Krüppel-like factor 4 regulates membranous and endochondral ossification

Deletion of Trps1 regulatory elements recapitulates postnatal hip joint abnormalities and growth retardation of Trichorhinophalangeal syndrome in mice

Trichorhinophalangeal syndrome (TRPS) is a genetic disorder caused by point mutations or deletions in the gene-encoding transcription factor TRPS1. TRPS patients display a range of skeletal dysplasias, including reduced jaw size, short stature, and a cone-shaped digit epiphysis. Certain TRPS patients experience early onset coxarthrosis that leads to a devastating drop in their daily activities. The etiologies of congenital skeletal abnormalities of TRPS were revealed through the analysis of Trps1 mutant mouse strains. However, early postnatal lethality in Trps1 knockout mice has hampered the study of postnatal TRPS pathology. Here, through epigenomic analysis we identified two previously uncharacterized candidate gene regulatory regions in the first intron of Trps1. We deleted these regions, either individually or simultaneously, and examined their effects on skeletal morphogenesis. Animals that were deleted individually for either region displayed only modest phenotypes. In contrast, the Trps1Δint/Δint mouse strain with simultaneous deletion of both genomic regions exhibit postnatal growth retardation. This strain displayed delayed secondary ossification center formation in the long bones and misshaped hip joint development that resulted in acetabular dysplasia. Reducing one allele of the Trps1 gene in Trps1Δint mice resulted in medial patellar dislocation that has been observed in some patients with TRPS. Our novel Trps1 hypomorphic strain recapitulates many postnatal pathologies observed in human TRPS patients, thus positioning this strain as a useful animal model to study postnatal TRPS pathogenesis. Our observations also suggest that Trps1 gene expression is regulated through several regulatory elements, thus guaranteeing robust expression maintenance in skeletal cells.

Krüppel-like factors family in health and disease

Krüppel-like factors (KLFs) are a family of basic transcription factors with three conserved Cys2/His2 zinc finger domains located in their C-terminal regions. It is acknowledged that KLFs exert complicated effects on cell proliferation, differentiation, survival, and responses to stimuli. Dysregulation of KLFs is associated with a range of diseases including cardiovascular disorders, metabolic diseases, autoimmune conditions, cancer, and neurodegenerative diseases. Their multidimensional roles in modulating critical pathways underscore the significance in both physiological and pathological contexts. Recent research also emphasizes their crucial involvement and complex interplay in the skeletal system. Despite the substantial progress in understanding KLFs and their roles in various cellular processes, several research gaps remain. Here, we elucidated the multifaceted capabilities of KLFs on body health and diseases via various compliable signaling pathways. The associations between KLFs and cellular energy metabolism and epigenetic modification during bone reconstruction have also been summarized. This review helps us better understand the coupling effects and their pivotal functions in multiple systems and detailed mechanisms of bone remodeling and develop potential therapeutic strategies for the clinical treatment of pathological diseases by targeting the KLF family.

KLF transcription factors in bone diseases

Krüppel-like factors (KLFs) are crucial in the development of bone disease. They are a family of zinc finger transcription factors that are unusual in containing three highly conserved zinc finger structural domains interacting with DNA. It has been discovered that it engages in various cell functions, including proliferation, apoptosis, autophagy, stemness, invasion and migration, and is crucial for the development of human tissues. In recent years, the role of KLFs in bone physiology and pathology has received adequate attention. In addition to regulating the normal growth and development of the musculoskeletal system, KLFs participate in the pathological process of the bones and joints and are intimately linked to several skeletal illnesses, such as osteoarthritis (OA), rheumatoid arthritis (RA), osteoporosis (OP) and osteosarcoma (OS). Consequently, targeting KLFs has emerged as a promising therapeutic approach for an array of bone disorders. In this review, we summarize the current literature on the importance of KLFs in the emergence and regulation of bone illnesses, with a particular emphasis on the pertinent mechanisms by which KLFs regulate skeletal diseases. We also discuss the need for KLFs-based medication-targeted treatment. These endeavours offer new perspectives on the use of KLFs in bone disorders and provide prognostic biomarkers, therapeutic targets and possible drug candidates for bone diseases.

CUL4B orchestrates mesenchymal stem cell commitment by epigenetically repressing KLF4 and C/EBPδ

Dysregulated lineage commitment of mesenchymal stem cells (MSCs) contributes to impaired bone formation and an imbalance between adipogenesis and osteogenesis during skeletal aging and osteoporosis. The intrinsic cellular mechanism that regulates MSC commitment remains unclear. Here, we identified Cullin 4B (CUL4B) as a critical regulator of MSC commitment. CUL4B is expressed in bone marrow MSCs (BMSCs) and downregulated with aging in mice and humans. Conditional knockout of Cul4b in MSCs resulted in impaired postnatal skeletal development with low bone mass and reduced bone formation. Moreover, depletion of CUL4B in MSCs aggravated bone loss and marrow adipose accumulation during natural aging or after ovariectomy. In addition, CUL4B deficiency in MSCs reduced bone strength. Mechanistically, CUL4B promoted osteogenesis and inhibited adipogenesis of MSCs by repressing KLF4 and C/EBPδ expression, respectively. The CUL4B complex directly bound to Klf4 and Cebpd and epigenetically repressed their transcription. Collectively, this study reveals CUL4B-mediated epigenetic regulation of the osteogenic or adipogenic commitment of MSCs, which has therapeutic implications in osteoporosis.

