Targeting the chemokine receptor CXCR4 with histamine analog to reduce inflammation in juvenile arthritis

Introduction

Among immune cells, activated monocytes play a detrimental role in chronic and viral-induced inflammatory pathologies, particularly in Juvenile Idiopathic Arthritis (JIA), a childhood rheumatoid arthritis (RA) disease. The uncontrolled activation of monocytes and excessive production of inflammatory factors contribute to the damage of bone-cartilage joints. Despite the moderate beneficial effect of current therapies and clinical trials, there is still a need for alternative strategies targeting monocytes to treat RA.

Methods

To explore such an alternative strategy, we investigated the effects of targeting the CXCR4 receptor using the histamine analog clobenpropit (CB). Monocytes were isolated from the blood and synovial fluids of JIA patients to assess CB's impact on their production of key inflammatory cytokines. Additionally, we administered daily intraperitoneal CB treatment to arthritic mice to evaluate its effects on circulating inflammatory cytokine levels, immune cell infiltrates, joints erosion, and bone resorption, as indicators of disease progression.

Results

Our findings demonstrated that CXCR4 targeting with CB significantly inhibited the spontaneous and induced-production of key inflammatory cytokines by monocytes isolated from JIA patients. Furthermore, CB treatment in a mouse model of collagen-induce arthritis resulted in a significant decrease in circulating inflammatory cytokine levels, immune cell infiltrates, joints erosion, and bone resorption, leading to a reduction in disease progression.

Discussion

In conclusion, targeting CXCR4 with the small amino compound CB shows promise as a therapeutic option for chronic and viral-induced inflammatory diseases, including RA. CB effectively regulated inflammatory cytokine production of monocytes, presenting a potential targeted approach with potential advantages over current therapies. These results warrant further research and clinical trials to explore the full therapeutic potential of targeting CXCR4 with CB-like molecules in the management of various inflammatory diseases.

Titanium dioxide and carbon black nanoparticles disrupt neuronal homeostasis via excessive activation of cellular prion protein signaling

Background

Epidemiological emerging evidence shows that human exposure to some nanosized materials present in the environment would contribute to the onset and/or progression of Alzheimer’s disease (AD). The cellular and molecular mechanisms whereby nanoparticles would exert some adverse effects towards neurons and take part in AD pathology are nevertheless unknown.

Results

Here, we provide the prime evidence that titanium dioxide (TiO₂) and carbon black (CB) nanoparticles (NPs) bind the cellular form of the prion protein (PrP^C), a plasma membrane protein well known for its implication in prion diseases and prion-like diseases, such as AD. The interaction between TiO₂- or CB-NPs and PrP^C at the surface of neuronal cells grown in culture corrupts PrP^C signaling function. This triggers PrP^C-dependent activation of NADPH oxidase and subsequent production of reactive oxygen species (ROS) that alters redox equilibrium. Through PrP^C interaction, NPs also promote the activation of 3-phosphoinositide-dependent kinase 1 (PDK1), which in turn provokes the internalization of the neuroprotective TACE α-secretase. This diverts TACE cleavage activity away from (i) TNFα receptors (TNFR), whose accumulation at the plasma membrane augments the vulnerability of NP-exposed neuronal cells to TNFα -associated inflammation, and (ii) the amyloid precursor protein APP, leading to overproduction of neurotoxic amyloid Aβ40/42 peptides. The silencing of PrP^C or the pharmacological inhibition of PDK1 protects neuronal cells from TiO₂- and CB-NPs effects regarding ROS production, TNFα hypersensitivity, and Aβ rise. Finally, we show that dysregulation of the PrP^C-PDK1-TACE pathway likely occurs in the brain of mice injected with TiO₂-NPs by the intra-cerebro-ventricular route as we monitor a rise of TNFR at the cell surface of several groups of neurons located in distinct brain areas.

Conclusion

Our in vitro and in vivo study thus posits for the first time normal cellular prion protein PrP^C as being a neuronal receptor of TiO₂- and CB-NPs and identifies PrP^C-coupled signaling pathways by which those nanoparticles alter redox equilibrium, augment the intrinsic sensitivity of neurons to neuroinflammation, and provoke a rise of Aβ peptides. By identifying signaling cascades dysregulated by TiO₂- and CB-NPs in neurons, our data shed light on how human exposure to some NPs might be related to AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00490-x.

Loss of prion protein control of glucose metabolism promotes neurodegeneration in model of prion diseases

Corruption of cellular prion protein (PrPC) function(s) at the plasma membrane of neurons is at the root of prion diseases, such as Creutzfeldt-Jakob disease and its variant in humans, and Bovine Spongiform Encephalopathies, better known as mad cow disease, in cattle. The roles exerted by PrPC, however, remain poorly elucidated. With the perspective to grasp the molecular pathways of neurodegeneration occurring in prion diseases, and to identify therapeutic targets, achieving a better understanding of PrPC roles is a priority. Based on global approaches that compare the proteome and metabolome of the PrPC expressing 1C11 neuronal stem cell line to those of PrPnull-1C11 cells stably repressed for PrPC expression, we here unravel that PrPC contributes to the regulation of the energetic metabolism by orienting cells towards mitochondrial oxidative degradation of glucose. Through its coupling to cAMP/protein kinase A signaling, PrPC tones down the expression of the pyruvate dehydrogenase kinase 4 (PDK4). Such an event favors the transfer of pyruvate into mitochondria and its conversion into acetyl-CoA by the pyruvate dehydrogenase complex and, thereby, limits fatty acids β-oxidation and subsequent onset of oxidative stress conditions. The corruption of PrPC metabolic role by pathogenic prions PrPSc causes in the mouse hippocampus an imbalance between glucose oxidative degradation and fatty acids β-oxidation in a PDK4-dependent manner. The inhibition of PDK4 extends the survival of prion-infected mice, supporting that PrPSc-induced deregulation of PDK4 activity and subsequent metabolic derangements contribute to prion diseases. Our study posits PDK4 as a potential therapeutic target to fight against prion diseases.

