In vitro astrocyte and cerebral endothelial cell response to electrospun poly(epsilon-caprolactone) mats of different architecture

A Hybrid Nanofiber/Paper Cell Culture Platform for Building a 3D Blood-brain Barrier Model

The blood brain barrier (BBB) protects the central nervous system from toxins and pathogens in the blood by regulating permeation of molecules through the barrier interface. In vitro BBB models described to date reproduce some aspects of BBB functionality, but also suffer from incomplete phenotypic expression of brain endothelial traits, difficulty in reproducibility and fabrication, or overall cost. To address these limitations, we describe a three-dimensional (3D) BBB model based on a hybrid paper/nanofiber scaffold. The cell culture platform utilizes lens paper as a framework to accommodate 3D culture of astrocytes. An electrospun nanofiber layer is coated onto one face of the paper to mimic the basement membrane and support growth of an organized two-dimensional layer of endothelial cells (ECs). Human induced pluripotent stem cell-derived ECs and astrocytes are co-cultured to develop a human BBB model. Morphological and spatial organization of model are validated with confocal microscopy. Measurements of transendothelial resistance and permeability demonstrate the BBB model develops a high-quality barrier and responds to hyperosmolar treatments. RNA-sequencing shows introduction of astrocytes both regulates EC tight junction proteins and improves endothelial phenotypes related to vasculogenesis. This model shows promise as a model platform for future in vitro studies of the BBB.

Electrospun Fiber Scaffolds for Engineering Glial Cell Behavior to Promote Neural Regeneration

Electrospinning is a fabrication technique used to produce nano- or micro- diameter fibers to generate biocompatible, biodegradable scaffolds for tissue engineering applications. Electrospun fiber scaffolds are advantageous for neural regeneration because they mimic the structure of the nervous system extracellular matrix and provide contact guidance for regenerating axons. Glia are non-neuronal regulatory cells that maintain homeostasis in the healthy nervous system and regulate regeneration in the injured nervous system. Electrospun fiber scaffolds offer a wide range of characteristics, such as fiber alignment, diameter, surface nanotopography, and surface chemistry that can be engineered to achieve a desired glial cell response to injury. Further, electrospun fibers can be loaded with drugs, nucleic acids, or proteins to provide the local, sustained release of such therapeutics to alter glial cell phenotype to better support regeneration. This review provides the first comprehensive overview of how electrospun fiber alignment, diameter, surface nanotopography, surface functionalization, and therapeutic delivery affect Schwann cells in the peripheral nervous system and astrocytes, oligodendrocytes, and microglia in the central nervous system both in vitro and in vivo. The information presented can be used to design and optimize electrospun fiber scaffolds to target glial cell response to mitigate nervous system injury and improve regeneration.

Biomaterial Approaches to Modulate Reactive Astroglial Response

Over several decades, biomaterial scientists have developed materials to spur axonal regeneration and limit secondary injury and tested these materials within preclinical animal models. Rarely, though, are astrocytes examined comprehensively when biomaterials are placed into the injury site. Astrocytes support neuronal function in the central nervous system. Following an injury, astrocytes undergo reactive gliosis and create a glial scar. The astrocytic glial scar forms a dense barrier which restricts the extension of regenerating axons through the injury site. However, there are several beneficial effects of the glial scar, including helping to reform the blood-brain barrier, limiting the extent of secondary injury, and supporting the health of regenerating axons near the injury site. This review provides a brief introduction to the role of astrocytes in the spinal cord, discusses astrocyte phenotypic changes that occur following injury, and highlights studies that explored astrocyte changes in response to biomaterials tested within in vitro or in vivo environments. Overall, we suggest that in order to improve biomaterial designs for spinal cord injury applications, investigators should more thoroughly consider the astrocyte response to such designs.

