mKlf7, a potential transcriptional regulator of TrkA nerve growth factor receptor expression in sensory and sympathetic neurons

Krüppel-like factors in bone biology

The krüppel-like factor (KLF) family is a group of zinc finger transcription factors and contributes to different cellular processes such as differentiation, proliferation, migration, and apoptosis. While different studies show the roles of this family in skeletal development-specifically in chondrocyte and osteocyte development and bone homeostasis-there are few reviews summarizing their importance. To fill this gap, this review discusses current knowledge on different functions of the KLF family during skeletal development, including their roles in stem cell maintenance and differentiation, cell apoptosis, and cell cycle. To understand the importance of the KLF family, we also review genotype-phenotype correlations in different animal models. We also discuss how KLF proteins function through different signaling pathways and display their paramount importance in skeletal development. To highlight their roles in cartilage- or bone-related cells, we also use single-cell RNA sequencing publicly available data on mouse hindlimb. We also challenge our knowledge of how the KLF family is epigenetically regulated-e.g., using DNA methylation, histone modifications, and noncoding RNAs-during chondrocyte and osteocyte development.

Differential Expression Profiling of microRNAs in Human Placenta-Derived Mesenchymal Stem Cells Cocultured with Grooved Porous Hydroxyapatite Scaffolds

Scaffold materials used for bone defect repair are often limited by osteogenic efficacy. Moreover, microRNAs (miRNAs) are involved in regulating the expression of osteogenic-related genes. In previous studies, we verified the enhancement of osteogenesis using a grooved porous hydroxyapatite scaffold (HAG). In the present study, we analyzed the contribution of HAG to the osteogenic differentiation of human placenta-derived mesenchymal stem cells (hPMSCs) from the perspective of miRNA differential expression. Furthermore, results showed that miRNAs were differentially expressed in the osteogenic differentiation of hPMSCs cocultured with HAG. In detail, 16 miRNAs were significantly upregulated and 29 miRNAs were downregulated with HAG. In addition, bioinformatics analyses showed that the differentially expressed miRNAs were enriched in a variety of biological processes, including signal transduction, cell metabolism, cell junctions, cell development and differentiation, and that they were associated with osteogenic differentiation through axon guidance, mitogen-activated protein kinase, and the transforming growth factor beta signaling pathway. Furthermore, multiple potential target genes of these miRNAs were closely related to osteogenic differentiation. Importantly, overexpression of miR-146a-5p (an upregulated miRNA) promoted the osteogenic differentiation of hPMSCs, and miR-145-5p overexpression (a downregulated miRNA) inhibited the osteogenic differentiation of hPMSCs.

Influencers in the Somatosensory System: Extrinsic Control of Sensory Neuron Phenotypes

Somatosensory neurons in dorsal root ganglia (DRG) comprise several main subclasses: high threshold nociceptors/thermoceptors, high- and low-threshold mechanoreceptors, and proprioceptors. Recent years have seen an explosion in the identification of molecules that underlie the functional diversity of these sensory modalities. They also have begun to reveal the developmental mechanisms that channel the emergence of this subtype diversity, solidifying the importance of peripheral instructive signals. Somatic sensory neurons collectively serve numerous essential physiological and protective roles, and as such, an increased understanding of the processes that underlie the specialization of these sensory subtypes is not only biologically interesting but also clinically relevant.

DNA methylation of CpG sites in the chicken KLF7 promoter and Exon 2 in association with mRNA expression in abdominal adipose tissue and blood metabolic indicators

BACKGROUND

Our previous study found that chicken KLF7 was an important regulator in formation of adipose tissue. In the present study, we analyzed the association for DNA methylation in chicken KLF7 with its transcripts of abdominal adipose tissue and blood metabolic indicators.

RESULTS

The KLF7 transcripts of the adipose tissue of Chinese yellow broilers were associated with age (F = 6.67, P = 0.0035). In addition, the KLF7 transcripts were negatively correlated with blood glucose levels (r = - 0.61841, P = 0.0140). The DNA methylation levels of 26 CpG loci in the chicken KLF7 promoter and Exon 2 were studied by Sequenom MassArray. A total of 22 valid datasets were obtained. None of them was significantly different in relation to age (P > 0.05). However, the DNA methylation levels in the promoter were lower than those in Exon 2 (T = 40.74, P < 0.01). Correlation analysis showed that the DNA methylation levels of PCpG6 and E2CpG9 were significantly correlated with KLF7 transcripts and blood high-density lipoprotein levels, respectively, and many CpG loci were correlated with each other (P < 0.05). The methylation data were subjected to principal component analysis and factor analysis. The six principal components (z1-z6) were extracted and named Factors 1-6, respectively. Factor analysis showed that Factor 1 had a higher load on the loci in the promoter, and Factors 2-6 loaded highly on quite different loci in Exon 2. Correlation analysis showed that only z1 was significantly correlated to KLF7 transcripts (P < 0.05). In addition, an established regression equation between z1 and KLF7 transcripts was built, and the contribution of z1 to the variation on KLF7 transcripts was 34.29%.

CONCLUSIONS

In conclusion, the KLF7 transcripts of chicken abdominal adipose tissue might be inhibited by DNA methylation in the promoter, and it might be related to the DNA methylation level of PCpG6.

