Cytoskeletal disassembly and cell rounding promotes adipogenesis from ES cells

Cytoskeletal rearrangement precedes nucleolar remodeling during adipogenesis

Differentiation of adipose progenitor cells into mature adipocytes entails a dramatic reorganization of the cellular architecture to accommodate lipid storage into cytoplasmic lipid droplets. Lipid droplets occupy most of the adipocyte volume, compressing the nucleus beneath the plasma membrane. How this cellular remodeling affects sub-nuclear structure, including size and number of nucleoli, remains unclear. We describe the morphological remodeling of the nucleus and the nucleolus during in vitro adipogenic differentiation of primary human adipose stem cells. We find that cell cycle arrest elicits a remodeling of nucleolar structure which correlates with a decrease in protein synthesis. Strikingly, triggering cytoskeletal rearrangements mimics the nucleolar remodeling observed during adipogenesis. Our results point to nucleolar remodeling as an active, mechano-regulated mechanism during adipogenic differentiation and demonstrate a key role of the actin cytoskeleton in defining nuclear and nucleolar architecture in differentiating human adipose stem cells.

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).

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.

Comprehensive single-shot biophysical cytometry using simultaneous quantitative phase imaging and Brillouin spectroscopy

Single-cell analysis, or cytometry, is a ubiquitous tool in the biomedical sciences. Whereas most cytometers use fluorescent probes to ascertain the presence or absence of targeted molecules, biophysical parameters such as the cell density, refractive index, and viscosity are difficult to obtain. In this work, we combine two complementary techniques-quantitative phase imaging and Brillouin spectroscopy-into a label-free image cytometry platform capable of measuring more than a dozen biophysical properties of individual cells simultaneously. Using a geometric simplification linked to freshly plated cells, we can acquire the cellular diameter, volume, refractive index, mass density, non-aqueous mass, fluid volume, dry volume, the fractional water content of cells, both by mass and by volume, the Brillouin shift, Brillouin linewidth, longitudinal modulus, longitudinal viscosity, the loss modulus, and the loss tangent, all from a single acquisition, and with no assumptions of underlying parameters. Our methods are validated across three cell populations, including a control population of CHO-K1 cells, cells exposed to tubulin-disrupting nocodazole, and cells under hypoosmotic shock. Our system will unlock new avenues of research in biophysics, cell biology, and medicine.

Adipose cells and tissues soften with lipid accumulation while in diabetes adipose tissue stiffens

Adipose tissue expansion involves both differentiation of new precursors and size increase of mature adipocytes. While the two processes are well balanced in healthy tissues, obesity and diabetes type II are associated with abnormally enlarged adipocytes and excess lipid accumulation. Previous studies suggested a link between cell stiffness, volume and stem cell differentiation, although in the context of preadipocytes, there have been contradictory results regarding stiffness changes with differentiation. Thus, we set out to quantitatively monitor adipocyte shape and size changes with differentiation and lipid accumulation. We quantified by optical diffraction tomography that differentiating preadipocytes increased their volumes drastically. Atomic force microscopy (AFM)-indentation and -microrheology revealed that during the early phase of differentiation, human preadipocytes became more compliant and more fluid-like, concomitant with ROCK-mediated F-actin remodelling. Adipocytes that had accumulated large lipid droplets were more compliant, and further promoting lipid accumulation led to an even more compliant phenotype. In line with that, high fat diet-induced obesity was associated with more compliant adipose tissue compared to lean animals, both for drosophila fat bodies and murine gonadal adipose tissue. In contrast, adipose tissue of diabetic mice became significantly stiffer as shown not only by AFM but also magnetic resonance elastography. Altogether, we dissect relative contributions of the cytoskeleton and lipid droplets to cell and tissue mechanical changes across different functional states, such as differentiation, nutritional state and disease. Our work therefore sets the basis for future explorations on how tissue mechanical changes influence the behaviour of mechanosensitive tissue-resident cells in metabolic disorders.

The integrin beta1 modulator Tirofiban prevents adipogenesis and obesity by the overexpression of integrin-linked kinase: a pre-clinical approach in vitro and in vivo

Background

Obesity is caused by the enlargement of the white adipose tissue (WAT) depots, characterized by the hypertrophic enlargement of malfunctioning adipocytes within WAT which increases the storage of triglycerides (TG) in the lipid droplets (LD). Adipogenesis pathways as well as the expression and activity of some extracellular matrix receptors integrins are upregulated. Integrinβ1 (INTB1) is the main isoform involved in WAT remodeling during obesity and insulin resistance-related diseases. We recently described Integrin Linked Kinase (ILK), a scaffold protein recruited by INTB1, as an important mediator of WAT remodeling and insulin resistance. As the few approved drugs to fight obesity have brought long-term cardiovascular side effects and given that the consideration of INTB1 and/or ILK modulation as anti-obesogenic strategies remains unexplored, we aimed to evaluate the anti-obesogenic capacity of the clinically approved anticoagulant Tirofiban (TF), stated in preclinical studies as a cardiovascular protector.

