Regulation of human mesenchymal stem cell differentiation by TREM-2

Recent Advances in Hydrogel Technology in Delivering Mesenchymal Stem Cell for Osteoarthritis Therapy

Osteoarthritis (OA), a chronic joint disease affecting over 500 million individuals globally, is characterized by the destruction of articular cartilage and joint inflammation. Conventional treatments are insufficient for repairing damaged joint tissue, necessitating novel therapeutic approaches. Mesenchymal stem cells (MSCs), with their potential for differentiation and self-renewal, hold great promise as a treatment for OA. However, challenges such as MSC viability and apoptosis in the ischemic joint environment hinder their therapeutic effectiveness. Hydrogels with biocompatibility and degradability offer a three-dimensional scaffold that support cell viability and differentiation, making them ideal for MSC delivery in OA treatment. This review discusses the pathological features of OA, the properties of MSCs, the challenges associated with MSC therapy, and methods for hydrogel preparation and functionalization. Furthermore, it highlights the advantages of hydrogel-based MSC delivery systems while providing insights into future research directions and the clinical potential of this approach.

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

The role of toll-like receptors in orchestrating osteogenic differentiation of mesenchymal stromal cells and osteoimmunology

Osteoimmunology is a concept involving molecular and cellular crosstalk between the skeletal and immune systems. Toll-like receptors (TLRs) are widely expressed both on mesenchymal stromal cells (MSCs), the hematopoietic cells, and immune cells in the osteogenic microenvironment for bone development or repair. TLRs can sense both exogenous pathogen-associated molecular patterns (PAMPs) derived from microorganisms, and damage-associated molecular patterns (DAMPs) derived from normal cells subjected to injury, inflammation, or cell apoptosis under physiological or pathological conditions. Emerging studies reported that TLR signaling plays an important role in bone remodeling by directly impacting MSC osteogenic differentiation or osteoimmunology. However, how to regulate TLR signaling is critical and remains to be elucidated to promote the osteogenic differentiation of MSCs and new bone formation for bone tissue repair. This review outlines distinct TLR variants on MSCs from various tissues, detailing the impact of TLR pathway activation or inhibition on MSC osteogenic differentiation. It also elucidates TLR pathways' interplay with osteoclasts, immune cells, and extracellular vesicles (EVs) derived from MSCs. Furthermore, we explore biomaterial-based activation to guide MSCs' osteogenic differentiation. Therefore, understanding TLRs' role in this context has significant implications for advancing bone regeneration and repair strategies.

Tanshinone ΙΙA-Incubated Mesenchymal Stem Cells Inhibit Lipopolysaccharide-Induced Inflammation of N9 Cells through TREM2 Signaling Pathway

Our previous study found that incubating mesenchymal stem cells (MSC) with tanshinone IIA (TIIA) before transplantation could significantly increase the inhibitory effect of MSC on neuroinflammation. Here, we investigated the possible mechanism of this effect. N9 cells and MSC were inoculated at a ratio of 1 : 1 into a Transwell coculture system. MSC were inoculated into the upper chamber, and N9 cells were inoculated into the lower chamber. In this experiment, N9 cells were treated with 1 μg/mL lipopolysaccharide (LPS) for 24 hours to induce inflammation, MSC were treated with 10 μM TIIA for 48 hours to prepare TIIA-incubated MSC (TIIA-MSC), and TREM2 siRNA was used to silence the TREM2 gene in MSC. The changes in IL-1β, IL-6, and TNF-α were evaluated by Western blotting. We found that LPS significantly increased the levels of IL-1β, IL-6, and TNF-α. While both MSC and TIIA-MSC downregulated the levels of (P = 0.092, P = 0.002), IL-6 (P = 0.014, P < 0.001), and TNF-α (P = 0.044, P = 0.003), TIIA-MSC downregulated IL-6 more significantly (P = 0.026). In addition, silencing TREM2 reduced the ability of TIIA-MSC to attenuate IL-6 (P = 0.005) and TNF-α (P = 0.033). These data suggest that the enhanced anti-inflammatory effect of TIIA-MSC on LPS-induced N9 cells may be mediated through the TREM2 signaling pathway.

