BET bromodomain inhibitors regulate keratinocyte plasticity.

Although most acute skin wounds heal rapidly, non-healing skin ulcers represent an increasing and substantial unmet medical need that urgently requires effective therapeutics. Keratinocytes resurface wounds to re-establish the epidermal barrier by transitioning to an activated, migratory state, but this ability is lost in dysfunctional chronic wounds. Small-molecule regulators of keratinocyte plasticity with the potential to reverse keratinocyte malfunction in situ could offer a novel therapeutic approach in skin wound healing. Utilizing high-throughput phenotypic screening of primary keratinocytes, we identify such small molecules, including bromodomain and extra-terminal domain (BET) protein family inhibitors (BETi). BETi induce a sustained activated, migratory state in keratinocytes in vitro, increase activation markers in human epidermis ex vivo and enhance skin wound healing in vivo. Our findings suggest potential clinical utility of BETi in promoting keratinocyte re-epithelialization of skin wounds. Importantly, this novel property of BETi is exclusively observed after transient low-dose exposure, revealing new potential for this compound class.

Growth factors engineered for super-affinity to the extracellular matrix enhance tissue healing.

Growth factors (GFs) are critical in tissue repair, but their translation to clinical use has been modest. Physiologically, GF interactions with extracellular matrix (ECM) components facilitate localized and spatially regulated signaling; therefore, we reasoned that the lack of ECM binding in their clinically used forms could underlie the limited translation. We discovered that a domain in placenta growth factor-2 (PlGF-2(123-144)) binds exceptionally strongly and promiscuously to ECM proteins. By fusing this domain to the GFs vascular endothelial growth factor-A, platelet-derived growth factor-BB, and bone morphogenetic protein-2, we generated engineered GF variants with super-affinity to the ECM. These ECM super-affinity GFs induced repair in rodent models of chronic wounds and bone defects that was greatly enhanced as compared to treatment with the wild-type GFs, demonstrating that this approach may be useful in several regenerative medicine applications.

Extracellular matrix deposition and scaffold biodegradation in an in vitro three-dimensional model of bone by X-ray computed microtomography.

The development of an in vitro model of bone and the optimization of tools for determining the biological processes occurring during bone repair remains a major goal in the field of bone tissue engineering. Recently, a model based on a three-dimensional co-culture of osteoblasts and osteoclast precursors in Skelite(TM) scaffolds was developed. Although induction of osteoblast and osteoclast differentiation was observed, a complete evaluation of bone deposition and biodegradation processes was missing due to technical limitations. In the current study, both X-ray computed microtomography and histological analysis were used to monitor these two key biological processes in the same in vitro model. Either osteoblasts or a combination of osteoblasts and osteoclasts were seeded on Skelite(TM) scaffolds. Scaffold biodegradation and increased bone deposition together with a more organized extracellular matrix were observed in the co-cultures, highlighting the role of osteoclasts in the determination and regulation of bone deposition. Results confirmed the potential and relevance of co-culturing osteoblasts and osteoclasts to resemble native tissue. The combination of X-ray computed microtomography and histology presented in this study could be useful in future studies for the validation and development of new in vitro culture systems for bone tissue engineering.

Fibronectin binding modulates CXCL11 activity and facilitates wound healing.

Engineered biomatrices offer the potential to recapitulate the regenerative microenvironment, with important implications in tissue repair. In this context, investigation of the molecular interactions occurring between growth factors, cytokines and extracellular matrix (ECM) has gained increasing interest. Here, we sought to investigate the possible interactions between the ECM proteins fibronectin (FN) and fibrinogen (Fg) with the CXCR3 ligands CXCL9, CXCL10 and CXCL11, which are expressed during wound healing. New binding interactions were observed and characterized. Heparin-binding domains within Fg (residues 15-66 of the ß chain, Fg ß15-66) and FN (FNI1-5, but not FNIII12-14) were involved in binding to CXCL10 and CXCL11 but not CXCL9. To investigate a possible influence of FN and Fg interactions with CXCL11 in mediating its role during re-epithelialization, we investigated human keratinocyte migration in vitro and wound healing in vivo in diabetic db/db mice. A synergistic effect on CXCL11-induced keratinocyte migration was observed when cells were treated with CXCL11 in combination with FN in a transmigration assay. Moreover, wound healing was enhanced in full thickness excisional wounds treated with fibrin matrices functionalized with FN and containing CXCL11. These findings highlight the importance of the interactions occurring between cytokines and ECM and point to design concepts to develop functional matrices for regenerative medicine.

Tenascin C promiscuously binds growth factors via its fifth fibronectin type III-like domain.

