Control of chondrocyte gene expression by actin dynamics: a novel role of cholesterol/Ror-α signalling in endochondral bone growth

Identification of Diagnostic Markers in Synovial Tissue of Osteoarthritis by Weighted Gene Coexpression Network

Osteoarthritis (OA) is a serious threat to human health. However, the etiology and pathogenesis of the disease are not fully understood. Most researchers believe that the degeneration and imbalance of articular cartilage, extracellular matrix, and subchondral bone are the fundamental causes of osteoarthritis. However, recent studies have shown that synovial lesions may precede cartilage, which may be an important precipitating factor in the early stage of OA and the whole course of the disease. This study aimed to conduct an analysis based on sequence data from the Gene Expression Omnibus (GEO) database to investigate the presence of effective biomarkers in the synovial tissue of osteoarthritis for the diagnosis and control of OA progression. In this study, the differentially expressed OA-related genes (DE-OARGs) in osteoarthritis synovial tissues were extracted in the GSE55235 and GSE55457 datasets using the Weighted Gene Co-expression Network Analysis (WGCNA) and limma. Least-Absolute Shrinkage and Selection Operator (LASSO) algorithm was used to select the diagnostic genes based on the DE-OARGs by glmnet package. 7 genes were selected as diagnostic genes including SAT1, RLF, MAFF, SIK1, RORA, ZNF529, and EBF2. Subsequently, the diagnostic model was constructed and the results of the Area Under the Curve (AUC) demonstrated that the diagnostic model had high diagnostic performance for OA. Additionally, among the 22 immune cells of the Cell type Identification By Estimating Relative Subsets Of RNA Transcripts (CIBERSORT) and the 24 immune cells of the single sample Gene Set Enrichment Analysis (ssGSEA), 3 immune cells and 5 immune cells were different between the OA and normal samples, respectively. The expression trends of the 7 diagnostic genes were consistent in the GEO datasets and the results of the real-time reverse transcription PCR (qRT-PCR). The results of this study demonstrate that these diagnostic markers have important significance in the diagnosis and treatment of OA, and will provide further evidence for the clinical and functional studies of OA.

Nutrient metabolism of the nucleus pulposus: A literature review

Cells take in, consume, and synthesize nutrients for numerous physiological functions. This includes not only energy production but also macromolecule biosynthesis, which will further influence cellular signaling, redox homeostasis, and cell fate commitment. Therefore, alteration in cellular nutrient metabolism is associated with pathological conditions. Intervertebral discs, particularly the nucleus pulposus (NP), are avascular and exhibit unique metabolic preferences. Clinical and preclinical studies have indicated a correlation between intervertebral degeneration (IDD) and systemic metabolic diseases such as diabetes, obesity, and dyslipidemia. However, a lack of understanding of the nutrient metabolism of NP cells is masking the underlying mechanism. Indeed, although previous studies indicated that glucose metabolism is essential for NP cells, the downstream metabolic pathways remain unknown, and the potential role of other nutrients, like amino acids and lipids, is understudied. In this literature review, we summarize the current understanding of nutrient metabolism in NP cells and discuss other potential metabolic pathways by referring to a human NP transcriptomic dataset deposited to the Gene Expression Omnibus, which can provide us hints for future studies of nutrient metabolism in NP cells and novel therapies for IDD.

Identifying the key genes regulating mesenchymal stem cells chondrogenic differentiation: an in vitro study

Background

Mesenchymal stem cells (MSCs) possess the potential to differentiate into chondrocytes, which makes them an ideal source for healing cartilage defects. Here, we seek to identify the essential genes participating in MSCs chondrogenesis.

Methods

Human MSCs were induced for chondrogenesis for 7, 14, and 21 days using a high-density micromass culture system, and RNA was extracted for RNA-seq.

Results

A total of 6247 differentially expressed genes (DEGs) were identified on day 7, and 85 DEGs were identified on day 14. However, no significant DEGs was identified on day 21. The top 30 DEGs at day 7, including COL9A3, COL10A1, and CILP2, are closely related to extracellular matrix organization. While the top 30 DEGs at day 14 revealed that inflammation-related genes were enriched, including CXCL8, TLR2, and CCL20. We also conducted protein–protein interaction (PPI) networks analysis using the search tool for the retrieval of interacting genes (STRING) database and identified key hub genes, including CXCL8, TLR2, CCL20, and MMP3. The transcriptional factors were also analyzed, identifying the top 5 TFs: LEF1, FOXO1, RORA, BHLHE41, and SOX5. We demonstrated one particular TF, RORA, in promoting early MSCs chondrogenesis.