Kruppel-like Factors in Skeletal Physiology and Pathologies

Kruppel-like factors (KLFs) belong to a large group of zinc finger-containing transcription factors with amino acid sequences resembling the Drosophila gap gene Krüppel. Since the first report of molecular cloning of the KLF family gene, the number of KLFs has increased rapidly. Currently, 17 murine and human KLFs are known to play crucial roles in the regulation of transcription, cell proliferation, cellular differentiation, stem cell maintenance, and tissue and organ pathogenesis. Recent evidence has shown that many KLF family molecules affect skeletal cells and regulate their differentiation and function. This review summarizes the current understanding of the unique roles of each KLF in skeletal cells during normal development and skeletal pathologies.

Pregnane X receptor (PXR) represses osteoblast differentiation through repression of the Hedgehog signaling pathway

The pregnane X receptor (PXR, NR1I2) belongs to the nuclear receptor family and functions as a xenobiotic and endobiotic sensor by binding to various molecules through its relatively flexible ligand-binding domain. In addition to these well-known canonical roles, we previously reported that PXR represses osteoblast differentiation. However, the mechanisms underlying the PXR-mediated repression of osteoblast differentiation remains unknown. In this study, we analyzed the changes in global gene expression profiles induced by PXR in calvarial osteoblasts cultured in standard fetal bovine serum (in which PXR induces repression of differentiation), and in those cultured in charcoal-stripped fetal bovine serum (in which PXR does not induce repression of differentiation). The comparison revealed that PXR attenuated the Hedgehog-mediated signaling in culture conditions that induced PXR-mediated repression of differentiation. Real-time PCR analysis showed that PXR repressed the Hedgehog signaling-induced genes such as Gli1 and Hhip, and conversely induced the Hedgehog signaling-repressed genes such as Cdon, Boc, and Gas1. Activation of Smo-mediated signaling in osteoblasts following treatment with a Smo agonist (SAG) significantly restored Gli-mediated transcriptional activity and osteoblast differentiation. Our results demonstrate the osteoblast-autonomous effects of PXR and identify a novel regulation of Hedgehog signaling by nuclear receptors.

Oxidative Stress-Induced Hypermethylation of KLF5 Promoter Mediated by DNMT3B Impairs Osteogenesis by Diminishing the Interaction with β-Catenin

Aims: Emerging evidence suggests that the pathogenesis of osteoporosis, characterized by impaired osteogenesis, is shifting from estrogen centric to oxidative stress. Our previous studies have shown that the zinc-finger transcription factor krüppel-like factor 5 (KLF5) plays a key role in the degeneration of nucleus pulposus and cartilage. However, its role in osteoporosis remains unknown. We aimed to investigate the effect and mechanism of KLF5 on osteogenesis under oxidative stress. Results: First, KLF5 was required for osteogenesis and stimulated osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). KLF5 was hypermethylated and downregulated in ovariectomy-induced osteoporosis mice and in BMSCs treated with H2O2. Interestingly, DNA methyltransferases 3B (DNMT3B) upregulation mediated the hypermethylation of KLF5 induced by oxidative stress, thereby impairing osteogenic differentiation. The inhibition of KLF5 hypermethylation using DNMT3B siRNA or 5-AZA-2-deoxycytidine (5-AZA) protected osteogenic differentiation of BMSCs from oxidative stress. Regarding the downstream mechanism, KLF5 induced β-catenin expression. More importantly, KLF5 promoted the nuclear translocation of β-catenin, which was mediated by the armadillo repeat region of β-catenin. Consistently, oxidative stress-induced KLF5 hypermethylation inhibited osteogenic differentiation by reducing the expression and nuclear translocation of β-catenin. Innovation: We describe the novel effect and mechanism of KLF5 on osteogenesis under oxidative stress, which is linked to osteoporosis for the first time. Conclusion: Our results suggested that oxidative stress-induced hypermethylation of KLF5 mediated by DNMT3B impairs osteogenesis by diminishing the interaction with β-catenin, which is likely to contribute to osteoporosis. Targeting the hypermethylation of KLF5 might be a new strategy for the treatment of osteoporosis. Antioxid. Redox Signal. 35, 1-20.

Hyaluronan Synthases' Expression and Activity Are Induced by Fluid Shear Stress in Bone Marrow-Derived Mesenchymal Stem Cells

During biomineralization, the cells generating the biominerals must be able to sense the external physical stimuli exerted by the growing mineralized tissue and change their intracellular protein composition according to these stimuli. In molluscan shell, the myosin-chitin synthases have been suggested to be the link for this communication between cells and the biomaterial. Hyaluronan synthases (HAS) belong to the same enzyme family as chitin synthases. Their product hyaluronan (HA) occurs in the bone and is supposed to have a regulatory function during bone regeneration. We hypothesize that HASes’ expression and activity are controlled by fluid-induced mechanotransduction as it is known for molluscan chitin synthases. In this study, bone marrow-derived human mesenchymal stem cells (hMSCs) were exposed to fluid shear stress of 10 Pa. The RNA transcriptome was analyzed by RNA sequencing (RNAseq). HA concentrations in the supernatants were measured by ELISA. The cellular structure of hMSCs and HAS2-overexpressing hMSCs was investigated after treatment with shear stress using confocal microscopy. Fluid shear stress upregulated the expression of genes that encode proteins belonging to the HA biosynthesis and bone mineralization pathways. The HAS activity appeared to be induced. Knowledge about the regulation mechanism governing HAS expression, trafficking, enzymatic activation and quality of the HA product in hMSCs is essential to understand the biological role of HA in the bone microenvironment.

