Electrospun fiber-mediated delivery of neurotrophin-3 mRNA for neural tissue engineering applications

Aligned electrospun fibers provide topographical cues and local therapeutic delivery to facilitate robust peripheral nerve regeneration. mRNA delivery enables transient expression of desired proteins that promote axonal regeneration. However, no prior work delivers mRNA from electrospun fibers for peripheral nerve regeneration applications. Here, we developed the first aligned electrospun fibers to deliver pseudouridine-modified (Ψ) neurotrophin-3 (NT-3) mRNA (ΨNT-3mRNA) to primary Schwann cells and assessed NT-3 secretion and bioactivity. We first electrospun aligned poly(L-lactic acid) (PLLA) fibers and coated them with the anionic substrates dextran sulfate sodium salt (DSS) or poly(3,4-dihydroxy-L-phenylalanine) (pDOPA). Cationic lipoplexes containing ΨNT-3mRNA complexed to JetMESSENGER® were then immobilized to the fibers, resulting in detectable ΨNT-3mRNA release for 28 days from all fiber groups investigated (PLLA+mRNA, 0.5DSS4h+mRNA, and 2pDOPA4h+mRNA). The 2pDOPA4h+mRNA group significantly increased Schwann cell secretion of NT-3 for 21 days compared to control PLLA fibers (p < 0.001-0.05) and, on average, increased Schwann cell secretion of NT-3 by ≥ 2-fold compared to bolus mRNA delivery from the 1µgBolus+mRNA and 3µgBolus+mRNA groups. The 2pDOPA4h+mRNA fibers supported Schwann cell secretion of NT-3 at levels that significantly increased dorsal root ganglia (DRG) neurite extension by 44% (p < 0.0001) and neurite area by 64% (p < 0.001) compared to control PLLA fibers. The data show that the 2pDOPA4h+mRNA fibers enhance the ability of Schwann cells to promote neurite growth from DRG, demonstrating this platform's potential capability to improve peripheral nerve regeneration. STATEMENT OF SIGNIFICANCE: Aligned electrospun fibers enhance axonal regeneration by providing structural support and guidance cues, but further therapeutic stimulation is necessary to improve functional outcomes. mRNA delivery enables the transient expression of therapeutic proteins, yet achieving local, sustained delivery remains challenging. Previous work shows that genetic material delivery from electrospun fibers improves regeneration; however, mRNA delivery has not been explored. Here, we examine mRNA delivery from aligned electrospun fibers to enhance neurite outgrowth. We show that immobilization of NT-3mRNA/JetMESSENGER® lipoplexes to aligned electrospun fibers functionalized with pDOPA enables local, sustained NT-3mRNA delivery to Schwann cells, increasing Schwann cell secretion of NT-3 and enhancing DRG neurite outgrowth. This study displays the potential benefits of electrospun fiber-mediated mRNA delivery platforms for neural tissue engineering.

Multivariate analysis reveals topography dependent relationships amongst neurite morphological features from dorsal root ganglia neurons

Objective.Nerve guidance scaffolds containing anisotropic architectures provide topographical cues to direct regenerating axons through an injury site to reconnect the proximal and distal end of an injured nerve or spinal cord. Previousin vitrocultures of individual neurons revealed that fiber characteristics such as fiber diameter and inter-fiber spacing alter neurite morphological features, such as total neurite length, the longest single neurite, branching density, and the number of primary neurites. However, the relationships amongst these four neurite morphological features have never been studied on fibrous topographies using multivariate analysis.Approach.In this study, we cultured dissociated dorsal root ganglia on aligned, fibrous scaffolds and flat, isotropic films and evaluated the univariate and multivariate differences amongst these four neurite morphological features.Main results.Univariate analysis showed that fibrous scaffolds increase the length of the longest neurite and decrease branching density compared to film controls. Further, multivariate analysis revealed that, regardless of scaffold type, overall neurite length increases due to a compromise between the longest extending neurite, branching density, and the number of primary neurites. Additionally, multivariate analysis indicated that neurite branching is more independent of the other neurite features when neurons were cultured on films but that branching is strongly related to the other neurite features when cultured on fibers.Significance.These findings are significant as they are the first evidence that aligned topographies affect the relationships between neurite morphological features. This study provides a foundation for analyzing how individual neurite morphology may relate to neural regeneration on a macroscopic scale and provide information that may be used to optimize nerve guidance scaffolds.