Methods

Fully differentiated adipocytes originating from C3H10T1/2 were exposed to TF and were co-treated with specific INTB1 blockers or with siRNA-based knockdown ILK expression. Lipid-specific dyes were used to determine the TG content in LD. The genetic expression pattern of ILK, pro-inflammatory cytokines (MCP1, IL6), adipogenesis (PPARγ, Leptin), thermogenesis (UCP1), proliferation (PCNA), lipid metabolism (FASN, HSL, ATGL), and metabolite transporters (FABP4, FAT, AQP7) were detected using quantitative PCR. Cytoskeletal actin polymerization was detected by confocal microscopy. Immunoblotting was performed to detect INTB1 phosphorylation at Thr788/9 and ILK activity as phosphorylation levels of protein kinase B (AKT) in Ser473 and glycogen synthase kinase 3β (GSK3β) at Ser9. TF was intraperitoneally administered once per day to wildtype and ILK knockdown mice (cKDILK) challenged with a high-fat diet (HFD) or control diet (STD) for 2 weeks. Body and WAT weight gains were compared. The expression of ILK and other markers was determined in the visceral epididymal (epi) and inguinal subcutaneous (sc) WAT.

Results

TF reduced TG content and the expression of adipogenesis markers and transporters in adipocytes, while UCP-1 expression was increased and the expression of lipases, cytokines or PCNA was not affected. Mechanistically, TF rapidly increased and faded the intracellular phosphorylation of INTB1 but not AKT or GSK3β. F-actin levels were rapidly decreased, and INTB1 blockade avoided the TF effect. After 24 h, ILK expression and phosphorylation rates of AKT and GSK3β were upregulated, while ILK silencing increased TG content. INTB1 blockade and ILK silencing avoided TF effects on the TG content and the transcriptional expression of PPARγ and UCP1. In HFD-challenged mice, the systemic administration of TF for several days reduced the weight gain on WAT depots. TF reduced adipogenesis and pro-inflammatory biomarkers and increased lipolysis markers HSL and FAT in epiWAT from HFD, while increased UCP1 in scWAT. In both WATs, TF upregulated ILK expression and activity, while no changes were observed in other tissues. In HFD-fed cKDILK, the blunted ILK in epiWAT worsened weight gain and avoided the anti-obesogenic effect of in vivo TF administration.

Conclusions

ILK downregulation in WAT can be considered a biomarker of obesity establishment. Via an INTB1-ILK axis, TF restores malfunctioning hypertrophied WAT by changing the expression of adipocyte-related genes, increasing ILK expression and activity, and reducing TG storage. TF prevents obesity, a property to be added to its anticoagulant and cardiovascular protective advantages.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00746-1.

Simvastatin-Incorporated Drug Delivery Systems for Bone Regeneration

Local drug delivery systems composed of biomaterials and osteogenic substances provide promising strategies for the reconstruction of large bone defects. In recent years, simvastatin has been studied extensively for its pleiotropic effects other than lowering of cholesterol, including its ability to induce osteogenesis and angiogenesis. Accordingly, several studies of simvastatin incorporated drug delivery systems have been performed to demonstrate the feasibility of such systems in enhancing bone regeneration. Therefore, this review explores the molecular mechanisms by which simvastatin affects bone metabolism and angiogenesis. The simvastatin concentrations that promote osteogenic differentiation are analyzed. Furthermore, we summarize and discuss a variety of simvastatin-loaded drug delivery systems that use different loading methods and materials. Finally, current shortcomings of and future development directions for simvastatin-loaded drug delivery systems are summarized. This review provides various advanced design strategies for simvastatin-incorporated drug delivery systems that can enhance bone regeneration.

Zc3h10 regulates adipogenesis by controlling translation and F-actin/mitochondria interaction

The commitment of mesenchymal stem cells to preadipocytes is stimulated by hormonal induction. Preadipocytes induced to differentiate repress protein synthesis, remodel their cytoskeleton, and increase mitochondrial function to support anabolic pathways. These changes enable differentiation into mature adipocytes. Our understanding of the factors that coordinately regulate the early events of adipocyte differentiation remains incomplete. Here, by using multipronged approaches, we have identified zinc finger CCCH-type containing 10 (Zc3h10) as a critical regulator of the early stages of adipogenesis. Zc3h10 depletion in preadipocytes resulted in increased protein translation and impaired filamentous (F)-actin remodeling, with the latter detrimental effect leading to mitochondrial and metabolic dysfunction. These defects negatively affected differentiation to mature adipocytes. In contrast, Zc3h10 overexpression yielded mature adipocytes with remarkably increased lipid droplet size. Overall, our study establishes Zc3h10 as a fundamental proadipogenic transcription factor that represses protein synthesis and promotes F-actin/mitochondria dynamics to ensure proper energy metabolism and favor lipid accumulation.

