Heterogeneity of Microglia Phenotypes: Developmental, Functional and Some Therapeutic Considerations.

BACKGROUND: Microglia play a pivotal role in maintaining homeostasis in complex brain environment. They first exist as amoeboid microglial cells (AMCs) in the developing brain, but with brain maturation, they transform into ramified microglial cells (RMCs). In pathological conditions, microglia are activated and have been classified into M1 and M2 phenotypes. The roles of AMCs, RMCs and M1/M2 microglia phenotypes especially in pathological conditions have been the focus of many recent studies. METHODS: Here, we review the early development of the AMCs and RMCs and discuss their specific functions with reference to their anatomic locations, immunochemical coding etc. M1 and M2 microglia phenotypes in different neuropathological conditions are also reviewed. RESULTS: Activated microglia are engaged in phagocytosis, production of proinflammatory mediators, trophic factors and synaptogenesis etc. Prolonged microglia activation, however, can cause damage to neurons and oligodendrocytes. The M1 and M2 phenotypes featured prominently in pathological conditions are discussed in depth. Experimental evidence suggests that microglia phenotype is being modulated by multiple factors including external and internal stimuli, local demands, epigenetic regulation, and herbal compounds. CONCLUSION: Prevailing views converge that M2 polarization is neuroprotective. Thus, proper therapeutic designs including the use of anti-inflammatory drugs, herbal agents may be beneficial in suppression of microglial activation, especially M1 phenotype, for amelioration of neuroinflammation in different neuropathological conditions. Finally, recent development of radioligands targeting 18 kDa translocator protein (TSPO) in activated microglia may hold great promises clinically for early detection of brain lesion with the positron emission tomography.

Herbal Compounds with Special Reference to Gastrodin as Potential Therapeutic Agents for Microglia Mediated Neuroinflammation.

BACKGROUND: Activated microglia play a pivotal role neurodegenerative diseases by producing a variety of proinflammatory mediators including tumor necrosis factor-alpha (TNF-α), interleukin- 1beta (IL-1ß) and nitric oxide (NO) that are toxic to neurons and oligodendrocytes. METHODS: In view of the above, suppression of microglia mediated neuroinflammation is deemed a therapeutic strategy for neurodegenerative diseases. Several potential Chinese herbal extracts have been reported to exert neuroprotective effects against neurodegenerative diseases targeting specifically at the activated microglia. In this connection, the phenolic glucoside gastrodin, a main constituent of the Chinese herbal medicine Gastrodia rhizoma, produced widely in the local community exhibits potential neuroprotective effects through suppression of neurotoxic proinflammatory mediators. RESULTS: Here, we first review the roles of activated microglia in different brain diseases. The effects of gastrodin on activated microglia are then considered. We have identified gastrodin as a putative therapeutic agent as it has been found to suppress microglial activation thus ameliorating neuroinflammation. More importantly, gastrodin downregulates the expression of renin angiotensin system (RAS) and production of proinflammatory mediators. Remarkably, gastrodin promotes Sirtuin 3 (Sirt3) up-regulation and nicotinamide adenine dinucleotide phosphate oxidase-2 (NOX-2) down-regulation after ischemichypoxia in activated microglia mediated by AT1 or AT2 receptors which are angiotensin II receptors subtypes, indicating a possible molecular link between RAS and Sirt3 survival genes. CONCLUSION: This review summarizes the beneficial effects of gastrodin acting on activated microglia along with other herbal compounds. Its efficacy in neuroprotection is consistent with some common herbal products in China.

Tail Nerve Electrical Stimulation and Electro-Acupuncture Can Protect Spinal Motor Neurons and Alleviate Muscle Atrophy after Spinal Cord Transection in Rats.

