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Dissertation (Ph.D.).--University of Texas Graduate School of Biomedical Sciences at San Antonio, 2007. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--Cornell University, January, 2009. / Vita. Includes bibliographical references (leaves 149-179)

Diss. (sammanfattning) Stockholm : Karol. inst., 2005. / Härtill 4 uppsatser.

Melanosome biogenesis is a highly regulated endosomal maturation process wherein structural fibers harbouring immature melanosomes acquires its biosynthetic proteins through the secretory pathway and finally matures into a functional organelle. These processes were shown to be dependent on several cytosolic protein complexes such as AP (adaptor protein)-1, AP-3, BLOC (biogenesis of lysosome-related organelles complex)-1, -2 and -3; in addition to kinesin motor KIF13A and Rab GTPases 7, 32 or 38. Mutations in the subunits of these complexes or Rab38 result into defective melanosome maturation leading to occulocutaneous albinism, a clinical phenotype commonly observed in Hermansky-Pudlak syndrome (HPS). Moreover, molecular function of these complexes in regulating the biogenesis of melanosome is partially known. The delivery of cargo to maturing melanosomal membranes requires fusion machinery that includes Rab GTPases, tethering factors and SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) proteins. However, the SNAREs involved in the transport of cargo to melanosomes is poorly understood. In this study entitled as “understanding the SNARE dynamics during melanosome biogenesis” we focus on functional role of endosomal Qa-SNARE protein, Syntaxin 13 (formally called STX12, herein referred to as STX13) in the organelle biogenesis and its transport in and out of melanosome. Moreover, these studies show that STX13-mediated cargo transport require a melanosomal membrane localized R-SNARE VAMP7 and these SNAREs are interdependent on each other in regulating their steady state distribution. In addition, this study illustrated the possible mechanism of SNARE recycling which occurs indirectly through AP-3 complex. Thus, these studies underscore the STX13‟s role in cargo transport to maturating melanosomes and its trafficking routes to and from the melanosomes. Chapter-I describes the literature review on melanosome biogenesis; Chapter-II lists the experimental procedures used in this study and Chapter-III to V focuses on results and discussion, segregated into three sections. Chapter-III: Screening and identification of endosomal SNAREs involved in the trafficking of melanosomal proteins. Our preliminary RNAi screen for SNAREs involved in melanosome biogenesis revealed STX13 as one of the Qa-SNARE affecting pigmentation and cargo transport. STX13, a recycling endosomal SNARE has been reported to interact with pallidin, a subunit of BLOC-1; however the functional role of this interaction in pigment formation is unknown. In addition, previous studies from our lab have shown that STX13 colocalize with endosomal Rab11 and partially with EEA1- or Rab5-positive organelles in melanocytes. Together, these observations insinuated us to characterize the functional role of STX13 in melanosome biogenesis. Upon STX13 inactivation, wild type mouse melanocytes showed hypopigmentation due to mistargeting of cargo such as TYRP1 and TYR to lysosomes. Knockdown of STX13 dramatically decrease the population of immature and mature melanosomes. Moreover, STX13 associate with the melanosome cargo on endosomal tubular structures. In addition, deletion of regulatory domain in STX13 increases the cargo transport to melanosomes due to its increased SNARE activity. This is possibly due to loss in intracellular regulation of SNARE occur through multiple factors such as SM (Sec1p/Munc18) proteins. Together this data suggests that STX13 mediates cargo transport to melanosomes from recycling endosomes. Chapter-IV: Functional characterization of the SNAREs involved in melanosomal maturation. Several in vitro studies have shown that a set of four SNAREs such as Qa, Qb, Qc (or Qbc) and R control the membrane fusion event duing the cargo transport. Additionally, this process is further regulated by SM proteins in in vivo. Electron microscopic studies in melanocytes have shown that melanosomal proteins were delivered to the melanosomal membrane through recycling endosomal tubular domains. Moreover, our RNAi screen show that STX13 possibly acts as Qa-SNARE in mediating the fusion events between melanosomal membranes and the endosomal tubular or vesicular intermediates. However, the role of other SNAREs for this membrane transport is unknown. It has been shown that the expression of VAMP family SNAREs such as VAMP3, VAMP7 and VAMP8 increased with melanogenesis upon differentiation of melanoma cells. VAMPs belong to the class of R-SNAREs, in which VAMP7 is known to interact with VARP (abbreviation) and AP-3 (mediates the trafficking of TYR) separately, and these molecules are known to regulate the cargo transport to melanosomes. However, the precise role of VAMP7 in pigment granule maturation is unknown. Therefore, we set out to characterize the functional role of VAMP7 in melanosome biogenesis. VAMP7 has been shown to localizes to multiple sub-cellular compartments and regulate the several transport steps in other cell types. Our study found that GFP-epitope tagged either human or rat VAMP7 localize to melanosomes at steady state in wild type mouse melanocytes. Knockdown of VAMP7 causes hypopigmentation of melanocytes and