Electrospun fiber surface nanotopography influences astrocyte-mediated neurite outgrowth

Aligned, electrospun fiber scaffolds provide topographical guidance for regenerating neurons and glia after central nervous system injury. To date, no study has explored how fiber surface nanotopography affects astrocyte response to fibrous scaffolds. Astrocytes play important roles in the glial scar, the blood brain barrier, and in maintaining homeostasis in the central nervous system. In this study, electrospun poly L-lactic acid fibers were engineered with smooth, pitted, or divoted surface nanotopography. Cortical or spinal cord primary rat astrocytes were cultured on the surfaces for either 1 or 3 d to examine the astrocyte response over time. The results showed that cortical astrocytes were significantly shorter and broader on the pitted and divoted fibers compared to those on smooth fibers. However, spinal cord astrocyte morphology was not significantly altered by the surface features. These findings indicate that astrocytes from unique anatomical locations respond differently to the presence of nanotopography. Western blot results show that the differences in morphology were not associated with significant changes in glial fibrillary acidicprotein (GFAP) or vinculin in either astrocyte population, suggesting that surface pits and divots do not induce a reactive phenotype in either cortical or spinal cord astrocytes. Finally, astrocytes were co-cultured with dorsal root ganglia to determine how the surfaces affected astrocyte-mediated neurite outgrowth. Astrocytes cultured on the fibers for shorter periods of time (1 d) generally supported longer neurite outgrowth. Pitted and divoted fibers restricted spinal cord astrocyte-mediated neurite outgrowth, while smooth fibers increased 3 d spinal cord astrocyte-mediated neurite outgrowth. In total, fiber surface nanotopography can influence astrocyte elongation and influence the capability of astrocytes to direct neurites. Therefore, fiber surface characteristics should be carefully controlled to optimize astrocyte-mediated axonal regeneration.

The Horizon of Materiobiology: A Perspective on Material-Guided Cell Behaviors and Tissue Engineering

Although the biological functions of cell and tissue can be regulated by biochemical factors (e.g., growth factors, hormones), the biophysical effects of materials on the regulation of biological activity are receiving more attention. In this Review, we systematically summarize the recent progress on how biomaterials with controllable properties (e.g., compositional/degradable dynamics, mechanical properties, 2D topography, and 3D geometry) can regulate cell behaviors (e.g., cell adhesion, spreading, proliferation, cell alignment, and the differentiation or self-maintenance of stem cells) and tissue/organ functions. How the biophysical features of materials influence tissue/organ regeneration have been elucidated. Current challenges and a perspective on the development of novel materials that can modulate specific biological functions are discussed. The interdependent relationship between biomaterials and biology leads us to propose the concept of "materiobiology", which is a scientific discipline that studies the biological effects of the properties of biomaterials on biological functions at cell, tissue, organ, and the whole organism levels. This Review highlights that it is more important to develop ECM-mimicking biomaterials having a self-regenerative capacity to stimulate tissue regeneration, instead of attempting to recreate the complexity of living tissues or tissue constructs ex vivo. The principles of materiobiology may benefit the development of novel biomaterials providing combinative bioactive cues to activate the migration of stem cells from endogenous reservoirs (i.e., cell niches), stimulate robust and scalable self-healing mechanisms, and unlock the body's innate powers of regeneration.

Electrospun gelatin biopapers as substrate for in vitro bilayer models of blood-brain barrier tissue

Gaining a greater understanding of the blood-brain barrier (BBB) is critical for improvement in drug delivery, understanding pathologies that compromise the BBB, and developing therapies to protect the BBB. In vitro human tissue models are valuable tools for studying these issues. The standard in vitro BBB models use commercially available cell culture inserts to generate bilayer co-cultures of astrocytes and endothelial cells (EC). Electrospinning can be used to produce customized cell culture substrates with optimized material composition and mechanical properties with advantages over off-the-shelf materials. Electrospun gelatin is an ideal cell culture substrate because it is a natural polymer that can aid cell attachment and be modified and degraded by cells. Here, we have developed a method to produce cell culture inserts with electrospun gelatin "biopaper" membranes. The electrospun fiber diameter and cross-linking method were optimized for the growth of primary human endothelial cell and primary human astrocyte bilayer co-cultures to model human BBB tissue. BBB co-cultures on biopaper were characterized via cell morphology, trans-endothelial electrical resistance (TEER), and permeability to FITC-labeled dextran and compared to BBB co-cultures on standard cell culture inserts. Over longer culture periods (up to 21 days), cultures on the optimized electrospun gelatin biopapers were found to have improved TEER, decreased permeability, and permitted a smaller separation between co-cultured cells when compared to standard PET inserts.