Spinal cord injury (SCI) often results in death of spinal neurons and atrophy of muscles which they govern. Thus, following SCI, reorganizing the lumbar spinal sensorimotor pathways is crucial to alleviate muscle atrophy. Tail nerve electrical stimulation (TANES) has been shown to activate the central pattern generator (CPG) and improve the locomotion recovery of spinal contused rats. Electroacupuncture (EA) is a traditional Chinese medical practice which has been proven to have a neural protective effect. Here, we examined the effects of TANES and EA on lumbar motor neurons and hindlimb muscle in spinal transected rats, respectively. From the third day postsurgery, rats in the TANES group were treated 5 times a week and those in the EA group were treated once every other day. Four weeks later, both TANES and EA showed a significant impact in promoting survival of lumbar motor neurons and expression of choline acetyltransferase (ChAT) and ameliorating atrophy of hindlimb muscle after SCI. Meanwhile, the expression of neurotrophin-3 (NT-3) in the same spinal cord segment was significantly increased. These findings suggest that TANES and EA can augment the expression of NT-3 in the lumbar spinal cord that appears to protect the motor neurons as well as alleviate muscle atrophy.

Scutellarin Attenuates Microglia-Mediated Neuroinflammation and Promotes Astrogliosis in Cerebral Ischemia - A Therapeutic Consideration.

Neuroinflammation plays an important role in different brain diseases including acute brain injuries such as cerebral ischemic stroke and chronic neurodegenerative diseases e.g. Alzheimer's disease etc. The central player in this is the activated microglia, which produce substantial amounts of proinflammatory mediators that may exacerbate the disease. Associated with microglia activation is astrogliosis characterized by hypertrophic astrocytes with increased expression of proinflammatory cytokines, neurotrophic factors, stem cell, neuronal and proliferation markers, all these are crucial for reconstruction of damaged tissue and ultimate restoration of neurological functions. Here, we review the roles of activated microglia and reactive astrocytes in brain diseases with special reference to cerebral ischemia, and the effects of scutellarin, a Chinese herbal extract on both glial cells. We first reviewed the close spatial relation between activated microglia and reactive astrocytes as it suggests that both glial cells work in concert for tissue reconstruction and repair. Secondly, we have identified scutellarin as a putative therapeutic agent as it has been found to not only suppress microglial activation thus ameliorating neuroinflammation, but also enhance astrocytic reaction. In the latter, scutellarin amplified the astrocytic reaction by upregulating the expression of neurotrophic factors among others thus indicating its neuroprotective role. Remarkably, the effects of scutellarin on reactive astrocytes were mediated by activated microglia supporting a functional "cross-talk" between the two glial types. This review highlights some of our recent findings taking into consideration of others demonstrating the beneficial effects of scutellarin on both glial cell types in cerebral ischemia as manifested by improvement of neurological functions.

Effects of electroacupuncture and the retinoid X receptor (RXR) signalling pathway on oligodendrocyte differentiation in the demyelinated spinal cord of rats.

OBJECTIVES: In spinal cord demyelination, some oligodendrocyte precursor cells (OPCs) remain in the demyelinated region but have a reduced capacity to differentiate into oligodendrocytes. This study investigated whether 'Governor Vessel' (GV) electroacupuncture (EA) would promote the differentiation of endogenous OPCs into oligodendrocytes by activating the retinoid X receptor γ (RXR-γ)-mediated signalling pathway. METHODS: Adult rats were microinjected with ethidium bromide (EB) into the T10 spinal cord to establish a model of spinal cord demyelination. EB-injected rats remained untreated (EB group, n=26) or received EA treatment (EB+EA group, n=26). A control group (n=26) was also included that underwent dural exposure without EB injection. After euthanasia at 7 days (n=5 per group), 15 days (n=8 per group) or 30 days (n=13 per group), protein expression of RXR-γ in the demyelinated spinal cord was evaluated by immunohistochemistry and Western blotting. In addition, OPCs derived from rat embryonic spinal cord were cultured in vitro, and exogenous 9-cis-RA (retinoic acid) and RXR-γ antagonist HX531 were administered to determine whether RA could activate RXR-γ and promote OPC differentiation. RESULTS: EA was found to increase the numbers of both OPCs and oligodendrocytes expressing RXR-γ and RALDH2, and promote remyelination in the remyelinated spinal cord. Exogenous 9-cis-RA enhanced the differentiation of OPCs into mature oligodendrocytes by activating RXR-γ. CONCLUSIONS: The results suggest that EA may activate RXR signalling to promote the differentiation of OPCs into oligodendrocytes in spinal cord demyelination.

Scutellarin as a Potential Therapeutic Agent for Microglia-Mediated Neuroinflammation in Cerebral Ischemia.