misroutes the cargo to lysosomes. Further, the inactivation of VAMP7 in melanocytes phenocopies the STX13 depletion, suggesting both the SNAREs are required for the melanosome biogenesis. In addition, knockdown of STX13 target the VAMP7 to lysosomes; while inactivation of VAMP7 affect the localization of STX13 to recycling tubular structures. Subsequently, the dominant active mutants of STX13 were not able to rescue the pigmentation or cargo transport defects in VAMP7 knockdown melanocytes. Together, the data suggests that STX13 functions from recycling endosomes and VAMP7 on melanosome membrane for the transport of cargo to melanosomes Chapter-V: Understanding the mechanism of STX13 recycling during melanosome biogenesis. At steady state, SNAREs are localized to the membranes of specific organelles where they mediate or regulate the membrane fusion. During this process, three or two Q-SNAREs on one membrane (in a trans-SNARE complex, possibly formed by Qa, Qb, Qc or Qbc) interact with a R-SNARE on another member to form a SNAREpin complex. Post-fusion, SNAREs are disassembled by SNAP and NSF proteins and then recycled back to the original compartment for next round of fusion. Here, we address the mechanism of post-fusion recycling of STX13 from melanosomes to endosomes. Previous studies have shown that STX13 mislocalize to melanosomes in AP-3-deficient melanocytes, suggesting a role for AP-3 in recycling the SNARE from melanosomes. Bioinformatic analysis of the N-terminal region of STX13 revealed the presence of two canonical adaptor binding motifs 3YGP6L and KETNE80L81L, resembling the tyrosine-based (YXXø) and dileucine-based motif [DE]XXXL[LI], recognized by several adaptor proteins. Point mutagenesis of these motifs in STX13 had no effect on their steady state distribution indicating that STX13 possibly uses non-canonical residues for its recycling. Further, deletion of the N-terminal region (either 1-129 or 14-129 aa) in STX13 redistributes the SNARE to melanosomes. Moreover, the activity and the trafficking of recycling defective STX13 mutants are dependent on another HPS complex, BLOC-2 and the SNARE, VAMP7. Absence of 1-129 region in STX13 or mutations in the subunits of AP-3 perturbs the steady state localization of STX13 suggesting an indirect role for AP-3 in recycling of STX13 to endosome via non canonical motifs present in its 1-129 aa region.

Protein Tyrosine Phosphatases (PTPs) initiate, modulate and terminate key cellular processes by dephosphorylating phosphotyrosine (pY) residues on signaling proteins. The coordinated action of PTPs with their cognate tyrosine kinases is crucial for the maintenance of cellular homeostasis. Five Receptor Tyrosine Phosphatases (RPTPs) DLAR, PTP99A, PTP69D,PTP10D and PTP52F are involved in the axon guidance process of the fruit-fly Drosophila melanogaster. The receptors in these RPTPs comprise of Cell Adhesion Molecules (CAMs) whilethe cytosolic region contains the catalytic PTP domains. Extensive studies on the genetic interactions between these RPTPs reveal that these five RPTPs collaborate, compete or are partially redundant in some developmental contexts. While the genetic interactions between these RPTPs are well characterized, the role of domain-domain interactions and the mechanism(s) of substrate recognition are poorly understood. The aim of this study was to understand the molecular basis for these interactions using a combination of biophysical, biochemical and structural biology tools. This thesis is organized as follows: Chapter 1: The introductory chapter of this thesis highlights the mechanistic issues in signal transduction with an emphasis on the role of the RPTPs in the neuro-development of Drosophila melanogaster. The first part of this chapter describes the structural features and the catalytic mechanism of the PTP domain. This is followed by a description of the mechanisms that modulate the activity of a PTP domain. The latter part of the chapter summarizes the role ofthese RPTPs in axon guidance of Drosophila melanogaster. The interactions between the RPTPsbased on genetic data provide a mechanistic hypothesis that could be examined in vitro. The studies described in the subsequent chapters of this thesis were performed to evaluate this hypothesis. Chapter 2: This chapter reports our observations on the so-called construct dependence on the expression of recombinant PTP domains in Escherichia coli. This chapter details the strategies used to obtain recombinant PTP domains in a soluble form suitable for biochemical and structural studies. This study involved substantial optimization in the size of the protein and overexpression strategies to avoid inclusion-body formation. Five strains of E. coli as well as three variations in purification tags viz., poly-histidine peptide attachments at the N-and C-termini and a construct with Glutathione-S-transferase at the N-terminus were examined. In this study, we observed that inclusion of a 45 residue stretch at the N-terminus was crucial for the over-expression of the PTP domains, influencing both the solubility and the stability of these recombinant proteins. While the addition of negatively charged residues in the N-terminal extension could partially rationalize the improvement in the solubility of these constructs, conventional parameters like the proportion of order-promoting residues or the aliphatic index did not correlate with the improved biochemical characteristics. The findings in this chapter suggest that the inclusion of additional