Serotonin (5-HT) inhibits Factor XIII-A-mediated plasma fibronectin matrix assembly and crosslinking in osteoblast cultures via direct competition with transamidation

Emerging roles of nerve-bone axis in modulating skeletal system

Over the past decades, emerging evidence in the literature has demonstrated that the innervation of bone is a crucial modulator for skeletal physiology and pathophysiology. The nerve-bone axis sparked extensive preclinical and clinical investigations aimed at elucidating the contribution of nerve-bone crosstalks to skeleton metabolism, homeostasis, and injury repair through the perspective of skeletal neurobiology. To date, peripheral nerves have been widely reported to mediate bone growth and development and fracture healing via the secretion of neurotransmitters, neuropeptides, axon guidance factors, and neurotrophins. Relevant studies have further identified several critical neural pathways that stimulate profound alterations in bone cell biology, revealing a complex interplay between the skeleton and nerve systems. In addition, inspired by nerve-bone crosstalk, novel drug delivery systems and bioactive materials have been developed to emulate and facilitate the process of natural bone repair through neuromodulation, eventually boosting osteogenesis for ideal skeletal tissue regeneration. Overall, this work aims to review the novel research findings that contribute to deepening the current understanding of the nerve-bone axis, bringing forth some schemas that can be translated into the clinical scenario to highlight the critical roles of neuromodulation in the skeletal system.

Identification of Factor XIII β-Sandwich Residues Mediating Glutamine Substrate Binding and Activation Peptide Cleavage

BACKGROUND

 Factor XIII (FXIII) forms covalent crosslinks across plasma and cellular substrates and has roles in hemostasis, wound healing, and bone metabolism. FXIII activity is implicated in venous thromboembolism (VTE) and is a target for developing pharmaceuticals, which requires understanding FXIII - substrate interactions. Previous studies proposed the β-sandwich domain of the FXIII A subunit (FXIII-A) exhibits substrate recognition sites.

MATERIAL AND METHODS

 Recombinant FXIII-A proteins (WT, K156E, F157L, R158Q/E, R171Q, and R174E) were generated to identify FXIII-A residues mediating substrate recognition. Proteolytic (FXIII-A*) and non-proteolytic (FXIII-A°) forms were analyzed for activation and crosslinking activities toward physiological substrates using SDS-PAGE and MALDI-TOF MS.

RESULTS

 All FXIII-A* variants displayed reduced crosslinking abilities compared to WT for Fbg αC (233 - 425), fibrin, and actin. FXIII-A* WT activity was greater than A°, suggesting the binding site is more exposed in FXIII-A*. With Fbg αC (233 - 425), FXIII-A* variants R158Q/E, R171Q, and R174E exhibited decreased activities approaching those of FXIII-A°. However, with a peptide substrate, FXIII-A* WT and variants showed similar crosslinking suggesting the recognition site is distant from the catalytic site. Surprisingly, FXIII-A R158E and R171Q displayed slower thrombin activation than WT, potentially due to loss of crucial H-bonding with neighboring activation peptide (AP) residues.

CONCLUSION

 In conclusion, FXIII-A residues K156, F157, R158, R171, and R174 are part of a binding site for physiological substrates [fibrin (α and γ) and actin]. Moreover, R158 and R171 control AP cleavage during thrombin activation. These investigations provide new molecular details on FXIII - substrate interactions that control crosslinking abilities.

Role of Serotonylation and SERT Posttranslational Modifications in Alzheimer's Disease Pathogenesis

