Differential regulation of microglial activation by propentofylline via cAMP signaling

Emerging phosphodiesterase inhibitors for treatment of neurodegenerative diseases

Neurodegenerative diseases (NDs) cause progressive loss of neuron structure and ultimately lead to neuronal cell death. Since the available drugs show only limited symptomatic relief, NDs are currently considered as incurable. This review will illustrate the principal roles of the signaling systems of cyclic adenosine and guanosine 3',5'-monophosphates (cAMP and cGMP) in the neuronal functions, and summarize expression/activity changes of the associated enzymes in the ND patients, including cyclases, protein kinases, and phosphodiesterases (PDEs). As the sole enzymes hydrolyzing cAMP and cGMP, PDEs are logical targets for modification of neurodegeneration. We will focus on PDE inhibitors and their potentials as disease-modifying therapeutics for the treatment of Alzheimer's disease, Parkinson's disease, and Huntington's disease. For the overlapped but distinct contributions of cAMP and cGMP to NDs, we hypothesize that dual PDE inhibitors, which simultaneously regulate both cAMP and cGMP signaling pathways, may have complementary and synergistic effects on modifying neurodegeneration and thus represent a new direction on the discovery of ND drugs.

Phosphodiesterase Inhibitors for Alzheimer's Disease: A Systematic Review of Clinical Trials and Epidemiology with a Mechanistic Rationale

BACKGROUND

Preclinical studies, clinical trials, and reviews suggest increasing 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) with phosphodiesterase inhibitors is disease-modifying in Alzheimer's disease (AD). cAMP/protein kinase A (PKA) and cGMP/protein kinase G (PKG) signaling are disrupted in AD. cAMP/PKA and cGMP/PKG activate cAMP response element binding protein (CREB). CREB binds mitochondrial and nuclear DNA, inducing synaptogenesis, memory, and neuronal survival gene (e.g., brain-derived neurotrophic factor) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α). cAMP/PKA and cGMP/PKG activate Sirtuin-1, which activates PGC1α. PGC1α induces mitochondrial biogenesis and antioxidant genes (e.g.,Nrf2) and represses BACE1. cAMP and cGMP inhibit BACE1-inducing NFκB and tau-phosphorylating GSK3β.

OBJECTIVE AND METHODS

We review efficacy-testing clinical trials, epidemiology, and meta-analyses to critically investigate whether phosphodiesteraseinhibitors prevent or treat AD.

RESULTS

Caffeine and cilostazol may lower AD risk. Denbufylline and sildenafil clinical trials are promising but preliminary and inconclusive. PF-04447943 and BI 409,306 are ineffective. Vinpocetine, cilostazol, and nicergoline trials are mixed. Deprenyl/selegiline trials show only short-term benefits. Broad-spectrum phosphodiesterase inhibitor propentofylline has been shown in five phase III trials to improve cognition, dementia severity, activities of daily living, and global assessment in mild-to-moderate AD patients on multiple scales, including the ADAS-Cogand the CIBIC-Plus in an 18-month phase III clinical trial. However, two books claimed based on a MedScape article an 18-month phase III trial failed, so propentofylline was discontinued. Now, propentofylline is used to treat canine cognitive dysfunction, which, like AD, involves age-associated wild-type Aβ deposition.

CONCLUSION

Phosphodiesterase inhibitors may prevent and treat AD.

Propentofylline decreases hypothalamic astrogliosis induced by hypercaloric diet in the rat

ABSTRACT Obesity is associated with a chronic and low-grade inflammatory response in the hypothalamus, where astrogliosis occurs with the upregulation of the astrocyte structural protein GFAP. As propentofylline (PPF) has inhibitory effects on astrocyte and microglial activation during inflammation, this study aimed to investigate if this xanthine derivative could decrease the astrocyte reaction induced by a hypercaloric diet (HD). Male Wistar rats were divided into four groups: NDS – rats receiving a normocaloric diet (ND) and daily saline solution; NDP – rats receiving ND and daily PPF (12.5 mg/kg/day, intraperitoneal route); HDS – rats receiving HD and saline solution, HDP – rats receiving HD and PPF. On the 21st day, rats were anesthetized, and perfused, and brains were collected for GFAP immunohistochemical study in the hypothalamus. Results showed that HD induced increased weight gain and hypothalamic astrogliosis. Propentofylline decreased the expression of GFAP in the HDP group, although it did not affect the weight gain induced by this diet.