Genome-wide association analysis of idiopathic epilepsy in the Belgian shepherd

Background

Idiopathic epilepsy (IE) is a common neurological disorder in the domestic dog, and is defined as repeated seizure activity having no identifiable underlying cause. Some breeds, such as the Belgian shepherd dog, have a greater prevalence of the disorder. Previous studies in this and other breeds have identified ADAM23 as a gene that confers risk of IE, although additional loci are known to exist. The present study sought to identify additional loci that influence IE in the Belgian shepherd dog.

Results

Genome-wide association studies (GWAS) revealed a significant association between IE and CFA 14 (p < 1.03 E− 08) and a suggestive association on CFA 37 (p < 2.91 E− 06) in a region in linkage disequilibrium with ADAM23. Logistic regression identified a 2-loci model that demonstrated interaction between the two chromosomal regions that when combined predicted IE risk with high sensitivity.

Conclusions

Two interacting loci, one each on CFAs 14 and 37, predictive of IE in the Belgian shepherd were identified. The loci are adjacent to potential candidate genes associated with neurological function. Further exploration of the region is warranted to identify causal variants underlying the association. Additionally, although the two loci were very good at predicting IE, they failed to capture all the risk, indicating additional loci or incomplete penetrance are also likely contributing to IE expression in the Belgian shepherd dog.

KLF7 overexpression in bone marrow stromal stem cells graft transplantation promotes sciatic nerve regeneration

OBJECTIVE

Our previous study demonstrated that the transcription factor, Krüppel-like Factor 7 (KLF7), stimulates axon regeneration following peripheral nerve injury. In the present study, we used a gene therapy approach to overexpress KLF7 in bone marrow-derived stem/stromal cells (BMSCs) as support cells, combined with acellular nerve allografts (ANAs) and determined the potential therapeutic efficacy of a KLF7-transfected BMSC nerve graft transplantation in a rodent model for sciatic nerve injury and repair.

APPROACH

We efficiently transfected BMSCs with adeno-associated virus (AAV)-KLF7, which were then seeded in ANAs for bridging sciatic nerve defects.

MAIN RESULTS

KLF7 overexpression promotes proliferation, survival, and Schwann-like cell differentiation of BMSCs in vitro. In vivo, KLF7 overexpression promotes transplanted BMSCs survival and myelinated fiber regeneration in regenerating ANAs; however, KLF7 did not improve Schwann-like cell differentiation of BMSCs within in the nerve grafts. KLF7-BMSCs significantly upregulated expression and secretion of neurotrophic factors by BMSCs, including nerve growth factor, ciliary neurotrophic factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor in regenerating ANA. KLF7-BMSCs also improved motor axon regeneration, and subsequent neuromuscular innervation and prevention of muscle atrophy. These benefits were associated with increased motor functional recovery of regenerating ANAs.

SIGNIFICANCE

Our findings suggest that KLF7-BMSCs promoted peripheral nerve axon regeneration and myelination, and ultimately, motor functional recovery. The mechanism of KLF7 action may be related to its ability to enhance transplanted BMSCs survival and secrete neurotrophic factors rather than Schwann-like cell differentiation. This study provides novel foundational data connecting the benefits of KLF7 in neural injury and repair to BMSC biology and function, and demonstrates a potential combination approach for the treatment of injured peripheral nerve via nerve graft transplant.

Inhibition of KLF7-Targeting MicroRNA 146b Promotes Sciatic Nerve Regeneration

A previous study has indicated that Krüppel-like factor 7 (KLF7), a transcription factor that stimulates Schwann cell (SC) proliferation and axonal regeneration after peripheral nerve injury, is a promising therapeutic transcription factor in nerve injury. We aimed to identify whether inhibition of microRNA-146b (miR-146b) affected SC proliferation, migration, and myelinated axon regeneration following sciatic nerve injury by regulating its direct target KLF7. SCs were transfected with miRNA lentivirus, miRNA inhibitor lentivirus, or KLF7 siRNA lentivirus in vitro. The expression of miR146b and KLF7, as well as SC proliferation and migration, were subsequently evaluated. In vivo, an acellular nerve allograft (ANA) followed by injection of GFP control vector or a lentiviral vector encoding an miR-146b inhibitor was used to assess the repair potential in a model of sciatic nerve gap. miR-146b directly targeted KLF7 by binding to the 3'-UTR, suppressing KLF7. Up-regulation of miR-146b and KLF7 knockdown significantly reduced the proliferation and migration of SCs, whereas silencing miR-146b resulted in increased proliferation and migration. KLF7 protein was localized in SCs in which miR-146b was expressed in vivo. Similarly, 4 weeks after the ANA, anti-miR-146b increased KLF7 and its target gene nerve growth factor cascade, promoting axonal outgrowth. Closer analysis revealed improved nerve conduction and sciatic function index score, and enhanced expression of neurofilaments, P0 (anti-peripheral myelin), and myelinated axon regeneration. Our findings provide new insight into the regulation of KLF7 by miR-146b during peripheral nerve regeneration and suggest a potential therapeutic strategy for peripheral nerve injury.

