Identification of Small Molecule Inhibitors and Ligand Directed Degraders of Calcium/Calmodulin Dependent Protein Kinase Kinase 1 and 2 (CaMKK1/2)

CaMKK2 signals through AMPK-dependent and AMPK-independent pathways to trigger cellular outputs including proliferation, differentiation, and migration, resulting in changes to metabolism, bone mass accrual, neuronal function, hematopoiesis, and immunity. CAMKK2 is upregulated in tumors including hepatocellular carcinoma, prostate, breast, and gastric cancer, and genetic deletion in myeloid cells results in increased antitumor immunity in several syngeneic models. Validation of the biological roles of CaMKK2 has relied on genetic deletion or small molecule inhibitors with activity against several biological targets. We sought to generate selective inhibitors and degraders to understand the biological impact of inhibiting catalytic activity and scaffolding and the potential therapeutic benefits of targeting CaMKK2. We report herein selective, ligand-efficient inhibitors and ligand-directed degraders of CaMKK2 that were used to probe immune and tumor intrinsic biology. These molecules provide two distinct strategies for ablating CaMKK2 signaling in vitro and in vivo.

Hematopoietic Stem Cell Niches Produce Lineage-Instructive Signals to Control Multipotent Progenitor Differentiation

Hematopoietic stem cells (HSCs) self-renew in bone marrow niches formed by mesenchymal progenitors and endothelial cells expressing the chemokine CXCL12, but whether a separate niche instructs multipotent progenitor (MPP) differentiation remains unclear. We show that MPPs resided in HSC niches, where they encountered lineage-instructive differentiation signals. Conditional deletion of the chemokine receptor CXCR4 in MPPs reduced differentiation into common lymphoid progenitors (CLPs), which decreased lymphopoiesis. CXCR4 was required for CLP positioning near Interleukin-7⁺ (IL-7) cells and for optimal IL-7 receptor signaling. IL-7⁺ cells expressed CXCL12 and the cytokine SCF, were mesenchymal progenitors capable of differentiation into osteoblasts and adipocytes, and comprised a minor subset of sinusoidal endothelial cells. Conditional Il7 deletion in mesenchymal progenitors reduced B-lineage committed CLPs, while conditional Cxcl12 or Scf deletion from IL-7⁺ cells reduced HSC and MPP numbers. Thus, HSC maintenance and multilineage differentiation are distinct cell lineage decisions that are both controlled by HSC niches.

Deletion of Rac in Mature Osteoclasts Causes Osteopetrosis, an Age-Dependent Change in Osteoclast Number, and a Reduced Number of Osteoblasts In Vivo

Rac1 and Rac2 are thought to have important roles in osteoclasts. Therefore, mice with deletion of both Rac1 and Rac2 in mature osteoclasts (DKO) were generated by crossing Rac1(flox/flox) mice with mice expressing Cre in the cathepsin K locus and then mating these animals with Rac2(-/-) mice. DKO mice had markedly impaired tooth eruption. Bone mineral density (BMD) was increased 21% to 33% in 4- to 6-week-old DKO mice at all sites when measured by dual-energy X-ray absorptiometry (DXA) and serum cross-linked C-telopeptide (CTx) was reduced by 52%. The amount of metaphyseal trabecular bone was markedly increased in DKO mice, but the cortices were very thin. Spinal trabecular bone mass was increased. Histomorphometry revealed significant reductions in both osteoclast and osteoblast number and function in 4- to 6-week-old DKO animals. In 14- to 16-week-old animals, osteoclast number was increased, although bone density was further increased. DKO osteoclasts had severely impaired actin ring formation, an impaired ability to generate acid, and reduced resorptive activity in vitro. In addition, their life span ex vivo was reduced. DKO osteoblasts expressed normal differentiation markers except for the expression of osterix, which was reduced. The DKO osteoblasts mineralized normally in vitro, indicating that the in vivo defect in osteoblast function was not cell autonomous. Confocal imaging demonstrated focal disruption of the osteocytic dendritic network in DKO cortical bone. Despite these changes, DKO animals had a normal response to treatment with once-daily parathyroid hormone (PTH). We conclude that Rac1 and Rac2 have critical roles in skeletal metabolism.

