Regulation of Small GTPase Prenylation in the Nervous System

Srebf2 mediates successful optic nerve axon regeneration via the mevalonate synthesis pathway

BACKGROUND

Axon regeneration within the mammalian central nervous system is extremely limited. In optic neuropathy conditions like glaucoma, the inability of retinal ganglion cell (RGC) axons to regenerate is a major impediment to functional recovery. In contrast, adult teleost fish such as zebrafish can fully regenerate RGC axons enabling visual recovery from optic nerve (ON) injury making it an ideal model to probe the mechanisms of successful axon regeneration.

METHODS

Laser Capture Microdissection followed by RNA-sequencing (LCM-seq) was used to identify genes and pathways differentially expressed in RGCs during ON regeneration. We validate these findings by in situ hybridization and qRT-PCR. Using loss- and gain-of-function experiments we demonstrate the necessity of srebf2 for efficient ON regeneration and recovery of visual function. Finally, we use LCM-seq coupled with experimental manipulations to identify downstream srebf2 target genes and test the role of hmgcra/b and mevalonate in this process. Statistical analysis was performed using Student's t-test, two-way ANOVA, or repeated measures with appropriate post-hoc tests where applicable.

RESULTS

LCM-seq comparison of uninjured versus 3-day post ON injury RGCs identified significant upregulation of the cholesterol synthesis pathway during axon regeneration. The master regulator of this pathway, the transcription factor srebf2, is upregulated throughout the regeneration phase. Chemical inhibition or morpholino-based gene knockdown of srebf2 decreased axon regeneration into the ON and optic tectum and delayed recovery of visual behavior over the course of normal optic nerve regeneration without causing a significant loss of RGCs. Constitutively active srebf2 can fully rescue axon regeneration and visual behavior losses caused by inhibition of endogenous srebf2 but does not accelerate regeneration compared to the control group. LCM-seq confirms the expected regulation of predicted srebf2 target genes after loss- or gain-of-function in vivo. Downstream of srebf2, hmgcra/b knockdown or simvastatin treatment delayed axon regeneration and this effect was rescued by supplemental mevalonate. Mevalonate treatment alone was sufficient to accelerate ON regeneration.

CONCLUSIONS

These results demonstrate that srebf2 and the downstream mevalonate synthesis pathway plays an important role in regulating efficient axon regeneration in the zebrafish visual system. Involvement of this pathway should be closely examined in failed mammalian ON regeneration.

New insights on the regulators and inhibitors of RhoA-ROCK signalling in Parkinson's disease

A multifaceted and widely prevalent neurodegenerative disease, Parkinson's disease (PD) is typified by the loss of dopaminergic neurons in the midbrain. The discovery of novel treatment(s) that can reverse or halt the course of the disease progression along with identifying the most reliable biomarker(s) in PD remains the crucial concern. RhoA in its active state has been demonstrated to interact with three distinct domains located in the central coiled-coil region of ROCK. RhoA appears to activate effectors most frequently by breaking the intramolecular autoinhibitory connections, which releases functional domains from the effector protein. Additionally, RhoA is highly expressed in the nervous system and it acts as a central molecule for its several downstream effector proteins in multiple signalling pathways both in neurons and glial cells. Mitochondrial dysfunction, vesicle transport malfunction and aggregation of α-Synuclein, a presynaptic neuronal protein genetically and neuropathologically associated with PD. While the RhoA-ROCK signalling pathway appears to have a significant role in PD symptoms, suggesting it could be a promising target for therapeutic interventions. Thus, this review article addresses the potential involvement of the RhoA-ROCK signalling system in the pathophysiology of neurodegenerative illnesses, with an emphasis on its biology and function. We also provide an overview of the state of research on RhoA regulation and its downstream biological activities, focusing on the role of RhoA signalling in neurodegenerative illnesses and the potential benefits of RhoA inhibition as a treatment for neurodegeneration.

