A Hock

Structural basis for autoinhibition and phosphorylation-dependent activation of c-Cbl

Cbls are RING ubiquitin ligases that attenuate receptor tyrosine kinase (RTK) signal transduction. Cbl ubiquitination activity is stimulated by phosphorylation of a linker helix region (LHR) tyrosine residue. To elucidate the mechanism of activation, we determined the structures of human CBL, a CBL−substrate peptide complex and a phosphorylated-Tyr371-CBL−E2−substrate peptide complex, and we compared them with the known structure of a CBL−E2−substrate peptide complex. Structural and biochemical analyses show that CBL adopts an autoinhibited RING conformation, where the RINGs E2-binding surface associates with CBL to reduce E2 affinity. Tyr371 phosphorylation activates CBL by inducing LHR conformational changes that eliminate autoinhibition, flip the RING domain and E2 into proximity of the substrate-binding site and transform the RING domain into an enhanced E2-binding module. This activation is required for RTK ubiquitination. Our results present a mechanism for regulation of c-Cbls activity by autoinhibition and phosphorylation-induced activation.

Regulation of the p53 pathway by ubiquitin and related proteins.

The p53 tumour suppressor protein is subject to many levels of control, including modification with ubiquitin and related proteins such as SUMO and NEDD8. These modifications regulate p53 at a number of levels, including control of protein turnover, alterations in sub-cellular localization and changes in the ability to regulate gene expression. Numerous E3 ligases that can mediate these modifications of p53 have been described, some of which promote conjugation with more than one ubiquitin-like protein. Understanding the complexity of this mechanism of p53 regulation will help in the development of therapeutic drugs that function to modulate these events.

Non-water-suppressed proton MR spectroscopy improves spectral quality in the human spinal cord

Magnetic resonance spectroscopy enables insight into the chemical composition of spinal cord tissue. However, spinal cord magnetic resonance spectroscopy has rarely been applied in clinical work due to technical challenges, including strong susceptibility changes in the region and the small cord diameter, which distort the lineshape and limit the attainable signal to noise ratio. Hence, extensive signal averaging is required, which increases the likelihood of static magnetic field changes caused by subject motion (respiration, swallowing), cord motion, and scanner‐induced frequency drift. To avoid incoherent signal averaging, it would be ideal to perform frequency alignment of individual free induction decays before averaging. Unfortunately, this is not possible due to the low signal to noise ratio of the metabolite peaks. In this article, frequency alignment of individual free induction decays is demonstrated to improve spectral quality by using the high signal to noise ratio water peak from non‐water‐suppressed proton magnetic resonance spectroscopy via the metabolite cycling technique. Electrocardiography (ECG)‐triggered point resolved spectroscopy (PRESS) localization was used for data acquisition with metabolite cycling or water suppression for comparison. A significant improvement in the signal to noise ratio and decrease of the Cramér Rao lower bounds of all metabolites is attained by using metabolite cycling together with frequency alignment, as compared to water‐suppressed spectra, in 13 healthy volunteers. Magn Reson Med, 2013.

Small molecules that bind the Mdm2 RING stabilize and activate p53

p53 is a tumor suppressor that responds to a variety of stresses such as oncogenes and DNA damage by activating its transcriptional targets to allow repair or elimination of damaged cells. In the absence of stress signals, p53 needs to be kept in check and this is achieved by the E3 ligase MDM2. For tumors that retain wild-type p53, therapeutic strategies aimed at removing the inhibitory activity of MDM2 on p53 are under development and to date have focused on drugs that prevent the binding of p53 to MDM2. Here, we report the analysis of a group of synthetic analogs derived from 5-deazaflavin compounds previously identified in a screen as inhibitors of MDM2 autoubiquitination. Using measurement of surface plasmon resonance, we demonstrated that active 5-deazaflavin analogs bind to the MDM2 RING, whereas inactive compounds show no binding. In cellular assays, these active MDM2 RING binding compounds inhibited the ubiquitination of p53, stabilized p53, led to increased expression of p53 targets and caused corresponding cell cycle effects. Deazaflavin analogs therefore function to activate p53 through a novel mechanism, by inhibiting the E3 ligase activity of MDM2 in a manner that involves binding to the MDM2 RING.

