A Review of the Factors Affecting the Design, Specification and Material Selection of Screws for Use in Orthopaedic Surgery

The literature on all aspects of design and performance of surgical screws has been reviewed. This information has then been used to optimize the design of surgical bone screws. Finally, the design of a 4 mm cortical bone screw has been considered in detail.

Functional interactions between mu opioid and alpha 2A-adrenergic receptors

Adrenergic and opioid receptors belong to the rhodopsin family of G-protein coupled receptors, couple to analogous signal transduction pathways, and affect the nociceptive system. Although a number of previous studies have reported functional interactions between these two receptors, the basis for this has not been well explored. We propose that direct receptor-receptor interactions could account, in part, for opioid-adrenergic cross-talk. In this report, we have addressed this using biophysical, biochemical, and pharmacological studies. We show that mu opioid and alpha2A adrenergic receptors reside in close proximity in live cells using the bioluminescence resonance energy transfer assay. These receptors colocalize to proximal dendrites in primary hippocampal neurons. mu-alpha2A Receptor complexes can be isolated from heterologous cells or primary neurons coexpressing these receptors. In these cells, the activation of either mu or alpha2A receptor leads to a significant increase in the level of immunoprecipitable mu-alpha2A complexes, whereas activation of both receptors leads to a significant decrease. The implications of these effects on signaling were examined using the agonist-mediated increase in G-protein activity and mitogen-activated protein kinase activity. We find that activation of either mu or alpha2A receptors leads to an increase in the extent of signaling, whereas activation of both receptors leads to a decrease. The increase in signaling by individual ligands and decrease by a combination of ligands is also seen in primary spinal cord neurons endogenously expressing these receptors. Taken together, these results suggest that physical associations between mu and alpha2A receptors could play a role in the functional interactions between these receptors.

G-protein-coupled receptor dimerization: modulation of receptor function

G-protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in the human genome that respond to a plethora of signals, including neurotransmitters, peptide hormones, and odorants, to name a few. They couple to second messenger signaling cascade mechanisms via heterotrimeric G-proteins. Recently, many studies have revealed that GPCRs exist as dimers, which may be present as homo- or heterodimers/oligomers. These recent findings have been met with skepticism, since they are contradictory to the dogma that GPCRs function as monomers. Although the existence of GPCR dimers/oligomers was predicted from early pharmacological and biochemical studies, further studies to critically evaluate this phenomenon were impeded by the lack of appropriate reagents. The availability of cDNAs for GPCRs, of highly selective ligands and of antibodies for these receptors has made it possible to visualize and investigate the functional effects of GPCR oligomers. Pharmacological studies, along with biochemical techniques, such as cross-linking and immunoprecipitation with differentially epitope-tagged receptors, have been employed to demonstrate the oligomerization of a number of GPCRs. Moreover, recent biophysical techniques, such as bioluminescence and fluorescence resonance energy transfer, now make it possible to examine GPCR dimerization/oligomerization in living cells. In this review, we provide a brief overview of some of the techniques employed to describe GPCR dimers, and we discuss their respective limitations. We also examine the implications of dimerization/oligomerization on GPCR function. In addition, we discuss domains of the receptors that are thought to facilitate dimerization/oligomerization. Finally, we consider recent evidence for the subcellular localization of the dimer/oligomer assembly.

G protein coupled receptor dimerization: implications in modulating receptor function

Protein-protein interactions are involved in the regulation of a large number of biological processes. It is well established that a variety of cell surface receptors interact with each other to form dimers, and that this is essential for their activation. Although the existence of G protein coupled receptor dimers was predicted from early pharmacological and biochemical analysis, solid evidence supporting dimerization has come within the past few years following the cloning of G protein coupled receptor cDNAs. Using differential epitope tagging and selective immunoisolation of receptor complexes, dimerization of a number of G protein coupled receptors including members of the rhodopsin, secretin, and metabotropic glutamate receptor families have been reported. More recently fluorescence or bioluminescence resonance energy transfer techniques have been used to examine dimerization of these receptors in live cells. These studies have found that whereas in some cases there is an agonist induced increase in the level of dimers, in others there is a decrease or no change in dimer levels. Several recent studies have also reported the ability of related members of G protein coupled receptors to heterodimerize. These heterodimers exhibit distinct physical and functional properties. Examination of possible sites of interactions between receptors has implicated a role for extracellular, transmembrane and/or C-terminal region in dimerization. The functional consequences of dimerization, explored mainly using mutant receptors, have demonstrated a role in modulating agonist affinity, efficacy, and/or trafficking properties. Thus dimerization appears to be a universal phenomenon that provides an additional mechanism for modulation of receptor function as well as cross-talk between G protein coupled receptors.

Oligomerization of opioid receptors with 2-adrenergic receptors: A role in trafficking and mitogen-activated protein kinase activation

G-protein-coupled receptors (GPCRs) have recently joined the list of cell surface receptors that dimerize. Dimerization has been shown to alter the ligand-binding, signaling, and trafficking properties of these receptors. Recent studies have shown that GPCRs heterodimerize with closely related members, resulting in the modulation of their function. In this study, we have attempted to determine whether members of GPCR superfamilies that couple to different families of G-proteins can associate and form oligomers. We chose the beta2 adrenergic receptor that couples to stimulatory G-proteins and delta & kappa opioid receptors that couple to inhibitory G-proteins. beta2 and delta receptors undergo robust agonist-mediated endocytosis, whereas kappa receptors do not. We find that when coexpressed, beta2 receptors can form heteromeric complexes with both delta and kappa receptors. This heterooligomerization does not significantly alter the ligand binding or coupling properties of the receptors. However, it affects the trafficking properties of the receptors. For example, we find that delta receptors, when coexpressed with beta2 receptors, undergo isoproterenol-mediated endocytosis. Conversely, beta2 receptors in these cells undergo etorphine-mediated endocytosis. However, beta2 receptors, when coexpressed with kappa receptors, undergo neither opioid- nor isoproterenol-mediated endocytosis. Moreover, these cells exhibit a substantial decrease in the isoproterenol-induced phosphorylation of mitogen-activated protein kinases. Taken together, these results provide direct evidence of heteromerization of GPCRs that couple to different types of G-proteins, which results in the modulation of receptor trafficking and signal transduction.

