Integration column: microwell arrays for mammalian cell culture

Microwell arrays have emerged as robust and versatile alternatives to conventional mammalian cell culture substrates. Using standard microfabrication processes, biomaterials surfaces can be topographically patterned to comprise high-density arrays of micron-sized cavities with desirable geometry. Hundreds to thousands of individual cells or cell colonies with controlled size and shape can be trapped in these cavities by simple gravitational sedimentation. Efficient long-term cell confinement allows for parallel analyses and manipulation of cell fate during in vitro culture. These live-cell arrays have already found applications in cell biology, for example to probe the effect of cell colony size on embryonic stem cell differentiation, to dissect the heterogeneity in single cell proliferation kinetics of neural or hematopoietic stem/progenitor cell populations, or to elucidate the role of cell shape on cell function. Here, we highlight the key applications of these platforms, hopefully inspiring biologists to apply these systems for their own studies.

Artificial stem cell niches

Stem cells are characterized by their dual ability to reproduce themselves (self-renew) and specialize (differentiate), yielding a plethora of daughter cells that maintain and regenerate tissues. In contrast to their embryonic counterparts, adult stem cells retain their unique functions only if they are in intimate contact with an instructive microenvironment, termed stem cell niche. In these niches, stem cells integrate a complex array of molecular signals that, in concert with induced cell-intrinsic regulatory networks, control their function and balance their numbers in response to physiologic demands. This progress report provides a perspective on how advanced materials technologies could be used (i) to engineer and systematically analyze specific aspects of functional stem cells niches in a controlled fashion in vitro and (ii) to target stem cell niches in vivo. Such "artificial niches" constitute potent tools for elucidating stem cell regulatory mechanisms with the capacity to directly impact the development of novel therapeutic strategies for tissue regeneration.

Micropatterning of hydrogels by soft embossing

Conventional in situ hydrogel micropatterning techniques work successfully for relatively stiff hydrogels, but they often result in locally damaged surfaces upon demolding in the case of soft and fragile polymer networks formed at low precursor concentration. To overcome this limitation, we have developed a versatile method, termed soft embossing, for the topographical micropatterning of fragile chemically cross-linked polymer hydrogels. Soft embossing is based on the imprinting of a microstructured template into a gel surface that is only partially cross-linked. Free functional groups continue to be consumed and upon complete cross-linking irreversibly confine the microstructure on the gel surface. Here we identify and optimize the parameters that control the soft embossing process and show that this method allows the fabrication of desired topographies with good fidelity. Finally, one of the produced gel micropatterns, an array of microwells, was successfully utilized forculturing and analyzing live single hematopoietic stem cells. Confining the stem cells to their microwells allowed for efficient quantification of their growth potential during in vitro culturing.

Integration column: artificial ECM: expanding the cell biology toolbox in 3D

Many crucial cellular processes in our tissues are governed by complex, spatio-temporally regulated interactions between cells and their extracellular matrix (ECM). These interactions can be studied using well-established 3D in vitro model systems such as collagen gels or Matrigel generated from native ECM macromolecular components. Recent advances in the molecular design of 'smart' synthetic biomaterials have generated artificial ECM (aECM) that mimic some of the key structural and biochemical characteristics of their naturally derived counterparts and, thanks to their synthetic origin, promise to overcome some complexities of the latter. Here I will discuss emerging approaches in aECM design, hopefully inspiring cell biologists to apply these systems to address their specific biological question.

Perturbation of single hematopoietic stem cell fates in artificial niches

Hematopoietic stem cells (HSCs) are capable of extensive self-renewal in vivo and are successfully employed clinically to treat hematopoietic malignancies, yet are in limited supply as in culture this self-renewal capacity is lost. Using an approach at the interface of stem cell biology and bioengineering, here we describe a novel platform of hydrogel microwell arrays for assessing the effects of either secreted or tethered proteins characteristic of the in vivo microenvironment, or niche, on HSC fate in vitro. Time-lapse microscopic analyses of single cells were crucial to overcoming inevitable heterogeneity of FACS-enriched HSCs. A reduction in proliferation kinetics or an increase in asynchronous division of single HSCs in microwells in response to specific proteins (Wnt3a and N-Cadherin) correlated well with subsequent serial long-term blood reconstitution in mice in vivo. Single cells that divided once in the presence of a given protein were capable of in vivo reconstitution, providing evidence of self-renewal divisions of HSCs in vitro. These results validate the hydrogel microwell platform as a broadly applicable paradigm for dissecting the regulatory role of specific signals within a complex stem cell niche.