Microglia in Cancer: For Good or for Bad?

Glioblastoma is a malignant tumor of astrocytic origin that is highly invasive, proliferative and angiogenic. Despite current advances in multimodal therapies, such as surgery, radio- and chemotherapy, the outcome for patients with glioblastoma is nearly always fatal. The glioblastoma microenvironment has a tremendous influence over the tumor growth and spread. Microglia and macrophages are abundant cells in the tumor mass. Increasing evidence indicates that glioblastoma recruits these cell populations and signals in a way that microglia and macrophages are subverted to promote tumor progression. In this chapter, we discuss some aspects of the interaction between microglia and glioblastoma, consequences of this interaction for tumor progression and the possibility of microglial cells being used as therapeutic vectors, which opens up new alternatives for the development of GBM therapies targeting microglia.

Phosphodiesterase inhibitors as therapeutics for traumatic brain injury

Developing therapeutics for traumatic brain injury remains a challenge for all stages of recovery. The pathological features of traumatic brain injury are diverse, and it remains an obstacle to be able to target the wide range of pathologies that vary between traumatic brain injured patients and that evolve during recovery. One promising therapeutic avenue is to target the second messengers cAMP and cGMP with phosphodiesterase inhibitors due to their broad effects within the nervous system. Phosphodiesterase inhibitors have the capability to target different injury mechanisms throughout the time course of recovery after brain injury. Inflammation and neuronal death are early targets of phosphodiesterase inhibitors, and synaptic dysfunction and circuitry remodeling are late potential targets of phosphodiesterase inhibitors. This review will discuss how signaling through cyclic nucleotides contributes to the pathology of traumatic brain injury in the acute and chronic stages of recovery. We will review our current knowledge of the successes and challenges of using phosphodiesterase inhibitors for the treatment of traumatic brain injury and conclude with important considerations in developing phosphodiesterase inhibitors as therapeutics for brain trauma.

Evaluation of Injured Axons Using Two-Photon Excited Fluorescence Microscopy after Spinal Cord Contusion Injury in YFP-H Line Mice

Elucidation of the process of degeneration of injured axons is important for the development of therapeutic modules for the treatment of spinal cord injuries. The aim of this study was to establish a method for time-lapse observation of injured axons in living animals after spinal cord contusion injury. YFP (yellow fluorescent protein)-H transgenic mice, which we used in this study, express fluorescence in their nerve fibers. Contusion damage to the spinal cord at the 11th vertebra was performed by IH (Infinite Horizon) impactor, which applied a pressure of 50 kdyn. The damaged spinal cords were re-exposed during the observation period under anesthesia, and then observed by two-photon excited fluorescence microscopy, which can observe deep regions of tissues including spinal cord axons. No significant morphological change of injured axons was observed immediately after injury. Three days after injury, the number of axons decreased, and residual axons were fragmented. Seven days after injury, only fragments were present in the damaged tissue. No hind-limb movement was observed during the observation period after injury. Despite the immediate paresis of hind-limbs following the contusion injury, the morphological degeneration of injured axons was delayed. This method may help clarification of pathophysiology of axon degeneration and development of therapeutic modules for the treatment of spinal cord injury.

