Destruxin E decreases Beta-amyloid generation by reducing colocalization of beta-amyloid-cleaving enzyme 1 and beta-amyloid protein precursor

Characteristics of membrane transport, metabolism, and target protein binding of cyclic depsipeptide destruxin E in HeLa cells

Cyclic peptides have attracted attention as new modalities for drug development owing to their unique pharmacokinetic and pharmacodynamic properties. Destruxin E, a 19-membered cyclodepsipeptide, is a promising candidate drug for cancer therapy. The purpose of the present study was to clarify the molecular mechanisms underlying membrane transport, metabolism, and the binding for target molecules of destruxin E in human cervical carcinoma HeLa cells used as a model of cancer cells. The influx transport and the intracellular metabolism of destruxin E were non-saturable and saturable, respectively, at up to 10 μM. The intracellular amounts of destruxin E and destruxin E-diol after incubation of destruxin E with the cells significantly decreased at 4 °C compared to those at 37 °C. Destruxin E-diol, but not destruxin E, undergoes efflux transport out of cells via P-gp/MDR1/ABCB1 and BCRP/ABCG2. The epoxide hydrolase EPHX2 functions as a potent metabolizing enzyme that can convert the epoxide of destruxin E to the destruxin E-diol. Treatment with an EPHX2 inhibitor increased the intracellular destruxin E levels and enhanced the inhibitory activity of vacuolar type-H+ ATPase. These results suggest that epoxide hydrolase could be a regulatory factor for intracellular destruxin E levels and its pharmacological activity.

Identification of the oosporein biosynthesis gene cluster in an entomopathogenic fungus Blackwellomyces cardinalis

Blackwellomyces cardinalis (≡ Cordyceps cardinalis) is an entomopathogenic fungus that hosts lepidopteran insect larvae. Oosporein, produced by Bl. cardinalis, is a red secondary metabolite that is also produced by other entomopathogens and is known to contribute to entomopathogenic activity. In this study, a homologous region of the oosporein biosynthesis gene cluster (BcOpS cluster) was found from the genome sequence of Bl. cardinalis strain NBRC 103832. Within the cluster, a putative transcription factor gene BcOpS3 was deleted by homologous recombination. The deletion strain (ΔBcOpS3) did not produce oosporein. Real-time qPCR analysis showed that the expression of all genes was either lost or greatly reduced compared to the wild type strain (WT). Infection assay using silkworms showed that the virulence of the ΔBcOpS3 strain was not different from that of the WT strain. We compared the expression levels of antimicrobial peptide genes in silkworm infected with these strains, and found that the increased expression of the cecA gene in WT was not observed in the ΔBcOpS3 strain, suggesting that the immune response of the silkworm was altered.

Endosomal Acid-Base Homeostasis in Neurodegenerative Diseases

Neurodegenerative disorders are debilitating and largely untreatable conditions that pose a significant burden to affected individuals and caregivers. Overwhelming evidence supports a crucial preclinical role for endosomal dysfunction as an upstream pathogenic hub and driver in Alzheimer's disease (AD) and related neurodegenerative disorders. We present recent advances on the role of endosomal acid-base homeostasis in neurodegeneration and discuss evidence for converging mechanisms. The strongest genetic risk factor in sporadic AD is the ε4 allele of Apolipoprotein E (ApoE4), which potentiates pre-symptomatic endosomal dysfunction and prominent amyloid beta (Aβ) pathology, although how these pathways are linked mechanistically has remained unclear. There is emerging evidence that the Christianson syndrome protein NHE6 is a prominent ApoE4 effector linking endosomal function to Aβ pathologies. By functioning as a dominant leak pathway for protons, the Na⁺/H⁺ exchanger activity of NHE6 limits endosomal acidification and regulates β-secretase (BACE)-mediated Aβ production and LRP1 receptor-mediated Aβ clearance. Pathological endosomal acidification may impact both Aβ generation and clearance mechanisms and emerges as a promising therapeutic target in AD. We also offer our perspective on the complex role of endosomal acid-base homeostasis in the pathogenesis of neurodegeneration and its therapeutic implications for neuronal rescue and repair strategies.

Altered membrane rigidity via enhanced endogenous cholesterol synthesis drives cancer cell resistance to destruxins

Destruxins, secondary metabolites of entomopathogenic fungi, exert a wide variety of interesting characteristics ranging from antiviral to anticancer effects. Although their mode of action was evaluated previously, the molecular mechanisms of resistance development are unknown. Hence, we have established destruxin-resistant sublines of HCT116 colon carcinoma cells by selection with the most prevalent derivatives, destruxin (dtx)A, dtxB and dtxE. Various cell biological and molecular techniques were applied to elucidate the regulatory mechanisms underlying these acquired and highly stable destruxin resistance phenotypes. Interestingly, well-known chemoresistance-mediating ABC efflux transporters were not the major players. Instead, in dtxA- and dtxB-resistant cells a hyper-activated mevalonate pathway was uncovered resulting in increased de-novo cholesterol synthesis rates and elevated levels of lanosterol, cholesterol as well as several oxysterol metabolites. Accordingly, inhibition of the mevalonate pathway at two different steps, using either statins or zoledronic acid, significantly reduced acquired but also intrinsic destruxin resistance. Vice versa, cholesterol supplementation protected destruxin-sensitive cells against their cytotoxic activity. Additionally, an increased cell membrane adhesiveness of dtxA-resistant as compared to parental cells was detected by atomic force microscopy. This was paralleled by a dramatically reduced ionophoric capacity of dtxA in resistant cells when cultured in absence but not in presence of statins. Summarizing, our results suggest a reduced ionophoric activity of destruxins due to cholesterol-mediated plasma membrane re-organization as molecular mechanism underlying acquired destruxin resistance in human colon cancer cells. Whether this mechanism might be valid also in other cell types and organisms exposed to destruxins e.g. as bio-insecticides needs to be evaluated.

