Identification of a novel phosphonocarboxylate inhibitor of Rab geranylgeranyl transferase that specifically prevents Rab prenylation in osteoclasts and macrophages

Lipid metabolic vulnerabilities of multiple myeloma

Multiple myeloma (MM) is the second most common hematological malignancy worldwide, characterized by abnormal proliferation of malignant plasma cells within a tumor-permissive bone marrow microenvironment. Metabolic dysfunctions are emerging as key determinants in the pathobiology of MM. In this review, we highlight the metabolic features of MM, showing how alterations in various lipid pathways, mainly involving fatty acids, cholesterol and sphingolipids, affect the growth, survival and drug responsiveness of MM cells, as well as their cross-talk with other cellular components of the tumor microenvironment. These findings will provide a new path to understanding the mechanisms underlying how lipid vulnerabilities may arise and affect the phenotype of malignant plasma cells, highlighting novel druggable pathways with a significant impact on the management of MM.

Brain-Type Creatine Kinase Release from Cultured Osteoclasts Exposed to Neridronate in Children Affected by Osteogenesis Imperfecta Type 1

Brain-type creatine kinase (CK-BB) increases during osteoclastogenesis, with high circulating amounts in type I osteogenesis imperfecta (OI) following treatment with neridronate, a bisphosphonate able to inhibit osteoclast activity and survival. The aim of this study was to demonstrate the correlation between osteoclastogenesis and CK-BB release from OI patients' osteoclasts treated with different concentrations of neridronate. Our patients showed reduced bone quality, increased levels of CTX I, a marker of bone resorption, and decreased levels of OPG, an inhibitor of osteoclastogenesis. In OI patients, the presence of MCSF and RANKL determined an increased secretion of CK-BB from osteoclasts (p = 0.04) compared with control conditions without these cytokines; interestingly, in the absence of these factors, the secretion of CK-BB is significantly elevated at 3 µmol/L compared with 0.03 and 1 µmol/L (p = 0.007). In healthy donors' cultures, the higher concentration of CK-BB can be detected following stimulation with 3 µmol/L neridronate compared with the untreated condition both with and without MCSF and RANKL (p = 0.03 and p = 0.006, respectively). Consistently, in osteoclast cultures, neridronate treatment is associated with a decrease in multinucleated TRAP+ cells, together with morphology changes typical of apoptosis. Consistently, in the media of the same osteoclast cultures, we demonstrated a significant increase in caspase-3 levels. In conclusion, our findings support the idea that CK-BB levels increase in the serum of OI-treated patients.

Targeting the Isoprenoid Biosynthetic Pathway in Multiple Myeloma

Multiple myeloma (MM) is a plasma cell malignancy for which there is currently no cure. While treatment options for MM have expanded over the last two decades, all patients will eventually become resistant to current therapies. Thus, there is an urgent need for novel therapeutic strategies to treat MM. The isoprenoid biosynthetic pathway (IBP) is responsible for the post-translational modification of proteins belonging to the Ras small GTPase superfamily, such as Ras, Rho and Rab family members. Given the important roles these GTPase proteins play in various cellular processes, there is significant interest in the development of inhibitors that disturb their prenylation and consequently their activity in MM cells. Numerous preclinical studies have demonstrated that IBP inhibitors have anti-MM effects, including the induction of apoptosis in MM cells and inhibition of osteoclast activity. Some IBP inhibitors have made their way into the clinic. For instance, nitrogenous bisphosphonates are routinely prescribed for the management MM bone disease. Other IBP inhibitors, including statins and farnesyltransferase inhibitors, have been evaluated in clinical trials for MM, while there is substantial preclinical investigation into geranylgeranyl diphosphate synthase inhibitors. Here we discuss recent advances in the development of IBP inhibitors, assess their mechanism of action and evaluate their potential as anti-MM agents.

Therapeutic Targeting of Rab GTPases: Relevance for Alzheimer’s Disease

Rab GTPases (Rabs) are small proteins that play crucial roles in vesicle transport and membrane trafficking. Owing to their widespread functions in several steps of vesicle trafficking, Rabs have been implicated in the pathogenesis of several disorders, including cancer, diabetes, and multiple neurodegenerative diseases. As treatments for neurodegenerative conditions are currently rather limited, the identification and validation of novel therapeutic targets, such as Rabs, is of great importance. This review summarises proof-of-concept studies, demonstrating that modulation of Rab GTPases in the context of Alzheimer’s disease (AD) can ameliorate disease-related phenotypes, and provides an overview of the current state of the art for the pharmacological targeting of Rabs. Finally, we also discuss the barriers and challenges of therapeutically targeting these small proteins in humans, especially in the context of AD.

Rational design, optimization, and biological evaluation of novel α-Phosphonopropionic acids as covalent inhibitors of Rab geranylgeranyl transferase

Rab geranylgeranyltransferase (GGTase-II, RGGT) catalyses the post-translational modification of eukaryotic Rab GTPases, proteins implicated in several pathologies, including cancer, diabetes, neurodegenerative, and infectious diseases. Thus, RGGT inhibitors are believed to be a potential platform for the development of drugs and tools for studying processes related to the abnormal activity of Rab GTPases. Here, a series of new α-phosphonocarboxylates have been prepared in the first attempt of rational design of covalent inhibitors of RGGT derived from non-covalent inhibitors. These compounds were equipped with electrophilic groups capable of binding cysteines, which are present in the catalytic cavity of RGGT. A few of these analogues have shown micromolar activity against RGGT, which correlated with their ability to inhibit the proliferation of the HeLa cancer cell line. The proposed mechanism of this inhibitory activity was rationalised by molecular docking and mass spectrometric measurements, supported by stability and reactivity studies.

Ras and Rab Interactor 3: From Cellular Mechanisms to Human Diseases

Ras and Rab interactor 3 (RIN3) functions as a Guanine nucleotide Exchange Factor (GEF) for some members of the Rab family of small GTPase. By promoting the activation of Rab5, RIN3 plays an important role in regulating endocytosis and endocytic trafficking. In addition, RIN3 activates Ras, another small GTPase, that controls multiple signaling pathways to regulate cellular function. Increasing evidence suggests that dysregulation of RIN3 activity may contribute to the pathogenesis of several disease conditions ranging from Paget’s Disease of the Bone (PDB), Alzheimer’s Disease (AD), Chronic Obstructive Pulmonary Disease (COPD) and to obesity. Recent genome-wide association studies (GWAS) identified variants in the RIN3 gene to be linked with these disease conditions. Interestingly, some variants appear to be missense mutations in the functional domains of the RIN3 protein while most variants are located in the noncoding regions of the RIN3 gene, potentially altering its gene expression. However, neither the protein structure of RIN3 nor its exact function(s) (except for its GEF activity) has been fully defined. Furthermore, how the polymorphisms/variants contribute to disease pathogenesis remain to be understood. Herein, we examine, and review published studies in an attempt to provide a better understanding of the physiological function of RIN3; More importantly, we construct a framework linking the polymorphisms/variants of RIN3 to altered cell signaling and endocytic traffic, and to potential disease mechanism(s).