ERK5 negatively regulates Kruppel-like factor 4 and promotes osteogenic lineage cell proliferation in response to MEK5 overexpression or fluid shear stress

Aim of the study: Fluid shear stress (FSS) plays a critical role in osteoblast proliferation via extracellular signal-regulated kinase 5 (ERK5). Kruppel-like factor 4 (KLF4) knockout robustly enhances bone formation due to increased osteoblast differentiation and mineralization. However, the effect of KLF4 on osteoblast proliferation is unresolved. Therefore, the aim of our study was to investigate the effect of KLF4 on osteogenic lineage cell proliferation and the relationship between KLF4 and ERK5. Materials and methods: MC3T3-E1 cells were treated with FSS and/or KLF4 siRNA, cell viability was accessed by Edu labeling and CCK-8 assay, and proliferative gene expression were assessed by PCR array. Bone marrow stromal cells (BMSCs) were infected with adenovirus expressing KLF4 and/or constitutively active MEK5, cell viability was evaluated using crystal violet staining, colony formation assay, and cell WST1 assay. The levels of KLF4 and ERK5 phosphorylation were identified through qRT-PCR and western blot, respectively. Results: KLF4 expression was significantly down-regulated by FSS exposure, however, this was reversed by ERK5 siRNA. KLF4 overexpression inhibited colony formation efficiency and cell viability in BMSCs. Adenoviruses expressing constitutively active MEK5 increased ERK5 phosphorylation, which inhibited KLF4 expression, and promoted BMSC proliferation. FSS-induced osteoblast proliferation also involved elevation of Cyclin B2 and Cdc14b as well as repressed expression of P27. Conclusions: KLF4 negatively regulates osteogenic lineage cell proliferation, and ERK5 negatively regulates KLF4 expression and promotes osteogenic lineage cell proliferation.

BMP2-dependent gene regulatory network analysis reveals Klf4 as a novel transcription factor of osteoblast differentiation

Transcription factors (TFs) regulate the expression of target genes, inducing changes in cell morphology or activities needed for cell fate determination and differentiation. The BMP signaling pathway is widely regarded as one of the most important pathways in vertebrate skeletal biology, of which BMP2 is a potent inducer, governing the osteoblast differentiation of bone marrow stromal cells (BMSCs). However, the mechanism by which BMP2 initiates its downstream transcription factor cascade and determines the direction of differentiation remains largely unknown. In this study, we used RNA-seq, ATAC-seq, and animal models to characterize the BMP2-dependent gene regulatory network governing osteoblast lineage commitment. Sp7-Cre; Bmp2fx/fx mice (BMP2-cKO) were generated and exhibited decreased bone density and lower osteoblast number (n > 6). In vitro experiments showed that BMP2-cKO mouse bone marrow stromal cells (mBMSCs) had an impact on osteoblast differentiation and deficient cell proliferation. Osteogenic medium induced mBMSCs from BMP2-cKO mice and control were subjected to RNA-seq and ATAC-seq analysis to reveal differentially expressed TFs, along with their target open chromatin regions. Combined with H3K27Ac CUT&Tag during osteoblast differentiation, we identified 2338 BMP2-dependent osteoblast-specific active enhancers. Motif enrichment assay revealed that over 80% of these elements were directly targeted by RUNX2, DLX5, MEF2C, OASIS, and KLF4. We deactivated Klf4 in the Sp7 + lineage to validate the role of KLF4 in osteoblast differentiation of mBMSCs. Compared to the wild-type, Sp7-Cre; Klf4fx/+ mice (KLF4-Het) were smaller in size and had abnormal incisors resembling BMP2-cKO mice. Additionally, KLF4-Het mice had fewer osteoblasts and decreased osteogenic ability. RNA-seq and ATAC-seq revealed that KLF4 mainly "co-bound" with RUNX2 to regulate downstream genes. Given the significant overlap between KLF4- and BMP2-dependent NFRs and enriched motifs, our findings outline a comprehensive BMP2-dependent gene regulatory network specifically governing osteoblast differentiation of the Sp7 + lineage, in which Klf4 is a novel transcription factor.

Bi-fated tendon-to-bone attachment cells are regulated by shared enhancers and KLF transcription factors

The mechanical challenge of attaching elastic tendons to stiff bones is solved by the formation of a unique transitional tissue. Here, we show that murine tendon-to-bone attachment cells are bi-fated, activating a mixture of chondrocyte and tenocyte transcriptomes, under regulation of shared regulatory elements and Krüppel-like factors (KLFs) transcription factors. High-throughput bulk and single-cell RNA sequencing of humeral attachment cells revealed expression of hundreds of chondrogenic and tenogenic genes, which was validated by in situ hybridization and single-molecule ISH. ATAC sequencing showed that attachment cells share accessible intergenic chromatin areas with either tenocytes or chondrocytes. Epigenomic analysis revealed enhancer signatures for most of these regions. Transgenic mouse enhancer reporter assays verified the shared activity of some of these enhancers. Finally, integrative chromatin and motif analyses and transcriptomic data implicated KLFs as regulators of attachment cells. Indeed, blocking expression of both Klf2 and Klf4 in developing limb mesenchyme impaired their differentiation.