Mitigating neuroinflammation in cognitive areas: exploring the impact of HMG-CoA reductase inhibitor

Existing literature suggests that infection-specific mechanisms may play a significant role in the onset and progression of dementia, as opposed to the broader phenomenon of systemic inflammation. In addition, 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase inhibitors have been proposed as a potential therapeutic approach for sepsis, given their anti-inflammatory and antioxidant properties. We investigated the neuroprotective effect of an HMG-CoA reductase inhibitor (simvastatin) by analyzing neurodegenerative markers, mitochondrial respiration, and neuronal tracing in the prefrontal cortex (PFC) and thalamic nucleus reuniens (RE) of sepsis survivor animals. Adult Wistar rats were subjected to sepsis by cecal ligation and puncture or left non-manipulated. The animals were treated with simvastatin or vehicle for 4 days before and 10 days after surgery. The treatment preserved the non-associative memory (P < 0.05), recovered expression of Smad-3 in the hippocampus (P < 0.05), and prevented increased expression of calpain-1 (hippocampus: P < 0.0001; PFC: P < 0.05) and GSKβ (hippocampus: P < 0.0001; PFC: P < 0.0001) in the brain structures of the sepsis survivor animals. These animals also showed mitochondrial dysfunction and decreased axon terminals in the RE. Simvastatin seems to restore energy metabolism by improving the electron transfer system (ETS) values in the hippocampus (P < 0.01) and the oxidative phosphorylation/ETS (P/E) ratio in the PFC (P < 0.05), in addition to preventing the reduction of axon terminals in survivor animals. These results suggest a potential neuroprotective effect and the importance of considering HMG-CoA reductase inhibitors as a possible adjuvant therapy in sepsis.

A subpellicular microtubule dynein transport machinery regulates ookinete morphogenesis for mosquito transmission of Plasmodium yoelii

The cortical cytoskeleton of subpellicular microtubules (SPMTs) supports the Plasmodium ookinete morphogenesis during mosquito transmission of malaria. SPMTs are hypothesized to function as the cytoskeletal tracks in motor-driven cargo transport for apical organelle and structure assembly in ookinetes. However, the SPMT-based transport motor has not been identified in the Plasmodium. The cytoplasmic dynein is the motor moving towards the minus end of microtubules (MTs) and likely be responsible for cargo transport to the apical part in ookinetes. Here we screen 7 putative dynein heavy chain (DHC) proteins in the P. yoelii and identify DHC3 showing peripheral localization in ookinetes. DHC3 is localized at SPMTs throughout ookinete morphogenesis. We also identify five other dynein subunits localizing at SPMTs. DHC3 disruption impairs ookinete development, shape, and gliding, leading to failure in mosquito infection of Plasmodium. The DHC3-deficient ookinetes display defective formation or localization of apical organelles and structures. Rab11A and Rab11B interact with DHC3 at SPMTs in a DHC3-dependent manner, likely functioning as the receptors for the cargoes driven by SPMT-dynein. Disturbing Rab11A or Rab11B phenocopies DHC3 deficiency in ookinete morphogenesis. Our study reveals an SPMT-based dynein motor driving the transport of Rab11A- and Rab11B-labeled cargoes in the ookinete morphogenesis of Plasmodium.

Effects of Simvastatin on RBL-2H3 Cell Degranulation

Hypercholesterolemia is a major complication of arteriosclerosis. Mast cells in arteriosclerosis plaques induce inflammatory reactions and promote arterial sclerosis. In this study, we evaluated the pharmacological effects of simvastatin (SV)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitors on the degranulation of rat basophilic leukemia (RBL)-2H3 cells, which are commonly used as mast cell models. SV significantly decreased the degranulation induced by three types of stimulation: antigen antibody reaction (Ag-Ab), thapsigargin (Tg) serosal endoplasmic reticulum calcium ATPase (SERCA) inhibitor, and A23187 calcium ionophore. SV had a stronger inhibitory effect on degranulation induced by Ag-Ab stimulation than the other two stimulations. However, SV did not inhibit increase of intracellular Ca²⁺ concentrations. Mevalonate or geranylgeraniol co-treatment with SV completely prevented the inhibitory effect of SV on the degranulation induced by these stimulations. Immunoblotting results showed that SV inhibited protein kinase C (PKC) delta translocation induced by Ag-Ab but not by Tg or A23187. SV induced a reduction in active Rac1, and actin filament rearrangement. In conclusion, SV inhibits RBL-2H3 cell degranulation by inhibiting downstream signaling pathways, including the sequential degranulation pathway. These inhibitory effects were completely reversed by the addition of geranylgeraniol and might be induced by changes in the translocation of the small guanosine 5'-triphosphatase (GTPase) families Rab and Rho, which are related to vesicular transport PKC delta translocation and actin filament formation, respectively. These changes are caused by the inhibition of HMG-CoA reductase by SV following the synthesis of geranylgeranyl pyrophosphates, which play important roles in the activation of small GTPases, Rab.

