Substratum-induced differentiation of human pluripotent stem cells reveals the coactivator YAP is a potent regulator of neuronal specification

Nuclear pores safeguard the integrity of the nuclear envelope

Nuclear pore complexes (NPCs) mediate nucleocytoplasmic exchange, which is essential for eukaryotes. Mutations in the central scaffolding components of NPCs are associated with genetic diseases, but how they manifest only in specific tissues remains unclear. This is exemplified in Nup133-deficient mouse embryonic stem cells, which grow normally during pluripotency, but differentiate poorly into neurons. Here, using an innovative in situ structural biology approach, we show that Nup133-/- mouse embryonic stem cells have heterogeneous NPCs with non-canonical symmetries and missing subunits. During neuronal differentiation, Nup133-deficient NPCs frequently disintegrate, resulting in abnormally large nuclear envelope openings. We propose that the elasticity of the NPC scaffold has a protective function for the nuclear envelope and that its perturbation becomes critical under conditions that impose an increased mechanical load onto nuclei.

Mechanical confinement matters: Unveiling the effect of two-photon polymerized 2.5D and 3D microarchitectures on neuronal YAP expression and neurite outgrowth

The effect of mechanical cues on cellular behaviour has been reported in multiple studies so far, and a specific aspect of interest is the role of mechanotransductive proteins in neuronal development. Among these, yes-associated protein (YAP) is responsible for multiple functions in neuronal development such as neuronal progenitor cells migration and differentiation while myocardin-related transcription factor A (MRTFA) facilitates neurite outgrowth and axonal pathfinding. Both proteins have indirectly intertwined fates via their signalling pathways. There is little literature investigating the roles of YAP and MRTFA in vitro concerning neurite outgrowth in mechanically confined microenvironments. Moreover, our understanding of their relationship in immature neurons cultured within engineered confined microenvironments is still lacking. In this study, we fabricated, via two-photon polymerization (2PP), 2.5D microgrooves and 3D polymeric microchannels, with a diameter range from 5 to 30 μm. We cultured SH-SY5Y cells and differentiated them into immature neuron-like cells on both 2.5D and 3D microstructures to investigate the effect of mechanical confinement on cell morphology and protein expression. In 2.5D microgrooves, both YAP and MRTFA nuclear/cytoplasmic (N/C) ratios exhibited maxima in the 10 μm grooves indicating a strong relation with mechanical-stress-inducing confinement. In 3D microchannels, both proteins' N/C ratio exhibited minima in presence of 5 or 10 μm channels, a behaviour that was opposite to the ones observed in the 2.5D microgrooves and that indicates how the geometry and mechanical confinement of 3D microenvironments are unique compared to 2.5D ones due to focal adhesion, actin, and nuclear polarization. Further, especially in presence of 2.5D microgrooves, cells featured an inversely proportional relationship between YAP N/C ratio and the average neurite length. Finally, we also cultured human induced pluripotent stem cells (hiPSCs) and differentiated them into cortical neurons on the microstructures for up to 2 weeks. Interestingly, YAP and MRTFA N/C ratios also showed a maximum around the 10 μm 2.5D microgrooves, indicating the physiological relevance of our study. Our results elucidate the possible differences induced by 2.5D and 3D confining microenvironments in neuronal development and paves the way for understanding the intricate interplay between mechanotransductive proteins and their effect on neural cell fate within engineered cell microenvironments.

Mechanosensitive Differentiation of Human iPS Cell-Derived Podocytes

Stem cell fate decisions, including proliferation, differentiation, morphological changes, and viability, are impacted by microenvironmental cues such as physical and biochemical signals. However, the specific impact of matrix elasticity on kidney cell development and function remains less understood due to the lack of models that can closely recapitulate human kidney biology. An established protocol to differentiate podocytes from human-induced pluripotent stem (iPS) cells provides a promising avenue to elucidate the role of matrix elasticity in kidney tissue development and lineage determination. In this study, we synthesized polyacrylamide hydrogels with different stiffnesses and investigated their ability to promote podocyte differentiation and biomolecular characteristics. We found that 3 kPa and 10 kPa hydrogels significantly support the adhesion, differentiation, and viability of podocytes. Differentiating podocytes on a more compliant (0.7 kPa) hydrogel resulted in significant cell loss and detachment. Further investigation of the mechanosensitive proteins yes-associated protein (YAP) and synaptopodin revealed nuanced molecular distinctions in cellular responses to matrix elasticity that may otherwise be overlooked if morphology and cell spreading alone were used as the primary metric for selecting matrices for podocyte differentiation. Specifically, hydrogels with kidney-like rigidities outperformed traditional tissue culture plates at modulating the molecular-level expression of active mechanosensitive proteins critical for podocyte health and function. These findings could guide the development of physiologically relevant platforms for kidney tissue engineering, disease modeling, and mechanistic studies of organ physiology and pathophysiology. Such advances are critical for realizing the full potential of in vitro platforms in accurately predicting human biological responses.

The Importance of Effective Ligand Concentration to Direct Epithelial Cell Polarity in Dynamic Hydrogels

Epithelial cysts and organoids are multicellular hollow structures formed by correctly polarized epithelial cells. Important in steering these cysts from single cells is the dynamic regulation of extracellular matrix presented ligands, and matrix dynamics. Here, control over the effective ligand concentration is introduced, decoupled from bulk and local mechanical properties, in synthetic dynamic supramolecular hydrogels formed through noncovalent crosslinking of supramolecular fibers. Control over the effective ligand concentration is realized by 1) keeping the ligand concentration constant, but changing the concentration of nonfunctionalized molecules or by 2) varying the ligand concentration, while keeping the concentration of non-functionalized molecules constant. The results show that in 2D, the effective ligand concentration within the supramolecular fibers rather than gel stiffness (from 0.1 to 8 kPa) regulates epithelial polarity. In 3D, increasing the effective ligand concentration from 0.5 × 10-3 to 2 × 10-3 m strengthens the effect of increased gel stiffness from 0.1 to 2 kPa, to synergistically yield more correctly polarized cysts. Through integrin manipulation, it is shown that epithelial polarity is regulated by tension-based homeostasis between cells and matrix. The results reveal the effective ligand concentration as influential factor in regulating epithelial polarity and provide insights on engineering of synthetic biomaterials for cell and organoid culture.

Substrate stiffness alters layer architecture and biophysics of human induced pluripotent stem cells to modulate their differentiation potential

Lineage-specific differentiation of human induced pluripotent stem cells (hiPSCs) relies on complex interactions between biochemical and physical cues. Here we investigated the ability of hiPSCs to undergo lineage commitment in response to inductive signals and assessed how this competence is modulated by substrate stiffness. We showed that Activin A-induced hiPSC differentiation into mesendoderm and its derivative, definitive endoderm, is enhanced on gel-based substrates softer than glass. This correlated with changes in tight junction formation and extensive cytoskeletal remodeling. Further, live imaging and biophysical studies suggested changes in cell motility and interfacial contacts underlie hiPSC layer reshaping on soft substrates. Finally, we repurposed an ultra-soft silicone gel, which may provide a suitable substrate for culturing hiPSCs at physiological stiffnesses. Our results provide mechanistic insight into how epithelial mechanics dictate the hiPSC response to chemical signals and provide a tool for their efficient differentiation in emerging stem cell therapies.

Interface-Mediated Neurogenic Signaling: The Impact of Surface Geometry and Chemistry on Neural Cell Behavior for Regenerative and Brain-Machine Interfacing Applications

Nanomaterial advancements have driven progress in central and peripheral nervous system applications such as tissue regeneration and brain-machine interfacing. Ideally, neural interfaces with native tissue shall seamlessly integrate, a process that is often mediated by the interfacial material properties. Surface topography and material chemistry are significant extracellular stimuli that can influence neural cell behavior to facilitate tissue integration and augment therapeutic outcomes. This review characterizes topographical modifications, including micropillars, microchannels, surface roughness, and porosity, implemented on regenerative scaffolding and brain-machine interfaces. Their impact on neural cell response is summarized through neurogenic outcome and mechanistic analysis. The effects of surface chemistry on neural cell signaling with common interfacing compounds like carbon-based nanomaterials, conductive polymers, and biologically inspired matrices are also reviewed. Finally, the impact of these extracellular mediated neural cues on intracellular signaling cascades is discussed to provide perspective on the manipulation of neuron and neuroglia cell microenvironments to drive therapeutic outcomes.

