Intranuclear Actin Structure Modulates Mesenchymal Stem Cell Differentiation

Unraveling the mechanism in l-Caldesmon regulating the osteogenic differentiation of PDLSCs: An innovative perspective

Maxillofacial bone defect is one of the common symptoms in maxillofacial, which affects the function and aesthetics of maxillofacial region. Periodontal ligament stem cells (PDLSCs) are extensively used in bone tissue engineering. The mechanism that regulates the osteogenic differentiation of PDLSCs remains not fully elucidated. Previous studies demonstrated that l-Caldesmon (l-CALD, or CALD1) might be involved in the osteogenic differentiation of PDLSCs. Here, the mechanism by which CALD1 regulates the osteogenic differentiation of PDLSCs is investigated. The osteogenic differentiation of PDLSCs is enhanced with Cald1 knockdown. Whole transcriptome sequencing (RNA-seq) analysis shows that bone morphogenetic proteins (BMP) signaling pathway and Wingless type (Wnt) pathway have significant change with Cald1 knockdown, and the expressions of Wnt-induced secreted protein 1 (WISP1), BMP2, Smad1/5/9, and p-Smad1/5/9 are significantly upregulated, while Glycogen synthase kinase 3β (GSK3β) and p-GSK3β are downregulated. In addition, subcutaneous implantation in nude mice shows that knockdown of Cald1 enhances the osteogenic differentiation of PDLSCs in vivo. Taken together, this study demonstrates that knockdown of Cald1 enhances the osteogenic differentiation of PDLSCs by BMP and Wnt signaling pathways, and provides a novel approach for subsequent clinical treatment.

Nuclear actin structure regulates chromatin accessibility

Polymerized β-actin may provide a structural basis for chromatin accessibility and actin transport into the nucleus can guide mesenchymal stem cell (MSC) differentiation. Using MSC, we show that using CK666 to inhibit Arp2/3 directed secondary actin branching results in decreased nuclear actin structure, and significantly alters chromatin access measured with ATACseq at 24 h. The ATAC-seq results due to CK666 are distinct from those caused by cytochalasin D (CytoD), which enhances nuclear actin structure. In addition, nuclear visualization shows Arp2/3 inhibition decreases pericentric H3K9me3 marks. CytoD, alternatively, induces redistribution of H3K27me3 marks centrally. Such alterations in chromatin landscape are consistent with differential gene expression associated with distinctive differentiation patterns. Further, knockdown of the non-enzymatic monomeric actin binding protein, Arp4, leads to extensive chromatin unpacking, but only a modest increase in transcription, indicating an active role for actin-Arp4 in transcription. These data indicate that dynamic actin remodeling can regulate chromatin interactions.

Epigenetic regulators controlling osteogenic lineage commitment and bone formation

Bone formation and homeostasis are controlled by environmental factors and endocrine regulatory cues that initiate intracellular signaling pathways capable of modulating gene expression in the nucleus. Bone-related gene expression is controlled by nucleosome-based chromatin architecture that limits the accessibility of lineage-specific gene regulatory DNA sequences and sequence-specific transcription factors. From a developmental perspective, bone-specific gene expression must be suppressed during the early stages of embryogenesis to prevent the premature mineralization of skeletal elements during fetal growth in utero. Hence, bone formation is initially inhibited by gene suppressive epigenetic regulators, while other epigenetic regulators actively support osteoblast differentiation. Prominent epigenetic regulators that stimulate or attenuate osteogenesis include lysine methyl transferases (e.g., EZH2, SMYD2, SUV420H2), lysine deacetylases (e.g., HDAC1, HDAC3, HDAC4, HDAC7, SIRT1, SIRT3), arginine methyl transferases (e.g., PRMT1, PRMT4/CARM1, PRMT5), dioxygenases (e.g., TET2), bromodomain proteins (e.g., BRD2, BRD4) and chromodomain proteins (e.g., CBX1, CBX2, CBX5). This narrative review provides a broad overview of the covalent modifications of DNA and histone proteins that involve hundreds of enzymes that add, read, or delete these epigenetic modifications that are relevant for self-renewal and differentiation of mesenchymal stem cells, skeletal stem cells and osteoblasts during osteogenesis.

Cytochalasins as Modulators of Stem Cell Differentiation

Regenerative medicine aims to identify new research strategies for the repair and restoration of tissues damaged by pathological or accidental events. Mesenchymal stem cells (MSCs) play a key role in regenerative medicine approaches due to their specific properties, such as the high rate of proliferation, the ability to differentiate into several cell lineages, the immunomodulatory potential, and their easy isolation with minimal ethical issues. One of the main goals of regenerative medicine is to modulate, both in vitro and in vivo, the differentiation potential of MSCs to improve their use in the repair of damaged tissues. Over the years, much evidence has been collected about the ability of cytochalasins, a large family of 60 metabolites isolated mainly from fungi, to modulate multiple properties of stem cells (SCs), such as proliferation, migration, and differentiation, by altering the organization of the cyto- and the nucleo-skeleton. In this review, we discussed the ability of two different cytochalasins, cytochalasins D and B, to influence specific SC differentiation programs modulated by several agents (chemical or physical) or intra- and extra-cellular factors, with particular attention to human MSCs (hMSCs).

Upregulation of NOR-1 in calcified human vascular tissues: impact on osteogenic differentiation and calcification

Cardiovascular calcification is a significant public health issue whose pathophysiology is not fully understood. NOR-1 regulates critical processes in cardiovascular remodeling, but its contribution to ectopic calcification is unknown. NOR-1 was overexpressed in human calcific aortic valves and calcified atherosclerotic lesions colocalizing with RUNX2, a factor essential for osteochondrogenic differentiation and calcification. NOR-1 and osteogenic markers were upregulated in calcifying human valvular interstitial cells (VICs) and human vascular smooth muscle cells (VSMCs). Gain- and loss-of-function approaches demonstrated that NOR-1 negatively modulates the expression of osteogenic genes relevant for the osteogenic transdifferentiation (RUNX2, IL-6, BMP2, and ALPL) and calcification of VICs. VSMCs from transgenic mice overexpressing NOR-1 in these cells (TgNOR-1VSMC) expressed lower basal levels of osteogenic genes (IL-6, BMP2, ALPL, OPN) than cells from WT littermates, and their upregulation by a high-phosphate osteogenic medium (OM) was completely prevented by NOR-1 transgenesis. Consistently, this was associated with a dramatic reduction in the calcification of both transgenic VSMCs and aortic rings from TgNOR-1VSMC mice exposed to OM. Atherosclerosis and calcification were induce in mice by the administration of AAV-PCSK9D374Y and a high-fat/high-cholesterol diet. Challenged-TgNOR-1VSMC mice exhibited decreased vascular expression of osteogenic markers, and both less atherosclerotic burden (assessed in whole aorta and lesion size in aortic arch and brachiocephalic artery) and less vascular calcification (assessed either by near-infrared fluorescence imaging or histological analysis) than WT mice. Our data indicate that NOR-1 negatively modulates the expression of genes critically involved in the osteogenic differentiation of VICs and VSMCs, thereby restraining ectopic cardiovascular calcification.

Actin polymerization inhibition by targeting ARPC2 affects intestinal stem cell homeostasis

Background

The rapid turnover of the intestinal epithelium is driven by the proliferation and differentiation of intestinal stem cells (ISCs). The dynamics of the F-actin cytoskeleton are critical for maintaining intercellular force and the signal transduction network. However, it remains unclear how direct interference with actin polymerization impacts ISC homeostasis. This study aims to reveal the regulatory effects of the F-actin cytoskeleton on the homeostasis of intestinal epithelium, as well as the potential risks of benproperine (BPP) as an anti-tumor drug.

Methods

Phalloidin fluorescence staining was utilized to test F-actin polymerization. Flow cytometry and IHC staining were employed to discriminate different types of intestinal epithelial cells. Cell proliferation was assessed through bromo-deoxyuridine (BrdU) and 5-ethynyl-2'-deoxyuridine (EdU) incorporation assays. The proliferation and differentiation of intestinal stem cells were replicated in vitro through organoid culture. Epithelial migration was evaluated through BrdU pulse labeling and chasing in mice.