Heterodimerization of mu and delta opioid receptors: A role in opiate synergy

Opiate analgesics are widely used in the treatment of severe pain. Because of their importance in therapy, different strategies have been considered for making opiates more effective while curbing their liability to be abused. Although most opiates exert their analgesic effects primarily via mu opioid receptors, a number of studies have shown that delta receptor-selective drugs can enhance their potency. The molecular basis for these findings has not been elucidated previously. In the present study, we examined whether heterodimerization of mu and delta receptors could account for the cross-modulation previously observed between these two receptors. We find that co-expression of mu and delta receptors in heterologous cells followed by selective immunoprecipitation results in the isolation of mu-delta heterodimers. Treatment of these cells with extremely low doses of certain delta-selective ligands results in a significant increase in the binding of a mu receptor agonist. Similarly, treatment with mu-selective ligands results in a significant increase in the binding of a delta receptor agonist. This robust increase is also seen in SKNSH cells that endogenously express both mu and delta receptors. Furthermore, we find that a delta receptor antagonist enhances both the potency and efficacy of the mu receptor signaling; likewise a mu antagonist enhances the potency and efficacy of the delta receptor signaling. A combination of agonists (mu and delta receptor selective) also synergistically binds and potentiates signaling by activating the mu-delta heterodimer. Taken together, these studies show that heterodimers exhibit distinct ligand binding and signaling characteristics. These findings have important clinical ramifications and may provide new foundations for more effective therapies.

Opioids and their complicated receptor complexes

No field more eagerly awaits a molecular clarification for G-protein coupled receptor (GPCR) dimerization than the opioid receptor field. Extensive evidence of pharmacological and functional interactions between opioid receptor types has primed this field for such a resolution. In retrospect, much of the data collected on synergy between different opioid receptor types may represent the functional correlate for the newly found opioid receptor dimerization. While previous reports of functional synergy have been, for the most part, consistent in demonstrating cross-regulation between two receptor types, the lack of highly receptor-selective ligands allowed skeptics to remain doubtful over the interpretations of these results. Today, two important developments in the opioid receptor field help reinvigorate the hypothesis of functional, cross-modulating opioid receptor complexes: (1) The existence of highly selective ligands which eliminate any possibility of cross-reactivity between receptor types, and (2) the discovery that opioid receptors and a number of other GPCRs exist as dimers in biochemical, functional and pharmacological assays. It is with these new tools that we seek to understand the mechanisms and implications of dimerization. Initial results of these studies have demonstrated that the dimerization of opioid receptors may help consolidate several pharmacological findings that have remained unanswered. In this review we present biochemical, pharmacological and functional evidence for opioid receptor complexes and add evidence from our recent studies on opioid receptor dimerization. We believe a thorough understanding of receptor dimerization is crucial in clarifying the mechanism of action of opioids and other drugs and may serve a more practical purpose in aiding the development of novel therapeutic drugs.

Kappa opioid receptor endocytosis by dynorphin peptides

Internalization and downregulation are important steps in the modulation of receptor function. Recent work with the beta2 adrenergic and opioid receptors have implicated these processes in receptor-mediated activation of mitogen-activated protein kinase (MAPK). We have used CHO cells expressing epitope-tagged rat kappa opioid receptors (rKORs) and prodynorphin-derived peptides to characterize the agonist-mediated endocytosis of rKORs and activation of MAPK. Kappa receptor-selective peptides induced receptor internalization and downregulation whereas nonpeptide agonists did not. An examination of the ability of dynorphin A-17-related peptides (lacking C-terminal amino acids) to promote KOR internalization, inhibition of adenylyl cyclase, and MAPK phosphorylation revealed that the N-terminal seven residues play an important role in eliciting these responses. Both dynorphin peptides and nonpeptide agonists induced rapid and robust phosphorylation of MAPKs. Taken together, these results point to a difference in the ability of dynorphin peptides and nonpeptide ligands to promote rKOR endocytosis and support the view that rKOR internalization is not required for MAPK activation.

G-protein-coupled receptor heterodimerization modulates receptor function

The opioid system modulates several physiological processes, including analgesia, the stress response, the immune response and neuroendocrine function. Pharmacological and molecular cloning studies have identified three opioid-receptor types, delta, kappa and mu, that mediate these diverse effects. Little is known about the ability of the receptors to interact to form new functional structures, the simplest of which would be a dimer. Structural and biochemical studies show that other G-protein-coupled receptors (GPCRs) interact to form homodimers. Moreover, two non-functional receptors heterodimerize to form a functional receptor, suggesting that dimerization is crucial for receptor function. However, heterodimerization between two fully functional receptors has not been documented. Here we provide biochemical and pharmacological evidence for the heterodimerization of two fully functional opioid receptors, kappa and delta. This results in a new receptor that exhibits ligand binding and functional properties that are distinct from those of either receptor. Furthermore, the kappa-delta heterodimer synergistically binds highly selective agonists and potentiates signal transduction. Thus, heterodimerization of these GPCRs represents a novel mechanism that modulates their function.