Peripheral nerve injuries (PNI) occur as the result of sudden trauma and can lead to life-long disability, reduced quality of life, and heavy economic and social burdens. Although the peripheral nervous system (PNS) has the intrinsic capacity to regenerate and regrow axons to a certain extent, current treatments frequently show incomplete recovery with poor functional outcomes, particularly for large PNI. Many surgical procedures are available to halt the propagation of nerve damage, and the choice of a procedure depends on the extent of the injury. In particular, recovery from large PNI gaps is difficult to achieve without any therapeutic intervention or some form of tissue/cell-based therapy. Autologous nerve grafting, considered the "gold standard" is often implemented for treatment of gap formation type PNI. Although these surgical procedures provide many benefits, there are still considerable limitations associated with such procedures as donor site morbidity, neuroma formation, fascicle mismatch, and scarring. To overcome such restrictions, researchers have explored various avenues to improve post-surgical outcomes. The most commonly studied methods include: cell transplantation, growth factor delivery to stimulate regenerating axons and implanting nerve guidance conduits containing replacement cells at the site of injury. Replacement cells which offer maximum benefits for the treatment of PNI, are Schwann cells (SCs), which are the peripheral glial cells and in part responsible for clearing out debris from the site of injury. Additionally, they release growth factors to stimulate myelination and axonal regeneration. Both primary SCs and genetically modified SCs enhance nerve regeneration in animal models; however, there is no good source for extracting SCs and the only method to obtain SCs is by sacrificing a healthy nerve. To overcome such challenges, various cell types have been investigated and reported to enhance nerve regeneration.In this review, we have focused on cell-based strategies aimed to enhance peripheral nerve regeneration, in particular the use of mesenchymal stem cells (MSCs). Mesenchymal stem cells are preferred due to benefits such as autologous transplantation, routine isolation procedures, and paracrine and immunomodulatory properties. Mesenchymal stem cells have been transplanted at the site of injury either directly in their native form (undifferentiated) or in a SC-like form (transdifferentiated) and have been shown to significantly enhance nerve regeneration. In addition to transdifferentiated MSCs, some studies have also transplanted ex-vivo genetically modified MSCs that hypersecrete growth factors to improve neuroregeneration.

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Because of the limitations imposed by traditional two-dimensional (2D) cultures, biomaterials have become a major focus in neural and tissue engineering to study cell behavior in vitro. 2D systems fail to account for interactions between cells and the surrounding environment; these cell−matrix interactions are important to guide cell differentiation and influence cell behavior such as adhesion and migration. Biomaterials provide a unique approach to help mimic the native microenvironment in vivo. In this study, a novel microfluidic technique is used to encapsulate adult rat hippocampal stem/ progenitor cells (AHPCs) within alginate-based fibrous hydrogels. To our knowledge, this is the first study to encapsulate AHPCs within a fibrous hydrogel. Alginate-based hydrogels were cultured for 4 days in vitro and recovered to investigate the effects of a 3D environment on the stem cell fate. Post recovery, cells were cultured for an additional 24 or 72 h in vitro before fixing cells to determine if proliferation and neuronal differentiation were impacted after encapsulation. The results indicate that the 3D environment created within a hydrogel is one factor promoting AHPC proliferation and neuronal differentiation (19.1 and 13.5%, respectively); however, this effect is acute. By 72 h post recovery, cells had similar levels of proliferation and neuronal differentiation (10.3 and 8.3%, respectively) compared to the control conditions. Fibrous hydrogels may better mimic the natural micro-environment present in vivo and be used to encapsulate AHPCs, enhancing cell proliferation and selective differentiation. Understanding cell behavior within 3D scaffolds may lead to the development of directed therapies for central nervous system repair and rescue.

This study investigated the effect of electrical stimuli parameters using graphene-based devices for the transdifferentiation of genetically engineered brain-derived neurotrophic factor (BDNF) hypersecreting mesenchymal stem cells (BDNF-MSCs) into neuronal or glial lineages. The results suggest that BDNF-MSCs have the tendency to transdifferentiate into both neuronal and Schwann cell (SC)-like phenotypes at lower voltages (25-50 mV). However, as the applied voltage changed from 25 to 100 mV at 50 Hz, the transdifferentiation of BDNF-MSCs yielded more into SC-like phenotypes and resulted in complete transdifferentiation into SC-like phenotypes at 100 mV and 50 Hz. With an increase in voltage to 100 mV, the complete transdifferentiation to SC-like phenotypes also resulted in enhanced paracrine activity leading to total secretion of nerve growth factor (NGF) up to 50 ng/mL with pronounced biological activity, causing neurite extension of 4 μm/cell on PC12-TrkB cells. Moreover, 90% of the transdifferentiated cells demonstrated significant myelination potential. The contact co-culture of BDNF-MSCs with adult hippocampal progenitor cells (AHPCs) in the presence of electrical stimuli resulted in differentiation of BDNF-MSCs into SC-like phenotypes accompanied by synergistic neurite extension of AHPCs. Overall, this study demonstrates the possibility of controlling simultaneous and spatial differentiation of MSCs into selected neuronal and glial lineages at desired ratios via changes in electrical stimuli through graphene-based devices and can contribute to the development of novel cell-based strategies for nervous system rescue and repair. Lay Summary This work evaluates the effect of different electrical stimuli conditions applied through inkjet-printed and laser-annealed graphene-based interdigitated circuits on the differentiation behavior of mesenchymal stem cells. Our results suggested that it is possible to spatially and locally control the differentiation of mesenchymal stem cells into final lineage type (glial or neuronal) by manipulating the electrical stimuli. The future work will include the control of stem cell differentiation and fate commitment in an in vivo model using electrical stimuli.

N‐acetylaspartylglutamate (NAAG) is a common neurotransmitter in the mammalian nervous system; however, it has never been reported in the nervous system of the fruit fly, Drosophila melanogaster. Using antiserum against NAAG, we localized NAAG‐like immunoreactivity to neurons in the ventral nerve cord and to type Is glutamatergic nerve terminals in larval neuromuscular junctions. Using liquid chromatography tandem mass spectrometry (LC‐MS), we failed to find NAAG but found the related peptide N‐acetylaspartylglutamylglutamate (NAAG2) in Drosophila CNS and body wall tissue. This is the first report of any NAAG‐family peptide in the nervous system of Drosophila and is also the first report of NAAG2 being present in a much higher concentration than NAAG in the nervous system of any species. Thus, the larval fruit fly presents an interesting model for the study of the functional role of NAAG2 of which very little is known—especially in the absence of an abundance of NAAG.