Results

The F-actin content was observed to significantly increase as crypt cells migrated into the villus region. Additionally, actin polymerization in secretory cells, especially in Paneth cells (PCs), was much higher than that in neighboring ISCs. Treatment with the newly identified actin-related protein 2/3 complex subunit 2 (ARPC2) inhibitor BPP led to a dose-dependent increase or inhibition of intestinal organoid growth in vitro and crypt cell proliferation in vivo. Compared with the vehicle group, BPP treatment decreased the expression of Lgr5 ISC feature genes in vivo and in organoid culture. Meanwhile, PC differentiation derived from ISCs and progenitors was decreased by inhibition of F-actin polymerization. Mechanistically, BPP-induced actin polymerization inhibition may activate the Yes1-associated transcriptional regulator pathway, which affects ISC proliferation and differentiation. Accordingly, BPP treatment affected intestinal epithelial cell migration in a dose-dependent manner.

Conclusion

Our findings indicate that the regulation of cytoskeleton reorganization can affect ISC homeostasis. In addition, inhibiting ARPC2 with the Food and Drug Administration-approved drug BPP represents a novel approach to influencing the turnover of intestinal epithelial cells.

Mechanotransduction in tissue engineering: Insights into the interaction of stem cells with biomechanical cues

Stem cells in their natural microenvironment are exposed to biochemical and biophysical cues emerging from the extracellular matrix (ECM) and neighboring cells. In particular, biomechanical forces modulate stem cell behavior, biological fate, and early developmental processes by sensing, interpreting, and responding through a series of biological processes known as mechanotransduction. Local structural changes in the ECM and mechanics are driven by reciprocal activation of the cell and the ECM itself, as the initial deposition of matrix proteins sequentially affects neighboring cells. Recent studies on stem cell mechanoregulation have provided insight into the importance of biomechanical signals on proper tissue regeneration and function and have shown that precise spatiotemporal control of these signals exists in stem cell niches. Against this background, the aim of this work is to review the current understanding of the molecular basis of mechanotransduction by analyzing how biomechanical forces are converted into biological responses via cellular signaling pathways. In addition, this work provides an overview of advanced strategies using stem cells and biomaterial scaffolds that enable precise spatial and temporal control of mechanical signals and offer great potential for the fields of tissue engineering and regenerative medicine will be presented.

HSPB7 oppositely regulates human mesenchymal stromal cell-derived osteogenesis and adipogenesis

BACKGROUND

Recent evidence suggests that accumulation of marrow adipose tissue induced by aberrant lineage allocation of bone marrow-derived mesenchymal stromal cells (BMSCs) contributes to the pathophysiologic processes of osteoporosis. Although master regulators of lineage commitment have been well documented, molecular switches between osteogenesis and adipogenesis are largely unknown.

METHODS

HSPB7 gene expression during osteogenic and adipogenic differentiation of BMSCs was evaluated by qPCR and Western blot analyses. Lentiviral-mediated knockdown or overexpression of HSPB7 and its deletion constructs were used to assess its function. The organization of cytoskeleton was examined by immunofluorescent staining. ALP activity, calcium assay, Alizarin Red S staining and Oil Red O staining were performed in vitro during osteoblast or adipocyte differentiation. SB431542 and Activin A antibody were used to identify the mechanism of Activin A in the regulation of osteogenic differentiation in BMSCs.

RESULTS

In this study, we identified HSPB7 capable of oppositely regulating osteogenic and adipogenic differentiation of BMSCs. HSPB7 silencing promoted adipogenesis while reducing osteogenic differentiation and mineralization. Conversely, overexpression of HSPB7 strongly enhanced osteogenesis, but no effect was observed on adipogenic differentiation. Deletion of the N-terminal or C-terminal domain of HSPB7 led to decreased osteoblastic potency and mineralization. Mechanistically, our data showed that Activin A is a downstream target participating in HSPB7 knockdown-mediated osteogenic inhibition.

CONCLUSIONS

Our findings suggest that HSPB7 plays a positive role in driving osteoblastic differentiation, and with the capability in maintaining the osteo-adipogenesis balance. It holds great promise as a potential therapeutic target in the treatment of bone metabolic diseases.

The dependences of mesenchymal stem cells commitments on the size, concentration, internalization and exposure time of Iron Oxide Nanoparticles through F-actin, Lamin A and ROS

Though magnetic iron oxide nanoparticles (IONPs) are approved for clinical use as contrast agents for MR imaging in United States and Europe, and are widely used to label cells in research, the relationship between IONPs and mesenchymal stem cells (MSCs) is not fully addressed. Here the effects of consistently appeared γ-Fe₂ O₃ on the lineage commitment of MSCs were studied to optimize applications of IONPs in MSCs upon verification of viability. 30 nm 10 μg/mL induced highest promotions on osteogenesis, while 30 and 50 nm of 100 μg/mL elicited most chondrogensis in 14 days, where the effects on ALP, GAG and SOX9 appeared after 7 days, while on RUNX2 came out after 10 days. γ-Fe₂ O₃ enhanced intracellular and extracellular Fe³⁺ and ROS, modulated F-actin and decreased Lamin A of MSCs at different time scale. The disturbances of F-actin, Lamin A or ROS altered the effects of γ-Fe₂ O₃ on MSC differentiation. Our results demonstrate that different size, concentration and modulation of γ-Fe₂ O₃ are needed in its MSC applications for bone and cartilage tissues. Furthermore, an undocumented phenomenon that the modulation of F-actin affected the Lamin A expression in MSCs was observed.

Influence of culture conditions on the secretome of mesenchymal stem cells derived from feline adipose tissue: Proteomics approach

This study aimed to describe the secretome of mesenchymal stem cells derived from feline adipose tissue (AD-MSCs) and compare the effects of different culture conditions on AD-MSC proteomics using a shotgun approach. Adipose tissue was collected from 5 female cats and prepared to culture. Conditioned media was collected at third passage, in which the cells were cultured under 4 conditions, normoxia with fetal bovine serum (N + FBS), hypoxia with FBS (H + FBS), normoxia without FBS (N - FBS), and hypoxia without FBS (H - FBS). Then, the secretome was concentrated and prepared for proteomic approaches. Secretomes cultured with FBS-free medium had more than twice identified proteins in comparison with the secretomes cultured with FBS. In contrast, hypoxic conditions did not increase protein amount and affected only a small proteome fraction. Relevant proteins were related to the extracellular matrix promoting environmental modulation, influencing cell signaling pathways, and providing a suitable environment for cell proliferation and maintenance. Moreover, other proteins were also related to cell adhesion, migration and morphogenesis. Culture conditions can influence protein abundance in AD-MSC secretome, and can give also more specificity to cell and cell-free treatments for different diseases.

DRR1 promotes neuroblastoma cell differentiation by regulating CREB expression

BACKGROUND

Neuroblastoma is the most common cancer in infants and the most common extracranial solid tumor in childhood. DRR1 was identified to be downregulated in poorly differentiated ganglion cells from neuroblastoma model mice. However, the roles of DRR1 in neuroblastoma remain largely unclear.

METHODS

The neuroblastoma cells were induced to differentiate, and the expression of DRR1 was detected. The expression of the neuroblastoma cell differentiation markers was analyzed in DRR1 shRNA- or DRR1-expressing vector-treated neuroblastoma cells. The downstream genes of DRR1 were screened with ChIP-seq assay. Finally, TNB1 cells were infected with DRR1 shRNA and CREB expressing vector containing lentivirus, and the expression of the cell differentiation markers, cell cycle distribution and tumor growth were analyzed.

RESULTS

The expression of DRR1 was increased in differentiated neuroblastoma cells, and downregulation of DRR1 expression inhibited the differentiation of neuroblastoma cells. Further experiments indicated that CREB is a candidate downstream gene of DRR1, and it mediates neuroblastoma cell differentiation. Moreover, overexpression of CREB rescued the effect of DRR1 shRNA on cell differentiation, cell cycle distribution and tumor growth in neuroblastoma.

CONCLUSIONS

DRR1-CREB axis modulates the differentiation of neuroblastoma cells and is associated with the outcome of neuroblastoma patients.

IMPACT

DRR1 is involved in regulation of the differentiation of neuroblastoma. Binding with actin is essential for DRR1 to regulate neuroblastoma cell differentiation. CREB is a candidate downstream gene of DRR1 in regulating of the differentiation of neuroblastoma.