parameters like secondary structure propensities apart from rigid domain predictions could play a crucial role in obtaining a soluble recombinant protein upon expression in E. coli. Chapter 3: This chapter reports the crystal structure of the PTP domain of PTP10D and PTP10Dsubstrate/inhibitor complexes. These structural studies revealed aromatic ring stacking interactions that mediate substrate recruitment into the PTP active site. In particular, these studies revealed the role of conserved aromatic residue in Motif 1 (Phenylalaline 76 in case ofPTP10D). Mutation of Phenylalanine 76 residue to a Leucine (similar to the mutation found in the inactive distal PTP domains in other bi-domain PTPs) resulted in a sixty-fold decrease in the catalytic efficiency of the enzyme. Fluorescence kinetic measurements to monitor ligand binding showed a three fold increase in the half time of enzyme-ligand complex formation. These studies highlight the role of the KNRY loop in substrate recruitment at the active of the PTP domain and the role of this segment in modulating the kinetics of the enzyme-substrate complex formation. Chapter 4: This chapter describes a strategy to utilize protein-protein interaction data to identify putative peptide substrates for a given protein. This study was performed in collaboration with Shameer Khader and Prof. R. Sowdhamini at the National Center for Biological Sciences (NCBS).This integrated search approach, called ‘PeptideMine’ was developed into a web-server for experimental and computational biologists. The Peptide Mine strategy combines sequence searches in the 'interacting sequence space' of a protein using sequence patterns or functional motifs. A compilation of indices that describe the chemical and solubility properties of potential peptide substrates to facilitate investigation by in vitro or in silico studies is also obtained from this server. The biological significance of such a design-strategy was examined in the context of protein-peptide interactions in the case of RPTPs of Drosophila melanogaster. Chapter 5: In this chapter, we report an analysis of the influence of the membrane distal (D2) domain on the catalytic activity and substrate specificity of the membrane proximal (D1) domain using two bi-domain RPTPs as a model system. Biochemical studies reveal contrasting roles for the D2 domain of the Drosophila Leukocyte antigen Related (DLAR) and Protein Tyrosine Phosphatase on Drosophila chromosome band 99A (PTP99A). While D2 lowers the catalytic activity of the D1 domain in DLAR, the D2 domain of PTP99A leads to an increase in the catalytic activity of its D1 domain. Substrate specificity, on the other hand, is cumulative, whereby the individual specificities of the D1 and D2 domains contribute to the substrate specificity of these two-domain enzymes. Molecular dynamics simulations on structural models of DLAR and PTP99A revealed a conformational rationale for the experimental observations. These studies suggested that concerted structural changes mediate inter-domain communication resulting in either inhibitory or activating effects of the membrane distal PTP domain on the catalytic activity of the membrane proximal PTP domain. Chapter 6: This chapter describes biochemical studies to understand the role of the D2 domain of PTP99A. While the catalytic activity of PTP99A is localized to its membrane proximal (D1)domain, the inactive membrane distal (D2) domain influences the catalytic activity of the D1domain. Phosphatase activity, monitored using small molecule as well as peptide substrates, suggested that the D2 domain activates D1. Thermodynamic measurements on the bi-domain(D1-D2 protein) as well as single domain PTP99A protein constructs suggest that the presence of the inactive D2 domain influences the stability of the bi-domain protein. The mechanism by which the D2 domain activates and stabilizes the bi-domain protein is governed by a few interactions at the inter-domain interface. In particular, we note that mutating Lys990 at the interface attenuates inter-domain communication. This residue is located at a structurally equivalent position to the so-called allosteric site of a canonical PTP, PTP1B. These observations suggest functional optimization in bi-domain RPTPs wherein the inactive PTP domain modulates the catalytic activity of the bi-domain enzyme. Chapter 7: This chapter summarizes the experimental and computational studies on the Drosophila melanogaster PTP domains. The salient features of the experimental data that revealed hitherto uncharacterized sequence-structure relationships in the conserved PTP domain are highlighted. The latter part of this chapter briefly suggests the scope of future research in this area based on some of the findings reported in this thesis. Appendix : This thesis has an appendix section with four parts. These comprise of technical details and auxiliary work that was not included in the main text of the thesis. Appendix I describes cloning strategies, purification protocols and a list of all recombinant proteins used in this study. Appendix II describes the standardization of the ‘Three Phase partitioning’ protocol for refolding and solubilization of protein from inclusion bodies. Appendix III includes theimmunochemical work performed to elucidate the localization of PTP10D in Drosophila embryos. Appendix IV describes the work on a Quercetin 2,3 Dioxygenase from Bacillus subtilis with an emphasis on the role of metal ions in modulating catalytic activity in this class of proteins.