The Cellular Prion Protein-ROCK Connection: Contribution to Neuronal Homeostasis and Neurodegenerative Diseases

Amyloid-based neurodegenerative diseases such as prion, Alzheimer's, and Parkinson's diseases have distinct etiologies and clinical manifestations, but they share common pathological events. These diseases are caused by abnormally folded proteins (pathogenic prions PrP^Sc in prion diseases, β-amyloids/Aβ and Tau in Alzheimer's disease, α-synuclein in Parkinson's disease) that display β-sheet-enriched structures, propagate and accumulate in the nervous central system, and trigger neuronal death. In prion diseases, PrP^Sc-induced corruption of the physiological functions exerted by normal cellular prion proteins (PrP^C) present at the cell surface of neurons is at the root of neuronal death. For a decade, PrP^C emerges as a common cell surface receptor for other amyloids such as Aβ and α-synuclein, which relays, at least in part, their toxicity. In lipid-rafts of the plasma membrane, PrP^C exerts a signaling function and controls a set of effectors involved in neuronal homeostasis, among which are the RhoA-associated coiled-coil containing kinases (ROCKs). Here we review (i) how PrP^C controls ROCKs, (ii) how PrP^C-ROCK coupling contributes to neuronal homeostasis, and (iii) how the deregulation of the PrP^C-ROCK connection in amyloid-based neurodegenerative diseases triggers a loss of neuronal polarity, affects neurotransmitter-associated functions, contributes to the endoplasmic reticulum stress cascade, renders diseased neurons highly sensitive to neuroinflammation, and amplifies the production of neurotoxic amyloids.

(Sub)stellar companions shape the winds of evolved stars

Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe.

Production of seedable Amyloid-β peptides in model of prion diseases upon PrPSc-induced PDK1 overactivation

The presence of amyloid beta (Aβ) plaques in the brain of some individuals with Creutzfeldt-Jakob or Gertsmann-Straussler-Scheinker diseases suggests that pathogenic prions (PrP^Sc) would have stimulated the production and deposition of Aβ peptides. We here show in prion-infected neurons and mice that deregulation of the PDK1-TACE α-secretase pathway reduces the Amyloid Precursor Protein (APP) α-cleavage in favor of APP β-processing, leading to Aβ40/42 accumulation. Aβ predominates as monomers, but is also found as trimers and tetramers. Prion-induced Aβ peptides do not affect prion replication and infectivity, but display seedable properties as they can deposit in the mouse brain only when seeds of Aβ trimers are co-transmitted with PrP^Sc. Importantly, brain Aβ deposition accelerates death of prion-infected mice. Our data stress that PrP^Sc, through deregulation of the PDK1-TACE-APP pathway, provokes the accumulation of Aβ, a prerequisite for the onset of an Aβ seeds-induced Aβ pathology within a prion-infectious context.

Aβ plaques have been detected in brains of patients with prion diseases. Here, using mice, the authors show that prion infection enhances Aβ production via a PDK1-TACE mechanism and that brain deposition of Aβ induced by Aβ seeds co-transmitted with PrP^Sc contributes to mortality in prion disease.

Multifaceted Regulations of the Serotonin Transporter: Impact on Antidepressant Response

Serotonin transporter, SERT (SLC64A for solute carrier family 6, member A4), is a twelve transmembrane domain (TMDs) protein that assumes the uptake of serotonin (5-HT) through dissipation of the Na⁺ gradient established by the electrogenic pump Na/K ATPase. Abnormalities in 5-HT level and signaling have been associated with various disorders of the central nervous system (CNS) such as depression, obsessive-compulsive disorder, anxiety disorders, and autism spectrum disorder. Since the 50s, SERT has raised a lot of interest as being the target of a class of antidepressants, the Serotonin Selective Reuptake Inhibitors (SSRIs), used in clinics to combat depressive states. Because of the refractoriness of two-third of patients to SSRI treatment, a better understanding of the mechanisms regulating SERT functions is of priority. Here, we review how genetic and epigenetic regulations, post-translational modifications of SERT, and specific interactions between SERT and a set of diverse partners influence SERT expression, trafficking to and away from the plasma membrane and activity, in connection with the neuronal adaptive cell response to SSRI antidepressants.

Alcohol exposure promotes DNA methyltransferase DNMT3A upregulation through reactive oxygen species-dependent mechanisms

Abundant evidence has accumulated showing that fetal alcohol exposure broadly modifies DNA methylation profiles in the brain. DNA methyltransferases (DNMTs), the enzymes responsible for DNA methylation, are likely implicated in this process. However, their regulation by ethanol exposure has been poorly addressed. Here, we show that alcohol exposure modulates DNMT protein levels through multiple mechanisms. Using a neural precursor cell line and primary mouse embryonic fibroblasts (MEFs), we found that ethanol exposure augments the levels of Dnmt3a, Dnmt3b, and Dnmt3l transcripts. We also unveil similar elevation of mRNA levels for other epigenetic actors upon ethanol exposure, among which the induction of lysine demethylase Kdm6a shows heat shock factor dependency. Furthermore, we show that ethanol exposure leads to specific increase in DNMT3A protein levels. This elevation not only relies on the upregulation of Dnmt3a mRNA but also depends on posttranscriptional mechanisms that are mediated by NADPH oxidase-dependent production of reactive oxygen species (ROS). Altogether, our work underlines complex regulation of epigenetic actors in response to alcohol exposure at both transcriptional and posttranscriptional levels. Notably, the upregulation of DNMT3A emerges as a prominent molecular event triggered by ethanol, driven by the generation of ROS.