Histamine H3 receptor in primary mouse microglia inhibits chemotaxis, phagocytosis, and cytokine secretion

Histamine is a physiological amine which initiates a multitude of physiological responses by binding to four known G-protein coupled histamine receptor subtypes as follows: histamine H1 receptor (H1 R), H2 R, H3 R, and H4 R. Brain histamine elicits neuronal excitation and regulates a variety of physiological processes such as learning and memory, sleep-awake cycle and appetite regulation. Microglia, the resident macrophages in the brain, express histamine receptors; however, the effects of histamine on critical microglial functions such as chemotaxis, phagocytosis, and cytokine secretion have not been examined in primary cells. We demonstrated that mouse primary microglia express H2 R, H3 R, histidine decarboxylase, a histamine synthase, and histamine N-methyltransferase, a histamine metabolizing enzyme. Both forskolin-induced cAMP accumulation and ATP-induced intracellular Ca(2+) transients were reduced by the H3 R agonist imetit but not the H2 R agonist amthamine. H3 R activation on two ubiquitous second messenger signalling pathways suggests that H3 R can regulate various microglial functions. In fact, histamine and imetit dose-dependently inhibited microglial chemotaxis, phagocytosis, and lipopolysaccharide (LPS)-induced cytokine production. Furthermore, we confirmed that microglia produced histamine in the presence of LPS, suggesting that H3 R activation regulate microglial function by autocrine and/or paracrine signalling. In conclusion, we demonstrate the involvement of histamine in primary microglial functions, providing the novel insight into physiological roles of brain histamine.

Effects of propentofylline on CNS remyelination in the rat brainstem

Propentofylline (PPF) is a xanthine derivative with pharmacological effects distinct from those of the classical methylxanthines. It depresses activation of microglial cells and astrocytes which is associated with neuronal damage during neural inflammation and hypoxia. The aim of this study was to evaluate whether PPF had the capacity of affecting glial cells behavior during the process of demyelination and remyelination following ethidium bromide (EB) gliotoxic injury. EB injection into the CNS is commonly used as an experimental demyelinating model inducing local oligodendroglial and astrocytic death, which results in primary demyelination, blood-brain barrier and glia limitans disruption and Schwann cells invasion. Sixty Wistar rats were divided into four different groups receiving 10 microlitres of 0.1% EB or 0.9% saline solution into the cisterna pontis and treated or not with the xanthine. PPF treatment was done using 12.5 mg/kg/day by the intraperitonial route for 31 days of the experimental period. The rats were euthanized from 7 to 31 days after EB injection and brainstem sections were collected and processed for light and transmission electron microscopy studies. Results from both groups were compared by using a semi-quantitative method developed for documenting in semithin sections the extent and nature of remyelination of demyelinating lesions. Results showed that PPF administration after EB injection significantly increased both oligodendroglial and Schwann cell remyelination at 31 days (mean remyelination scores of 3.67 ± 0.5 for oligodendrocytes and 1.27 ± 0.49 for Schwann cells) compared to untreated animals (scores of 3.19 ± 0.57 and 0.90 ± 0.33, respectively).

Bradykinin decreases nitric oxide release from microglia via inhibition of cyclic adenosine monophosphate signaling

Bradykinin (BK) is a major potent inflammatory mediator outside the central nervous system. In Alzheimer's disease, BK release and BK receptor expression in brain tissues are upregulated relatively early during the course of the disease. Hence, BK was believed to promote neuroinflammation. However, BK was recently reported to possess anti-inflammatory and neuroprotective roles. Exposure of BV2 microglial cell line to BK lead to a decrease in NO release from unstimulated cells as well as a dose-dependent attenuation, mediated by both B1 and B2 receptors, in lipopolysaccharide (LPS)-induced NO production. In this study we examined whether cyclic adenosine monophosphate (cAMP) signaling is involved in BK-mediated effect in microglial nitric oxide (NO) production. A protein kinase A (PKA) inhibitor mimicked the effects of BK, while cAMP elevating agents antagonized BK-mediated NO decrease. Moreover, BK inhibited the activation of cAMP responsive element binding protein (CREB). In addition, BK protected microglial cells from death triggered by combinations of LPS and each of the cAMP elevating agents. Finally, the addition of Gαi protein inhibitor abrogated the effects of BK on NO release, and the expression of Gαi protein in the plasma membrane was induced by BK. These results suggest that BK-mediated reduction in microglial NO production depends on coupling to Gi protein and also involves inhibition of cAMP-PKA-CREB signaling.

