Hepatitis C virus RNA is 5'-capped with flavin adenine dinucleotide.

RNA viruses have evolved elaborate strategies to protect their genomes, including 5' capping. However, until now no RNA 5' cap has been identified for hepatitis C virus1,2 (HCV), which causes chronic infection, liver cirrhosis and cancer3. Here we demonstrate that the cellular metabolite flavin adenine dinucleotide (FAD) is used as a non-canonical initiating nucleotide by the viral RNA-dependent RNA polymerase, resulting in a 5'-FAD cap on the HCV RNA. The HCV FAD-capping frequency is around 75%, which is the highest observed for any RNA metabolite cap across all kingdoms of life4-8. FAD capping is conserved among HCV isolates for the replication-intermediate negative strand and partially for the positive strand. It is also observed in vivo on HCV RNA isolated from patient samples and from the liver and serum of a human liver chimeric mouse model. Furthermore, we show that 5'-FAD capping protects RNA from RIG-I mediated innate immune recognition but does not stabilize the HCV RNA. These results establish capping with cellular metabolites as a novel viral RNA-capping strategy, which could be used by other viruses and affect anti-viral treatment outcomes and persistence of infection.

JFH1-based Core-NS2 genotype variants of HCV with genetic stability in vivo and in vitro: Important tools in the evaluation of virus neutralization.

BACKGROUND AND AIMS: HCV infection continues to be a major global health burden despite effective antiviral treatments. The urgent need for a protective vaccine is hindered by the scarcity of suitable HCV-permissive animal models tractable in vaccination and challenge studies. Currently, only antibody neutralization studies in infectious cell culture systems or studies of protection by passive immunization of human liver chimeric mice offer the possibility to evaluate the effect of vaccine-induced antibodies. However, differences between culture-permissive and in vivo-permissive viruses make it a challenge to compare analyses between platforms. To address this problem, we aimed at developing genotype-specific virus variants with genetic stability both in vitro and in vivo. APPROACH AND RESULTS: We demonstrated infection of human liver chimeric mice with cell culture-adapted HCV JFH1-based Core-NS2 recombinants of genotype 1-6, with a panel of 10 virus strains used extensively in neutralization and receptor studies. Clonal re-engineering of mouse-selected mutations resulted in virus variants with robust replication both in Huh7.5 cells and human liver chimeric mice, with genetic stability. Furthermore, we showed that, overall, these virus variants have similar in vitro neutralization profiles as their parent strains and demonstrated their use for in vivo neutralization studies. CONCLUSIONS: These mouse-selected HCV recombinants enable the triage of new vaccine-relevant antibodies in vitro and further allow characterization of protection from infection in vivo using identical viruses in human liver chimeric mice. As such, these viruses will serve as important resources in testing novel antibodies and can thus guide strategies to develop an efficient protective vaccine against HCV infection.

Full-length sequence analysis of hepatitis C virus genotype 3b strains and development of an in vivo infectious 3b cDNA clone.

IMPORTANCE: HCV genotype 3b is a difficult-to-treat subtype, associated with accelerated progression of liver disease and resistance to antivirals. Moreover, its prevalence has significantly increased among persons who inject drugs posing a serious risk of transmission in the general population. Thus, more genetic information and antiviral testing systems are required to develop novel therapeutic options for this genotype 3 subtype. We determined the complete genomic sequence and complexity of three genotype 3b isolates, which will be beneficial to study its biology and evolution. Furthermore, we developed a full-length in vivo infectious cDNA clone of genotype 3b and showed its robustness and genetic stability in human-liver chimeric mice. This is, to our knowledge the first reported infectious cDNA clone of HCV genotype 3b and will provide a valuable tool to evaluate antivirals and neutralizing antibodies in vivo, as well as in the development of infectious cell culture systems required for further research.

Molecular Determinants of Mouse Adaptation of Rat Hepacivirus.