Enhanced GLT-1 mediated glutamate uptake and migration of primary astrocytes directed by fibronectin-coated electrospun poly-L-lactic acid fibers

Bioengineered fiber substrates are increasingly studied as a means to promote regeneration and remodeling in the injured central nervous system (CNS). Previous reports largely focused on the ability of oriented scaffolds to bridge injured regions and direct outgrowth of axonal projections. In the present work, we explored the effects of electrospun microfibers on the migration and physiological properties of brain astroglial cells. Primary rat astrocytes were cultured on either fibronectin-coated poly-L-lactic acid (PLLA) films, fibronectin-coated randomly oriented PLLA electrospun fibers, or fibronectin-coated aligned PLLA electrospun fibers. Aligned PLLA fibers strongly altered astrocytic morphology, orienting cell processes, actin microfilaments, and microtubules along the length of the fibers. On aligned fibers, astrocytes also significantly increased their migration rates in the direction of fiber orientation. We further investigated if fiber topography modifies astrocytic neuroprotective properties, namely glutamate and glutamine transport and metabolism. This was done by quantifying changes in mRNA expression (qRT-PCR) and protein levels (Western blotting) for a battery of relevant biomolecules. Interestingly, we found that cells grown on random and/or aligned fibers increased the expression levels of two glutamate transporters, GLAST and GLT-1, and an important metabolic enzyme, glutamine synthetase, as compared to the fibronectin-coated films. Functional assays revealed increases in glutamate transport rates due to GLT-1 mediated uptake, which was largely determined by the dihydrokainate-sensitive GLT-1. Overall, this study suggests that aligned PLLA fibers can promote directed astrocytic migration, and, of most importance, our in vitro results indicate for the first time that electrospun PLLA fibers can positively modify neuroprotective properties of glial cells by increasing rates of glutamate uptake.

Effects of humidity and solution viscosity on electrospun fiber morphology

Electrospinning is a popular technique to fabricate tissue engineering scaffolds due to the exceptional tunability of fiber morphology that can be used to control scaffold mechanical properties, degradation rate, and cell behavior. Although the effects of modulating processing or solution parameters on fiber morphology have been extensively studied, there remains limited understanding of the impact of environmental parameters such as humidity. To address this gap, three polymers (poly(ethylene glycol) [PEG], polycaprolactone [PCL], and poly(carbonate urethane) [PCU]) were electrospun at a range of relative humidities (RH = 5%-75%) and the resulting fiber architecture characterized with scanning electron microscopy. Low relative humidity (< 50%) resulted in fiber breakage for all three polymers due to decreased electrostatic discharge from the jet. At high relative humidity (> 50%), three distinct effects were observed based on individual polymer properties. An increase in fiber breakage and loss of fiber morphology occurred in the PEG system as a result of increased water absorption at high relative humidity. In contrast, surface pores on PCL fibers were observed and hypothesized to have formed via vapor-induced phase separation. Finally, decreased PCU fiber collection occurred at high humidity likely due to increased electrostatic discharge. These findings highlight that the effects of relative humidity on electrospun fiber morphology are dependent on polymer hydrophobicity, solvent miscibility with water, and solvent volatility. An additional study was conducted to highlight that small changes in molecular weight can strongly influence solution viscosity and resulting fiber morphology. We propose that solution viscosity rather than concentration is a more useful parameter to report in electrospinning methodology to enable reproduction of findings. In summary, this study further elucidates key mechanisms in electrospun fiber formation that can be utilized to fabricate tissue engineering scaffolds with tunable and reproducible properties.