Bisphosphonates: The role of chemistry in understanding their biological actions and structure-activity relationships, and new directions for their therapeutic use

The bisphosphonates ((HO)2P(O)CR1R2P(O)(OH)2, BPs) were first shown to inhibit bone resorption in the 1960s, but it was not until 30 years later that a detailed molecular understanding of the relationship between their varied chemical structures and biological activity was elucidated. In the 1990s and 2000s, several potent bisphosphonates containing nitrogen in their R2 side chains (N-BPs) were approved for clinical use including alendronate, risedronate, ibandronate, and zoledronate. These are now mostly generic drugs and remain the leading therapies for several major bone-related diseases, including osteoporosis and skeletal-related events associated with bone metastases. The early development of chemistry in this area was largely empirical and only a few common structural features related to strong binding to calcium phosphate were clear. Attempts to further develop structure-activity relationships to explain more dramatic pharmacological differences in vivo at first appeared inconclusive, and evidence for mechanisms underlying cellular effects on osteoclasts and macrophages only emerged after many years of research. The breakthrough came when the intracellular actions on the osteoclast were first shown for the simpler bisphosphonates, via the in vivo formation of P-C-P derivatives of ATP. The synthesis and biological evaluation of a large number of nitrogen-containing bisphosphonates in the 1980s and 1990s led to the key discovery that the antiresorptive effects of these more complex analogs on osteoclasts result mostly from their potency as inhibitors of the enzyme farnesyl diphosphate synthase (FDPS/FPPS). This key branch-point enzyme in the mevalonate pathway of cholesterol biosynthesis is important for the generation of isoprenoid lipids that are utilized for the post-translational modification of small GTP-binding proteins essential for osteoclast function. Since then, it has become even more clear that the overall pharmacological effects of individual bisphosphonates on bone depend upon two key properties: the affinity for bone mineral and inhibitory effects on biochemical targets within bone cells, in particular FDPS. Detailed enzyme-ligand crystal structure analysis began in the early 2000s and advances in our understanding of the structure-activity relationships, based on interactions with this target within the mevalonate pathway and related enzymes in osteoclasts and other cells have continued to be the focus of research efforts to this day. In addition, while many members of the bisphosphonate drug class share common properties, now it is more clear that chemical modifications to create variations in these properties may allow customization of BPs for different uses. Thus, as the appreciation for new potential opportunities with this drug class grows, new chemistry to allow ready access to an ever-widening variety of bisphosphonates continues to be developed. Potential new uses of the calcium phosphate binding mechanism of bisphosphonates for the targeting of other drugs to the skeleton, and effects discovered on other cellular targets, even at non-skeletal sites, continue to intrigue scientists in this research field.

Synthesis of the 6-Substituted Imidazo[1,2-a]Pyridine-3-yl-2- Phosphonopropionic Acids as Potential Inhibitors of Rab Geranylgeranyl Transferase

Twelve phosphonopropionates derived from 2-hydroxy-3-imidazo[1,2-a]pyridin-3-yl-2-phosphonopropionic acid (3-IPEHPC) were synthesized and evaluated for their activity as inhibitors of protein geranylgeranylation. The nature of the substituent in the C6 position of imidazo[1,2-a]pyridine ring was responsible for the compound's activity against Rab geranylgeranyl transferase (RGGT). The most active inhibitors disrupted Rab11A prenylation in the human cervical carcinoma HeLa cell line. The esterification of carboxylic acid in the phosphonopropionate moiety turned the inhibitor into an inactive analog.

Rab GTPases in Osteoclastic Bone Resorption and Autophagy

Small guanosine triphosphate hydrolases (GTPases) of the Rab family are involved in plasma membrane delivery, fusion events, and lysosomal and autophagic degradation pathways, thereby regulating signaling pathways and cell differentiation and function. Osteoclasts are bone-resorbing cells that maintain bone homeostasis. Polarized vesicular trafficking pathways result in the formation of the ruffled border, the osteoclast’s resorptive organelle, which also assists in transcytosis. Here, we reviewed the different roles of Rab GTPases in the endomembrane machinery of osteoclasts and in bone diseases caused by the dysfunction of these proteins, with a particular focus on autophagy and bone resorption. Understanding the molecular mechanisms underlying osteoclast-related bone disease development is critical for developing and improving therapies.

Molecular mechanisms of action of bisphosphonates and new insights into their effects outside the skeleton

Bisphosphonates (BP) are a class of calcium-binding drug used to prevent bone resorption in skeletal disorders such as osteoporosis and metastatic bone disease. They act by selectively targeting bone-resorbing osteoclasts and can be grouped into two classes depending on their intracellular mechanisms of action. Simple BPs cause osteoclast apoptosis after cytoplasmic conversion into toxic ATP analogues. In contrast, nitrogen-containing BPs potently inhibit FPP synthase, an enzyme of the mevalonate (cholesterol biosynthesis) pathway. This results in production of a toxic metabolite (ApppI) and the loss of long-chain isoprenoid lipids required for protein prenylation, a process necessary for the function of small GTPase proteins essential for the survival and activity of osteoclasts. In this review we provide a state-of-the-art overview of these mechanisms of action and a historical perspective of how they were discovered. Finally, we challenge the long-held dogma that BPs act only in the skeleton and highlight recent studies that reveal insights into hitherto unknown effects on tumour-associated and tissue-resident macrophages.

Insights about the structure of farnesyl diphosphate synthase (FPPS) and the activity of bisphosphonates on the proliferation and ultrastructure of Leishmania and Giardia

Background

The enzyme farnesyl diphosphate synthase (FPPS) is positioned in the intersection of different sterol biosynthesis pathways such as those producing isoprenoids, dolichols and ergosterol. FPPS is ubiquitous in eukaryotes and is inhibited by nitrogen-containing bisphosphonates (N-BP). N-BP activity and the mechanisms of cell death as well as damage to the ultrastructure due to N-BP has not yet been investigated in Leishmania infantum and Giardia. Thus, we evaluated the effect of N-BP on cell viability and ultrastructure and then performed structural modelling and phylogenetic analysis on the FPPS enzymes of Leishmania and Giardia.

Methods

We performed multiple sequence alignment with MAFFT, phylogenetic analysis with MEGA7, and 3D structural modelling for FPPS with Modeller 9.18 and on I-Tasser server. We performed concentration curves with N-BP in Leishmania promastigotes and Giardia trophozoites to estimate the IC₅₀via the MTS/PMS viability method. The ultrastructure was evaluated by transmission electron microscopy, and the mechanism of cell death by flow cytometry.