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No / Heterozygous germline defects in a gene encoding a type II receptor for bone morphogenetic proteins (BMPR-II) underlie the majority of inherited cases of the vascular disorder known as pulmonary arterial hypertension (PAH). However, the precise molecular consequences of PAH causing mutations on the function of the receptor complex remain unclear. We employed novel enzymatic and fluorescence activity based techniques to assess the impact of PAH mutations on pre-mRNA splicing, nonsense-mediated decay (NMD) and receptor complex interactions. We demonstrate that nonsense and frameshift mutations trigger NMD, providing further evidence that haplo-insufficiency is a major molecular consequence of disease-related BMPR2 mutations. We identified heterogeneous functional defects in BMPR-II activity, including impaired type I receptor phosphorylation, receptor interactions and altered receptor complex stoichiometry leading to perturbation of downstream signalling pathways. Importantly, these studies demonstrate that the intracellular domain of BMPR-II is both necessary and sufficient for receptor complex interaction. Finally and to address the potential for resolution of stoichiometric balance, we investigated an agent that promotes translational readthrough of a BMPR2 nonsense reporter construct without interfering with the NMD pathway. We propose that stoichiometric imbalance, due to either haplo-insufficiency or loss of optimal receptor-receptor interactions impairs BMPR-II mediated signalling in PAH. Taken together, these studies have identified an important target for early therapeutic intervention in familial PAH.

All cells are equipped to sense changes in their environment and make adaptive responses according to the stimuli. Signal recognition usually occurs at the cell membrane (with the exception of steroid signalling) where the ligand, which can be a small molecule, a peptide or a protein, binds its cognate receptor. This results in a change in the conformation of the receptor which in turn can regulate the production of second messengers. Second messengers can now modulate specific pathways which control gene expression and modify various aspects of cell behaviour. The signalling cascade is terminated by the removal of second messenger and/or by desensitisation of the receptor to the extracellular signal. Cyclic guanosine monophosphate (cGMP) was first identified in the rat urine and since then has emerged as an important second messenger regulating diverse cell processes. Subsequent to its discovery in mammalian cells, enzymes responsible for its synthesis (guanylyl cyclases), hydrolysis (phosphodiesterases) and its most common effectors (cGMP-dependent protein kinases) were identified. Guanylyl cyclases exist in two forms, cytosolic and membrane bound. Both have a conserved guanylyl cyclase domain, but differ in their choice of ligands, overall structure and tissue localization. It is now known that cytosolic and the membrane-bound forms are involved in eliciting distinct cellular responses. Receptor guanylyl cyclase C (GC-C) was identified as the target for a family of heat-stable enterotoxin toxins (ST) produced by enterotoxigenic E.coli. Stable toxin-mediated diarrhoeas are observed frequently in infants and contribute significantly to the incidence of Travellers’ Diarrhea. Early studies demonstrated that the effects of ST were mediated by an increase in intracellular cGMP levels in intestinal cells, and the receptor for ST was almost exclusively expressed in the apical microvilli of the intestinal brush-border epithelia. Effectors of cGMP in intestinal cells include protein kinase G (PKG), cyclic nucleotide gated ion channel 3 (CNG), and the cystic fibrosis transmembrane conductance regulator (CFTR). ST is an exogenous ligand which serves as a hyperagonist for GC-C, in comparison with the endogenous ligands guanylin and uroguanylin, which maintain fluid-ion homeostasis in the intestinal epithelia. The GC-C/cGMP signal transduction pathway also modulates intestinal cell proliferation along the crypt-villus axis by exerting a cytostatic effect on the epithelial cells, thereby regulating their turnover and neoplastic transformation. The current study describes in molecular detail two signalling pathways, one impinging on and one emerging from GC-C, which regulate colonic cell proliferation. The first part identifies the cross-talk and cross-regulation of GC-C and c-src. The second part delves into the molecular basis of GC-C/cGMP-mediated cytostasis and its effect on colonic tumorigenesis. Cross-talk between signalling pathways