Electrospun polycaprolactone scaffolds with tailored porosity using two approaches for enhanced cellular infiltration

The impact of mat porosity of polycaprolactone (PCL) electrospun fibers on the infiltration of neuron-like PC12 cells was evaluated using two different approaches. In the first method, bi-component aligned fiber mats were fabricated via the co-electrospinning of PCL with polyethylene oxide (PEO). Variation of the PEO flow rate, followed by selective removal of PEO from the PCL/PEO mesh, allowed for control of the porosity of the resulting scaffold. In the second method, aligned fiber mats were fabricated from various concentrations of PCL solutions to generate fibers with diameters between 0.13 ± 0.06 and 9.10 ± 4.1 μm. Of the approaches examined, the variation of PCL fiber diameter was found to be the better method for increasing the infiltration of PC12 cells, with the optimal infiltration into the ca. 1.5-mm-thick meshes observed for the mats with the largest fiber diameters, and hence largest pore sizes.

Directionality and bipolarity of olfactory ensheathing cells on electrospun nanofibers

Aim: As a preliminary to the construction of olfactory ensheathing cells (OECs) bearing scaffold for bridging larger lesions in the spinal cord, we have investigated the response of purified cultured OECs to nanoscale fibers of varying diameter using US FDA-approved, biodegradable poly(lactic-co-glycolic-acid). Materials & methods: Conventional electrospinning produced fibers of approximately 700 nm diameter (nano-700) while nanocomposite electrospinning with quantum dots produced significantly more uniform fibers of a reduced diameter to approximately 237 nm (nano-250). OECs from adult rat were FACS purified, cultured at low density on either a flat surface or a meshwork of randomly orientated nano-700 and nano-250 fibers, and assessed using cytomorphometric analysis of immunofluorescent confocal images and by scanning electron microscopy. Results & conclusion: Compared with a flat surface, culture on a nano-700 mesh increases cell attachment. Cells change from rounded to stellate forms in random orientation. Further size reduction to the nano-250 favors bipolarity in cells with unidirectional orientation as observed in the case when transplanted OECs were used to bridge areas of damage in rat spinal cords.

Original submitted 26 August 2011; Revised submitted 28 October 2011; Published online 25 May 2012

Nanofibrous collagen nerve conduits for spinal cord repair

Nerve regeneration in an injured spinal cord is often restricted, contributing to the devastating outcome of neurologic impairment below the site of injury. Although implantation of tissue-engineered scaffolds has evolved as a potential treatment method, the outcomes remain sub-optimal. One possible reason may be the lack of topographical signals from these constructs to provide contact guidance to invading cells or regrowing axons. Nanofibers mimic the natural extracellular matrix architecturally and may therefore promote physiologically relevant cellular phenotypes. In this study, the potential application of electrospun collagen nanofibers (diameter=208.2±90.4 nm) for spinal cord injury (SCI) treatment was evaluated in vitro and in vivo. Primary rat astrocytes and dorsal root ganglias (DRGs) were seeded on collagen-coated glass cover slips (two-dimensional [2D] substrate controls), and randomly oriented or aligned collagen fibers to evaluate scaffold topographical effects on astrocyte behavior and neurite outgrowth, respectively. When cultured on collagen nanofibers, astrocyte proliferation and expression of glial fibrillary acidic protein (GFAP) were suppressed as compared to cells on 2D controls at days 3 (p<0.05) and 7 (p<0.01). Aligned fibers resulted in elongated astrocytes (elongation factor >4, p<0.01) and directed the orientation of neurite outgrowth from DRGs along fiber axes. In the contrast, neurites emanated radially on randomly oriented collagen fibers. By forming collagen scaffolds into spiral tubular structures, we demonstrated the feasibility of using electrospun nanofibers for the treatment of acute SCI using a rat hemi-section model. At days 10 and 30 postimplantation, extensive cellular penetration into the constructs was observed regardless of fiber orientation. However, scaffolds with aligned fibers appeared more structurally intact at day 30. ED1 immunofluorescent staining revealed macrophage invasion by day 10, which decreased significantly by day 30. Neural fiber sprouting as evaluated by neurofilament staining was observed as early as day 10. In addition, GFAP immunostained astrocytes were found only at the boundary of the lesion site, and no astrocyte accumulation was observed in the implantation area at any time point. These findings indicate the feasibility of fabricating 3D spiral constructs using electrospun collagen fibers and demonstrated the potential of these scaffolds for SCI repair.