Prostaglandins limit nuclear actin to control nucleolar function during oogenesis

Prostaglandins (PGs), locally acting lipid signals, regulate female reproduction, including oocyte development. However, the cellular mechanisms of PG action remain largely unknown. One cellular target of PG signaling is the nucleolus. Indeed, across organisms, loss of PGs results in misshapen nucleoli, and changes in nucleolar morphology are indicative of altered nucleolar function. A key role of the nucleolus is to transcribe ribosomal RNA (rRNA) to drive ribosomal biogenesis. Here we take advantage of the robust, in vivo system of Drosophila oogenesis to define the roles and downstream mechanisms whereby PGs regulate the nucleolus. We find that the altered nucleolar morphology due to PG loss is not due to reduced rRNA transcription. Instead, loss of PGs results in increased rRNA transcription and overall protein translation. PGs modulate these nucleolar functions by tightly regulating nuclear actin, which is enriched in the nucleolus. Specifically, we find that loss of PGs results in both increased nucleolar actin and changes in its form. Increasing nuclear actin, by either genetic loss of PG signaling or overexpression of nuclear targeted actin (NLS-actin), results in a round nucleolar morphology. Further, loss of PGs, overexpression of NLS-actin or loss of Exportin 6, all manipulations that increase nuclear actin levels, results in increased RNAPI-dependent transcription. Together these data reveal PGs carefully balance the level and forms of nuclear actin to control the level of nucleolar activity required for producing fertilization competent oocytes.

Cytochalasin B Influences Cytoskeletal Organization and Osteogenic Potential of Human Wharton's Jelly Mesenchymal Stem Cells

Among perinatal stem cells of the umbilical cord, human Wharton's jelly mesenchymal stem cells (hWJ-MSCs) are of great interest for cell-based therapy approaches in regenerative medicine, showing some advantages over other MSCs. In fact, hWJ-MSCs, placed between embryonic and adult MSCs, are not tumorigenic and are harvested with few ethical concerns. Furthermore, these cells can be easily cultured in vitro, maintaining both stem properties and a high proliferative rate for several passages, as well as trilineage capacity of differentiation. Recently, it has been demonstrated that cytoskeletal organization influences stem cell biology. Among molecules able to modulate its dynamics, Cytochalasin B (CB), a cyto-permeable mycotoxin, influences actin microfilament polymerization, thus affecting several cell properties, such as the ability of MSCs to differentiate towards a specific commitment. Here, we investigated for the first time the effects of a 24 h-treatment with CB at different concentrations (0.1-3 μM) on hWJ-MSCs. CB influenced the cytoskeletal organization in a dose-dependent manner, inducing changes in cell number, proliferation, shape, and nanomechanical properties, thus promoting the osteogenic commitment of hWJ-MSCs, as confirmed by the expression analysis of osteogenic/autophagy markers.

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

EVL Promotes Osteo-/Odontogenic Differentiation of Dental Pulp Stem Cells via Activating JNK Signaling Pathway

Objective

Human dental pulp stem cells (hDPSCs) were recognized as a suitable and promising source of stem cells in dental pulp regeneration. However, the mechanism by which hDPSCs differentiation into osteo-/odontogenic lineage remains unclear. Ena/VASP-like protein (EVL) has been found to be involved in diverse biological processes. In this study, we explored the role and underlying mechanism of EVL in osteo-/odontogenic differentiation of hDPSCs.

Methods

Expression of EVL was detected in hDPSCs by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western blot (WB) analyses during osteo-/odontogenic differentiation. The function of EVL in osteo-/odontogenic differentiation and involvement of MAPK signaling pathways were evaluated by alkaline phosphatase (ALP) staining and activity, alizarin red staining (ARS), and qRT-PCR and western blot analyses.

Results

The expression of EVL was upregulated during osteo-/odontogenic differentiation of hDPSCs. Overexpression of EVL significantly increased osteo-/odontogenic capacity of hDPSCs, which was reflected in increased alkaline phosphatase (ALP) staining, ALP activity, mineralized nodule formation, and the expressions of genes related to osteo-/odontogenic differentiation, while downregulation of EVL inhibited it. In addition, EVL activated the JNK pathway and phosphorylation of p38 MAPK during differentiation procedure of hDPSCs. The EVL-enhanced differentiation of DPSCs was suppressed by blocking the JNK pathway, rather than the p38 MAPK pathway.

Conclusion

EVL promotes the osteo-/odontogenic differentiation of hDPSCs by activating the JNK pathway, providing a future target for osteo-/odontogenic differentiation and dental pulp regeneration.

Changes in the mechanical properties of human mesenchymal stem cells during differentiation

A thorough understanding of the changes in mechanical property behind intracellular biophysical and biochemical processes during differentiation of human mesenchymal stem cells (hMSCs) is helpful to direct and enhance the commitment of cells to a particular lineage. In this study, displacement creep of the mesenchymal cell lineages (osteogenic, chondrogenic and adipogenic hMSCs) were determined by using atomic force microscopy, which was then used to determine their mechanical properties. We found that at any stages of differentiation, the mesenchymal cell lineages are linear viscoelastic materials and well matched with a simple power-law creep compliance. In addition, the viscoelasticity of mesenchymal cell lineages showed different trends during differentiation. The adipogenic hMSCs showed continuous softening at all stages. The osteogenic and chondrogenic hMSCs only continuously soften and become more fluid-like in the early stage of differentiation, and get stiffened and less fluid-like in the later stage. These findings will help more accurately imitate cellular biomechanics in the microenvironment, and provided an important reference in the biophysics biomimetic design of stem cell differentiation.

The skeleton in a physical world

All organisms exist within a physical space and respond to physical forces as part of daily life. In higher organisms, the skeleton is critical for locomotion in the physical environment, providing a carapace upon which the animal can move to accomplish functions necessary for living. As such, the skeleton has responded evolutionarily, and does in real-time, to physical stresses placed on it to ensure that its structure supports its function in the sea, in the air, and on dry land. In this article, we consider how those cells responsible for remodeling skeletal structure respond to mechanical force including load magnitude, frequency, and cyclicity, and how force rearranges cellular structure in turn. The effects of these forces to balance the mesenchymal stem cell supply of bone-forming osteoblasts and energy storing adipocytes are addressed. That this phenotypic switching is achieved at the level of both gene transactivation and alteration of structural epigenetic controls of gene expression is considered. Finally, as clinicians, we consider this information as it applies to a prescriptive for intelligent exercise.

Comprehensive bioinformatics analysis reveals the prognostic value, predictive value, and immunological roles of ANLN in human cancers

Anillin (ANLN) is a unique scaffolding, actin-binding protein, which is essential for the integrity and ingression of the cleavage furrow. It is mainly involved in the cytokinesis process, while its role in various tumors has not been fully addressed and remains largely elusive. To provide a thorough perspective of ANLN’s roles among diverse malignancies, we conducted a comprehensive, pan-cancer analysis about ANLN, including but not limited to gene expression levels, prognostic value, biological functions, interacting proteins, immune-related analysis, and predictive value. As a result, when compared to normal tissues, ANLN expression is elevated in most cancers, and its expression also differs in different immune subtypes and molecular subtypes in diverse cancers. In addition, in 17 types of cancer, ANLN expression is increased in early tumor stages, and higher ANLN expression predicts worse survival outcomes in more than ten cancers. Furthermore, ANLN shows close correlations with the infiltration levels of most immune cells, and enrichment analysis using ANLN co-expressed genes reveals that ANLN plays essential roles in cell cycle, mitosis, cellular senescence, and p53 signaling pathways. In the final, ANLN exhibits high accuracy in predicting many cancers, and subsequent multivariate analysis suggests ANLN could be an independent prognostic factor in specific cancer types. Taken together, ANLN is proved to be a novel and promising biomarker for its excellent predictive utility, promising prognostic value, and potential immunological roles in pan-cancer. Targeting ANLN might be an attractive approach to tumor treatment.

Sculpting Rupture-Free Nuclear Shapes in Fibrous Environments

Cytoskeleton-mediated force transmission regulates nucleus morphology. How nuclei shaping occurs in fibrous in vivo environments remains poorly understood. Here suspended nanofiber networks of precisely tunable (nm-µm) diameters are used to quantify nucleus plasticity in fibrous environments mimicking the natural extracellular matrix. Contrary to the apical cap over the nucleus in cells on 2-dimensional surfaces, the cytoskeleton of cells on fibers displays a uniform actin network caging the nucleus. The role of contractility-driven caging in sculpting nuclear shapes is investigated as cells spread on aligned single fibers, doublets, and multiple fibers of varying diameters. Cell contractility increases with fiber diameter due to increased focal adhesion clustering and density of actin stress fibers, which correlates with increased mechanosensitive transcription factor Yes-associated protein (YAP) translocation to the nucleus. Unexpectedly, large- and small-diameter fiber combinations lead to teardrop-shaped nuclei due to stress fiber anisotropy across the cell. As cells spread on fibers, diameter-dependent nuclear envelope invaginations that run the nucleus's length are formed at fiber contact sites. The sharpest invaginations enriched with heterochromatin clustering and sites of DNA repair are insufficient to trigger nucleus rupture. Overall, the authors quantitate the previously unknown sculpting and adaptability of nuclei to fibrous environments with pathophysiological implications.