Tenascin C (TNC) is an extracellular matrix protein that is upregulated during development as well as tissue remodeling. TNC is comprised of multiple independent folding domains, including 15 fibronectin type III-like (TNCIII) domains. The fifth TNCIII domain (TNCIII5) has previously been shown to bind heparin. Our group has shown that the heparin-binding fibronectin type III domains of fibronectin (FNIII), specifically FNIII12-14, possess affinity towards a large number of growth factors. Here, we show that TNCIII5 binds growth factors promiscuously and with high affinity. We produced recombinant fragments of TNC representing the first five TNCIII repeats (TNCIII1-5), as well as subdomains, including TNCIII5, to study interactions with various growth factors. Multiple growth factors of the platelet-derived growth factor (PDGF) family, the fibroblast growth factor (FGF) family, the transforming growth factor beta (TGF-ß) superfamily, the insulin-like growth factor binding proteins (IGF-BPs), and neurotrophins were found to bind with high affinity to this region of TNC, specifically to TNCIII5. Surface plasmon resonance was performed to analyze the kinetics of binding of TNCIII1-5 with TGF-ß1, PDGF-BB, NT-3, and FGF-2. The promiscuous yet high affinity of TNC for a wide array of growth factors, mediated mainly by TNCIII5, may play a role in multiple physiological and pathological processes involving TNC.

Engineering the regenerative microenvironment with biomaterials.

Modern synthetic biomaterials are being designed to integrate bioactive ligands within hydrogel scaffolds for cells to respond and assimilate within the matrix. These advanced biomaterials are only beginning to be used to simulate the complex spatio-temporal control of the natural healing microenvironment. With increasing understanding of the role of growth factors and cytokines and their interactions with components of the extracellular matrix, novel biomaterials are being developed that more closely mimic the natural healing environments of tissues, resulting in increased efficacy in applications of tissue repair and regeneration. Herein, the important aspects of the healing microenvironment, and how these features can be incorporated within innovative hydrogel scaffolds, are presented.

Engineering the growth factor microenvironment with fibronectin domains to promote wound and bone tissue healing.

Although growth factors naturally exert their morphogenetic influences within the context of the extracellular matrix microenvironment, the interactions among growth factors, their receptors, and other extracellular matrix components are typically ignored in clinical delivery of growth factors. We present an approach for engineering the cellular microenvironment to greatly accentuate the effects of vascular endothelial growth factor-A (VEGF-A) and platelet-derived growth factor-BB (PDGF-BB) for skin repair, and of bone morphogenetic protein-2 (BMP-2) and PDGF-BB for bone repair. A multifunctional recombinant fragment of fibronectin (FN) was engineered to comprise (i) a factor XIIIa substrate fibrin-binding sequence, (ii) the 9th to 10th type III FN repeat (FN III9-10) containing the major integrin-binding domain, and (iii) the 12th to 14th type III FN repeat (FN III12-14), which binds growth factors promiscuously, including VEGF-A165, PDGF-BB, and BMP-2. We show potent synergistic signaling and morphogenesis between α5ß1 integrin and the growth factor receptors, but only when FN III9-10 and FN III12-14 are proximally presented in the same polypeptide chain (FN III9-10/12-14). The multifunctional FN III9-10/12-14 greatly enhanced the regenerative effects of the growth factors in vivo in a diabetic mouse model of chronic wounds (primarily through an angiogenic mechanism) and in a rat model of critical-size bone defects (through a mesenchymal stem cell recruitment mechanism) at doses where the growth factors delivered within fibrin only had no significant effects.

The development of tissue-engineered bone of different origin through endochondral and intramembranous ossification following the implantation of mesenchymal stem cells and osteoblasts in a murine model.

The study of host cell recruitment by implanted exogenous cells is one of the novel challenges in tissue engineering. We previously reported the development of tissue-engineered bone deposited by host cells in porous ceramic scaffolds seeded with murine mesenchymal stem cells (MSC) and implanted in immunocompromised mice. To better highlight the contribution of host cells to the development of the engineered tissue and to investigate whether the capacity to recruit host cells was dependent on the donor cell commitment, we implanted ceramic scaffolds seeded with either murine GFP labeled MSC or GFP labeled osteoblasts (OB) into immunocompromised mice. Although we observed formation of bone in all scaffolds, the origin of bone cells and the ossification type were strictly dependent on the nature and commitment of the seeded cells. MSC implants led to formation of bone of host origin through the activation of an endochondral ossification process while an intramembranous ossification directly performed by the seeded cells was observed in OB implants. Moreover, we observed an increased vascularization in MSC implants due to the higher capacity of MSC to recruit host CD31+ endothelial cells. The relationship between this enhanced vascularization and the type of ossification is discussed.

The recruitment of two consecutive and different waves of host stem/progenitor cells during the development of tissue-engineered bone in a murine model.