Results

The nitrogen-containing bisphosphonate risedronate had stronger anti-proliferative activity in Leishmania compared to other N-BPs with an IC₅₀ of 13.8 µM, followed by ibandronate and alendronate with IC₅₀ values of 85.1 µM and 112.2 µM, respectively. The effect of N-BPs was much lower on trophozoites of Giardia than Leishmania (IC₅₀ of 311 µM for risedronate). Giardia treated with N-BP displayed concentric membranes around the nucleus and nuclear pyknosis. Leishmania had mitochondrial swelling, myelin figures, double membranes, and plasma membrane blebbing. The same population labelled with annexin-V and 7-AAD had a loss of membrane potential (TMRE), indicative of apoptosis. Multiple sequence alignments and structural alignments of FPPS proteins showed that Giardia and Leishmania FPPS display low amino acid identity but possess the conserved aspartate-rich motifs.

Conclusions

Giardia and Leishmania FPPS enzymes are phylogenetically distant but display conserved protein signatures. The N-BPs effect on FPPS was more pronounced in Leishmania than Giardia. This might be due to general differences in metabolism and differences in the FPPS catalytic site.

Novel benzimidazole phosphonates as potential inhibitors of protein prenylation

Benzimidazole carboxyphosphonates and bisphosphonates have been prepared and evaluated for their activity as inhibitors of protein prenylation or isoprenoid biosynthesis. The nature of the phosphonate head group was found to dictate enzyme specificity. The lead carboxyphosphonate inhibits geranylgeranyl transferase II while its corresponding bisphosphonate analogue potently inhibits farnesyl diphosphate synthase. The most active inhibitors effectively disrupted protein prenylation in human multiple myeloma cells.

In Vivo Evaluation of Isoprenoid Triazole Bisphosphonate Inhibitors of Geranylgeranyl Diphosphate Synthase: Impact of Olefin Stereochemistry on Toxicity and Biodistribution

The enzyme geranylgeranyl diphosphate synthase (GGDPS) synthesizes the 20-carbon isoprenoid geranylgeranyl pyrophosphate, which is used in geranylgeranylation reactions. We have demonstrated that GGDPS inhibitors in multiple myeloma (MM) cells disrupt Rab geranylgeranylation, leading to inhibition of monoclonal protein trafficking, induction of the unfolded protein response pathway (UPR), and apoptosis. We have previously reported preclinical studies with the GGDPS inhibitor VSW1198, which is a mixture of homogeranyl/homoneryl triazole bisphosphonates. Additional structure-function efforts have led to development of the α-methylated derivatives RAM2093 (homogeranyl) and RAM2061 (homoneryl). As little is known regarding the impact of olefin stereochemistry on drug properties in vivo, we pursued additional preclinical evaluation of RAM2093 and RAM2061. In MM cell lines, both isomers induce activation of UPR/apoptotic markers in a concentration-dependent manner and with similar potency. Single-dose testing in CD-1 mice identified a maximum tolerated i.v. dose of 0.5 mg/kg for RAM2061 and 0.3 mg/kg for RAM2093. Liver toxicity was the primary barrier to dose escalation for both compounds. Disruption of geranylgeranylation in vivo was confirmed after multidose administration of either compound. Pharmacokinetic studies revealed plasma terminal half-lives of 29.2 ± 6 (RAM2061) and 22.1 ± 4 hours (RAM2093). Relative to RAM2061, RAM2093 levels were significantly higher in liver tissue but not in other tissues. Using MM.1S flank xenografts, we observed a significant reduction in tumor growth in mice treated with RAM2061 relative to controls. Collectively, these studies reveal olefin stereochemistry-dependent effects on GGDPS inhibitor biodistribution and confirm the in vivo efficacy of this novel therapeutic approach. SIGNIFICANCE STATEMENT: These studies reveal olefin stereochemistry-dependent effects on the in vivo properties of two novel triazole bisphosphonate inhibitors of geranylgeranyl diphosphate synthase and demonstrate the therapeutic potential of this class of inhibitors for the treatment of multiple myeloma.

Rescue bisphosphonate treatment of alveolar bone improves extraction socket healing and reduces osteonecrosis in zoledronate-treated mice

Bisphosphonate (BP)-related osteonecrosis of the jaw, previously known as BRONJ, now referred to more broadly as medication-related osteonecrosis of the jaw (MRONJ), is a morbid condition that represents a significant risk for oncology patients who have received high dose intravenous (IV) infusion of a potent nitrogen containing BP (N-BP) drug. At present, no clinical procedure is available to prevent or effectively treat MRONJ. Although the pathophysiological basis is not yet fully understood, legacy adsorbed N-BP in jawbone has been proposed to be associated with BRONJ by one or more mechanisms. We hypothesized that removal of the pre-adsorbed N-BP drug common to these pathological mechanisms from alveolar bone could be an effective preventative/therapeutic strategy. This study demonstrates that fluorescently labeled BP pre-adsorbed on the surface of murine maxillo-cranial bone in vivo can be displaced by subsequent application of other BPs. We previously described rodent BRONJ models involving the combination of N-BP treatment such as zoledronate (ZOL) and dental initiating factors such as tooth extraction. We further refined our mouse model by using gel food during the first 7 days of the tooth extraction wound healing period, which decreased confounding food pellet impaction problems in the open boney socket. This refined mouse model does not manifest BRONJ-like severe jawbone exposure, but development of osteonecrosis around the extraction socket and chronic gingival inflammation are clearly exhibited. In this study, we examined the effect of benign BP displacement of legacy N-BP on tooth extraction wound healing in the in vivo model. Systemic IV administration of a low potency BP (lpBP: defined as inactive at 100 μM in a standard protein anti-prenylation assay) did not significantly attenuate jawbone osteonecrosis. We then developed an intra-oral formulation of lpBP, which when injected into the gingiva adjacent to the tooth prior to extraction, dramatically reduced the osteocyte necrosis area. Furthermore, the tooth extraction wound healing pattern was normalized, as evidenced by timely closure of oral soft tissue without epithelial hyperplasia, significantly reduced gingival inflammation and increased new bone filling in the extraction socket. Our results are consistent with the hypothesis that local application of a rescue BP prior to dental surgery can decrease the amount of a legacy N-BP drug in proximate jawbone surfaces below the threshold that promotes osteocyte necrosis. This observation should provide a conceptual basis for a novel strategy to improve socket healing in patients treated with BPs while preserving therapeutic benefit from anti-resorptive N-BP drug in vertebral and appendicular bones.