A function of fascin1 in the colony formation of mouse embryonic stem cells

Fascin1 is known to participate in the migration of cancer cells by binding to actin filaments. Recent studies evidenced that fascin1 also modulates processes such as the tumorigenesis and maintenance of pluripotency genes in cancer stem cells. However, the function of fascin1 in embryonic stem cells remains unclear. In this article, we report that fascin1 is highly expressed and widely distributed in mouse embryonic stem cells (mESCs), which are regulated by JAK-STAT3 and β-catenin. We found that the overexpression of fascin1 impairs the formation of mESC colonies via the downregulation of intercellular adhesion molecules, and that mimicking the dephosphorylated mutation of fascin1 or inhibiting phosphorylation with Gö6983 significantly enhances colony formation. Hyperphosphorylated fascin1 can promote the maintenance of pluripotency in mESCs via nuclear localization and suppressing DNA methyltransferase expression. Our findings demonstrate a novel function of fascin1, as a vital regulator, in the colony formation and pluripotency of mESCs and provide insights into the molecular mechanisms underlying embryonic stem cell self-organization and development in vitro.

Actin and myosin II modulate differentiation of pluripotent stem cells

Use of stem cell-based therapies in tissue engineering and regenerative medicine is hindered by efficient means of directed differentiation. For pluripotent stem cells, an initial critical differentiation event is specification to one of three germ lineages: endoderm, mesoderm, and ectoderm. Differentiation is known to be regulated by numerous extracellular and intracellular factors, but the role of the cytoskeleton during specification, or early differentiation, is still unknown. In these studies, we used agonists and antagonists to modulate actin polymerization and the actin-myosin molecular motor during spontaneous differentiation of embryonic stem cells in embryoid bodies. We found that inhibiting either actin polymerization or actin-myosin interactions led to a decrease in differentiation to the mesodermal lineage and an increase in differentiation to the endodermal lineage. Thus, targeting processes that regulate cytoskeletal tension may be effective in enhancing or inhibiting differentiation towards cells of the endodermal or mesodermal lineages, which include hepatocytes, islets, cardiomyocytes, endothelial cells, and osteocytes. Therefore, these fundamental findings demonstrate that modulation of the cytoskeleton may be useful in production for a range of cell-based therapies, including for liver, pancreatic, cardiac, vascular, and orthopedic applications.

Mesenchymal stem cell fate following non-viral gene transfection strongly depends on the choice of delivery vector

Controlling the phenotype of mesenchymal stem cells (MSCs) through the delivery of regulatory genes is a promising strategy in tissue engineering (TE). Essential to effective gene delivery is the choice of gene carrier. Non-viral delivery vectors have been extensively used in TE, however their intrinsic effects on MSC differentiation remain poorly understood. The objective of this study was to investigate the influence of three different classes of non-viral gene delivery vectors: (1) cationic polymers (polyethylenimine, PEI), (2) inorganic nanoparticles (nanohydroxyapatite, nHA) and (3) amphipathic peptides (RALA peptide) on modulating stem cell fate after reporter and therapeutic gene delivery. Despite facilitating similar reporter gene transfection efficiencies, these nanoparticle-based vectors had dramatically different effects on MSC viability, cytoskeletal morphology and differentiation. After reporter gene delivery (pGFP or pLUC), the nHA and RALA vectors supported an elongated MSC morphology, actin stress fibre formation and the development of mature focal adhesions, while cells appeared rounded and less tense following PEI transfection. These changes in MSC morphology correlated with enhanced osteogenesis following nHA and RALA transfection and adipogenesis following PEI transfection. When therapeutic genes encoding for transforming growth factor beta 3 (TGF-β3) and/or bone morphogenic protein 2 (BMP2) were delivered to MSCs, nHA promoted osteogenesis in 2D culture and the development of an endochondral phenotype in 3D culture, while RALA was less osteogenic and appeared to promote a more stable hyaline cartilage-like phenotype. In contrast, PEI failed to induce robust osteogenesis or chondrogenesis of MSCs, despite effective therapeutic protein production. Taken together, these results demonstrate that the differentiation of MSCs through the application of non-viral gene delivery strategies depends not only on the gene delivered, but also on the gene carrier itself.

STATEMENT OF SIGNIFICANCE

Nanoparticle-based non-viral gene delivery vectors have been extensively used in regenerative medicine, however their intrinsic effects on mesenchymal stem cell (MSC) differentiation remain poorly understood. This paper demonstrates that different classes of commonly used non-viral vectors are not inert and they have a strong effect on cell morphology, stress fiber formation and gene transcription in MSCs, which in turn modulates their capacity to differentiate towards osteogenic, adipogenic and chondrogenic lineages. These results also point to the need for careful and tissue-specific selection of nanoparticle-based delivery vectors to prevent undesired phenotypic changes and off-target effects when delivering therapeutic genes to damaged or diseased tissues.

New Bioengineering Breakthroughs and Enabling Tools in Regenerative Medicine

PURPOSE OF REVIEW

In this review, we provide a general overview of recent bioengineering breakthroughs and enabling tools that are transforming the field of regenerative medicine (RM). We focus on five key areas that are evolving and increasingly interacting including mechanobiology, biomaterials and scaffolds, intracellular delivery strategies, imaging techniques, and computational and mathematical modeling.