Enhancing the reliability and throughput of neurosphere culture on hydrogel microwell arrays

The neurosphere assay is the standard retrospective assay to test the self-renewal capability and multipotency of neural stem cells (NSCs) in vitro. However, it has recently become clear that not all neurospheres are derived from a NSC and that on conventional cell culture substrates, neurosphere motility may cause frequent neurosphere "merging" [Nat Methods 2006;3:801-806; Stem Cells 2007;25:871-874]. Combining biomimetic hydrogel matrix technology with microengineering, we developed a microwell array platform on which NSC fate and neurosphere formation can be unequivocally attributed to a single founding cell. Using time-lapse microscopy and retrospective immunostaining, the fate of several hundred single NSCs was quantified. Compared with conventional neurosphere culture methods on plastic dishes, we detected a more than 100% increase in single NSC viability on soft hydrogels. Effective confinement of single proliferating cells to microwells led to neurosphere formation of vastly different sizes, a high percentage of which showed stem cell phenotypes after one week in culture. The reliability and increased throughput of this platform should help to better elucidate the function of sphere-forming stem/progenitor cells independent of their proliferation dynamics. Disclosure of potential conflicts of interest is found at the end of this article.

Biomolecular Hydrogels Formed and Degraded via Site-Specific Enzymatic Reactions

We present polymeric hydrogel biomaterials that are biomimetic both in their synthesis and degradation. The design of oligopeptide building blocks with dual enzymatic responsiveness allows us to create polymer networks that are formed and functionalized via enzymatic reactions and are degradable via other enzymatic reactions, both occurring under physiological conditions. The activated transglutaminase enzyme factor XIIIa was utilized for site-specific coupling of prototypical cell adhesion ligands and for simultaneous cross-linking of hydrogel networks from factor XIIIa substrate-modified multiarm poly(ethylene glycol) macromers. Ligand incorporation is nearly quantitative and thus controllable, and does not alter the network's macroscopic properties over a concentration range that elicits specific cell adhesion. Living mammalian cells can be encapsulated in the gels without any noticeable decrease in viability. The degradation of gels can be engineered to occur, for example, via cell-secreted matrix metalloproteinases, thus rendering these gels interesting for biomedical applications such as drug delivery systems or smart implants for in situ tissue engineering.

Mechanisms of 3-D migration and matrix remodeling of fibroblasts within artificial ECMs

The elucidation of molecular cell-extracellular matrix (ECM) interactions regulating tissue dynamics necessitates straightforward model systems that can dissect the associated physiological complexity into a smaller number of distinct interactions. Here we employ a previously developed artificial ECM model system to study dynamic cell-matrix interactions involved in proteolytic three-dimensional (3-D) migration and matrix remodeling at the level of single cells. Quantitative time-lapse microscopy of primary human fibroblasts exposed to exogenous physiological matrix metalloproteinase (MMP) inhibitors revealed that 3-D migration is dependent on cell seeding density and occurred via highly localized MMP- and tissue inhibitor of metalloproteinases-2-dependent processes. Stimulation of cells by tumor necrosis factor alpha led to a striking augmentation in fibroblast migration that was accompanied by induction of alphaVbeta3 integrin expression. In long-term cultures, extensive localized cellular matrix remodeling resulted in the morphogenesis of single cells into interconnected multicellular networks. Therefore, these tailor-made artificial ECMs can replicate complex 3-D cell-matrix interactions involved in tissue development and regeneration, an important step in the design of next-generation synthetic biomaterials for tissue engineering.

Part II: Fibroblasts preferentially migrate in the direction of principal strain

A growing body of evidence suggests that the sensory information from the cytoskeleton and integrins may be responsible for guiding migration during mechano- and haptotaxis. However, the dual function of these subcellular structures as mechano-sensors and -actuators is only partially understood. Using a new cell chamber described in the preceding companion paper (Ref to part I, Raeber et al. 2007a) we investigated the migration response of adhesion-dependent fibroblasts embedded 3-dimensionally within synthetic protease-sensitive poly(ethylene glycol) hydrogels to stepwise and cyclic mechanical loads. To that end, we developed a spatially and temporally resolved migration analysis technique capable of providing estimates of statistical cell migration parameters along and perpendicular to the main strain direction. Fibroblasts reoriented themselves in the direction of principal strain, increased their proteolytic migration activity and moved preferentially parallel to the principal strain axis. These results point to a possible correlation between planes of iso-strain and migration direction.