The lack of robust immunocompetent animal models for hepatitis C virus (HCV) impedes vaccine development and studies of immune responses. Norway rat hepacivirus (NrHV) infection in rats shares HCV-defining characteristics, including hepatotropism, chronicity, immune responses, and aspects of liver pathology. To exploit genetic variants and research tools, we previously adapted NrHV to prolonged infection in laboratory mice. Through intrahepatic RNA inoculation of molecular clones of the identified variants, we here characterized four mutations in the envelope proteins responsible for mouse adaptation, including one disrupting a glycosylation site. These mutations led to high-titer viremia, similar to that observed in rats. In 4-week-old mice, infection was cleared after around 5 weeks compared to 2 to 3 weeks for nonadapted virus. In contrast, the mutations led to persistent but attenuated infection in rats, and they partially reverted, accompanied by an increase in viremia. Attenuated infection in rat but not mouse hepatoma cells demonstrated that the characterized mutations were indeed mouse adaptive rather than generally adaptive across species and that species determinants and not immune interactions were responsible for attenuation in rats. Unlike persistent NrHV infection in rats, acute resolving infection in mice was not associated with the development of neutralizing antibodies. Finally, infection of scavenger receptor B-I (SR-BI) knockout mice suggested that adaptation to mouse SR-BI was not a primary function of the identified mutations. Rather, the virus may have adapted to lower dependency on SR-BI, thereby potentially surpassing species-specific differences. In conclusion, we identified specific determinants of NrHV mouse adaptation, suggesting species-specific interactions during entry. IMPORTANCE A prophylactic vaccine is required to achieve the World Health Organization's objective for hepatitis C virus elimination as a serious public health threat. However, the lack of robust immunocompetent animal models supporting hepatitis C virus infection impedes vaccine development as well as studies of immune responses and viral evasion. Hepatitis C virus-related hepaciviruses were discovered in a number of animal species and provide useful surrogate infection models. Norway rat hepacivirus is of particular interest, as it enables studies in rats, an immunocompetent and widely used small laboratory animal model. Its adaptation to robust infection also in laboratory mice provides access to a broader set of mouse genetic lines and comprehensive research tools. The presented mouse-adapted infectious clones will be of utility for reverse genetic studies, and the Norway rat hepacivirus mouse model will facilitate studies of hepacivirus infection for in-depth characterization of virus-host interactions, immune responses, and liver pathology.

Neutralization and receptor use of infectious culture-derived rat hepacivirus as a model for HCV.

BACKGROUND AND AIMS: Lack of tractable immunocompetent animal models amenable to robust experimental challenge impedes vaccine efforts for HCV. Infection with rodent hepacivirus from Rattus norvegicus (RHV-rn1) in rats shares HCV-defining characteristics, including liver tropism, chronicity, and pathology. RHV in vitro cultivation would facilitate genetic studies on particle production, host factor interactions, and evaluation of antibody neutralization guiding HCV vaccine approaches. APPROACH AND RESULTS: We report an infectious reverse genetic cell culture system for RHV-rn1 using highly permissive rat hepatoma cells and adaptive mutations in the E2, NS4B, and NS5A viral proteins. Cell culture-derived RHV-rn1 particles (RHVcc) share hallmark biophysical characteristics of HCV and are infectious in mice and rats. Culture adaptive mutations attenuated RHVcc in immunocompetent rats, and the mutations reverted following prolonged infection, but not in severe combined immunodeficiency (SCID) mice, suggesting that adaptive immune pressure is a primary driver of reversion. Accordingly, sera from RHVcc-infected SCID mice or the early acute phase of immunocompetent mice and rats were infectious in culture. We further established an in vitro RHVcc neutralization assay, and observed neutralizing activity of rat sera specifically from the chronic phase of infection. Finally, we found that scavenger receptor class B type I promoted RHV-rn1 entry in vitro and in vivo. CONCLUSIONS: The RHV-rn1 infectious cell culture system enables studies of humoral immune responses against hepacivirus infection. Moreover, recapitulation of the entire RHV-rn1 infectious cycle in cell culture will facilitate reverse genetic studies and the exploration of tropism and virus-host interactions.