Membrane trafficking in osteoclasts and implications for osteoporosis

Osteoclasts are large multinucleated cells exquisitely adapted to resorb bone matrix. Like other eukaryotes, osteoclasts possess an elaborate ensemble of intracellular organelles through which solutes, proteins and other macromolecules are trafficked to their target destinations via membrane-bound intermediaries. During bone resorption, membrane trafficking must be tightly regulated to sustain the structural and functional polarity of the osteoclasts’ membrane domains. Of these, the ruffled border (RB) is most characteristic, functioning as the osteoclasts' secretory apparatus. This highly convoluted organelle is classically considered to be formed by the targeted fusion of acidic vesicles with the bone-facing plasma membrane. Emerging findings disclose new evidence that the RB is far more complex than previously envisaged, possessing discrete subdomains that are serviced by several intersecting endocytic, secretory, transcytotic and autophagic pathways. Bone-resorbing osteoclasts therefore serve as a unique model system for studying polarized membrane trafficking. Recent advances in high-resolution microscopy together with the convergence of genetic and cell biological studies in humans and in mice have helped illuminate the major membrane trafficking pathways in osteoclasts and unmask the core molecular machinery that governs these distinct vesicle transport routes. Among these, small Rab GTPases, their binding partners and members of the endocytic sorting nexin family have emerged as critical regulators. This mini review summarizes our current understanding of membrane trafficking in osteoclasts, the key molecular participants, and discusses how these transport machinery may be exploited for the development of new therapies for metabolic disorders of bone-like osteoporosis.

The Many Faces of Rap1 GTPase

This review addresses the issue of the numerous roles played by Rap1 GTPase (guanosine triphosphatase) in different cell types, in terms of both physiology and pathology. It is one among a myriad of small G proteins with endogenous GTP-hydrolyzing activity that is considerably stimulated by posttranslational modifications (geranylgeranylation) or guanine nucleotide exchange factors (GEFs), and inhibited by GTPase-activating proteins (GAPs). Rap1 is a ubiquitous protein that plays an essential role in the control of metabolic processes, such as signal transduction from plasma membrane receptors, cytoskeleton rearrangements necessary for cell division, intracellular and substratum adhesion, as well as cell motility, which is needed for extravasation or fusion. We present several examples of how Rap1 affects cells and organs, pointing to possible molecular manipulations that could have application in the therapy of several diseases.

Isoprenoids and protein prenylation: implications in the pathogenesis and therapeutic intervention of Alzheimer's disease

The mevalonate-isoprenoid-cholesterol biosynthesis pathway plays a key role in human health and disease. The importance of this pathway is underscored by the discovery that two major isoprenoids, farnesyl and geranylgeranyl pyrophosphate, are required to modify an array of proteins through a process known as protein prenylation, catalyzed by prenyltransferases. The lipophilic prenyl group facilitates the anchoring of proteins in cell membranes, mediating protein-protein interactions and signal transduction. Numerous essential intracellular proteins undergo prenylation, including most members of the small GTPase superfamily as well as heterotrimeric G proteins and nuclear lamins, and are involved in regulating a plethora of cellular processes and functions. Dysregulation of isoprenoids and protein prenylation is implicated in various disorders, including cardiovascular and cerebrovascular diseases, cancers, bone diseases, infectious diseases, progeria, and neurodegenerative diseases including Alzheimer's disease (AD). Therefore, isoprenoids and/or prenyltransferases have emerged as attractive targets for developing therapeutic agents. Here, we provide a general overview of isoprenoid synthesis, the process of protein prenylation and the complexity of prenylated proteins, and pharmacological agents that regulate isoprenoids and protein prenylation. Recent findings that connect isoprenoids/protein prenylation with AD are summarized and potential applications of new prenylomic technologies for uncovering the role of prenylated proteins in the pathogenesis of AD are discussed.

Synthesis and Biological Evaluation of Imidazole-Bearing α-Phosphonocarboxylates as Inhibitors of Rab Geranylgeranyl Transferase (RGGT)

Rab geranylgeranyl transferase (RGGT) is an interesting therapeutic target, as it ensures proper functioning of Rab GTPases, a class of enzymes responsible for the regulation of vesicle trafficking. Relying on our previous studies, we synthesized a set of new α-phosphonocarboxylic acids as potential RGGT inhibitors, with emphasis on the elaboration of imidazole-containing analogues. We identified two compounds with activity similar to that of previously reported RGGT inhibitors, showing structural similarity to imidazo[1,2-a]pyridine-containing analogues in terms of their substitution pattern. Interestingly, analogues of the N-series, derived from another phosphonocarboxylate RGGT inhibitor, 2-fluoro-3-(1H-imidazol-1-yl)-2-phosphonopropanoic acid, turned out to be inactive in our model, indicating that an additional substituent localized at positions C2 or C4 of the imidazole ring, may adversely affect the potency against the targeted enzyme.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Identification of the Privileged Position in the Imidazo[1,2-a]pyridine Ring of Phosphonocarboxylates for Development of Rab Geranylgeranyl Transferase (RGGT) Inhibitors

Members of the Rab GTPase family are master regulators of vesicle trafficking. When disregulated, they are associated with a number of pathological states. The inhibition of RGGT, an enzyme responsible for post-translational geranylgeranylation of Rab GTPases represents one way to control the activity of these proteins. Because the number of molecules modulating RGGT is limited, we combined molecular modeling with biological assays to ascertain how modifications of phosphonocarboxylates, the first reported RGGT inhibitors, rationally improve understanding of their structure-activity relationship. We have identified the privileged position in the core scaffold of the imidazo[1,2-a]pyridine ring, which can be modified without compromising compounds' potency. Thus modified compounds are micromolar inhibitors of Rab11A prenylation, simultaneously being inactive against Rap1A/Rap1B modification, with the ability to inhibit proliferation of the HeLa cancer cell line. These findings were rationalized by molecular docking, which recognized interaction of phosphonic and carboxylic groups as decisive in phosphonocarboxylate localization in the RGGT binding site.

A supplement-free osteoclast-osteoblast co-culture for pre-clinical application

There is increasing demand for efficient and physiological in vitro cell culture systems suitable for testing new pharmaceutical drugs or for evaluating materials for tissue regeneration. In particular, co-cultures of two or more tissue-relevant cell types have the advantage to study the response of cells on diverse parameters in a more natural environment with respect to physiological complexity. We developed a direct bone cell co-culture system using human peripheral blood monocytes (hPBMC) and human bone marrow stromal cells (hBMSC) as osteoclast/osteoblast precursor cells, respectively, strictly avoiding external supplements for the induction of differentiation. The sophisticated direct hPBMC/hBMSC co-culture was characterized focusing on osteoclast function and was compared with two indirect approaches. Only in the direct co-culture, hPBMC were triggered by hBMSC into osteoclastogenesis and became active resorbing osteoclasts. Bisphosphonates and sulfated glycosaminoglycans were used to examine the suitability of the co-culture system for evaluating the influence of certain effectors on bone healing and bone regeneration and the contribution of each cell type thereby. The results show that the investigated substances had more pronounced effects on both osteoblasts and osteoclasts in the co-culture system than in respective monocultures.