Advances in 3D tissue models for neural engineering: self-assembled versus engineered tissue models

Neural tissue engineering has emerged as a promising field that aims to create functional neural tissue for therapeutic applications, drug screening, and disease modelling. It is becoming evident in the literature that this goal requires development of three-dimensional (3D) constructs that can mimic the complex microenvironment of native neural tissue, including its biochemical, mechanical, physical, and electrical properties. These 3D models can be broadly classified as self-assembled models, which include spheroids, organoids, and assembloids, and engineered models, such as those based on decellularized or polymeric scaffolds. Self-assembled models offer advantages such as the ability to recapitulate neural development and disease processes in vitro, and the capacity to study the behaviour and interactions of different cell types in a more realistic environment. However, self-assembled constructs have limitations such as lack of standardised protocols, inability to control the cellular microenvironment, difficulty in controlling structural characteristics, reproducibility, scalability, and lengthy developmental timeframes. Integrating biomimetic materials and advanced manufacturing approaches to present cells with relevant biochemical, mechanical, physical, and electrical cues in a controlled tissue architecture requires alternate engineering approaches. Engineered scaffolds, and specifically 3D hydrogel-based constructs, have desirable properties, lower cost, higher reproducibility, long-term stability, and they can be rapidly tailored to mimic the native microenvironment and structure. This review explores 3D models in neural tissue engineering, with a particular focus on analysing the benefits and limitations of self-assembled organoids compared with hydrogel-based engineered 3D models. Moreover, this paper will focus on hydrogel based engineered models and probe their biomaterial components, tuneable properties, and fabrication techniques that allow them to mimic native neural tissue structures and environment. Finally, the current challenges and future research prospects of 3D neural models for both self-assembled and engineered models in neural tissue engineering will be discussed.

Kidney Disease Modeling with Organoids and Organs-on-Chips

Kidney disease is a global health crisis affecting more than 850 million people worldwide. In the United States, annual Medicare expenditures for kidney disease and organ failure exceed $81 billion. Efforts to develop targeted therapeutics are limited by a poor understanding of the molecular mechanisms underlying human kidney disease onset and progression. Additionally, 90% of drug candidates fail in human clinical trials, often due to toxicity and efficacy not accurately predicted in animal models. The advent of ex vivo kidney models, such as those engineered from induced pluripotent stem (iPS) cells and organ-on-a-chip (organ-chip) systems, has garnered considerable interest owing to their ability to more accurately model tissue development and patient-specific responses and drug toxicity. This review describes recent advances in developing kidney organoids and organ-chips by harnessing iPS cell biology to model human-specific kidney functions and disease states. We also discuss challenges that must be overcome to realize the potential of organoids and organ-chips as dynamic and functional conduits of the human kidney. Achieving these technological advances could revolutionize personalized medicine applications and therapeutic discovery for kidney disease.

Assessment of human embryonic stem cells differentiation into definitive endoderm lineage on the soft substrates

Pluripotent stem cells (PSCs) hold enormous potential for treating multiple diseases owing to their ability to self-renew and differentiate into any cell type. Albeit possessing such promising potential, controlling their differentiation into a desired cell type continues to be a challenge. Recent studies suggest that PSCs respond to different substrate stiffness and, therefore, can differentiate towards some lineages via Hippo pathway. Human PSCs can also differentiate and self-organize into functional cells, such as organoids. Traditionally, human PSCs are differentiated on stiff plastic or glass plates towards definitive endoderm and then into functional pancreatic progenitor cells in the presence of soluble growth factors. Thus, whether stiffness plays any role in differentiation towards definitive endoderm from human pluripotent stem cells (hPSCs) remains unclear. Our study found that the directed differentiation of human embryonic stem cells towards endodermal lineage on the varying stiffness did not differ from the differentiation on stiff plastic dishes. We also observed no statistical difference between the expression of yes-associated protein (YAP) and phosphorylated YAP. Furthermore, we demonstrate that lysophosphatidic acid, a YAP activator, enhanced definitive endoderm formation, whereas verteporfin, a YAP inhibitor, did not have the significant effect on the differentiation. In summary, our results suggest that human embryonic stem cells may not differentiate in response to changes in stiffness, and that such cues may not have as significant impact on the level of YAP. Our findings indicate that more research is needed to understand the direct relationship between biophysical forces and hPSCs differentiation.

Spatial cell fate manipulation of human pluripotent stem cells by controlling the microenvironment using photocurable hydrogel

Human pluripotent stem cells (hPSCs) dynamically respond to their chemical and physical microenvironment, dictating their behavior. However, conventional in vitro studies predominantly employ plastic culture wares, which offer a simplified representation of the in vivo microenvironment. Emerging evidence underscores the pivotal role of mechanical and topological cues in hPSC differentiation and maintenance. In this study, we cultured hPSCs on hydrogel substrates with spatially controlled stiffness. The use of culture substrates that enable precise manipulation of spatial mechanical properties holds promise for better mimicking in vivo conditions and advancing tissue engineering techniques. We designed a photocurable polyethylene glycol-polyvinyl alcohol (PVA-PEG) hydrogel, allowing the spatial control of surface stiffness and geometry at a micrometer scale. This versatile hydrogel can be functionalized with various extracellular matrix proteins. Laminin 511-functionalized PVA-PEG gel effectively supports the growth and differentiation of hPSCs. Moreover, by spatially modulating the stiffness of the patterned gel, we achieved spatially selective cell differentiation, resulting in the generation of intricate patterned structures.

Substrate Stiffness Modulates Stemness and Differentiation of Rabbit Corneal Endothelium Through the Paxillin-YAP Pathway

Purpose

To explore the role of substrate stiffness and the mechanism beneath corneal endothelial cells' (CECs') stemness maintenance and differentiation.

Methods

CECs were divided into central zone (8 mm trephined boundary) and peripheral zone (8 mm trephined edge with attached limbal). Two zones were analyzed by hematoxylin-eosin staining and scanning electron microscopy for anatomic structure. The elastic modulus of Descemet's membrane (DM) was analyzed by atomic force microscopy. Compressed type I collagen gels with different stiffness were constructed as an in vitro model system to test the role of stiffness on phenotype using cultured rabbit CECs. Cell morphology, expression and intracellular distribution of Yes-associated protein (YAP), differentiation (ZO-1, Na+/K+-ATPase), stemness (FOXD3, CD34, Sox2, Oct3/4), and endothelial-mesenchymal transition (EnMT) markers were analyzed by immunofluorescence, quantitative RT-PCR, and Western blot.

Results

The results showed that the peripheral area of rabbit and human DM is softer than the central area ex vivo. Using the biomimetic extracellular matrix collagen gels in vitro model, we then demonstrated that soft substrate weakens the differentiation and EnMT in the culture of CECs. It was further proved by the inhibitor experiment that soft substrate enhances stemness maintenance via inhibition of paxillin-YAP signaling, which was activated on a stiff substrate.

Conclusions

Our findings confirm that substrate stiffness modulates the stemness maintenance and differentiation of CECs and suggest a potential strategy for CEC-based corneal tissue engineering.

Mechanics in the nervous system: From development to disease

Physical forces are ubiquitous in biological processes across scales and diverse contexts. This review highlights the significance of mechanical forces in nervous system development, homeostasis, and disease. We provide an overview of mechanical signals present in the nervous system and delve into mechanotransduction mechanisms translating these mechanical cues into biochemical signals. During development, mechanical cues regulate a plethora of processes, including cell proliferation, differentiation, migration, network formation, and cortex folding. Forces then continue exerting their influence on physiological processes, such as neuronal activity, glial cell function, and the interplay between these different cell types. Notably, changes in tissue mechanics manifest in neurodegenerative diseases and brain tumors, potentially offering new diagnostic and therapeutic target opportunities. Understanding the role of cellular forces and tissue mechanics in nervous system physiology and pathology adds a new facet to neurobiology, shedding new light on many processes that remain incompletely understood.