Protective role of cellular prion protein against TNFα-mediated inflammation through TACE α-secretase

Although cellular prion protein PrP^C is well known for its implication in Transmissible Spongiform Encephalopathies, its functions remain elusive. Combining in vitro and in vivo approaches, we here show that PrP^C displays the intrinsic capacity to protect neuronal cells from a pro-inflammatory TNFα noxious insult. Mechanistically, PrP^C coupling to the NADPH oxidase-TACE α-secretase signaling pathway promotes TACE-mediated cleavage of transmembrane TNFα receptors (TNFRs) and the release of soluble TNFR, which limits the sensitivity of recipient cells to TNFα. We further show that PrP^C expression is necessary for TACE α-secretase to stay at the plasma membrane in an active state for TNFR shedding. Such PrP^C control of TACE localization depends on PrP^C modulation of β1 integrin signaling and downstream activation of ROCK-I and PDK1 kinases. Loss of PrP^C provokes TACE internalization, which in turn cancels TACE-mediated cleavage of TNFR and renders PrP^C-depleted neuronal cells as well as PrP^C knockout mice highly vulnerable to pro-inflammatory TNFα insult. Our work provides the prime evidence that in an inflammatory context PrP^C adjusts the response of neuronal cells targeted by TNFα through TACE α-secretase. Our data also support the view that abnormal TACE trafficking and activity in prion diseases originate from a-loss-of-PrP^C cytoprotective function.

Pulp stem cells: implication in reparative dentin formation

Many dental pulp stem cells are neural crest derivatives essential for lifelong maintenance of tooth functions and homeostasis as well as tooth repair. These cells may be directly implicated in the healing process or indirectly involved in cell-to-cell diffusion of paracrine messages to resident (pulpoblasts) or nonresident cells (migrating mesenchymal cells). The identity of the pulp progenitors and the mechanisms sustaining their regenerative capacity remain largely unknown. Taking advantage of the A4 cell line, a multipotent stem cell derived from the molar pulp of mouse embryo, we investigated the capacity of these pulp-derived precursors to induce in vivo the formation of a reparative dentin-like structure upon implantation within the pulp of a rodent incisor or a first maxillary molar after surgical exposure. One month after the pulp injury alone, a nonmineralized fibrous matrix filled the mesial part of the coronal pulp chamber. Upon A4 cell implantation, a mineralized osteodentin was formed in the implantation site without affecting the structure and vitality of the residual pulp in the central and distal parts of the pulp chamber. These results show that dental pulp stem cells can induce the formation of reparative dentin and therefore constitute a useful tool for pulp therapies. Finally, reparative dentin was also built up when A4 progenitors were performed by alginate beads, suggesting that alginate is a suitable carrier for cell implantation in teeth.

Essential Roles of Dopamine and Serotonin in Tooth Repair: Functional Interplay Between Odontogenic Stem Cells and Platelets

Characterizing stem cell intrinsic functions is an ongoing challenge for cell therapies. Here, we report that two independent A4 and H8 stem cell lines isolated from mouse molar pulp display the overall functions of bioaminergic cells. Both clones produce neurotrophins and synthesize, catabolize, store, and transport serotonin (5-hydroxytryptamine [5-HT]) and dopamine (DA). They express 5-HT1D,2B,7 and D1,3 autoreceptors, which render pulpal stem cells competent to respond to circulating 5-HT and DA. We show that injury-activated platelets are the source of systemic 5-HT and DA necessary for dental repair since natural dentin reparation is impaired in two rat models with monoamine storage-deficient blood platelets. Moreover, selective inhibition of either D1, D3, 5-HT2B, or 5-HT7 receptor within the pulp of wild-type rat molars after lesion alters the reparative process. Altogether our data argue that 5-HT and DA coreleased by pulp injury-activated platelets are critical for stem cell-mediated dental repair through 5-HT and DA receptor signalings.

Comparative studies between mice molars and incisors are required to draw an overview of enamel structural complexity

In the field of dentistry, the murine incisor has long been considered as an outstanding model to study amelogenesis. However, it clearly appears that enamel from wild type mouse incisors and molars presents several structural differences. In incisor, exclusively radial enamel is observed. In molars, enamel displays a high level of complexity since the inner part is lamellar whereas the outer enamel shows radial and tangential structures. Recently, the serotonin 2B receptor (5-HT_2BR) was shown to be involved in ameloblast function and enamel mineralization. The incisors from 5HT_2BR knockout (KO) mice exhibit mineralization defects mostly in the outer maturation zone and porous matrix network in the inner zone. In the molars, the mutation affects both secretory and maturation stages of amelogenesis since pronounced alterations concern overall enamel structures. Molars from 5HT_2BR KO mice display reduction in enamel thickness, alterations of inner enamel architecture including defects in Hunter-Schreger Bands arrangements, and altered maturation of the outer radial enamel. Differences of enamel structure were also observed between incisor and molar from other KO mice depleted for genes encoding enamel extracellular matrix proteins. Thus, upon mutation, enamel analysis based exclusively on incisor defects would be biased. In view of the functional relationship between enamel structure and tooth morphogenesis, identification of molecular actors involved in amelogenesis requires comparative studies between mice molars and incisors.