Enzymatic formation of modular cell-instructive fibrin analogs for tissue engineering

The molecular engineering of cell-instructive artificial extracellular matrices is a powerful means to control cell behavior and enable complex processes of tissue formation and regeneration. This work reports on a novel method to produce such smart biomaterials by recapitulating the crosslinking chemistry and the biomolecular characteristics of the biopolymer fibrin in a synthetic analog. We use activated coagulation transglutaminase factor XIIIa for site-specific coupling of cell adhesion ligands and engineered growth factor proteins to multiarm poly(ethylene glycol) macromers that simultaneously form proteolytically sensitive hydrogel networks in the same enzyme-catalyzed reaction. Growth factor proteins are quantitatively incorporated and released upon cell-derived proteolytic degradation of the gels. Primary stromal cells can invade and proteolytically remodel these networks both in an in vitro and in vivo setting. The synthetic ease and potential to engineer their physicochemical and bioactive characteristics makes these hybrid networks true alternatives for fibrin as provisional drug delivery platforms in tissue engineering.

Molecularly engineered PEG hydrogels: a novel model system for proteolytically mediated cell migration

Model systems mimicking the extracellular matrix (ECM) have greatly helped in quantifying cell migration in three dimensions and elucidated the molecular determinants of cellular motility in morphogenesis, regeneration, and disease progression. Here we tested the suitability of proteolytically degradable synthetic poly(ethylene glycol) (PEG)-based hydrogels as an ECM model system for cell migration research and compared this designer matrix with the two well-established ECM mimetics fibrin and collagen. Three-dimensional migration of dermal fibroblasts was quantified by time-lapse microscopy and automated single-cell tracking. A broadband matrix metalloproteinase (MMP) inhibitor and tumor necrosis factor-alpha, a potent MMP-inducer in fibroblasts, were used to alter MMP regulation. We demonstrate a high sensitivity of migration in synthetic networks to both MMP modulators: inhibition led to an almost complete suppression of migration in PEG hydrogels, whereas MMP upregulation increased the fraction of migrating cells significantly. Conversely, migration in collagen and fibrin proved to be less sensitive to the above MMP modulators, as their fibrillar architecture allowed for MMP-independent migration through preexisting pores. The possibility of molecularly recapitulating key functions of the natural extracellular microenvironment and the improved protease sensitivity makes PEG hydrogels an interesting model system that allows correlation between protease activity and cell migration.

Bovine primary chondrocyte culture in synthetic matrix metalloproteinase-sensitive poly(ethylene glycol)-based hydrogels as a scaffold for cartilage repair

A poly(ethylene glycol) (PEG)-based hydrogel was used as a scaffold for chondrocyte culture. Branched PEG-vinylsulfone macromers were end-linked with thiol-bearing matrix metalloproteinase (MMP)-sensitive peptides (GCRDGPQGIWGQDRCG) to form a three-dimensional network in situ under physiologic conditions. Both four- and eight-armed PEG macromer building blocks were examined. Increasing the number of PEG arms increased the elastic modulus of the hydrogels from 4.5 to 13.5 kPa. PEG-dithiol was used to prepare hydrogels that were not sensitive to degradation by cell-derived MMPs. Primary bovine calf chondrocytes were cultured in both MMP-sensitive and MMP-insensitive hydrogels, formed from either four- or eight-armed PEG. Most (>90%) of the cells inside the gels were viable after 1 month of culture and formed cell clusters. Gel matrices with lower elastic modulus and sensitivity to MMP-based matrix remodeling demonstrated larger clusters and more diffuse, less cell surface-constrained cell-derived matrix in the chondron, as determined by light and electron microscopy. Gene expression experiments by real-time RT-PCR showed that the expression of type II collagen and aggrecan was increased in the MMP-sensitive hydrogels, whereas the expression level of MMP-13 was increased in the MMP-insensitive hydrogels. These results indicate that cellular activity can be modulated by the composition of the hydrogel. This study represents one of the first examples of chondrocyte culture in a bioactive synthetic material that can be remodeled by cellular protease activity.