Neurodegenerative Diseases: From Dysproteostasis, Altered Calcium Signalosome to Selective Neuronal Vulnerability to AAV-Mediated Gene Therapy

Despite intense research into the multifaceted etiology of neurodegenerative diseases (ND), they remain incurable. Here we provide a brief overview of several major ND and explore novel therapeutic approaches. Although the cause (s) of ND are not fully understood, the accumulation of misfolded/aggregated proteins in the brain is a common pathological feature. This aggregation may initiate disruption of Ca⁺⁺ signaling, which is an early pathological event leading to altered dendritic structure, neuronal dysfunction, and cell death. Presently, ND gene therapies remain unidimensional, elusive, and limited to modifying one pathological feature while ignoring others. Considering the complexity of signaling cascades in ND, we discuss emerging therapeutic concepts and suggest that deciphering the molecular mechanisms involved in dendritic pathology may broaden the phenotypic spectrum of ND treatment. An innovative multiplexed gene transfer strategy that employs silencing and/or over-expressing multiple effectors could preserve vulnerable neurons before they are lost. Such therapeutic approaches may extend brain health span and ameliorate burdensome chronic disease states.

Prenylation Defects and Oxidative Stress Trigger the Main Consequences of Neuroinflammation Linked to Mevalonate Pathway Deregulation

The cholesterol biosynthesis represents a crucial metabolic pathway for cellular homeostasis. The end products of this pathway are sterols, such as cholesterol, which are essential components of cell membranes, precursors of steroid hormones, bile acids, and other molecules such as ubiquinone. Furthermore, some intermediates of this metabolic system perform biological activity in specific cellular compartments, such as isoprenoid molecules that can modulate different signal proteins through the prenylation process. The defects of prenylation represent one of the main causes that promote the activation of inflammation. In particular, this mechanism, in association with oxidative stress, induces a dysfunction of the mitochondrial activity. The purpose of this review is to describe the pleiotropic role of prenylation in neuroinflammation and to highlight the consequence of the defects of prenylation.

RhoA Signaling in Neurodegenerative Diseases

Ras homolog gene family member A (RhoA) is a small GTPase of the Rho family involved in regulating multiple signal transduction pathways that influence a diverse range of cellular functions. RhoA and many of its downstream effector proteins are highly expressed in the nervous system, implying an important role for RhoA signaling in neurons and glial cells. Indeed, emerging evidence points toward a role of aberrant RhoA signaling in neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. In this review, we summarize the current knowledge of RhoA regulation and downstream cellular functions with an emphasis on the role of RhoA signaling in neurodegenerative diseases and the therapeutic potential of RhoA inhibition in neurodegeneration.

Prodrugs of pyrophosphates and bisphosphonates: disguising phosphorus oxyanions

Pyrophosphates have important functions in living systems and thus pyrophosphate-containing molecules and their more stable bisphosphonate analogues have the potential to be used as drugs for treating many diseases including cancer and viral infections. Both pyrophosphates and bisphosphonates are polyanionic at physiological pH and, whilst this is essential for their biological activity, it also limits their use as therapeutic agents. In particular, the high negative charge density of these compounds prohibits cell entry other than by endocytosis, prevents transcellular oral absorption and causes sequestration to bone. Therefore, prodrug strategies have been developed to temporarily disguise the charges of these compounds. This review examines the various systems that have been used to mask the phosphorus-containing moieties of pyrophosphates and bisphosphonates and also illustrates the utility of such prodrugs.