Btbd7 is essential for region-specific epithelial cell dynamics and branching morphogenesis in vivo.

Branching morphogenesis of developing organs requires coordinated but poorly understood changes in epithelial cell-cell adhesion and cell motility. We report that Btbd7 is a crucial regulator of branching morphogenesis in vivo. Btbd7 levels are elevated in peripheral cells of branching epithelial end buds, where it enhances cell motility and cell-cell adhesion dynamics. Genetic ablation of Btbd7 in mice disrupts branching morphogenesis of salivary gland, lung and kidney. Btbd7 knockout results in more tightly packed outer bud cells, which display stronger E-cadherin localization, reduced cell motility and decreased dynamics of transient cell separations associated with cleft formation; inner bud cells remain unaffected. Mechanistic analyses using in vitro MDCK cells to mimic outer bud cell behavior establish that Btbd7 promotes loss of E-cadherin from cell-cell adhesions with enhanced migration and transient cell separation. Btbd7 can enhance E-cadherin ubiquitination, internalization, and degradation in MDCK and peripheral bud cells for regulating cell dynamics. These studies show how a specific regulatory molecule, Btbd7, can function at a local region of developing organs to regulate dynamics of cell adhesion and motility during epithelial branching morphogenesis.

Replicons of a Rodent Hepatitis C Model Virus Permit Selection of Highly Permissive Cells.

Animal hepaciviruses represent promising surrogate models for hepatitis C virus (HCV), for which there are no efficient immunocompetent animal models. Experimental infection of laboratory rats with rodent hepacivirus isolated from feral Rattus norvegicus (RHV-rn1) mirrors key aspects of HCV infection in humans, including chronicity, hepatitis, and steatosis. Moreover, RHV has been adapted to infect immunocompetent laboratory mice. RHV in vitro systems have not been developed but would enable detailed studies of the virus life cycle crucial for designing animal experiments to model HCV infection. Here, we established efficient RHV-rn1 selectable subgenomic replicons with and without reporter genes. Rat and mouse liver-derived cells did not readily support the complete RHV life cycle, but replicon-containing cell clones could be selected with and without acquired mutations. Replication was significantly enhanced by mutations in NS4B and NS5A and in cell clones cured of replicon RNA. These mutations increased RHV replication of both mono- and bicistronic constructs, and CpG/UpA-dinucleotide optimization of reporter genes allowed replication. Using the replicon system, we show that the RHV-rn1 NS3-4A protease cleaves a human mitochondrial antiviral signaling protein reporter, providing a sensitive readout for virus replication. RHV-rn1 replication was inhibited by the HCV polymerase inhibitor sofosbuvir and high concentrations of HCV NS5A antivirals but not by NS3 protease inhibitors. The microRNA-122 antagonist miravirsen inhibited RHV-rn1 replication, demonstrating the importance of this HCV host factor for RHV. These novel RHV in vitro systems will be useful for studies of tropism, molecular virology, and characterization of virus-host interactions, thereby providing important complements to in vivo systems.IMPORTANCE A vaccine against hepatitis C virus (HCV) is crucial for global control of this important pathogen, which induces fatal human liver diseases. Vaccine development has been hampered by the lack of immunocompetent animal models. Discovery of rodent hepacivirus (RHV) enabled establishment of novel surrogate animal models. These allow robust infection and reverse genetic and immunization studies of laboratory animals, which develop HCV-like chronicity. Currently, there are no RHV in vitro systems available to study tropism and molecular virology. Here, we established the first culture systems for RHV, recapitulating the intracellular phase of the virus life cycle in vitro These replicon systems enabled identification of replication-enhancing mutations and selection of cells highly permissive to RHV replication, which allow study of virus-host interactions. HCV antivirals targeting NS5A, NS5B, and microRNA-122 efficiently inhibited RHV replication. Hence, several important aspects of HCV replication are shared by the rodent virus system, reinforcing its utility as an HCV model.