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is implicated mainly in Alzheimer's disease (AD) and reported to be responsible for several processes and roles in the human body, such as regulating sleep, food intake, sexual behavior, anxiety, and drug abuse. It is synthesized from the amino acid tryptophan. Serotonin also functions as a signal between neurons to mature, survive, and differentiate. It plays a crucial role in neuronal plasticity, including cell migration and cell contact formation. Various psychiatric disorders, such as depression, schizophrenia, autism, and Alzheimer's disease, have been linked to an increase in serotonin-dependent signaling during the development of the nervous system. Recent studies have found 5-HT and other monoamines embedded in the nuclei of various cells, including immune cells, the peritoneal mast, and the adrenal medulla. Evidence suggests these monoamines to be involved in widespread intracellular regulation by posttranslational modifications (PTMs) of proteins. Serotonylation is the calcium-dependent process in which 5-HT forms a long-lasting covalent bond to small cytoplasmic G-proteins by endogenous transglutaminase 2 (TGM2). Serotonylation plays a role in various biological processes. The purpose of our article is to summarize historical developments and recent advances in serotonin research and serotonylation in depression, aging, AD, and other age-related neurological diseases. We also discussed several of the latest developments with Serotonin, including biological functions, pathophysiological implications and therapeutic strategies to treat patients with depression, dementia, and other age-related conditions.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

Plasma serotonin precursors and metabolite are correlated with bone mineral density and bone turnover markers in patients with postmenopausal osteoporosis

BACKGROUND

Serotonin (5-HT) precursors regulate bone remodeling. This study aims to investigate the correlation of plasma 5-HT precursors and metabolite with bone mineral density (BMD) and bone turnover markers in postmenopausal osteoporosis (PMOP) patients.

METHODS

The age, body mass index (BMI), and years since menopause (YSM) were documented for 348 postmenopausal women in normal/osteopenia/osteoporosis (OP) groups, with lumbar spine and femoral neck BMD measured. Serum bone turnover markers (PINP/β-CTX) and plasma 5-HT, 5-HT precursors (Trp/5-HTP) and metabolite (5-HIAA) were measured by ELISA. OP patients were allocated to high/low expression groups following ROC analysis of 5-HT/Trp/5-HTP/5-HIAA. The relationship of plasma 5-HT/Trp/5-HTP/5-HIAA, BMD, and bone turnover markers with PMOP was analyzed using logistic regression analysis. The correlation of plasma 5-HT/Trp/5-HTP/5-HIAA with BMD and bone turnover markers was analyzed using Pearson's correlation analysis, followed by logistic regression analysis of the relationship between plasma 5-HT/Trp/5-HTP/5-HIAA and BMD, bone turnover markers and PMOP.

RESULTS

BMI, YSM, BMD and PINP, and β-CTX levels differed among groups. Levels of plasma 5-HT precursors/metabolite were increased in OP patients. Individuals with high 5-HT precursors/metabolite levels had low BMD and high PINP/β-CTX levels. The 5-HT precursors/metabolite negatively-correlated with BMD and positively-correlated with PINP/β-CTX. BMI, YSM, BMD, and PINP/β-CTX/Trp/5-HTP/5-HT related to PMOP and were independent risk factors for OP.

CONCLUSION

Plasma 5-HT precursors and metabolite negatively-correlate with BMD and positively-correlate with PINP/β-CTX in PMOP patients. Peripheral 5-HT precursors and metabolite level may be a new direction of treatment of PMOP and bone metabolism-related disorders.

Advances of Blood Coagulation Factor XIII in Bone Healing

The biologic process of bone healing is complicated, involving a variety of cells, cytokines, and growth factors. As a result of bone damage, the activation of a clotting cascade leads to hematoma with a high osteogenic potential in the initial stages of healing. A major factor involved in this course of events is clotting factor XIII (FXIII), which can regulate bone defect repair in different ways during various stages of healing. Autografts and allografts often have defects in clinical practice, making the development of advanced materials that support bone regeneration a critical requirement. Few studies, however, have examined the promotion of bone healing by FXIII in combination with biomaterials, in particular, its effect on blood coagulation and osteogenesis. Therefore, we mainly summarized the role of FXIII in promoting bone regeneration by regulating the extracellular matrix and type I collagen, bone-related cells, angiogenesis, and platelets, and described the research progress of FXIII = related biomaterials on osteogenesis. This review provides a reference for investigators to explore the mechanism by which FXIII promotes bone healing and the combination of FXIII with biomaterials to achieve targeted bone tissue repair.