Conclusions

Taken together, our results suggested that these DEGs may have a complex effect on MSCs chondrogenesis both synergistically and solitarily.

Expanding horizons of achondroplasia treatment: current options and future developments

Activating mutations in the FGFR3 receptor tyrosine kinase lead to most prevalent form of genetic dwarfism in humans, the achondroplasia. Many features of the complex function of FGFR3 in growing skeleton were characterized, which facilitated identification of therapy targets, and drove progress toward treatment. In August 2021, the vosoritide was approved for treatment of achondroplasia, which is based on a stable variant of the C-natriuretic peptide. Other drugs may soon follow, as several conceptually different inhibitors of FGFR3 signaling progress through clinical trials. Here, we review the current achondroplasia therapeutics, describe their mechanisms, and illuminate motivations leading to their development. We also discuss perspectives of curing achondroplasia, and options for repurposing achondroplasia drugs for dwarfing conditions unrelated to FGFR3.

Inhibition of nuclear receptor RORα attenuates cartilage damage in osteoarthritis by modulating IL-6/STAT3 pathway

Osteoarthritis (OA) is characterized by cartilage destruction, chronic inflammation, and local pain. Evidence showed that retinoic acid receptor-related orphan receptor-α (RORα) is crucial in cartilage development and OA pathogenesis. Here, we investigated the role and molecular mechanism of RORα, an important member of the nuclear receptor family, in regulating the development of OA pathologic features. Investigation into clinical cartilage specimens showed that RORα expression level is positively correlated with the severity of OA and cartilage damage. In an in vivo OA model induced by anterior crucial ligament transaction, intra-articular injection of si-Rora adenovirus reversed the cartilage damage. The expression of cartilage matrix components type II collagen and aggrecan were elevated upon RORα blockade. RNA-seq data suggested that the IL-6/STAT3 pathway is significantly downregulated, manifesting the reduced expression level of both IL-6 and phosphorylated STAT3. RORα exerted its effect on IL-6/STAT3 signaling in two different ways, including interaction with STAT3 and IL-6 promoter. Taken together, our findings indicated the pivotal role of the RORα/IL-6/STAT3 axis in OA progression and confirmed that RORα blockade improved the matrix catabolism in OA chondrocytes. These results may provide a potential treatment target in OA therapy.

Inverse Agonist of Retinoid-Related Orphan Receptor-Alpha Prevents Apoptosis and Degeneration in Nucleus Pulposus Cells via Upregulation of YAP

Intervertebral disc degenerative disease (IDD) is the most common degenerative spine disease, which leads to chronic low back pain and symptoms in the lower extremities. In this study, we found that RORα, a member of the retinoid-related orphan receptor family, is significantly elevated in nucleus pulposus tissue in IDD patients. The elevation of RORα is associated with increased apoptosis of nucleus pulposus (NP) cells. Therefore, we applicated a well-established inverse agonist of RORα, SR3335, to investigate its role in regulating NP cell metabolism and apoptosis. To further investigate the mechanism that SR3335 regulates the pathogenesis of IDD in vitro, tumor necrosis factor alpha (TNF-α) stimulation was used in human NP cells to mimic the hostile environment that leads to degeneration. We found that SR3335 treatment reversed the trend of increased apoptosis in NP cells induced by TNF-α treatment. Next, TNF-α treatment upregulated the expression of type II collagen and aggrecan and downregulated MMP13 (matrix-degrading enzyme matrix metalloproteinase 13) and ADAMTS4 (a disintegrin and metalloproteinase with thrombospondin motifs 4). However, these effects were reversed after SR3335 treatment. Furthermore, we find that SR3335 mediated the effect in NP cells by regulating the YAP signaling pathway, especially by affecting the phosphorylation state of YAP. In conclusion, the reduction of matrix degradation enzymes and apoptosis upon SR3335 treatment suggests that SR3335 is a promising drug in reversing the deleterious microenvironment in IDD patients.

Effects of ultrasound therapy on the synovial fluid proteome in a rabbit surgery-induced model of knee osteoarthritis

Background

Ultrasound (US) therapy may improve osteoarthritis symptoms. We investigated the effects of US on the synovial fluid (SF) proteome in a rabbit knee osteoarthritis (KOA) model to explore its therapeutic mechanisms.

