Gérard Brunel

Absence of the lysophosphatidic acid receptor LPA1 results in abnormal bone development and decreased bone mass

Lysophosphatidic acid (LPA) is a lipid mediator that acts in paracrine systems via interaction with a subset of G protein-coupled receptors (GPCRs). LPA promotes cell growth and differentiation, and has been shown to be implicated in a variety of developmental and pathophysiological processes. At least 6 LPA GPCRs have been identified to date: LPA1-LPA6. Several studies have suggested that local production of LPA by tissues and cells contributes to paracrine regulation, and a complex interplay between LPA and its receptors, LPA1 and LPA4, is believed to be involved in the regulation of bone cell activity. In particular, LPA1 may activate both osteoblasts and osteoclasts. However, its role has not as yet been examined with regard to the overall status of bone in vivo. We attempted to clarify this role by defining the bone phenotype of LPA1((-/-)) mice. These mice demonstrated significant bone defects and low bone mass, indicating that LPA1 plays an important role in osteogenesis. The LPA1((-/-)) mice also presented growth and sternal and costal abnormalities, which highlights the specific roles of LPA1 during bone development. Microcomputed tomography and histological analysis demonstrated osteoporosis in the trabecular and cortical bone of LPA1((-/-)) mice. Finally, bone marrow mesenchymal progenitors from these mice displayed decreased osteoblastic differentiation. These results suggest that LPA1 strongly influences bone development both qualitatively and quantitatively and that, in vivo, its absence results in decreased osteogenesis with no clear modification of osteoclasis. They open perspectives for a better understanding of the role of the LPA/LPA1 paracrine pathway in bone pathophysiology.

TLR2 sensing of F. nucleatum and S. sanguinis distinctly triggered gingival innate response

Gingival tissue faces constant exposure to micro-organisms. It functions as part of the host response, an anti-microbial barrier that recognizes and discriminates between commensal and pathogenic bacteria. This study aimed to evaluate and compare the effects of cell wall extracts from different periodontal bacteria, commensals Streptococcus sanguinis and Fusobacterium nucleatum and the pathogen Porphyromonas gingivalis, on the innate immune response of gingival keratinocytes and the role of TLR2 in regulating this. We assayed mRNA levels to determine the expression of human beta-defensins (hbetaD2, hbetaD3), interleukin-1alpha, -1beta, 6 and 8 and matrix metalloproteinase-9. F. nucleatum extracts induced beta-defensin and inflammatory marker mRNA expression at higher levels than P. gingivalis. Extracts from the Gram-positive commensal S. sanguinis did not upregulate the host response. TLR2 extinction inhibited the upregulation of beta-defensin and cytokine transcripts by F. nucleatum extracts but, in contrast, led to a weak induction of hbetaD3 after challenge with S. sanguinis extracts. Although F. nucleatum strongly induces innate immune and inflammatory mediators, S. sanguinis limits their expression through TLR2. Together, our data demonstrate that gingival keratinocytes recognize and discriminate between Gram-positive and Gram-negative commensal extracts, in part through TLR2, to activate different signaling pathways of the innate immune host response.

Antimicrobial peptide modulation in a differentiated reconstructed gingival epithelium

Gingival innate immunity has been studied by using biopsies and normal or transformed epithelial cell monolayers. To overcome individual biological variabilities and as a physiological alternative, we have proposed using a reconstructed tissue equivalent. In this study, we investigated the functionality and the stage of differentiation of a reconstructed human gingival epithelium. We also characterized this epithelium at the molecular level to investigate its differentiation stage compared with native human gingival epithelium. The expression levels and localization of markers related to proteins and lipids of well-differentiated stratified epithelium, such as cytokeratins, cornified envelope proteins and enzymes, or to factors in lipid synthesis and trafficking were examined. Immunohistochemistry revealed similar localization patterns in both types of epithelia and mRNA quantification showed a close resemblance of their expression profiles. We further revealed that, like native gingiva, reconstructed gingival epithelium was able to respond to pro-inflammatory or lipopolysaccharide stimuli by producing antimicrobial peptides hβD-2, hβD-3 or LL-37. Finally, we demonstrated that reconstructed human gingival epithelium, as a model, was good enough to be proposed as a functional equivalent for native human gingival epithelium in order to study the regulation of gingival innate immunity against periodontal infections.

Impact of Calcium Phosphate Particle Morphology on Osteoconduction: an In Vivo Study

Apatite/β−TCP particles exhibiting non-conventional urchin-like morphology were prepared by hydrothermal synthesis. Their implantation in the rat calvarium was followed during 60 days. A total absence of osteoconduction was observed despite a favorable chemical composition, stressing the fundamental role of particle morphology on bone regeneration. Results are discussed in relation with other literature data. Possible explanations include the disfavored accumulation of biological mediators due to the acicular shape of the particles and/or a limited accessibility for cells.