Chemical approaches to inhibitors of isoprenoid biosynthesis: targeting farnesyl and geranylgeranyl pyrophosphate synthases

The chemical synthesis of farnesyl and geranylgeranyl pyrophosphate synthase inhibitors are surveyed.

Calmodulin interacts with Rab3D and modulates osteoclastic bone resorption

Calmodulin is a highly versatile protein that regulates intracellular calcium homeostasis and is involved in a variety of cellular functions including cardiac excitability, synaptic plasticity and signaling transduction. During osteoclastic bone resorption, calmodulin has been reported to concentrate at the ruffled border membrane of osteoclasts where it is thought to modulate bone resorption activity in response to calcium. Here we report an interaction between calmodulin and Rab3D, a small exocytic GTPase and established regulator osteoclastic bone resorption. Using yeast two-hybrid screening together with a series of protein-protein interaction studies, we show that calmodulin interacts with Rab3D in a calcium dependent manner. Consistently, expression of a calcium insensitive form of calmodulin (i.e. CaM1234) perturbs calmodulin-Rab3D interaction as monitored by bioluminescence resonance energy transfer (BRET) assays. In osteoclasts, calmodulin and Rab3D are constitutively co-expressed during RANKL-induced osteoclast differentiation, co-occupy plasma membrane fractions by differential gradient sedimentation assay and colocalise in the ruffled border as revealed by confocal microscopy. Further, functional blockade of calmodulin-Rab3D interaction by calmidazolium chloride coincides with an attenuation of osteoclastic bone resorption. Our data imply that calmodulin- Rab3D interaction is required for efficient bone resorption by osteoclasts in vitro.

Mechanisms for autophagy modulation by isoprenoid biosynthetic pathway inhibitors in multiple myeloma cells

Multiple myeloma (MM) is characterized by the production of monoclonal protein (MP). We have shown previously that disruption of the isoprenoid biosynthetic pathway (IBP) causes a block in MP secretion through a disruption of Rab GTPase activity, leading to an enhanced unfolded protein response and subsequent apoptosis in MM cells. Autophagy is induced by cellular stressors including nutrient deprivation and ER stress. IBP inhibitors have been shown to have disparate effects on autophagy. Here we define the mechanisms underlying the differential effects of IBP inhibitors on autophagic flux in MM cells utilizing specific pharmacological inhibitors. We demonstrate that IBP inhibition induces a net increase in autophagy as a consequence of disruption of isoprenoid biosynthesis which is not recapitulated by direct geranylgeranyl transferase inhibition. IBP inhibitor-induced autophagy is a cellular defense mechanism as treatment with the autophagy inhibitor bafilomycin A1 enhances the cytotoxic effects of GGPP depletion, but not geranylgeranyl transferase inhibition. Immunofluorescence microscopy studies revealed that IBP inhibitors disrupt ER to Golgi trafficking of monoclonal light chain protein and that this protein is not a substrate for alternative degradative pathways such as aggresomes and autophagosomes. These studies support further development of specific GGTase II inhibitors as anti-myeloma agents.

Mevalonate kinase deficiency leads to decreased prenylation of Rab GTPases

Mevalonate kinase deficiency (MKD) is caused by mutations in a key enzyme of the mevalonate–cholesterol biosynthesis pathway, leading to recurrent autoinflammatory disease characterised by enhanced release of interleukin-1β (IL-1β). It is currently believed that the inflammatory phenotype of MKD is triggered by temperature-sensitive loss of mevalonate kinase activity and reduced biosynthesis of isoprenoid lipids required for the prenylation of small GTPase proteins. However, previous studies have not clearly shown any change in protein prenylation in patient cells under normal conditions. With lymphoblast cell lines from two compound heterozygous MKD patients, we used a highly sensitive in vitro prenylation assay, together with quantitative mass spectrometry, to reveal a subtle accumulation of unprenylated Rab GTPases in cells cultured for 3 days or more at 40 °C compared with 37 °C. This included a 200% increase in unprenylated Rab7A, Rab14 and Rab1A. Inhibition of sterol regulatory element-binding protein (SREBP) activation by fatostatin led to more pronounced accumulation of unprenylated Rab proteins in MKD cells but not parent cells, suggesting that cultured MKD cells may partially overcome the loss of isoprenoid lipids by SREBP-mediated upregulation of enzymes required for isoprenoid biosynthesis. Furthermore, while inhibition of Rho/Rac/Rap prenylation promoted the release of IL-1β, specific inhibition of Rab prenylation by NE10790 had no effect in human peripheral blood mononuclear cells or human THP-1 monocytic cells. These studies demonstrate for the first time that mutations in mevalonate kinase can lead to a mild, temperature-induced defect in the prenylation of small GTPases, but that loss of prenylated Rab GTPases is not the cause of enhanced IL-1β release in MKD.

In Vitro Effects of Aminobisphosphonates on Vγ9Vδ2 T Cell Activation and Differentiation

In this study we have evaluated the in vitro effects of four different aminobisphosphonates, alendronate, risedronate, neridronate and zoledronate, on Vγ9Vδ2 T cell activation and differentiation. All tested aminobisphosphonates induce an IL-2-dependent activation and expansion of Vγ9Vδ2 T lymphocytes in primary PBMC cultures of healthy donors. Most notably, they also determine a different distribution of Vγ9Vδ2 T cell subsets, with decrease of Tnaive and TCM cells and increase of TEM and TEMRA Vγ9Vδ2 cells, indicating that in vitro treatment with aminobisphosphonates induces Vγ9Vδ2 T lymphocytes to differentiate towards an effector/cytotoxic phenotype. Accordingly, Vγ9Vδ2 T lymphocytes cultured with aminobisphosphonates and IL-2 showed a major content of IFN-γ and acquired the ability to kill tumor target cells.

Evolutionary comparison of prenylation pathway in kinetoplastid Leishmania and its sister Leptomonas

Background

Leptomonas is monogenetic kinetoplastid parasite of insects and is primitive in comparison to Leishmania. Comparative studies of these two kinetoplastid may share light on the evolutionary transition to dixenous parasitism in Leishmania. In order to adapt and survive within two hosts, Leishmania species must have acquired virulence factors in addition to mechanisms that mediate susceptibility/resistance to infection in the pathology associated with disease. Rab proteins are key mediators of vesicle transport and contribute greatly to the evolution of complexity of membrane transport system. In this study we used our whole genome sequence data of these two divergent kinetoplastids to analyze the orthologues/paralogues of Rab proteins.