The physiology, pathology and potential therapeutic application of serotonylation

The classical neurotransmitter serotonin or 5-hydroxytryptamine (5-HT), synthesized from tryptophan, can be produced both centrally and peripherally. Through binding to functionally distinct receptors, serotonin is profoundly implicated in a number of fundamental physiological processes and pathogenic conditions. Recently, serotonin has been found covalently incorporated into proteins, a newly identified post-translational modification termed serotonylation. Transglutaminases (TGMs), especially TGM2, are responsible for catalyzing the transamidation reaction by transferring serotonin to the glutamine residues of target proteins. Small GTPases, extracellular matrix protein fibronectin, cytoskeletal proteins and histones are the most reported substrates for serotonylation, and their functions are triggered by this post-translational modification. This Review highlights the roles of serotonylation in physiology and diseases and provides perspectives for pharmacological interventions to ameliorate serotonylation for disease treatment.

Factor XIIIa inhibitors as potential novel drugs for venous thromboembolism

Human factor XIIIa (FXIIIa) is a multifunctional transglutaminase with a significant role in hemostasis. FXIIIa catalyzes the last step in the coagulation process. It stabilizes the blood clot by cross-linking the α- and γ-chains of fibrin. It also protects the newly formed clot from plasmin-mediated fibrinolysis, primarily by cross-linking α2-antiplasmin to fibrin. Furthermore, FXIIIa is a major determinant of clot size and clot's red blood cells content. Therefore, inhibitors targeting FXIIIa have been considered to develop a new generation of anticoagulants to prevent and/or treat venous thromboembolism. Several inhibitors of FXIIIa have been discovered or designed including active site and allosteric site small molecule inhibitors as well as natural and modified polypeptides. This work reviews the structural, biochemical, and pharmacological aspects of FXIIIa inhibitors so as to advance their molecular design to become more clinically relevant.

Assessment of expression and specific activities of transglutaminases TG1, TG2, and FXIII-A during osteoclastogenesis

Osteoclasts are large multinucleated bone-resorbing cells derived from monocyte/macrophage lineage. Macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL) drive the multi-stage osteoclastogenesis. Transglutaminases (TGs) are Ca²⁺- and thiol-dependent acyl transferases and protein crosslinking enzymes. TG enzyme family contains eight catalytically active enzymes TG1-7 and Factor XIII-A (FXIII-A). Recent studies have shown that TG1, TG2, and FXIII-A are present in osteoclasts and that TG2 and FXIII-A regulate osteoclastogenesis. In this study, we examined gene and protein expression and specific activities of TG1, TG2, and FXIII-A during osteoclastogenesis using "Hitomi peptides" in a day-by-day manner. We report that TG activities are highest in the differentiation and early fusion phases and then decrease dramatically. TG activities were upregulated by M-CSF and downregulated by addition of RANKL. FXIII-A was dramatically downregulated by RANKL, suggesting its involvement in M-CSF-mediated precursor commitment phase. TG1 and TG2 proteins were present throughout osteoclastogenesis, suggesting that they may have functions in both differentiation and fusion. In summary, the three TGs likely exert distinct functions at different stages of osteoclastogenesis. Our work also demonstrates that the "Hitomi peptides" are highly specific tools for detection of distinct TGs in a system where multiple TGs are present.

Serotonylation: Serotonin Signaling and Epigenetics

Serotonylation, the covalent linkage of serotonin to proteins has been discovered more than 60 years ago but only recently the mechanisms and first functions have been elucidated. It has been found that transglutaminases (TG) such as TG2 and the blood coagulation factor XIIIa are the enzymes which catalyze the linkage of serotonin and other monoamines to distinct glutamine (Gln) residues of target proteins. The first target proteins, small G-proteins and extracellular matrix constituents, were found in platelets and are pivotally involved in platelet aggregation and the formation of thrombi. The serotonylation of the same proteins is also involved in insulin secretion and in the proliferation of pulmonary vascular smooth muscle cells and thereby in the pathogenesis of pulmonary arterial hypertension (PAH). Recently histones have been described as targets of serotonylation opening the area of transcriptional control to this posttranslational protein modification. Future studies will certainly reveal further target proteins, signaling pathways, cellular processes, and diseases, in which serotonylation or, more general, monoaminylation is important.