is believed to play a key role in regulating cell physiology. Phosphorylation of signalling molecules by protein kinases is frequently used as a means of achieving this cross-regulation. Aberrant hyperactivation of the c-src tyrosine kinase is an early event in the progression of colorectal cancer, and activated c-src specifically phosphorylates a number of proteins in the cell. It was found that c-src can phosphorylate GC-C in T84 colorectal carcinoma cells, as well as in the rat intestinal epithelia. Tyrosine phosphorylation of GC-C resulted in attenuation of ligand-mediated cGMP production; an effect which was reversed by chemical or transcriptional knockdown of c-src. These effects were found to be cell line-independent and relied only on the extent of c-src expression and activation in the cell. Mutational analysis revealed GC-C to be phosphorylated on a conserved tyrosine residue (Y820) in the guanylyl cyclase domain. The sequence of GC-C around Y820 allowed for efficient phosphorylation by c-src, and indeed, kinase assays indicated that the affinity of c-src for the GC-C Y820 peptide was one of the highest reported till date. A phospho-mimetic mutation at this site, which mimics a constitutively phosphorylated receptor, resulted in a sharp reduction of guanylyl cyclase activity of the receptor, reiterating the inhibitory role of Y820 phosphorylation on GC-C activity. Phosphorylation of GC-C at Y820 generated a docking site for the SH2 domain of c-src which could interact and thereby co-localize with GC-C on the cell membrane. Intriguingly, this interaction resulted in activation of c-src, setting-up a feed-forward loop of inhibitory GC-C phosphorylation and c-src activation. Treatment of colorectal carcinoma cells with ligands for GC-C reduces cell proliferation and inhibits tumorigenesis. It was observed that this cytostatic effect can be modulated by the status of c-src activation, and consequently, the fraction of tyrosine phosphorylated GC-C in these cells. Since activation of c-src is a frequent event in intestinal neoplasia, phosphorylation of GC-C by active c-src may be one of the means by which the cytostatic effects of GC-C agonists (guanylin and uroguanylin) in the intestine are bypassed, thereby leading to cancer progression. Colonisation of the gut with enteropathogenic microorganisms induces secretion of IFNγ from the host mucosal immune system, which subsequently activates c-src in intestinal epithelial cells. Ligand-stimulated activity of GC-C was found to be reduced in IFNγ treated cells. This could be one of the host defence mechanisms initiated in response to enterotoxigenic E. coli infection. These results provide the first evidence of cross-talk between a receptor guanylyl cyclase and a tyrosine kinase that results in heterologous desensitisation of the receptor. Populations with a higher incidence of enterotoxigenic E.coli infections appear to be protected from intestinal neoplasia. It was found that mice lacking GC-C, and therefore unable to respond to ST, displayed an increased cell proliferation in colonic crypts and enhanced carcinogen-induced aberrant crypt foci formation, which is a surrogate marker for colorectal carcinogenesis. However, pharmacological elevation of cGMP was able to efficiently induce cytostasis even in GC-C knockout mice, indicating a key role for cGMP in regulating colonic cell proliferation. Through microarray analyses, genes regulated by ST-induced GC-C activation in T84 colorectal carcinoma cells were identified. Genes involved in a number of cellular pathways were differentially expressed, including those involved in signal transduction, protein and solute secretion, transcriptional regulation and extracellular matrix formation. One of the genes found to be significantly up-regulated was the cell-cycle inhibitor, p21. The increase in p21 expression was validated at both the transcript and protein level. This p53-independent up-regulation of p21 was coupled to the activation of the cGMP-responsive kinase, PKGII, since knockdown of PKGII using specific siRNAs abolished ST-induced p21 induction. Activation of PKGII led to phosphorylation and activation of the stress responsive p38 MAPK. Similar to what was seen following knockdown of PKGII, inhibition of p38 MAPK activity attenuated the up-regulation of p21 in response to cGMP, indicating that PKGII and p38 MAPK could be a part of a pathway regulating p21 expression. It was found that active p38 MAPK phosphorylated the ubiquitous transcription factor SP1, enhancing its occupancy at the proximal p21 promoter. Therefore, SP1 could be one of the factors linking cGMP to transcription of the