Cytochalasin B Modulates Nanomechanical Patterning and Fate in Human Adipose-Derived Stem Cells

Cytoskeletal proteins provide architectural and signaling cues within cells. They are able to reorganize themselves in response to mechanical forces, converting the stimuli received into specific cellular responses. Thus, the cytoskeleton influences cell shape, proliferation, and even differentiation. In particular, the cytoskeleton affects the fate of mesenchymal stem cells (MSCs), which are highly attractive candidates for cell therapy approaches due to their capacity for self-renewal and multi-lineage differentiation. Cytochalasin B (CB), a cyto-permeable mycotoxin, is able to inhibit the formation of actin microfilaments, resulting in direct effects on cell biological properties. Here, we investigated for the first time the effects of different concentrations of CB (0.1–10 μM) on human adipose-derived stem cells (hASCs) both after 24 h (h) of CB treatment and 24 h after CB wash-out. CB influenced the metabolism, proliferation, and morphology of hASCs in a dose-dependent manner, in association with progressive disorganization of actin microfilaments. Furthermore, the removal of CB highlighted the ability of cells to restore their cytoskeletal organization. Finally, atomic force microscopy (AFM) revealed that cytoskeletal changes induced by CB modulated the viscoelastic properties of hASCs, influencing their stiffness and viscosity, thereby affecting adipogenic fate.

Architectural control of mesenchymal stem cell phenotype through nuclear actin

There is growing appreciation that architectural components of the nucleus regulate gene accessibility by altering chromatin organization. While nuclear membrane connector proteins link the mechanosensitive actin cytoskeleton to the nucleoskeleton, actin’s contribution to the inner architecture of the nucleus remains enigmatic. Control of actin transport into the nucleus, plus the presence of proteins that control actin structure (the actin tool-box) within the nucleus, suggests that nuclear actin may support biomechanical regulation of gene expression. Cellular actin structure is mechanoresponsive: actin cables generated through forces experienced at the plasma membrane transmit force into the nucleus. We posit that dynamic actin remodeling in response to such biomechanical cues provides a novel level of structural control over the epigenetic landscape. We here propose to bring awareness to the fact that mechanical forces can promote actin transfer into the nucleus and control structural arrangements as illustrated in mesenchymal stem cells, thereby modulating lineage commitment.

Mechanically Induced Nuclear Shuttling of β-Catenin Requires Co-transfer of Actin

Mesenchymal stem cells (MSCs) respond to environmental forces with both cytoskeletal re-structuring and activation of protein chaperones of mechanical information, β-catenin, and yes-associated protein 1 (YAP1). To function, MSCs must differentiate between dynamic forces such as cyclic strains of extracellular matrix due to physical activity and static strains due to ECM stiffening. To delineate how MSCs recognize and respond differently to both force types, we compared effects of dynamic (200 cycles × 2%) and static (1 × 2% hold) strain on nuclear translocation of β-catenin and YAP1 at 3 hours after force application. Dynamic strain induced nuclear accumulation of β-catenin, and increased cytoskeletal actin structure and cell stiffness, but had no effect on nuclear YAP1 levels. Critically, both nuclear actin and nuclear stiffness increased along with dynamic strain-induced β-catenin transport. Augmentation of cytoskeletal structure using either static strain or lysophosphatidic acid did not increase nuclear content of β-catenin or actin, but induced robust nuclear increase in YAP1. As actin binds β-catenin, we considered whether β-catenin, which lacks a nuclear localization signal, was dependent on actin to gain entry to the nucleus. Knockdown of cofilin-1 (Cfl1) or importin-9 (Ipo9), which co-mediate nuclear transfer of G-actin, prevented dynamic strain-mediated nuclear transfer of both β-catenin and actin. In sum, dynamic strain induction of actin re-structuring promotes nuclear transport of G-actin, concurrently supporting nuclear access of β-catenin via mechanisms used for actin transport. Thus, dynamic and static strain activate alternative mechanoresponses reflected by differences in the cellular distributions of actin, β-catenin, and YAP1.

At the nuclear envelope of bone mechanobiology

The nuclear envelope and nucleoskeleton are emerging as signaling centers that regulate how physical information from the extracellular matrix is biochemically transduced into the nucleus, affecting chromatin and controlling cell function. Bone is a mechanically driven tissue that relies on physical information to maintain its physiological function and structure. Disorder that present with musculoskeletal and cardiac symptoms, such as Emery-Dreifuss muscular dystrophies and progeria, correlate with mutations in nuclear envelope proteins including Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, Lamin A/C, and emerin. However, the role of nuclear envelope mechanobiology on bone function remains underexplored. The mesenchymal stem cell (MSC) model is perhaps the most studied relationship between bone regulation and nuclear envelope function. MSCs maintain the musculoskeletal system by differentiating into multiple cell types including osteocytes and adipocytes, thus supporting the bone's ability to respond to mechanical challenge. In this review, we will focus on how MSC function is regulated by mechanical challenges both in vitro and in vivo within the context of bone function specifically focusing on integrin, β-catenin and YAP/TAZ signaling. The importance of the nuclear envelope will be explored within the context of musculoskeletal diseases related to nuclear envelope protein mutations and nuclear envelope regulation of signaling pathways relevant to bone mechanobiology in vitro and in vivo.

Actin polymerization state regulates osteogenic differentiation in human adipose-derived stem cells

BACKGROUND

Actin is an essential cellular protein that assembles into microfilaments and regulates numerous processes such as cell migration, maintenance of cell shape, and material transport.

METHODS

In this study, we explored the effect of actin polymerization state on the osteogenic differentiation of human adipose-derived stem cells (hASCs). The hASCs were treated for 7 days with different concentrations (0, 1, 5, 10, 20, and 50 nM) of jasplakinolide (JAS), a reagent that directly polymerizes F-actin. The effects of the actin polymerization state on cell proliferation, apoptosis, migration, and the maturity of focal adhesion-related proteins were assessed. In addition, western blotting and alizarin red staining assays were performed to assess osteogenic differentiation.

RESULTS

Cell proliferation and migration in the JAS (0, 1, 5, 10, and 20 nM) groups were higher than in the control group and the JAS (50 nM) group. The FAK, vinculin, paxillin, and talin protein expression levels were highest in the JAS (20 nM) group, while zyxin expression was highest in the JAS (50 nM) group. Western blotting showed that osteogenic differentiation in the JAS (0, 1, 5, 10, 20, and 50 nM) group was enhanced compared with that in the control group, and was strongest in the JAS (50 nM) group.

CONCLUSIONS

In summary, our data suggest that the actin polymerization state may promote the osteogenic differentiation of hASCs by regulating the protein expression of focal adhesion-associated proteins in a concentration-dependent manner. Our findings provide valuable information for exploring the mechanism of osteogenic differentiation in hASCs.

SETD4 in the Proliferation, Migration, Angiogenesis, Myogenic Differentiation and Genomic Methylation of Bone Marrow Mesenchymal Stem Cells

Epigenetic modification is a crucial mechanism affecting the biological function of stem cells. SETD4 is a histone methyltransferase, and its biological role in bone marrow mesenchymal stem cells (BMSCs) is currently unknown. In this study, we employed CRISPR/Cas9 technology edited mouse model and found that SETD4 knockout significantly promoted the proliferation of BMSCs, impaired BMSCs migration and differentiation potentials of lineages of cardiacmyocyte and smooth muscle cell, and even the angiogenesis via paracrine of VEGF. Through Reduced Representation Bisulfite Sequencing (RRBS) method, we verified that the overall genomic methylation of BMSCs in the SETD4 knockout group only was decreased by 0.47 % compared with wild type. However, the changed genomic methylation covers a total of 96,331 differential methylated CpG sites and 8,692 differential methylation regions (DMRs), with part of them settled in promoter regions. Bioinformatic analysis revealed that differential CpG islands and DMRs in promoter impacted 270 GO functions and 34 KEGG signaling pathways, with some closely related to stem cell biology. Mechanismly, SETD4 knockout inhibited sets of monomethylases and dimethylases for histone lysine, along with significant changes in some factors including Nkx2.5, Gata4, Gli2, Grem2, E2f7, Map7, Nr2f2 and Shox2 that associated with stem cell biology. These results are the first to reveal that even though SETD4 changes the genome's overall methylation to a limited extent in BMSCs, it still affects the numerous cellular functions and signaling pathways, implying SETD4-altered genomic methylation serves a crucial molecular role in BMSCs' biological functions.

