Spider silk for xeno-free long-term self-renewal and differentiation of human pluripotent stem cells

Artificial spider silk supports and guides neurite extension in vitro

Surgical intervention with the use of autografts is considered the gold standard to treat peripheral nerve injuries. However, a biomaterial that supports and guides nerve growth would be an attractive alternative to overcome problems with limited availability, morbidity at the site of harvest, and nerve mismatches related to autografts. Native spider silk is a promising material for construction of nerve guidance conduit (NGC), as it enables regeneration of cm-long nerve injuries in sheep, but regulatory requirements for medical devices demand synthetic materials. Here, we use a recombinant spider silk protein (NT2RepCT) and a functionalized variant carrying a peptide derived from vitronectin (VN-NT2RepCT) as substrates for nerve growth support and neurite extension, using a dorsal root ganglion cell line, ND7/23. Two-dimensional coatings were benchmarked against poly-d-lysine and recombinant laminins. Both spider silk coatings performed as the control substrates with regards to proliferation, survival, and neurite growth. Furthermore, NT2RepCT and VN-NT2RepCT spun into continuous fibers in a biomimetic spinning set-up support cell survival, neurite growth, and guidance to an even larger extent than native spider silk. Thus, artificial spider silk is a promising biomaterial for development of NGCs.

A new cell-sized support for 3D cell cultures based on recombinant spider silk fibers

It is now generally accepted that 2D cultures cannot accurately replicate the rich environment and complex tissue architecture that exists in vivo, and that classically cultured cells tend to lose their original function. Growth of spheroids as opposed to 2D cultures on plastic has now been hailed as an efficient method to produce quantities of high-quality cells for cancer research, drug discovery, neuroscience, and regenerative medicine. We have developed a new recombinant protein that mimics dragline spidersilk and that self-assembles into cell-sized coils. These have high thermal and shelf-life stability and can be readily sterilized and stored for an extended period of time. The fibers are flexible, elastic, and biocompatible and can serve as cell-sized scaffold for the formation of 3D cell spheroids. As a proof of concept, recombinant spidersilk was integrated as a scaffold in spheroids of three cell types: primary rat hepatocytes, human mesenchymal stem cells, and mouse L929 cells. The scaffolds significantly reduced spheroid shrinkage and unlike scaffold-free spheroids, spheroids did not disintegrate over the course of long-term culture. Cells in recombinant spidersilk spheroids showed increased viability, and the cell lines continued to proliferate for longer than control cultures without spidersilk. The spidersilk also supported biological functions. Recombinant spidersilk primary hepatocyte spheroids exhibited 2.7-fold higher levels of adenosine triphosphate (ATP) continued to express and secrete albumin and exhibited significantly higher basal and induced CYP3A activity for at least 6 weeks in culture, while control spheroids without fibers stopped producing albumin after 27 days and CPY3A activity was barely detectable after 44 days. These results indicate that recombinant spidersilk can serve as a useful tool for long-term cell culture of 3D cell spheroids and specifically that primary hepatocytes can remain active in culture for an extended period of time which could be of great use in toxicology testing.

In Vitro Study of Human Immune Responses to Hyaluronic Acid Hydrogels, Recombinant Spidroins and Human Neural Progenitor Cells of Relevance to Spinal Cord Injury Repair

Scaffolds of recombinant spider silk protein (spidroin) and hyaluronic acid (HA) hydrogel hold promise in combination with cell therapy for spinal cord injury. However, little is known concerning the human immune response to these biomaterials and grafted human neural stem/progenitor cells (hNPCs). Here, we analyzed short- and long-term in vitro activation of immune cells in human peripheral blood mononuclear cells (hPBMCs) cultured with/without recombinant spidroins, HA hydrogels, and/or allogeneic hNPCs to assess potential host-donor interactions. Viability, proliferation and phenotype of hPBMCs were analyzed using NucleoCounter and flow cytometry. hPBMC viability was confirmed after exposure to the different biomaterials. Short-term (15 h) co-cultures of hPBMCs with spidroins, but not with HA hydrogel, resulted in a significant increase in the proportion of activated CD69+ CD4+ T cells, CD8+ T cells, B cells and NK cells, which likely was caused by residual endotoxins from the Escherichia coli expression system. The observed spidroin-induced hPBMC activation was not altered by hNPCs. It is resource-effective to evaluate human compatibility of novel biomaterials early in development of the production process to, when necessary, make alterations to minimize rejection risk. Here, we present a method to evaluate biomaterials and hPBMC compatibility in conjunction with allogeneic human cells.