Designing electrospun fiber platforms for efficient delivery of genetic material and genome editing tools

Electrospun fibers are versatile biomaterial platforms with great potential to support regeneration. Electrospun fiber characteristics such as fiber diameter, degree of alignment, rate of degradation, and surface chemistry enable the creation of unique, tunable scaffolds for various drug or gene delivery applications. The delivery of genetic material and genome editing tools via viral and non-viral vectors are approaches to control cellular protein production. However, immunogenicity, off-target effects, and low delivery efficiencies slow the progression of gene delivery strategies to clinical settings. The delivery of genetic material from electrospun fibers overcomes such limitations by allowing for localized, tunable delivery of genetic material. However, the process of electrospinning is harsh, and care must be taken to retain genetic material bioactivity. This review presents an up-to-date summary of strategies to incorporate genetic material onto or within electrospun fiber platforms to improve delivery efficiency and enhance the regenerative potential of electrospun fibers for various tissue engineering applications.

Acute Dose-Dependent Neuroprotective Effects of Poly(pro-17β-estradiol) in a Mouse Model of Spinal Contusion Injury

17β-Estradiol (E2) confers neuroprotection in preclinical models of spinal cord injury when administered systemically. The goal of this study was to apply E2 locally to the injured spinal cord for a sustained duration using poly(pro-E2) film biomaterials. Following contusive spinal cord injury in adult male mice, poly(pro-E2) films were implanted subdurally and neuroprotection was assessed using immunohistochemistry 7 days after injury and implantation. In these studies, poly(pro-E2) films modestly improved neuroprotection without affecting the inflammatory response when compared to the injured controls. To increase the E2 dose released, bolus-releasing poly(pro-E2) films were fabricated by incorporating unbound E2 into the poly(pro-E2) films. However, compared to the injured controls, bolus-releasing poly(pro-E2) films did not significantly enhance neuroprotection or limit inflammation at either 7 or 21 days post-injury. Future work will focus on developing poly(pro-E2) biomaterials capable of more precisely releasing therapeutic doses of E2.

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.

Conventional immunomarkers stain a fraction of astrocytes in vitro: A comparison of rat cortical and spinal cord astrocytes in naïve and stimulated cultures

Astrocytes are responsible for a wide variety of essential functions throughout the central nervous system. The protein markers glial fibrillary acidic protein (GFAP), glutamate aspartate transporter (GLAST), glutamate transporter-1 (GLT-1), glutamine synthetase (GS), 10-formyltetrahydrofolate dehydrogenase (ALDH1L1), and the transcription factor SOX9 are routinely used to label astrocytes in primary rodent cultures. However, GLAST, GLT-1, GS, and SOX9 are also produced by microglia and oligodendrocytes and GFAP, GLAST, GLT-1, and GS production levels are affected by astrocyte phenotypic changes associated with reactive astrogliosis. No group has performed a comprehensive immunocytochemical evaluation to quantify the percentage of cells labeled by these markers in vitro, nor compared changes in staining between cortex- and spinal cord-derived cells in naïve and stimulated cultures. Here, we quantified the percentage of cells positively stained for these six markers in astrocyte, microglia, and oligodendrocyte cultures isolated from neonatal rat cortices and spinal cords. Additionally, we incubated the astrocytes with transforming growth factor (TGF)-β1 or TGF-β3 to determine if the labeling of these markers is altered by these stimuli. We found that only SOX9 in cortical cultures and ALDH1L1 in spinal cord cultures labeled more than 75% of the cells in naïve and stimulated astrocyte cultures and stained less than 5% of the cells in microglia and oligodendrocyte cultures. Furthermore, significantly more cortical than spinal cord astrocytes stained for GFAP, GLAST, and ALDH1L1 in naïve cultures, whereas significantly more spinal cord than cortical astrocytes stained for GLAST and GS in TGF-β1-treated cultures. These findings are important as variability in marker staining may lead to misinterpretation of the astrocyte response in cocultures, migration assays, or engineered disease models.