Neurogenic Cell Behavior in 3D Culture Enhanced Within a Highly Compliant Synthetic Hydrogel Platform Formed via Competitive Crosslinking

Purpose

Scaffold materials that better support neurogenesis are still needed to improve cell therapy outcomes for neural tissue damage. We have used a modularly tunable, highly compliant, degradable hydrogel to explore the impacts of hydrogel compliance stiffness on neural differentiation. Here we implemented competitive matrix crosslinking mechanics to finely tune synthetic hydrogel moduli within soft tissue stiffnesses, a range much softer than typically achievable in synthetic crosslinked hydrogels, providing a modularly controlled and ultrasoft 3D culture model which supports and enhances neurogenic cell behavior.

Methods

Soluble competitive allyl monomers were mixed with proteolytically-degradable poly(ethylene glycol) diacrylate derivatives and crosslinked to form a matrix, and resultant hydrogel stiffness and diffusive properties were evaluated. Neural PC12 cells or primary rat fetal neural stem cells (NSCs) were encapsulated within the hydrogels, and cell morphology and phenotype were investigated to understand cell-matrix interactions and the effects of environmental stiffness on neural cell behavior within this model.

Results

Addition of allyl monomers caused a concentration-dependent decrease in hydrogel compressive modulus from 4.40 kPa to 0.26 kPa (natural neural tissue stiffness) without influencing soluble protein diffusion kinetics through the gel matrix. PC12 cells encapsulated in the softest hydrogels showed significantly enhanced neurite extension in comparison to PC12s in all other hydrogel stiffnesses tested. Encapsulated neural stem cells demonstrated significantly greater spreading and elongation in 0.26 kPa alloc hydrogels than in 4.4 kPa hydrogels. When soluble growth factor deprivation (for promotion of neural differentiation) was evaluated within the neural stiffness gels (0.26 kPa), NSCs showed increased neuronal marker expression, indicating early enhancement of neurogenic differentiation.

Conclusions

Implementing allyl-acrylate crosslinking competition reduced synthetic hydrogel stiffness to provide a supportive environment for 3D neural tissue culture, resulting in enhanced neurogenic behavior of encapsulated cells. These results indicate the potential suitability of this ultrasoft hydrogel system as a model platform for further investigating environmental factors on neural cell behavior.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00794-2.

Harnessing mechanobiology for kidney organoid research

Recently, organoids have emerged as revolutionizing tools with the unprecedented potential to recreate organ-specific microanatomy in vitro. Upon their derivation from human pluripotent stem cells (hPSCs), organoids reveal the blueprints of human organogenesis, further allowing the faithful recapitulation of their physiology. Nevertheless, along with the evolution of this field, advanced research exposed the organoids' shortcomings, particularly regarding poor reproducibility rates and overall immatureness. To resolve these challenges, many studies have started to underscore the relevance of mechanical cues as a relevant source to induce and externally control hPSCs differentiation. Indeed, established organoid generation protocols from hPSCs have mainly relyed on the biochemical induction of fundamental signalling pathways present during kidney formation in mammals, whereas mechanical cues have largely been unexplored. This review aims to discuss the pertinence of (bio) physical cues within hPSCs-derived organoid cultures, while deciphering their effect on morphogenesis. Moreover, we will explore state-of-the-art mechanobiology techniques as revolutionizing means for understanding the underlying role of mechanical forces in biological processes in organoid model systems.

The ad hoc chemical design of random PBS-based copolymers influences the activation of cardiac differentiation while altering the HYPPO pathway target genes in hiPSCs

Cardiac tissue engineering is a cutting-edge technology aiming to replace irreversibly damaged cardiac tissue and restore contractile functionality. However, cardiac tissue engineering porous and perfusable scaffolds to enable oxygen supply in vitro and eventually promote angiogenesis in vivo are still desirable. Two fully-aliphatic random copolymers of poly(butylene succinate) (PBS), poly(butylene succinate/Pripol), P(BSBPripol), and poly(butylene/neopentyl glycol succinate), P(BSNS), containing two different subunits, neopentyl glycol and Pripol 1009, were successfully synthesized and then electrospun in tridimentional fibrous mats. The copolymers show different thermal and mechanical behaviours as result of their chemical structure. In particular, copolymerization led to a reduction in crystallinity and consequently PBS stiffness, reaching values of elastic modulus very close to those of soft tissues. Then, to check the biological suitability, human induced Pluripotent Stem Cells (hiPSCs) were directly seeded on both PBS-based copolymeric scaffolds. The results confirmed the ability of both the scaffolds to sustain cell viability and to maintain their stemness during cell expansion. Furthermore, gene expression and immunofluorescence analysis showed that P(BSBPripol) scaffold promoted an upregulation of the early cardiac progenitor and later-stage markers with a simultaneously upregulation of HYPPO pathway gene expression, crucial for mechanosensing of cardiac progenitor cells. These results suggest that the correct ad-hoc chemical design and, in turn, the mechanical properties of the matrix, such as substrate stiffness, together with surface porosity, play a critical role in regulating the behaviour of cardiac progenitors, which ultimately offers valuable insights into the development of novel bio-inspired scaffolds for cardiac tissue regeneration.

Bioactive and chemically defined hydrogels with tunable stiffness guide cerebral organoid formation and modulate multi-omics plasticity in cerebral organoids

Organoids are an emerging technology with great potential in human disease modelling, drug development, diagnosis, tissue engineering, and regenerative medicine. Organoids as 3D-tissue culture systems have gained special attention in the past decades due to their ability to faithfully recapitulate the complexity of organ-specific tissues. Despite considerable successes in culturing physiologically relevant organoids, their real-life applications are currently limited by challenges such as scarcity of an appropriate biomimetic matrix. Peptide amphiphiles (PAs) due to their well-defined chemistry, tunable bioactivity, and extracellular matrix (ECM)-like nanofibrous architecture represent an attractive material scaffold for organoids development. Using cerebral organoids (COs) as exemplar, we demonstrate the possibility to create bio-instructive hydrogels with tunable stiffness ranging from 0.69 kPa to 2.24 kPa to culture and induce COs growth. We used orthogonal chemistry involving oxidative coupling and supramolecular interactions to create two-component hydrogels integrating the bio-instructive activity and ECM-like nanofibrous architecture of a laminin-mimetic PAs (IKVAV-PA) and tunable crosslinking density of hyaluronic acid functionalized with tyramine (HA-Try). Multi-omics technology including transcriptomics, proteomics, and metabolomics reveals the induction and growth of COs in soft HA-Tyr hydrogels containing PA-IKVAV such that the COs display morphology and biomolecular signatures similar to those grown in Matrigel scaffolds. Our materials hold great promise as a safe synthetic ECM for COs induction and growth. Our approach represents a well-defined alternative to animal-derived matrices for the culture of COs and might expand the applicability of organoids in basic and clinical research. STATEMENT OF SIGNIFICANCE: Synthetic bio-instructive materials which display tissue-specific functionality and nanoscale architecture of the native extracellular matrix are attractive matrices for organoids development. These synthetic matrices are chemically defined and animal-free compared to current gold standard matrices such as Matrigel. Here, we developed hydrogel matrices with tunable stiffness, which incorporate laminin-mimetic peptide amphiphiles to grow and expand cerebral organoids. Using multi-omics tools, the present study provides exciting data on the effects of neuro-inductive cues on the biomolecular profiles of brain organoids.

Tunable hydrogel viscoelasticity modulates human neural maturation

Human-induced pluripotent stem cells (hiPSCs) have emerged as a promising in vitro model system for studying neurodevelopment. However, current models remain limited in their ability to incorporate tunable biomechanical signaling cues imparted by the extracellular matrix (ECM). The native brain ECM is viscoelastic and stress-relaxing, exhibiting a time-dependent response to an applied force. To recapitulate the remodelability of the neural ECM, we developed a family of protein-engineered hydrogels that exhibit tunable stress relaxation rates. hiPSC-derived neural progenitor cells (NPCs) encapsulated within these gels underwent relaxation rate-dependent maturation. Specifically, NPCs within hydrogels with faster stress relaxation rates extended longer, more complex neuritic projections, exhibited decreased metabolic activity, and expressed higher levels of genes associated with neural maturation. By inhibiting actin polymerization, we observed decreased neuritic projections and a concomitant decrease in neural maturation gene expression. Together, these results suggest that microenvironmental viscoelasticity is sufficient to bias human NPC maturation.