Antinociceptive interaction of (±)-CPP and propentofylline in monoarthritic rats

Introduction

Multiple studies have shown that glial cells of the spinal cord, such as astrocytes and microglia, have close contact with neurons, suggesting the term tripartite synapse. In these synapses, astrocytes surrounding neurons contribute to neuronal excitability and synaptic transmission, thereby increasing nociception and thus the persistence of chronic pain. Conversely, the N-methyl-D-aspartate (NMDA) receptor is crucial in the generation and maintenance of chronic pain. It has multiple sites of modulation. One is the site of recognition of extracellular neurotransmitter (glutamate), which can be blocked by competitive antagonists such as (3-(2-carboxipiperazin-4)1-propyl phosphonic acid), (±)-CPP, resulting in a blockade of the calcium current and thus the intracellular transduction process. In the present study, we investigated whether the potential antinociceptive effect of glial inhibition produced by propentofylline (PPF) can be enhanced when combined with an NMDA-receptor inhibitor such as (±)-CPP.

Methods

We used Sprague-Dawley monoarthritic rats. The monoarthritis was induced by injection of complete Freund adjuvant in the right tibiotarsal joint. Four weeks later, rats were treated with PPF (1, 10, 30, and 100 μg/10 μl) intrathecally (i.t.) for 10 days, injected once with (±)-CPP (2.5, 5, 12.5, 25, 50, and 100 μg/10 μl, i.t.), or both treatments combined. The antinociceptive effect was evaluated on day 11 for PPF and immediately to (±)-CPP, by assessing the vocalization threshold to mechanical stimulation of the arthritic paw.

Results

The data indicate that intrathecal administration of increasing concentrations of (±)-CPP or PPF produced a significant dose-dependent antinociceptive effect with respect to monoarthritic rats receiving saline. The linear regression analysis showed that the dose that produces 30% of maximal effect (ED₃₀) for i.t. (±)-CPP was 3.97 μg, and 1.42 μg for i.t. PPF. The administration of the PPF and (±)-CPP combination in fixed proportions of ED₃₀ produced a dose-dependent antinociceptive effect, showing an interaction of the supraadditive type.

Conclusions

The results suggest that glia inhibitors can synergically potentiate the effect of glutamate blockers for the treatment of chronic inflammatory pain.

Postinjury treatment with rolipram increases hemorrhage after traumatic brain injury

The pathology caused by traumatic brain injury (TBI) is exacerbated by the inflammatory response of the injured brain. Two proinflammatory cytokines that contribute to inflammation after TBI are tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). From previous studies using the parasagittal fluid-percussion brain injury model, we reported that the anti-inflammatory drug rolipram, a phosphodiesterase 4 inhibitor, reduced TNF-α and IL-1β levels and improved histopathological outcome when administered 30 min prior to injury. We now report that treatment with (±)-rolipram given 30 min after injury significantly reduced TNF-α levels in the cortex and hippocampus. However, postinjury administration of (±)-rolipram significantly increased cortical contusion volume and increased atrophy of the cortex compared with vehicle-treated animals at 10 days postinjury. Thus, despite the reduction in proinflammatory cytokine levels, histopathological outcome was worsened with post-TBI (±)-rolipram treatment. Further histological analysis of (±)-rolipram-treated TBI animals revealed significant hemorrhage in the contused brain. Given the well-known role of (±)-rolipram of increasing vasodilation, it is likely that (±)-rolipram worsened outcome after fluid-percussion brain injury by causing increased bleeding.