The cerebral ischemia is one of the most common diseases in the central nervous system that causes progressive disability or even death. In this connection, the inflammatory response mediated by the activated microglia is believed to play a central role in this pathogenesis. In the event of brain injury, activated microglia can clear the cellular debris and invading pathogens, release neurotrophic factors, etc., but in chronic activation microglia may cause neuronal death through the release of excessive inflammatory mediators. Therefore, suppression of microglial over-reaction and microglia-mediated neuroinflammation is deemed to be a therapeutic strategy of choice for cerebral ischemic damage. In the search for potential herbal extracts that are endowed with the property in suppressing the microglial activation and amelioration of neuroinflammation, attention has recently been drawn to scutellarin, a Chinese herbal extract. Here, we review the roles of activated microglia and the effects of scutellarin on activated microglia in pathological conditions especially in ischemic stroke. We have further extended the investigation with special reference to the effects of scutellarin on Notch signaling, one of the several signaling pathways known to be involved in microglial activation. Furthermore, in light of our recent experimental evidence that activated microglia can regulate astrogliosis, an interglial "cross-talk" that was amplified by scutellarin, it is suggested that in designing of a more effective therapeutic strategy for clinical management of cerebral ischemia both glial types should be considered collectively.

Protective effects of ginseng on neurological disorders.

Ginseng (Order: Apiales, Family: Araliaceae, Genus: Panax) has been used as a traditional herbal medicine for over 2000 years, and is recorded to have antianxiety, antidepressant and cognition enhancing properties. The protective effects of ginseng on neurological disorders are discussed in this review. Ginseng species and ginsenosides, and their intestinal metabolism and bioavailability are briefly introduced. This is followed by molecular mechanisms of effects of ginseng on the brain, including glutamatergic transmission, monoamine transmission, estrogen signaling, nitric oxide (NO) production, the Keap1/Nrf2 adaptive cellular stress pathway, neuronal survival, apoptosis, neural stem cells and neuroregeneration, microglia, astrocytes, oligodendrocytes and cerebral microvessels. The molecular mechanisms of the neuroprotective effects of ginseng in Alzheimer's disease (AD) including ß-amyloid (Aß) formation, tau hyperphosphorylation and oxidative stress, major depression, stroke, Parkinson's disease and multiple sclerosis are presented. It is hoped that this discussion will stimulate more studies on the use of ginseng in neurological disorders.

Combination of electroacupuncture and grafted mesenchymal stem cells overexpressing TrkC improves remyelination and function in demyelinated spinal cord of rats.

This study attempted to graft neurotrophin-3 (NT-3) receptor (TrkC) gene modified mesenchymal stem cells (TrkC-MSCs) into the demyelinated spinal cord and to investigate whether electroacupuncture (EA) treatment could promote NT-3 secretion in the demyelinated spinal cord as well as further enhance grafted TrkC-MSCs to differentiate into oligodendrocytes, remyelination and functional recovery. Ethidium bromide (EB) was microinjected into the spinal cord of rats at T10 to establish a demyelinated model. Six groups of animals were prepared for the experiment: the sham, PBS, MSCs, MSCs+EA, TrkC-MSCs and TrkC-MSCs+EA groups. The results showed that TrkC-MSCs graft combined with EA treatment (TrkC-MSCs+EA group) significantly increased the number of OPCs and oligodendrocyte-like cells differentiated from MSCs. Immunoelectron microscopy showed that the oligodendrocyte-like cells differentiated from TrkC-MSCs formed myelin sheaths. Immunofluorescence histochemistry and Western blot analysis indicated that TrkC-MSCs+EA treatment could promote the myelin basic protein (MBP) expression and Kv1.2 arrangement trending towards the normal level. Furthermore, behavioural test and cortical motor evoked potentials detection demonstrated a significant functional recovery in the TrkC-MSCs+EA group. In conclusion, our results suggest that EA treatment can increase NT-3 expression, promote oligodendrocyte-like cell differentiation from TrkC-MSCs, remyelination and functional improvement of demyelinated spinal cord.

Electroacupuncture Promotes the Differentiation of Transplanted Bone Marrow Mesenchymal Stem Cells Preinduced With Neurotrophin-3 and Retinoic Acid Into Oligodendrocyte-Like Cells in Demyelinated Spinal Cord of Rats.