Results

During change of lifestyle from monogenetic (Leptomonas) to digenetic (Leishmania), we found that the prenyl machinery remained unchanged. Geranylgeranyl transferase-I (GGTase-I) was absent in both Leishmania and its sister Leptomonas. Farnesyltransferase (FTase) and geranylgeranyl transferase-II (GGTase-II) were identified for protein prenylation. We predict that activity of the missing alpha-subunit (α-subunit) of GGTase-II in Leptomonas was probably contributed by the α-subunit of FTase, while beta-subunit (β-subunit) of GGTase-II was conserved and indicated functional conservation in the evolution of these two kinetoplastids. Therefore the β-subunit emerges as an excellent target for compounds inhibiting parasite activity in clinical cases of co-infections. We also confirmed that during the evolution to digenetic life style in Leishmania, the parasite acquired capabilities to evade drug action and maintain parasite virulence in the host with the incorporation of short-chain dehydrogenase/reductase (SDR/MDR) superfamily in Rab genes.

Conclusion

Our study based on whole genome sequences is the first to build comparative evolutionary analysis and identification of prenylation proteins in Leishmania and its sister Leptomonas. The information presented in our present work has importance for drug design targeted to kill L. donovani in humans but not affect the human form of the prenylation enzymes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0538-3) contains supplementary material, which is available to authorized users.

N-Oxide derivatives of 3-(3-pyridyl)-2-phosphonopropanoic acids as potential inhibitors of Rab geranylgeranylation

The N-oxide derivatives of [2-(3-pyridinyl)-1-hydroxyethylidene-1,1-phosphonocarboxylic acid (or PEHPC) and [2-(3-pyridinyl)-1-ethylidene-1,1-phosphonocarboxylic acid (or PEPC) have been prepared and evaluated for their activity against several enzymes which utilize isoprenoids. The parent pyridines are known inhibitors of GGTase II, but the N-oxide derivatives show no improvement in biological activity in assays with the isolated enzyme. However, the PEHPC N-oxide did induce significant accumulation of intracellular light chain in myeloma cells, consistent with inhibition of Rab geranylgeranylation.

Induction of creatine kinase release from cultured osteoclasts via the pharmacological action of aminobisphosphonates

An increase of serum creatine kinase (CK) has been observed in clinical studies of nitrogen-containing aminobisphosphonates (N-BPs). Osteoclasts are thought to be the source of the CK, but there is no clear evidence for the hypothesis. In this study, CK release from rabbit osteoclasts induced by N-BPs was examined in an in vitro culture system. Rabbit bone-derived cells were cultured for 3 days on the N-BPs pretreated cortical bone slices. CK activity in the culture medium was measured at 3 days of culture. The CK activity was increased with all N-BPs at concentrations at which showed antiresorptive activity over 60% inhibition of C-terminal cross-linking telopeptide of type I collagen (CTX-1) release. The maximum induction of CK activity was 2.6 times the control level. The lowest N-BP concentration inducing CK release was 3 times lower than that required to decrease the osteoclast number. Bafilomycin A1, an inhibitor of vacuolar H⁺-ATPase, abrogated all N-BP actions, including CK release. Bone-derived cells except osteoclasts were insensitive to bafilomycin A1, suggesting that osteoclasts were the source of CK. Regarding the time course, CK release occurred after a 1 day lag time and increased steadily until day 3 of culture. These results show that CK release is induced by N-BPs from osteoclasts at concentrations at which N-BPs show antiresorptive activity over 60% inhibition of CTX-1 release in vitro. These findings explain the mechanism of the CK increase induced by clinical use of N-BPs.

Rab-NANOPS: FRET biosensors for Rab membrane nanoclustering and prenylation detection in mammalian cells

Rab proteins constitute the largest subfamily of Ras-like small GTPases. They are central to vesicular transport and organelle definition in eukaryotic cells. Unlike their Ras counterparts, they are not a hallmark of cancer. However, a number of diseases, including cancer, show a misregulation of Rab protein activity. As for all membrane-anchored signaling proteins, correct membrane organization is critical for Rabs to operate. In this chapter, we provide a detailed protocol for the use of a flow cytometry-based Fluorescence Resonance Energy Transfer (FRET)-biosensors assay, which allows to detect changes in membrane anchorage, subcellular distribution, and of the nanoscale organization of Rab-GTPases in mammalian cell lines. This assay is high-throughput amenable and can therefore be utilized in chemical-genomic and drug discovery efforts.

The regulation of osteoclast function and bone resorption by small GTPases

Osteoclasts are multinucleated cells that are responsible for resorption of bone, and increased activity of these cells is associated with several common bone diseases, including postmenopausal osteoporosis. Upon adhesion to bone, osteoclasts become polarized and reorganise their cytoskeleton and membrane to form unique domains including the sealing zone (SZ), which is a dense ring of F-actin-rich podosomes delimiting the ruffled border (RB), where protons and proteases are secreted to demineralise and degrade the bone matrix, respectively. These processes are dependent on the activity of small GTPases. Rho GTPases are well known to control the organization of F-actin and adhesion structures of different cell types, affecting subsequently their migration. In osteoclasts, RhoA, Rac, Cdc42, RhoU and also Arf6 regulate podosome assembly and their organization into the SZ. By contrast, the formation of the RB involves vesicular trafficking pathways that are regulated by the Rab family of GTPases, in particular lysosomal Rab7. Finally, osteoclast survival is dependent on the activity of Ras GTPases. The correct function of almost all these GTPases is absolutely dependent on post-translational prenylation, which enables them to localize to specific target membranes. Bisphosphonate drugs, which are widely used in the treatment of bone diseases such as osteoporosis, act by preventing the prenylation of small GTPases, resulting in the loss of the SZ and RB and therefore inhibition of osteoclast activity, as well as inducing osteoclast apoptosis. In this review we summarize current understanding of the role of specific prenylated small GTPases in osteoclast polarization, function and survival.

Synthesis and characterization of novel phosphonocarboxylate inhibitors of RGGT

Phosphonocarboxylate (PC) analogs of the anti-osteoporotic drugs, bisphosphonates, represent the first class of selective inhibitors of Rab geranylgeranyl transferase (RabGGTase, RGGT), an enzyme implicated in several diseases including ovarian, breast and skin cancer. Here we present the synthesis and biological characterization of an extended set of this class of compounds, including lipophilic derivatives of the known RGGT inhibitors. From this new panel of PCs, we have identified an inhibitor of RGGT that is of similar potency as the most active published phosphonocarboxylate, but of higher selectivity towards this enzyme compared to prenyl pyrophosphate synthases. New insights into structural requirements are also presented, showing that only PC analogs of the most potent 3rd generation bisphosphonates inhibit RGGT. In addition, the first phosphonocarboxylate-derived GGPPS inhibitor is reported.