Functional differences of Toll-like receptor 4 in osteogenesis, adipogenesis and chondrogenesis in human bone marrow-derived mesenchymal stem cells

Multipotent human bone marrow-derived mesenchymal stem cells (hMSCs) are promising candidates for bone and cartilage regeneration. Toll-like receptor 4 (TLR4) is expressed by hMSCs and is a receptor for both exogenous and endogenous danger signals. TLRs have been shown to possess functional differences based on the species (human or mouse) they are isolated from therefore, the effects of knockdown of TLR4 were evaluated in humans during the differentiation of MSCs into bone, fat and chondrocyte cells in vitro. We investigated the expression profile of TLR4 during the differentiation of hMSCs into three different lineages on days 7, 14 and 21 and assessed the differentiation potential of the cells in the presence of lipopolysaccharide (LPS, as an exogenous agonist) and fibronectin fragment III-1c (FnIII-1c, as an endogenous agonist). TLR4 expression increased following the induction of hMSC differentiation into all three lineages. Alkaline phosphatase activity revealed that FnIII-1c accelerated calcium deposition on day 7, whereas LPS increased calcium deposition on day 14. Chondrogenesis increased in the presence of LPS; however, FnIII-1c acted as a reducer in the late stage. TLR4 silencing led to decreased osteogenesis and increased adipogenesis. Furthermore, Wnt5a expression was inversely related to chondrogenesis during the late stage of differentiation. We suggest that understanding the functionality of TLR4 (in the presence of pathogen or stress signal) during the differentiation of hMSCs into three lineages would be useful for MSC-based treatments.

TLR activation inhibits the osteogenic potential of human periodontal ligament stem cells through Akt signaling in a Myd88- or TRIF-dependent manner

BACKGROUND

This study investigated the effects of Toll-like receptors (TLRs) on human periodontal ligament stem cells (hPDLSCs) osteogenic differentiation and the associated mechanisms.

METHODS

TLR1, TLR3, TLR4, and TLR6 expression in hPDLSCs was evaluated by real-time reverse transcriptase polymerase chain reaction (RT-PCR) and flow cytometry, whereas their functional roles were assessed based on nuclear factor (NF)-κB activation and proinflammatory cytokine expression. The osteogenic effects of these TLRs were analyzed by alkaline phosphatase (ALP) staining, ALP activity, and alizarin red staining. The roles of Myd88, TRIF, and downstream molecules mitogen-activated protein kinases (MAPKs) and protein kinase B (Akt) in TLR-mediated impaired osteogenic differentiation were examined by real-time RT-PCR and western blotting using specific small interfering RNA siRNA and pharmacologic inhibitors. The involvement of Akt activation in restoring TLR1-, 4-, and 6-mediated osteogenic suppression was verified using the Akt activator SC-79.

RESULTS

TLR1, TLR3, TLR4, and TLR6 were highly expressed functionally in hPDLSCs and high doses of TLR ligands inhibited osteogenic potential. Furthermore, blocking Myd88 partly rescued the decrease in osteogenesis mediated by TLR1, TLR4, and TLR6 activation by enhancing Akt phosphorylation; likewise, TRIF suppression partially rescued lipopolysaccharide (LPS)-mediated osteogenic inhibition through ERK and Akt activation. Moreover, Akt activation restored the TLR-mediated inhibition of hPDLSC osteogenic differentiation.

CONCLUSIONS

High doses of TLR1, TLR4, and TLR6 ligands suppress hPDLSC osteogenic differentiation by inhibiting Akt activation through Myd88- or TRIF-dependent signaling pathways. Blocking these adaptors or reactivating Akt could restore the TLR-mediated decrease in hPDLSC osteogenesis, and might be an ideal strategy for periodontitis treatment.