Regenerative Effect of Platelet Concentrates in Oral and Craniofacial Regeneration

Platelet concentrates (PCs) are biological autologous products derived from the patient's whole blood and consist mainly of supraphysiologic concentration of platelets and growth factors (GFs). These GFs have anti-inflammatory and healing enhancing properties. Overall, PCs seem to enhance bone and soft tissue healing in alveolar ridge augmentation, periodontal surgery, socket preservation, implant surgery, endodontic regeneration, sinus augmentation, bisphosphonate related osteonecrosis of the jaw (BRONJ), osteoradionecrosis, closure of oroantral communication (OAC), and oral ulcers. On the other hand, no effect was reported for gingival recession and guided tissue regeneration (GTR) procedures. Also, PCs could reduce pain and inflammatory complications in temporomandibular disorders (TMDs), oral ulcers, and extraction sockets. However, these effects have been clinically inconsistent across the literature. Differences in study designs and types of PCs used with variable concentration of platelets, GFs, and leucocytes, as well as different application forms and techniques could explain these contradictory results. This study aims to review the clinical applications of PCs in oral and craniofacial tissue regeneration and the role of their molecular components in tissue healing.

Transglutaminase Activity Determines Nuclear Localization of Serotonin Immunoreactivity in the Early Embryos of Invertebrates and Vertebrates

Serotonin (5-HT) is a key player in many physiological processes in both the adult organism and developing embryo. One of the mechanisms for 5-HT-mediated effects is covalent binding of 5-HT to the target proteins catalyzed by transglutaminases (serotonylation). Despite the implication in a variety of physiological processes, the involvement of serotonylation in embryonic development remains unclear. Here we tested the hypothesis that 5-HT serves as a substrate for transglutaminase-mediated transamidation of the nuclear proteins in the early embryos of both vertebrates and invertebrates. For this, we demonstrated that the level of serotonin immunoreactivity (5-HT-ir) in cell nuclei increases upon the elevation of 5-HT concentration in embryos of sea urchins, mollusks, and teleost fish. Consistently, pharmacological inhibition of transglutaminase activity resulted in the reduction of both brightness and nuclear localization of anti-5-HT staining. We identified specific and bright 5-HT-ir within nuclei attributed to a subset of different cell types: ectodermal and endodermal, macro- and micromeres, and blastoderm. Western blot and dot blot confirmed the presence of 5-HT-ir epitopes in the normal embryos of all the species examined. The experimental elevation of 5-HT level led to the enhancement of 5-HT-ir-related signal on blots in a species-specific manner. The obtained results demonstrate that 5-HT is involved in transglutaminase-dependent monoaminylation of nuclear proteins and suggest nuclear serotonylation as a possible regulatory mechanism during early embryonic development. The results reveal that this pathway is conserved in the development of both vertebrates and invertebrates.

Evaluation of jawbone morphology and bone density indices in panoramic radiographs of selective serotonin reuptake inhibitor users: a preliminary study

OBJECTIVES:

To assess the influence of selective serotonin reuptake inhibitor (SSRI) use on jawbone and bone mineral density by retrospective analysis of panoramic radiographs.

METHODS:

Radiographic and clinical records were sourced from the Division of Orthodontics and TMJD, Eastman Institute for Oral Health, University of Rochester. Randomly selected adults (20-65 years) were categorized into: "Active" (with history of SSRI use of >6 months) and a "Control" group. Panoramic indices: Klemetti index (KI), panoramic mandibular index, antegonial notching index, condylar pathology, mandibular cortical width (MCW)⁠ and mean ramus height were recorded. Frequency-weighted Χ2 tests and multinomial regression controlling for age and gender were applied to categorical indices (KI, condylar pathology, antegonial notching index). Multivariate generalized linear modeling was applied to mean ramus height, MCW and panoramic mandibular index. Multiple regression analyses determined: (a) panoramic indices that best predicted SSRI use, and (b) independent predictors of KI category.