Assembly of FN-silk with laminin-521 to integrate hPSCs into a three-dimensional culture for neural differentiation

Three-dimensional (3D) neural tissue cultures recapitulate the basic concepts during development and disease better than what can be obtained using conventional two-dimensional cultures. Here, we use a recombinant spider silk protein functionalized with a cell binding motif from fibronectin (FN-silk) in combination with a human recombinant laminin 521 (LN-521) to create a fully defined stem cell niche in 3D. A novel method to assemble silk blended with LN-521 together with human pluripotent stem cells (hPSC) is used to create centimeter-sized foams, which upon cultivation develop into 3D cell constructs supported by a microfibrillar network. After initial cell expansion, neural differentiation was induced to form a homogenous layer of continuous neuroectodermal tissue that allows further differentiation into neuronal subtypes. The silk-supported 3D cell constructs could then be detached from the bottom of the well and cultured as floating entities, where cells appeared in distinctive radial organization resembling early neural tube. This shows that the neural progenitors retain their cellular self-organization ability in the FN-silk/LN-521-supported 3D culture. Calcium imaging demonstrated spontaneous activity, which is important for the formation of neuronal networks. Together, the results show that hPSCs integrated into FN-silk/LN-521 foam develop into neural progenitors and that these stay viable during long-term differentiations. FN-silk/LN-521 also supports morphogenesis mimicking the human brain development and can serve as base for engineering of hPSC-derived neural tissue.

Spider Silk for Tissue Engineering Applications

Due to its properties, such as biodegradability, low density, excellent biocompatibility and unique mechanics, spider silk has been used as a natural biomaterial for a myriad of applications. First clinical applications of spider silk as suture material go back to the 18th century. Nowadays, since natural production using spiders is limited due to problems with farming spiders, recombinant production of spider silk proteins seems to be the best way to produce material in sufficient quantities. The availability of recombinantly produced spider silk proteins, as well as their good processability has opened the path towards modern biomedical applications. Here, we highlight the research on spider silk-based materials in the field of tissue engineering and summarize various two-dimensional (2D) and three-dimensional (3D) scaffolds made of spider silk. Finally, different applications of spider silk-based materials are reviewed in the field of tissue engineering in vitro and in vivo.

Single cell analysis of autism patient with bi-allelic NRXN1-alpha deletion reveals skewed fate choice in neural progenitors and impaired neuronal functionality

We generated human iPS derived neural stem cells and differentiated cells from healthy control individuals and an individual with autism spectrum disorder carrying bi-allelic NRXN1-alpha deletion. We investigated the expression of NRXN1-alpha during neural induction and neural differentiation and observed a pivotal role for NRXN1-alpha during early neural induction and neuronal differentiation. Single cell RNA-seq pinpointed neural stem cells carrying NRXN1-alpha deletion shifting towards radial glia-like cell identity and revealed higher proportion of differentiated astroglia. Furthermore, neuronal cells carrying NRXN1-alpha deletion were identified as immature by single cell RNA-seq analysis, displayed significant depression in calcium signaling activity and presented impaired maturation action potential profile in neurons investigated with electrophysiology. Our observations propose NRXN1-alpha plays an important role for the efficient establishment of neural stem cells, in neuronal differentiation and in maturation of functional excitatory neuronal cells.