Plithotaxis, a collective cell migration, regulates the sliding of proliferating pulp cells located in the apical niche

Using the proliferating cell nuclear antigen (PCNA) immunostaining, we previously identified, after pulp exposure, three zones of proliferating cells in the rat molar pulp. Zones I and II were in the crown near the pulp. Zone III was near the apex revealing a recruitment of mitotic cells at distance from the lesion. To gain further insight into the spatio-temporal evolution of proliferating pulp cells of zone III, we performed a longitudinal study of PCNA staining in rat molar mesial root at 3, 8, and 15 d after pulp exposure associated to implantation of unloaded or amelogenin loaded agarose beads. At day 3 after implantation, PCNA-positive cells were located in the central part of the radicular pulp. At day 8, PCNA-labeled cells were aligned in the lateral part of the pulp beneath the odontoblast/sub-odontoblast layer. At day 15, PCNA labeling became undetectable in the root and was located in the coronal pulp. These results suggest that after pulp exposure, PCNA-positive cells may migrate from the central part of the radicular pulp to the sub-odontoblast cell layer and then from the apical root to the crown. Electron microscopy and immunostaining analysis showed that pulpal cells were linked by desmosome-like and gap-junctions. Extracellular matrix was composed of thin collagen fibrils associated with glycosaminoglycans favoring cell mobility. These data suggest that the syncytium-like structure formed by pulp radicular cells may be a pre-request for plithotaxis, a collective cell migration process. This emergent mechanism may govern pulp healing and regeneration after dental lesion.

A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor

The cellular prion protein, PrP^C, is a glycosylphosphatidylinositol-anchored protein, abundant in lipid rafts and highly expressed in the brain. While PrP^C is much studied for its involvement under its abnormal PrP^Sc isoform in Transmissible Spongiform Encephalopathies, its physiological role remains unclear. Here, we report that GSK3β, a multifunctional kinase whose inhibition is neuroprotective, is a downstream target of PrP^C signalling in serotonergic neuronal cells. We show that the PrP^C-dependent inactivation of GSK3β is relayed by a caveolin-Lyn platform located on neuronal cell bodies. Furthermore, the coupling of PrP^C to GSK3β potentiates serotonergic signalling by altering the distribution and activity of the serotonin 1B receptor (5-HT_1BR), a receptor that limits neurotransmitter release. In vivo, our data reveal an increased GSK3β kinase activity in PrP-deficient mouse brain, as well as sustained 5-HT_1BR activity, whose inhibition promotes an anxiogenic behavioural response. Collectively, our data unveil a new facet of PrP^C signalling that strengthens neurotransmission.

Deletion of serotonin 2B receptor provokes structural alterations of mouse dental tissues

Rampant caries and periodontal diseases occur in patients treated with antidepressants such as serotonin reuptake inhibitors (SRIs; e.g., Prozac) which target the serotonin transporter (SERT). As the serotonin 2B receptor (5HT2BR) regulates SERT functionality and capacity to recognize SRIs, we investigated the potential role of 5HT2BR on dental tissues by exploiting 5HT2BR knockout (KO) mice. Compared to wild-type (WT) mice, several structural differences were identified in the teeth of KO mice. In the molar of KO mice, rod curvatures and twisting were altered compared to WT mice, suggesting involvement of 5HT2BR at early stages of enamel formation. The volume of the KO enamel layer was also reduced, and larger porosities were observed in the prismatic enamel, with smaller crystallite thickness. Crystallite pattern disorganization and occlusal abrasion were enhanced in female KO mice, indicating a sexual dimorphism. In the incisor, no difference was detected in the width of the enamel layer between KO and WT mice; however, enamel maturation differed in absence of 5HT2BR. Specifically, the outer aprismatic enamel border was 1.5- to 2-fold larger in KO compared to WT mice, together with a decreased etching pattern. Finally, although no noticeable difference was observed in dentin, the micro-CT three-dimensional pulp reconstruction evidenced a decrease in both length and width of dentin formation in the root canals of the KO versus WT mice. These data provide evidence that 5HT2BR-mediated signaling pathways are involved in enamel formation and dentinogenesis.

Enamel alterations in serotonin 2B receptor knockout mice

The role of the serotonin 2B receptor (5-HT(2B) R) in enamel formation and mineralization was explored in adult 5HT(2B) R knockout (KO) mice compared with wild-type (WT) mice. In the molar, quantitative data obtained by micro-computed tomography imaging showed that the overall volume of the enamel layer was firmly reduced in KO mice. Defective mineralization was ascertained by energy-dispersive X-ray microanalysis. We also observed, using scanning electron microscopy, that parazones in the KO mice included two or three helicoidally twisted rods within Hunter-Schreger bands, instead of a single rod, as found in the WT mice. Minor disturbances were also detected in the incisors of KO mice. Structural modifications, thinner enamel crystallites, and porosities observed in KO mice indicate that the 5-HT(2B) R-mediated signaling pathways as part of the enamel formation process. These data provide a basis for evaluating the role of 5-HT(2B) R in ameloblast functions. Defects observed in the mineralization and structure of enamel in KO mice highlight that the 5-HT(2B) R interferes with the mechanisms directing amelogenesis.