RECENT FINDINGS

Mechanobiology plays an increasingly important role in tissue regeneration and design of therapies. This knowledge is aiding the design of more precise and effective biomaterials and scaffolds. Likewise, this enhanced precision is enabling ways to communicate with and stimulate cells down to their genome. Novel imaging technologies are permitting visualization and monitoring of all these events with increasing resolution from the research stages up to the clinic. Finally, algorithmic mining of data and soft matter physics and engineering are creating growing opportunities to predict biological scenarios, device performance, and therapeutic outcomes.

SUMMARY

We have found that the development of these areas is not only leading to revolutionary technological advances but also enabling a conceptual leap focused on targeting regenerative strategies in a holistic manner. This approach is bringing us ever more closer to the reality of personalized and precise RM.

Systems view of adipogenesis via novel omics-driven and tissue-specific activity scoring of network functional modules

The investigation of the complex processes involved in cellular differentiation must be based on unbiased, high throughput data processing methods to identify relevant biological pathways. A number of bioinformatics tools are available that can generate lists of pathways ranked by statistical significance (i.e. by p-value), while ideally it would be desirable to functionally score the pathways relative to each other or to other interacting parts of the system or process. We describe a new computational method (Network Activity Score Finder - NASFinder) to identify tissue-specific, omics-determined sub-networks and the connections with their upstream regulator receptors to obtain a systems view of the differentiation of human adipocytes. Adipogenesis of human SBGS pre-adipocyte cells in vitro was monitored with a transcriptomic data set comprising six time points (0, 6, 48, 96, 192, 384 hours). To elucidate the mechanisms of adipogenesis, NASFinder was used to perform time-point analysis by comparing each time point against the control (0 h) and time-lapse analysis by comparing each time point with the previous one. NASFinder identified the coordinated activity of seemingly unrelated processes between each comparison, providing the first systems view of adipogenesis in culture. NASFinder has been implemented into a web-based, freely available resource associated with novel, easy to read visualization of omics data sets and network modules.

Simvastatin enhances Rho/actin/cell rigidity pathway contributing to mesenchymal stem cells' osteogenic differentiation

Recent studies have indicated that statins induce osteogenic differentiation both in vitro and in vivo. The molecular mechanism of statin-stimulated osteogenesis is unknown. Activation of RhoA signaling increases cytoskeletal tension, which plays a crucial role in the osteogenic differentiation of mesenchymal stem cells. We thus hypothesized that RhoA signaling is involved in simvastatin-induced osteogenesis in bone marrow mesenchymal stem cells. We found that although treatment with simvastatin shifts localization of RhoA protein from the membrane to the cytosol, the treatment still activates RhoA dose-dependently because it reduces the association with RhoGDIα. Simvastatin also increased the expression of osteogenic proteins, density of actin filament, the number of focal adhesions, and cellular tension. Furthermore, disrupting actin cytoskeleton or decreasing cell rigidity by using chemical agents reduced simvastatin-induced osteogenic differentiation. In vivo study also confirms that density of actin filament is increased in simvastatin-induced ectopic bone formation. Our study is the first to demonstrate that maintaining intact actin cytoskeletons and enhancing cell rigidity are crucial in simvastatin-induced osteogenesis. The results suggested that simvastatin, which is an osteoinductive factor and acts by increasing actin filament organization and cell rigidity combined with osteoconductive biomaterials, may benefit stem-cell-based bone regeneration.

Lipid-overloaded enlarged adipocytes provoke insulin resistance independent of inflammation

In obesity, adipocyte hypertrophy and proinflammatory responses are closely associated with the development of insulin resistance in adipose tissue. However, it is largely unknown whether adipocyte hypertrophy per se might be sufficient to provoke insulin resistance in obese adipose tissue. Here, we demonstrate that lipid-overloaded hypertrophic adipocytes are insulin resistant independent of adipocyte inflammation. Treatment with saturated or monounsaturated fatty acids resulted in adipocyte hypertrophy, but proinflammatory responses were observed only in adipocytes treated with saturated fatty acids. Regardless of adipocyte inflammation, hypertrophic adipocytes with large and unilocular lipid droplets exhibited impaired insulin-dependent glucose uptake, associated with defects in GLUT4 trafficking to the plasma membrane. Moreover, Toll-like receptor 4 mutant mice (C3H/HeJ) with high-fat-diet-induced obesity were not protected against insulin resistance, although they were resistant to adipose tissue inflammation. Together, our in vitro and in vivo data suggest that adipocyte hypertrophy alone may be crucial in causing insulin resistance in obesity.

Environmental physical cues determine the lineage specification of mesenchymal stem cells

BACKGROUND

Physical cues of cellular environment affect cell fate and differentiation. For example, an environment with high stiffness drives mesenchymal stem cells (MSCs) to undergo osteogenic differentiation, while low stiffness leads to lipogenic differentiation. Such effects could be independent of chemical/biochemical inducers.