Distinct time courses of microglial and astrocytic hyperactivation and the glial contribution to pain hypersensitivity in a facial cancer model

Although recent evidence suggests that central glial hyperactivation is involved in cancer-induced persistent pain, the time course of this hyperactivation and the glial contribution to pain hypersensitivity remain unclear. The present study investigated the time-dependent spatial changes of microglial and astrocytic hyperactivation in the trigeminocervical complex, which consists of the medullary (MDH) and upper cervical (UCDH) dorsal horns, and pain-related behaviors in a rat facial cancer model in which Walker 256B-cells are inoculated into the vibrissal pad. In this model, the tumors grew within the vibrissal pad, from which sensory nerve fibers project into the MDH, but did not expand into the infraorbital region, from which fibers project into the UCDH. Nevertheless, mechanical allodynia and thermal hyperalgesia were observed not only in the vibrissal pad but also in the infraorbital region. Western blotting and immunofluorescence studies indicated that microglia were widely activated in the trigeminocervical complex on day 4 and gradually inactivated by day 11. In contrast, astrocytes were only activated in the MDH on day 4; the hyperactivation later expanded into the UCDH. Daily administration of the glial hyperactivation inhibitor propentofylline beginning on day 4 suppressed the glial hyperactivation on later days. Propentofylline treatment largely prevented allodynia/hyperalgesia in the infraorbital region beginning on day 5, although established allodynia/hyperalgesia in the vibrissal pad was less sensitive to the treatment. These results suggest that central glial hyperactivation, transient microglial hyperactivation and persistent astrocytic hyperactivation, contributes to the development of pain hypersensitivity but not to the maintenance of pain in this model.

Propentofylline decreases tumor growth in a rodent model of glioblastoma multiforme by a direct mechanism on microglia

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain cancer, with a median survival of less than 2 years after diagnosis. The tumor microenvironment plays a critical role in tumor invasion and progression. Microglia and infiltrating macrophages are the most abundant immune cells in the tumor. In the present study, we demonstrate that systemic propentofylline (PPF), an atypical methylxanthine with central nervous system (CNS) glial modulating and anti-inflammatory actions, significantly decreased tumor growth in a CNS-1 rat model of GBM by targeting microglia and not tumor cells. Rats received tumor injections of 1 × 10(5) CNS-1 cells in the right striatum with daily intraperitonial injections of PPF (50 mg/kg) or saline beginning the day of tumor injection. PPF did not cause apoptosis or decrease proliferation of CNS-1 tumor cells. Furthermore, we demonstrate, using in vitro methods, that PPF decreased microglial migration toward CNS-1 tumor cells and decreased MMP-9 expression. The effects of PPF were shown to be specific to microglia and not peripheral macrophages. These results support a differential functional role of resident microglia and infiltrating macrophages in the brain tumor environment. Our data highlight microglia as a crucial target for future therapeutic development and present PPF as a possible drug for treatment of human GBM.

High-throughput assay for the identification of compounds regulating osteogenic differentiation of human mesenchymal stromal cells

Human mesenchymal stromal cells are regarded as the golden standard for cell-based therapies. They present multilineage differentiation potential and trophic and immunosuppressive abilities, making them the best candidate for clinical applications. Several molecules have been described to increase bone formation and were mainly discovered by candidate approaches towards known signaling pathways controlling osteogenesis. However, their bone forming potential is still limited, making the search for novel molecules a necessity. High-throughput screening (HTS) not only allows the screening of a large number of diverse chemical compounds, but also allows the discovery of unexpected signaling pathways and molecular mechanisms for a certain application, even without the prior knowledge of the full molecular pathway. Typically HTS is performed in cell lines, however, in this manuscript we have performed a phenotypical screen on more clinically relevant human mesenchymal stromal cells, as a proof of principle that HTS can be performed in those cells and can be used to find small molecules that impact stem cell fate. From a library of pharmacologically active small molecules, we were able to identify novel compounds with increased osteogenic activity. These compounds allowed achieving levels of bone-specific alkaline phosphatase higher than any other combination previously known. By combining biochemical techniques, we were able to demonstrate that a medium to high-throughput phenotypic assay can be performed in academic research laboratories allowing the discovery of novel molecules able to enhance stem cell differentiation.