Concomitant multipotent and unipotent dental pulp progenitors and their respective contribution to mineralised tissue formation

Upon in vitro induction or in vivo implantation, the stem cells of the dental pulp display hallmarks of odontoblastic, osteogenic, adipogenic or neuronal cells. However, whether these phenotypes result from genuine multipotent cells or from coexistence of distinct progenitors is still an open question. Furthermore, determining whether a single cell-derived progenitor is capable of undergoing a differentiation cascade leading to tissue repair in situ is important for the development of cell therapy strategies. Three clonal pulp precursor cell lines (A4, C5, H8), established from embryonic ED18 first molars of mouse transgenic for a recombinant plasmid adeno-SV40, were induced to differentiate towards the odonto/osteogenic, chondrogenic or adipogenic programme. Expression of phenotypic markers of each lineage was evaluated by RT-PCR, histochemistry or immunocytochemistry. The clones were implanted into mandibular incisors or calvaria of adult mice. The A4 clone was capable of being recruited towards at least 3 mesodermal lineages in vitro and of contributing to dentin-like or bone formation, in vivo, thus behaving as a multipotent cell. In contrast, the C5 and H8 clones displayed a more restricted potential. Flow cytometric analysis revealed that isolated monopotent and multipotent clones could be distinguished by a differential expression of CD90. Altogether, isolation of these clonal lines allowed demonstrating the coexistence of multipotential and restricted-lineage progenitors in the mouse pulp. These cells may further permit unravelling specificities of the different types of pulp progenitors, hence facilitating the development of cell-based therapies of the dental pulp or other cranio-facial tissues.

Raphe-mediated signals control the hippocampal response to SRI antidepressants via miR-16

Serotonin reuptake inhibitor (SRI) antidepressants such as fluoxetine (Prozac), promote hippocampal neurogenesis. They also increase the levels of the bcl-2 protein, whose overexpression in transgenic mice enhances adult hippocampal neurogenesis. However, the mechanisms underlying SRI-mediated neurogenesis are unclear. Recently, we identified the microRNA miR-16 as an important effector of SRI antidepressant action in serotonergic raphe and noradrenergic locus coeruleus (LC). We show here that miR-16 mediates adult neurogenesis in the mouse hippocampus. Fluoxetine, acting on serotonergic raphe neurons, decreases the amount of miR-16 in the hippocampus, which in turn increases the levels of the serotonin transporter (SERT), the target of SRI, and that of bcl-2 and the number of cells positive for Doublecortin, a marker of neuronal maturation. Neutralization of miR-16 in the hippocampus further exerts an antidepressant-like effect in behavioral tests. The fluoxetine-induced hippocampal response is relayed, in part, by the neurotrophic factor S100β, secreted by raphe and acting via the LC. Fluoxetine-exposed serotonergic neurons also secrete brain-derived neurotrophic factor, Wnt2 and 15-Deoxy-delta12,14-prostaglandin J2. These molecules are unable to mimic on their own the action of fluoxetine and we show that they act synergistically to regulate miR-16 at the hippocampus. Of note, these signaling molecules are increased in the cerebrospinal fluid of depressed patients upon fluoxetine treatment. Thus, our results demonstrate that miR-16 mediates the action of fluoxetine by acting as a micromanager of hippocampal neurogenesis. They further clarify the signals and the pathways involved in the hippocampal response to fluoxetine, which may help refine therapeutic strategies to alleviate depressive disorders.

miR-16 targets the serotonin transporter: a new facet for adaptive responses to antidepressants

The serotonin transporter (SERT) ensures the recapture of serotonin and is the pharmacological target of selective serotonin reuptake inhibitor (SSRI) antidepressants. We show that SERT is a target of microRNA-16 (miR-16). miR-16 is expressed at higher levels in noradrenergic than in serotonergic cells; its reduction in noradrenergic neurons causes de novo SERT expression. In mice, chronic treatment with the SSRI fluoxetine (Prozac) increases miR-16 levels in serotonergic raphe nuclei, which reduces SERT expression. Further, raphe exposed to fluoxetine release the neurotrophic factor S100β, which acts on noradrenergic cells of the locus coeruleus. By decreasing miR-16, S100β turns on the expression of serotonergic functions in noradrenergic neurons. Based on pharmacological and behavioral data, we propose that miR-16 contributes to the therapeutic action of SSRI antidepressants in monoaminergic neurons.

Serotonergic 5-HT(2B) receptor controls tissue-nonspecific alkaline phosphatase activity in osteoblasts via eicosanoids and phosphatidylinositol-specific phospholipase C

In previous studies, we observed that mice knocked out for the serotonin-2B receptor (5-HT(2B)R) show defects in bone homeostasis. The present work focuses on the downstream targets relaying the anabolic function of this receptor in osteoblasts. A functional link between the 5-HT(2B)R and the activity of the tissue-nonspecific alkaline phosphatase (TNAP) is established using the C1 osteoprogenitor cell line. During C1 osteogenic differentiation, both 5-HT(2B)R and TNAP mRNA translations are delayed with respect to extracellular matrix deposition. Once the receptor is expressed, it constitutively controls TNAP activity at a post-translational level along the overall period of mineral deposition. Indeed, pharmacological inhibition of the 5-HT(2B)R intrinsic activity or shRNA-mediated 5-HT(2B)R knockdown prevents TNAP activation, but not its mRNA translation. In contrast, agonist stimulation of the receptor further increases TNAP activity during the initial mineralization phase. Building upon our previous observations that the 5-HT(2B)R couples with the phospholipase A2 pathway and prostaglandin production at the beginning of mineral deposition, we show that the 5-HT(2B)R controls leukotriene synthesis via phospholipase A2 at the terminal stages of C1 differentiation. These two 5-HT(2B)R-dependent eicosanoid productions delineate distinct time windows of TNAP regulation during the osteogenic program. Finally, prostaglandins or leukotrienes are shown to relay the post-translational activation of TNAP via stimulation of the phosphatidylinositol-specific phospholipase C. In agreement with the above findings, primary calvarial osteoblasts from 5-HT(2B)R-null mice exhibit defects in TNAP activity.

