Expression of Functional γ-Aminobutyric Acid Type A Receptors in Schwann-Like Adult Stem Cells

Progesterone exerts a neuroprotective action in a Parkinson's disease human cell model through membrane progesterone receptor α (mPRα/PAQR7)

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide, and current treatment options are unsatisfactory on the long term. Several studies suggest a potential neuroprotective action by female hormones, especially estrogens. The potential role of progestogens, however, is less defined, and no studies have investigated the potential involvement of membrane progesterone receptors (mPRs). In the present study, the putative neuroprotective role for mPRs was investigated in SH-SY5Y cells, using two established pharmacological treatments for cellular PD models, 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium (MPP+). Our results show that both the physiologic agonist progesterone and the specific mPR agonist Org OD 02-0 were effective in reducing SH-SY5Y cell death induced by 6-OHDA and MPP⁺, whereas the nuclear PR agonist promegestone (R5020) and the GABA_A receptor agonist muscimol were ineffective. Experiments performed with gene silencing technology and selective pharmacological agonists showed that mPRα is the isoform responsible for the neuroprotective effects we observed. Further experiments showed that the PI3K-AKT and MAP kinase signaling pathways are involved in the mPRα-mediated progestogen neuroprotective action in SH-SY5Y cells. These findings suggest that mPRα could play a neuroprotective role in PD pathology and may be a promising target for the development of therapeutic strategies for PD prevention or management.

Membrane Progesterone Receptor α (mPRα/PAQR7) Promotes Survival and Neurite Outgrowth of Human Neuronal Cells by a Direct Action and Through Schwann Cell-like Stem Cells

We recently showed that membrane progesterone receptor α (mPRα/PAQR7) promotes pro-regenerative effects in Schwann cell-like adipose stem cells (SCL-ASC), an alternative model to Schwann cells for the promotion of peripheral nerve regeneration. In this study, we investigated how mPRα activation with the mPR-specific agonist Org OD 02-0 in SCL-ASC affected regenerative parameters in two neuronal cell lines, IMR-32 and SH-SY-5Y. In a series of conditioned medium experiments, we found that mPR activation of SCL-ASC led to increased neurite outgrowth, protection from cell death and increased expression of peripheral nerve regeneration markers (CREB3, ATF3, GAP43) in neuronal cell lines. These effects were stronger than the ones observed with the conditioned medium from untreated SCL-ASC. The addition of Org OD 02-0 to the untreated cell medium mimicked the effects of mPR activation of SCL-ASC on cell death, but not on neurite outgrowth. Therefore, the effect of Org OD 02-0 on neurite outgrowth is SCL-ASC-dependent, while its effect on cell survivability is likely due to the direct activation of mPRs on neuronal cells. SCL-ASC transfection with mPRα siRNA showed that this isoform is responsible for the beneficial effect on neurite outgrowth. Further experiments showed that SCL-ASC-dependent outcomes likely involved the release of BDNF and IGF-2 from these cells. The beneficial mPRα effect on neurite outgrowth was confirmed in co-culture conditions. These findings strengthen the hypothesis that mPRα could play a pro-regenerative role in SCL-ASC and be a therapeutic target for the promotion of peripheral nerve regeneration.

Schwann Cell-Like Cells: Origin and Usability for Repair and Regeneration of the Peripheral and Central Nervous System

Functional recovery after neurotmesis, a complete transection of the nerve fiber, is often poor and requires a surgical procedure. Especially for longer gaps (>3 mm), end-to-end suturing of the proximal to the distal part is not possible, thus requiring nerve graft implantation. Artificial nerve grafts, i.e., hollow fibers, hydrogels, chitosan, collagen conduits, and decellularized scaffolds hold promise provided that these structures are populated with Schwann cells (SC) that are widely accepted to promote peripheral and spinal cord regeneration. However, these cells must be collected from the healthy peripheral nerves, resulting in significant time delay for treatment and undesired morbidities for the donors. Therefore, there is a clear need to explore the viable source of cells with a regenerative potential similar to SC. For this, we analyzed the literature for the generation of Schwann cell-like cells (SCLC) from stem cells of different origins (i.e., mesenchymal stem cells, pluripotent stem cells, and genetically programmed somatic cells) and compared their biological performance to promote axonal regeneration. Thus, the present review accounts for current developments in the field of SCLC differentiation, their applications in peripheral and central nervous system injury, and provides insights for future strategies.