The Na+/H+ exchanger NHE6 modulates endosomal pH to control processing of amyloid precursor protein in a cell culture model of Alzheimer disease

Early intervention may be key to safe and effective therapies in patients with Alzheimer disease. Endosomal dysfunction is an early step in neurodegeneration. Endosomes are a major site of production of Aβ peptide from the processing of amyloid precursor protein (APP) by clipping enzymes (β- and γ-secretases). The β-secretase enzyme BACE1 requires acidic lumen pH for optimum function, and acid pH promotes Aβ aggregation. The Na(+)/H(+) exchanger NHE6 provides a leak pathway for protons, limiting luminal acidification by proton pumps. Like APP, NHE6 expression was induced upon differentiation of SH-SY5Y neuroblastoma cells and localized to an endosomal compartment. Therefore, we investigated whether NHE6 expression altered APP localization and processing in a stably transfected cell culture model of human APP expression. We show that co-expression with NHE6 or treatment with the Na(+)/H(+) ionophore monensin shifted APP away from the trans-Golgi network into early and recycling endosomes in HEK293 cells. NHE6 alkalinized the endosomal lumen, similar to monensin, and significantly attenuated APP processing and Aβ secretion. In contrast, Aβ production was elevated upon NHE6 knockdown. We show that NHE6 transcript and protein levels are lowered in Alzheimer brains relative to control. These findings, taken together with emerging genetic evidence linking endosomal Na(+)/H(+) exchangers with Alzheimer disease, suggest that proton leak pathways may regulate Aβ generation and contribute to disease etiology.

Unveiling the biosynthetic puzzle of destruxins in Metarhizium species

Insect pathogenic fungi produce a plethora of insecticidally and pharmaceutically active compounds, including 39 cyclohexadepsipeptide destruxins (dtxs). Even though dtxs were first discovered more than 50 y ago, the genes responsible for their biosynthesis were unknown until this study. Based on our comparative genomic information and targeted gene disruptions, we report the gene cluster for dtx biosynthesis in the insect pathogen Metarhizium robertsii. The nonribosomal peptide synthetase DtxS1 has six adenylation domains, two of which are capable of selecting different amino acids to synthesize dtx B and its analogs. The cytochrome P450 enzyme DtxS2 converts dtx B into other dtxs by a chain of reactions, each producing a new derivative. The aldo-keto reductase DtxS3 and aspartic acid decarboxylase DtxS4 are responsible for the conversion and provision of the first and last substrates for the dtx assembly line, respectively. Insect bioassays showed that dtxs could suppress both cellular and humoral immune responses thereby assisting fungal propagation in insects. The differing abilities of Metarhizium species to produce toxins is dependent on the presence of the dtxS1 gene. The toxigenic species are capable of killing multiple orders of insects, whereas the nontoxigenic Metarhizium spp. have narrow host ranges. Thus, the acquisition or retention of the dtx biosynthesis gene cluster in Metarhizium lineages has been coordinated with the evolution of fungal host specificity. The data from this study will facilitate the development of dtxs as bioinsecticides or pharmaceuticals.

Development and applications of destruxins: a review

The insecticidal and phytotoxic activities of destruxins (dtxs) have been well studied. The cyclodepsipeptides, which are dtxs mainly isolated from the fungus Metarhizium anisopliae and other fungi, have been well characterized in vitro and in vivo. A succession of important function, such as antitumoral, antiviral, insecticidal, cytotoxic, immunosuppressant, phytotoxic, and antiproliferative effects have been observed. To date, 39 dtxs derivatives have been identified. Dtxs possess a variety of biological activities, including acting as virulence factors for specific insects, a V-ATPase inhibitor that provides a basis for the development of new drug to against osteoporosis, cancer, or biological control agents, etc. Here, we focus on some of the research progress made on understanding dtxs during the last decade, introduce some of the newly identified dtx members, especially from M. anisopliae, and give an overview of the applications of dtxs. Using the dtxs to learn about and moderate biological events has advanced significantly during the past year. We believe that several ongoing dtx application fields may benefit from the reviewed information herein.

Synthesis, structure determination, and biological evaluation of destruxin E

The total synthesis of destruxin E (1) has been achieved for the first time, and the stereochemistry of its chiral center at the epoxide has been determined to be (S). The cyclization precursor 3a was synthesized by solid-phase peptide synthesis. Macrolactonization of 3a utilizing MNBA-DMAPO, followed by formation of the epoxide, then furnished destruxin E. Its diastereomer, epi-destruxin E (2), was also synthesized in the same manner. Furthermore, the biological evaluation indicated that destruxin E exhibits V-ATPase inhibitory activity 10-fold greater than that of epi-destruxin E.

Neurite-like structures induced by mevalonate pathway blockade are due to the stability of cell adhesion foci and are enhanced by the presence of APP

Epidemiological studies have shown an association between statin use and a decreased risk of dementia. However, the mechanism by which this beneficial effect is brought about is unclear. In the context of Alzheimer's disease, at least three possibilities have been studied; reduction in amyloid beta peptide (Abeta) production, the promotion of alpha-secretase cleavage and positive effects on neurite outgrowth. By investigating the effects of mevalonate pathway blockade on neurite outgrowth using real-time imaging, we found that rather than promote the production of neurite extensions, inhibition rapidly induced cell rounding. Crucially, neurite-like structures were generated through the persistence of cell-cell and cell-substrate adhesions and not through a mechanism of positive outgrowth. This effect can be strikingly enhanced by the over-expression of human amyloid precursor protein and is isoprenoid rather than cholesterol dependent.