Methods

Sixteen healthy 6-month-old New Zealand white rabbits (eight male, eight female), weighing 2.5–3.0 kg, were randomly divided into groups A and B with eight rabbits per group. Both groups were subjected to right anterior cruciate ligament transaction. Six weeks after surgery, we treated the operated knee joint of group A rabbits with US and of group B rabbits with sham US for 2 weeks. The proteomes of knee joint SF from groups A and B rabbits were then analyzed using a label-free mass spectrometry (MS) quantification method.

Results

We identified 19 protein sequences annotated by 361 Gene Ontology (GO) items. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) database of rabbit protein sequences, we then annotated the KO numbers of homologous/similar proteins to 32 relevant KEGG pathways. We extracted 10 significantly differentially expressed proteins among the 32 relevant KEGG messages/metabolism pathways. The proteins whose levels were decreased were apolipoprotein A-I (AopA-1), transferrin (TF), carboxypeptidase B2 (CBP2), arylesterase/paraoxonase (PON), fibrinogen alpha chain, and alpha-2-macroglobulin (A2M). The proteins whose levels were increased were molecular chaperone HtpG/heat shock proteins (htpG, HSP90A), decorin (DCN), pyruvate kinase (PK, pyk), and fatty acid-binding protein 4/adipocyte (FABP4, aP2).

Conclusions

US therapy can alter protein levels in SF, which can decrease AopA-1, TF, CBP2, PON, fibrinogen alpha chain and A2M protein levels, and increase HtpG/HSP90A, DCN, PK/PKY, and FABP4/aP2 protein levels in SF of KOA, suggesting that the therapeutic mechanisms of US therapy on KOA may occur through changes in the SF proteome.

ECM signaling in cartilage development and endochondral ossification

During cartilage development chondrocytes undergo a multi-step process characterized by consecutive changes in cell morphology and gene expression. Cell proliferation, polarity, differentiation, and migration are influenced by chemical and mechanical signaling between the extracellular matrix (ECM) and the cell. Several structurally diverse transmembrane receptors such as integrins, discoidin domain receptor 2 (DDR 2), and CD44 mediate the crosstalk between cells and their ECM. However, the contribution of cell-matrix interactions during early chondrogenesis and further cartilage development through cell receptors and their signal transduction pathways is still not fully understood. Determination of receptor signaling pathways and the function of downstream targets will aid in a better understanding of musculoskeletal pathologies such as chondrodysplasia, and the development of new approaches for the treatment of cartilage disorders. We will summarize recent findings, linking cell receptors and their potential signaling pathways to the control of chondrocyte behavior during early chondrogenesis and endochondral ossification.

Synergistic co-regulation and competition by a SOX9-GLI-FOXA phasic transcriptional network coordinate chondrocyte differentiation transitions

The growth plate mediates bone growth where SOX9 and GLI factors control chondrocyte proliferation, differentiation and entry into hypertrophy. FOXA factors regulate hypertrophic chondrocyte maturation. How these factors integrate into a Gene Regulatory Network (GRN) controlling these differentiation transitions is incompletely understood. We adopted a genome-wide whole tissue approach to establish a Growth Plate Differential Gene Expression Library (GP-DGEL) for fractionated proliferating, pre-hypertrophic, early and late hypertrophic chondrocytes, as an overarching resource for discovery of pathways and disease candidates. De novo motif discovery revealed the enrichment of SOX9 and GLI binding sites in the genes preferentially expressed in proliferating and prehypertrophic chondrocytes, suggesting the potential cooperation between SOX9 and GLI proteins. We integrated the analyses of the transcriptome, SOX9, GLI1 and GLI3 ChIP-seq datasets, with functional validation by transactivation assays and mouse mutants. We identified new SOX9 targets and showed SOX9-GLI directly and cooperatively regulate many genes such as Trps1, Sox9, Sox5, Sox6, Col2a1, Ptch1, Gli1 and Gli2. Further, FOXA2 competes with SOX9 for the transactivation of target genes. The data support a model of SOX9-GLI-FOXA phasic GRN in chondrocyte development. Together, SOX9-GLI auto-regulate and cooperate to activate and repress genes in proliferating chondrocytes. Upon hypertrophy, FOXA competes with SOX9, and control toward terminal differentiation passes to FOXA, RUNX, AP1 and MEF2 factors.