Expression and Function of GABA Receptors in Myelinating Cells

Myelin facilitates the fast transmission of nerve impulses and provides metabolic support to axons. Differentiation of oligodendrocyte progenitor cells (OPCs) and Schwann cell (SC) precursors is critical for myelination during development and myelin repair in demyelinating disorders. Myelination is tightly controlled by neuron-glia communication and requires the participation of a wide repertoire of signals, including neurotransmitters such as glutamate, ATP, adenosine, or γ-aminobutyric acid (GABA). GABA is the main inhibitory neurotransmitter in the central nervous system (CNS) and it is also present in the peripheral nervous system (PNS). The composition and function of GABA receptors (GABARs) are well studied in neurons, while their nature and role in glial cells are still incipient. Recent studies demonstrate that GABA-mediated signaling mechanisms play relevant roles in OPC and SC precursor development and function, and stand out the implication of GABARs in oligodendrocyte (OL) and SC maturation and myelination. In this review, we highlight the evidence supporting the novel role of GABA with an emphasis on the molecular identity of the receptors expressed in these glial cells and the possible signaling pathways involved in their actions. GABAergic signaling in myelinating cells may have potential implications for developing novel reparative therapies in demyelinating diseases.

Muscarinic receptors modulate Nerve Growth Factor production in rat Schwann-like adipose-derived stem cells and in Schwann cells

Regenerative capability of the peripheral nervous system after injury is enhanced by Schwann cells (SCs) producing several growth factors. The clinical use of SCs in nerve regeneration strategies is hindered by the necessity of removing a healthy nerve to obtain the therapeutic cells. Adipose-derived stem cells (ASCs) can be chemically differentiated towards a SC-like phenotype (dASCs), and represent a promising alternative to SCs. Their physiology can be further modulated pharmacologically by targeting receptors for neurotransmitters such as acetylcholine (ACh). In this study, we compare the ability of rat dASCs and native SCs to produce NGF in vitro. We also evaluate the ability of muscarinic receptors, in particular the M2 subtype, to modulate NGF production and maturation from the precursor (proNGF) to the mature (mNGF) form. For the first time, we demonstrate that dASCs produce higher basal levels of proNGF and mature NGF compared to SCs. Moreover, muscarinic receptor activation, and in particular M2 subtype stimulation, modulates NGF production and maturation in both SCs and dASCs. Indeed, both cell types express both proNGF A and B isoforms, as well as mNGF. After M2 receptor stimulation, proNGF-B (25 kDa), which is involved in apoptotic processes, is strongly reduced at transcript and protein level. Thus, we demonstrate that dASCs possess a stronger neurotrophic potential compared to SCs. ACh, via M2 muscarinic receptors, contributes to the modulation and maturation of NGF, improving the regenerative properties of dASCs.

M2 receptors activation modulates cell growth, migration and differentiation of rat Schwann-like adipose-derived stem cells

Schwann cells (SCs) play a central role in peripheral nervous system physiology and in the response to axon injury. The ability of SCs to proliferate, secrete growth factors, modulate immune response, migrate and re-myelinate regenerating axons has been largely documented. However, there are several restrictions hindering their clinical application, such as the difficulty in collection and a slow in vitro expansion. Adipose-derived stem cells (ASCs) present good properties for peripheral nerve regenerative medicine. When exposed to specific growth factors in vitro, they can acquire a SC-like phenotype (dASCs) expressing key SCs markers and assuming spindle-shaped morphology. Nevertheless, the differentiated phenotype is unstable and several strategies, including pharmacological stimulation, are being studied to improve differentiation outcomes. Cholinergic receptors are potential pharmacological targets expressed in glial cells. Our previous work demonstrated that muscarinic cholinergic receptors, in particular M2 subtype, are present in SCs and are able to modulate several physiological processes. In the present work, muscarinic receptors expression was characterised and the effects mediated by M2 muscarinic receptor were evaluated in rat dASCs. M2 receptor activation, by the preferred agonist arecaidine propargyl ester (APE), caused a reversible arrest of dASCs cell growth, supported by the downregulation of proteins involved in the maintenance of cell proliferation and upregulation of proteins involved in the differentiation (i.e., c-Jun and Egr-2), without affecting cell survival. Moreover, M2 receptor activation in dASCs enhances a pronounced spindle-shaped morphology, supported by Egr2 upregulation, and inhibits cell migration. Our data clearly demonstrate that rat dASCs express functional muscarinic receptors, in particular M2 subtype, which is able to modulate their physiological and morphological processes, as well as SCs differentiation. These novel findings could open new opportunities for the development of combined cell and pharmacological therapies for peripheral nerve regeneration, harnessing the potential of dASCs and M2 receptors.