Magnetic Nanofibrous Hydrogels for Dynamic Control of Stem Cell Differentiation

The extracellular matrix in tissue consists of complex heterogeneous soft materials with hierarchical structure and dynamic mechanical properties dictating cell and tissue level function. In many natural matrices, there are nanofibrous structures that serve to guide cell activity and dictate the form and function of tissue. Synthetic hydrogels with integrated nanofibers can mimic the structural properties of native tissue; however, model systems with dynamic mechanical properties remain elusive. Here we demonstrate modular nanofibrous hydrogels that can be reversibly stiffened in response to applied magnetic fields. Iron oxide nanoparticles were incorporated into gelatin nanofibers through electrospinning, followed by chemical stabilization and fragmentation. These magnetoactive nanofibers can be mixed with virtually any hydrogel material and reversibly stiffen the matrix at a low fiber content (≤3%). In contrast to previous work, where a large quantity of magnetic material disallowed cell encapsulation, the low nanofiber content allows matrix stiffening with cells in 3D. Using adipose derived stem cells, we show how nanofibrous matrices are beneficial for both osteogenesis and adipogenesis, where stiffening the hydrogel with applied magnetic fields enhances osteogenesis while discouraging adipogenesis. Skeletal myoblast progenitors were used as a model of tissue morphogenesis with matrix stiffening augmenting myogenesis and multinucleated myotube formation. The ability to reversibly stiffen fibrous hydrogels through magnetic stimulation provides a useful tool for studying nanotopography and dynamic mechanics in cell culture, with a scope for stimuli responsive materials for tissue engineering.

Functional bioengineered tissue models of neurodegenerative diseases

Aging-associated neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases remain poorly understood and no disease-modifying treatments exist despite decades of investigation. Predominant in vitro (e.g., 2D cell culture, organoids) and in vivo (e.g., mouse) models of these diseases are insufficient mimics of human brain tissue structure and function and of human neurodegenerative pathobiology, and have thus contributed to this collective translational failure. This has been a longstanding challenge in the field, and new strategies are required to address both fundamental and translational needs. Bioengineered tissue culture models constitute a class of promising alternatives, as they can overcome the low cell density, poor nutrient exchange, and long term culturability limitations of existing in vitro models. Further, they can reconstruct the structural, mechanical, and biochemical cues of native brain tissue, providing a better mimic of human brain tissues for in vitro pathobiological investigation and drug development. We discuss bioengineering techniques for the generation of these neurodegenerative tissue models, including biomaterials-, organoid-, and microfluidics-based approaches, and design considerations for their construction. To aid the development of the next generation of functional neurodegenerative disease models, we discuss approaches to incorporate greater cellular diversity and simulate aging processes within bioengineered brain tissues.

Hippo Pathway in Schwann Cells and Regeneration of Peripheral Nervous System

Hippo pathway is an evolutionarily conserved signaling pathway comprising a series of MST/LATS kinase complexes. Its key transcriptional coactivators YAP and TAZ regulate transcription factors such as TEAD family to direct gene expression. The regulation of Hippo pathway, especially the nuclear level change of YAP and TAZ, significantly influences the cell fate switching from proliferation to differentiation, regeneration, and postinjury repair. This review outlines the main findings of Hippo pathway in peripheral nerve development, regeneration, and tumorigenesis, especially the studies in Schwann cells. We also summarize other roles of Hippo pathway in damage repair of the peripheral nerve system and discuss the potential future research which probably contributes to novel therapeutic strategies.

Guideline for design of substrate stiffness for mesenchymal stem cell culture based on heterogeneity of YAP and RUNX2 responses

Mesenchymal stem cells (MSCs) have the potential for self-renewal and multipotency to differentiate into various lineages. Thus, they are of great interest in regenerative medicine as a cell source for tissue engineering. Substrate stiffness is one of the most extensively studied exogenous physical factors; however, consistent results have not always been reported for controlling MSCs. Conventionally used stiff culture substrates, such as tissue-culture polystyrene and glass, enhance nuclear localization of a mechanotransducer YAP and a pre-osteogenic transcription factor RUNX2, and bias MSCs towards the osteogenic lineage, even without osteogenic-inducing soluble factors. The mechanosensitive nature and intrinsic heterogeneity present challenges for obtaining reproducible results. This review summarizes the heterogeneity in human MSC response, specifically, nuclear/cytoplasmic localization changes in the mechanotransducer yes-associated protein (YAP) and the osteogenic transcription factor RUNX2, in response to substrate stiffness. In addition, a perspective on the intracellular factors attributed to response heterogeneity is discussed. The optimal range of stiffness parameters, Young's modulus, for MSC expansion culture to suppress osteogenic differentiation bias through the suppression of YAP and RUNX2 nuclear localization, and cell cycle progression is likely to be surprisingly narrow for a cell population from an identical donor and vary among cell populations from different donors. We believe that characterization of the heterogeneity of MSCs and understanding their biological meaning is an exciting research direction to establish guidelines for the design of culture substrates for the sophisticated control of MSC properties.

Mobility mediates maturation: Synthetic substrates to enhance neural differentiation

The maturation of human induced pluripotent stem cell (hiPSC)-derived neurons in 2D is dependent upon cell attachment, spreading, and pathfinding across a biomaterial substrate. In this issue of Cell Stem Cell, Álvarez et al.¹ demonstrate that highly mobile supramolecular scaffolds facilitate long-term hiPSC-derived motor neuron culture, increase maturation-related phenotypes, and recapitulate disease-relevant pathologies.

Transitions in development - an interview with Samira Musah

Samira Musah is an Assistant Professor in the Departments of Biomedical Engineering and Medicine at Duke University, USA. Samira's research focuses on leveraging pluripotent stem cells, bioengineering and organ-on-a-chip technologies to understand more about human kidney development, disease and therapy. We met with Samira over Microsoft Teams to hear more about her path to independence, mentors and her love of yoga.

Surface Engineering of Auxetic Scaffolds for Neural and Vascular Differentiation from Human Pluripotent Stem Cells

Auxetic materials are the materials that can display negative Poisson's ratio that describes the degree to which a material contracts (or expands) transversally when axially strained. Human stem cells sense the mechanical properties of the microenvironment, including material surface properties, stiffness, and Poisson's ratio. In this study, six different auxetic polyurethane (PU) foams with different elastic modulus (0.7-1.8 kPa) and Poisson's ratio (-0.1 to -0.5) are used to investigate lineage specification of human induced pluripotent stem cells (hiPSCs). The surfaces of the foams are modified with chitosan or heparin to enhance the adhesion and proliferation of hiPSCs. Then, the vascular and neural differentiation of hiPSCs are investigated on different foams with distinct elastic modulus and Poisson's ratio. With different auxetic foams, cells show differential adherent density and differentiation capacity. Chitosan and heparin surface functionalization promote the hindbrain and hippocampal markers, but not forebrain markers during neural patterning of hiPSCs. Properly surface engineered auxetic scaffolds can also promote vascular differentiation of hiPSCs. This study represents a versatile and multifunctional scaffold fabrication approach and can lead to a suitable system for establishing hiPSC culture models in applications of neurovascular disease modeling and drug screening.