Transplantation of bone marrow mesenchymal stem cells (MSCs) promotes functional recovery in multiple sclerosis (MS) patients and in a murine model of MS. However, there is only a modicum of information on differentiation of grafted MSCs into oligodendrocyte-like cells in MS. The purpose of this study was to transplant neurotrophin-3 (NT-3) and retinoic acid (RA) preinduced MSCs (NR-MSCs) into a demyelinated spinal cord induced by ethidium bromide and to investigate whether EA treatment could promote NT-3 secretion in the demyelinated spinal cord. We also sought to determine whether increased NT-3 could further enhance NR-MSCs overexpressing the tyrosine receptor kinase C (TrkC) to differentiate into more oligodendrocyte-like cells, resulting in increased remyelination and nerve conduction in the spinal cord. Our results showed that NT-3 and RA increased transcription of TrkC mRNA in cultured MSCs. EA increased NT-3 levels and promoted differentiation of oligodendrocyte-like cells from grafted NR-MSCs in the demyelinated spinal cord. There was evidence of myelin formation by grafted NR-MSCs. In addition, NR-MSC transplantation combined with EA treatment (the NR-MSCs + EA group) reduced demyelination and promoted remyelination. Furthermore, the conduction of cortical motor-evoked potentials has improved compared to controls. Together, our data suggest that preinduced MSC transplantation combined with EA treatment not only increased MSC differentiation into oligodendrocyte-like cells forming myelin sheaths, but also promoted remyelination and functional improvement of nerve conduction in the demyelinated spinal cord.

Folic acid supplementation changes the fate of neural progenitors in mouse embryos of hyperglycemic and diabetic pregnancy.

Folic acid has been shown to decrease the incidence of neural tube defects (NTDs) in normal and hyperglycemic conditions, but the influence of folic acid on the development of central nervous system is not fully understood. Here, we aimed to explore the effects of folic acid, especially high dose of folic acid, on the characteristics of neural progenitors in embryos of hyperglycemic and diabetic mouse. Hyperglycemic and diabetic pregnant mice were given 3 mg/kg or 15 mg/kg folic acid from embryonic day 0.5 (E0.5) and were euthanased on E11.5, E13.5 or E18.5. The incidence of NTDs at E13.5 was counted. The proliferation, apoptosis and differentiation of neural progenitors and neuronal migration were determined using BrdU incorporation assay, TUNEL assay, immunofluorescence, Western blot and real-time reverse transcriptase polymerase chain reaction. Both normal and high doses of folic acid decreased the incidence of NTDs, promoted proliferation and reduced apoptosis of neuroepithelial cells in embryos of hyperglycemic and diabetic mice. Importantly, folic acid, especially at high dose, might affect the premature differentiation of neural progenitors in embryos of hyperglycemic and diabetic pregnancy. This may be attributed to changes of messenger RNA expression levels of some basic-helix-loop-helix transcription factors. In addition, folic acid might be involved in regulating neuronal migration in embryos of hyperglycemic and diabetic pregnancy. These findings suggest that periconceptional supplementation of folic acid, especially at high dose, may be a double-edged sword because it may result in undesirable outcomes affecting both the neuronal and glial differentiation in hyperglycemic and diabetic pregnancy.

Melatonin promotes proliferation and differentiation of neural stem cells subjected to hypoxia in vitro.

Melatonin, an endogenously produced neurohormone secreted by the pineal gland, has a variety of physiological functions and neuroprotective effects. It can modulate the functions of neural stem cells (NSCs) including proliferation and differentiation in embryonic brain tissue but its effect and mechanism on the stem cells in hypoxia remains to be explored. Here, we show that melatonin stimulates proliferation of NSCs during hypoxia. Additionally, it also promoted the differentiation of NSCs into neurons. However, it did not appear to exert an obvious effect on the differentiation of astrocytes. The present results have further shown that the promotional effect of NSCs proliferation by melatonin involved the MT1 receptor and increased phosphorylation of ERK1/2. The effect of melatonin on differentiation of NSCs is linked to altered expression of differentiation-related genes. In the light of these findings, it is suggested that melatonin may be beneficial as a supplement for treatment of neonatal hypoxic-ischemic brain injury for promoting the proliferation and differentiation of NSCs.