Brain Cholesterol Biosynthetic Pathway Is Altered in a Preclinical Model of Fragile X Syndrome

Fragile X Syndrome (FXS) is the most frequent form of inherited X-linked pathology, associated with an intellectual and developmental disability, and currently considered the first monogenic cause of autism spectrum disorder (ASD). Low levels of total cholesterol reported in the serum of FXS patients, and evidence that FMRP targets a subset of mRNAs encoding proteins of lipid synthesis and transport suggests that the cholesterol metabolism impairments could be involved in FXS. Thus, the aim of the presented work was to investigate the modulations of the cholesterol biosynthetic pathway and its end-products in a recently developed Fmr1-Δexon 8 rat model of FXS. Here, we show that this experimental model mimics what is found in FXS patients, exhibiting a lower serum cholesterol content, accompanied by a reduction in food intake and body weight compared to WT animals. Moreover, alterations of proteins committed to cholesterol synthesis and uptake have been observed in the amygdala, prefrontal cortex and nucleus accumbens. Interestingly, the end-products show a brain region-dependent modulation in Fmr1-Δexon 8 rats. Overall, our results demonstrate that the cholesterol biosynthetic pathway is altered in some brain regions of this preclinical model of FXS. This finding has relevance for future studies to delve deeper into the involvement of this metabolic process in FXS, and thus its possible role as a therapeutic target.

Cholesterol synthesis inhibition or depletion in axon regeneration

Cholesterol is biosynthesized by all animal cells. Beyond its metabolic role in steroidogenesis, it is enriched in the plasma membrane where it has key structural and regulatory functions. Cholesterol is thus presumably important for post-injury axon regrowth, and this notion is supported by studies showing that impairment of local cholesterol reutilization impeded regeneration. However, several studies have also shown that statins, inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase, are enhancers of axon regeneration, presumably acting through an attenuation of the mevalonate isoprenoid pathway and consequent reduction in protein prenylation. Several recent reports have now shown that cholesterol depletion, as well as inhibition of cholesterol synthesis per se, enhances axon regeneration. Here, I discussed these findings and propose some possible underlying mechanisms. The latter would include possible disruptions to axon growth inhibitor signaling by lipid raft-localized receptors, as well as other yet unclear neuronal survival signaling process enhanced by cholesterol lowering or depletion.

Beyond Lipid-Lowering: Effects of Statins on Cardiovascular and Cerebrovascular Diseases and Cancer

The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, also known as statins, are administered as first-line therapy for hypercholesterolemia, both as primary and secondary prevention. Besides the lipid-lowering effect, statins have been suggested to inhibit the development of cardiovascular disease through anti-inflammatory, antioxidant, vascular endothelial function-improving, plaque-stabilizing, and platelet aggregation-inhibiting effects. The preventive effect of statins on atherothrombotic stroke has been well established, but statins can influence other cerebrovascular diseases. This suggests that statins have many neuroprotective effects in addition to lowering cholesterol. Furthermore, research suggests that statins cause pro-apoptotic, growth-inhibitory, and pro-differentiation effects in various malignancies. Preclinical and clinical evidence suggests that statins inhibit tumor growth and induce apoptosis in specific cancer cell types. The pleiotropic effects of statins on cardiovascular and cerebrovascular diseases have been well established; however, the effects of statins on cancer patients have not been fully elucidated and are still controversial. This review discusses the recent evidence on the effects of statins on cardiovascular and cerebrovascular diseases and cancer. Additionally, this study describes the pharmacological action of statins, focusing on the aspect of ‘beyond lipid-lowering’.

Rab3a, a small GTP-binding protein, is required for the stabilization of the murine leukaemia virus Gag protein

We recently identified a CD63-interacting protein to understand the role of CD63 in virion production of the human immunodeficiency virus type 1, and we have found that Rab3a forms a complex with CD63. In this study, we analysed the effect of Rab3a on virion production of the murine leukaemia virus (MLV), which is another member of the retrovirus family. We found that Rab3a silencing induced lysosomal degradation of the MLV Gag protein, and recovery of the Rab3a expression restored the level of the Gag protein through a complex formation of MLV Gag and Rab3a, indicating that Rab3a is required for MLV Gag protein expression. In contrast, CD63 silencing decreased the infectivity of released virions but had no effect on virion production, indicating that CD63 facilitates the infectivity of released MLV particles. Although Rab3a induced CD63 degradation in uninfected cells, the complex of MLV Gag and Rab3a suppressed the Rab3a-mediated CD63 degradation in MLV-infected cells. Finally, we found that the MLV Gag protein interacts with Rab3a to stabilize its own protein and CD63 that facilitates the infectivity of released MLV particles. Considering the involvement of Rab3a in lysosome trafficking to the plasma membrane, it may also induce cell surface transport of the MLV Gag protein.