Kruppel-like factor 4 upregulates matrix metalloproteinase 13 expression in chondrocytes via mRNA stabilization

Matrix metalloproteinase 13 (MMP13) is indispensable for normal skeletal development and is also a principal proteinase responsible for articular joint pathologies. MMP13 mRNA level needs to be tightly regulated in both positive and negative manners to achieve normal development and also to prevent joint destruction. We showed previously that Kruppel-like factor 4 (KLF4) strongly induces the expression of members of the MMP family of genes including that for MMP13 in cultured chondrocytes. Through expression-based screening of approximately 400 compounds, we identified several that efficiently downregulated MMP13 gene expression induced by KLF4. Compounds grouped as topoisomerase inhibitors (transcriptional inhibitors) downregulated MMP13 expression levels, which proved the validity of our screening method. In this screening, trichostatin A (TSA) was identified as one of the most potent repressors. Mechanistically, increased MMP13 mRNA levels induced by KLF4 were not mainly caused by increased rates of RNA polymerase II-mediated MMP13 transcription, but arose from escaping mRNA decay. TSA treatment almost completely blunted the effect of KLF4. Importantly, KLF4 was detected in chondrocytes at the joint destruction sites in a rodent model of osteoarthritis. Our results partially explain how KLF4 regulates numerous proteinase gene expressions simultaneously in chondrocytes. Also, these observations suggest that modulation of KLF4 activity or expression could be a novel therapeutic target for osteoarthritis.

Direct conversion of mouse embryonic fibroblast to osteoblast cells using hLMP-3 with Yamanaka factors

Large bone defects and bone loss after fractures remain significant challenges for orthopedic surgeons. Our study aims to find an available, applicable and biological treatment for bone regeneration overcoming the limitations in ESC/iPSC technology. We directly reprogrammed the mouse embryonic fibroblast (MEF) into osteoblast cells using different combinations of Yamanaka factors with human lim mineralization protein-3 (hLMP-3). LMP is an intracellular LIM-domain protein acting as an effective positive regulator of the osteoblast differentiation. After transduction, cells were cultured in osteogenic medium, and then examined for osteoblast formation. The expression of osteogenic markers (BMP2, Runx2 and Osterix) during reprogramming and in vitro mineralization assay revealed that the best reprogramming cocktail was (c-Myc - Oct4) with hLMP-3. In addition, both immunofluorescent staining and western blot analysis confirmed that osteocalcin (OCN) expression increased in the cells treated with the c-Myc/Oct4/hLMP3 cocktail than using hLMP-3 alone. Furthermore, this reprogramming cocktail showed efficient healing in an induced femoral bone defect in rat animal model one month after transplantation. In the present study, we reported for the first time the effect of combining Yamanaka factors with hLMP-3 to induce osteoblast cells from MEF both in vitro and in vivo.

Lentiviral vector expression of Klf4 enhances chondrogenesis and reduces hypertrophy in equine chondrocytes

Monolayer expansion of chondrocytes in culture results in the dedifferentiation of chondrocytes with inferior cartilage specific extracellular matrix synthesis and proliferation when compared with its native counterpart. We aimed to enhance chondrocyte proliferation and articular cartilage specific gene expression through ectopic expression of the major pluripotency transcription factors (Oct4, Sox2, Klf4 and c-Myc). We also aimed to provide insights to the modulation of TGFβ receptor mRNA with Klf4 overexpression. Equine chondrocytes pooled from three donors were transduced with lentiviral vectors expressing the induced pluripotency factors, Oct4, Sox2. Klf4 and c-Myc (OSKM), singly, or in combination or together with green fluorescent protein (GFP) as a control. Klf4 and c-Myc overexpressing chondrocytes showed a significant increase in mitosis when compared to the control (P < 0.01 and P < 0.0001 respectively). Furthermore, overexpression of Klf4 or OSKM in three dimensional (3D) culture of equine chondrocytes resulted in a significant increase in Col2a1 mRNA levels relative to the controls (P < 0.05 and P < 0.01 respectively) while all transcription factors significantly lowered the mRNA of the fibrocartilage marker Col1a1. We also employed a Col2a1 promoter driven GFP reporter for real time monitoring of Col2a1 gene activation in 3D micromass culture, which showed significantly higher promoter activity when cultures were treated with the growth factor TGFβ3 (P < 0.05). The chondrogenic properties of Klf4 transduced chondrocytes at a lower passage (P4) showed significant increases in Sox9 (P < 0.001), Col2a1 (P < 0.05) and TGFβ receptor I (P < 0.05) and II (P < 0.001) expression relative to the DS-Red expressing control. The chondrocyte dedifferentiation marker Col1a1 and hypertrophic marker Col10a1 were significantly downregulated with the inclusion of Klf4 (P < 0.01 and P < 0.05 respectively). In Conclusion, chondrogenic re-differentiation and proliferation of equine chondrocytes is promoted through ectopic expression of Klf4 while suppressing chondrocyte dedifferentiation.