Propentofylline: glial modulation, neuroprotection, and alleviation of chronic pain

Propentofylline is a unique methylxanthine with clear cyclic AMP, phosphodiesterase, and adenosine actions, including enhanced synaptic adenosine signaling. Both in vitro and in vivo studies have demonstrated profound neuroprotective, antiproliferative, and anti-inflammatory effects of propentofylline. Propentofylline has shown efficacy in preclinical models of stroke, opioid tolerance, and acute and chronic pain. Clinically, propentofylline has shown efficacy in degenerative and vascular dementia, and as a potential adjuvant treatment for schizophrenia and multiple sclerosis. Possible mechanisms of action include a direct glial modulation to decrease a reactive phenotype, decrease glial production and release of damaging proinflammatory factors, and enhancement of astrocyte-mediated glutamate clearance. This chapter reviews the literature that supports a myriad of protective actions of this small molecule and implicates propentofylline as a potential therapeutic for the treatment of chronic pain. From these studies, we propose a CNS multipartite synaptic action of propentofylline that includes modulation of pre- and postsynaptic neurons, astrocytes, and microglia in the treatment of chronic pain syndromes, including, but not limited to, neuropathic pain.

Down-regulation of astrocytic GLAST by microglia-related inflammation is abrogated in dibutyryl cAMP-differentiated cultures

The influence of neuroinflammation on glutamate uptake by glial cells was examined after exposing primary cultures of rat astrocytes to conditioned culture medium from lipopolysaccharide-activated microglia. While such treatment triggered an inflammatory response in astrocytes, as revealed by the induction of cytokine expression, a significant decrease in GLAST expression and activity was observed after 72 h. This regulation of glutamate transporter was not observed with medium from naive microglia, but was mimicked by direct addition of tumor necrosis factor-alpha (TNF-alpha), a major cytokine released from activated microglia. Hence, on its own, TNF-alpha also triggered inflammation in astrocyte cultures, highlighting complex cross-talk between astrocytes and microglia in inflammatory conditions. This putatively detrimental regulation of GLAST in response to inflammation was also studied in cells exposed to dibutyryl cAMP, recognized as a model of astrocytes exhibiting a typical differentiated or activated phenotype. In this model, the conditioned culture medium from activated microglia, as well as TNF-alpha, were found to increase glutamate uptake capacity. Consistently, both of these treatments caused only modest induction of an inflammatory response in dibutyryl cAMP-matured astrocytes as compared to undifferentiated astrocytes. Together, these results suggest that differentiated/activated astrocytes are endowed with the capacity to confront inflammatory insults and that drugs influencing the astrocytes phenotype would deserve further consideration in the treatment of neurological disorders.

Dynamic regulation of spinal pro-inflammatory cytokine release in the rat in vivo following peripheral nerve injury

Spinal release of cytokines may play a critical role in the maladapted nociceptive signaling underlying chronic pain states. In order to investigate this biology, we have developed a novel 'high flux' intrathecal microdialysis approach in combination with multiplex bead-based immunoassay technology to concurrently monitor the spinal release of interleukin (IL)-1beta, IL-6 and tumour necrosis factor (TNF)alpha in rats with unilateral sciatic nerve chronic constriction injury (CCI). Intrathecal microdialysis was performed under isoflurane/N(2)O anaesthesia in rats with confirmed mechanical hypersensitivity. In a first study, C-fiber strength electrical stimulation of the operated nerve in neuropathic rats was found to evoke a dramatic increase in IL-1beta efflux ( approximately 15-fold) that was significantly greater than that observed in the sham-operated group. Spinal IL-6 efflux was also responsive to primary afferent stimulation, whereas TNFalpha was not. In a second study, treatment with the glial inhibitor propentofylline for 7days normalized CCI-induced mechanical hypersensitivity. In the same animals, this treatment also significantly reduced intrathecal IL-1beta, IL-6 and TNFalpha and prevented afferent stimulation-evoked cytokine release of both IL-1beta and IL-6. These results provide support for glia as the source of the majority of intrathecal IL-1beta, IL-6 and TNFalpha that accompanies mechanical hypersensitivity in the CCI rat. Moreover, our studies demonstrate the ability of a neurone-glia signaling mechanism to dynamically modulate this release and support a role of spinal IL-1beta in the phasic transmission of abnormal pain signals.