The cellular prion protein interacts with the tissue non-specific alkaline phosphatase in membrane microdomains of bioaminergic neuronal cells

Background

The cellular prion protein, PrP^C, is GPI anchored and abundant in lipid rafts. The absolute requirement of PrP^C in neurodegeneration associated to prion diseases is well established. However, the function of this ubiquitous protein is still puzzling. Our previous work using the 1C11 neuronal model, provided evidence that PrP^C acts as a cell surface receptor. Besides a ubiquitous signaling function of PrP^C, we have described a neuronal specificity pointing to a role of PrP^C in neuronal homeostasis. 1C11 cells, upon appropriate induction, engage into neuronal differentiation programs, giving rise either to serotonergic (1C11^5-HT) or noradrenergic (1C11^NE) derivatives.

Methodology/Principal Findings

The neuronal specificity of PrP^C signaling prompted us to search for PrP^C partners in 1C11-derived bioaminergic neuronal cells. We show here by immunoprecipitation an association of PrP^C with an 80 kDa protein identified by mass spectrometry as the tissue non-specific alkaline phosphatase (TNAP). This interaction occurs in lipid rafts and is restricted to 1C11-derived neuronal progenies. Our data indicate that TNAP is implemented during the differentiation programs of 1C11^5-HT and 1C11^NE cells and is active at their cell surface. Noteworthy, TNAP may contribute to the regulation of serotonin or catecholamine synthesis in 1C11^5-HT and 1C11^NE bioaminergic cells by controlling pyridoxal phosphate levels. Finally, TNAP activity is shown to modulate the phosphorylation status of laminin and thereby its interaction with PrP.

Conclusion/Significance

The identification of a novel PrP^C partner in lipid rafts of neuronal cells favors the idea of a role of PrP in multiple functions. Because PrP^C and laminin functionally interact to support neuronal differentiation and memory consolidation, our findings introduce TNAP as a functional protagonist in the PrP^C-laminin interplay. The partnership between TNAP and PrP^C in neuronal cells may provide new clues as to the neurospecificity of PrP^C function.

PKBα is required for adipose differentiation of mouse embryonic fibroblasts

Protein kinase Bα (PKBα) is a key regulator of metabolism, proliferation and differentiation. We have explored the role of PKBα in adipogenesis using wild-type and PKBα-knockout mouse embryonic fibroblasts (MEFs) and show that lack of PKBα prevents MEF differentiation into adipocytes. Expression of ectopic PKBα in PKBα-deficient cells restores adipogenesis. We identified 80 genes whose expression was upregulated in wild-type MEFs during adipogenesis but whose expression was significantly reduced in PKBα-deficient MEFs under the same conditions. Significantly, the regulator of adipogenesis Krüppel-like transcription factor 15 gene expression was downregulated in PKBα-deficient MEFs but could be restored by expressing an active PKBα in the deficient cells. The level of lipocalin 2, renin 1 and receptor-activity-modifying protein 3 genes expressed by adipose cells was also decreased in PKBα-deficient MEFs, and are inhibited by LY294002 treatment during early adipocyte differentiation of 3T3-L1 cells. The results underscore an essential role for PKBα in the transcriptional program required for adipogenesis.

Dosage-dependent effects of Akt1/protein kinase Balpha (PKBalpha) and Akt3/PKBgamma on thymus, skin, and cardiovascular and nervous system development in mice

Akt/protein kinase B (PKB) plays a critical role in the regulation of metabolism, transcription, cell migration, cell cycle progression, and cell survival. The existence of viable knockout mice for each of the three isoforms suggests functional redundancy. We generated mice with combined mutant alleles of Akt1 and Akt3 to study their effects on mouse development. Here we show that Akt1-/- Akt3+/- mice display multiple defects in the thymus, heart, and skin and die within several days after birth, while Akt1+/- Akt3-/- mice survive normally. Double knockout (Akt1-/-) Akt3-/-) causes embryonic lethality at around embryonic days 11 and 12, with more severe developmental defects in the cardiovascular and nervous systems. Increased apoptosis was found in the developing brain of double mutant embryos. These data indicate that the Akt1 gene is more essential than Akt3 for embryonic development and survival but that both are required for embryo development. Our results indicate isoform-specific and dosage-dependent effects of Akt on animal survival and development.

Physiological functions of protein kinase B/Akt

The genetic manipulation of mice has become an essential and elegant method for studying the function of proteins in physiology, and for testing the veracity of information obtained from cell culture experiments. During the past few years, a variety of transgenic and knockout mouse models of PKB (protein kinase B)/Akt have been generated and investigated. In this paper, we focus on the phenotypes of these PKB/Akt overexpression and mutant mice that may help to elucidate the functions exerted by PKB/Akt in mammals.