RESULTS:

64 SSRI users and 48 Controls were assessed. SSRI users had significantly higher odds of having worse KI status than normal [mildly to moderately eroded cortex: odds ratio (OR) = 2.926, 95% CI (1.07-8.04) and severely eroded cortex: OR = 19.86, 95% CI (3.91-100.69)], more frequent flat condylar anatomy (right side: p = 0.009, left side: p < 0.001) but greater ramus height (p = 0.001) and mandibular cortical width (p = 0.032). Age, gender, SSRI use each significantly impacted KI. Only SSRI use significantly impacted condylar pathology, ramus height and MCW. KI category (OR = 1.3) was the best panoramic predictor of SSRI use. Conversely, KI category C3 was significantly predicted by SSRI use (OR = 31.2, p = 0.002), female gender (17.5, p = 0.006), and severe antegonial notching (OR = 1289, p < 0.001).

CONCLUSIONS:

SRRI use was significantly associated with worse panoramic morphometric indices: KI, condylar pathology, ramus height, and MCW, where KI was its strongest predictor. Worse KI was independently predicted by SSRI use.

Transglutaminase in Receptor and Neurotransmitter-Regulated Functions

Transglutaminases (TGs) and especially TG2 play important roles in neurotransmitter and receptor signaling pathways. Three different mechanisms by which TG2 interacts with neurotransmitter and receptor signaling systems will be discussed in this review. The first way in which TG2 interacts with receptor signaling is via its function as a guanine nucleotide binding protein (G-protein) coupling to G-protein coupled receptors (GPCRs) to activate down-stream signaling pathways. TG2 can exist in a least two conformations, a closed GTP-bound conformation and an open calcium-bound conformation. In the closed GTP-bound conformation, TG2 is capable of functioning as a G-protein for GPCRs. In the open calcium-bound conformation, TG2 catalyzes a transamidation reaction cross-linking proteins or catalyzing the covalent binding of a mono- or polyamine to a protein. The second mechanism is regulation of the transamidation reaction catalyzed by TG2 via receptor stimulation which can increase local calcium concentrations and thereby increase transamidation reactions. The third way in which TG2 plays a role in neurotransmitter and receptor signaling systems is via its use of monoamine neurotransmitters as a substrate. Monoamine neurotransmitters including serotonin can be substrates for transamidation to a protein often a small G-protein (also known as a small GTPase) resulting in activation of the small G-protein. The transamidation of a monoamine neurotransmitter or serotonin has been designated as monoaminylation or more specifically serotonylation, respectively. Other proteins are also targets for monoaminylation such as fibronectin and cytoskeletal proteins. These receptor and neurotransmitter-regulated reactions by TG2 play roles in physiological and key pathophysiological processes.

Regulation of Bone Metabolism by Serotonin

The processes of bone growth and turnover are tightly regulated by the actions of various signaling molecules, including hormones, growth factors, and cytokines. Imbalances in these processes can lead to skeletal disorders such as osteoporosis or high bone mass disease. It is becoming increasingly clear that serotonin can act through a number of mechanisms, and at different locations in the body, to influence the balance between bone formation and resorption. Its actions on bone metabolism can vary, based on its site of synthesis (central or peripheral) as well as the cells and subtypes of receptors that are activated. Within the central nervous system, serotonergic neurons act via the hypothalamus to suppress sympathetic input to the bone. Since sympathetic input inhibits bone formation, brain serotonin has a net positive effect on bone growth. Gut-derived serotonin is thought to inhibit bone growth by attenuating osteoblast proliferation via activation of receptors on pre-osteoblasts. There is also evidence that serotonin can be synthesized within the bone and act to modulate bone metabolism. Osteoblasts, osteoclasts, and osteocytes all have the machinery to synthesize serotonin, and they also express the serotonin-reuptake transporter (SERT). Understanding the roles of serotonin in the tightly balanced system of bone modeling and remodeling is a clinically relevant goal. This knowledge can clarify bone-related side effects of drugs that affect serotonin signaling, including serotonin-specific reuptake inhibitors (SSRIs) and receptor agonists and antagonists, and it can potentially lead to therapeutic approaches for alleviating bone pathologies.