Mesenchymal Stem Cell Functionalization for Enhanced Therapeutic Applications

IMPACT STATEMENT

Culture expansion of MSCs has detrimental effects on various cell characteristics and attributes (e.g., phenotypic changes and senescence), which, in addition to inherent interdonor variability, negatively impact the standardization and reproducibility of their therapeutic potential. The identification of innate distinct functional MSC subpopulations, as well as the description of ex vivo protocols aimed at maintaining phenotypes and enhancing specific functions have the potential to overcome these limitations. The incorporation of those approaches into cell-based therapy would significantly impact the field, as more reproducible clinical outcomes may be achieved.

TREM-2 negatively regulates LPS-mediated inflammatory response in rat bone marrow-derived MSCs

To the best of our knowledge, our previous study demonstrated the expression of triggering receptor expressed on myeloid cells 2 (TREM-2) in human bone marrow mesenchymal stem cells (MSCs) for the first time. However, the inflammation regulatory role of TREM-2 in MSCs remain elusive. The aim of the present study was to investigate the immune regulation and the underlying mechanism of TREM-2 in rat bone marrow MSCs. MSCs were divided into three groups: ^NullMSCs, ^TREM-2MSCs, and ^NormMSCs. TREM-2 was expressed in MSCs at the mRNA and protein level. Following stimulation by lipopolysaccharide (LPS), the gene transcription levels of TREM-2 and inflammatory cytokines were increased. The expression levels of inflammatory cytokines, including tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), in the ^TREM-2MSCs lentiviral vector group were significantly downregulated, and the expression of IL-10 was significantly upregulated compared with the controls. Western blot analysis revealed that TREM-2 downregulated the LPS-induced inflammatory response in MSCs, which was probably associated with regulating AKT serine/threonine kinase and p38 mitogen-activated protein kinase downstream signaling proteins. The results of the current study demonstrated that TREM-2 negatively regulates the LPS-mediated inflammatory response in MSCs suggesting that TREM-2 is a potential target of immune regulation in rat MSCs.

Impact of inflammation on early hematopoiesis and the microenvironment

Steady-state hematopoiesis is maintained by slowly dividing, self-renewing hematopoietic stem cells (HSCs) and their offspring, lineage-specified downstream progenitors in bone marrow (BM). It was previously thought that hematopoietic stresses such as infection or other inflammatory stimuli, are mostly recognized by terminally differentiated immune cells, i.e., front-line defenders at the local site of reaction, and that they produce factors that directly act on hematopoietic stem and progenitors (HSPCs) in BM and subsequently stimulate them to rebuild and sustain the hemato-lymphatic system. However, accumulating evidence now indicates that primitive HSPCs, as well as microenvironmental cells in BM are also able to sense systemically migrating hematopoietic stress signals, and respond by orchestrating on-site hematopoiesis via direct and indirect mechanisms. While inflammation has many beneficial roles in activating the immune system for defense or facilitating tissue repair, it also shows detrimental effects if sustained chronically, i.e., might lead to HSPC damage as bone marrow failure or leukemia. Thus, inflammation requires tight control of initiation and termination in time and space dependent manner.

TREM2 deficiency impairs chemotaxis and microglial responses to neuronal injury

Sequence variations in the triggering receptor expressed on myeloid cells 2 (TREM2) have been linked to an increased risk for neurodegenerative disorders such as Alzheimer's disease and frontotemporal lobar degeneration. In the brain, TREM2 is predominantly expressed in microglia. Several disease-associated TREM2 variants result in a loss of function by reducing microglial phagocytosis, impairing lipid sensing, preventing binding of lipoproteins and affecting shielding of amyloid plaques. We here investigate the consequences of TREM2 loss of function on the microglia transcriptome. Among the differentially expressed messenger RNAs in wild-type and Trem2^-/- microglia, gene clusters are identified which represent gene functions in chemotaxis, migration and mobility. Functional analyses confirm that loss of TREM2 impairs appropriate microglial responses to injury and signals that normally evoke chemotaxis on multiple levels. In an ex vivo organotypic brain slice assay, absence of TREM2 reduces the distance migrated by microglia. Moreover, migration towards defined chemo-attractants is reduced upon ablation of TREM2 and can be rescued by TREM2 re-expression. In vivo, microglia lacking TREM2 migrate less towards injected apoptotic neurons, and outgrowth of microglial processes towards sites of laser-induced focal CNS damage in the somatosensory cortex is slowed. The apparent lack of chemotactic stimulation upon depletion of TREM2 is consistent with a stable expression profile of genes characterizing the homoeostatic signature of microglia.