MT3-MMP Promotes Excitatory Synapse Formation by Promoting Nogo-66 Receptor Ectodomain Shedding.

Cell-surface molecules are dynamically regulated at the synapse to assemble and disassemble adhesive contacts that are important for synaptogenesis and for tuning synaptic transmission. Metalloproteinases dynamically regulate cellular behaviors through the processing of cell surface molecules. In the present study, we evaluated the role of membrane-type metalloproteinases (MT-MMPs) in excitatory synaptogenesis. We find that MT3-MMP and MT5-MMP are broadly expressed in the mouse cerebral cortex and that MT3-MMP loss-of-function interferes with excitatory synapse development in dissociated cortical neurons and in vivo We identify Nogo-66 receptor (NgR1) as an MT3-MMP substrate that is required for MT3-MMP-dependent synapse formation. Introduction of the shed ectodomain of NgR1 is sufficient to accelerate excitatory synapse formation in dissociated cortical neurons and in vivo Together, our findings support a role for MT3-MMP-dependent shedding of NgR1 in regulating excitatory synapse development.SIGNIFICANCE STATEMENT In this study, we identify MT3-MMP, a membrane-bound zinc protease, to be necessary for the development of excitatory synapses in cortical neurons. We identify Nogo-66 receptors (NgR1) as a downstream target of MT3-MMP proteolytic activity. Furthermore, processing of surface NgR1 by MT3-MMP generates a soluble ectodomain fragment that accelerates the formation of excitatory synapses. We propose that MT3-MMP activity and NgR1 shedding could stimulate circuitry remodeling in the adult brain and enhance functional connectivity after brain injury.

A CCR2 macrophage endocytic pathway mediates extravascular fibrin clearance in vivo.

Extravascular fibrin deposition accompanies many human diseases and causes chronic inflammation and organ damage, unless removed in a timely manner. Here, we used intravital microscopy to investigate how fibrin is removed from extravascular space. Fibrin placed into the dermis of mice underwent cellular endocytosis and lysosomal targeting, revealing a novel intracellular pathway for extravascular fibrin degradation. A C-C chemokine receptor type 2 (CCR2)-positive macrophage subpopulation constituted the majority of fibrin-uptaking cells. Consequently, cellular fibrin uptake was diminished by elimination of CCR2-expressing cells. The CCR2-positive macrophage subtype was different from collagen-internalizing M2-like macrophages. Cellular fibrin uptake was strictly dependent on plasminogen and plasminogen activator. Surprisingly, however, fibrin endocytosis was unimpeded by the absence of the fibrin(ogen) receptors, αMß2 and ICAM-1, the myeloid cell integrin-binding site on fibrin or the endocytic collagen receptor, the mannose receptor. The study identifies a novel fibrin endocytic pathway engaged in extravascular fibrin clearance and shows that interstitial fibrin and collagen are cleared by different subsets of macrophages employing distinct molecular pathways.

WNT1-induced Secreted Protein-1 (WISP1), a Novel Regulator of Bone Turnover and Wnt Signaling.