GABA-B1 Receptor-Null Schwann Cells Exhibit Compromised In Vitro Myelination

GABA-B receptors are important for Schwann cell (SC) commitment to a non-myelinating phenotype during development. However, the P0-GABA-B1^fl/fl conditional knockout mice, lacking the GABA-B1 receptor specifically in SCs, also presented axon modifications, suggesting SC non-autonomous effects through the neuronal compartment. In this in vitro study, we evaluated whether the specific deletion of the GABA-B1 receptor in SCs may induce autonomous or non-autonomous cross-changes in sensory dorsal root ganglia (DRG) neurons. To this end, we performed an in vitro biomolecular and transcriptomic analysis of SC and DRG neuron primary cultures from P0-GABA-B1^fl/fl mice. We found that cells from conditional P0-GABA-B1^fl/fl mice exhibited proliferative, migratory and myelinating alterations. Moreover, we found transcriptomic changes in novel molecules that are involved in peripheral neuron-SC interaction.

Improving the glial differentiation of human Schwann-like adipose-derived stem cells with graphene oxide substrates

There is urgent need to improve the clinical outcome of peripheral nerve injury. Many efforts are directed towards the fabrication of bioengineered conduits, which could deliver stem cells to the site of injury to promote and guide peripheral nerve regeneration. The aim of this study is to assess whether graphene and related nanomaterials can be useful in the fabrication of such conduits. A comparison is made between graphene oxide (GO) and reduced GO substrates. Our results show that the graphene substrates are highly biocompatible, and the reduced GO substrates are more effective in increasing the gene expression of the biomolecules involved in the regeneration process compared to the other substrates studied.

In vitro and in vivo evaluation of silk fibroin functionalized with GABA and allopregnanolone for Schwann cell and neuron survival

AIM

This in vitro and in vivo study reports on silk fibroin (SF) scaffold, functionalized for in situ delivery of GABA and/or allopregnanolone (ALLO), as biomaterial for potential application in tissue engineering and nerve regeneration.

MATERIALS & METHODS

We evaluated the feasibility to design 2D scaffolds (films) made of regenerated Bombyx mori SF, functionalized with GABA and/or ALLO to enhance in vitro biological functions, health, survival and growth of Schwann cells and sensitive neurons of the dorsal root ganglia.

RESULTS & CONCLUSION

Our 2D-SF film showed an efficient loading and controllable release of drugs promoting nerve regeneration. SF functionalized film may be helpful for the development of bioengineered conduits and, in principle, have great potential for long-gap nerve injury repair.

Src and phospho-FAK kinases are activated by allopregnanolone promoting Schwann cell motility, morphology and myelination