In the development of the mammalian growth plate, while several transcription factors are individually well known for their key roles in regulating phases of chondrocyte differentiation, there is little information on how they interact and cooperate with each other. We took an unbiased genome wide approach to identify the transcription factors and signaling pathways that play dominant roles in the chondrocyte differentiation cascade. We developed a searchable library of differentially expressed genes, GP-DGEL, which has fine spatial resolution and global transcriptomic coverage for discovery of processes, pathways and disease candidates. Our work identifies a novel regulatory mechanism that integrates the action of three transcription factors, SOX9, GLI and FOXA. SOX9-GLI auto-regulate and cooperate to activate and repress genes in proliferating chondrocytes. Upon entry into prehypertrophy, FOXA competes with SOX9, and control of hypertrophy passes to FOXA, RUNX, AP1 and MEF2 factors.

Achondroplasia: Development, pathogenesis, and therapy

Autosomal dominant mutations in fibroblast growth factor receptor 3 (FGFR3) cause achondroplasia (Ach), the most common form of dwarfism in humans, and related chondrodysplasia syndromes that include hypochondroplasia (Hch), severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN), and thanatophoric dysplasia (TD). FGFR3 is expressed in chondrocytes and mature osteoblasts where it functions to regulate bone growth. Analysis of the mutations in FGFR3 revealed increased signaling through a combination of mechanisms that include stabilization of the receptor, enhanced dimerization, and enhanced tyrosine kinase activity. Paradoxically, increased FGFR3 signaling profoundly suppresses proliferation and maturation of growth plate chondrocytes resulting in decreased growth plate size, reduced trabecular bone volume, and resulting decreased bone elongation. In this review, we discuss the molecular mechanisms that regulate growth plate chondrocytes, the pathogenesis of Ach, and therapeutic approaches that are being evaluated to improve endochondral bone growth in people with Ach and related conditions. Developmental Dynamics 246:291-309, 2017. © 2016 Wiley Periodicals, Inc.

Liver X Receptor activation delays chondrocyte hypertrophy during endochondral bone growth

OBJECTIVE

Activation of the Liver X Receptor (LXR) has recently been identified as a therapeutic strategy for osteoarthritis (OA). Human OA articular cartilage explants show decreased LXR expression, and LXRβ-null mice display OA-like symptoms. LXR agonist administration to OA articular cartilage explants suppresses proteoglycan degradation and restores LXR-activated transcription. We aimed to investigate the effect of LXR activation on chondrocyte differentiation to elucidate the molecular mechanisms behind its protection against OA.

METHOD

The specific LXR agonist, GW3965, was used to examine the effect of LXR activation on chondrocyte differentiation. Tibia organ cultures were used to examine the effect of LXR activation on bone growth and growth plate morphology, followed by immunohistochemical analysis. In ATDC5 and micromass cultures, chondrocyte differentiation was examined through cellular staining and proliferation assays. Various chondrogenic markers were analyzed by real-time reverse-transcription polymerase chain reaction (qRT-PCR) in micromass RNA.

RESULTS

Chondrocyte hypertrophy was suppressed by GW3965 treatment, as shown by decreased hypertrophic zone length in the tibial growth plate, decreased alkaline phosphatase staining in ATDC5 and micromass cultures, and down regulation of Col10a1, Mmp13 and Runx2 expression. Increased proliferation in treated ATDC5 cells and up-regulation of Col2a1 expression in treated micromass cultures suggest hypertrophy is suppressed secondary to prolonged proliferation. Decreased p57 levels in treated growth plates suggest this to be due to cell-cycle exit delay.

CONCLUSION

Our findings regarding LXR's role in cartilage development provide insight into how LXR activation prevents cartilage breakdown, further solidifying its potential as a therapeutic target of OA.

Identification of novel regulators of osteoblast matrix mineralization by time series transcriptional profiling

Bone mineralization is a carefully orchestrated process, regulated by a number of promoters and inhibitors that function to ensure effective hydroxyapatite formation. Here we sought to identify new regulators of this process through a time series microarray analysis of mineralising primary osteoblast cultures over a 27 day culture period. To our knowledge this is the first microarray study investigating murine calvarial osteoblasts cultured under conditions that permit both physiological extracellular matrix mineralization through the formation of discrete nodules and the terminal differentiation of osteoblasts into osteocytes. RT-qPCR was used to validate and expand the microarray findings. We demonstrate the significant up-regulation of >6,000 genes during the osteoblast mineralization process, the highest-ranked differentially expressed genes of which were those dominated by members of the PPAR-γ signalling pathway, namely Adipoq, Cd36 and Fabp4. Furthermore, we show that the inhibition of this signalling pathway promotes matrix mineralisation in these primary osteoblast cultures. We also identify Cilp, Phex, Trb3, Sox11, and Psat1 as novel regulators of matrix mineralization. Further studies examining the precise function of the identified genes and their interactions will advance our understanding of the mechanisms underpinning biomineralization.