Emerging roles of cytoskeletal proteins in regulating gene expression and genome organization during differentiation

In the eukaryotic cell nucleus, cytoskeletal proteins are emerging as essential players in nuclear function. In particular, actin regulates chromatin as part of ATP-dependent chromatin remodeling complexes, it modulates transcription and it is incorporated into nascent ribonucleoprotein complexes, accompanying them from the site of transcription to polyribosomes. The nuclear actin pool is undistinguishable from the cytoplasmic one in terms of its ability to undergo polymerization and it has also been implicated in the dynamics of chromatin, regulating heterochromatin segregation at the nuclear lamina and maintaining heterochromatin levels in the nuclear interiors. One of the next frontiers is, therefore, to determine a possible involvement of nuclear actin in the functional architecture of the cell nucleus by regulating the hierarchical organization of chromatin and, thus, genome organization. Here, we discuss the repertoire of these potential actin functions and how they are likely to play a role in the context of cellular differentiation.

Cytochalasin D Promotes Osteogenic Differentiation of MC3T3-E1 Cells via p38-MAPK Signaling Pathway

BACKGROUND

Bone defect caused by trauma, tumor resection, infection or congenital malformation is a common clinical disease. Bone tissue engineering is regarded as a promising way of bone defect reconstruction. Thus, agents that can promote osteogenesis have received great attention. Cytochalasin D (Cyto D), a metabolite derived from molds, proves to be able to modify actin, reorganize cytoskeleton, and then promote the osteogenic differentiation.

OBJECTIVE

The purpose of this study was to explore the effect and mechanism of Cyto D on osteogenic differentiation of mouse pre-osteoblast MC3T3-E1 cells.

METHODS

The optimum concentration of Cyto D was explored. The osteogenic differentiation of MC3T3-E1 cells induced by Cyto D was assessed by alkaline phosphatase (ALP) staining, Alizarin Red S (ARS) staining, western blotting and quantitative real-time polymerase chain reaction (RT-qPCR). In addition, a specific pathway inhibitor was utilized to explore whether MAPK pathways were involved in this process.

RESULTS

The results showed that the optimized concentration of action was 10-2µg/ml. The expression of Runx2, OCN and OSX was up-regulated by the supplement of Cyto D. ALP activity, calcium deposition, and phosphorylation level of p38 protein were also improved. Inhibition of the pathway significantly reduced the activation of p38, and the expression of osteogenic-related genes.

CONCLUSION

Cyto D can promote the osteogenic differentiation of MC3T3 cells via the p38-MAPK signaling pathway, but not the ERK1/2 or JNK, and it is a potential agent to improve the osteogenesis of MC3T3 cells.

Nuclear actin: The new normal

The presence of actin in the nucleus has historically been a highly contentious issue. It is now, however, well accepted that actin has physiologically important roles in the nucleus. In this Review, we describe the evolution of our thinking about actin in the nucleus starting with evidence supporting its involvement in transcription, chromatin remodeling and intranuclear movements. We also review the growing literature on the mechanisms that regulate the import and export of actin and how post-translational modifications of actin could regulate nuclear actin. We end with an extended discussion of the role of nuclear actin in the repair of DNA double stranded breaks.

A glance on the role of actin in osteogenic and adipogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) have the capacity to differentiate into multiple lineages including osteogenic and adipogenic lineages. An increasing number of studies have indicated that lineage commitment by MSCs is influenced by actin remodeling. Moreover, actin has roles in determining cell shape, nuclear shape, cell spreading, and cell stiffness, which eventually affect cell differentiation. Osteogenic differentiation is promoted in MSCs that exhibit a large spreading area, increased matrix stiffness, higher levels of actin polymerization, and higher density of stress fibers, whereas adipogenic differentiation is prevalent in MSCs with disrupted actin networks. In addition, the mechanical properties of F-actin empower cells to sense and transduce mechanical stimuli, which are also reported to influence differentiation. Various biomaterials, mechanical, and chemical interventions along with pathogen-induced actin alteration in the form of polymerization and depolymerization in MSC differentiation were studied recently. This review will cover the role of actin and its modifications through the use of different methods in inducing osteogenic and adipogenic differentiation.

Low Intensity Vibrations Augment Mesenchymal Stem Cell Proliferation and Differentiation Capacity during in vitro Expansion

A primary component of exercise, mechanical signals, when applied in the form of low intensity vibration (LIV), increases mesenchymal stem cell (MSC) osteogenesis and proliferation. While it is generally accepted that exercise effectively combats the deleterious effects of aging in the musculoskeletal system, how long-term exercise affects stem cell aging, which is typified by reduced proliferative and differentiative capacity, is not well explored. As a first step in understanding the effect of long-term application of mechanical signals on stem cell function, we investigated the effect of LIV during in vitro expansion of MSCs. Primary MSCs were subjected to either a control or to a twice-daily LIV regimen for up to sixty cell passages (P60) under in vitro cell expansion conditions. LIV effects were assessed at both early passage (EP) and late passage (LP). At the end of the experiment, P60 cultures exposed to LIV maintained a 28% increase of cell doubling and a 39% reduction in senescence-associated β-galactosidase activity (p < 0.01) but no changes in telomere lengths and p16^INK4a levels were observed. Prolonged culture-associated decreases in osteogenic and adipogenic capacity were partially protected by LIV in both EP and LP groups (p < 0.05). Mass spectroscopy of late passage MSC indicated a synergistic decrease of actin and microtubule cytoskeleton-associated proteins in both control and LIV groups while LIV induced a recovery of proteins associated with oxidative reductase activity. In summary, our findings show that the application of long-term mechanical challenge (+LIV) during in vitro expansion of MSCs for sixty passages significantly alters MSC proliferation, differentiation and structure. This suggests LIV as a potential tool to investigate the role of physical activity during aging.

β-Catenin Preserves the Stem State of Murine Bone Marrow Stromal Cells Through Activation of EZH2

During bone marrow stromal cell (BMSC) differentiation, both Wnt signaling and the development of a rigid cytoskeleton promote commitment to the osteoblastic over adipogenic lineage. β-catenin plays a critical role in the Wnt signaling pathway to facilitate downstream effects on gene expression. We show that β-catenin was additive with cytoskeletal signals to prevent adipogenesis, and β-catenin knockdown promoted adipogenesis even when the actin cytoskeleton was depolymerized. β-catenin also prevented osteoblast commitment in a cytoskeletal-independent manner, with β-catenin knockdown enhancing lineage commitment. Chromatin immunoprecipitation (ChIP)-sequencing demonstrated binding of β-catenin to the promoter of enhancer of zeste homolog 2 (EZH2), a key component of the polycomb repressive complex 2 (PRC2) complex that catalyzes histone methylation. Knockdown of β-catenin reduced EZH2 protein levels and decreased methylated histone 3 (H3K27me3) at osteogenic loci. Further, when EZH2 was inhibited, β-catenin's anti-differentiation effects were lost. These results indicate that regulating EZH2 activity is key to β-catenin's effects on BMSCs to preserve multipotentiality. © 2020 American Society for Bone and Mineral Research.

Role of Nucleoporins and Transport Receptors in Cell Differentiation

Bidirectional molecular movements between the nucleus and cytoplasm take place through nuclear pore complexes (NPCs) embedded in the nuclear membrane. These macromolecular structures are composed of several nucleoporins, which form seven different subcomplexes based on their biochemical affinity. These nucleoporins are integral components of the complex, not only allowing passive transport but also interacting with importin, exportin, and other molecules that are required for transport of protein in various cellular processes. Transport of different proteins is carried out either dependently or independently on transport receptors. As well as facilitating nucleocytoplasmic transport, nucleoporins also play an important role in cell differentiation, possibly by their direct gene interaction. This review will cover the general role of nucleoporins (whether its dependent or independent) and nucleocytoplasmic transport receptors in cell differentiation.