Oxysterols and EBI2 promote osteoclast precursor migration to bone surfaces and regulate bone mass homeostasis

The mechanisms guiding cells toward bone surfaces are generally unknown. Here, Nevius et al. show that the Gαi protein–coupled receptor EBI2 is expressed in mouse osteoclast precursors to guide these cells toward bone surfaces. Defective EBI2 signaling increased bone mass in male mice and protected female mice from age- and estrogen deficiency–induced osteoporosis.

Bone surfaces attract hematopoietic and nonhematopoietic cells, such as osteoclasts (OCs) and osteoblasts (OBs), and are targeted by bone metastatic cancers. However, the mechanisms guiding cells toward bone surfaces are essentially unknown. Here, we show that the Gαi protein–coupled receptor (GPCR) EBI2 is expressed in mouse monocyte/OC precursors (OCPs) and its oxysterol ligand 7α,25-dihydroxycholesterol (7α,25-OHC) is secreted abundantly by OBs. Using in vitro time-lapse microscopy and intravital two-photon microscopy, we show that EBI2 enhances the development of large OCs by promoting OCP motility, thus facilitating cell–cell interactions and fusion in vitro and in vivo. EBI2 is also necessary and sufficient for guiding OCPs toward bone surfaces. Interestingly, OCPs also secrete 7α,25-OHC, which promotes autocrine EBI2 signaling and reduces OCP migration toward bone surfaces in vivo. Defective EBI2 signaling led to increased bone mass in male mice and protected female mice from age- and estrogen deficiency–induced osteoporosis. This study identifies a novel pathway involved in OCP homing to the bone surface that may have significant therapeutic potential.

EBI2 guides osteoclast precursors to bone niches and regulates osteoclastogenesis and bone mass homeostasis. (P5099)

Osteoclasts (OCs) are bone-lining cells derived from the fusion of monocytic precursors. Their exclusive positioning in contact with bone enables OC differentiation and the remodeling of bone outgrowth. However, the guidance cues that osteoclast precursors (OCP) follow for positioning at the bone surface remain poorly characterized. We demonstrate that EBI2 (Gpr183) was abundantly expressed in OCPs, and that autocrine EBI2 signaling was necessary for differentiation into mature osteoclasts in vitro. The expression of EBI2 and its ligands suggested that EBI2 signaling regulates OCP motility. Time lapse microscopy of OCP differentiation revealed reduced velocity and displacement of EBI2-deficient OCPs and mature OCs in vitro. Mice deficient in EBI2 or EBI2-ligands were osteopetrotic under homeostasis, and were protected from osteoporosis induced by estrogen deficiency. As osteoblasts (OBs) control osteoclast differentiation we hypothesized that they also recruit OCPs to bone niches. Indeed, OBs expressed EBI2-ligand synthesis enzymes, Ch25h and Cyp7b1, and secreted EBI2 ligands in vitro. We further demonstrate that EBI2-deficient OCPs were unable to efficiently migrate towards bone, and differentiated into smaller sized osteoclasts in vivo. Remarkably, overexpression of EBI2 was sufficient for directing OCPs to bone surfaces. Combined, these studies revealed a novel EBI2 controlled bone marrow niche, and identified a new candidate for the treatment of bone disorders.