SCOPE OF REVIEW

Stiffness and/or topography of cellular environment can control MSC differentiation and fate determination. In addition, physical factors such as tension, which resulted from profound cytoskeleton reorganization during MSC differentiation, affect the gene expression essential for the differentiation. Although physical cues control MSC lineage specification probably by reorganizing and tuning cytoskeleton, the full mechanism is largely unclear. It also remains elusive how physical signals are sensed by cells and transformed into biochemical and biological signals. More importantly, it becomes pivotal to define explicitly the physical cue(s) essential for cell differentiation and fate decision. With a focus on MSC, we present herein current understanding of the interplay between i) physical cue and factors and ii) MSC differentiation and fate determination.

MAJOR CONCLUSIONS

Biophysical cues can initiate or strengthen the biochemical signaling for MSC fate determination and differentiation. Physical properties of cellular environment direct the structural adaptation and functional coupling of the cells to their environment.

GENERAL SIGNIFICANCE

These observations not only open a simple avenue to engineer cell fate in vitro, but also start to reveal the physical elements that regulate and determine cell fate.

Evaluation of the effects of boron nitride nanotubes functionalized with gum arabic on the differentiation of rat mesenchymal stem cells

The biocompatibility of boron nitride nanotubes with rat mesenchymal stem cells has been evaluated in terms of cell proliferation and differentiation.

Rho-associated protein kinases play an important role in the differentiation of rat adipose-derived stromal cells into cardiomyocytes in vitro

Adipose-derived stromal cells (ADSCs) represent a readily available abundant supply of mesenchymal stem cells and have the ability to differentiate into cardiomyocytes in mice and human, making ADSCs a promising source of cardiomyocytes for transplantation. However, there has been no report of differentiation of rat ADSCs into cardiomyocytes. In addition, signaling pathways in the differentiation process from ADSCs to cardiomyocytes are unknown. In this study, we first demonstrated that rat ADSCs spontaneously differentiated into cardiomyocytes in vitro, when cultured on a complete medium formulation MethoCult GF M3534. These differentiated cells possessed cardiomyocyte phenotype and expressed cardiac markers. Moreover, these cells showed open excitation-contracting coupling and Ca²⁺ transient and contracted spontaneously. The role of Rho-associated protein kinases (ROCKs) in the differentiation process was then studied by using ROCK-specific inhibitor Y-27632 and ROCK siRNAs. These agents changed the arrangement of cytoskeleton and diminished appearance of cardiomyocyte phenotype, accompanied by inhibition of c-Jun N-terminal kinase (JNK) phosphorylation and promotion of Akt phosphorylation. Collectively, this is the first study to demonstrate that rat ADSCs could spontaneously differentiate into cardiomyocytes in vitro and ROCKs play an important role in the differentiation of ADSCs into beating cardiomyocytes in conjunction of the PI3K/Akt pathway and the JNK pathway.

Adipocyte stiffness increases with accumulation of lipid droplets

Adipogenesis and increase in fat tissue mass are mechanosensitive processes and hence should be influenced by the mechanical properties of adipocytes. We evaluated subcellular effective stiffnesses of adipocytes using atomic force microscopy (AFM) and interferometric phase microscopy (IPM), and we verified the empirical results using finite element (FE) simulations. In the AFM studies, we found that the mean ratio of stiffnesses of the lipid droplets (LDs) over the nucleus was 0.83 ± 0.14, from which we further evaluated the ratios of LDs over cytoplasm stiffness, as being in the range of 2.5 to 8.3. These stiffness ratios, indicating that LDs are stiffer than cytoplasm, were verified by means of FE modeling, which simulated the AFM experiments, and provided good agreement between empirical and model-predicted structural behavior. In the IPM studies, we found that LDs mechanically distort their intracellular environment, which again indicated that LDs are mechanically stiffer than the surrounding cytoplasm. Combining these empirical and simulation data together, we provide in this study evidence that adipocytes stiffen with differentiation as a result of accumulation of LDs. Our results are relevant to research of adipose-related diseases, particularly overweight and obesity, from a mechanobiology and cellular mechanics perspectives.

Factor XIII-A transglutaminase acts as a switch between preadipocyte proliferation and differentiation

Factor XIII-A (FXIII-A) transglutaminase (TG) was recently identified as a potential causative obesity gene in human white adipose tissue (WAT). Here, we have examined the role of TG activity and the role of protein crosslinking in adipogenesis. Mouse WAT and preadipocytes showed abundant TG activity arising from FXIII-A. FXIII-A was localized to the cell surface and acted as a negative regulator of adipogenesis by promoting assembly of fibronectin (FN) from plasma into preadipocyte extracellular matrix. This modulated cytoskeletal dynamics and maintained the preadipocyte state. FXIII-A-assembled plasma FN (pFN) matrix promoted preadipocyte proliferation and potentiated the proproliferative effects of insulin (INS) while suppressing the prodifferentiating INS signaling. FXIII-A-deficient mouse embryonic fibroblasts showed increased lipid accumulation and decreased proliferation as well as decreased pFN assembly into extracellular matrix. Thus, FXIII-A serves as a preadipocyte-bound proliferation/differentiation switch that mediates effects of hepatocyte-produced circulating pFN.