Adriamycin-Induced Podocyte Injury Disrupts the YAP-TEAD1 Axis and Downregulates Cyr61 and CTGF Expression

The most severe forms of kidney diseases are often associated with irreversible damage to the glomerular podocytes, the highly specialized epithelial cells that encase glomerular capillaries and regulate the removal of toxins and waste from the blood. Several studies revealed significant changes to podocyte cytoskeletal structure during disease onset, suggesting possible roles of cellular mechanosensing in podocyte responses to injury. Still, this topic remains underexplored partly due to the lack of appropriate in vitro models that closely recapitulate human podocyte biology. Here, we leveraged our previously established method for the derivation of mature podocytes from human induced pluripotent stem cells (hiPSCs) to help uncover the roles of yes-associated protein (YAP), a transcriptional coactivator and mechanosensor, in podocyte injury response. We found that while the total expression levels of YAP remain relatively unchanged during Adriamycin (ADR)-induced podocyte injury, the YAP target genes connective tissue growth factor (CTGF) and cysteine-rich angiogenic inducer 61 (Cyr61) are significantly downregulated. Intriguingly, TEAD1 is significantly downregulated in podocytes injured with ADR. By examining multiple independent modes of cellular injury, we found that CTGF and Cyr61 expression are downregulated only when podocytes were exposed to molecules known to disrupt the cell's mechanical integrity or cytoskeletal structure. To our knowledge, this is the first report that the YAP-TEAD1 signaling axis is disrupted when stem cell-derived human podocytes experience biomechanical injury. Together, these results could help improve the understanding of kidney disease mechanisms and highlight CTGF and Cyr61 as potential therapeutic targets or biomarkers for patient stratification.

Engineered cell culture microenvironments for mechanobiology studies of brain neural cells

The biomechanical properties of the brain microenvironment, which is composed of different neural cell types, the extracellular matrix, and blood vessels, are critical for normal brain development and neural functioning. Stiffness, viscoelasticity and spatial organization of brain tissue modulate proliferation, migration, differentiation, and cell function. However, the mechanical aspects of the neural microenvironment are largely ignored in current cell culture systems. Considering the high promises of human induced pluripotent stem cell- (iPSC-) based models for disease modelling and new treatment development, and in light of the physiological relevance of neuromechanobiological features, applications of in vitro engineered neuronal microenvironments should be explored thoroughly to develop more representative in vitro brain models. In this context, recently developed biomaterials in combination with micro- and nanofabrication techniques 1) allow investigating how mechanical properties affect neural cell development and functioning; 2) enable optimal cell microenvironment engineering strategies to advance neural cell models; and 3) provide a quantitative tool to assess changes in the neuromechanobiological properties of the brain microenvironment induced by pathology. In this review, we discuss the biological and engineering aspects involved in studying neuromechanobiology within scaffold-free and scaffold-based 2D and 3D iPSC-based brain models and approaches employing primary lineages (neural/glial), cell lines and other stem cells. Finally, we discuss future experimental directions of engineered microenvironments in neuroscience.

Organoids as tools for fundamental discovery and translation-a Keystone Symposia report

Complex three-dimensional in vitro organ-like models, or organoids, offer a unique biological tool with distinct advantages over two-dimensional cell culture systems, which can be too simplistic, and animal models, which can be too complex and may fail to recapitulate human physiology and pathology. Significant progress has been made in driving stem cells to differentiate into different organoid types, though several challenges remain. For example, many organoid models suffer from high heterogeneity, and it can be difficult to fully incorporate the complexity of in vivo tissue and organ development to faithfully reproduce human biology. Successfully addressing such limitations would increase the viability of organoids as models for drug development and preclinical testing. On April 3-6, 2022, experts in organoid development and biology convened at the Keystone Symposium "Organoids as Tools for Fundamental Discovery and Translation" to discuss recent advances and insights from this relatively new model system into human development and disease.

Hippo Signaling in the Ovary: Emerging Roles in Development, Fertility, and Disease

Emerging studies indicate that the Hippo pathway, a highly conserved pathway that regulates organ size control, plays an important role in governing ovarian physiology, fertility, and pathology. Specific to the ovary, the spatiotemporal expression of the major components of the Hippo signaling cascade are observed throughout the reproductive lifespan. Observations from multiple species begin to elucidate the functional diversity and molecular mechanisms of Hippo signaling in the ovary in addition to the identification of interactions with other signaling pathways and responses to various external stimuli. Hippo pathway components play important roles in follicle growth and activation, as well as steroidogenesis, by regulating several key biological processes through mechanisms of cell proliferation, migration, differentiation, and cell fate determination. Given the importance of these processes, dysregulation of the Hippo pathway contributes to loss of follicular homeostasis and reproductive disorders such as polycystic ovary syndrome (PCOS), premature ovarian insufficiency, and ovarian cancers. This review highlights what is currently known about the Hippo pathway core components in ovarian physiology, including ovarian development, follicle development, and oocyte maturation, while identifying areas for future research to better understand Hippo signaling as a multifunctional pathway in reproductive health and biology.

Role of YAP as a Mechanosensing Molecule in Stem Cells and Stem Cell-Derived Hematopoietic Cells

Yes-associated protein (YAP) and WW domain-containing transcription regulator protein 1 (WWTR1, also known as TAZ) are transcriptional coactivators in the Hippo signaling pathway. Both are well-known regulators of cell proliferation and organ size control, and they have significant roles in promoting cell proliferation and differentiation. The roles of YAP and TAZ in stem cell pluripotency and differentiation have been extensively studied. However, the upstream mediators of YAP and TAZ are not well understood. Recently, a novel role of YAP in mechanosensing and mechanotransduction has been reported. The present review updates information on the regulation of YAP by mechanical cues such as extracellular matrix stiffness, fluid shear stress, and actin cytoskeleton tension in stem cell behaviors and differentiation. The review explores mesenchymal stem cell fate decisions, pluripotent stem cells (PSCs), self-renewal, pluripotency, and differentiation to blood products. Understanding how cells sense their microenvironment or niche and mimic those microenvironments in vitro could improve the efficiency of producing stem cell products and the efficacy of the products.

Biomaterials and bioengineering to guide tissue morphogenesis in epithelial organoids

Organoids are self-organized and miniatured in vitro models of organs and recapitulate key aspects of organ architecture and function, leading to rapid progress in understanding tissue development and disease. However, current organoid culture systems lack accurate spatiotemporal control over biochemical and physical cues that occur during in vivo organogenesis and fail to recapitulate the complexity of organ development, causing the generation of immature organoids partially resembling tissues in vivo. Recent advances in biomaterials and microengineering technologies paved the way for better recapitulation of organ morphogenesis and the generation of anatomically-relevant organoids. For this, understanding the native ECM components and organization of a target organ is essential in providing rational design of extracellular scaffolds that support organoid growth and maturation similarly to the in vivo microenvironment. In this review, we focus on epithelial organoids that resemble the spatial distinct structure and function of organs lined with epithelial cells including intestine, skin, lung, liver, and kidney. We first discuss the ECM diversity and organization found in epithelial organs and provide an overview of developing hydrogel systems for epithelial organoid culture emphasizing their key parameters to determine cell fates. Finally, we review the recent advances in tissue engineering and microfabrication technologies including bioprinting and microfluidics to overcome the limitations of traditional organoid cultures. The integration of engineering methodologies with the organoid systems provides a novel approach for instructing organoid morphogenesis via precise spatiotemporal modulation of bioactive cues and the establishment of high-throughput screening platforms.

Stretchable mesh microelectronics for the biointegration and stimulation of human neural organoids

Advances in tridimensional (3D) culture approaches have led to the generation of organoids that recapitulate cellular and physiological features of domains of the human nervous system. Although microelectrodes have been developed for long-term electrophysiological interfaces with neural tissue, studies of long-term interfaces between microelectrodes and free-floating organoids remain limited. In this study, we report a stretchable, soft mesh electrode system that establishes an intimate in vitro electrical interface with human neurons in 3D organoids. Our mesh is constructed with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based electrically conductive hydrogel electrode arrays and elastomeric poly(styrene-ethylene-butylene-styrene) (SEBS) as the substrate and encapsulation materials. This mesh electrode can maintain a stable electrochemical impedance in buffer solution under 50% compressive and 50% tensile strain. We have successfully cultured pluripotent stem cell-derived human cortical organoids (hCO) on this polymeric mesh for more than 3 months and demonstrated that organoids readily integrate with the mesh. Using simultaneous stimulation and calcium imaging, we show that electrical stimulation through the mesh can elicit intensity-dependent calcium signals comparable to stimulation from a bipolar stereotrode. This platform may serve as a tool for monitoring and modulating the electrical activity of in vitro models of neuropsychiatric diseases.