Assembly of functionalized silk together with cells to obtain proliferative 3D cultures integrated in a network of ECM-like microfibers

Tissues are built of cells integrated in an extracellular matrix (ECM) which provides a three-dimensional (3D) microfiber network with specific sites for cell anchorage. By genetic engineering, motifs from the ECM can be functionally fused to recombinant silk proteins. Such a silk protein, FN-silk, which harbours a motif from fibronectin, has the ability to self-assemble into networks of microfibers under physiological-like conditions. Herein we describe a method by which mammalian cells are added to the silk solution before assembly, and thereby get uniformly integrated between the formed microfibers. In the resulting 3D scaffold, the cells are highly proliferative and spread out more efficiently than when encapsulated in a hydrogel. Elongated cells containing filamentous actin and defined focal adhesion points confirm proper cell attachment to the FN-silk. The cells remain viable in culture for at least 90 days. The method is also scalable to macro-sized 3D cultures. Silk microfibers formed in a bundle with integrated cells are both strong and extendable, with mechanical properties similar to that of artery walls. The described method enables differentiation of stem cells in 3D as well as facile co-culture of several different cell types. We show that inclusion of endothelial cells leads to the formation of vessel-like structures throughout the tissue constructs. Hence, silk-assembly in presence of cells constitutes a viable option for 3D culture of cells integrated in a ECM-like network, with potential as base for engineering of functional tissue.

Transcriptome and Proteome Profiling of Neural Induced Pluripotent Stem Cells from Individuals with Down Syndrome Disclose Dynamic Dysregulations of Key Pathways and Cellular Functions

Down syndrome (DS) or trisomy 21 (T21) is a leading genetic cause of intellectual disability. To gain insights into dynamics of molecular perturbations during neurogenesis in DS, we established a model using induced pluripotent stem cells (iPSC) with transcriptome profiles comparable to that of normal fetal brain development. When applied on iPSCs with T21, transcriptome and proteome signatures at two stages of differentiation revealed strong temporal dynamics of dysregulated genes, proteins and pathways belonging to 11 major functional clusters. DNA replication, synaptic maturation and neuroactive clusters were disturbed at the early differentiation time point accompanied by a skewed transition from the neural progenitor cell stage and reduced cellular growth. With differentiation, growth factor and extracellular matrix, oxidative phosphorylation and glycolysis emerged as major perturbed clusters. Furthermore, we identified a marked dysregulation of a set of genes encoded by chromosome 21 including an early upregulation of the hub gene APP, supporting its role for disturbed neurogenesis, and the transcription factors OLIG1, OLIG2 and RUNX1, consistent with deficient myelination and neuronal differentiation. Taken together, our findings highlight novel sequential and differentiation-dependent dynamics of disturbed functions, pathways and elements in T21 neurogenesis, providing further insights into developmental abnormalities of the DS brain.

Xeno-free and feeder-free culture and differentiation of human embryonic stem cells on recombinant vitronectin-grafted hydrogels

Xeno-free culture and cardiomyocyte differentiation of human embryonic stem cells on vitronectin-grafted hydrogels by adjusting surface charge and elasticity.

Mechanistic Analysis of Physicochemical Cues in Promoting Human Pluripotent Stem Cell Self-Renewal and Metabolism

We have previously reported that a porous membrane of polyethylene terephthalate (PET) enables significant augmentation of human pluripotent stem cell (hPSC) proliferation and differentiation. The interaction between hPSCs and the PET surface induces β-catenin-mediated wingless/integrated (Wnt) signaling, leading to upregulation of the expression of adhesion molecules in hPSCs. In this study, we sought to unveil mechanisms underlying the role of the PET membrane in hPSC self-renewal and metabolism. We discovered that physicochemical cues of the PET membrane considerably alter hPSC metabolism by increasing the cell yield and suppressing the generation of toxic byproduct, indicating an effective cell self-renewal and a less apoptotic culture environment in the membrane culture system. Furthermore, we discovered that a caspase-8 medicated apoptotic pathway plays a profound role in obstructing hPSCs grown on a traditional tissue culture plate (TCP). Treating hPSCs seeded on a TCP surface with a caspase-8 inhibitor significantly suppressed cellular apoptotic pathway and improved cell proliferation and metabolism. Our experimental results provided valuable insights into signal pathways influencing hPSC self-renewal during routine maintenance and expansion, which would shed light on large-scale preparation of hPSCs for clinical applications.