Angiogenesis plays a central role in bone regeneration, not only for the transport of nutrients, but also for locally directing skeletal stem/progenitor cells. Following ectopic implantation of porous ceramic cubes seeded with mouse GFP-labeled mesenchymal stem cells (MSC) into syngenic mice, we investigated the cascade of events leading to bone formation. Implants harvested at different times were enzymatically digested to generate single-cell suspensions. Recovered cells were sorted to separate GFP+implanted MSC and host recruited GFP- cells. We isolated and characterized two different waves of cells, migrating from the host to the MSC-seeded ceramic. The first migrated cell population, recovered 7 days after implantation, was enriched in CD31+endothelial progenitors, while the second one, recruited at day 11, was enriched in CD146+pericyte-like cells. Both populations were not recruited into the scaffold following implantation of a non-MSC seeded ceramic. Pericyte-like cell mobilization was dependent on the first migrated endothelial cell population. Pericyte-like cells retained properties distinctive of stem cells, such as capacity of performing a high number of in vitro cell divisions and showed an osteogenic potential. Studies on the cross talk between implanted exogenous MSC and resident stem/progenitor cells could open new perspectives for future clinical applications.

Rolly protein (ROLP)-Epb4.1/3: a potential protein-protein interaction relevant for the maintenance of cell adhesion.

We recently described Rolly Protein (ROLP), a small protein synthesized by substrate-adherent cells in a broad range of tissues. In a first set of experiments performed taking advantage of bone forming tibial cartilage as an experimental model we showed that ROLP transcription is associated to cells in an active proliferation state, whereas its downregulation is observed when cell proliferation decreases. Taking advantage of siRNA technology we also documented the expression modulation of some apoptosis-related genes in ROLP-silenced cells. In this work we search for the possible molecular interactors of ROLP by using both the antibody array approach as well as the co-immunoprecipitation approach. Results suggest the occurrence of an interaction of ROLP with Erythrocyte membrane Protein Band 4.1/3 (Epb4.1/3), an oncosuppressor downregulated in tumor development and in metastatic tissues; in addition we report experimental results that keep in line also with a potential interaction of ROLP with other PDZ-containing proteins. We also present experimental evidences supporting a role played by ROLP in cell adhesion thus supporting the existence of a biologically relevant link between ROLP and Epb4.1/3. We here suggest that ROLP might exert its biological role cooperating with Epb4.1/3, a protein that is involved in biological pathways that are often inhibited in tumor metastasis. Given the role of Epb4.1/3 in contrasting cancerogenesis we think that its cooperation with ROLP might be relevant in cancer studies and deserves further investigation.

Osteoblast and osteoclast differentiation in an in vitro three-dimensional model of bone.

There is increasing interest in developing new in vitro tissue models using typical tissue engineering approaches. This study was designed to (1) develop a novel three-dimensional (3D) in vitro model of bone by seeding murine primary osteoblasts and osteoclast precursors on a resorbable porous ceramic scaffold based on silicon-stabilized tricalcium phosphate (Skelite), and (2) investigate bone cell interactions in a 3D environment mimicking an in vivo condition and compare it to traditional two-dimensional (2D) cultures. Murine primary osteoblasts from C57Bl6/J mice and osteoclast precursors from C57Bl/6-Tg(ACTB-EGFP)1Osb/J mice were co-cultured on 3D Skelite scaffolds and on standard plastic culture dishes. The differentiation of these cells in both culture conditions was compared by histology (hematoxylin-eosin staining and polarized light analysis), immunohistochemistry (collagen type I), and gene expression analysis by real-time PCR for Runt-related transcription factor 2, osterix, osteocalcin, cathepsin K, and tartrate resistant acid phosphatase. To analyze and compare bone turnover in 3D and 2D co-cultures, we evaluated the modulation of RANKL and OPG mRNA expression. We observed an enhancement of osteoblast differentiation in the 3D mineralized environment that in turn promoted earlier osteoclast differentiation. In this paper, we also report that the increased osteoblast differentiation in the 3D model led to a deposition of extracellular matrix that faithfully reflected the morphology of bone tissue.

New small nuclear RNA gene-like transcriptional units as sources of regulatory transcripts.

By means of a computer search for upstream promoter elements (distal sequence element and proximal sequence element) typical of small nuclear RNA genes, we have identified in the human genome a number of previously unrecognized, putative transcription units whose predicted products are novel noncoding RNAs with homology to protein-coding genes. By elucidating the function of one of them, we provide evidence for the existence of a sense/antisense-based gene-regulation network where part of the polymerase III transcriptome could control its polymerase II counterpart.

Proliferative arrest and activation of apoptosis related genes in Rolly Protein-silenced cells.

Here we describe a novel small polypeptide expressed in chick embryo and mouse adult tissues referred to as Rolly Protein (Rolp), expressed at the highest levels in tibial cartilage and lung respectively. Investigating its putative role in cartilage differentiation we found that its expression is restricted to proliferative stages consistently with a decreased proliferation rate observed in Rolp-silenced cells. Additional functional studies demonstrate that inhibition of Rolp expression causes a transcription modulation of genes involved in apoptosis. The results here provided strongly suggest an active role of Rolp in the control of cell proliferation and apoptosis.