WISP1/CCN4 (hereafter referred to as WISP1), a member of the CCN family, is found in mineralized tissues and is produced by osteoblasts and their precursors. In this study, Wisp1-deficient (Wisp1(-/-)) mice were generated. Using dual-energy x-ray absorptiometry, we showed that by 3 months, the total bone mineral density of Wisp1(-/-) mice was significantly lower than that of WT mice. Further investigation by micro-computed tomography showed that female Wisp1(-/-) mice had decreased trabecular bone volume/total volume and that both male and female Wisp1(-/-) mice had decreased cortical bone thickness accompanied by diminished biomechanical strength. The molecular basis for decreased bone mass in Wisp1(-/-) mice arises from reduced bone formation likely caused by osteogenic progenitors that differentiate poorly compared with WT cells. Osteoclast precursors from Wisp1(-/-) mice developed more tartrate-resistant acid phosphatase-positive cells in vitro and in transplants, suggesting that WISP1 is also a negative regulator of osteoclast differentiation. When bone turnover (formation and resorption) was induced by ovariectomy, Wisp1(-/-) mice had lower bone mineral density compared WT mice, confirming the potential for multiple roles for WISP1 in controlling bone homeostasis. Wisp1(-/-) bone marrow stromal cells had reduced expression of ß-catenin and its target genes, potentially caused by WISP1 inhibition of SOST binding to LRP6. Taken together, our data suggest that the decreased bone mass found in Wisp1(-/-) mice could potentially be caused by an insufficiency in the osteodifferentiation capacity of bone marrow stromal cells arising from diminished Wnt signaling, ultimately leading to altered bone turnover and weaker biomechanically compromised bones.

The membrane-anchored serine protease prostasin (CAP1/PRSS8) supports epidermal development and postnatal homeostasis independent of its enzymatic activity.

The membrane-anchored serine protease prostasin (CAP1/PRSS8) is part of a cell surface proteolytic cascade that is essential for epithelial barrier formation and homeostasis. Here, we report the surprising finding that prostasin executes these functions independent of its own enzymatic activity. Prostasin null (Prss8(-/-)) mice lack barrier formation and display fatal postnatal dehydration. In sharp contrast, mice homozygous for a point mutation in the Prss8 gene, which causes the substitution of the active site serine within the catalytic histidine-aspartate-serine triad with alanine and renders prostasin catalytically inactive (Prss8(Cat-/Cat-) mice), develop barrier function and are healthy when followed for up to 20 weeks. This striking difference could not be explained by genetic modifiers or by maternal effects, as these divergent phenotypes were displayed by Prss8(-/-) and Prss8(Cat-/Cat-) mice born within the same litter. Furthermore, Prss8(Cat-/Cat-) mice were able to regenerate epidermal covering following cutaneous wounding. This study provides the first demonstration that essential in vivo functions of prostasin are executed by a non-enzymatic activity of this unique membrane-anchored serine protease.

Targeting a single function of the multifunctional matrix metalloprotease MT1-MMP: impact on lymphangiogenesis.

The group of matrix metalloproteases (MMPs) is responsible for multiple processes of extracellular matrix remodeling in the healthy body but also for matrix and tissue destruction during cancer invasion and metastasis. The understanding of the contributions from each individual MMP, both in healthy and pathological events, has been complicated by the lack of specific inhibitors and the fact that some of the potent MMPs are multifunctional enzymes. These factors have also hampered the setup of therapeutic strategies targeting MMP activity. A tempting target is the membrane-associated MT1-MMP, which has well-documented importance in matrix degradation but which takes part in more than one pathway in this regard. In this report, we describe the selective targeting of a single function of this enzyme by means of a specific monoclonal antibody against MT1-MMP, raised in an MT1-MMP knock-out mouse. The antibody blocks the enzyme ability to activate proMMP-2 without interfering with the collagenolytic function or the general proteolytic activity of MT1-MMP. Using this antibody, we have shown that the MT1-MMP-catalyzed activation of proMMP-2 is involved in the outgrowth of cultured lymphatic endothelial cells in a collagen matrix in vitro, as well as in lymphatic vessel sprouting assayed ex vivo. This is the first example of the complete inactivation of a single function of a multifunctional MMP and the use of this strategy to pursue its role.

An inactivated SARS-CoV-2 vaccine induced cross-neutralizing persisting antibodies and protected against challenge in small animals.