Lipid transport and metabolism in healthy and osteoarthritic cartilage

Cartilage is an avascular tissue and cartilage metabolism depends on molecule diffusion from synovial fluid and subchondral bone. Thus, nutrient availability is limited by matrix permeability according to the size and charge of the molecules. Matrix composition limits the access of molecules to chondrocytes, determining cell metabolism and cartilage maintenance. Lipids are important nutrients in chondrocyte metabolism and are available for these cells through de novo synthesis but also through diffusion from surrounding tissues. Cartilage status and osteoarthritis development depend on lipid availability. This paper reviews lipid transport and metabolism in cartilage. We also analyze signalling pathways directly mediated by lipids and those that involve mTOR pathways, both in normal and osteoarthritic cartilage.

SIK3 is essential for chondrocyte hypertrophy during skeletal development in mice

Chondrocyte hypertrophy is crucial for endochondral ossification, but the mechanism underlying this process is not fully understood. We report that salt-inducible kinase 3 (SIK3) deficiency causes severe inhibition of chondrocyte hypertrophy in mice. SIK3-deficient mice showed dwarfism as they aged, whereas body size was unaffected during embryogenesis. Anatomical and histological analyses revealed marked expansion of the growth plate and articular cartilage regions in the limbs, accumulation of chondrocytes in the sternum, ribs and spine, and impaired skull bone formation in SIK3-deficient mice. The primary phenotype in the skeletal tissue of SIK3-deficient mice was in the humerus at E14.5, where chondrocyte hypertrophy was markedly delayed. Chondrocyte hypertrophy was severely blocked until E18.5, and the proliferative chondrocytes occupied the inside of the humerus. Consistent with impaired chondrocyte hypertrophy in SIK3-deficient mice, native SIK3 expression was detected in the cytoplasm of prehypertrophic and hypertrophic chondrocytes in developing bones in embryos and in the growth plates in postnatal mice. HDAC4, a crucial repressor of chondrocyte hypertrophy, remained in the nuclei in SIK3-deficient chondrocytes, but was localized in the cytoplasm in wild-type hypertrophic chondrocytes. Molecular and cellular analyses demonstrated that SIK3 was required for anchoring HDAC4 in the cytoplasm, thereby releasing MEF2C, a crucial facilitator of chondrocyte hypertrophy, from suppression by HDAC4 in nuclei. Chondrocyte-specific overexpression of SIK3 induced closure of growth plates in adulthood, and the SIK3-deficient cartilage phenotype was rescued by transgenic SIK3 expression in the humerus. These results demonstrate an essential role for SIK3 in facilitating chondrocyte hypertrophy during skeletogenesis and growth plate maintenance.

Reduced chondrocyte proliferation, earlier cell cycle exit and increased apoptosis in neuronal nitric oxide synthase-deficient mice

OBJECTIVE

Nitric oxide (NO) has been implicated in the local regulation of bone metabolism. However, the contribution made by specific nitric oxide synthase (NOS) enzymes to skeletal development is unclear. The objective of this study was to examine the effects of inactivation of neuronal nitric oxide synthase (nNOS) on cartilage development in mice.

DESIGN

Mice carrying a null mutation in the nNOS gene were used to address our objectives. Histological staining, immunohistochemistry and in situ analyses were employed along with real-time reverse transcriptase - polymerase chain reaction (RT-PCR).

RESULTS

nNOS-null mice show transient growth retardation and shorter long bones. nNOS-deficient growth plates show a reduction in replicating cells. Reduced chondrocyte numbers may in part be due to slower cell cycle progression and premature cell cycle exit caused by decreased cyclin D1 and increased p57 expression in mutants. In addition, apoptosis was increased as shown by increased cleaved-caspase 3 staining in hypertrophic chondrocytes in mutants. Real-time PCR demonstrated that expression of early chondrocyte markers such as Sox genes was reduced in mutant mice, while expression of prehypertrophic markers such as RORα was increased. Histological sections also demonstrated thinner cortical bone, fewer trabeculae and reduced mineralization in mutant mice.

CONCLUSIONS

These data identify an important role of nNOS in chondrocyte proliferation and endochondral bone growth and demonstrate that nNOS coordinates cell cycle exit and chondrocyte differentiation in cartilage development.

