Rac1-PAK1 regulation of Rab11 cycling promotes junction destabilization

Targeting guanine nucleotide exchange factors for novel cancer drug discovery

INTRODUCTION

Guanine nucleotide exchange factors (GEFs) regulate the activation of small GTPases (G proteins) of the Ras superfamily proteins controlling cellular functions. Ras superfamily proteins act as 'molecular switches' that are turned 'ON' by guanine exchange. There are five major groups of Ras family GTPases: Ras, Ran, Rho, Rab and Arf, with a variety of different GEFs regulating their GTP loading. GEFs have been implicated in various diseases including cancer. This makes GEFs attractive targets to modulate signaling networks controlled by small GTPases.

AREAS COVERED

In this review, the roles and mechanisms of GEFs in malignancy are outlined. The mechanism of guanine exchange activity by GEFs on a small GTPase is illustrated. Then, some examples of GEFs that are significant in cancer are presented with a discussion on recent progress in therapeutic targeting efforts using a variety of approaches.

EXPERT OPINION

Recently, GEFs have emerged as potential therapeutic targets for novel cancer drug development. Targeting small GTPases is challenging; thus, targeting their activation by GEFs is a promising strategy. Most GEF-targeted drugs are still in preclinical development. A deeper biological understanding of the underlying mechanisms of GEF activity and utilizing advanced technology are necessary to enhance drug discovery for GEFs in cancer.

Activation of cerebral Ras-related C3 botulinum toxin substrate (Rac) 1 promotes post-ischemic stroke functional recovery in aged mice

Brain functional impairment after stroke is common; however, the molecular mechanisms of post-stroke recovery remain unclear. It is well-recognized that age is the most important independent predictor of poor outcomes after stroke as older patients show poorer functional outcomes following stroke. Mounting evidence suggests that axonal regeneration and angiogenesis, the major forms of brain plasticity responsible for post-stroke recovery, diminished with advanced age. Previous studies suggest that Ras-related C3 botulinum toxin substrate (Rac) 1 enhances stroke recovery as activation of Rac1 improved behavior recovery in a young mice stroke model. Here, we investigated the role of Rac1 signaling in long-term functional recovery and brain plasticity in an aged (male, 18 to 22 months old C57BL/6J) brain after ischemic stroke. We found that as mice aged, Rac1 expression declined in the brain. Delayed overexpression of Rac1, using lentivirus encoding Rac1 injected day 1 after ischemic stroke, promoted cognitive (assessed using novel object recognition test) and sensorimotor (assessed using adhesive removal tests) recovery on days 14-28. This was accompanied by the increase of neurite and proliferative endothelial cells in the peri-infarct zone assessed by immunostaining. In a reverse approach, pharmacological inhibition of Rac1 by intraperitoneal injection of Rac1 inhibitor NSC23766 for 14 successive days after ischemic stroke worsened the outcome with the reduction of neurite and proliferative endothelial cells. Furthermore, Rac1 inhibition reduced the activation of p21-activated kinase 1, the protein level of brain-derived neurotrophic factor, and increased the protein level of glial fibrillary acidic protein in the ischemic brain on day 28 after stroke. Our work provided insight into the mechanisms behind the diminished plasticity after cerebral ischemia in aged brains and identified Rac1 as a potential therapeutic target for improving functional recovery in the older adults after stroke.

Biochemical Structure and Function of TRAPP Complexes in the Cardiac System

Trafficking protein particle (TRAPP) is well reported to play a role in the trafficking of protein products within the Golgi and endoplasmic reticulum. Dysfunction in TRAPP has been associated with disorders in the nervous and cardiovascular systems, but the majority of literature focuses on TRAPP function in the nervous system solely. Here, we highlight the known pathways of TRAPP and hypothesize potential impacts of TRAPP dysfunction on the cardiovascular system, particularly the role of TRAPP as a guanine-nucleotide exchange factor for Rab1 and Rab11. We also review the various cardiovascular phenotypes associated with changes in TRAPP complexes and their subunits.

Bayesian Machine Learning Enables Identification of Transcriptional Network Disruptions Associated with Drug-Resistant Prostate Cancer

Survival rates of patients with metastatic castration-resistant prostate cancer (mCRPC) are low due to lack of response or acquired resistance to available therapies, such as abiraterone (Abi). A better understanding of the underlying molecular mechanisms is needed to identify effective targets to overcome resistance. Given the complexity of the transcriptional dynamics in cells, differential gene expression analysis of bulk transcriptomics data cannot provide sufficient detailed insights into resistance mechanisms. Incorporating network structures could overcome this limitation to provide a global and functional perspective of Abi resistance in mCRPC. Here, we developed TraRe, a computational method using sparse Bayesian models to examine phenotypically driven transcriptional mechanistic differences at three distinct levels: transcriptional networks, specific regulons, and individual transcription factors (TF). TraRe was applied to transcriptomic data from 46 patients with mCRPC with Abi-response clinical data and uncovered abrogated immune response transcriptional modules that showed strong differential regulation in Abi-responsive compared with Abi-resistant patients. These modules were replicated in an independent mCRPC study. Furthermore, key rewiring predictions and their associated TFs were experimentally validated in two prostate cancer cell lines with different Abi-resistance features. Among them, ELK3, MXD1, and MYB played a differential role in cell survival in Abi-sensitive and Abi-resistant cells. Moreover, ELK3 regulated cell migration capacity, which could have a direct impact on mCRPC. Collectively, these findings shed light on the underlying transcriptional mechanisms driving Abi response, demonstrating that TraRe is a promising tool for generating novel hypotheses based on identified transcriptional network disruptions.

SIGNIFICANCE

The computational method TraRe built on Bayesian machine learning models for investigating transcriptional network structures shows that disruption of ELK3, MXD1, and MYB signaling cascades impacts abiraterone resistance in prostate cancer.

Nectin stabilization at adherens junctions is counteracted by Rab5a-dependent endocytosis

Cell-cell junctions undergo constant remodeling, which is crucial for the control of vascular integrity. Indeed, transport of junctional components such as cadherins is understood in increasing depth. However, little is known about the respective pathways regulating localization of nectin at cell-cell junctions. Here, we performed an siRNA-based screen of vesicle regulators of the RabGTPase family, leading to the identification of a novel role for Rab5a in the endocytosis nectin-2 at adherens junctions of primary human endothelial cells (HUVEC). Confocal microscopy experiments revealed disordered nectin-2 localization at adherens junctions upon Rab5a depletion. In addition, internalized nectin-2 was shown to prominently localize to Rab5a-positive vesicles in both fixed and living cells. As shown previously, nectin-2 stabilization at junctions is achieved via drebrin-dependent coupling to the subcortical actin cytoskeleton. Consistently, depletion of drebrin in this study leads to enhanced internalization of nectin-2 from junctions. Strikingly, simultaneous silencing of Rab5a and drebrin restored the junctional localization of nectin-2, pointing to Rab5a as counteracting the drebrin-dependent stabilization of nectin-2 at adherens junctions. This mechanism could be further validated by transendothelial resistance measurements. Collectively, our results identify Rab5a as a key player in the endocytosis of nectin-2 and thus in the regulation of adherens junction integrity in primary human endothelial cells.