Recombinant Spider Silk Functionalized Silkworm Silk Matrices as Potential Bioactive Wound Dressings and Skin Grafts

Silk is considered to be a potential biomaterial for a wide number of biomedical applications. Silk fibroin (SF) can be retrieved in sufficient quantities from the cocoons produced by silkworms. While it is easy to formulate into scaffolds with favorable mechanical properties, the natural SF does not contain bioactive functions. Spider silk proteins, on the contrary, can be produced in fusion with bioactive protein domains, but the recombinant procedures are expensive, and large-scale production is challenging. We combine the two types of silk to fabricate affordable, functional tissue-engineered constructs for wound-healing applications. Nanofibrous mats and microporous scaffolds made of natural silkworm SF are used as a bulk material that are top-coated with the recombinant spider silk protein (4RepCT) in fusion with a cell-binding motif, antimicrobial peptides, and a growth factor. For this, the inherent silk properties are utilized to form interactions between the two silk types by self-assembly. The intended function, that is, improved cell adhesion, antimicrobial activity, and growth factor stimulation, could be demonstrated for the obtained functionalized silk mats. As a skin prototype, SF scaffolds coated with functionalized silk are cocultured with multiple cell types to demonstrate formation of a bilayered tissue construct with a keratinized epidermal layer under in vitro conditions. The encouraging results support this strategy of fabrication of an affordable bioactive SF-spider silk-based biomaterial for wound dressings and skin substitutes.

An in vitro model of lissencephaly: expanding the role of DCX during neurogenesis

Lissencephaly comprises a spectrum of brain malformations due to impaired neuronal migration in the developing cerebral cortex. Classical lissencephaly is characterized by smooth cerebral surface and cortical thickening that result in seizures, severe neurological impairment and developmental delay. Mutations in the X-chromosomal gene DCX, encoding doublecortin, is the main cause of classical lissencephaly. Much of our knowledge about DCX-associated lissencephaly comes from post-mortem analyses of patient's brains, mainly since animal models with DCX mutations do not mimic the disease. In the absence of relevant animal models and patient brain specimens, we took advantage of induced pluripotent stem cell (iPSC) technology to model the disease. We established human iPSCs from two males with mutated DCX and classical lissencephaly including smooth brain and abnormal cortical morphology. The disease was recapitulated by differentiation of iPSC into neural cells followed by expression profiling and dissection of DCX-associated functions. Here we show that neural stem cells, with absent or reduced DCX protein expression, exhibit impaired migration, delayed differentiation and deficient neurite formation. Hence, the patient-derived iPSCs and neural stem cells provide a system to further unravel the functions of DCX in normal development and disease.

Biomedical Applications of Recombinant Silk-Based Materials

Silk is mostly known as a luxurious textile, which originates from silkworms first cultivated in China. A deeper look into the variety of silk reveals that it can be used for much more, in nature and by humanity. For medical purposes, natural silks were recognized early as a potential biomaterial for surgical threads or wound dressings; however, as biomedical engineering advances, the demand for high-performance, naturally derived biomaterials becomes more pressing and stringent. A common problem of natural materials is their large batch-to-batch variation, the quantity available, their potentially high immunogenicity, and their fast biodegradation. Some of these common problems also apply to silk; therefore, recombinant approaches for producing silk proteins have been developed. There are several research groups which study and utilize various recombinantly produced silk proteins, and many of these have also investigated their products for biomedical applications. This review gives a critical overview over of the results for applications of recombinant silk proteins in biomedical engineering.