Virtual screening for small-molecule pathway regulators by image-profile matching

Identifying the chemical regulators of biological pathways is a time-consuming bottleneck in developing therapeutics and research compounds. Typically, thousands to millions of candidate small molecules are tested in target-based biochemical screens or phenotypic cell-based screens, both expensive experiments customized to each disease. Here, our uncustomized, virtual, profile-based screening approach instead identifies compounds that match to pathways based on the phenotypic information in public cell image data, created using the Cell Painting assay. Our straightforward correlation-based computational strategy retrospectively uncovered the expected, known small-molecule regulators for 32% of positive-control gene queries. In prospective, discovery mode, we efficiently identified new compounds related to three query genes and validated them in subsequent gene-relevant assays, including compounds that phenocopy or pheno-oppose YAP1 overexpression and kill a Yap1-dependent sarcoma cell line. This image-profile-based approach could replace many customized labor- and resource-intensive screens and accelerate the discovery of biologically and therapeutically useful compounds.

Soft substrates promote direct chemical reprogramming of fibroblasts into neurons

Fibroblasts can be directly reprogrammed via a combination of small molecules to generate induced neurons (iNs), bypassing intermediate stages. This method holds great promise for regenerative medicine; however, it remains inefficient. Recently, studies have suggested that physical cues may improve the direct reprogramming of fibroblasts into neurons, but the underlying mechanisms remain to be further explored, and the physical factors reported to date do not exhibit the full properties of the extracellular matrix (ECM). Previous in vitro studies mainly used rigid polystyrene dishes, while one of the characteristics of the native in-vivo environment of neurons is the soft nature of brain ECM. The reported stiffness of brain tissue is very soft ranging between 100 Pa and 3 kPa, and the effect of substrate stiffness on direct neuronal reprogramming has not been explored. Here, we show for the first time that soft substrates substantially improved the production efficiency and quality of iNs, without needing to co-culture with glial cells during reprogramming, producing more glutamatergic neurons with electrophysiological functions in a shorter time. Transcriptome sequencing indicated that soft substrates might promote glutamatergic neuron reprogramming through integrins, actin cytoskeleton, Hippo signalling pathway, and regulation of mesenchymal-to-epithelial transition, and competing endogenous RNA network analysis provided new targets for neuronal reprogramming. We demonstrated that soft substrates may promote neuronal reprogramming by inhibiting microRNA-615-3p-targeting integrin subunit beta 4. Our findings can aid the development of regenerative therapies and help improve our understanding of neuronal reprogramming. STATEMENT OF SIGNIFICANCE: : First, we have shown that low stiffness promotes direct reprogramming on the basis of small molecule combinations. To the best of our knowledge, this is the first report on this type of method, which may greatly promote the progress of neural reprogramming. Second, we found that miR-615-3p may interact with ITGB4, and the soft substrates may promote neural reprogramming by inhibiting microRNA (miR)-615-3p targeting integrin subunit beta 4 (ITGB4). We are the first to report on this mechanism. Our findings will provide more functional neurons for subsequent basic and clinical research in neurological regenerative medicine, and will help to improve the overall understanding of neural reprogramming. This work also provides new ideas for the design of medical biomaterials for nerve regeneration.

Soft substrate maintains stemness and pluripotent stem cell-like phenotype of human embryonic stem cells under defined culture conditions

Human embryonic stem cells (hESCs) are derived from the inner cell mass (ICM) of the pre-implantation blastocyst. Prior to embryo implantation, the ICM cells are surrounded by trophoblasts which have mechanical stiffness ranging from Pascal (Pa) to kilopascal (kPa). However, under in vitro conditions these cells are cultured on stiff tissue culture treated plastic plates (TCP) which have stiffness of approximately 1 gigapascal (GPa). This obvious dichotomy motivated us to investigate the fate of hESCs cultured on softer substrate, and to probe if the hESCs undergo differentiation or they retain pluripotency on soft substrates. We investigated the expression of pluripotency markers, and lineage-specific markers; we particularly looked at the expression of transcriptional coactivator YAP (Yes-associated protein), an important mediator of extracellular matrix (ECM) mechanical cues and a known downstream transducer of Hippo pathway. Downregulation of YAP has been correlated to the loss of multipotency of human mesenchymal stem cells (hMSCs) and pluripotency in mouse ESCs (mESCs); but we report that hESCs maintain their stemness on soft substrate of varying stiffness. Our findings revealed that on soft substrate hESCs express pluripotency markers and does not undergo substrate-mediated differentiation. Interestingly we show that hESCs maintained basal level of YAP expression for cell survival and proliferation, but YAP expression does not correlate directly with pluripotency in hESCs. To summarize, our results show that hESCs retain their stemness on soft substrate despite downregulation of YAP.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10616-022-00537-z.

Automatic differentiation of human induced pluripotent stem cells toward synchronous neural networks on an arrayed monolayer of nanofiber membrane

Automatic differentiation of human-induced pluripotent stem cells (hiPSCs) facilitates the generation of cortical neural networks and studies of brain functions. Here, we present a method of directed differentiation of hiPSCs with a substrate made of a honeycomb microframe and a monolayer of crosslinked gelatin nanofibers in the form of an array of nanofiber membranes. Neural precursor cells (NPCs) were firstly derived from hiPSCs and then placed on the nanofiber membranes for automatically controlled neural differentiation over a long period. Due to the strong modulation of the substrate stiffness and permeability, most cells were found in the center area of the honeycomb compartments, giving rise to regular and inter-connected cortical neural clusters. More importantly, the neural activities of the clusters were synchronized proving the reliability of the method. Our results showed that the self-organization, as well as the neural activities of differentiating neural cells, were more efficient in the nanofiber membrane compared to the types of the substrate such as glass and nanofiber-covered glass. In addition to the inherent advantages such as manpower saving and fewer risks of contamination and human error, automatic differentiation avoided undesired shaking which might have critical effects on the formation of synchronous neural clusters. STATEMENT OF SIGNIFICANCE: : Synchronization of cortical neural activities is essential for information processing and human cognition. By automated differentiation of human induced pluripotent stem cells on arrayed monolayer of nanofiber membrane, synchronous neural clusters could be formed. Such an approach would allow creating a variety of neural networks with regular and interconnected clusters for systematic studies of human cortical functions.

Molecular regulation of trophoblast stem cell self-renewal and giant cell differentiation by the Hippo components YAP and LATS1

Background

Trophoblast stem cells (TSCs), the precursors of trophoblast cells of placenta, possess the potential to differentiate into various trophoblastic subtypes in vitro. Establishment of extraembryonic trophoblastic lineage is preceded by the “outside versus inside” positional information in preimplantation embryos, critically synchronized by the Hippo components. Abundant expression of Hippo effector YAP in TSCs and differentiated cells with paucity of information on Hippo regulation of TSC proliferation/differentiation led us test the hypothesis that Hippo dynamics is one of the regulators of  TSC proliferation/differentiation.

Methods

Blastocyst-derived murine TSCs were used. Dynamics of Hippo components were analyzed using immunofluorescence, western blotting, immunoprecipitation, qRT-PCR. Interaction studies were performed using full-length and deletion constructs. BrdU incorporation assay, flow cytometry-based polyploidy analysis and confocal microscopy were used to decipher the underlying mechanism.

Results

YAP translocates to the nucleus in TSCs and utilizes its WW₂ domain to interact with the PPQY motif of the stemness factor, CDX2. YAP limits TSC proliferation with associated effect on CDX2 target CyclinD1. Trophoblast giant cells (TGC) differentiation is associated with cytoplasmic retention of YAP, heightened pYAP^Ser127, decrease in the level of the core Hippo component, LATS1, which thereby impedes LATS1-LIMK2 association. Decreased LATS1-LIMK2 complex formation in TGCs was associated with elevated pLIMK2^Thr505 as well as its target pCOFILIN^Ser3. Precocious overexpression of LATS1 during trophoblast differentiation decreased TGC marker, Prl2c2, diminished pLIMK2^Thr505 and inactive COFILIN (pCOFILIN^Ser3) while COFILIN-phosphatase, CHRONOPHIN remained unchanged. LATS1 overexpression inhibited trophoblast endoreduplication with smaller-sized TGC-nuclei, lower ploidy level and disintegrated actin filaments. Inhibition of LIMK2 activity recapitulated the effects of LATS1 overexpression in trophoblast cells.

