Structural insight into the stabilization of microtubules by taxanes

Paclitaxel (Taxol^®) is a taxane and a first-line chemotherapeutic drug that stabilizes microtubules. While the interaction of paclitaxel with microtubules is well described, the current lack of high-resolution structural information on a tubulin-taxane complex precludes a comprehensive description of the binding determinants that affect the drug's mechanism of action. Here, we solved the crystal structure of the core baccatin III moiety of paclitaxel lacking the C13 side chain in complex with tubulin at 1.9 Å resolution. Based on this information, we engineered two tailor-made taxanes with modified C13 side chains, solved their crystal structures in complex with tubulin, and analyzed their effects along with those of paclitaxel, docetaxel, and baccatin III on the microtubule lattice by X-ray fiber diffraction. We then compared high-resolution structures of ligand-bound tubulin and microtubule complexes with apo forms and used molecular dynamics simulations to understand the consequences of taxane binding to tubulin as well as to simplified protofilament and microtubule-lattice models. Our combined approach sheds light on three mechanistic questions. Firstly, taxanes bind better to microtubules as compared to unassembled tubulin due to a dual structural mechanism: Tubulin assembly is linked to a conformational reorganization of the bM loop, which otherwise occludes ligand access to the taxane site, while the bulky C13 side chains preferentially recognize the microtubule-assembled over the unassembled conformational state of tubulin. Second, the occupancy of the taxane site by a ligand has no influence on the straightness of tubulin protofilaments. Finally, the longitudinal expansion of the microtubule lattices arises from the accommodation of the taxane core within the site, a process that is, however, not related to the microtubule stabilization mechanism of taxanes, as all analogs tested expand the microtubule lattice, despite the fact that one of them, Baccatin III, is biochemically inactive. In conclusion, our combined experimental and computational approach allowed us to describe the tubulin-taxane interaction in atomic detail and assess the structural determinants for binding.

Targeted inhibition of ubiquitin signaling reverses metabolic reprogramming and suppresses glioblastoma growth

Glioblastoma multiforme (GBM) is the most frequent and aggressive form of primary brain tumor in the adult population; its high recurrence rate and resistance to current therapeutics urgently demand a better therapy. Regulation of protein stability by the ubiquitin proteasome system (UPS) represents an important control mechanism of cell growth. UPS deregulation is mechanistically linked to the development and progression of a variety of human cancers, including GBM. Thus, the UPS represents a potentially valuable target for GBM treatment. Using an integrated approach that includes proteomics, transcriptomics and metabolic profiling, we identify praja2, a RING E3 ubiquitin ligase, as the key component of a signaling network that regulates GBM cell growth and metabolism. Praja2 is preferentially expressed in primary GBM lesions expressing the wild-type isocitrate dehydrogenase 1 gene (IDH1). Mechanistically, we found that praja2 ubiquitylates and degrades the kinase suppressor of Ras 2 (KSR2). As a consequence, praja2 restrains the activity of downstream AMP-dependent protein kinase in GBM cells and attenuates the oxidative metabolism. Delivery in the brain of siRNA targeting praja2 by transferrin-targeted self-assembling nanoparticles (SANPs) prevented KSR2 degradation and inhibited GBM growth, reducing the size of the tumor and prolonging the survival rate of treated mice. These data identify praja2 as an essential regulator of cancer cell metabolism, and as a potential therapeutic target to suppress GBM growth.

The E3 ubiquitin ligase praja2  is expressed in glioblastoma (GBM), targets the kinase suppressor of Ras 2 for degradation and attenuates oxidative metabolism. Nanoparticle-mediated delivery of praja2 siRNA to GBM-bearing mice reduces tumour size.

Insight into the role of copper in the promoted photocatalytic removal of NO using Zn2-xCuxCr-CO3 layered double hydroxide

In this work the ability of Zn_2-xCu_xCr-CO₃ layered double hydroxides (LDHs) as highly efficient DeNOx photocatalysts was studied. LDHs with x = 0, 0.2 and 0.4 were prepared using a coprecipitation method. The samples were characterized by different techniques such as XRD, XPS, FT-IR, ICP-MS, TG, SBET, SEM and Diffuse reflectance (DR). The increased amount of copper ions in the LDH layers gave rise to slight changes in the structure and morphology and an important variation of the optical properties of the LDHs. The prepared ZnCuCr-CO₃ photocatalysts exhibited favourable conversion efficiency (51%) and an extraordinary selectivity (97%) for the photochemical NO abatement. The photochemical mechanism was elucidated from DOS, EPR, Femtosecond transient absorption and in-situ DRIFTS studies. The results suggested that the presence of Cu²⁺ ions in the LDH framework introduced new states in the valence band states, thus favouring the production and mobility of e^-/h⁺ charge carriers and a greater production of ⋅O₂^- and ⋅OH.