Targeting the mevalonate cascade as a new therapeutic approach in heart disease, cancer and pulmonary disease

The cholesterol biosynthesis pathway, also known as the mevalonate (MVA) pathway, is an essential cellular pathway that is involved in diverse cell functions. The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR) is the rate-limiting step in cholesterol biosynthesis and catalyzes the conversion of HMG-CoA to MVA. Given its role in cholesterol and isoprenoid biosynthesis, the regulation of HMGCR has been intensely investigated. Because all cells require a steady supply of MVA, both the sterol (i.e. cholesterol) and non-sterol (i.e. isoprenoid) products of MVA metabolism exert coordinated feedback regulation on HMGCR through different mechanisms. The proper functioning of HMGCR as the proximal enzyme in the MVA pathway is essential under both normal physiologic conditions and in many diseases given its role in cell cycle pathways and cell proliferation, cholesterol biosynthesis and metabolism, cell cytoskeletal dynamics and stability, cell membrane structure and fluidity, mitochondrial function, proliferation, and cell fate. The blockbuster statin drugs ('statins') directly bind to and inhibit HMGCR, and their use for the past thirty years has revolutionized the treatment of hypercholesterolemia and cardiovascular diseases, in particular coronary heart disease. Initially thought to exert their effects through cholesterol reduction, recent evidence indicates that statins also have pleiotropic immunomodulatory properties independent of cholesterol lowering. In this review we will focus on the therapeutic applications and mechanisms involved in the MVA cascade including Rho GTPase and Rho kinase (ROCK) signaling, statin inhibition of HMGCR, geranylgeranyltransferase (GGTase) inhibition, and farnesyltransferase (FTase) inhibition in cardiovascular disease, pulmonary diseases (e.g. asthma and chronic obstructive pulmonary disease (COPD)), and cancer.

The Roles of Small GTPases in Osteoclast Biology

The adult skeleton undergoes bone remodeling that consists of bone formation by osteoblasts and bone resorption by osteoclasts. When the amount of bone resorbed is greater than the amount of new bone formed, low bone mass results, putting individuals at increased risk for osteoporosis and osteoporotic bone fracture. Nitrogenous bisphosphonates (NBPs) are the most common first line treatment for conditions of low bone mass. NBPs reduce osteoclast bone resorption by impairing the post-translational modification of small GTPases. Small GTPases play crucial roles in the differentiation, function, and survival of osteoclasts. Understanding the roles of individual small GTPases in osteoclast biology may lead to more targeted therapies for the treatment of low bone mass. In this review, we discuss recent investigations into the in vivo effects of individual GTPase deletion in osteoclasts and the molecular roles for small GTPases in osteoclast biology.

Targeting HSP90 and monoclonal protein trafficking modulates the unfolded protein response, chaperone regulation and apoptosis in myeloma cells

Multiple myeloma is characterized by the production of substantial quantities of monoclonal protein. We have previously demonstrated that select inhibitors of the isoprenoid biosynthetic pathway (IBP) induce apoptosis of myeloma cells via inhibition of Rab geranylgeranylation, leading to disruption of monoclonal protein trafficking and induction of the unfolded protein response (UPR) pathway. Heat-shock protein 90 (HSP90) inhibitors disrupt protein folding and are currently under clinical investigation in myeloma. The effects of combining IBP and HSP90 inhibitors on cell death, monoclonal protein trafficking, the UPR and chaperone regulation were investigated in monoclonal protein-producing cells. An enhanced induction of cell death was observed following treatment with IBP and HSP90 inhibitors, which occurred through both ER stress and non-ER stress pathways. The HSP90 inhibitor 17-AAG abrogated the effects of the IBP inhibitors on intracellular monoclonal protein levels and localization as well as induction of the UPR in myeloma cells. Disparate effects on chaperone expression were observed in myeloma vs amyloid light chain cells. Here we demonstrate that the novel strategy of targeting MP trafficking in concert with HSP90 enhances myeloma cell death via a complex modulation of ER stress, UPR, and cell death pathways.

Novel functions for Rab GTPases in multiple aspects of tumour progression

Rab GTPases are master regulators of intracellular trafficking and, in recent years, their role in the control of different aspects of tumour progression has emerged. In the present review, we show that Rab GTPases are disregulated in many cancers and have central roles in tumour cell migration, invasion, proliferation, communication with stromal cells and the development of drug resistance. As a consequence, Rab proteins may be novel potential candidates for the development of anticancer drugs and, in this context, the preliminary results obtained with an inhibitor of Rab function are also discussed.

Prenyltransferase Inhibitors: Treating Human Ailments from Cancer to Parasitic Infections

The posttranslational modification of protein prenylation is a covalent lipid modification on the C-terminus of substrate proteins that serves to enhance membrane affinity. Oncogenic proteins such as Ras have this modification and significant effort has been placed into developing inhibitors of the prenyltransferase enzymes for clinical therapy. In addition to cancer therapy, prenyltransferase inhibitors have begun to find important therapeutic uses in other diseases, including progeria, hepatitis C and D, parasitic infections, and other maladies. This review will trace the evolution of prenyltransferase inhibitors from their initial use as cancer therapeutics to their expanded applications for other diseases.

Triazole-based inhibitors of geranylgeranyltransferase II

A small set of triazole bisphosphonates has been prepared and tested for the ability to inhibit geranylgeranyltransferase II (GGTase II). The compounds were prepared through use of click chemistry to assemble a central triazole that links a polar head group to a hydrophobic tail. The resulting compounds were tested for their ability to inhibit GGTase II in an in vitro enzyme assay and also were tested for cytotoxic activity in an MTT assay with the human myeloma RPMI-8226 cell line. The most potent enzyme inhibitor was the triazole with a geranylgeranyl tail, which suggests that inhibitors that can access the enzyme region that holds the isoprenoid tail will display greater activity.

Cannabinoids and bone: endocannabinoids modulate human osteoclast function in vitro

BACKGROUND AND PURPOSE

Both CB(1) and CB(2) cannabinoid receptors have been shown to play a role in bone metabolism. Crucially, previous studies have focussed on the effects of cannabinoid ligands in murine bone cells. This study aimed to investigate the effects of cannabinoids on human bone cells in vitro.

EXPERIMENTAL APPROACH

Quantitative RT-PCR was used to determine expression of cannabinoid receptors and liquid chromatography-electrospray ionization tandem mass spectrometry was used to determine the presence of endocannabinoids in human bone cells. The effect of cannabinoids on human osteoclast formation, polarization and resorption was determined by assessing the number of cells expressing α(v) β(3) or with F-actin rings, or measurement of resorption area.

KEY RESULTS

Human osteoclasts express both CB(1) and CB(2) receptors. CB(2) expression was significantly higher in human monocytes compared to differentiated osteoclasts. Furthermore, the differentiation of human osteoclasts from monocytes was associated with a reduction in 2-AG levels and an increase in anandamide (AEA) levels. Treatment of osteoclasts with LPS significantly increased levels of AEA. Nanomolar concentrations of AEA and the synthetic agonists CP 55 940 and JWH015 stimulated human osteoclast polarization and resorption; these effects were attenuated in the presence of CB(1) and/or CB(2) antagonists.