Aligned Fingolimod-Releasing Electrospun Fibers Increase Dorsal Root Ganglia Neurite Extension and Decrease Schwann Cell Expression of Promyelinating Factors

Researchers are investigating the use of biomaterials with aligned guidance cues, like those provided by aligned electrospun fibers, to facilitate axonal growth across critical-length peripheral nerve defects. To enhance the regenerative outcomes further, these aligned fibers can be designed to provide local, sustained release of therapeutics. The drug fingolimod improved peripheral nerve regeneration in preclinical rodent models by stimulating a pro-regenerative Schwann cell phenotype and axonal growth. However, the systemic delivery of fingolimod for nerve repair can lead to adverse effects, so it is necessary to develop a means of providing sustained delivery of fingolimod local to the injury. Here we created aligned fingolimod-releasing electrospun fibers that provide directional guidance cues in combination with the local, sustained release of fingolimod to enhance neurite outgrowth and stimulate a pro-regenerative Schwann cell phenotype. Electrospun fiber scaffolds were created by blending fingolimod into poly(lactic-co-glycolic acid) (PLGA) at a w/w% (drug/polymer) of 0.0004, 0.02, or 0.04%. We examined the effectiveness of these scaffolds to stimulate neurite extension in vitro by measuring neurite outgrowth from whole and dissociated dorsal root ganglia (DRG). Subsequently, we characterized Schwann cell migration and gene expression in vitro. The results show that drug-loaded PLGA fibers released fingolimod for 28 days, which is the longest reported release of fingolimod from electrospun fibers. Furthermore, the 0.02% fingolimod-loaded fibers enhanced neurite outgrowth from whole and dissociated DRG neurons, increased Schwann cell migration, and reduced the Schwann cell expression of promyelinating factors. The in vitro findings show the potential of the aligned fingolimod-releasing electrospun fibers to enhance peripheral nerve regeneration and serve as a basis for future in vivo studies.

Coating Topologically Complex Electrospun Fibers with Nanothin Silk Fibroin Enhances Neurite Outgrowth in Vitro

Electrospun poly-l-lactic acid (PLLA) fibers are commonly used for tissue engineering applications because of their uniform morphology, and their efficacy can be further enhanced via surface modification. In this study, we aimed to increase neurite outgrowth along electrospun fibers by coating with silk fibroin (SF), a bioinert protein derived from Bombyx mori cocoon threads, shown to be neurocompatible. Aligned PLLA fibers were electrospun with smooth, pitted, and divoted surface nanotopographies and coated with SF by immersion in coating solution for either 12 or 24 h. Specifically, thin-film coatings of SF were generated by leveraging the controlled self-assembly of SF in aqueous conditions that promote β-sheet assembly. For both 12- and 24-h coatings, Congo Red staining for β-sheet structures confirmed the presence of SF coatings on PLLA fibers. Confocal imaging of fluorescein-labeled SF further demonstrated a homogeneous coating formation on PLLA fibers. No change in the water contact angle of the surfaces was observed after coating; however, an increase in the isoelectric point (pI) to values comparable with the theoretical pI of SF was seen. Notably, there was a significant trend of increased dorsal root ganglia (DRG) adhesion on scaffolds coated with SF, as well as greater neurite outgrowth on pitted and divoted fibers that had been coated with SF. Ultimately, this work demonstrated that thin-film SF coatings formed by self-assembly uniformly coat electrospun fibers, providing a new strategy to increase the neuroregenerative capacity of electrospun scaffolds. To our knowledge, this is the first instance of biomedical modification of topologically complex substrates using noncovalent methods.

Vastly extended drug release from poly(pro-17β-estradiol) materials facilitates in vitro neurotrophism and neuroprotection

Central nervous system (CNS) injuries persist for years, and currently there are no therapeutics that can address the complex injury cascade that develops over this time-scale. 17β-estradiol (E2) has broad tropism within the CNS, targeting and inducing beneficial phenotypic changes in myriad cells following injury. To address the unmet need for vastly prolonged E2 release, we report first-generation poly(pro-E2) biomaterial scaffolds that release E2 at nanomolar concentrations over the course of 1-10 years via slow hydrolysis in vitro. As a result of their finely tuned properties, these scaffolds demonstrate the ability to promote and guide neurite extension ex vivo and protect neurons from oxidative stress in vitro. The design and testing of these materials reported herein demonstrate the first step towards next-generation implantable biomaterials with prolonged release and excellent regenerative potential.