Structural model for differential cap maturation at growing microtubule ends

Microtubules (MTs) are hollow cylinders made of tubulin, a GTPase responsible for essential functions during cell growth and division, and thus, key target for anti-tumor drugs. In MTs, GTP hydrolysis triggers structural changes in the lattice, which are responsible for interaction with regulatory factors. The stabilizing GTP-cap is a hallmark of MTs and the mechanism of the chemical-structural link between the GTP hydrolysis site and the MT lattice is a matter of debate. We have analyzed the structure of tubulin and MTs assembled in the presence of fluoride salts that mimic the GTP-bound and GDP•P_i transition states. Our results challenge current models because tubulin does not change axial length upon GTP hydrolysis. Moreover, analysis of the structure of MTs assembled in the presence of several nucleotide analogues and of taxol allows us to propose that previously described lattice expansion could be a post-hydrolysis stage involved in P_i release.

Proteolysis of MOB1 by the ubiquitin ligase praja2 attenuates Hippo signalling and supports glioblastoma growth

Human glioblastoma is the most frequent and aggressive form of brain tumour in the adult population. Proteolytic turnover of tumour suppressors by the ubiquitin–proteasome system is a mechanism that tumour cells can adopt to sustain their growth and invasiveness. However, the identity of ubiquitin–proteasome targets and regulators in glioblastoma are still unknown. Here we report that the RING ligase praja2 ubiquitylates and degrades Mob, a core component of NDR/LATS kinase and a positive regulator of the tumour-suppressor Hippo cascade. Degradation of Mob through the ubiquitin–proteasome system attenuates the Hippo cascade and sustains glioblastoma growth in vivo. Accordingly, accumulation of praja2 during the transition from low- to high-grade glioma is associated with significant downregulation of the Hippo pathway. These findings identify praja2 as a novel upstream regulator of the Hippo cascade, linking the ubiquitin proteasome system to deregulated glioblastoma growth.

Tumour suppressors can be inactivated in cancer not only as a result of mutation, but also by proteolytic degradation. Here the authors show that, during glioma development, the accumulation of the ubiquitin ligase praja2 sustains tumour growth by degrading MOB1—a core component of the Hippo pathway.

Primary peripheral PNET/Ewing's sarcoma arising in the meninges, confirmed by the presence of the rare translocation t(21;22) (q22;q12)

Peripheral primitive neuroectodermal tumor/Ewing's sarcoma (ES) (pPNET/ES) of intracranial origin are very rare. These tumors are characterized by specific translocations involving a gene on chromosome 22q12, the most common being t(11;22) (q24;q12). We report a case of 37-year-old man with pPNET/ES arising in the meninges and bearing the rare translocation t(21;22) (q22;q12). The tumor was composed of sheets and nests of monotonous small cells with round to oval nuclei, finely dispersed chromatin, small nucleolus and scant cytoplasm. We discuss the importance of the differential diagnosis with central primitive neuroectodermal tumors (cPNET).

Evaluation status and prognostic significance of O6-methylguanine-DNA methyltransferase (MGMT) promoter methylation in pediatric high grade gliomas

INTRODUCTION

In this study, we investigated the prognostic and predictive value of MGMT promoter methylation and protein expression in 30 pediatric high grade gliomas (pHGG).

METHODS

MGMT promoter methylation was assayed by methylation-specific polymerase chain reaction (MSP), whereas MGMT protein expression was evaluated by immunohistochemistry (IHC).

RESULTS

MGMT promoter methylation was observed in 7/24 (30%) cases, whereas MGMT protein overexpression was found in 19/28 (68%) cases with similar distribution in grade III and grade IV gliomas. Median survival of methylated and unmethylated patients was 16 and 8 months, respectively. Moreover, overall survival and progression-free survival showed a trend toward reduction in patients with unmethylation (p = 0.9 and p = 0.7, respectively). For MGMT protein expression, the median survival was 8.5 and 17 months for patients with MGMT overexpression or low expression, respectively. Although these two groups did not show statistically significant differences in terms of overall survival or progression-free survival (p = 0.8 and p = 0.7, respectively), there was a significant correlation between MGMT protein expression and MGMT promoter methylation status (p = 0.01).

CONCLUSIONS

Our findings indicate that, in pHGG, (a) MGMT promoter methylation is less frequent than in adult malignant gliomas, (b) there is a high correlation between MGMT MSP and MGMT IHC, and (c) as in adults, MGMT status is associated with prognosis, although this observation has to be statistically validated on larger series of patients.

Features critical for membrane binding revealed by DivIVA crystal structure

DivIVA is a conserved protein in Gram-positive bacteria that localizes at the poles and division sites, presumably through direct sensing of membrane curvature. DivIVA functions as a scaffold and is vital for septum site selection during vegetative growth and chromosome anchoring during sporulation. DivIVA deletion causes filamentous growth in Bacillus subtilis, whereas overexpression causes hyphal branching in Streptomyces coelicolor. We have determined the crystal structure of the N-terminal (Nt) domain of DivIVA, and show that it forms a parallel coiled-coil. It is capped with two unique crossed and intertwined loops, exposing hydrophobic and positively charged residues that we show here are essential for membrane binding. An intragenic suppressor introducing a positive charge restores membrane binding after mutating the hydrophobic residues. We propose that the hydrophobic residues insert into the membrane and that the positively charged residues bind to the membrane surface. A low-resolution crystal structure of the C-terminal (Ct) domain displays a curved tetramer made from two parallel coiled-coils. The Nt and Ct parts were then merged into a model of the full length, 30 nm long DivIVA protein.