Recent studies on neural tube defects in embryos of diabetic pregnancy: an overview.

Maternal diabetes develops in 2-6% of total pregnancies, depending on geographical and ethnic background. About 10% of fetuses from diabetic pregnancy display congenital malformations in various organ systems including cardiovascular, gastrointestinal, genitourinary and neurological systems, among which the neural tube defects (NTDs) such as anencephaly, holoprosencephaly and syntelencephaly were more frequently demonstrated. Recent studies by the Diabetes Control and Complications Trial Research Group show that tight glycemic control early in pregnancy decreases the progression of a number of diabetic complications. However, it appears that the pre-existing tissue damage cannot be reversed even after normoglycemic levels are achieved during pregnancy. In recent years, considerable efforts have been made to investigate the etiology of birth defects among infants of diabetic mothers. It has been shown that diabetes-induced fetal abnormalities are accompanied by some metabolic disturbances including elevated superoxide dismutase (SOD) activity, reduced levels of myoinositol and arachidonic acid and inhibition of the pentose phosphate shunt pathway. Moreover, the frequency of fetal malformations in diabetic pregnancy has been reported to be markedly reduced by dietary supplements of antioxidants such as vitamin E, vitamin C and butylated hy- droxytoluene, suggesting that oxidative stress is involved in the etiology of fetal dysmorphogenesis. Furthermore, several experimental studies have shown that NTDs in embryos of diabetic mice are associated with altered expression of genes, which control development of the neural tube. In this review, recent findings of possible molecular mechanisms which cause morphological changes during neural tube development in embryos of diabetic pregnancy are discussed.

Folic acid supplementation affects apoptosis and differentiation of embryonic neural stem cells exposed to high glucose.

Folic acid (FA) supplementation has been shown to be extremely effective in reducing the occurrence of neural tube defects (NTDs), one of the most common birth defects associated with diabetic pregnancy. However, the antiteratogenic mechanism of FA in diabetes-induced NTDs is unclear. This study investigated the neuroprotective mechanism of FA in neural stem cells (NSCs) exposed to high glucose in vitro. The undifferentiated or differentiated NSCs were cultured in normal D-glucose concentration (NG) or high D-glucose concentration (HG) with or without FA. FA supplementation significantly decreased apoptosis induced by HG and lowered the expression of p53 in the nucleus of undifferentiated NSCs exposed to HG. Administration of FA in differentiated NSCs did not alter their precocious differentiation induced by HG. The increased mRNA expression levels of the basic helix-loop-helix factors including Neurog1, Neurog2, NeuroD2, Mash1, Id1, Id2, and Hes5 in the presence of HG were not significantly affected by FA. The present results provided a cellular mechanism by which FA supplementation may have a potential role in prevention of NTDs in diabetic pregnancies. On the other hand, FA increased the mRNA expression levels of the above transcription factors and accelerated the differentiation of NSCs in the NG medium, suggesting that it may adversely affect the normal differentiation of NSCs. Therefore, the timing and dose of FA would be critical factors in considering FA supplementation in normal maternal pregnancy.

Role of GABA in electro-acupuncture therapy on cerebral ischemia induced by occlusion of the middle cerebral artery in rats.

This study investigated the possible involvement of gamma-aminobutyric acid (GABA) in the therapeutic effect of cerebral ischemia by electro-acupuncture (EA) using the rat model with middle cerebral artery occlusion (MCAO). By immunohistochemistry, the changes of GABA expression level in the primary infarct area and its penumbral regions were examined. The changes in infarct area and survival neuron percentages were also assessed using haematoxylin and eosin stained sections after picrotoxin (PTX) injection, a GABA receptor's antagonist. Our results showed that EA markedly decreased the ischemic damaged areas in the cerebral cortex and hippocampus. Concomitant to this was an up-regulation of GABA immunoexpression in MCAO rats with EA treatment (P < 0.05). Furthermore, injection of PTX in rats subjected to MCAO or MCAO followed by EA treatment increased the infarct area and decreased survival cell percentage significantly when compared with those without PTX injection. In the light of these findings, it is suggested that EA on specific and established acupoints that are commonly used in clinical management of cerebral ischemia may have elicited an up-regulated expression of GABA that would have a neuroprotective effect.