Vaccines have relieved the public health burden of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and globally inactivated vaccines are most widely used. However, poor vaccination accessibility and waning immunity maintain the pandemic, driving emergence of variants. We developed an inactivated SARS-CoV-2 (I-SARS-CoV-2) vaccine based on a viral isolate with the Spike mutation D614G, produced in Vero cells in a scalable bioreactor, inactivated with ß-propiolactone, purified by membrane-based steric exclusion chromatography, and adjuvanted with MF59-like adjuvant AddaVax. I-SARS-CoV-2 and a derived split vaccine induced persisting neutralizing antibodies in mice; moreover, lyophilized antigen was immunogenic. Following homologous challenge, I-SARS-CoV-2 immunized hamsters were protected against disease and lung pathology. In contrast with reports for widely used vaccines, hamster plasma similarly neutralized the homologous and the Delta (B.1.617.2) variant viruses, whereas the Omicron (B.1.1.529) variant was neutralized less efficiently. Applied bioprocessing approaches offer advantages regarding scalability and production, potentially benefitting worldwide vaccine coverage.

Membrane-type MMPs are indispensable for placental labyrinth formation and development.

The membrane-type matrix metalloproteinases (MT-MMPs) are essential for pericellular matrix remodeling in late stages of development, as well as in growth and tissue homeostasis in postnatal life. Although early morphogenesis is perceived to involve substantial tissue remodeling, the roles of MT-MMPs in these processes are only partially characterized. Here we explore the functions of 2 prominently expressed MT-MMPs, MT1-MMP and MT2-MMP, and describe their roles in the process of placental morphogenesis. The fetal portion of the placenta, in particular the labyrinth (LA), displays strong overlapping expression of MT1-MMP and MT2-MMP, which is critical for syncytiotrophoblast formation and in turn for fetal vessels. Disruption of trophoblast syncytium formation consequently leads to developmental arrest with only a few poorly branched fetal vessels entering the LA causing embryonic death at embryonic day 11.5. Through knockdown of MMP expression, we demonstrate that either MT1-MMP or MT2-MMP is crucial specifically during development of the LA. In contrast, knockdown of MT-MMP activity after LA formation is compatible with development to term and postnatal life. Taken together these data identify essential but interchangeable roles for MT1-MMP or MT2-MMP in placental vasculogenesis and provide the first example of selective temporal and spatial MMP activity required for development of the mouse embryo.

MT1-mmp: a collagenase essential for tumor cell invasive growth.

The manuscript discussed in this preview describes that reconstituted three-dimensional extracellular matrices such as fibrillar collagen and fibrin exert stringent territorial growth control on cells. The authors show that tumor cells are able to escape the matrix-enforced growth control effect (entrapment) by pericellular proteolysis mediated by MT1-MMP, a membrane bound matrix metalloproteinase capable of directly cleaving both type I collagen and fibrin but not by other, soluble matrix metalloprotinases. These data convincingly demonstrate one way that tumor cells orchestrate proteolysis to invade surrounding tissues.

Novel HCV Genotype 4d Infectious Systems and Assessment of Direct-Acting Antivirals and Antibody Neutralization.

Hepatitis C virus (HCV) genotype 4 is highly prevalent in the Middle East and parts of Africa. Subtype 4d has recently spread among high-risk groups in Europe. However, 4d infectious culture systems are not available, hampering studies of drugs, as well as neutralizing antibodies relevant for HCV vaccine development. We determined the consensus 4d sequence from a chronic hepatitis C patient by next-generation sequencing, generated a full-length clone thereof (pDH13), and demonstrated that pDH13 RNA-transcripts were viable in the human-liver chimeric mouse model, but not in Huh7.5 cells. However, a JFH1-based DH13 Core-NS5A 4d clone encoding A1671S, T1785V, and D2411G was viable in Huh7.5 cells, with efficient growth after inclusion of 10 additional substitutions [4d(C5A)-13m]. The efficacies of NS3/4A protease- and NS5A- inhibitors against genotypes 4a and 4d were similar, except for ledipasvir, which is less potent against 4d. Compared to 4a, the 4d(C5A)-13m virus was more sensitive to neutralizing monoclonal antibodies AR3A and AR5A, as well as 4a and 4d patient plasma antibodies. In conclusion, we developed the first genotype 4d infectious culture system enabling DAA efficacy testing and antibody neutralization assessment critical to optimization of DAA treatments in the clinic and for vaccine design to combat the HCV epidemic.