Defined and Scalable Differentiation of Human Oligodendrocyte Precursors from Pluripotent Stem Cells in a 3D Culture System

Oligodendrocyte precursor cells (OPCs) offer considerable potential for the treatment of demyelinating diseases and injuries of the CNS. However, generating large quantities of high-quality OPCs remains a substantial challenge that impedes their therapeutic application. Here, we show that OPCs can be generated from human pluripotent stem cells (hPSCs) in a three-dimensional (3D), scalable, and fully defined thermoresponsive biomaterial system. We used CRISPR/Cas9 to create a NKX2.2-EGFP human embryonic stem cell reporter line that enabled fine-tuning of early OPC specification and identification of conditions that markedly increased the number of OLIG2⁺ and NKX2.2⁺ cells generated from hPSCs. Transplantation of 50-day-old OPCs into the brains of NOD/SCID mice revealed that progenitors generated in 3D without cell selection or purification subsequently engrafted, migrated, and matured into myelinating oligodendrocytes in vivo. These results demonstrate the potential of harnessing lineage reporter lines to develop 3D platforms for rapid and large-scale production of OPCs.

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A defined and scalable 3D system accelerates the differentiation of OPCs from hPSCs

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A NKX2.2-EGFP hESC reporter line enables optimization of OPC differentiation

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3D-derived OPCs engraft, migrate, and mature after implantation into NOD/SCID mice

Xeno-free culture of human pluripotent stem cells on oligopeptide-grafted hydrogels with various molecular designs

Establishing cultures of human embryonic (ES) and induced pluripotent (iPS) stem cells in xeno-free conditions is essential for producing clinical-grade cells. Development of cell culture biomaterials for human ES and iPS cells is critical for this purpose. We designed several structures of oligopeptide-grafted poly (vinyl alcohol-co-itaconic acid) hydrogels with optimal elasticity, and prepared them in formations of single chain, single chain with joint segment, dual chain with joint segment, and branched-type chain. Oligopeptide sequences were selected from integrin- and glycosaminoglycan-binding domains of the extracellular matrix. The hydrogels grafted with vitronectin-derived oligopeptides having a joint segment or a dual chain, which has a storage modulus of 25 kPa, supported the long-term culture of human ES and iPS cells for over 10 passages. The dual chain and/or joint segment with cell adhesion molecules on the hydrogels facilitated the proliferation and pluripotency of human ES and iPS cells.

Biological Effects of Culture Substrates on Human Pluripotent Stem Cells

In recent years, as human pluripotent stem cells (hPSCs) have been commonly cultured in feeder-free conditions, a number of cell culture substrates have been applied or developed. However, the functional roles of these substrates in maintaining hPSC self-renewal remain unclear. Here in this review, we summarize the types of these substrates and their effect on maintaining hPSC self-renewal. Endogenous extracellular matrix (ECM) protein expression has been shown to be crucial in maintaining hPSC self-renewal. These ECM molecules interact with integrin cell-surface receptors and transmit their cellular signaling. We discuss the possible effect of integrin-mediated signaling pathways on maintaining hPSC self-renewal. Activation of integrin-linked kinase (ILK), which transmits ECM-integrin signaling to AKT (also known as protein kinase B), has been shown to be critical in maintaining hPSC self-renewal. Also, since naïve pluripotency has been widely recognized as an alternative pluripotent state of hPSCs, we discuss the possible effects of culture substrates and integrin signaling on naïve hPSCs based on the studies of mouse embryonic stem cells. Understanding the role of culture substrates in hPSC self-renewal and differentiation enables us to control hPSC behavior precisely and to establish scalable or microfabricated culture technologies for regenerative medicine and drug development.

Efficient passage of human pluripotent stem cells on spider silk matrices under xeno-free conditions

Human pluripotent stem cells (hPSCs) hold great promise for applications in regenerative medicine and pharmaceutical development. Such applications require cell culture methods and reagents that are chemically defined, xeno-free, scalable, and low-cost. Herein, we describe non-mechanical passaging of hPSCs on spider silk films under chemically defined and xeno-free conditions. The cells were dissociated into single cells or small aggregates using Accutase or enzyme-free dissociation buffer and then passaged to spider silk films, where they expanded in monolayers until they covered the surface. Cells cultured over 10 passages on spider silk film remained karyotypically normal and pluripotent. In conclusion, a novel method for passaging dissociated hPSCs under conditions that are compatible with clinical applications is presented. The method is cost-efficient and may be useful for both research and clinical applications.