Dimensionality changes actin network through lamin A/C and zyxin

Mechanosensing proteins have mainly been investigated in 2D culture platforms, while understanding their regulation in 3D enviroments is critical for tissue engineering. Among mechanosensing proteins, the actin cytoskeleton plays a key role in human mesenchymal stromal cells (hMSCs) activity, but its regulation in 3D tissue engineered scaffolds remains poorly studied. Here, we show that human mesenchymal stromal cells (hMSCs) cultured on 3D electrospun scaffolds made of a stiff material do not form actin stress fibers, contrary to hMSCs on 2D films of the same material. On 3D electrospun and additive manufactured scaffolds, hMSCs also displayed fewer focal adhesions, lower lamin A and C expression and less YAP1 nuclear localization and myosin light chain phosphorylation. Together, this strongly suggests that dimensionality prevents the build-up of cellular tension, even on stiff materials. Knock down of either lamin A and C or zyxin resulted in fewer stress fibers in the cell center. Zyxin knock down reduced lamin A and C expression, but not vice versa, showing that this signal chain starts from the outside of the cell. Lineage commitment was not affected by the lack of these important osteogenic proteins in 3D, as all cells committed to osteogenesis in bi-potential medium. Our study demonstrates that dimensionality changes the actin cytoskeleton through lamin A and C and zyxin, and highlights the difference in the regulation of lineage commitment in 3D enviroments. Together, these results can have important implications for future scaffold design for both stiff- and soft tissue engineering constructs.

The insulin-PI3K-Rac1 axis contributes to terminal adipocyte differentiation through regulation of actin cytoskeleton dynamics

Adipocyte differentiation is accompanied by a pronounced change in the actin cytoskeleton characterized by the reorganization of filamentous (F)-actin stress fibers into cortical F-actin structures. We previously showed that depolymerization of F-actin stress fibers induced by inactivation of RhoA-ROCK (Rho-associated kinase) signaling acts as a trigger for adipocyte differentiation. The relevance and underlying mechanism of the formation of cortical F-actin structures from depolymerized actin during adipocyte differentiation have remained unclear, however. We have now examined the mechanistic relation between actin dynamics and adipogenic induction. Transient exposure to the actin-depolymerizing agent latrunculin A (LatA) supported the formation of adipocyte-associated cortical actin structures and the completion of terminal adipocyte differentiation in the presence of insulin, whereas long-term exposure to LatA prevented such actin reorganization as well as terminal adipogenesis. Moreover, these effects of insulin were prevented by inhibition of phosphatidylinositol 3-kinase (PI3K)-Rac1 signaling and the actin-related protein 2/3 (Arp2/3) complex which is a critical component of the cortical actin networks. Our findings thus suggest that the insulin-PI3K-Rac1 axis leads to the formation of adipocyte-associated cortical actin structures which is essential for the completion of adipocyte differentiation.

Anillin regulates breast cancer cell migration, growth, and metastasis by non-canonical mechanisms involving control of cell stemness and differentiation

Background

Breast cancer metastasis is driven by a profound remodeling of the cytoskeleton that enables efficient cell migration and invasion. Anillin is a unique scaffolding protein regulating major cytoskeletal structures, such as actin filaments, microtubules, and septin polymers. It is markedly overexpressed in breast cancer, and high anillin expression is associated with poor prognosis. The aim of this study was to investigate the role of anillin in breast cancer cell migration, growth, and metastasis.

Methods

CRISPR/Cas9 technology was used to deplete anillin in highly metastatic MDA-MB-231 and BT549 cells and to overexpress it in poorly invasive MCF10AneoT cells. The effects of anillin depletion and overexpression on breast cancer cell motility in vitro were examined by wound healing and Matrigel invasion assays. Assembly of the actin cytoskeleton and matrix adhesion were evaluated by immunofluorescence labeling and confocal microscopy. In vitro tumor development was monitored by soft agar growth assays, whereas cancer stem cells were examined using a mammosphere formation assay and flow cytometry. The effects of anillin knockout on tumor growth and metastasis in vivo were determined by injecting control and anillin-depleted breast cancer cells into NSG mice.

Results

Loss-of-function and gain-of-function studies demonstrated that anillin is necessary and sufficient to accelerate migration, invasion, and anchorage-independent growth of breast cancer cells in vitro. Furthermore, loss of anillin markedly attenuated primary tumor growth and metastasis of breast cancer in vivo. In breast cancer cells, anillin was localized in the nucleus; however, knockout of this protein affected the cytoplasmic/cortical events, e.g., the organization of actin cytoskeleton and cell-matrix adhesions. Furthermore, we observed a global transcriptional reprogramming of anillin-depleted breast cancer cells that resulted in suppression of their stemness and induction of the mesenchymal to epithelial trans-differentiation. Such trans-differentiation was manifested by the upregulation of basal keratins along with the increased expression of E-cadherin and P-cadherin. Knockdown of E-cadherin restored the impaired migration and invasion of anillin-deficient breast cancer cells.

Conclusion

Our study demonstrates that anillin plays essential roles in promoting breast cancer growth and metastatic dissemination in vitro and in vivo and unravels novel functions of anillin in regulating breast cancer stemness and differentiation.

Knockdown of formin mDia2 alters lamin B1 levels and increases osteogenesis in stem cells

Nuclear actin plays a critical role in mediating mesenchymal stem cell (MSC) fate commitment. In marrow-derived MSCs, the principal diaphanous-related formin Diaph3 (mDia2) is present in the nucleus and regulates intranuclear actin polymerization, whereas Diaph1 (mDia1) is localized to the cytoplasm and controls cytoplasmic actin polymerization. We here show that mDia2 can be used as a tool to query actin-lamin nucleoskeletal structure. Silencing mDia2 affected the nucleoskeletal lamin scaffold, altering nuclear morphology without affecting cytoplasmic actin cytoskeleton, and promoted MSC differentiation. Attempting to target intranuclear actin polymerization by silencing mDia2 led to a profound loss in lamin B1 nuclear envelope structure and integrity, increased nuclear height, and reduced nuclear stiffness without compensatory changes in other actin nucleation factors. Loss of mDia2 with the associated loss in lamin B1 promoted Runx2 transcription and robust osteogenic differentiation and suppressed adipogenic differentiation. Hence, mDia2 is a potent tool to query intranuclear actin-lamin nucleoskeletal structure, and its presence serves to retain multipotent stromal cells in an undifferentiated state.

Advanced glycation end-products regulate extracellular matrix-adipocyte metabolic crosstalk in diabetes

The adipose tissue extracellular matrix (ECM) regulates adipocyte cellular metabolism and is altered in obesity and type 2 diabetes, but mechanisms underlying ECM-adipocyte metabolic crosstalk are poorly defined. Advanced glycation end-product (AGE) formation is increased in diabetes. AGE alter tissue function via direct effects on ECM and by binding scavenger receptors on multiple cell types and signaling through Rho GTPases. Our goal was to determine the role and underlying mechanisms of AGE in regulating human ECM-adipocyte metabolic crosstalk. Visceral adipocytes from diabetic and non-diabetic humans with obesity were studied in 2D and 3D-ECM culture systems. AGE is increased in adipose tissue from diabetic compared to non-diabetic subjects. Glycated collagen 1 and AGE-modified ECM regulate adipocyte glucose uptake and expression of AGE scavenger receptors and Rho signaling mediators, including the DIAPH1 gene, which encodes the human Diaphanous 1 protein (hDia1). Notably, inhibition of hDia1, but not scavenger receptors RAGE or CD36, attenuated AGE-ECM inhibition of adipocyte glucose uptake. These data demonstrate that AGE-modification of ECM contributes to adipocyte insulin resistance in human diabetes, and implicate hDia1 as a potential mediator of AGE-ECM-adipocyte metabolic crosstalk.