Kruppel-Like Factor 4 represses osteoblast differentiation via ciliary Hedgehog signaling

Primary cilia are appendages observed in most types of cells, and serve as cellular antennae for sensing environmental signals. Evidence is accumulating that correct ciliogenesis and ciliary functions are indispensable for normal skeletal development by regulating signaling pathways important for bone development. However, whether ciliogenesis is regulated by bone-related factors in osteoblasts is largely unknown. Here we show that Kruppel-Like Factor 4 (KLF4), which is known to repress osteoblast differentiation, supports the formation and maintenance of cilia in cultured osteoblasts; however, the length of the cilia observed in KLF4-induced cells were significantly shorter compared to the control cells. Basal Hedgehog signaling was repressed by KLF4. Significantly, activating Hedgehog signaling using a Smoothened agonist significantly rescued osteoblast mineralization and osteoblastic gene expressions. Global gene expression analysis showed that KLF4 induced number of genes including the nuclear receptor, Pregnane X receptor (PXR), and PXR repressed calvarial osteoblast mineralization and repressed Gli1 expression similar as the effect observed by inducing KLF4. Our results implicate that KLF4 plays important roles for maintaining osteoblasts in an immature state by repressing basal activation of the Hedgehog signaling.

2 dyn/cm2 shear force upregulates kruppel-like factor 4 expression in human chondrocytes to inhibit the interleukin-1β-activated nuclear factor-κB

The shear force effect on human chondrocytes is time and magnitude dependent. Recently, kruppel-like factor (KLF) 4 has been identified as a pleiotropic protein and its activity in cells is dependent on different stimuli and/or cell types. The role of KLF4 in chondrocytes is still unclear and there has been no report determining whether shear force regulates KLF4 levels in chondrocytes. Hence, this study was carried out to investigate the role of KLF4 in human chondrocytes under shear force stimulation and the underlying mechanism. Human primary and SW1353 chondrocytes were used in this study. The shear forces at 2, 5, or 15 dyn/cm² intensity were applied to both types of human chondrocytes. The specific small interfering RNAs, activators, and inhibitors were used to study the detailed mechanism of shear force. The presented results showed that 2, but not 5 and 15, dyn/cm² shear force increases KLF4 expression in human primary and SW1353 chondrocytes. Extracellular signal-regulated kinase 5 induced peroxisome proliferator-activated receptor γ transcription activity to increase KLF4 transcription. Moreover, the KLF4 induction in human chondrocytes in response to 2 dyn/cm² shear force could attenuate interleukin (IL)-1β-stimulated nuclear factor-κB activation. These results elucidate the role of KLF4 in antagonizing the effect of IL-1β in human chondrocytes under 2 dyn/cm² shear force stimulation and provide a possible mechanism to demonstrate the protection of moderate forces or exercises in cartilage.

Kruppel-like factor 4 (KLF-4) plays a crucial role in simvastatin (SVT)-induced differentiation of rabbit articular chondrocytes

Simvastatin is a cholesterol-lowing reagent that is derived synthetically from the fermentation of Aspergillus terreus. Recently, SVT has been shown to possess a protective effect of chondrocytes. Kruppel-like factor 4 (KLF-4) is a zinc finger transcription factor that plays crucial roles during the development and maintenance of multiple organs. However, the roles of KLF-4 in chondrocytes have not been well unknown. Here, we investigated whether KLF-4 regulates SVT-caused differentiated phenotype of chondrocytes. A KLF-4 cDNA or KLF-4 siRNA was transfected into SVT-treated chondrocytes. Western blot analysis, RT-PCR and immunofluorescence staining analyzed expression of type II collagen and SOX-9, marker proteins of differentiation. The results showed overexpression of KLF-4 accelerates SVT-induced type II collagen expression, as determined by western blot analysis and causes sulfated-proteoglycan synthesis, as detected by Alcian blue staining. RT-PCR revealed that ectopic expression of KLF-4 induces SVT-caused SOX-9, a transcription factor of type II collagen, expression. Transfection of KLF-4 siRNA reversed SVT-caused type II collagen and SOX-9 expression and inhibited SVT-induced sulfated proteoglycan production. This study indicates that KLF-4 plays critical role in SVT-caused chondrocytes differentiation.

Kruppel-like factor 4 regulates matrix metalloproteinase and aggrecanase gene expression in chondrocytes

Kruppel-like factor 4 (KLF4) is a zinc finger transcription factor that plays crucial roles during the development and maintenance of multiple organs. We and others have previously shown that KLF4 is involved in bone modeling and remodeling but roles played by KLF4 during skeletogenesis are still not fully understood. Here, we show that KLF4 is expressed in the epiphyseal growth plate and articular chondrocytes. Most articular chondrocytes expressed KLF4 in embryos but it localized only in a subset of superficial zone cells in postnatal mice. When KLF4 was overexpressed in chondrocytes in vitro, it severely repressed chondrocytic gene expressions. Global gene expression profiling of KLF4-transduced chondrocytes revealed matrix degrading proteinases of the matrix metalloproteinase and disintegrin and metalloproteinase with thrombospondin-1 domain families within the group of upregulated genes. Proteinase induction by KLF4 was alleviated by Trichostatin A treatment suggesting the possible involvement of epigenetic mechanisms on proteinase induction by KLF4. These results indicate the possible involvement of KLF4 in physiological and pathological aspects during cartilage development and maintenance.