Krüppel-like factors in mammalian stem cells and development

Krüppel-like factors (KLFs) are a family of zinc-finger transcription factors that are found in many species. Recent studies have shown that KLFs play a fundamental role in regulating diverse biological processes such as cell proliferation, differentiation, development and regeneration. Of note, several KLFs are also crucial for maintaining pluripotency and, hence, have been linked to reprogramming and regenerative medicine approaches. Here, we review the crucial functions of KLFs in mammalian embryogenesis, stem cell biology and regeneration, as revealed by studies of animal models. We also highlight how KLFs have been implicated in human diseases and outline potential avenues for future research.

KLF7-transfected Schwann cell graft transplantation promotes sciatic nerve regeneration

Our former study demonstrated that Krüppel-like Factor 7 (KLF7) is a transcription factor that stimulates axonal regeneration after peripheral nerve injury. Currently, we used a gene therapy approach to overexpress KLF7 in Schwann cells (SCs) and assessed whether KLF7-transfected SCs graft could promote sciatic nerve regeneration. SCs were transfected by adeno-associated virus 2 (AAV2)-KLF7 in vitro. Mice were allografted by an acellular nerve (ANA) with either an injection of DMEM (ANA group), SCs (ANA+SCs group) or AAV2-KLF7-transfected SCs (ANA+KLF7-SCs group) to assess repair of a sciatic nerve gap. The results indicate that KLF7 overexpression promoted the proliferation of both transfected SCs and native SCs. The neurite length of the dorsal root ganglia (DRG) explants was enhanced. Several beneficial effects were detected in the ANA+KLF7-SCs group including an increase in the compound action potential amplitude, sciatic function index score, enhanced expression of PKH26-labeling transplant SCs, peripheral myelin protein 0, neurofilaments, S-100, and myelinated regeneration nerve. Additionally, HRP-labeled motoneurons in the spinal cord, CTB-labeled sensory neurons in the DRG, motor endplate density and the weight ratios of target muscles were increased by the treatment while thermal hyperalgesia was diminished. Finally, expression of KLF7, NGF, GAP43, TrkA and TrkB were enhanced in the grafted SCs, which may indicate that several signal pathways may be involved in conferring the beneficial effects from KLF7 overexpression. We concluded that KLF7-overexpressing SCs promoted axonal regeneration of the peripheral nerve and enhanced myelination, which collectively proved KLF-SCs as a novel therapeutic strategy for injured nerves.

Krüpple-like factors 7 and 6a mRNA expression in adult zebrafish central nervous system

Krüpple-like factors (KLFs) are transcription factors with zinc finger DNA binding domains known to play important roles in brain development and central nervous system (CNS) regeneration. There is little information on KLFs expression in adult vertebrate CNS. In this study, we used in situ hybridization to examine Klf7 mRNA (klf7) and Klf6a mRNA (klf6a) expression in adult zebrafish CNS. Both klfs exhibit wide and similar expression in the zebrafish CNS. Brain areas containing strongly labeled cells include the ventricular regions of the dorsomedial telencephalon, the ventromedial telencephalon, periventricular regions of the thalamus and hypothalamus, torus longitudinalis, stratum periventriculare of the optic tectum, granular regions of the cerebellar body and valvula, and superficial layers of the facial and vagal lobes. In the spinal cord, klf7- and klf6a-expressing cells are found in both the dorsal and ventral horns. Numerous sensory structures (e.g. auditory, lateral line, olfactory and visual) and several motor nuclei (e.g. oculomotor, trigeminal, and vagal motor nuclei) contain klf7- and/or klf6a-expressing cells. Our results may provide useful information for determining these Klfs in maintenance and/or function in adult CNS.

Releasing 'brakes' to nerve regeneration: intrinsic molecular targets

Restoring critical neuronal architecture after peripheral nerve injury is challenging. Although immediate regenerative responses to peripheral axon injury involve the synthesis of regeneration-associated proteins in neurons and Schwann cells, an unfavorable balance between growth facilitatory and growth inhibitory signaling impairs the growth continuum of injured peripheral nerves. Molecules involved with the signaling network of tumor suppressors play crucial roles in shifting the balance between growth and restraint during axon regeneration. An understanding of the molecular framework of tumor suppressor molecules in injured neurons and its impact on stage-specific regeneration events may expose therapeutic intervention points. In this review we discuss how signaling networks of the specific tumor suppressors PTEN, Rb1, p53, p27 and p21 are altered in injured peripheral nerves and how this impacts peripheral nerve regeneration. Insights into the roles and importance of these pathways may open new avenues for improving the neurological deficits associated with nerve injury.

Neurotrophin signalling and transcription programmes interactions in the development of somatosensory neurons

Somatosensory neurons of the dorsal root ganglia are generated from multipotent neural crest cells by a process of progressive specification and differentiation. Intrinsic transcription programmes active in somatosensory neuron progenitors and early post-mitotic neurons drive the cell-type expression of neurotrophin receptors. In turn, signalling by members of the neurotrophin family controls expression of transcription factors that regulate neuronal sub-type specification. This chapter explores the mechanisms by which this crosstalk between neurotrophin signalling and transcription programmes generates the diverse functional sub-types of somatosensory neurons found in the mature animal.