The selective modulation of endothelial cell mobility on RGD peptide containing surfaces by YIGSR peptides

The ability of the biomimetic peptides YIGSR, PHSRN and RGD to selectively affect adhesion and migration of human microvascular endothelial cells (MVEC) and vascular smooth muscle cells (HVSMC) was evaluated. Cell mobility was quantified by time-lapse video microscopy of single cells migrating on peptide modified surfaces. Polyethylene glycol (PEG) hydrogels modified with YIGSR or PHSRN allowed only limited adhesion and no spreading of MVEC and HVSMC. However, when these peptides were individually combined with the strong cell binding peptide RGD in PEG hydrogels, the YIGSR peptide was found to selectively enhance the migration of MVEC by 25% over that of MVEC on RGD alone (p<0.05). No corresponding effect was observed for HVSMC. This suggests that the desired response of specific cell types to tissue engineering scaffolds could be optimized through a combinatory approach to the use of biomimetic peptides.

Synthetic biomaterials as instructive extracellular microenvironments for morphogenesis in tissue engineering

New generations of synthetic biomaterials are being developed at a rapid pace for use as three-dimensional extracellular microenvironments to mimic the regulatory characteristics of natural extracellular matrices (ECMs) and ECM-bound growth factors, both for therapeutic applications and basic biological studies. Recent advances include nanofibrillar networks formed by self-assembly of small building blocks, artificial ECM networks from protein polymers or peptide-conjugated synthetic polymers that present bioactive ligands and respond to cell-secreted signals to enable proteolytic remodeling. These materials have already found application in differentiating stem cells into neurons, repairing bone and inducing angiogenesis. Although modern synthetic biomaterials represent oversimplified mimics of natural ECMs lacking the essential natural temporal and spatial complexity, a growing symbiosis of materials engineering and cell biology may ultimately result in synthetic materials that contain the necessary signals to recapitulate developmental processes in tissue- and organ-specific differentiation and morphogenesis.

Biopolymeric delivery matrices for angiogenic growth factors

The development of new therapeutic approaches that aim to help the body exert its natural mechanisms for vascularized tissue growth (therapeutic angiogenesis) has become one of the most active areas of tissue engineering. Through basic research, several growth factor families and cytokines that are capable to induce physiological blood vessel formation have been identified. Indeed, preclinical and clinical investigations have indicated that therapeutic administration of angiogenic factors, such as the prototypic vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF), to sites of ischemia in the heart or the limb can improve regional blood flow. For new and lasting tissue vascularization, prolonged tissue exposure to these factors could be critical. Furthermore, as shown for VEGF, dosage must be tightly controlled, as excess amounts of VEGF can cause severe vascular leakage and hypotension. This review emphasizes natural and synthetic polymer matrices with respect to their development as vehicles for local and controlled delivery of angiogenic proteins, such as VEGF and bFGF, and their clinical applicability. In the dawn of experimental vascular engineering, new biomaterial schemes for clinical growth factor administration that take better account of biological principles of angiogenic growth factor function and the cell biological basis necessary to produce functional vasculature are evolving. Alongside their base function as protective embedment for angiogenic growth factors, these new classes of bioactive polymers are engineered with additional functionalities that better preserve growth factor activity and more closely mimic the in vivo release mechanisms and profiles of angiogenic growth factors from the extracellular matrix (ECM). Consequently, the preparation of both natural or completely synthetic materials with biological characteristics of the ECM has become central to many tissue engineering approaches that aim to deliver growth factors in a therapeutically efficient mode. Another promising venue to improve angiogenic performance is presented by biomaterials that allow sequential delivery of growth factors with complementary roles in blood vessel initiation and stabilization.

