bot. In line with this uptake of C3bot into Vim^-/- neurons was strongly decreased resulting in reduced ADP-ribosylation of RhoA and B as detected by an antibody recognizing selectively ADP-ribosylated RhoA/B. Again, uptake of C3bot into Vim^-/- neurons was rescued by addition of extracellular vimentin. In addition, in purified embryonic stem cell-derived motor neurons that are devoid of glial cells C3bot elicited axonotrophic effects confining neuronal vimentin as a binding partner. Primary neuronal cultures from vimentin knockout (KO) mice were used to study the impact of vimentin on axonal growth and internalization of C3bot. In contrast to wild type, vimentin knockout neurons were insensitive to the axonotrophic effects of C3bot. Application of extracellular vimentin (recombinant vimentin) to vimentin KO neurons completely restored the growth-promoting effects of C3bot. In line with this uptake of C3bot into vimentin KO neurons was strongly decreased resulting in reduced ADP-ribosylation of RhoA and B as detected by an antibody recognizing selectively ADP-ribosylated RhoA/B.

C3 exoenzyme impairs cell proliferation and apoptosis by altering the activity of transcription factors

C3 exoenzyme from C. botulinum is an ADP-ribosyltransferase that inactivates selectively RhoA, B, and C by coupling an ADP-ribose moiety. Rho-GTPases are involved in various cellular processes, such as regulation of actin cytoskeleton, cell proliferation, and apoptosis. Previous studies of our group with the murine hippocampal cell line HT22 revealed a C3-mediated inhibition of cell proliferation after 48 h and a prevention of serum-starved cells from apoptosis. For both effects, alterations of various signaling pathways are already known, including also changes on the transcriptional level. Investigations on the transcriptional activity in HT22 cells treated with C3 for 48 h identified five out of 48 transcription factors namely Sp1, ATF2, E2F-1, CBF, and Stat6 with a significantly regulated activity. For validation of identified transcription factors, studies on the protein level of certain target genes were performed. Western blot analyses exhibited an enhanced abundance of Sp1 target genes p21 and COX-2 as well as an increase in phosphorylation of c-Jun. In contrast, the level of p53 and apoptosis-inducing GADD153, a target gene of ATF2, was decreased. Our results reveal that C3 regulates the transcriptional activity of Sp1 and ATF2 resulting downstream in an altered protein abundance of various target genes. As the affected proteins are involved in the regulation of cell proliferation and apoptosis, thus the C3-mediated anti-proliferative and anti-apoptotic effects are consequences of the Rho-dependent alterations of the activity of certain transcriptional factors.

Proteome Alterations of Hippocampal Cells Caused by Clostridium botulinum C3 Exoenzyme

C3bot from Clostridium botulinum is a bacterial mono-ADP-ribosylating enzyme, which transfers an ADP-ribose moiety onto the small GTPases Rho A/B/C. C3bot and the catalytic inactive mutant (C3E174Q) cause axonal and dendritic growth as well as branching in primary hippocampal neurons. In cultured murine hippocampal HT22 cells, protein abundances were analyzed in response to C3bot or C3E174Q treatment using a shotgun proteomics approach. Proteome analyses were performed at four time points over 6 days. More than 4000 protein groups were identified at each time point and quantified in triplicate analyses. On day one, 46 proteins showed an altered abundance, and after 6 days, more than 700 proteins responded to C3bot with an up- or down-regulation. In contrast, C3E174Q had no provable impact on protein abundance. Protein quantification was verified for several proteins by multiple reaction monitoring. Data analysis of altered proteins revealed different cellular processes that were affected by C3bot. They are particularly involved in mitochondrial and lysosomal processes, adhesion, carbohydrate and glucose metabolism, signal transduction, and nuclear proteins of translation and ribosome biogenesis. The results of this study gain novel insights into the function of C3bot in hippocampal cells.