Schwann cells' (SCs) development and maturation require coordinate and complementary activation of several signals and intracellular pathways. Among factors controlling these processes, the signalling intermediates Src tyrosine kinase and focal adhesion kinase (FAK) are relevant for SCs', participating in regulation of their adhesion, motility and migration. Recently, the progesterone metabolite allopregnanolone (ALLO) was proved to be synthesized by SCs, whereas it acts autocrinally on SCs motility and proliferation, which are crucial processes for nerve development, maturation and regeneration. Herein, we investigate the hypothesis that the molecular mechanisms behind the ALLO's action on SCs involve the signalling intermediates Src and FAK. We first demonstrated that ALLO 10^-6  M regulates SCs morphology, motility and myelination, also increasing the internode distance in the in vitro myelination model of neuron/SCs co-culture. ALLO's actions were mediated by the modulation of Src/FAK pathway, since they were counteracted by PP2 10^-5  M, a selective inhibitor of Src kinase. Then, we proved that Src/FAK activation in SCs involves GABA-A dependent mechanisms and actin re-arrangements. In conclusion, our findings are the first to corroborate the importance of the neuroactive steroid ALLO in regulating SCs development and maturation via the Src and phospho-FAK signalling activation. Cover Image for this issue: doi: 10.1111/jnc.13795.

The regenerative role of adipose-derived stem cells (ADSC) in plastic and reconstructive surgery

The potential use of stem cell-based therapies for the repair and regeneration of various tissues and organs offers a paradigm shift in plastic and reconstructive surgery. The use of either embryonic stem cells (ESC) or induced pluripotent stem cells (iPSC) in clinical situations is limited because of regulations and ethical considerations even though these cells are theoretically highly beneficial. Adult mesenchymal stem cells appear to be an ideal stem cell population for practical regenerative medicine. Among these cells, adipose-derived stem cells (ADSC) have the potential to differentiate the mesenchymal, ectodermal and endodermal lineages and are easy to harvest. Additionally, adipose tissue yields a high number of ADSC per volume of tissue. Based on this background knowledge, the purpose of this review is to summarise and describe the proliferation and differentiation capacities of ADSC together with current preclinical data regarding the use of ADSC as regenerative tools in plastic and reconstructive surgery.

Overexpression of Glutamate Decarboxylase in Mesenchymal Stem Cells Enhances Their Immunosuppressive Properties and Increases GABA and Nitric Oxide Levels

The neurotransmitter GABA has been recently identified as a potent immunosuppressive agent that targets both innate and adaptive immune systems and prevents disease progression of several autoimmunity models. Mesenchymal stem cells (MSCs) are self-renewing progenitor cells that differentiate into various cell types under specific conditions, including neurons. In addition, MSC possess strong immunosuppressive capabilities. Upon cytokine priming, undifferentiated MSC suppress T-cell proliferation via cell-to-cell contact mechanisms and the secretion of soluble factors like nitric oxide, prostaglandin E2 and IDO. Although MSC and MSC-derived neuron-like cells express some GABAergic markers in vitro, the role for GABAergic signaling in MSC-mediated immunosuppression remains completely unexplored. Here, we demonstrate that pro-inflammatory cytokines selectively regulate GAD-67 expression in murine bone marrow-MSC. However, expression of GAD-65 is required for maximal GABA release by MSC. Gain of function experiments using GAD-67 and GAD-65 co-expression demonstrates that GAD increases immunosuppressive function in the absence of pro-inflammatory licensing. Moreover, GAD expression in MSC evokes an increase in both GABA and NO levels in the supernatants of co-cultured MSC with activated splenocytes. Notably, the increase in NO levels by GAD expression was not observed in cultures of isolated MSC expressing GAD, suggesting crosstalk between these two pathways in the setting of immunosuppression. These results indicate that GAD expression increases MSC-mediated immunosuppression via secretion of immunosuppressive agents. Our findings may help reconsider GABAergic activation in MSC for immunological disorders.

Human Schwann-like cells derived from adipose-derived mesenchymal stem cells rapidly de-differentiate in the absence of stimulating medium