The VR1 ligand capsaicin confers protection from autoimmune diabetes by suppressing autoreactive T cells in an APC dependent manner (50.38)

Sensory nerve fibers innervating the pancreas play a critical role in the etiology of autoimmune diabetes. These sensory neurons express a nonselective cation channel vanilloid receptor 1 (VR1) which has been identified as a receptor for acid, temperatures > 43°C and Capsaicin (CP), the principal pungent ingredient in chili peppers. We have reported that dendritic cells (DCs), a key cell type in immune responses, have the receptor for CP, and engagement of this receptor has powerful immune consequences. However very little is known about the function of VR1 expressing pancreas resident antigen presenting cells (APCs) including DCs in Type 1 diabetes. Here we have explored the immunomodulatory activity of CP in the non-obese diabetic (NOD) mouse and found that oral administration of CP delays the onset of diabetes and reduces the incidence of diabetes in NOD mice in a dose dependent manner. Moreover, splenocytes from CP treated mice are able to transfer protection in recipient NOD mice. CP administration attenuates the proliferation of pancreatic antigen-specific autoreactive T cells in pancreatic lymph nodes (PLN) both in vivo and in vitro in two distinct mouse models of diabetes, NOD and RIP-OVA. We have further found that this CP dependent modulation of autoimmunity requires both VR1 expression and a population of myeloid CD11b+ cells present in the PLN. These results reveal that CP is an immunologically potent ligand that when administered orally suppresses autoimmune diabetes by inhibiting the proliferation of autoreactive T cells in an APC dependent manner.

CD4+CD25+Foxp3+ T regulatory cell stimulation by high dose gp96 is costimulation and APC independent (131.3)

The immunogenicity of heat shock protein-peptide complexes (HSPs) against tumors and pathogens is initiated when the HSP engages its receptors, CD91 and maybe others, on antigen presenting cells (APCs). This allows APCs to obtain antigen in the form of peptides, and acquire signals that indicate for them to mature, express co-stimulation and migrate (1). However, high doses of HSPs tend to negate ongoing T cell responses (2), an effect shown to involve generation of a CD4+ suppressor response (3). This immune suppression has been demonstrated to protect mice from autoimmune conditions including experimental autoimmune encephalomyelitis (EAE) and the NOD model of diabetes (3). Here we provide evidence that highly enriched CD4+CD25+Foxp3+ T regulatory cells proliferate up to 3 fold more in the presence of the HSP gp96 and IL-2 compared to IL-2 alone. LPS or albumin were used as controls and did not support proliferation. This effect is APC and co-stimulation independent and may occur via CD91 which is expressed on T regulatory cells or by another receptor. Overall CD4+CD25+Foxp3+ T regulatory cells are stimulated directly by HSPs and may be the mechanism by which high doses of HSPs induce a suppressor response.

This study was supported by NIH grant CA84479-02 and by a research agreement with Antigenics Inc.

Peroxisome Proliferator-Activated Receptor γ (PPARγ) and Immunoregulation: Enhancement of Regulatory T Cells through PPARγ-Dependent and -Independent Mechanisms

Peroxisome proliferator-activated receptor (PPAR)gamma is a nuclear hormone receptor primarily characterized for its effect on insulin metabolism. PPARgamma ligands, used to treat human type 2 diabetes, also down-regulate most immune system cells including APCs and pathogenic T cells. These effects putatively underlie the efficacy of PPARgamma ligands in treating animal models of autoimmunity, leading to projections of therapeutic potential in human autoimmunity. However, the relationship between PPARgamma ligands and CD4+CD25+ regulatory T cells (Tregs) has not been examined. Specifically, no studies have examined the role of Tregs in mediating the in vivo immunoregulatory effects of PPARgamma ligands, and there have been no investigations of the use of PPARgamma ligands to treat autoimmunity in the absence of Tregs. We now characterize the novel relationship between ciglitazone, a thiazolidinedione class of PPARgamma ligand, and both murine natural Tregs (nTregs) and inducible Tregs (iTregs). In vitro, ciglitazone significantly enhances generation of iTregs in a PPARgamma-independent manner. Surprisingly, and contrary to the current paradigm, we find that, in a model of graft-vs-host disease, the immunotherapeutic effect of ciglitazone requires the presence of nTregs that express PPARgamma. Overall, our results indicate that, unlike its down-regulatory effect on other cells of the immune system, ciglitazone has an enhancing effect on both iTregs and nTregs, and this finding may have important implications for using PPARgamma ligands in treating human autoimmune disease.