Mesenchymal Stromal Cells and Toll-Like Receptor Priming: A Critical Review

Mesenchymal Stromal Cells (MSCs) are potential cellular candidates for several immunotherapy purposes. Their multilineage potential and immunomodulatory properties make them interesting tools for the treatment of various immunological diseases. However, depending on the local microenvironment, diverse biological functions of MSCs can be modulated. Indeed, during infections such as obtained following TLR-agonist engagement (called as TLR priming), the phenotype, multilineage potential, hematopoietic support and immunomodulatory capacity of MSCs can present critical changes, which could further affect their therapeutic potential. Thus, for appropriate clinical application of MSCs, it is important to well know and understand these effects in particular during infectious episodes and to find the suitable experimental settings to study that. Pre-stimulation of MSCs with a specific TLR ligand may serve as an effective priming step to modulate one of its function to achieve a desired therapeutic issue.

Inflamm-Aging of Hematopoiesis, Hematopoietic Stem Cells, and the Bone Marrow Microenvironment

All hematopoietic and immune cells are continuously generated by hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) through highly organized process of stepwise lineage commitment. In the steady state, HSCs are mostly quiescent, while HPCs are actively proliferating and contributing to daily hematopoiesis. In response to hematopoietic challenges, e.g., life-threatening blood loss, infection, and inflammation, HSCs can be activated to proliferate and engage in blood formation. The HSC activation induced by hematopoietic demand is mediated by direct or indirect sensing mechanisms involving pattern recognition receptors or cytokine/chemokine receptors. In contrast to the hematopoietic challenges with obvious clinical symptoms, how the aging process, which involves low-grade chronic inflammation, impacts hematopoiesis remains undefined. Herein, we summarize recent findings pertaining to functional alternations of hematopoiesis, HSCs, and the bone marrow (BM) microenvironment during the processes of aging and inflammation and highlight some common cellular and molecular changes during the processes that influence hematopoiesis and its cells of origin, HSCs and HPCs, as well as the BM microenvironment. We also discuss how age-dependent alterations of the immune system lead to subclinical inflammatory states and how inflammatory signaling might be involved in hematopoietic aging. Our aim is to present evidence supporting the concept of “Inflamm-Aging,” or inflammation-associated aging of hematopoiesis.

Triggering receptor expressed on myeloid cells and 5'adenosine monophosphate-activated protein kinase in the inflammatory response: a potential therapeutic target

INTRODUCTION

The events in the cellular and molecular signaling triggered during inflammation mitigate tissue healing. The metabolic check-point control mediated by 5'-adenosine monophosphate-activated protein kinase (AMPK) is crucial for switching the cells into an activated state capable of mediating inflammatory events. The cell metabolism involved in the inflammatory response represents a potential therapeutic target for the pharmacologic management of inflammation. Areas covered: In this article, a critical review is presented on triggering receptor expressed on myeloid cell (TREM) receptors and their role in the inflammatory responses, as well as homeostasis between different TREM molecules and their regulation. Additionally, we discussed the relationship between TREM and AMPK to identify novel targets to limit the inflammatory response. Literature search was carried out from the National Library of Medicine's Medline database (using PubMed as the search engine) and Google Scholar and identified relevant studies up to 30 March 2016 using inflammation, TREM, AMPK, as the key words. Expert commentary: The prevention of phenotype switching of immune cells during inflammation by targeting AMPK and TREM-1 could be beneficial for developing novel management strategies for inflammation and associated complications.