Cloning, tissue expression and polymorphisms of chicken Krüppel-like factor 7 gene

Krüppel-like factor 7 (KLF7) has been extensively studied in mammalian species, but its role in birds is still unclear. In the current study, cloning and sequencing showed that the full-length coding region of chicken KLF7 (Gallus gallus KLF7, gKLF7) was 891 bp long, encoding 296 amino acids. In addition, real-time RT-PCR analysis showed that gKLF7 was broadly expressed in all 15 chicken tissues selected, and its expression was significantly different in spleen, proventriculus, abdominal fat, brain, leg muscle, gizzard and heart between fat and lean broilers at 7 weeks of age. Additionally, one novel single nucleotide polymorphism (SNP), XM_426569.3: c. A141G, was identified in the second exon of gKLF7. Association analysis showed that this locus was significantly associated with fatness traits in Arbor Acres broiler random population and the eighth generation of Northeast Agricultural University broiler lines divergently selected for abdominal fat content (NEAUHLF) population (P < 0.05). These results suggest that gKLF7 might be a candidate gene for chicken fatness traits.

Isoform diversity and its importance for axon regeneration

Axon regeneration is a fundamental problem facing neuroscientists and clinicians. Failure of axon regeneration is caused by both extrinsic and intrinsic mechanisms. New techniques to examine gene expression such as Next Generation Sequencing of the Transcriptome (RNA-Seq) drastically increase our knowledge of both gene expression complexity (RNA isoforms) and gene expression regulation. By utilizing RNA-Seq, gene expression can now be defined at the level of isoforms, an essential step for understanding the mechanisms governing cell identity, growth and ultimately cellular responses to injury and disease.

Multiple transcription factor families regulate axon growth and regeneration

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP-responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c-Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ-light chain enhancer of activated B cells (NFκB), and Krüppel-like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

The transcription factor Cux2 marks development of an A-delta sublineage of TrkA sensory neurons

The developmental process and unique molecular identity between the many different types of dorsal root ganglion (DRG) sensory neurons generated during embryogenesis provide the cellular basis for the distinct perceptual modalities of somatosensation. The mechanisms leading to the generation of different types of nociceptive sensory neurons remain only partly understood. Here, we show that the transcription factor Cux2 is a novel marker of sensory neuron subpopulations of three main sublineages as defined by the expression of neurotrophic factor receptors TrkA, TrkB and TrkC. In particular, it is expressed in a subpopulation of early TrkA(+) neurons that arise during the early, Ngn1-independent initiated neurogenesis in the DRG. Postnatally, Cux2 marks a specific subtype of A-delta nociceptors as seen by expression of TrkA and NF200 but absence of TrpV1. Analysis of Cux2 mutant mice shows that Cux2 is not required for specification of Trk(+) neuronal subpopulations. However, Cux2 mutant mice are hypersensitive to mechanical, but not to heat or cold stimuli, consistent with a requirement in the process of specification of the mechanoreceptive neuron circuit. Hence, our results show that Cux2 is expressed and may participate in development of a specific subtype of myelinated TrkA(+) nociceptors.

Krüppel-like transcription factors in the nervous system: novel players in neurite outgrowth and axon regeneration

The Krüppel-like family of transcription factors (KLFs) have been widely studied in proliferating cells, though very little is known about their role in post-mitotic cells, such as neurons. We have recently found that the KLFs play a role in regulating intrinsic axon growth ability in retinal ganglion cells (RGCs), a type of central nervous system (CNS) neuron. Previous KLF studies in other cell types suggest that there may be cell-type specific KLF expression patterns, and that their relative expression allows them to compete for binding sites, or to act redundantly to compensate for another's function. With at least 15 of 17 KLF family members expressed in neurons, it will be important for us to determine how this complex family functions to regulate the intricate gene programs of axon growth and regeneration. By further characterizing the mechanisms of the KLF family in the nervous system, we may better understand how they regulate neurite growth and axon regeneration.

Transcription factor KLF7 regulates differentiation of neuroectodermal and mesodermal cell lineages

Previous gene targeting studies in mice have implicated the nuclear protein Krüppel-like factor 7 (KLF7) in nervous system development while cell culture assays have documented its involvement in cell cycle regulation. By employing short hairpin RNA (shRNA)-mediated gene silencing, here we demonstrate that murine Klf7 gene expression is required for in vitro differentiation of neuroectodermal and mesodermal cells. Specifically, we show a correlation of Klf7 silencing with down-regulation of the neuronal marker microtubule-associated protein 2 (Map2) and the nerve growth factor (NGF) tyrosine kinase receptor A (TrkA) using the PC12 neuronal cell line. Similarly, KLF7 inactivation in Klf7-null mice decreases the expression of the neurogenic marker brain lipid-binding protein/fatty acid-binding protein 7 (BLBP/FABP7) in neural stem cells (NSCs). We also report that Klf7 silencing is detrimental to neuronal and cardiomyocytic differentiation of embryonic stem cells (ESCs), in addition to altering the adipogenic and osteogenic potential of mouse embryonic fibroblasts (MEFs). Finally, our results suggest that genes that are key for self-renewal of undifferentiated ESCs repress Klf7 expression in ESCs. Together with previous findings, these results provide evidence that KLF7 has a broad spectrum of regulatory functions, which reflect the discrete cellular and molecular contexts in which this transcription factor operates.