Krüppel-like factor 4 (KLF4): What we currently know

Krüppel-like factor 4 (KLF4) is an evolutionarily conserved zinc finger-containing transcription factor that regulates diverse cellular processes such as cell growth, proliferation, and differentiation. Since its discovery in 1996, KLF4 has been gaining a lot of attention, particularly after it was shown in 2006 as one of four factors involved in the induction of pluripotent stem cells (iPSCs). Here we review the current knowledge about the different functions and roles of KLF4 in various tissue and organ systems.

KLF4 Knockdown Attenuates TBI-Induced Neuronal Damage through p53 and JAK-STAT3 Signaling

AIMS

Traumatic brain injury (TBI) is induced by complex primary and secondary mechanisms that give rise to cell death, inflammation, and neurological dysfunction. Understanding the mechanisms that drive neurological damage as well as those that promote repair can guide the development of therapeutic drugs for TBI. Kruppel-like factor 4 (KLF4) has been reported to negatively regulate axon regeneration of injured retinal ganglion cells (RGCs) through inhibition of JAK-STAT3 signaling. However, the role of KLF4 in TBI remains unreported. Reactive oxygen species (ROS)-induced neuronal death is a pathophysiological hallmark of TBI.

METHODS

In this study, we used H₂ O₂ -treated RGCs in vitro and the optic nerve crush model in vivo to simulate neuronal damage in TBI. The function of KLF4 in RGC survival and axon regeneration in these models was investigated. In addition, the effects of KLF4 knockdown on neuronal damage after a brain impact that mimics moderate TBI were studied.

RESULTS

The results show that H₂ O₂ induces p53-dependent apoptosis of RGCs in vitro through upregulation of KLF4. Additionally, KLF4 knockdown in vivo significantly enhances CNTF-induced axon regeneration of RGCs after optic nerve crush, and more importantly, prevents neuronal damage after a moderate brain impact in rats. Our Western blot analysis and immunoprecipitation assay results indicate that these effects of KLF4 knockdown are mediated by the p53 and JAK-STAT3 pathways.

CONCLUSION

These findings provide evidence that KLF4 plays an important role in the pathophysiology of TBI. Blocking KLF4 may be a potential therapeutic strategy for the treatment of TBI, either alone or in combination with agents that target complementary mechanisms.

Fate mapping of Trps1 daughter cells during cardiac development using novel Trps1-Cre mice

Tricho-rhino-phalangeal syndrome (TRPS) is a rare congenital disorder that is characterized by abnormal hair growth and skeletal deformities. These result in sparse hair, short stature, and early onset of joint problems. Recent reports have shown that a relatively high proportion of patients with TRPS exhibit a broad range of congenital heart defects. To determine the regulation of Trps1 transcription in vivo, we generated novel transgenic mice, which expressed Cre recombinase under the murine Trps1 proximal promoter sequence (Trps1-Cre). We crossed these mice with Cre reporter mice to identify Trps1 daughter cells. Labeled cells were observed in the appendicular joint tissue, dermal papilla of the hair follicles, cardiac valves, aortic sinus, atrial walls, and the interventricular septum. In situ analysis showed restricted Trps1 expression, which was observed in endocardial cushions of the outflow tract, and in leaflets of all mature cardiac valves. These results suggest that the Trps1 proximal promoter sequence contains some of the tissue-specific Trps1 regulatory region. Further, our findings partially explain why patients with TRPS show a broad range of congenital cardiac defects, although Trps1 expression is observed in a more restricted fashion. genesis 54:379-388, 2016. © 2016 Wiley Periodicals, Inc.

Successful induction of sclerostin in human-derived fibroblasts by 4 transcription factors and its regulation by parathyroid hormone, hypoxia, and prostaglandin E2

Sclerostin, coded by SOST, is a secretory protein that is specifically expressed in osteocytes and suppresses osteogenesis by inhibiting WNT signaling. The regulatory mechanism underlying SOST expression remains unclear mainly due to the absence of an adequate human cell model. Thus, we herein attempted to establish a cell model of human dermal fibroblasts in order to investigate the functions of sclerostin. We selected 20 candidate transcription factors (TFs) that induce SOST expression by analyzing gene expression patterns in the human sarcoma cell line, SaOS-2, between differentiation and maintenance cultures using microarrays. An effective set of TFs to induce SOST expression was sought by their viral transduction into fibroblasts, and a combination of four TFs: ATF3, KLF4, PAX4, and SP7, was identified as the most effective inducer of SOST expression. Quantitative PCR demonstrated that the expression levels of SOST in fibroblasts treated with the 4 TFs were 199- and 1439-fold higher than those of the control after 1-week and 4-week cultures, respectively. The level of sclerostin in the conditioned medium, as determined by ELISA, was 21.2pmol/l 4weeks after the transduction of the 4 TFs. Interestingly, the production of Dickkopf1 (DKK1), another secreted inhibitor of WNT signaling, was also increased by transduction of these 4 TFs. Parathyroid hormone (PTH) significantly suppressed the induced SOST by 38% and sclerostin by 82% that of the vehicle. Hypoxia increased the induced SOST by 62% that of normoxia. Furthermore, prostaglandin E2 (PGE2) increased SOST expression levels to 16-fold those of the vehicle. In conclusion, the efficient induction of SOST expression and sclerostin production was achieved in human dermal fibroblasts by the transduction of ATF3, KLF4, PAX4, and SP7, and the induced SOST and sclerostin were regulated by PTH, hypoxia, and PGE2. This model may contribute to elucidating the regulatory mechanisms underlying SOST expression and advancing drug development for metabolic bone diseases.