Long-term xeno-free culture of human pluripotent stem cells on hydrogels with optimal elasticity

The tentative clinical application of human pluripotent stem cells (hPSCs), such as human embryonic stem cells and human induced pluripotent stem cells, is restricted by the possibility of xenogenic contamination resulting from the use of mouse embryonic fibroblasts (MEFs) as a feeder layer. Therefore, we investigated hPSC cultures on biomaterials with different elasticities that were grafted with different nanosegments. We prepared dishes coated with polyvinylalcohol-co-itaconic acid hydrogels grafted with an oligopeptide derived from vitronectin (KGGPQVTRGDVFTMP) with elasticities ranging from 10.3 to 30.4 kPa storage moduli by controlling the crosslinking time. The hPSCs cultured on the stiffest substrates (30.4 kPa) tended to differentiate after five days of culture, whereas the hPSCs cultured on the optimal elastic substrates (25 kPa) maintained their pluripotency for over 20 passages under xeno-free conditions. These results indicate that cell culture matrices with optimal elasticity can maintain the pluripotency of hPSCs in culture.

Natural and Synthetic Materials for Self-Renewal, Long-Term Maintenance, and Differentiation of Induced Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) have attracted considerable attention from the public, clinicians, and scientists since their discovery in 2006, and raised huge expectations for regenerative medicine. One of the distinctive features of iPSCs is their propensity to differentiate into the cells of three germ lines in vitro and in vivo. The human iPSCs can be used to study the mechanisms underlying a disease and to monitor the disease progression, for testing drugs in vitro, and for cell therapy, avoiding many ethical and immunologic concerns. This technology offers the potential to take an individual approach to each patient and allows a more accurate diagnosis and specific treatment. However, there are several obstacles that impede the use of iPSCs. The derivation of fully reprogrammed iPSCs is expensive, time-consuming, and demands meticulous attention to many details. The use of biomaterials could increase the efficacy and safety while decreasing the cost of tissue engineering. The choice of a substrate utilized for iPSC culture is also important because cell-substrate contacts influence cellular behavior such as self-renewal, expansion, and differentiation. This Progress Report aims to summarize the advantages and drawbacks of natural and synthetic biomaterials, and to evaluate their role for maintenance and differentiation of iPSCs.

Human embryonic stem cell cultivation: historical perspective and evolution of xeno-free culture systems

Human embryonic stem cells (hESC) have emerged as attractive candidates for cell-based therapies that are capable of restoring lost cell and tissue function. These unique cells are able to self-renew indefinitely and have the capacity to differentiate in to all three germ layers (ectoderm, endoderm and mesoderm). Harnessing the power of these pluripotent stem cells could potentially offer new therapeutic treatment options for a variety of medical conditions. Since the initial derivation of hESC lines in 1998, tremendous headway has been made in better understanding stem cell biology and culture requirements for maintenance of pluripotency. The approval of the first clinical trials of hESC cells for treatment of spinal cord injury and macular degeneration in 2010 marked the beginning of a new era in regenerative medicine. Yet it was clearly recognized that the clinical utility of hESC transplantation was still limited by several challenges. One of the most immediate issues has been the exposure of stem cells to animal pathogens, during hESC derivation and during in vitro propagation. Initial culture protocols used co-culture with inactivated mouse fibroblast feeder (MEF) or human feeder layers with fetal bovine serum or alternatively serum replacement proteins to support stem cell proliferation. Most hESC lines currently in use have been exposed to animal products, thus carrying the risk of xeno-transmitted infections and immune reaction. This mini review provides a historic perspective on human embryonic stem cell culture and the evolution of new culture models. We highlight the challenges and advances being made towards the development of xeno-free culture systems suitable for therapeutic applications.

Physical cues of cell culture materials lead the direction of differentiation lineages of pluripotent stem cells

Differentiation methods of hPSCs into specific cell lineages. Differentiation of hPSCsviaEB formation (types AB, A–D) or without EB formation (types E–H).