A model for neuronal competition during development

We report that developmental competition between sympathetic neurons for survival is critically dependent on a sensitization process initiated by target innervation and mediated by a series of feedback loops. Target-derived nerve growth factor (NGF) promoted expression of its own receptor TrkA in mouse and rat neurons and prolonged TrkA-mediated signals. NGF also controlled expression of brain-derived neurotrophic factor and neurotrophin-4, which, through the receptor p75, can kill neighboring neurons with low retrograde NGF-TrkA signaling whereas neurons with high NGF-TrkA signaling are protected. Perturbation of any of these feedback loops disrupts the dynamics of competition. We suggest that three target-initiated events are essential for rapid and robust competition between neurons: sensitization, paracrine apoptotic signaling, and protection from such effects.

Specification of sensory neuron cell fate from the neural crest

How distinct cell fates are generated from initially homogeneous cell populations is a driving question in developmental biology. The neural crest is one such cell population that is capable of producing an incredible array of derivatives. Cells as different in function and form as the pigment cells in the skin or the neurons and glia of the peripheral nervous system are all derived from neural crest. How do these cells choose to migrate along distinct routes, populate defined regions of the embryo and differentiate into specific cell types? This chapter focuses on the development of one particular neural crest derivative, sensory neurons, as a model for studying these questions of cell fate specification. In the head, sensory neurons reside in the trigeminal and epibranchial ganglia, while in the trunk they form the spinal or dorsal root ganglia (DRG). The development of the DRG will be the main focus of this review. The neurons and glia of the DRG derive from trunk neural crest cells that coalesce at the lateral edge of the spinal cord (Fig. 1). These neural crest cells migrate along the same routes as neural crest cells that populate the autonomic sympathetic ganglia located along the dorsal aorta. Somehow DRG precursors must make the decision to stop and adopt a sensory fate adjacent to the spinal cord rather than continuing on to become part of the autonomic ganglia. Moreover, once the DRG precursors aggregate in their final positions there are still a number of fate choices to be made. The mature DRG is composed of many neurons with different morphologies and distinct biochemical properties as well as glial cells that support these neurons.

Ca2+, CREB and krüppel: a novel KLF7-binding element conserved in mouse and human TRKB promoters is required for CREB-dependent transcription

Brain-derived neurotrophic factor (BDNF) signaling through its receptor, trkB, is essential for the proper development and function of the nervous system. Here we identify a novel regulatory element designated TCaRE3 (TRKB Ca(2+) response element 3) required for CREB-dependent TRKB transcription in neurons. TCaRE3-inactivating mutations abolished both Ca(2+)- and cAMP-stimulated TRKB expression, despite the presence of upstream CREs. TCaRE3 mutations also reduced basal expression by at least 80%. Electrophoretic mobility shift assays revealed the presence of a neuronal nuclear factor able to bind TCaRE3 in a sequence-specific manner and we have identified krüppel-like factor 7 (KLF7) as a candidate TCaRE3 transcription factor. Importantly, despite limited overall sequence homology between the promoter regions of the human and mouse TRKB genes, TCaRE3 exhibits 100% sequence identity. Mutation analysis of the human TRKB promoter region demonstrated that the role of TCaRE3 is also conserved, suggesting that the functional interaction between CREB bound to the CREs and KLF7 bound to TCaRE3 is essential for the proper regulation of TRKB in neurons.

The transcription factor Runx3 represses the neurotrophin receptor TrkB during lineage commitment of dorsal root ganglion neurons

Runx3, a Runt domain transcription factor, determines neurotrophin receptor phenotype in dorsal root ganglion (DRG) neurons. Molecular mechanisms by which Runx3 controls distinct neurotrophin receptors are largely unknown. Here, we show that RUNX3 abolished mRNA induction of TRKB expression, and concomitantly altered the neurotrophin response in a differentiating neuroblastoma cell line. In contrast, RUNX3 did not play a significant role in TRKC regulation even under the relevant BMP signaling pathway. We identified putative regulatory elements of Ntrk2/NTRK2 (a gene that codes for TrkB) using an unbiased computational approach. One of these elements was a highly conserved intronic sequence that contains a cluster of Runx binding sites. In a primary culture of DRG neurons, endogenous Runx3 bound to the consensus cluster, which had repressor activity against the Ntrk2 promoter under the control of NT-3 signaling. Consistent with these findings, Runx3-deficient embryos showed an increased number of trkB+ DRG neurons and failed to maintain trkC expression. Taken together, Runx3 determines TrkC positive sensory neuron identities through the transcriptional repression of TrkB when Trk-BTrkC double positive neurons differentiate into TrkC single positive neurons.

Specification and connectivity of neuronal subtypes in the sensory lineage

During the development of the nervous system, many different types of neuron are produced. As well as forming the correct type of neuron, each must also establish precise connections. Recent findings show that, because of shared gene programmes, neuronal identity is intimately linked to and coordinated with axonal behaviour. Peripheral sensory neurons provide an excellent system in which to study these interactions. This review examines how neuronal diversity is created in the PNS and describes proteins that help to direct the diversity of neuronal subtypes, cell survival, axonal growth and the establishment of central patterns of modality-specific connections.