Type-3 metabotropic glutamate receptors negatively modulate bone morphogenetic protein receptor signaling and support the tumourigenic potential of glioma-initiating cells

Targeted-therapies enhancing differentiation of glioma-initiating cells (GICs) are potential innovative approaches to the treatment of malignant gliomas. These cells support tumour growth and recurrence and are resistant to radiotherapy and chemotherapy. We have found that GICs express mGlu3 metabotropic glutamate receptors. Activation of these receptors sustained the undifferentiated state of GICs in culture by negatively modulating the action of bone morphogenetic proteins, which physiologically signal through the phosphorylation of the transcription factors, Smads. The cross-talk between mGlu3 receptors and BMP receptors was mediated by the activation of the mitogen-activated protein kinase pathway. Remarkably, pharmacological blockade of mGlu3 receptors stimulated the differentiation of cultured GICs into astrocytes, an effect that appeared to be long lasting, independent of the growth conditions, and irreversible. In in vivo experiments, a 3-month treatment with the brain-permeant mGlu receptor antagonist, LY341495 limited the growth of infiltrating brain tumours originating from GICs implanted into the brain parenchyma of nude mice. While clusters of tumour cells were consistently found in the brain of control mice, they were virtually absent in a large proportion of mice treated with LY341495. These findings pave the way to a new non-cytotoxic treatment of malignant gliomas based on the use of mGlu3 receptor antagonists.

Two-hit model for progression of medulloblastoma preneoplasia in Patched heterozygous mice

Inactivation of one Ptc1 allele predisposes humans and mice to spontaneous medulloblastoma development, and irradiation of newborn Ptc1 heterozygous mice results in dramatic increase of medulloblastoma incidence. While a role for loss of wild-type (wt) Ptc1 (LOH) in radiation-induced medulloblastomas from Ptc1(neo67/+) mice is well established, the importance of this event in spontaneous medulloblastomas is still unclear. Here, we demonstrate that biallelic Ptc1 loss plays a crucial role in spontaneous medulloblastomas, as shown by high rate of wt Ptc1 loss in spontaneous tumors. In addition, remarkable differences in chromosomal events involving the Ptc1 locus in spontaneous and radiation-induced medulloblastomas suggest distinct mechanisms for Ptc1 loss. To assess when, during tumorigenesis, Ptc1 loss occurs, we characterized cerebellar abnormalities that precede tumor appearance in Ptc1(neo67/+) mice. We show that inactivation of only one copy of Ptc1 is sufficient to give rise to abnormal cerebellar proliferations with different degree of altered cell morphology, but lacking potential to progress to neoplasia. Furthermore, we identify biallelic Ptc1 loss as the event causally related to the transition from the preneoplastic stage to full blown medulloblastoma. These results underscore the utility of the Ptc1(neo67/+) mouse model for studies on the mechanisms of medulloblastoma and for development of new therapeutic strategies.

Structural insights into FtsZ protofilament formation

The prokaryotic tubulin homolog FtsZ polymerizes into a ring structure essential for bacterial cell division. We have used refolded FtsZ to crystallize a tubulin-like protofilament. The N- and C-terminal domains of two consecutive subunits in the filament assemble to form the GTPase site, with the C-terminal domain providing water-polarizing residues. A domain-swapped structure of FtsZ and biochemical data on purified N- and C-terminal domains show that they are independent. This leads to a model of how FtsZ and tubulin polymerization evolved by fusing two domains. In polymerized tubulin, the nucleotide-binding pocket is occluded, which leads to nucleotide exchange being the rate-limiting step and to dynamic instability. In our FtsZ filament structure the nucleotide is exchangeable, explaining why, in this filament, nucleotide hydrolysis is the rate-limiting step during FtsZ polymerization. Furthermore, crystal structures of FtsZ in different nucleotide states reveal notably few differences.

Filtering out contrast reversals for microscopy autofocus

Autofocus functions based on measurement of image resolution appear to be precise and robust for biological microscopy. However, the through-focus response of these functions previously exhibited unwanted local maxima, or side peaks. Here we report theoretical and experimental studies showing that side peaks are mainly a result of contrast reversals inherent in optical systems at mid-range frequencies. These contrast reversals are not present in frequencies near optical cutoff. Contrast reversals thus limit the lower cutoff for resolution measurement filters, whereas signal-to-noise limits the upper cutoff. These improved bandpass design criteria led to sharp, unimodal autofocus responses for all tested microscopy specimens.