Transcriptional background of beef marbling - novel genes implicated in intramuscular fat deposition

The purpose of this study was to identify novel marbling-related genes by comparison of the global gene expression in semitendinosus muscle of 15-month-old Limousin (LIM), Holstein-Friesian (HF) and Hereford (HER) bulls. Muscle of LIM was lean with low intramuscular fat (IMF) content (0.53%) unlike the marbled muscles of HER and HF characterized by higher amounts of IMF (1.10 and 0.81%, respectively). The comparison of muscle transcriptional profile between marbled and lean beef revealed significant differences in expression of 144 genes, presumably involved in consecutive stages of adipose tissue development, such as preadipocyte proliferation and differentiation, adipocyte maturation, lipid filling and lipid metabolism leading to increased IMF deposition and marbling development. Correlation coefficients and regression analysis for nine of them (gadd45a, pias3, ccrn4l, diras3, pou5f1, hoxa9, atp2a2 and pim1) validated by real-time qPCR confirmed their moderate-high correlation with IMF% and explained up to 70.5% of the total variability in IMF deposition in the bulls.

BSCL2/seipin regulates adipogenesis through actin cytoskeleton remodelling

Seipin regulates lipid homeostasis by preventing lipid droplet (LD) formation in non-adipocytes but promoting it in developing adipocytes. Here, we report that seipin interacts with 14-3-3β through its N- and C-termini. Expression of 14-3-3β is upregulated during adipogenesis, and its deletion results in defective adipogenesis without affecting key adipogenic transcription factors. We further identified the actin-severing protein cofilin-1 as an interacting partner to 14-3-3β. Cofilin-1 was spatiotemporally recruited by 14-3-3β in the cytoplasm during adipocyte differentiation. Extensive actin cytoskeleton remodelling, from stress fibres to cortical structures, was apparent during adipogenesis, but not under lipogenic conditions, indicating that actin cytoskeleton remodelling is only required for adipocyte development. Similar to seipin and 14-3-3β, cofilin-1 knockdown led to impaired adipocyte development. At the cellular level, differentiated cells with knockdown of cofilin-1, 14-3-3β or seipin continued to maintain relatively intact stress fibres, in contrast to cortical actin structure in control cells. Finally, 3T3-L1 cells expressing a severing-resistant actin mutant exhibited impaired adipogenesis. We propose that seipin regulates adipogenesis by recruiting cofilin-1 to remodel actin cytoskeleton through the 14-3-3β protein.

Modulating the actin cytoskeleton affects mechanically induced signal transduction and differentiation in mesenchymal stem cells

Mechanical interactions of mesenchymal stem cells (MSC) with the environment play a significant role in controlling the diverse biological functions of these cells. Mechanical forces are transduced by integrins to the actin cytoskeleton that functions as a scaffold to switch mechanical signals into biochemical pathways. To explore the significance of cytoskeletal mechanisms in human MSC we modulated the actin cytoskeleton using the depolymerising drugs cytochalasin D (CytD) and latrunculin A (LatA), as well as the stabilizing drug jasplakinolide (Jasp) and examined the activation of the signalling molecules ERK and AKT during mechanical loading. All three drugs provoked significant changes in cell morphology and organisation of the cytoskeleton. Application of mechanical forces to β1-integrin receptors using magnetic beads without deformation of the cell shape induced a phosphorylation of ERK and AKT. Of the two drugs that inhibited the cytoskeletal polymerization, LatA completely blocked the activation of ERK and AKT due to mechanical forces, whereas CytD inhibited the activation of AKT but not of ERK. Activation of both signalling molecules by integrin loading was not affected due to cell treatment with the cytoskeleton stabilizing drug Jasp. To correlate the effects of the drugs on mechanically induced activation of AKT and ERK with parameters of MSC differentiation, we studied ALP activity as a marker for osteogenic differentiation and examined the uptake of fat droplets as marker for adipogenic differentiation in the presence of the drugs. All three drugs inhibited ALP activity of MSC in osteogenic differentiation medium. Adipogenic differentiation was enhanced by CytD and Jasp, but not by LatA. The results indicate that modulation of the cytoskeleton using perturbing drugs can differentially modify both mechanically induced signal transduction and MSC differentiation. In addition to activation of the signalling molecules ERK and AKT, other cytoskeletal mechanisms are involved in MSC differentiation.

Surface bound amine functional group density influences embryonic stem cell maintenance

Gradient surfaces are highly effective tools to screen and optimize cell- surface interactions. Here, the response of embryonic stem (ES) cell colonies to plasma polymer gradient surfaces is investigated. Surface chemistry ranged from pure allylamine (AA) plasma polymer on one end of the gradient to pure octadiene (OD) plasma polymer on the other end. Optimal surface chemistry conditions for retention of pluripotency were identified. Expression of the stem cell markers alkaline phosphatase (AP) and Oct4 varied with the position of the ES cell colonies across the OD-AA plasma polymer gradient. Both markers were more strongly retained on the OD plasma polymer rich regions of the gradients. The observed variation of expression across the plasma polymer gradient increased with duration of stem cell culture. While maximum cell adhesion to the gradient substrate occurred at a nitrogen- to-carbon (N/C ratio) of approximately 0.1, Oct4 and AP expression was best retained at an N/C ratio < 0.04. Stem cell marker expression correlated with colony size and morphology: more compact, multilayered colonies with prominent F-actin staining arose as the N/C ratio decreased. Disruption of actin polymerization using Y-27632 ROCK inhibitor resulted in a collapse of the multilayer colony structure into monolayers with limited cell-cell contact. A corresponding decrease in expression of AP and Oct4 was observed. Oct4 expression along with 3D colony morphology was partially rescued on the OD plasma polymer rich regions of the gradient.