Conclusion

These results unveil a multilayered regulation of trophoblast self-renewal and differentiation by the Hippo components.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02844-w.

Decellularization of Full Heart—Optimizing the Classical Sodium-Dodecyl-Sulfate-Based Decellularization Protocol

Compared to cell therapy, where cells are injected into a defect region, the treatment of heart infarction with cells seeded in a vascularized scaffold bears advantages, such as an immediate nutrient supply or a controllable and persistent localization of cells. For this purpose, decellularized native tissues are a preferable choice as they provide an in vivo-like microenvironment. However, the quality of such scaffolds strongly depends on the decellularization process. Therefore, two protocols based on sodium dodecyl sulfate or sodium deoxycholate were tailored and optimized for the decellularization of a porcine heart. The obtained scaffolds were tested for their applicability to generate vascularized cardiac patches. Decellularization with sodium dodecyl sulfate was found to be more suitable and resulted in scaffolds with a low amount of DNA, a highly preserved extracellular matrix composition, and structure shown by GAG quantification and immunohistochemistry. After seeding human endothelial cells into the vasculature, a coagulation assay demonstrated the functionality of the endothelial cells to minimize the clotting of blood. Human-induced pluripotent-stem-cell-derived cardiomyocytes in co-culture with fibroblasts and mesenchymal stem cells transferred the scaffold into a vascularized cardiac patch spontaneously contracting with a frequency of 25.61 ± 5.99 beats/min for over 16 weeks. The customized decellularization protocol based on sodium dodecyl sulfate renders a step towards a preclinical evaluation of the scaffolds.

Home Away From Home: Bioengineering Advancements to Mimic the Developmental and Adult Stem Cell Niche

The inherent self-organizing capacity of pluripotent and adult stem cell populations has advanced our fundamental understanding of processes that drive human development, homeostasis, regeneration, and disease progression. Translating these principles into in vitro model systems has been achieved with the advent of organoid technology, driving innovation to harness patient-specific, cell-laden regenerative constructs that can be engineered to augment or replace diseased tissue. While developmental organization and regenerative adult stem cell niches are tightly regulated in vivo, in vitro analogs lack defined architecture and presentation of physicochemical cues, leading to the unhindered arrangement of mini-tissues that lack complete physiological mimicry. This review aims to highlight the recent integrative engineering approaches that elicit spatio-temporal control of the extracellular niche to direct the structural and functional maturation of pluripotent and adult stem cell derivatives. While the advances presented here leverage multi-pronged strategies ranging from synthetic biology to microfabrication technologies, the methods converge on recreating the biochemical and biophysical milieu of the native tissue to be modeled or regenerated.

Changes in chromatin accessibility landscape and histone H3 core acetylation during valproic acid-induced differentiation of embryonic stem cells

Directed differentiation of mouse embryonic stem cells (mESCs) or induced pluripotent stem cells (iPSCs) provides powerful models to dissect the molecular mechanisms leading to the formation of specific cell lineages. Treatment with histone deacetylase inhibitors can significantly enhance the efficiency of directed differentiation. However, the mechanisms are not well understood. Here, we use CUT&RUN in combination with ATAC-seq to determine changes in both histone modifications and genome-wide chromatin accessibility following valproic acid (VPA) exposure. VPA induced a significant increase in global histone H3 acetylation (H3K56ac), a core histone modification affecting nucleosome stability, as well as enrichment at loci associated with cytoskeletal organization and cellular morphogenesis. In addition, VPA altered the levels of linker histone H1 subtypes and the total histone H1/nucleosome ratio indicative of initial differentiation events. Notably, ATAC-seq analysis revealed changes in chromatin accessibility of genes involved in regulation of CDK serine/threonine kinase activity and DNA duplex unwinding. Importantly, changes in chromatin accessibility were evident at several key genomic loci, such as the pluripotency factor Lefty, cardiac muscle troponin Tnnt2, and the homeodomain factor Hopx, which play critical roles in cardiomyocyte differentiation. Massive parallel transcription factor (TF) footprinting also indicates an increased occupancy of TFs involved in differentiation toward mesoderm and endoderm lineages and a loss of footprints of POU5F1/SOX2 pluripotency factors following VPA treatment. Our results provide the first genome-wide analysis of the chromatin landscape following VPA-induced differentiation in mESCs and provide new mechanistic insight into the intricate molecular processes that govern departure from pluripotency and early lineage commitment.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Natural Membrane Differentiates Human Adipose-Derived Mesenchymal Stem Cells to Neurospheres by Mechanotransduction Related to YAP and AMOT Proteins

Adipose tissue-derived mesenchymal stem cells (ADMSCs) are promising candidates for regenerative medicine, as they have good cell yield and can differentiate into several cell lines. When induced to the neuronal differentiation, they form neurospheres composed of neural precursors (NPs) that can be an alternative in treating neurodegenerative diseases. This study aimed to characterize NPs from neurospheres obtained after seeding ADMSCs on a natural polyisoprene-based membrane. The ADMSCs were isolated from adipose tissue by enzymatic dissociation, were subjected to trilineage differentiation, and were characterized by flow cytometry for specific ADMSC surface markers. For neuronal differentiation, the cells were seeded on polystyrene flasks coated with the membrane and were characterized by immunocytochemistry and RT-PCR. The results demonstrated that the isolated cells showed characteristics of ADMSCs. At 15 to 25 days, ADMSCs seeded on the natural membrane developed neurospheres. Then, after dissociation, the cells demonstrated characteristic neuronal markers expressed on NPs: nestin, ß-III tubulin, GFAP, NeuN, and the YAP1/AMOT in the cytoplasm. In conclusion, it was demonstrated that this membrane differentiates the ADMSCs to NPs without any induction factors, and suggests that their differentiation mechanisms are related to mechanotransduction regulated by the YAP and AMOT proteins.

Controlled aggregation enhances immunomodulatory potential of mesenchymal stromal cell aggregates

Human mesenchymal stromal cells (MSCs) are promising candidates for cell therapy due to their ease of isolation and expansion and their ability to secrete antiapoptotic, pro-angiogenic, and immunomodulatory factors. Three-dimensional (3D) aggregation "self-activates" MSCs to augment their pro-angiogenic and immunomodulatory potential, but the microenvironmental features and culture parameters that promote optimal MSC immunomodulatory function in 3D aggregates are poorly understood. Here, we generated MSC aggregates via three distinct methods and compared them with regard to their (a) aggregate structure and (b) immunomodulatory phenotype under resting conditions and in response to inflammatory stimulus. Methods associated with fast aggregation kinetics formed aggregates with higher cell packing density and reduced extracellular matrix (ECM) synthesis compared to those with slow aggregation kinetics. While all three methods of 3D aggregation enhanced MSC expression of immunomodulatory factors compared to two-dimensional culture, different aggregation methods modulated cells' temporal expression of these factors. A Design of Experiments approach, in which aggregate size and aggregation kinetics were systematically covaried, identified a significant effect of both parameters on MSCs' ability to regulate immune cells. Compared to small aggregates formed with fast kinetics, large aggregates with slow assembly kinetics were more effective at T-cell suppression and macrophage polarization toward anti-inflammatory phenotypes. Thus, culture parameters including aggregation method, kinetics, and aggregate size influence both the structural properties of aggregates and their paracrine immunomodulatory function. These findings underscore the utility of engineering strategies to control properties of 3D MSC aggregates, which may identify new avenues for optimizing the immunomodulatory function of MSC-based cell therapies.