Regulating Craniofacial Development at the 3' End: MicroRNAs and Their Function in Facial Morphogenesis

Defects in craniofacial development represent a majority of observed human birth defects, occurring at a rate as high as 1:800 live births. These defects often occur due to changes in neural crest cell (NCC) patterning and development and can affect non-NCC-derived structures due to interactions between NCCs and the surrounding cell types. Proper craniofacial development requires an intricate array of gene expression networks that are tightly controlled spatiotemporally by a number of regulatory mechanisms. One of these mechanisms involves the action of microRNAs (miRNAs), a class of noncoding RNAs that repress gene expression by binding to miRNA recognition sequences typically located in the 3' UTR of target mRNAs. Recent evidence illustrates that miRNAs are crucial for vertebrate facial morphogenesis, with changes in miRNA expression leading to facial birth defects, including some in complex human syndromes such as 22q11 (DiGeorge Syndrome). In this review, we highlight the current understanding of miRNA biogenesis, the roles of miRNAs in overall craniofacial development, the impact that loss of miRNAs has on normal development and the requirement for miRNAs in the development of specific craniofacial structures, including teeth. From these studies, it is clear that miRNAs are essential for normal facial development and morphogenesis, and a potential key in establishing new paradigms for repair and regeneration of facial defects.

Regulatory gene networks that shape the development of adaptive phenotypic plasticity in a cichlid fish

Phenotypic plasticity is the ability of organisms with a given genotype to develop different phenotypes according to environmental stimuli, resulting in individuals that are better adapted to local conditions. In spite of their ecological importance, the developmental regulatory networks underlying plastic phenotypes often remain uncharacterized. We examined the regulatory basis of diet-induced plasticity in the lower pharyngeal jaw (LPJ) of the cichlid fish Astatoreochromis alluaudi, a model species in the study of adaptive plasticity. Through raising juvenile A. alluaudi on either a hard or soft diet (hard-shelled or pulverized snails) for between 1 and 8 months, we gained insight into the temporal regulation of 19 previously identified candidate genes during the early stages of plasticity development. Plasticity in LPJ morphology was first detected between 3 and 5 months of diet treatment. The candidate genes, belonging to various functional categories, displayed dynamic expression patterns that consistently preceded the onset of morphological divergence and putatively contribute to the initiation of the plastic phenotypes. Within functional categories, we observed striking co-expression, and transcription factor binding site analysis was used to examine the prospective basis of their coregulation. We propose a regulatory network of LPJ plasticity in cichlids, presenting evidence for regulatory crosstalk between bone and muscle tissues, which putatively facilitates the development of this highly integrated trait. Through incorporating a developmental time-course into a phenotypic plasticity study, we have identified an interconnected, environmentally responsive regulatory network that shapes the development of plasticity in a key innovation of East African cichlids.

Kruppel-like factor 4 expression in osteoblasts represses osteoblast-dependent osteoclast maturation

Kruppel-like factor 4 (KLF4) is a zinc-finger-type transcription factor with a restricted expression pattern during skeletal development. We have previously shown that KLF4 represses osteoblast mineralization concomitant with a down-regulation in the expression of a number of osteoblastic genes, both in vivo and in vitro. In addition to the cell-autonomous effects of KLF4 in osteoblasts, transgenic osteoblastic-KLF4 mice show severe defects in osteoclast maturation. Wild-type bone-marrow-derived macrophages co-cultured with KLF4-expressing osteoblasts exhibit reduced formation of multinuclear osteoclasts as compared with control cultures overexpressing green fluorescent protein. Significantly, the transduction of Runx2, a master regulator of osteoblastogenesis, together with KLF4 into osteoblasts restores the reduction in osteoclastogenesis induced by KLF4 alone. Various extracellular matrix molecules are down-regulated by KLF4 overexpression but this down-regulation can be partially restored by the co-transduction of Runx2. These results suggest that osteoblastic-KLF4 affects osteoclast maturation by regulating cell-matrix interactions and reinforce the importance of the regional down-regulation of KLF4 expression in the subset of osteoblasts for normal skeletal modeling and remodeling.