TRP Channels Regulation of Rho GTPases in Brain Context and Diseases

Neurological and neuropsychiatric disorders are mediated by several pathophysiological mechanisms, including developmental and degenerative abnormalities caused primarily by disturbances in cell migration, structural plasticity of the synapse, and blood-vessel barrier function. In this context, critical pathways involved in the pathogenesis of these diseases are related to structural, scaffolding, and enzymatic activity-bearing proteins, which participate in Ca²⁺- and Ras Homologs (Rho) GTPases-mediated signaling. Rho GTPases are GDP/GTP binding proteins that regulate the cytoskeletal structure, cellular protrusion, and migration. These proteins cycle between GTP-bound (active) and GDP-bound (inactive) states due to their intrinsic GTPase activity and their dynamic regulation by GEFs, GAPs, and GDIs. One of the most important upstream inputs that modulate Rho GTPases activity is Ca²⁺ signaling, positioning ion channels as pivotal molecular entities for Rho GTPases regulation. Multiple non-selective cationic channels belonging to the Transient Receptor Potential (TRP) family participate in cytoskeletal-dependent processes through Ca²⁺-mediated modulation of Rho GTPases. Moreover, these ion channels have a role in several neuropathological events such as neuronal cell death, brain tumor progression and strokes. Although Rho GTPases-dependent pathways have been extensively studied, how they converge with TRP channels in the development or progression of neuropathologies is poorly understood. Herein, we review recent evidence and insights that link TRP channels activity to downstream Rho GTPase signaling or modulation. Moreover, using the TRIP database, we establish associations between possible mediators of Rho GTPase signaling with TRP ion channels. As such, we propose mechanisms that might explain the TRP-dependent modulation of Rho GTPases as possible pathways participating in the emergence or maintenance of neuropathological conditions.

Rho GTPases in the Amygdala-A Switch for Fears?

Fear is a fundamental evolutionary process for survival. However, excess or irrational fear hampers normal activity and leads to phobia. The amygdala is the primary brain region associated with fear learning and conditioning. There, Rho GTPases are molecular switches that act as signaling molecules for further downstream processes that modulate, among others, dendritic spine morphogenesis and thereby play a role in fear conditioning. The three main Rho GTPases—RhoA, Rac1, and Cdc42, together with their modulators, are known to be involved in many psychiatric disorders that affect the amygdala′s fear conditioning mechanism. Rich2, a RhoGAP mainly for Rac1 and Cdc42, has been studied extensively in such regard. Here, we will discuss these effectors, along with Rich2, as a molecular switch for fears, especially in the amygdala. Understanding the role of Rho GTPases in fear controlling could be beneficial for the development of therapeutic strategies targeting conditions with abnormal fear/anxiety-like behaviors.

Bromo- and extraterminal domain protein inhibition improves immunotherapy efficacy in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) represents the majority of liver cancer and is the fourth most common cause of cancer-related death. Although advances in molecular targeted therapy have shown promise, none of these agents has yet demonstrated significant clinical benefit. Bromo- and extraterminal domain (BET) protein inhibitors have been considered potential therapeutic drugs for HCC but the biological activity remains unclear. This study found that BET protein inhibition did not effectively suppress the progression of HCC, using a transgenic HCC mouse model. Mechanistically, the BET protein inhibitor JQ1 upregulated the expression of programmed cell death-ligand 1 (PD-L1) on the plasma membrane in vivo and in vitro. Moreover, JQ1 enhanced the expression of Rab8A, which upregulated the expression of PD-L1 on the plasma membrane. This study also showed that JQ1 combined with anti-PD-L1 Ab effectively suppressed HCC progression, and this benefit was obtained by enhancing the activation and cytotoxic capabilities of CD8 T cells. These results revealed the crucial role and regulation of BET protein inhibition on the expression of PD-L1 in HCC. Thus, combining BET protein inhibition with immune checkpoint blockade offers an efficient therapeutic approach for HCC.