CONCLUSIONS

AND IMPLICATIONS Low concentrations of cannabinoids activate human osteoclasts in vitro. There is a dynamic regulation of the expression of the CB(2) receptor and the production of the endocannabinoids during the differentiation of human bone cells. These data suggest that small molecules modulating the endocannabinoid system could be important therapeutics in human bone disease.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Rab13 is upregulated during osteoclast differentiation and associates with small vesicles revealing polarized distribution in resorbing cells

Osteoclasts are bone-resorbing multinucleated cells that undergo drastic changes in their polarization due to heavy vesicular trafficking during the resorption cycle. These events require the precise orchestration of membrane traffic in order to maintain the unique characteristics of the different membrane domains in osteoclasts. Rab proteins are small GTPases involved in regulation of most, if not all, steps of vesicle trafficking. The investigators studied RAB genes in human osteoclasts and found that at least 26 RABs were expressed in osteoclasts. Out of these, RAB13 gene expression was highly upregulated during differentiation of human peripheral blood monocytic cells into osteoclasts. To study its possible function in osteoclasts, the investigators performed immunolocalization studies for Rab13 and various known markers of osteoclast vesicular trafficking. Rab13 localized to small vesicular structures at the superior parts of the osteoclast between the trans-Golgi network and basolateral membrane domain. Rab13 localization suggests that it is not involved in endocytosis or transcytosis of bone degradation products. In addition, Rab13 did not associate with early endosomes or recycling endosomes labeled with EEA1 or TRITC-conjugated transferrin, respectively. Its involvement in glucose transporter traffic was excluded as well. It is suggested that Rab13 is associated with a putative secretory function in osteoclasts.

Psoromic acid is a selective and covalent Rab-prenylation inhibitor targeting autoinhibited RabGGTase

Post-translational attachment of geranylgeranyl isoprenoids to Rab GTPases, the key organizers of intracellular vesicular transport, is essential for their function. Rab geranylgeranyl transferase (RabGGTase) is responsible for prenylation of Rab proteins. Recently, RabGGTase inhibitors have been proposed to be potential therapeutics for treatment of cancer and osteoporosis. However, the development of RabGGTase selective inhibitors is complicated by its structural and functional similarity to other protein prenyltransferases. Herein we report identification of the natural product psoromic acid (PA) that potently and selectively inhibits RabGGTase with an IC(50) of 1.3 μM. Structure-activity relationship analysis suggested a minimal structure involving the depsidone core with a 3-hydroxyl and 4-aldehyde motif for binding to RabGGTase. Analysis of the crystal structure of the RabGGTase:PA complex revealed that PA forms largely hydrophobic interactions with the isoprenoid binding site of RabGGTase and that it attaches covalently to the N-terminus of the α subunit. We found that in contrast to other protein prenyltransferases, RabGGTase is autoinhibited through N-terminal (α)His2 coordination with the catalytic zinc ion. Mutation of (α)His dramatically enhances the reaction rate, indicating that the activity of RabGGTase is likely regulated in vivo. The covalent binding of PA to the N-terminus of the RabGGTase α subunit seems to potentiate its interaction with the active site and explains the selectivity of PA for RabGGTase. Therefore, psoromic acid provides a new starting point for the development of selective RabGGTase inhibitors.

Influence of bone affinity on the skeletal distribution of fluorescently labeled bisphosphonates in vivo

Bisphosphonates are widely used antiresorptive drugs that bind to calcium. It has become evident that these drugs have differing affinities for bone mineral; however, it is unclear whether such differences affect their distribution on mineral surfaces. In this study, fluorescent conjugates of risedronate, and its lower-affinity analogues deoxy-risedronate and 3-PEHPC, were used to compare the localization of compounds with differing mineral affinities in vivo. Binding to dentine in vitro confirmed differences in mineral binding between compounds, which was influenced predominantly by the characteristics of the parent compound but also by the choice of fluorescent tag. In growing rats, all compounds preferentially bound to forming endocortical as opposed to resorbing periosteal surfaces in cortical bone, 1 day after administration. At resorbing surfaces, lower-affinity compounds showed preferential binding to resorption lacunae, whereas the highest-affinity compound showed more uniform labeling. At forming surfaces, penetration into the mineralizing osteoid was found to inversely correlate with mineral affinity. These differences in distribution at resorbing and forming surfaces were not observed at quiescent surfaces. Lower-affinity compounds also showed a relatively higher degree of labeling of osteocyte lacunar walls and labeled lacunae deeper within cortical bone, indicating increased penetration of the osteocyte canalicular network. Similar differences in mineralizing surface and osteocyte network penetration between high- and low-affinity compounds were evident 7 days after administration, with fluorescent conjugates at forming surfaces buried under a new layer of bone. Fluorescent compounds were incorporated into these areas of newly formed bone, indicating that "recycling" had occurred, albeit at very low levels. Taken together, these findings indicate that the bone mineral affinity of bisphosphonates is likely to influence their distribution within the skeleton.

Chemical biology of lipidated proteins

Many signaling proteins such as the members of the Ras superfamily of GTPases are posttranslationally modified by covalent attachment of lipid groups, which is crucial for the correct localization and function of these proteins. Numerous lipidated proteins are oncogens often found mutated in several human cancers. Therefore, several therapeutic strategies have been developed based on the inhibition of the enzymes involved in these lipidation steps. Here, we will summarize the results on protein lipidation inhibition, mainly focusing on the small molecules targeting the isoprenylation and acylation of proteins.

Generation of human osteoclasts from peripheral blood

Osteoclasts are multi-nucleated cells that have the unique ability to resorb calcified bone matrix. They derive from haematopoietic precursor cells, and can be generated in vitro by stimulation of peripheral blood mononuclear cells with the cytokines M-CSF and RANKL. In this chapter, we describe the method for generating human osteoclast from peripheral blood or buffy coats, as well as methods for studying both the differentiation and resorbing activity of these cells.

Geranylgeranyl diphosphate synthase: an emerging therapeutic target

Proteins modified post-translationally by geranylgeranylation have been implicated in numerous cellular processes related to human disease. In recent years, the study of protein geranylgeranylation has advanced tremendously in both cellular and animal models. The advances in our understanding of the biological roles of geranylgeranylated proteins have been paralleled by advances in the medicinal chemistry of geranylgeranylation inhibitors such as those that target geranylgeranyl transferases I and II and geranylgeranyl diphosphate synthase (GGDPS). Although these findings provide the rationale for further development of geranylgeranylation as a therapeutic target, more advanced studies on the efficacy of this approach in various disease models will be required to support translation to clinical studies. This article attempts to describe the advances in (and the challenges of) validation of GGDPS as a novel therapeutic target and assesses the advantages of targeting GGDPS relative to other enzymes involved in geranylgeranylation.