MT1-matrix metalloproteinase directs arterial wall invasion and neointima formation by vascular smooth muscle cells.

During pathologic vessel remodeling, vascular smooth muscle cells (VSMCs) embedded within the collagen-rich matrix of the artery wall mobilize uncharacterized proteolytic systems to infiltrate the subendothelial space and generate neointimal lesions. Although the VSMC-derived serine proteinases, plasminogen activator and plasminogen, the cysteine proteinases, cathepsins L, S, and K, and the matrix metalloproteinases MMP-2 and MMP-9 have each been linked to pathologic matrix-remodeling states in vitro and in vivo, the role that these or other proteinases play in allowing VSMCs to negotiate the three-dimensional (3-D) cross-linked extracellular matrix of the arterial wall remains undefined. Herein, we demonstrate that VSMCs proteolytically remodel and invade collagenous barriers independently of plasmin, cathepsins L, S, or K, MMP-2, or MMP-9. Instead, we identify the membrane-anchored matrix metalloproteinase, MT1-MMP, as the key pericellular collagenolysin that controls the ability of VSMCs to degrade and infiltrate 3-D barriers of interstitial collagen, including the arterial wall. Furthermore, genetic deletion of the proteinase affords mice with a protected status against neointimal hyperplasia and lumen narrowing in vivo. These studies suggest that therapeutic interventions designed to target MT1-MMP could prove beneficial in a range of human vascular disease states associated with the destructive remodeling of the vessel wall extracellular matrix.

Secreted frizzled related-protein 2 (Sfrp2) deficiency decreases adult skeletal stem cell function in mice.

In a previous transcriptomic study of human bone marrow stromal cells (BMSCs, also known as bone marrow-derived "mesenchymal stem cells"), SFRP2 was highly over-represented in a subset of multipotent BMSCs (skeletal stem cells, SSCs), which recreate a bone/marrow organ in an in vivo ectopic bone formation assay. SFRPs modulate WNT signaling, which is essential to maintain skeletal homeostasis, but the specific role of SFRP2 in BMSCs/SSCs is unclear. Here, we evaluated Sfrp2 deficiency on BMSC/SSC function in models of skeletal organogenesis and regeneration. The skeleton of Sfrp2-deficient (KO) mice is overtly normal; but their BMSCs/SSCs exhibit reduced colony-forming efficiency, reflecting low SSC self-renewal/abundancy. Sfrp2 KO BMSCs/SSCs formed less trabecular bone than those from WT littermates in the ectopic bone formation assay. Moreover, regeneration of a cortical drilled hole defect was dramatically impaired in Sfrp2 KO mice. Sfrp2-deficient BMSCs/SSCs exhibited poor in vitro osteogenic differentiation as measured by Runx2 and Osterix expression and calcium accumulation. Interestingly, activation of the Wnt co-receptor, Lrp6, and expression of Wnt target genes, Axin2, C-myc and Cyclin D1, were reduced in Sfrp2-deficient BMSCs/SSCs. Addition of recombinant Sfrp2 restored most of these activities, suggesting that Sfrp2 acts as a Wnt agonist. We demonstrate that Sfrp2 plays a role in self-renewal of SSCs and in the recruitment and differentiation of adult SSCs during bone healing. SFRP2 is also a useful marker of BMSC/SSC multipotency, and a factor to potentially improve the quality of ex vivo expanded BMSC/SSC products.