Transglutaminase activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics

Osteoclasts, bone resorbing cells, derive from monocyte/macrophage cell lineage. Increased osteoclast activity is responsible for bone destruction in diseases such as osteoporosis, periodontitis and rheumatoid arthritis. Transglutaminases (TGs), protein crosslinking enzymes, were recently found involved in osteoclastogenesis in vivo, however their mechanisms of action have remained unknown. In this study, we have investigated the role of TG activity in osteoclastogenesis in vitro using four TG inhibitors, NC9, Z006, T101, and monodansyl cadaverine. Our results showed that all TG inhibitors were capable of blocking the entire osteoclastogenesis process. The most potent of the inhibitors, NC9 when added to cultures at different phases of osteoclastogenesis, inhibited differentiation, migration, and fusion of pre-osteoclasts as well as resorption activity of mature osteoclasts. Further investigation into the mechanisms revealed that NC9 increased RhoA levels and blocked podosome belt formation suggesting that TG activity regulates actin dynamics in pre-osteoclasts. The inhibitory effect of NC9 on osteoclastogenesis as well as podosome belt formation was completely reversed with a Rho-family inhibitor Exoenzyme C3. Microtubule architecture, acetylation, and detyrosination of α-tubulin were not affected. Finally, we demonstrated that macrophages and osteoclasts expressed mRNA of three TGs:TG1, TG2, and Factor XIII-A which were all differentially regulated in these cells during differentiation. Immunofluoresence microscopic analysis showed that all three enzymes co-localized to podosomes in osteoclasts. Taken together, our data suggests that TG activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics and that this may involve contribution from all three TG enzymes.

Mineralization-inhibiting effects of transglutaminase-crosslinked polymeric osteopontin

Osteopontin (OPN) belongs to the SIBLING family (Small, Integrin-Binding LIgand N-linked Glycoproteins) of mineral-binding matrix proteins found in bones and teeth. OPN is a well-known inhibitor of matrix mineralization, and enzymatic modification of OPN can affect this inhibitory function. In bone, OPN exists both as a monomer and as a high-molecular-weight polymer - the latter is formed by transglutaminase-mediated crosslinking of glutamine and lysine residues in OPN to create homotypic protein assemblies. OPN can be covalently crosslinked by transglutaminase 2 (TG2) and Factor XIII-A. Polymeric OPN has increased binding to collagen and promotes osteoblast adhesion, but despite these initial observations, its role in mineralization is not clear. In this study, we investigated the effect of polymerized OPN on mineralization using a hydroxyapatite crystal growth assay and mineralizing MC3T3-E1 osteoblast cultures. In the cultures, endogenous polymeric OPN was detected after mineralization occurred. In cell-free conditions, TG2 was used to crosslink bovine OPN into its polymeric form, and atomic force microscopy and dynamic light scattering revealed variably-sized, large branched aggregates ranging across hundreds of nanometers. These OPN polymers inhibited the growth of hydroxyapatite crystals in solution at concentrations similar to monomeric OPN, although the crosslinking slightly reduced its inhibitory potency. When added to MC3T3-E1 osteoblast cultures, this exogenous polymeric OPN essentially did not inhibit mineralization when given during the later mineralization stages of culture; however, cultures treated early and then continuously with polymeric OPN throughout both the matrix assembly and mineral deposition stages showed reduced mineralization. Immunoblotting of protein extracts from these continuously treated cultures revealed exogenous OPN polymers incorporated into mature matrix that had not yet mineralized. These results suggest that in bone, the increased size and branched structure of crosslinked inhibitory polymeric OPN near the mineralization front could hinder it from accessing focal mineralization sites in the dense collagen-rich matrix, suggesting that OPN-crosslinking into polymers may represent a way to fine-tune the inhibitory potency of OPN on bone mineralization.