Identification of genes regulated by transcription factor KLF7 in differentiating olfactory sensory neurons

Gene targeting in mice has recently demonstrated that transcription factor KLF7 plays a critical role in neurite outgrowth and neuronal survival. Here we extended this genetic evidence by establishing the transcriptional profile of differentiating olfactory sensory neurons (OSNs) in Klf7(-/-) mice, and by identifying relevant genes that are directly regulated by KLF7. Functional clustering of DNA microarray data revealed that loss of KLF7 affects primarily the activity of genes involved in OSN differentiation, axonal growth, cytoskeletal dynamics, cell adhesion and synaptogenesis. Cell transfection experiments, on the other hand, demonstrated that the promoters of the genes encoding the OSN-specific OMP and the adhesion molecule L1 are both activated by KLF7 binding to CACCC motifs. Collectively, these results advance knowledge of transcriptional regulation of olfactory neurogenesis and KLF7 action.

Multiple pathways regulate intracellular shuttling of MoKA, a co-activator of transcription factor KLF7

MoKA is a novel F-box containing protein that interacts with and stimulates the activity of transcription factor KLF7, a regulator of neuronal differentiation. MoKA accumulates throughout the cell and predominantly in the cytosol, consistent with the presence of several putative nuclear localization and export signals (NLSs and NESs). The present study was designed to refine the identity and location of the sequences responsible for MoKA intracellular shuttling and transcriptional activity. Forced expression of fusion proteins in mammalian cells demonstrated that only one of three putative NLSs potentially recognized by karyopherin receptors is involved in nuclear localization of MoKA. By contrast, three distinct sequences were found to participate in mediating cytoplasmic accumulation. One of them is structurally and functionally related to the leucine-rich export signal that interacts with the exportin 1 (CRM1) receptor. The other two export signals instead display either a novel leucine-rich sequence or an undefined peptide motif, and both appear to act through CRM1-independent pathways. Finally, transcriptional analyses using the chimeric GAL4 system mapped the major activation domain of MoKA to a highly acidic sequence that resides between the NLS and NES clusters.

Brn3a and Klf7 cooperate to control TrkA expression in sensory neurons

The zinc finger protein Klf7 and POU homeodomain protein Brn3a are each required for efficient transcription of TrkA in primary sensory neurons. In this study, we examined whether these transcription factors act in concert to regulate TrkA expression. In vitro, Brn3a and Klf7 can synergistically activate the TrkA enhancer. In vivo, precursor cells that are destined to become TrkA(+) neurons are born. However, both Brn3a and Klf7 are dispensable for the initiation of TrkA expression. At E12.5, while TrkA expression is unaffected in Brn3a-/- trigeminal ganglia and only slightly decreased in Klf7-/- trigeminal ganglia, it is severely reduced in the double mutant Brn3a-/-;Klf7-/- trigeminal ganglia. At birth, all Trk(+) neurons are lost in Brn3a-/-;Klf7-/- trigeminal ganglia. We further demonstrate that the TrkA enhancer is inactive in Brn3a-/-;Klf7-/- trigeminal ganglia. Thus, cooperation between these two transcription factors is required for endogenous TrkA gene expression and the survival of nociceptive sensory neurons.

The zinc finger transcription factor Klf7 is required for TrkA gene expression and development of nociceptive sensory neurons

TrkA, the high affinity receptor for nerve growth factor (NGF), is essential for the development of nociceptive sensory and sympathetic neurons. The zinc finger transcription factor Klf7 interacts with an important cis element of the TrkA minimal enhancer and is coexpressed with TrkA in these neurons. We show that Klf7 binds to the endogenous TrkA minimal enhancer and can activate transcription from the TrkA minimal enhancer in a sequence-dependent manner. In Klf7(-/-) newborn mice, we find a significant reduction in sensory neurons due to increased apoptosis. The neuronal loss is restricted to nociceptive neurons that normally depend on TrkA for neurotrophic support, while other populations of somatosensory neurons appear normal. The reduction of TrkA expression in sensory neurons is a direct effect of Klf7 gene ablation, rather than a secondary effect of cell death. As a result, Klf7(-/-) mice have deficient response to noxious stimuli. Finally, removal of one TrkA allele exacerbates the loss of TrkA(+) neurons in Klf7(-/-) mice. Thus, Klf7 specifically regulates TrkA gene expression and is required for the development of a subset of nociceptive sensory neurons.

Transcription factor KLF7 is important for neuronal morphogenesis in selected regions of the nervous system

The Krüppel-like transcription factors (KLFs) are important regulators of cell proliferation and differentiation in several different organ systems. The mouse Klf7 gene is strongly active in postmitotic neuroblasts of the developing nervous system, and the corresponding protein stimulates transcription of the cyclin-dependent kinase inhibitor p21waf/cip gene. Here we report that loss of KLF7 activity in mice leads to neonatal lethality and a complex phenotype which is associated with deficits in neurite outgrowth and axonal misprojection at selected anatomical locations of the nervous system. Affected axon pathways include those of the olfactory and visual systems, the cerebral cortex, and the hippocampus. In situ hybridizations and immunoblots correlated loss of KLF7 activity in the olfactory epithelium with significant downregulation of the p21waf/cip and p27kip1 genes. Cotransfection experiments extended the last finding by documenting KLF7's ability to transactivate a reporter gene construct driven by the proximal promoter of p27kip1. Consistent with emerging evidence for a role of Cip/Kip proteins in cytoskeletal dynamics, we also documented p21waf/cip and p27kip1 accumulation in the cytoplasm of differentiating olfactory sensory neurons. KLF7 activity might therefore control neuronal morphogenesis in part by optimizing the levels of molecules that promote axon outgrowth.