First Report of Bacterial Canker of Walnut Caused by Brenneria nigrifluens in France

Since the summer of 2000, vertical oozing cankers have been observed on trunks and branches of Persian walnut trees (Juglans regia). Cvs. Fernor, Chandler, Mayette, and Hartley were the most frequently affected, but cvs. Lara and Franquette could also be affected. Brenneria nigrifluens (synonym Erwinia nigrifluens) (3) was isolated from diseased trees from 13 orchards and nurseries in southwestern (Aquitaine, Périgord, Charentes, and Quercy), southeastern (Grenoble areas), and western (near Angers) France. Cankers were observed on trunks and branches where brown-to-black exudates staining the bark appeared mainly in the summer. Isolations were performed primarily from exudates but also from infected tissues by using King's medium B. Colonies similar in appearance to Brenneria nigrifluens (1) were purified and characterized. Gram reaction, Kovac's oxidase, oxidative-fermentative metabolism, and urease activity were assayed for all isolates. API Biotype 100 kits (BioMérieux, Marcy l'Etoile, France) were used as recommended, except that incubations were at 28°C for 4 days. When compared with the reference strain (French Collection of Plant Pathogenic Bacteria (CFBP) 4998^T = National Collection of Plant Pathogenic Bacteria (NCPPB) 564^T = American Type Culture Collection (ATCC) 13028^T) from California, 14 isolated strains were identified as B. nigrifluens on the basis of physiological and biochemical characteristics. These 14 strains were deposited in the CFBP under Accession Nos. 6746 to 6759. Pathogenicity of three selected strains (CFBP 6746, 6747, and 6758) was confirmed by inoculating branches of 7-year-old walnut trees with 10⁸ CFU of each isolate introduced in wounds (2). The reference strain (CFBP 4998^T) and water were similarly inoculated as controls. Two and five months later, necrotic lesions were observed in the inner bark and dark lines were observed in internal wood, but no external cankers were observed on any trees inoculated with the local and reference strains. B. nigrifluens was reisolated from the dark lines in internal wood up to approximately 10 cm from the inoculation site. To our knowledge, this is the first report of this bacterium in France. References: (1) L. Hauben et al. Syst. Appl. Microbiol. 21:384, 1998. (2) M. Ridé and S. Ridé. Proc. Int. Conf. Plant Pathogenic Bacteria, 4th, 2:957, 1978. (3) E. E. Wilson et al. Phytopathology 47:669, 1957.

Partial rescue of insulin receptor-deficient mice by transgenic complementation with an activated insulin receptor in the liver

Insulin receptor (IR)-deficient mice develop severe diabetes mellitus, diabetic ketoacidosis (DKA) and liver steatosis and die within 1 week after birth. We examined in this work whether the metabolic phenotype of IR(-/-) mutants could be improved by transgenic complementation with IR selectively in the liver. We first generated transgenic mice expressing a human DNA complementary to RNA encoding a truncated constitutively activated form of IR (IRdelta) under the control of liver-specific phenylalanine hydroxylase (PAH) gene promoter. These mice presented more pronounced fasting hypoglycemia and showed slightly improved glucose tolerance as compared to controls. The transgenic mice were crossed with IR(+/-) mutants to generate IR(-/-) mice carrying the PAH-IRDelta transgene. Although such mutants developed glycosuria, DKA was delayed by more than 1 week and survival was prolonged to 8-20 days in approximately 10% of mice. In these partially rescued pups, serum glucose and triglyceride levels were lowered, hepatic glycogen stores were reconstituted and liver steatosis was absent as compared with pups which developed strong DKA and died earlier. Thus, lack of insulin action in the liver is responsible in large part for the metabolic disorders seen in IR(+/-) mice. This study should stimulate interest in therapeutic strategies aimed at improving hepatic function in diabetes.

Improved metabolic disorders of insulin receptor-deficient mice by transgenic overexpression of glucokinase in the liver

AIMS/HYPOTHESIS

Insulin receptor null mutant mice develop severe diabetes, ketoacidosis and liver steatosis and die within 1 week after birth. Since the liver plays an essential role in the control of glucose homeostasis, we examined in this work whether the metabolic disorders of insulin receptor-deficient mice could be improved upon restoration of hepatic glucose metabolism by transgenic constitutive overexpression of glucokinase selectively in the liver.

METHODS

We first generated transgenic mice overexpressing rat glucokinase cDNA under control of the liver-specific phenylalanine hydroxylase gene promoter. These transgenic mice were crossed with heterozygous insulin-receptor-null mutants to produce homozygous insulin-receptor-null mice overexpressing glucokinase in the liver.

RESULTS

The transgenic mice overexpressing glucokinase in the liver showed improved glucose tolerance and were mildly hypoglycaemic and hyperlipidaemic under starved conditions. The introduction of the glucokinase transgene in insulin receptor null mice did not prevent the development of glycosuria. However, ketoacidosis was delayed by more than 1 week and survival was prolonged to 10 to 16 days in 16% of the pups. In these longer surviving pups, serum glucose and triglyceride concentrations were lowered, hepatic glycogen stores were reconstituted and liver steatosis was absent as compared with the pups which had developed strong ketoacidosis and died earlier.

CONCLUSIONS/INTERPRETATION

These results show that overexpression of hepatic glucokinase can compensate, in part, for the metabolic disorders developed by insulin receptor-deficient mice. This shows the importance of improving hepatic function in diabetes and must revive interest in enhancement of glucokinase activity as a therapeutic strategy for the treatment of diabetes.