Regulation of adipogenesis by cytoskeleton remodelling is facilitated by acetyltransferase MEC-17-dependent acetylation of α-tubulin

Cytoskeleton remodelling is a prerequisite step for the morphological transition from preadipocytes to mature adipocytes. Although microtubules play a pivotal role in organizing cellular structure, regulation of microtubule dynamics during adipogenesis remains unclear. In the present paper we show that acetylation of α-tubulin is up-regulated during adipogenesis, and adipocyte development is dependent on α-tubulin acetylation, as expression of an acetylation-resistant α-tubulin mutant significantly inhibits adipogenesis. Moreover, acetylation of α-tubulin is under the control of the acetyltransferase MEC-17 and deacetylases SIRT2 (Sirtuin 2) and HDAC6 (histone deacetylase 6). Adipocyte development is inhibited in MEC-17-knockdown cells, but enhanced in MEC-17-overexpressing cells. Finally, we show that katanin, a microtubule-severing protein with enhanced activity on acetylated α-tubulin, is actively involved in adipogenesis. We propose that co-ordinated up-regulation of α-tubulin acetylation initiates cytoskeleton remodelling by promoting α-tubulin severing by katanin which, in turn, allows expansion of lipid droplets and accelerates the morphological transition toward mature adipocytes.

Effects of barium titanate nanoparticles on proliferation and differentiation of rat mesenchymal stem cells

Nanomaterials hold great promise in the manipulation and treatments of mesenchymal stem cells, since they allow the modulation of their properties and differentiation. However, systematic studies have to be carried out in order to assess their potential toxicological effects. The present study reports on biocompatibility evaluation of glycol-chitosan coated barium titanate nanoparticles (BTNPs) on rat mesenchymal stem cells (MSCs). BTNPs are a class of ceramic systems which possess interesting features for biological applications thanks to their peculiar dielectric and piezoelectric properties. Viability was evaluated up to 5 days of incubation (concentrations in the range 0-100 μg/ml) both quantitatively and qualitatively with specific assays. Interactions cells/nanoparticles were further investigated with analysis of the cytoskeleton conformation, with SEM and TEM imaging, and with AFM analysis. Finally, differentiation in adipocytes and osteocytes was achieved in the presence of high doses of BTNPs, thus highlighting the safety of these nanostructures towards mesenchymal stem cells.

Assessing embryonic stem cell response to surface chemistry using plasma polymer gradients

The control of cell-material interactions is the key to a broad range of biomedical interactions. Gradient surfaces have recently been established as tools allowing the high-throughput screening and optimization of these interactions. In this paper, we show that plasma polymer gradients can reveal the subtle influence of surface chemistry on embryonic stem cell behavior and probe the mechanisms by which this occurs. Lateral gradients of surface chemistry were generated by plasma polymerization of diethylene glycol dimethyl ether on top of a substrate coated with an acrylic acid plasma polymer using a tilted slide as a mask. Gradient surfaces were characterized by X-ray photoelectron spectroscopy, infrared microscopy mapping and profilometry. By changing the plasma polymerization time, the gradient profile could be easily manipulated. To demonstrate the utility of these surfaces for the screening of cell-material interactions, we studied the response of mouse embryonic stem (ES) cells to these gradients and compared the performance of different plasma polymerization times during gradient fabrication. We observed a strong correlation between surface chemistry and cell attachment, colony size and retention of stem cell markers. Cell adhesion and colony formation showed striking differences on gradients with different plasma polymer deposition times. Deposition time influenced the depth of the plasma film deposited and the relative position of surface functional group density on the substrate, but not the range of plasma-generated species.

RhoA/ROCK, cytoskeletal dynamics, and focal adhesion kinase are required for mechanical stretch-induced tenogenic differentiation of human mesenchymal stem cells

Human bone marrow mesenchymal stem cells (hMSCs) have the potential to differentiate into tendon/ligament-like lineages when they are subjected to mechanical stretching. However, the means through which mechanical stretch regulates the tenogenic differentiation of hMSCs remains unclear. This study examined the role of RhoA/ROCK, cytoskeletal organization, and focal adhesion kinase (FAK) in mechanical stretch-induced tenogenic differentiation characterized by the up-regulation of tendon-related marker gene expression. Our findings showed that RhoA/ROCK and FAK regulated mechanical stretch-induced realignment of hMSCs by regulating cytoskeletal organization and that RhoA/ROCK and cytoskeletal organization were essential to mechanical stretch-activated FAK phosphorylation at Tyr397. We also demonstrated that this process can be blocked by Y-27632 (a specific inhibitor of RhoA/ROCK), cytochalasin D (an inhibitor of cytoskeletal organization) or PF 573228 (a specific inhibitor of FAK). The results of this study suggest that RhoA/ROCK, cytoskeletal organization, and FAK compose a "signaling network" that senses mechanical stretching and drives mechanical stretch-induced tenogenic differentiation of hMSCs. This work provides novel insights regarding the mechanisms of tenogenesis in a stretch-induced environment and supports the therapeutic potential of hMSCs.