Role of Krüppel-like factor 4 and its binding proteins in vascular disease

Krüppel-like factor 4(KLF4) is a zinc-finger transcription factor that plays a key role in cellular differentiation and proliferation during normal development and in various diseases, such as cancer. The results of recent studies have revealed that KLF4 is expressed in multiple vascular cell types, including phenotypically modulated smooth muscle cells(SMCs), endothelial cells and monocytes/macrophages and contributes to the progression of vascular diseases by activating or repressing the transcription of multiple genes via its associations with a variety of partner proteins. For example, KLF4 decreases the expression of markers of SMC differentiation by interacting with serum response factor, ELK1 and histone deacetylases. KLF4 also suppresses SMC proliferation by associating with p53. In addition, KLF4 enhances arterial medial calcification in concert with RUNX2. Furthermore, endothelial KLF4 represses arterial inflammation by binding to nuclear factor-κB. This article summarizes the role of KLF4 in vascular disease with a particular focus on in vivo studies and reviews recent progress in our understanding of the regulatory mechanisms involved in KLF4- mediated gene transcription.

Kruppel-like factor 4 attenuates osteoblast formation, function, and cross talk with osteoclasts

KLF4 controls bone homeostasis by negatively regulating both osteoclast and osteoblast differentiation.

Osteoblasts not only control bone formation but also support osteoclast differentiation. Here we show the involvement of Kruppel-like factor 4 (KLF4) in the differentiation of osteoclasts and osteoblasts. KLF4 was down-regulated by 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) in osteoblasts. Overexpression of KLF4 in osteoblasts attenuated 1,25(OH)₂D₃-induced osteoclast differentiation in co-culture of mouse bone marrow cells and osteoblasts through the down-regulation of receptor activator of nuclear factor κB ligand (RANKL) expression. Direct binding of KLF4 to the RANKL promoter repressed 1,25(OH)₂D₃-induced RANKL expression by preventing vitamin D receptor from binding to the RANKL promoter region. In contrast, ectopic overexpression of KLF4 in osteoblasts attenuated osteoblast differentiation and mineralization. KLF4 interacted directly with Runx2 and inhibited the expression of its target genes. Moreover, mice with conditional knockout of KLF4 in osteoblasts showed markedly increased bone mass caused by enhanced bone formation despite increased osteoclast activity. Thus, our data suggest that KLF4 controls bone homeostasis by negatively regulating both osteoclast and osteoblast differentiation.

Deletion of Krüppel-like factor 4 in endothelial and hematopoietic cells enhances neointimal formation following vascular injury

Background

Krüppel‐like factor 4 (Klf4) is involved in a variety of cellular functions by activating or repressing the transcription of multiple genes. Results of previous studies showed that tamoxifen‐inducible global deletion of the Klf4 gene in mice accelerated neointimal formation following vascular injury, in part via enhanced proliferation of smooth muscle cells (SMCs). Because Klf4 is also expressed in non‐SMCs including endothelial cells (ECs), we determined if Tie2 promoter‐dependent deletion of Klf4 in ECs and hematopoietic cells affected injury‐induced neointimal formation.

Methods and Results

Klf4 conditional knockout (cKO) mice were generated by breeding Tie2‐Cre mice and Klf4 floxed mice, and their phenotype was analyzed after carotid ligation injury. Results showed that injury‐induced repression of SMC differentiation markers was unaffected by Tie2 promoter‐dependent Klf4 deletion. However, of interest, neointimal formation was significantly enhanced in Klf4‐cKO mice 21 days following carotid injury. Moreover, Klf4‐cKO mice exhibited an augmented proliferation rate, enhanced accumulation of macrophages and T lymphocytes, and elevated expression of cell adhesion molecules including vascular cell adhesion molecule–1 (Vcam1) and E‐selectin in injured arteries. Mechanistic analyses in cultured ECs revealed that Klf4 inhibited tumor necrosis factor‐α–induced expression of Vcam1 through blocking the binding of nuclear factor‐κB to the Vcam1 promoter.

Conclusions

These results provide evidence that Klf4 in non‐SMCs such as ECs regulates neointimal formation by repressing arterial inflammation following vascular injury.

KLF4 promoted odontoblastic differentiation of mouse dental papilla cells via regulation of DMP1

Odontoblasts, which derive from dental papilla, are a type of terminally differentiated matrix-secreting cells. Previous studies have identified various transcription factors involved in the differentiation process of odontoblasts. We have recently found that Krüppel-like factor 4 (Klf4) was expressed in the polarizing and elongating odontoblasts, but the function of Klf4 in the differentiation of odontoblasts is still unclear. We hypothesized Klf4 promoted the differentiation of odontoblasts by up-regulating some odontoblast-related genes. In this study, we found that the expression of Klf4 increased significantly during the odontoblastic differentiation of primary mouse dental papilla cells and the mouse dental papilla cell line-mDPC6T. Overexpression of Klf4 significantly up-regulated odontoblast-related genes, such as Dmp1, Dspp, and Alp, and promoted the accumulation of mineral nodules. Knock-down of Klf4 down-regulated expression of Dmp1, Dspp, and Alp, and inhibited mineral deposition. We applied in silico analysis and identified one target gene of Klf4-Dmp1. Based on further analysis of ChIP data, EMSA and dual luciferase activity assays, we confirmed that Klf4 was able to specifically bind to the Dmp1 promoter and transactivate its expression. Furthermore, forced expression of Dmp1 in the Klf4 knock-down mDPC6T cell line significantly recovered its odontoblastic differentiation ability. Our data confirmed our hypothesis that Klf4 promotes the differentiation of odontoblasts via the up-regulation of Dmp1.