Inducible nitric oxide synthase-nitric oxide signaling mediates the mitogenic activity of Rac1 during endochondral bone growth

Coordinated proliferation and differentiation of growth plate chondrocytes controls endochondral bone growth and final height in humans, and disruption of this process results in diseases of the growing and adult skeleton, such as chondrodysplasias or osteoarthritis. We had shown recently that chondrocyte-specific deletion of the gene Rac1 in mice leads to severe dwarfism due to reduced chondrocyte proliferation, but the molecular pathways involved remained unclear. Here, we demonstrate that Rac1-deficient chondrocytes have severely reduced levels of inducible nitric oxide synthase (iNOS) protein and nitric oxide (NO) production. NO donors reversed the proliferative effects induced by Rac1 deficiency, whereas inhibition of NO production mimicked the effects of Rac1 loss of function. Examination of the growth plate of iNOS-deficient mice revealed reduced chondrocyte proliferation and expression of cyclin D1, resembling the phenotype of Rac1-deficient growth plates. Finally, we demonstrate that Rac1-NO signaling inhibits the expression of ATF3, a known suppressor of cyclin D1 expression in chondrocytes. In conclusion, our studies identify the iNOS-NO pathway as a novel mediator of mitogenic Rac1 signaling and indicate that it could be a target for growth disorder therapies.

Transcriptional profiling reveals a role for RORalpha in regulating gene expression in obesity-associated inflammation and hepatic steatosis

Retinoid-related orphan receptor (ROR)α4 is the major RORα isoform expressed in adipose tissues and liver. In this study we demonstrate that RORα-deficient staggerer mice (RORα(sg/sg)) fed with a high-fat diet (HFD) exhibited reduced adiposity and hepatic triglyceride levels compared with wild-type (WT) littermates and were resistant to the development of hepatic steatosis, adipose-associated inflammation, and insulin resistance. Gene expression profiling showed that many genes involved in triglyceride synthesis and storage, including Cidec, Cidea, and Mogat1, were expressed at much lower levels in liver of RORα(sg/sg) mice. In contrast, overexpression of RORα in mouse hepatoma Hepa1-6 cells significantly increased the expression of genes that were repressed in RORα(sg/sg) liver, including Sult1b1, Adfp, Cidea, and ApoA4. ChIP and promoter analysis suggested that several of these genes were regulated directly by RORα. In addition to reduced lipid accumulation, inflammation was greatly diminished in white adipose tissue (WAT) of RORα(sg/sg) mice fed with an HFD. The infiltration of macrophages and the expression of many immune response and proinflammatory genes, including those encoding various chemo/cytokines, Toll-like receptors, and TNF signaling proteins, were significantly reduced in RORα(sg/sg) WAT. Moreover, RORα(sg/sg) mice fed with an HFD were protected from the development of insulin resistance. RORα(sg/sg) mice consumed more oxygen and produced more carbon dioxide, suggesting increased energy expenditure in this genotype. Our study indicates that RORα plays a critical role in the regulation of several aspects of metabolic syndrome. Therefore, RORα may provide a novel therapeutic target in the management of obesity and associated metabolic diseases.

Genetic loci that regulate healing and regeneration in LG/J and SM/J mice

MRL mice display unusual healing properties. When MRL ear pinnae are hole punched, the holes close completely without scarring, with regrowth of cartilage and reappearance of both hair follicles and sebaceous glands. Studies using (MRL/lpr x C57BL/6)F(2) and backcross mice first showed that this phenomenon was genetically determined and that multiple loci contributed to this quantitative trait. The lpr mutation itself, however, was not one of them. In the present study we examined the genetic basis of healing in the Large (LG/J) mouse strain, a parent of the MRL mouse and a strain that shows the same healing phenotype. LG/J mice were crossed with Small (SM/J) mice and the F(2) population was scored for healing and their genotypes determined at more than 200 polymorphic markers. As we previously observed for MRL and (MRL x B6)F(2) mice, the wound-healing phenotype was sexually dimorphic, with female mice healing more quickly and more completely than male mice. We found quantitative trait loci (QTLs) on chromosomes (Chrs) 9, 10, 11, and 15. The heal QTLs on Chrs 11 and 15 were linked to differential healing primarily in male animals, whereas QTLs on Chrs 9 and 10 were not sexually dimorphic. A comparison of loci identified in previous crosses with those in the present report using LG/J x SM/J showed that loci on Chrs 9, 11, and 15 colocalized with those seen in previous MRL crosses, whereas the locus on Chr 10 was not seen before and is contributed by SM/J.