Selective serotonin reuptake inhibitors and dental implant failure-A significant concern in elders?

OBJECTIVE

Depression is a significantly prevalent health concern in geriatric populations. Selective serotonin reuptake inhibitor drugs (SSRI) are the most commonly prescribed antidepressant agents, with increasing rates of prescription. The present report aimed to present a concise review of the current understanding regarding SSRI effects on bone and dental implant outcomes.

MATERIALS AND METHODS

A broad-based review and summary of literature pertaining to the effects of SSRI on bone metabolism and on dental implant survival was performed.

RESULTS

The available literature indicates that serotonin plays a significant role in bone metabolism and experimental reports demonstrate adverse impacts of SSRI on multiple pathways of bone metabolism. Early clinical reports suggest detrimental effects of SSRI on dental implant survival. The type of SSRI drug, dosage and host-related genetic and metabolic factors could be potential modulating factors. There is a paucity of data regarding SSRI usage and dental implant survival specific to geriatric cohorts.

CONCLUSION

As older individuals comprise a high-risk group for both high oral rehabilitation and SSRI use, clinicians should be aware the potential association between SSRI and dental implant failures. Well-designed investigations specific to geriatric cohorts are essential to understand the implications of SSRI use on dental implant prognosis.

Relationships between Bone Turnover and Energy Metabolism

It is well established that diabetes can be detrimental to bone health, and its chronic complications have been associated with an increased risk of osteoporotic fracture. However, there is growing evidence that the skeleton plays a key role in a whole-organism approach to physiology. The hypothesis that bone may be involved in the regulation of physiological functions, such as insulin sensitivity and energy metabolism, has been suggested. Given the roles of insulin, adipokines, and osteocalcin in these pathways, the need for a more integrative conceptual approach to physiology is emphasized. Recent findings suggest that bone plays an important role in regulating intermediary metabolism, being possibly both a target of diabetic complications and a potential pathophysiologic factor in the disease itself. Understanding the relationships between bone turnover and glucose metabolism is important in order to develop treatments that might reestablish energy metabolism and bone health. This review describes new insights relating bone turnover and energy metabolism that have been reported in the literature.

Serotonylation and Transamidation of Other Monoamines

Although serotonin was discovered over 65 years ago, it has been only within the past decade that serotonin was found to be involved in a covalent post-translational modification to proteins. The enzyme transglutaminase catalyzes the transamidation of serotonin to a protein-bound glutamine residue; the amino group of serotonin is covalently bound to the gamma carboxamide of glutamine. The term serotonylation is used to describe this transamidation reaction to serotonin. Not only can serotonin be a substrate for transamidation to proteins but also other monoamine neurotransmitters are substrates including histamine, dopamine, and noradrenaline. The term monoaminylation has been coined to describe the transamidation of monoamines to protein substrates. Small G proteins have emerged as the most common substrate for monoaminylation and are activated by this post-translational modification. Fibronectin and cytoskeletal proteins are also substrates for monoaminylation. Serotonylation and monoaminylation are involved in a number of physiological functions, including platelet activation, insulin release, smooth muscle contraction, and regulation of membrane localization of the serotonin transporter. Stimulation of 5-HT2A receptors increases serotonylation and activates the small G protein Rac1, which plays a role in dendritic spine regulation. Monoaminylation is implicated in pathophysiological processes as well such as diabetes and hypertension. The availability of monoamines for monoaminylation is altered by antidepressants that target serotonin transporters, noradrenaline transporters, or the enzymatic degradation of monoamines as well as drugs of abuse such as cocaine and amphetamines. Further research on monoaminylation is needed to elucidate its physiological and pathophysiological roles and to explore monoaminylation as a novel target for drug therapy.