Differential regulation of fatty acid amide hydrolase promoter in human immune cells and neuronal cells by leptin and progesterone

We have shown recently that in human T lymphocytes, leptin stimulates activity and expression of the endocannabinoid-degrading enzyme fatty acid amide hydrolase (FAAH), through STAT3 (signal transducer and activator of transcription 3) and its CRE (cAMP response element)-like transcriptional target in the FAAH promoter [Maccarrone, M., Di Rienzo, M., Finazzi-Agro, A., & Rossi, A. (2003) J. Biol. Chem. 278, 13318-13324]. We have also shown that progesterone, alone or additively with leptin, up-regulates the FAAH gene in human T-cells, through the Ikaros transcription factor [Maccarrone, M., Bari, M., Di Rienzo, M., Finazzi-Agro, A., & Rossi, A. (2003) J. Biol. Chem. 278, 32726-32732]. Here, we extend these observations to immortalized human lymphoma U937 cells, where stimulation of FAAH by leptin (up to approximately 300% of the controls) involves binding to a leptin receptor (Kd = 2.0 +/- 0.1 nm, Bmax = 382 +/- 5 fmol.mg protein(-1), apparent molecular mass of approximately 110 kDa), and stimulation by progesterone involves an intracellular receptor of approximately 120 kDa. Unlike FAAH, the other proteins of the endocannabinoid system are not modulated by the two hormones. Interestingly, human neuroblastoma CHP100 cells also have a leptin receptor (approximately 110 kDa, Kd = 2.2 +/- 0.2 nm, Bmax = 339 +/- 8 fmol.mg protein(-1)), a progesterone receptor (approximately 120 kDa), STAT3 and Ikaros, yet their FAAH is not activated by leptin or progesterone. These data, corroborated by transient expression and electrophoretic mobility-shift assays, demonstrate an unprecedented cell-specific regulation of the FAAH gene, which has important implications for the control of tone and activity of AEA along the neuroimmune axis.

A new role for the Kruppel-like transcription factor KLF6 as an inhibitor of c-Jun proto-oncoprotein function

Kruppel-like transcription factors (KLFs) represent one of the most diverse set of regulators in vertebrate organisms. KLF family members are involved in cell proliferation and differentiation control in normal as well as in pathological situations. Here, we demonstrate that KLF6 behaves as a functional antagonist of the c-Jun proto-oncoprotein. Thus, KLF6 overexpression downregulated c-Jun-dependent transcription and a physical interaction between c-Jun and KLF6 was detected. Moreover, cell proliferation induced by c-Jun was significantly decreased by KLF6. The inhibition of c-Jun functions correlates directly with c-Jun protein degradation induced by KLF6. We also show that all KLF6 effects on c-Jun were largely dependent on phorbol ester (TPA/ionomycin) extracellular stimulation, which enhanced KLF6 nuclear translocation and transcriptional activity and modified its phosphorylation status. Our data are consistent with a novel mechanism of KLF6's role as an inhibitor of cell proliferation by counteracting the function of the c-Jun proto-oncoprotein involving enhanced c-Jun degradation by the proteasome-dependent pathway, and further reinforces KLF6 as a potential tumor suppressor gene product.

Identification of MoKA, a novel F-box protein that modulates Krüppel-like transcription factor 7 activity

KLF7, a member of the Krüppel-like transcription factor family, is believed to regulate neurogenesis and cell cycle progression. Here, a yeast two-hybrid screen for KLF7 cofactors in the developing nervous system identified a novel 140-kDa protein named MoKA, for modulator of KLF7 activity. Interaction between MoKA and KLF7 was confirmed by the in vitro glutathione S-transferase pull-down assay and by coimmunoprecipitation of the proteins overexpressed in mammalian cells. Functional assays documented that MoKA is a KLF7 coactivator, and in situ hybridizations identified the developing nervous system and the adult testes as two sites of MoKA and Klf7 coexpression. Chromatin immunoprecipitation experiments demonstrated KLF7 binding to the p21(WAF1/Cip1) gene while transient transfection assays documented KLF7 stimulation of the p21(WAF1/Cip1) proximal promoter. Additional tests revealed that distinct structural motifs of MoKA direct interaction with KLF7 and shuttling between the nucleus and cytoplasm of asynchronously cycling cells. Altogether, our results strongly suggest that MoKA and KLF7 interact functionally to regulate gene expression during cell differentiation and identify the cell cycle regulator p21(WAF1/Cip1) as one of the targeted genes.