Finding a viable cell-based therapy to address peripheral nerve injury holds promise for enhancing the currently suboptimal microsurgical approaches to peripheral nerve repair. Autologous nerve grafting is the current gold standard for surgical repair of nerve gaps; however, this causes donor nerve morbidity in the patient, and the results remain unsatisfactory. Transplanting autologous Schwann cells (SCs) results in similar morbidity, as well as limited cell numbers and restricted potential for expansion in vitro. Adipose-derived stem cells (ASCs), 'differentiated' towards an SC-like phenotype in vitro (dASCs), have been presented as an alternative to SC therapies. The differentiation protocol stimulates ASCs to mimic the SC phenotype; however, the efficacy of dASCs in nerve repair is not yet convincing, and the practicality of the SC-like phenotype is unproven. Here, we examined the stability of dASCs by withdrawing differentiation medium for 72 h after the full 18-day differentiation protocol, and measuring changes in morphology, gene expression, and protein levels. Withdrawal of differentiation medium from dASCs resulted in a rapid reversion to stem cell-like characteristics. Quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay analyses demonstrated a significant reduction in gene and protein expression of growth factors that were expressed at high levels following 'differentiation'. Therefore, we question the relevance of differentiation to an SC-like phenotype, as withdrawal of differentiation medium, a model of transplantation into an injured nerve, results in rapid reversion of the dASC phenotype to stem cell-like characteristics. Further investigation into the differentiation process and the response of dASCs to an injured environment must be undertaken prior to the use of dASCs in peripheral nerve repair therapies.

Adipose derived stem cells and nerve regeneration

Injuries to peripheral nerves are common and cause life-changing problems for patients alongside high social and health care costs for society. Current clinical treatment of peripheral nerve injuries predominantly relies on sacrificing a section of nerve from elsewhere in the body to provide a graft at the injury site. Much work has been done to develop a bioengineered nerve graft, precluding sacrifice of a functional nerve. Stem cells are prime candidates as accelerators of regeneration in these nerve grafts. This review examines the potential of adipose-derived stem cells to improve nerve repair assisted by bioengineered nerve grafts.

Differentiation of adipose-derived stem cells into Schwann cell phenotype induces expression of P2X receptors that control cell death

Schwann cells (SCs) are fundamental for development, myelination and regeneration in the peripheral nervous system. Slow growth rate and difficulties in harvesting limit SC applications in regenerative medicine. Several molecules, including receptors for neurosteroids and neurotransmitters, have been suggested to be implicated in regulating physiology and regenerative potential of SCs. Adipose-derived stem cells (ASCs) can be differentiated into SC-like phenotype (dASC) sharing morphological and functional properties with SC, thus representing a valid SC alternative. We have previously shown that dASC express γ-aminobutyric-acid receptors, which modulate their proliferation and neurotrophic potential, although little is known about the role of other neurotransmitters in ASC. In this study, we investigated the expression of purinergic receptors in dASC. Using reverse transriptase (RT)-PCR, western blot analyses and immunocytochemistry, we have demonstrated that ASCs express P2X3, P2X4 and P2X7 purinoceptors. Differentiation of ASCs towards glial phenotype was accompanied by upregulation of P2X4 and P2X7 receptors. Using Ca(2+)-imaging techniques, we have shown that stimulation of purinoceptors with adenosine 5'-triphosphate (ATP) triggers intracellular Ca(2+) signals, indicating functional activity of these receptors. Whole-cell voltage clamp recordings showed that ATP and BzATP induced ion currents that can be fully inhibited with specific P2X7 antagonists. Finally, using cytotoxicity assays we have shown that the increase of intracellular Ca(2+) leads to dASC death, an effect that can be prevented using a specific P2X7 antagonist. Altogether, these results show, for the first time, the presence of functional P2X7 receptors in dASC and their link with critical physiological processes such as cell death and survival. The presence of these novel pharmacological targets in dASC might open new opportunities for the management of cell survival and neurotrophic potential in tissue engineering approaches using dASC for nerve repair.

Adipose-derived stem cells and nerve regeneration: promises and pitfalls

In order to improve the outcome of nerve regeneration following peripheral trauma injuries, the development of bioengineered nerve grafts has attracted great attention in the field of tissue engineering. Adult stem cells constitute the ideal alternative to Schwann cells (SCs) as transplantable cells in bioartificial nerve grafts. Among the various sources of stem cells with potential applications for regenerative medicine, the adipose tissue has been proven to be one of the most promising. Adipose-derived stem cells (ASCs) are easily obtained, rapidly expanded, show low immunogenicity, and can be differentiated into SCs in vitro. This chapter will focus on recent advances in the use of differentiated and undifferentiated ASCs for peripheral nerve regeneration, with a critical attention for the clinical exploitability of ASC in nerve repair strategies.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.