Engineering in vitro human neural tissue analogs by 3D bioprinting and electrostimulation

There is a fundamental need for clinically relevant, reproducible, and standardized in vitro human neural tissue models, not least of all to study heterogenic and complex human-specific neurological (such as neuropsychiatric) disorders. Construction of three-dimensional (3D) bioprinted neural tissues from native human-derived stem cells (e.g., neural stem cells) and human pluripotent stem cells (e.g., induced pluripotent) in particular is appreciably impacting research and conceivably clinical translation. Given the ability to artificially and favorably regulate a cell's survival and behavior by manipulating its biophysical environment, careful consideration of the printing technique, supporting biomaterial and specific exogenously delivered stimuli, is both required and advantageous. By doing so, there exists an opportunity, more than ever before, to engineer advanced and precise tissue analogs that closely recapitulate the morphological and functional elements of natural tissues (healthy or diseased). Importantly, the application of electrical stimulation as a method of enhancing printed tissue development in vitro, including neuritogenesis, synaptogenesis, and cellular maturation, has the added advantage of modeling both traditional and new stimulation platforms, toward improved understanding of efficacy and innovative electroceutical development and application.

Ultrashort Peptide Bioinks Support Automated Printing of Large-Scale Constructs Assuring Long-Term Survival of Printed Tissue Constructs

We report about rationally designed ultrashort peptide bioinks, overcoming severe limitations in current bioprinting procedures. Bioprinting is increasingly relevant in tissue engineering, regenerative and personalized medicine due to its ability to fabricate complex tissue scaffolds through an automated deposition process. Printing stable large-scale constructs with high shape fidelity and enabling long-term cell survival are major challenges that most existing bioinks are unable to solve. Additionally, they require chemical or UV-cross-linking for the structure-solidifying process which compromises the encapsulated cells, resulting in restricted structure complexity and low cell viability. Using ultrashort peptide bioinks as ideal bodylike but synthetic material, we demonstrate an instant solidifying cell-embedding printing process via a sophisticated extrusion procedure under true physiological conditions and at cost-effective low bioink concentrations. Our printed large-scale cell constructs and the chondrogenic differentiation of printed mesenchymal stem cells point to the strong potential of the peptide bioinks for automated complex tissue fabrication.

Biomaterials Regulate Mechanosensors YAP/TAZ in Stem Cell Growth and Differentiation

Tissue-resident stem cells are surrounded by a microenvironment known as 'stem cell niche' which is specific for each stem cell type. This niche comprises of cell-intrinsic and -extrinsic factors like biochemical and biophysical signals, which regulate stem cell characteristics and differentiation. Biochemical signals have been thoroughly studied however, the effect of biophysical signals on stem cell regulation is yet to be completely understood. Biomaterials have aided in addressing this issue since they can provide a defined and tuneable microenvironment resembling in vivo conditions. We review various biomaterials used in many studies which have shown a connection between biomaterial-generated mechanical signals and alteration in stem cell behaviour. Researchers probed to understand the mechanism of mechanotransduction and reported that the signals from the extracellular matrix regulate a transcription factor yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ), which is a downstream-regulator of the Hippo pathway and it transduces the mechanical signals inside the nucleus. We highlight the role of the YAP/TAZ as mechanotransducers in stem cell self-renewal and differentiation in response to substrate stiffness, also the possibility of mechanobiology as the emerging field of regenerative medicines and three-dimensional tissue printing.

Modulating the Electrical and Mechanical Microenvironment to Guide Neuronal Stem Cell Differentiation

The application of induced pluripotent stem cells (iPSCs) in disease modeling and regenerative medicine can be limited by the prolonged times required for functional human neuronal differentiation and traditional 2D culture techniques. Here, a conductive graphene scaffold (CGS) to modulate mechanical and electrical signals to promote human iPSC-derived neurons is presented. The soft CGS with cortex-like stiffness (≈3 kPa) and electrical stimulation (±800 mV/100 Hz for 1 h) incurs a fivefold improvement in the rate (14d) of generating iPSC-derived neurons over some traditional protocols, with an increase in mature cellular markers and electrophysiological characteristics. Consistent with other culture conditions, it is found that the pro-neurogenic effects of mechanical and electrical stimuli rely on RhoA/ROCK signaling and de novo ciliary neurotrophic factor (CNTF) production respectively. Thus, the CGS system creates a combined physical and continuously modifiable, electrical niche to efficiently and quickly generate iPSC-derived neurons.

YAP and TAZ Mediators at the Crossroad between Metabolic and Cellular Reprogramming

Cell reprogramming can either refer to a direct conversion of a specialized cell into another or to a reversal of a somatic cell into an induced pluripotent stem cell (iPSC). It implies a peculiar modification of the epigenetic asset and gene regulatory networks needed for a new cell, to better fit the new phenotype of the incoming cell type. Cellular reprogramming also implies a metabolic rearrangement, similar to that observed upon tumorigenesis, with a transition from oxidative phosphorylation to aerobic glycolysis. The induction of a reprogramming process requires a nexus of signaling pathways, mixing a range of local and systemic information, and accumulating evidence points to the crucial role exerted by the Hippo pathway components Yes-Associated Protein (YAP) and Transcriptional Co-activator with PDZ-binding Motif (TAZ). In this review, we will first provide a synopsis of the Hippo pathway and its function during reprogramming and tissue regeneration, then we introduce the latest knowledge on the interplay between YAP/TAZ and metabolism and, finally, we discuss the possible role of YAP/TAZ in the orchestration of the metabolic switch upon cellular reprogramming.

The Effect of Physical Cues of Biomaterial Scaffolds on Stem Cell Behavior

Stem cells have been sought as a promising cell source in the tissue engineering field due to their proliferative capacity as well as differentiation potential. Biomaterials have been utilized to facilitate the delivery of stem cells in order to improve their engraftment and long-term viability upon implantation. Biomaterials also have been developed as scaffolds to promote stem cell induced tissue regeneration. This review focuses on the latter where the biomaterial scaffold is designed to provide physical cues to stem cells in order to promote their behavior for tissue formation. Recent work that explores the effect of scaffold physical properties, topography, mechanical properties and electrical properties, is discussed. Although still being elucidated, the biological mechanisms, including cell shape, focal adhesion distribution, and nuclear shape, are presented. This review also discusses emerging areas and challenges in clinical translation.

Harnessing Mechanobiology for Tissue Engineering

A primary challenge in tissue engineering is to recapitulate both the structural and functional features of whole tissues and organs. In vivo, patterning of the body plan and constituent tissues emerges from the carefully orchestrated interactions between the transcriptional programs that give rise to cell types and the mechanical forces that drive the bending, twisting, and extensions critical to morphogenesis. Substantial recent progress in mechanobiology-understanding how mechanics regulate cell behaviors and what cellular machineries are responsible-raises the possibility that one can begin to use these insights to help guide the strategy and design of functional engineered tissues. In this perspective, we review and propose the development of different approaches, from providing appropriate extracellular mechanical cues to interfering with cellular mechanosensing machinery, to aid in controlling cell and tissue structure and function.

Modulating Functionalized Poly(ethylene glycol) Diacrylate Hydrogel Mechanical Properties through Competitive Crosslinking Mechanics for Soft Tissue Applications

Local mechanical stiffness influences cell behavior, and thus cell culture scaffolds should approximate the stiffness of the tissue type from which the cells are derived. In synthetic hydrogels, this has been difficult to achieve for very soft tissues such as neural. This work presents a method for reducing the stiffness of mechanically and biochemically tunable synthetic poly(ethylene glycol) diacrylate hydrogels to within the soft tissue stiffness regime by altering the organization of the crosslinking sites. A soluble allyl-presenting monomer, which has a higher propensity for chain termination than acrylate monomers, was introduced into the PEG-diacrylate hydrogel precursor solution before crosslinking, resulting in acrylate-allyl competition and a reduction in gel compressive modulus from 5.1 ± 0.48 kPa to 0.32 ± 0.09 kPa. Both allyl monomer concentration and chemical structure were shown to influence the effectiveness of competition and change in stiffness. Fibroblast cells demonstrated a 37% reduction in average cell spread area on the softest hydrogels produced as compared to cells on control hydrogels, while the average percentage of neural cells extending neurites increased by 41% on these hydrogels, demonstrating the potential for this technology to serve as a soft tissue culture system.