Functional analysis of the regulatory regions of the matrilin-1 gene in transgenic mice reveals modular arrangement of tissue-specific control elements

Matrilin-1 is a non-collagenous protein, which functions in the organization of the extracellular matrix by forming collagen-dependent and -independent filamentous networks. It is secreted primarily by chondrocytes in a characteristic spatial, temporal and developmental stage-specific pattern during skeletogenesis. As a first step to define the tissue- and site-specific regulatory regions of the chicken matrilin-1 gene in vivo, we generated transgenic mice harboring various promoter and intronic fragments fused to the LacZ reporter gene. Histological analysis of the transgene expression pattern during ontogenic development revealed specific X-gal staining in most primordial elements of endochondral bones of transgenic mouse lines carrying either the long promoter between -2011 and +67 or the intronic fragment with a short promoter between -338 and +1819. The cartilage-specific activity of the latter transgene, however, was accompanied with variable ectopic expression pattern in neural and other tissues depending on the site of integration. The presence of both promoter upstream and intronic elements was necessary for the high level transgene activity in all chondrogenic tissues and for the extraskeletal transgene expression pattern resembling the most to that of the chicken matrilin-1 gene, e.g. expression in the eye, and lack of expression in the diminishing notochord and nucleus pulposus. The activity of the transgenes was restricted to the columnar proliferating and pre-hypertrophic chondrocytes visualized by BrdU incorporation and distribution of phosphorylated Sox9, respectively. DNA elements between -2011 and -338 also mediated ectopic LacZ expression in cells of neural crest origin. These results suggest that an interplay of modularly arranged cartilage- and neural crest-specific DNA elements control the expression of the matrilin-1 gene. The dispersal of cartilage-specific elements in the promoter upstream and intronic regions shows similarity to the transcriptional regulation of the Col11a2 gene.

The SHB adapter protein is required for efficient multilineage differentiation of mouse embryonic stem cells

The SH2 domain-containing adapter protein SHB transmits signals from receptor tyrosine kinases regulating diverse processes such as apoptosis and differentiation. To elucidate a role for SHB in cell differentiation, wild-type and R522K (inactive SH2 domain-mutant) SHB were transfected and expressed in mouse embryonic stem (ES) cells. Microarray analysis using Affymetrix U74A chips on undifferentiated ES cells and expression of selected differentiation markers after generation of embryoid bodies were subsequently assessed. Wild-type SHB altered the expression of 16 genes in undifferentiated ES cells, many of which have been found to relate to neural cell function. R522K-SHB altered the expression of 128 genes in undifferentiated ES cells, the majority of which were decreased, including several transcription factors related to development. When grown as embryoid bodies, after 4 days R522K-SHB ES cells were already found to display a different morphological appearance, with an impaired cavity formation that occurred in the absence of altered OCT4 expression. This impairment was reversed by exogenous addition of Matrigel. In addition, R522K-SHB embryoid bodies displayed reduced mRNA contents of the liver protein albumin, the pancreatic proteins amylase, glucagon and insulin after 20 days of differentiation. Matrigel did not restore the impaired expression of albumin in the R522K-SHB cells. Expression of the mesodermal marker cardiac actin and the neural marker neurofilament heavy chain alpha was not affected by wild-type or R522K-SHB overexpression. It is concluded that SHB is required for efficient differentiation of ES cells into embryoid bodies with normal cavities and cells belonging to endodermal lineages.

The effects of chronic ethanol consumption on neurotrophins and their receptors in the rat hippocampus and basal forebrain

Damage to the basal forebrain frequently results in deficits in learning and memory. Mnenonic dysfunction also occurs following prolonged ethanol consumption in humans and in animal models of chronic ethanol intake, accompanied by specific abnormalities in synaptic transmission between the basal forebrain and hippocampus. The integrity of at least some of the reciprocal neuronal connections between these brain regions is influenced by target-derived neurotrophic factors. We used a semiquantitative reverse transcription polymerase chain reaction technique to measure the messenger RNA for neurotrophins BDNF and NGF, and for their receptors trkB, trkA, and the low affinity receptor, p75(NTR) in the hippocampus and basal forebrain of rats after 28 weeks of alcohol consumption without malnutrition. This chronic ethanol treatment (CET) resulted in a marked and selective reduction in basal forebrain trkA mRNA. Western blotting revealed a similar reduction of basal forebrain trkA protein. CET effects on basal forebrain trkA may reflect impaired NGF signaling that could compromise septohippocampal synaptic connections, cholinergic differentiation, and emergent functional abilities dependent on these properties.

Developmental expression of mouse Krüppel-like transcription factor KLF7 suggests a potential role in neurogenesis

To identify potential functions for the Krüppel-like transcription factor KLF7, we have determined the spatiotemporal pattern of gene expression during embryogenesis and in the adult organism. We show that the profile of Klf7 expression predominantly involves the central and peripheral nervous systems and is broadly identified by three separate phases. The first phase occurs early in embryogenesis with increasingly strong expression in the spinal cord, notably in motor neurons of the ventral horn, in dorsal root ganglia, and in sympathetic ganglia. The second robust phase of Klf7 expression is confined to the early postnatal cerebral cortex and is downregulated thereafter. The third phase is characterized by high and sustained expression in the adult cerebellum and dorsal root ganglia. Functionally, these three phases coincide with establishment of neuronal phenotype in embryonic spinal cord, with synaptogenesis and development of mature synaptic circuitry in the postnatal cerebral cortex, and with survival and/or maintenance of function of adult sensory neurons and cerebellar granule cells. Consistent with Klf7 expression in newly formed neuroblasts, overexpression of the gene in cultured fibroblasts and neuroblastoma cells repressed cyclin D1, activated p21, and led to G1 growth arrest. Based on these data, we argue for multiple potential functions for KLF7 in the developing and adult nervous system; they include participating in differentiation and maturation of several neuronal subtypes and in phenotypic maintenance of mature cerebellar granule cells and dorsal root ganglia.