Lineage Commitment, Signaling Pathways, and the Cytoskeleton Systems in Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) from adult tissues are promising candidates for personalized cell therapy and tissue engineering. Significant progress was achieved in our understanding of the regulation of MSCs proliferation and differentiation by different cues during the past years. Proliferation and differentiation of MSCs are sensitive to the extracellular matrix (ECM) properties, physical cues, and chemical signaling. Sheath stress, matrix stiffness, surface adhesiveness, and micro- and nanotopography define cell shape and dictate lineage commitment of MSCs even in the absence of specific chemical signals. We discuss mechanotransduction as the major route from ECM through the cytoskeleton toward signaling pathways and gene expression. All components of the cytoskeleton from primary cilium and focal adhesions (FAs) to actin, microtubules (MTs), and intermediate filaments (IFs) are involved in the mechanotransduction. Differentiation of MSCs is regulated via the complex network of interrelated signaling pathways, including RhoA/ROCK, Akt/Erk, and YAP/TAZ effectors of Hippo pathway. These pathways could be regulated both by chemical and mechanical stimuli. Attenuation of these pathways in MSCs results in specific changes in FAs and actin cytoskeleton. Besides, differentiation of MSCs affects MTs and IFs. Recent findings highlight the role of intranuclear actin in the regulation of transcription factors in response to mechanical environmental stimuli. Alterations of cytoskeletal components reflect the MSC senescence state and their migratory capacity. In this review, we discuss the relationships between the molecular interactions in signaling pathways and morphological response of cytoskeletal components and reveal the complex interrelations between cytoskeleton systems and signaling pathways during lineage commitment of MSCs. Impact Statement This review describes the complex network of relationships between mechanical and biochemical stimuli in mesenchymal stem cells (MSC) and their balance which defines the morphological changes of cell shape due to rearrangement of cytoskeletal systems during lineage commitment of MSCs.

Cell adhesion and culture medium dependent changes in the high frequency mechanical vibration induced proliferation, osteogenesis, and intracellular organization of human adipose stem cells

High frequency (HF) mechanical vibration appears beneficial for in vitro osteogenesis of mesenchymal stem cells (MSCs). However, the current mechanobiological understanding of the method remains insufficient. We designed high-throughput stimulators to apply horizontal or vertical high magnitude HF (HMHF; 2.5 G_peak, 100 Hz) vibration on human adipose stem cells (hASCs). We analyzed proliferation, alkaline phosphatase (ALP) activity, mineralization, and effects on the actin cytoskeleton and nuclei using immunocytochemical stainings. Proliferation was studied on a standard tissue culture plastic (sTCP) surface and on an adhesion supporting tissue culture plastic (asTCP) surface in basal (BM) and osteogenic (OM) culture medium conditions. We discovered that the improved cell adhesion was a prerequisite for vibration induced changes in the proliferation of hASCs. Similarly, the adhesion supporting surface enabled us to observe vibration initiated ALP activity and mineralization changes in OM condition. The horizontal vibration increased ALP activity, while vertical stimulation reduced ALP activity. However, mineralization was not enhanced by the HMHF vibration. We performed image-based analysis of actin and nuclei to obtain novel data of the intracellular-level responses to HF vibration in BM and OM conditions. Our quantitative results suggest that actin organizations were culture medium and stimulation direction dependent. Both stimulation directions decreased OM induced changes in nuclear size and elongation. Consequently, our findings of the nuclear deformations provide supportive evidence for the involvement of the nuclei in the mechanocoupling of HF vibration. Taken together, the results of this study enhanced the knowledge of the intracellular mechanisms of HF vibration induced osteogenesis of MSCs.

Cellular and pathophysiological consequences of Arp2/3 complex inhibition: role of inhibitory proteins and pharmacological compounds

The actin-related protein complex 2/3 (Arp2/3) generates branched actin networks important for many cellular processes such as motility, vesicular trafficking, cytokinesis, and intercellular junction formation and stabilization. Activation of Arp2/3 requires interaction with actin nucleation-promoting factors (NPFs). Regulation of Arp2/3 activity is achieved by endogenous inhibitory proteins through direct binding to Arp2/3 and competition with NPFs or by binding to Arp2/3-induced actin filaments and disassembly of branched actin networks. Arp2/3 inhibition has recently garnered more attention as it has been associated with attenuation of cancer progression, neurotoxic effects during drug abuse, and pathogen invasion of host cells. In this review, we summarize current knowledge on expression, inhibitory mechanisms and function of endogenous proteins able to inhibit Arp2/3 such as coronins, GMFs, PICK1, gadkin, and arpin. Moreover, we discuss cellular consequences of pharmacological Arp2/3 inhibition.

RASSF1A is required for the maintenance of nuclear actin levels

Nuclear actin participates in many essential cellular processes including gene transcription, chromatin remodelling and mRNA processing. Actin shuttles into and out the nucleus through the action of dedicated transport receptors importin-9 and exportin-6, but how this transport is regulated remains unclear. Here, we show that RASSF1A is a novel regulator of actin nucleocytoplasmic trafficking and is required for the active maintenance of nuclear actin levels through supporting binding of exportin-6 (XPO6) to RAN GTPase. RASSF1A (Ras association domain family 1 isoform A) is a tumour suppressor gene frequently silenced by promoter hypermethylation in all major solid cancers. Specifically, we demonstrate that endogenous RASSF1A localises to the nuclear envelope (NE) and is required for nucleocytoplasmic actin transport and the concomitant regulation of myocardin-related transcription factor A (MRTF-A), a co-activator of the transcription factor serum response factor (SRF). The RASSF1A/RAN/XPO6/nuclear actin pathway is aberrant in cancer cells where RASSF1A expression is lost and correlates with reduced MRTF-A/SRF activity leading to cell adhesion defects. Taken together, we have identified a previously unknown mechanism by which the nuclear actin pool is regulated and uncovered a previously unknown link of RASSF1A and MRTF-A/SRF in tumour suppression.

Combating osteoporosis and obesity with exercise: leveraging cell mechanosensitivity

Osteoporosis, a condition of skeletal decline that undermines quality of life, is treated with pharmacological interventions that are associated with poor adherence and adverse effects. Complicating efforts to improve clinical outcomes, the incidence of obesity is increasing, predisposing the population to a range of musculoskeletal complications and metabolic disorders. Pharmacological management of obesity has yet to deliver notable reductions in weight and debilitating complications are rarely avoided. By contrast, exercise shows promise as a non-invasive and non-pharmacological method of regulating both osteoporosis and obesity. The principal components of exercise - mechanical signals - promote bone and muscle anabolism while limiting formation and expansion of fat mass. Mechanical regulation of bone and marrow fat might be achieved by regulating functions of differentiated cells in the skeletal tissue while biasing lineage selection of their common progenitors - mesenchymal stem cells. An inverse relationship between adipocyte versus osteoblast fate selection from stem cells is implicated in clinical conditions such as childhood obesity and increased marrow adiposity in type 2 diabetes mellitus, as well as contributing to skeletal frailty. Understanding how exercise-induced mechanical signals can be used to improve bone quality while decreasing fat mass and metabolic dysfunction should lead to new strategies to treat chronic diseases such as osteoporosis and obesity.

Gene regulation through dynamic actin control of nuclear structure

Bone marrow mesenchymal stem cells exist in a multipotential state, where osteogenic and adipogenic genomes are silenced in heterochromatin at the inner nuclear leaflet. Physical force, generated in the marrow space during dynamic exercise exerts control overexpression of differentiation. Mesenchymal stem cells experience mechanical force through their cytoskeletal attachments to substrate, inducing signaling that alters gene expression. The generated force is further transferred from the cytoskeleton to the nucleoskeleton through tethering of actin to Linker of Nucleus and Cytoskeleton (LINC) complexes. Forces exerted on LINC alter the shape and placement of the nucleus within the cell, and are ultimately transferred into the nucleus. LINC complexes transverse the nuclear membrane and connect to the internal nucleoskeleton that is made up of lamin filaments and actin. Force transfer through LINC thus causes structural rearrangements of the nuclear scaffolding upon which chromosomes are arranged. Gene availability is not only modulated through heterochromatin remodeling enzymes and active transcription factors but also by control of nucleoskeletal structure and nuclear enzymes that mediate actin polymerization in the nucleus. Nuclear actin structure may be affected by similar force-activated pathways as those controlling the cytoplasmic actin cytoskeleton and represent a critical determinant of mesenchymal stem cell lineage commitment.