Genetic manipulation of insulin signaling, action and secretion in mice. Insights into glucose homeostasis and pathogenesis of type 2 diabetes

Non-insulin-dependent diabetes mellitus (NIDDM) is a complex heterogeneous polygenic disease characterized mainly by insulin resistance and pancreatic beta-cell dysfunction. In recent years, several genetically engineered mouse models have been developed for the study of the pathophysiological consequences of defined alterations in a single gene or in a set of candidate diabetogenes. These represent new tools that are providing invaluable insights into NIDDM pathogenesis. In this review, we highlight the lessons emerging from the study of some of the transgenic or knockout mice in which the expression of key actors in insulin signaling, action or secretion has been manipulated. In addition to contributing to our knowledge of the specific roles of individual genes in the control of glucose homeostasis, these studies have made it possible to address several crucial issues in NIDDM that have remained controversial or unanswered for a number of years.

Endocrine pancreas in insulin receptor-deficient mouse pups

Insulin receptor (IR)-deficient pups rapidly become hyperglycemic and hyperinsulinemic and die of diabetic ketoacidosis within a few days. Immunocytochemical analysis of the endocrine pancreas revealed that IR deficiency did not alter islet morphology or the number of beta-, alpha-, delta-, and pancreatic polypeptide (PP) cells. The lack of IR did not result in major changes in the expression of islet hormone genes or of beta-cell-specific marker genes encoding pancreas duodenum homeobox-containing transcription factor-1 (PDX-1), glucokinase (GCK), and GLUT2, as shown by reverse transcriptase-polymerase chain reaction analysis. The serum glucagon levels in IR-deficient and nondiabetic littermates were comparable. Finally, total insulin content in the pancreas of IR-deficient pups was gradually depleted, indicating sustained insulin secretion, not compensated for by increased insulin biosynthesis. These findings are discussed in light of recent results suggesting a role of IR in beta-cell function.

IGF-1 receptor as an alternative receptor for metabolic signaling in insulin receptor-deficient muscle cells

We have derived skeletal muscle cell lines from wild-type (wt) and insulin receptor (IR) knockout mice to unravel the metabolic potential of IGF-1 receptor (IGF-1R). Both wt and IR(-/-) myoblasts differentiated into myotubes with similar patterns of expression of muscle-specific genes such as MyoD, myogenin and MLC1A indicating that IR is not required for this process. Binding of 125I-IGF-1 on wt and IR(-/-) myotubes was similar showing that IGF-1R was not upregulated in the absence of IR. Stimulation of IR(-/-) myotubes with IGF-1 (10(-10) to 10(-7) M) increased glucose uptake and incorporation into glycogen, induced IRS-1 phosphorylation and activated PI 3-kinase and MAP kinase, two enzymes of major signaling pathways. These effects were comparable to those obtained with wt myotubes using insulin or IGF-1 or with IR(-/-) myotubes using insulin at higher concentrations. This study provides a direct evidence that IGF-1R can represent an alternative receptor for metabolic signaling in muscle cells.

Genetic engineering in mice: impact on insulin signalling and action

The expression of a number of genes encoding key players in insulin signalling and action, including insulin, insulin receptor (IR), downstream signalling molecules such as insulin receptor substrate-1 (IRS-1) and IRS-2, glucose transporters (GLUT4, GLUT2) and important metabolic enzymes such as glucokinase, has now been altered in transgenic or knockout mice. Such mice presented with phenotypes ranging from mild defects, revealing complementarity between key molecules or pathways, to severe diabetes with ketoacidosis and early postnatal death. Insulin action could also be improved by overproduction of proteins acting at regulatory steps. The development of diabetes by combining mutations, which alone do not lead to major metabolic alterations, validated the 'diabetogenes' concept of non-insulin-dependent diabetes mellitus. Genes encoding insulin-like growth factors (IGF-I and IGF-II) and their type I receptor (IGF-IR) have also been disrupted. It appears that although IR and IGF-IR are both capable of metabolic and mitogenic signalling, they are not fully redundant. However, IR could replace IGF-IR if efficiently activated by IGF-II. Studies with cell lines lacking IR or IGF-IR lend support to such conclusions. Concerning the issues of specificity and redundancy, studies with cell lines derived from IRS-1-deficient mice showed that IRS-1 and IRS-2 are also not completely interchangeable.

Genetic manipulation of insulin action and beta-cell function in mice

Transgenic and gene targeting approaches have now been applied to a number of genes in order to investigate the metabolic disorders that would result by manipulating insulin action or pancreatic beta-cell function in the mouse. The availability of such mutant mice will allow in the future to develop animal models in which the pathophysiologies resulting from polygenic defects might be reconstituted and studied in detail. Such animal models hopefully will lead to better understanding of complex polygenic diseases such as non-insulin-dependent diabetes mellitus (NIDDM).

Insulin receptor-deficient cells as a new tool for dissecting complex interplay in insulin and insulin-like growth factors

Cell systems derived from knockout mice for the insulin receptor (IR) or the IGF-1 receptor (IGF-1R) represent unique tools for dissecting complex interplay in the actions of insulin and insulin-like growth factors through their cognate versus non-cognate receptor. In this study, we used a fibroblast cell line derived from IR-deficient mice to investigate metabolic and mitogenic effects of IGF-1 and insulin. IGF-1 was able to stimulate glucose uptake, glucose incorporation into glycogen and thymidine incorporation in such cells. Phosphatidylinositol 3-kinase and mitogen-activated protein kinase, two enzymes of major metabolic-mitogenic signaling pathways, were activated upon stimulating these cells with IGF-1. All these effects were also achieved when IR-deficient cells were stimulated with insulin. Thus, IGF-1R can represent an alternative receptor through which insulin might exert some of its effects.