MMP-2 sensitive, VEGF-bearing bioactive hydrogels for promotion of vascular healing

We sought to develop bioactive hydrogels to facilitate arterial healing, e.g., after balloon angioplasty. Toward this end, we developed a new class of proteolytically sensitive, biologically active polyethylene glycol (PEG)-peptide hydrogels that can be formed in situ to temporarily protect the arterial injury from blood contact. Furthermore, we incorporated endothelial cell-specific biological signals with the goal of enhancing arterial reendothelialization. Here we demonstrate efficient endothelial cell anchorage and activation on PEG hydrogel matrices modified by conjugation with both the cell adhesive peptide motif RGD and an engineered variant of vascular endothelial growth factor (VEGF). By crosslinking peptide sequences for cleavage by MMP-2 into the polymer backbone, the hydrogels became sensitive to proteolytic degradation by cell-derived matrix metalloproteinases (MMPs). Analysis of molecular hallmarks associated with endothelial cell activation by VEGF-RGD hydrogel matrices revealed a 70% increase in production of the latent MMP-2 zymogen compared with PEG-peptide hydrogels lacking VEGF. By additional provision of transforming growth factor beta1 (TGF-beta1) within the PEG-peptide hydrogel, conversion of the latent MMP zymogen into its active form was demonstrated. As a result of MMP-2 activation, strongly enhanced hydrogel degradation by activated endothelial cells was observed. Our data illustrate the critical importance of growth factor activities for remodeling of synthetic biomaterials into native tissue, as it is desired in many applications of regenerative medicine. Functionalized PEG-peptide hydrogels could help restore the native vessel wall and improve the performance of angioplasty procedures.

Cell‐demanded release of VEGF from synthetic, biointeractive cell‐ingrowth matrices for vascularized tissue growth

Local, controlled induction of angiogenesis remains a challenge that limits tissue engineering approaches to replace or restore diseased tissues. We present a new class of bioactive synthetic hydrogel matrices based on poly(ethylene glycol) (PEG) and synthetic peptides that exploits the activity of vascular endothelial growth factor (VEGF) alongside the base matrix functionality for cellular ingrowth, that is, induction of cell adhesion by pendant RGD-containing peptides and provision of cell-mediated remodeling by cross-linking matrix metalloproteinase substrate peptides. By using a Michael-type addition reaction, we incorporated variants of VEGF121 and VEGF165 covalently within the matrix, available for cells as they invade and locally remodel the material. The functionality of the matrix-conjugated VEGF was preserved and was critical for in vitro endothelial cell survival and migration within the matrix environment. Consistent with a scheme of locally restricted availability of VEGF, grafting of these VEGF-modified hydrogel matrices atop the chick chorioallontoic membrane evoked strong new blood vessel formation precisely at the area of graft-membrane contact. When implanted subcutaneously in rats, these VEGF-containing matrices were completely remodeled into native, vascularized tissue. This type of synthetic, biointeractive matrix with integrated angiogenic growth factor activity, presented and released only upon local cellular demand, could become highly useful in a number of clinical healing applications of local therapeutic angiogenesis.

Synthesis and physicochemical characterization of end-linked poly(ethylene glycol)-co-peptide hydrogels formed by Michael-type addition

The synthesis of novel hybrid hydrogels by stepwise copolymerization of multiarm vinyl sulfone-terminated poly(ethylene glycol) macromers and alpha-omega cysteine oligopeptides via Michael-type additions is described. Cross-linking kinetics, studied by in situ rheometry, can be controlled by pH and the presence of charged amino acid residues in close proximity to the Cys, which modulates the pK(a) of the thiol group. These end-linked networks were characterized by their equilibrium swelling in water, by their viscoelastic properties in the swollen state, and by their soluble fraction. It was demonstrated that structure and properties are very sensitive to the preparation state including stoichiometry and precursor concentration and less sensitive to the pH during cross-linking. For each network the concentration of elastically active chains (nu) was calculated from experimentally determined sol fractions using Miller-Macosko theory and compared to values obtained from swelling and rheometry studies and by calculation from Flory's classical network models. Hydrogels were also prepared with varying macromer structures, and their properties were shown to respond to both macromer functionality and molecular weight.

Repair of bone defects using synthetic mimetics of collagenous extracellular matrices

We have engineered synthetic poly(ethylene glycol) (PEG)-based hydrogels as cell-ingrowth matrices for in situ bone regeneration. These networks contain a combination of pendant oligopeptide ligands for cell adhesion (RGDSP) and substrates for matrix metalloproteinase (MMP) as linkers between PEG chains. Primary human fibroblasts were shown to migrate within these matrices by integrin- and MMP-dependent mechanisms. Gels used to deliver recombinant human bone morphogenetic protein-2 (rhBMP-2) to the site of critical- sized defects in rat crania were completely infiltrated by cells and were remodeled into bony tissue within five weeks. Bone regeneration was dependent on the proteolytic sensitivity of the matrices and their architecture. The cell-mediated proteolytic invasiveness of the gels and entrapment of rhBMP-2 resulted in efficient and highly localized bone regeneration.