Differential role of actin-binding proteins in controlling the adipogenic differentiation of human CD105-positive Wharton's Jelly cells

BACKGROUND

Wharton's Jelly cells (WJCs) can be differentiated into adipocytes by cytoskeletal reorganisation in association with changes in the mechanical properties of cells.

METHODS

WJCs subjected to adipocyte induction were observed changes in the cell morphology and alterations in actin filament formation. Transfection with either small interfering RNAs (siRNAs) against formin-2 (FMN-2), tropomyosin-1 (Tm-1), caldesmon (CaD), and profilin (Pro) or a pcDNA6-gelsolin (GSN)-constructed vector in WJCs was used to establish their regulatory roles in controlling adipogenesis. Phenotypic transformation of the cell shape and changes in cell surface adhesion force were determined in WJCs after transformation.

RESULTS

The levels of protein and mRNA expression of β-actin and several key actin binding ptoteins (ABPs) were decreased during the early stage of adipogenic induction but were recovered in the later induction. The siFMN-2, siTm-1, siCaD, and siPro gene knockdown in WJCs caused a widening of the cell shape, while WJCs overexpressing GSN retained a fibroblast cell shape. For both transformations, atomic force microscopy revealed alterations in the biomechanical signals on the cell surface. However, the adipogenic potency was increased after siFMN-2, siTm-1, siCaD, and siPro gene knockdown and decreased during GSN overexpression.

CONCLUSIONS

siRNA gene knockdown of siFMN-2, siTm-1, siCaD, and siPro enhances the potency for WJCs commitment to adipocyte, while GSN overexpression modulates the PPAR-γ-independent pathway for the adipogenesis of WJCs.

GENERAL SIGNIFICANCE

The phenotypic changes associated with decreased ABP gene expression are critical for regulating the adipogenic differentiation of WJCs through the temporal control of actin filament organisation.

RPTPμ tyrosine phosphatase promotes adipogenic differentiation via modulation of p120 catenin phosphorylation

Adipocyte differentiation can be regulated by the combined activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). In particular, PTPs act as key regulators in differentiation-associated signaling pathways. We recently found that receptor-type PTPμ (RPTPμ) expression is markedly increased during the adipogenic differentiation of 3T3-L1 preadipocytes and mesenchymal stem cells. Here, we investigate the functional roles of RPTPμ and the mechanism of its involvement in the regulation of signal transduction during adipogenesis of 3T3-L1 cells. Depletion of endogenous RPTPμ by RNA interference significantly inhibited adipogenic differentiation, whereas RPTPμ overexpression led to an increase in adipogenic differentiation. Ectopic expression of p120 catenin suppressed adipocyte differentiation, and the decrease in adipogenesis by p120 catenin was recovered by introducing RPTPμ. Moreover, RPTPμ induced a decrease in the cytoplasmic p120 catenin expression by reducing its tyrosine phosphorylation level, consequently leading to enhanced translocation of Glut-4 to the plasma membrane. On the basis of these results, we propose that RPTPμ acts as a positive regulator of adipogenesis by modulating the cytoplasmic p120 catenin level. Our data conclusively demonstrate that differentiation into adipocytes is controlled by RPTPμ, supporting the utility of RPTPμ and p120 catenin as novel target proteins for the treatment of obesity.

Repressor transcription factor 7-like 1 promotes adipogenic competency in precursor cells

The identification of factors that define adipocyte precursor potential has important implications for obesity. Preadipocytes are fibroblastoid cells committed to becoming round lipid-laden adipocytes. In vitro, this differentiation process is facilitated by confluency, followed by adipogenic stimuli. During adipogenesis, a large number of cytostructural genes are repressed before adipocyte gene induction. Here we report that the transcriptional repressor transcription factor 7-like 1 (TCF7L1) binds and directly regulates the expression of cell structure genes. Depletion of TCF7L1 inhibits differentiation, because TCF7L1 indirectly induces the adipogenic transcription factor peroxisome proliferator-activated receptor γ in a manner that can be replaced by inhibition of myosin II activity. TCF7L1 is induced by cell contact in adipogenic cell lines, and ectopic expression of TCF7L1 alleviates the confluency requirement for adipocytic differentiation of precursor cells. In contrast, TCF7L1 is not induced during confluency of non-adipogenic fibroblasts, and, remarkably, forced expression of TCF7L1 is sufficient to commit non-adipogenic fibroblasts to an adipogenic fate. These results establish TCF7L1 as a transcriptional hub coordinating cell-cell contact with the transcriptional repression required for adipogenic competency.