Effects of serotonin 2A/1A receptor stimulation on social exclusion processing

Significance Social cognition critically impacts the development, progression, and treatment of psychiatric disorders. However, social cognition skills are insufficiently targeted by current treatment approaches. By applying a multimodal brain imaging strategy, the present study demonstrated the importance of the serotonin 2A/1A receptor system in the modulation of social exclusion processing. Understanding the biochemical underpinnings of the social rejection experience is important for increasing our knowledge about social/emotional processing and the related neural responses. The identification of relevant neural responses is in turn crucial for the efficacious management of disorders influenced by social factors. Our findings may help to diminish a knowledge gap that currently restrains the development of pharmacotherapies for sociocognitive deficits in psychiatric disorders. Social ties are crucial for physical and mental health. However, psychiatric patients frequently encounter social rejection. Moreover, an increased reactivity to social exclusion influences the development, progression, and treatment of various psychiatric disorders. Nevertheless, the neuromodulatory substrates of rejection experiences are largely unknown. The preferential serotonin (5-HT) 2A/1A receptor agonist, psilocybin (Psi), reduces the processing of negative stimuli, but whether 5-HT2A/1A receptor stimulation modulates the processing of negative social interactions remains unclear. Therefore, this double-blind, randomized, counterbalanced, cross-over study assessed the neural response to social exclusion after the acute administration of Psi (0.215 mg/kg) or placebo (Pla) in 21 healthy volunteers by using functional magnetic resonance imaging (fMRI) and resting-state magnetic resonance spectroscopy (MRS). Participants reported a reduced feeling of social exclusion after Psi vs. Pla administration, and the neural response to social exclusion was decreased in the dorsal anterior cingulate cortex (dACC) and the middle frontal gyrus, key regions for social pain processing. The reduced neural response in the dACC was significantly correlated with Psi-induced changes in self-processing and decreased aspartate (Asp) content. In conclusion, 5-HT2A/1A receptor stimulation with psilocybin seems to reduce social pain processing in association with changes in self-experience. These findings may be relevant to the normalization of negative social interaction processing in psychiatric disorders characterized by increased rejection sensitivity. The current results also emphasize the importance of 5-HT2A/1A receptor subtypes and the Asp system in the control of social functioning, and as prospective targets in the treatment of sociocognitive impairments in psychiatric illnesses.

Electrocardiogram‐triggered, higher order, projection‐based B0 shimming allows for fast and reproducible shim convergence in spinal cord 1H MRS

1H MRS allows insight into the chemical composition of the central nervous system. However, as a result of technical challenges, it has rarely been applied to the spinal cord. In particular, the strong susceptibility changes around the spinal cord and the pulsatile flow of the cerebrospinal fluid lead to distinct B0 field distortions which often considerably degrade the spectral quality. Hence, B0 shimming is one of the main challenges in 1H MRS of the spinal cord. Electrocardiogram (ECG)‐triggered, higher order, projection‐based B0 shimming was introduced and compared with both conventional projection‐based B0 shimming and B0 shimming based on ECG‐triggered, three‐dimensional B0 field mapping. The linewidth of the unsuppressed water peak was used to evaluate the reproducibility and the potential improvement to B0 homogeneity. The use of ECG‐triggered projection‐based B0 shimming in combination with ECG triggering during preparation phases and triggering during acquisition of the spectra is the most robust method and thus helps to improve the spectral quality for MRS of the spinal cord. Copyright

1H-MR Spectroscopy in the Human Spinal Cord

SUMMARY: MR spectroscopy allows insight into the chemical composition of human tissue noninvasively. Thereby it can help to better characterize pathologic processes affecting the spinal cord and may provide important clinical markers for differential diagnosis. However, due to technical challenges, it has been rarely applied to the spinal cord. The aim of this review was to summarize the technical development and clinical studies using MR spectroscopy in the spinal cord. Main challenges of applying MR spectroscopy in the spinal cord are discussed, and a description of a state-of-the-art scan protocol is given. In conclusion, MR spectroscopy is a promising tool for research and diagnosis of the spinal cord because it can provide additional information complementary to other noninvasive imaging methods. However, the application of MR spectroscopy in the spinal cord is not straightforward, and great care is required to attain optimal spectral quality.

Ubiquitination and proteasomal degradation of ATG12 regulates its proapoptotic activity

During macroautophagy, conjugation of ATG12 to ATG5 is essential for LC3 lipidation and autophagosome formation. Additionally, ATG12 has ATG5-independent functions in diverse processes including mitochondrial fusion and mitochondrial-dependent apoptosis. In this study, we investigated the regulation of free ATG12. In stark contrast to the stable ATG12–ATG5 conjugate, we find that free ATG12 is highly unstable and rapidly degraded in a proteasome-dependent manner. Surprisingly, ATG12, itself a ubiquitin-like protein, is directly ubiquitinated and this promotes its proteasomal degradation. As a functional consequence of its turnover, accumulation of free ATG12 contributes to proteasome inhibitor-mediated apoptosis, a finding that may be clinically important given the use of proteasome inhibitors as anticancer agents. Collectively, our results reveal a novel interconnection between autophagy, proteasome activity, and cell death mediated by the ubiquitin-like properties of ATG12.

Structural analysis of MDM2 RING separates degradation from regulation of p53 transcription activity

MDM2–MDMX complexes bind the p53 tumor-suppressor protein, inhibiting p53s transcriptional activity and targeting p53 for proteasomal degradation. Inhibitors that disrupt binding between p53 and MDM2 efficiently activate a p53 response, but their use in the treatment of cancers that retain wild-type p53 may be limited by on-target toxicities due to p53 activation in normal tissue. Guided by a novel crystal structure of the MDM2–MDMX–E2(UbcH5B)–ubiquitin complex, we designed MDM2 mutants that prevent E2–ubiquitin binding without altering the RING-domain structure. These mutants lack MDM2s E3 activity but retain the ability to limit p53′s transcriptional activity and allow cell proliferation. Cells expressing these mutants respond more quickly to cellular stress than cells expressing wild-type MDM2, but basal p53 control is maintained. Targeting the MDM2 E3-ligase activity could therefore widen the therapeutic window of p53 activation in tumors.

5-Deazaflavin derivatives as inhibitors of p53 ubiquitination by HDM2

Graphical abstract