Synthetic matrix metalloproteinase-sensitive hydrogels for the conduction of tissue regeneration: engineering cell-invasion characteristics

Synthetic hydrogels have been molecularly engineered to mimic the invasive characteristics of native provisional extracellular matrices: a combination of integrin-binding sites and substrates for matrix metalloproteinases (MMP) was required to render the networks degradable and invasive by cells via cell-secreted MMPs. Degradation of gels was engineered starting from a characterization of the degradation kinetics (k(cat) and K(m)) of synthetic MMP substrates in the soluble form and after crosslinking into a 3D hydrogel network. Primary human fibroblasts were demonstrated to proteolytically invade these networks, a process that depended on MMP substrate activity, adhesion ligand concentration, and network crosslinking density. Gels used to deliver recombinant human bone morphogenetic protein-2 to the site of critical defects in rat cranium were completely infiltrated by cells and remodeled into bony tissue within 4 wk at a dose of 5 microg per defect. Bone regeneration was also shown to depend on the proteolytic sensitivity of the matrices. These hydrogels may be useful in tissue engineering and cell biology as alternatives for naturally occurring extracellular matrix-derived materials such as fibrin or collagen.

Poly(ethylene glycol) block copolymers

The ubiquitous use of poly(ethylene glycol) in the biomaterials field has also boosted the research activity in the chemical derivatization of this polymer. We focused our interest on the preparation of tailor-made poly(ethylene glycol)-based structures and on the study of structure-activity relationships for its functionalization, as preliminary steps for the preparation of smart functional materials. More specifically, amphiphilic and cationic block copolymers were prepared for prospective use in the preparation of self-assembled carriers, and Michael-type addition of thiols onto acrylates was studied as a model for end-group reaction leading to hydrogel formation.

Systematic modulation of Michael-type reactivity of thiols through the use of charged amino acids

A quantitative structure-reactivity relationship for the Michael-type addition of thiols onto acrylates was determined. Several thiol-containing peptides were investigated by examining the correlation between the second-order rate constant of their addition onto PEG-diacrylate and the pK(a) of the thiols within a peptide. By introducing charged amino acids in close proximity to a cysteine, the pK(a) of the thiol was systematically modulated by electrostatic interactions. Positive charges from the amino acid arginine decreased the pK(a) of the thiol and accelerated the reaction with acrylates while negative charges from aspartic acids showed the opposite effect. A linear correlation between thiolate concentrations and kinetic constants was found, confirming the role of thiolates as the reactive species in this Michael-type reaction. The relevant factors influencing the reactivity were the sign and the number of the neighboring charges, while the position of these charges had little effect on reactivity. These results provide a basis for the rational design of peptides, where the kinetics and thus selectivity of protein/peptide conjugation with polymeric structures via Michael-type addition reactions can be controlled.

Protein delivery from materials formed by self-selective conjugate addition reactions

A new chemical cross-linking scheme was utilized for the formation of degradable poly(ethylene glycol) hydrogels suitable for the delivery of protein drugs. An aqueous solution containing a PEG-multiacrylate and solid particles of albumin was mixed with an aqueous solution containing a PEG-dithiol, rapidly producing a cross-linked hydrogel through a Michael-type addition reaction. For some formulations, it was observed that about 65% of the incorporated protein was released with zero-order kinetics over a period of about 4 days. By changing the functionality of the cross-linker, the release of protein could even be delayed for about 4 days, followed by zero-order release. The mechanism for release appeared to be a combination of slow dissolution of protein in the presence of PEG and hindered diffusion of protein through the gel. The cross-linking of the gels was studied rheometrically, and the hydrolytic degradation of the gels was characterized by measuring the swelling of the gels. Biochemical analysis of the released proteins demonstrated that the polymers reacted with each other, but not with proteins. Utilizing the Flory-Rehner and Peppas-Merrill equations, a framework for modeling the protein release from the gels is described.