Impact statement

Gene expression is controlled by nuclear structure which is modulated by both internal and external forces exerted on the nucleoskeleton. Extracellular forces experienced through the actin cytoskeleton are transmitted to the internal nucleoskeleton via Linker of Nucleus and Cytoskeleton (LINC) protein connections. LINC complexes directly alter nuclear shape and entry of molecules that regulate transcription. New mechanistic models indicate that nuclear actin is a dynamic component of the filamentous nucleoskeleton and modified by an intranuclear “actin toolbox”, a set of enzymes that regulate linear and branched polymerization of nuclear actin. External stimulation of both biomechanical and biochemical pathways alters nuclear actin structure and has profound effects on gene expression by controlling chromatin architecture and transcription factor access to gene targets. The available data indicate that nucleoskeletal control of gene expression is critical for self-renewal and mesenchymal lineage-allocation in stem cells.

Mechanical Roles of F-Actin in the Differentiation of Stem Cells: A Review

In the development and differentiation of stem cells, mechanical forces associated with filamentous actin (F-actin) play a crucial role. The present review aims to reveal the relationship among the chemical components, microscopic structures, mechanical properties, and biological functions of F-actin. Particular attention is given to the functions of the cytoplasmic and nuclear microfilament cytoskeleton and their regulation mechanisms in the differentiation of stem cells. The distributions of different types of actin monomers in mammal cells and the functions of actin-binding proteins are summarized. We discuss how the fate of stem cells is regulated by intra/extracellular mechanical and chemical cues associated with microfilament-related proteins, intercellular adhesion molecules, etc. In addition, we also address the differentiation-induced variation in the stiffness of stem cells and the correlation between the fate and geometric shape change of stem cells. This review not only deepens our understanding of the biophysical mechanisms underlying the fates of stem cells under different culture conditions but also provides inspirations for the tissue engineering of stem cells.

Brief Report From the 3rd International Meeting on Bone Marrow Adiposity (BMA 2017)

The 3rd International Meeting on Bone Marrow Adiposity (BMA) was held at the Olympic Museum in Lausanne, Switzerland, on August 31st and September 1st, 2017. This brief monograph summarizes the scientific contents of the meeting and highlights the birth of the International Bone Marrow Adiposity Society (BMAS).

The Mechanobiology of the Actin Cytoskeleton in Stem Cells during Differentiation and Interaction with Biomaterials

An understanding of the cytoskeleton's importance in stem cells is essential for their manipulation and further clinical application. The cytoskeleton is crucial in stem cell biology and depends on physical and chemicals signals to define its structure. Additionally, cell culture conditions will be important in the proper maintenance of stemness, lineage commitment, and differentiation. This review focuses on the following areas: the role of the actin cytoskeleton of stem cells during differentiation, the significance of cellular morphology, signaling pathways involved in cytoskeletal rearrangement in stem cells, and the mechanobiology and mechanotransduction processes implicated in the interactions of stem cells with different surfaces of biomaterials, such as nanotopography, which is a physical cue influencing the differentiation of stem cells. Also, cancer stem cells are included since it is necessary to understand the role of their mechanical properties to develop new strategies to treat cancer. In this context, to study the stem cells requires integrated disciplines, including molecular and cellular biology, chemistry, physics, and immunology, as well as mechanobiology. Finally, since one of the purposes of studying stem cells is for their application in regenerative medicine, the deepest understanding is necessary in order to establish safety protocols and effective cell-based therapies.

Validation of Osteogenic Properties of Cytochalasin D by High-Resolution RNA-Sequencing in Mesenchymal Stem Cells Derived from Bone Marrow and Adipose Tissues

Differentiation of mesenchymal stromal/stem cells (MSCs) involves a series of molecular signals and gene transcription events required for attaining cell lineage commitment. Modulation of the actin cytoskeleton using cytochalasin D (CytoD) drives osteogenesis at early timepoints in bone marrow-derived MSCs and also initiates a robust osteogenic differentiation program in adipose tissue-derived MSCs. To understand the molecular basis for these pronounced effects on osteogenic differentiation, we investigated global changes in gene expression in CytoD-treated murine and human MSCs by high-resolution RNA-sequencing (RNA-seq) analysis. A three-way bioinformatic comparison between human adipose tissue-derived MSCs (hAMSCs), human bone marrow-derived MSCs (hBMSCs), and mouse bone marrow-derived MSCs (mBMSCs) revealed significant upregulation of genes linked to extracellular matrix organization, cell adhesion and bone metabolism. As anticipated, the activation of these differentiation-related genes is accompanied by a downregulation of nuclear and cell cycle-related genes presumably reflecting cytostatic effects of CytoD. We also identified eight novel CytoD activated genes-VGLL4, ARHGAP24, KLHL24, RCBTB2, BDH2, SCARF2, ACAD10, HEPH-which are commonly upregulated across the two species and tissue sources of our MSC samples. We selected the Hippo pathway-related VGLL4 gene, which encodes the transcriptional co-factor Vestigial-like 4, for further study because this pathway is linked to osteogenesis. VGLL4 small interfering RNA depletion reduces mineralization of hAMSCs during CytoD-induced osteogenic differentiation. Together, our RNA-seq analyses suggest that while the stimulatory effects of CytoD on osteogenesis are pleiotropic and depend on the biological state of the cell type, a small group of genes including VGLL4 may contribute to MSC commitment toward the bone lineage.

Enhancer of zeste homolog 2 (Ezh2) controls bone formation and cell cycle progression during osteogenesis in mice

Epigenetic mechanisms control skeletal development and osteoblast differentiation. Pharmacological inhibition of the histone 3 Lys-27 (H3K27) methyltransferase enhancer of zeste homolog 2 (EZH2) in WT mice enhances osteogenesis and stimulates bone formation. However, conditional genetic loss of Ezh2 early in the mesenchymal lineage (i.e. through excision via Prrx1 promoter-driven Cre) causes skeletal abnormalities due to patterning defects. Here, we addressed the key question of whether Ezh2 controls osteoblastogenesis at later developmental stages beyond patterning. We show that Ezh2 loss in committed pre-osteoblasts by Cre expression via the osterix/Sp7 promoter yields phenotypically normal mice. These Ezh2 conditional knock-out mice (Ezh2 cKO) have normal skull bones, clavicles, and long bones but exhibit increased bone marrow adiposity and reduced male body weight. Remarkably, in vivo Ezh2 loss results in a low trabecular bone phenotype in young mice as measured by micro-computed tomography and histomorphometry. Thus, Ezh2 affects bone formation stage-dependently. We further show that Ezh2 loss in bone marrow-derived mesenchymal cells suppresses osteogenic differentiation and impedes cell cycle progression as reflected by decreased metabolic activity, reduced cell numbers, and changes in cell cycle distribution and in expression of cell cycle markers. RNA-Seq analysis of Ezh2 cKO calvaria revealed that the cyclin-dependent kinase inhibitor Cdkn2a is the most prominent cell cycle target of Ezh2 Hence, genetic loss of Ezh2 in mouse pre-osteoblasts inhibits osteogenesis in part by inducing cell cycle changes. Our results suggest that Ezh2 serves a bifunctional role during bone formation by suppressing osteogenic lineage commitment while simultaneously facilitating proliferative expansion of osteoprogenitor cells.

Sun-mediated mechanical LINC between nucleus and cytoskeleton regulates βcatenin nuclear access

βcatenin acts as a primary intracellular signal transducer for mechanical and Wnt signaling pathways to control cell function and fate. Regulation of βcatenin in the cytoplasm has been well studied but βcatenin nuclear trafficking and function remains unclear. In a previous study we showed that, in mesenchymal stem cells (MSC), mechanical blockade of adipogenesis relied on inhibition of βcatenin destruction complex element GSK3β (glycogen synthase kinase 3β) to increase nuclear βcatenin as well as the function of Linker of Cytoskeleton and Nucleoskeleton (LINC) complexes, suggesting that these two mechanisms may be linked. Here we show that shortly after inactivation of GSK3β due to either low intensity vibration (LIV), substrate strain or pharmacologic inhibition, βcatenin associates with the nucleoskeleton, defined as the insoluble nuclear fraction that provides structure to the integrated nuclear envelope, nuclear lamina and chromatin. Co-depleting LINC elements Sun-1 and Sun-2 interfered with both nucleoskeletal association and nuclear entry of βcatenin, resulting in decreased nuclear βcatenin levels. Our findings reveal that the insoluble structural nucleoskeleton actively participates in βcatenin dynamics. As the cytoskeleton transmits applied mechanical force to the nuclear surface to influence the nucleoskeleton and its LINC mediated interaction, our results suggest a pathway by which LINC mediated connectivity may play a role in signaling pathways that depend on nuclear access of βcatenin.