Stabilized Interleukin-4-Loaded Poly(lactic-co-glycolic) Acid Films Shift Proinflammatory Macrophages toward a Regenerative Phenotype in Vitro

Macrophages are immune cells involved in wound healing and tissue regeneration; however, the sustained presence of proinflammatory macrophages in wound sites impairs healing. In this study, we shifted peritoneal macrophage polarization away from a proinflammatory (M1) phenotype through exposure to stabilized interleukin-4 (IL-4) in poly(lactic-co-glycolic acid) films in combination with topographical guidance from electrospun poly-L-lactic acid fibers. To our knowledge, this was the first study to stabilize IL-4 with bovine serum albumin (BSA) within a biomaterial. When IL-4 was coloaded with BSA for stabilization, we saw increased IL-4 bioactivity compared to no added stabilization, trehalose stabilization, or murine serum albumin stabilization. We observed increased elongation of peritoneal macrophages, increased RNA expression of anti-inflammatory marker arginase-1, increased ratio of interleukin-10/interleukin- 12 p40 RNA, and decreased protein expression of proinflammatory markers (interleukin-12 p40 and RANTES) compared to controls. Taken together, these results suggest the macrophages were less proinflammatory and were a more pro-resolving phenotype. When stabilized with BSA, IL-4-loaded films effectively shift macrophage polarization state and are thus promising scaffolds to reduce inflammation within in vivo injury models.

Exploring the effects of electrospun fiber surface nanotopography on neurite outgrowth and branching in neuron cultures

Three aligned, electrospun fiber scaffolds with unique surface features were created from poly-L-lactic acid (PLLA). Fibers without surface nanotopography (smooth fibers), fibers with surface divots (shallow pits), and fibers with surface pits (deeper pits) were fabricated, and fiber alignment, diameter, and density were characterized using scanning electron microscopy (SEM). Whole dorsal root ganglia (DRG) were isolated from rats and placed onto uncoated fibers or fibers coated with laminin. On uncoated fibers, neurite outgrowth was restricted by fibers displaying divoted or pitted nanotopography when compared to neurite outgrowth on smooth fibers. However, neurites extending from whole DRG cultured on laminin-coated fibers were not restricted by divoted or pitted surface nanotopography. Thus, neurites extending on laminin-coated fibers were able to extend long neurites even in the presence of surface divots or pits. To further explore this result, individual neurons isolated from dissociated DRG were seeded onto laminin-coated smooth, pitted, or divoted fibers. Interestingly, neurons on pitted or divoted fibers exhibited a 1.5-fold increase in total neurite length, and a 2.3 or 2.7-fold increase in neurite branching compared to neurons on smooth fibers, respectively. Based on these findings, we conclude that fiber roughness in the form of pits or divots can promote extension and branching of long neurites along aligned electrospun fibers in the presence of an extracellular matrix protein coating. Thus, aligned, electrospun fibers can be crafted to not only direct the extension of axons but to induce unique branching morphologies.

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.

Solvent retention in electrospun fibers affects scaffold mechanical properties

Electrospinning is a robust material fabrication method allowing for fine control of mechanical, chemical, and functional properties in scaffold manufacturing. Electrospun fiber scaffolds have gained prominence for their potential in a variety of applications such as tissue engineering and textile manufacturing, yet none have assessed the impact of solvent retention in fibers on the scaffold's mechanical properties. In this study, we hypothesized that retained electrospinning solvent acts as a plasticizer, and gradual solvent evaporation, by storing fibers in ambient air, will cause significant increases in electrospun fiber scaffold brittleness and stiffness, and a significant decrease in scaffold toughness. Thermogravimetric analysis indicated solvent retention in PGA, PLCL, and PET fibers, and not in PU and PCL fibers. Differential scanning calorimetry revealed that polymers that were electrospun below their glass transition temperature (T g ) retained solvent and polymers electrospun above T g did not. Young's moduli increased and yield strain decreased for solventretaining PGA, PLCL, and PET fiber scaffolds as solvent evaporated from the scaffolds over a period of 14 days. Toughness and failure strain decreased for PGA and PET scaffolds as solvent evaporated. No significant differences were observed in the mechanical properties of PU and PCL scaffolds that did not retain solvent. These observations highlight the need to consider solvent retention following electrospinning and its potential effects on scaffold mechanical properties.