p21 mRNA. Chronic activation of GC-C led to nuclear accumulation of p21 in colonic cells, which entered a quiescent state. These cells arrested in the G1 phase of the cell cycle, consequent to p21-dependent inhibition of the G1 cyclin-CDK complexes. A fraction of these quiescent cells stochastically initiated a cGMP-dependent senescence programme and displayed all the hallmarks of senescent cells, including flattened cell morphology, expression of SA- galactosidase and formation of senescence-associated heterochromatic foci. Activation of senescence and loss of tumorigenicity in these cells was crucially dependent on the up-regulation of p21. This irreversible exit from the cell cycle due to cGMP-mediated activation of the PKGII/p38/p21 axis was well correlated with reduced colonic polyp formation in mice exposed to ST. In summary, these observations may provide a possible explanation for the low incidence of colorectal carcinoma seen in countries with a high incidence of ST-mediated diarrhoea. Interestingly, c-src mediated tyrosine phosphorylation of GC-C prevented p21 accumulation following ligand application. The findings described in this thesis may have important implications in understanding the molecular mechanisms involved in the progression and treatment of colorectal cancer.

Osteoclast-mediated focal articular bone erosion is a hallmark of rheumatoid arthritis, a disease of inflammation-induced bone loss. Inflammation in the bone microenvironment enhances osteoclast differentiation leading to bone erosion. Simultaneously, inflammation also inhibits osteoblast-mediated bone formation, further contributing to the net loss of bone. Previous studies have shown a paucity of mature osteoblasts at eroded bone surfaces correlating with suppression of bone formation and upregulation of antagonists of the Wnt pathway, a signaling cascade essential for osteoblast lineage commitment. Despite these observations, the exact pathogenesis of impaired bone formation in the setting of inflammation is not clearly understood. This dissertation aims to delineate the mechanisms by which inflammation suppresses osteoblast differentiation and activity in inflammatory arthritis. Specifically, this research elucidates how inflammation-induced alterations in the Wnt and bone morphogenetic protein (BMP) osteogenic signaling pathways contribute to bone loss and formation at distinct inflammatory microenvironments within the bone. Secondly, the means by which cellular mediators, including lymphocytes and macrophages, facilitate bone erosion and formation was addressed. Taken together, the research in this dissertation underscores the relationship between inflammation-induced bone loss and alterations in osteogenic signaling. Using an innovative murine inflammatory arthritis model, this study definitively demonstrates that resolving inflammation promotes osteoblast-mediated bone formation. Repair of erosions correlates with upregulation of synovial expression of Wnt10b, a Wnt agonist, and downregulation of sFRP1 and sFRP2, Wnt antagonists. This work also directly evaluates the contribution of sFRP1 to inflammation-induced bone destruction. Furthermore, this research demonstrates that expression of BMP3, a negative regulator of BMP signaling, is upregulated in osteoblasts by IL-17, a pro-inflammatory cytokine. BMP3-expressing osteoblasts are also observed at erosion sites in murine arthritis. Lastly, evaluation of the mediators of inflammation-induced periosteal bone formation implicates BMP2 as a means by which inflammation may positively regulate osteoblast function. This dissertation further elucidates the role of T cells and macrophages in the erosion and formation processes, respectively. In the absence of lymphocytes, bone erosion occurred normally, demonstrating that RANKL-expressing lymphocytes are not absolutely required for the bone erosion. Preliminary studies also suggest that M2 macrophages are potential mediators of bone formation via the expression of BMP2. In conclusion, this dissertation explores the ability of inflammation to act as a rheostat, which controls the fate of bone by modulating not only osteoclast differentiation, but also osteogenic signaling pathways and cellular mediators in the bone microenvironment. The soluble mediators and cell types identified in this research highlight novel mechanisms by which inflammation may regulate osteoblast activity within the bone microenvironment. Collectively, these data imply that strict control of inflammation may be necessary in order to create an anabolic environment that preserves bone architecture in diseases of inflammation-induced bone loss.