TNF-alpha and neuropathic pain--a review

Tumor necrosis factor alpha (TNF-α) was discovered more than a century ago, and its known roles have extended from within the immune system to include a neuro-inflammatory domain in the nervous system. Neuropathic pain is a recognized type of pathological pain where nociceptive responses persist beyond the resolution of damage to the nerve or its surrounding tissue. Very often, neuropathic pain is disproportionately enhanced in intensity (hyperalgesia) or altered in modality (hyperpathia or allodynia) in relation to the stimuli. At time of this writing, there is as yet no common consensus about the etiology of neuropathic pain - possible mechanisms can be categorized into peripheral sensitization and central sensitization of the nervous system in response to the nociceptive stimuli. Animal models of neuropathic pain based on various types of nerve injuries (peripheral versus spinal nerve, ligation versus chronic constrictive injury) have persistently implicated a pivotal role for TNF-α at both peripheral and central levels of sensitization. Despite a lack of success in clinical trials of anti-TNF-α therapy in alleviating the sciatic type of neuropathic pain, the intricate link of TNF-α with other neuro-inflammatory signaling systems (e.g., chemokines and p38 MAPK) has indeed inspired a systems approach perspective for future drug development in treating neuropathic pain.

The "toll" of opioid-induced glial activation: improving the clinical efficacy of opioids by targeting glia

Glial activation participates in the mediation of pain including neuropathic pain, due to release of neuroexcitatory, proinflammatory products. Glial activation is now known to occur in response to opioids as well. Opioid-induced glial activation opposes opioid analgesia and enhances opioid tolerance, dependence, reward and respiratory depression. Such effects can occur, not via classical opioid receptors, but rather via non-stereoselective activation of toll-like receptor 4 (TLR4), a recently recognized key glial receptor participating in neuropathic pain as well. This discovery identifies a means for separating the beneficial actions of opioids (opioid receptor mediated) from the unwanted side-effects (TLR4/glial mediated) by pharmacologically targeting TLR4. Such a drug should be a stand-alone therapeutic for treating neuropathic pain as well. Excitingly, with newly-established clinical trials of two glial modulators for treating neuropathic pain and improving the utility of opioids, translation from rats-to-humans now begins with the promise of improved clinical pain control.

Activation of p-38alpha MAPK contributes to neuronal hyperexcitability in caudal regions remote from spinal cord injury

In the present study, we examined whether activation of p-38alpha MAPK modulates mechanical allodynia and neuronal hyperexcitability, and if propentofylline (PPF, a glial modulator) modulates specifically localized activated p-38alpha MAPK expression in caudal regions remote from a low thoracic hemisection injury in rats. T13 spinal hemisection produces bilateral mechanical allodynia in hindpaws with evoked (in response to mechanical stimuli) neuronal hyperexcitability in lumbar spinal wide dynamic range (WDR) neurons compared to sham controls. The mechanical allodynia and the evoked activity of WDR neurons is attenuated by intrathecal and topical administration of SB203580, an inhibitor of p-38 MAPK activation, dose dependently (p<0.05); however, the spontaneous activity showed no significant differences compared to sham controls. After T13 spinal hemisection, significantly increased phosphorylated (activated form) p-38alpha MAPK expression was present in both superficial and deep dorsal horn neurons as well as in microglia, but not in astrocytes, in the lumbar spinal cord compared to sham controls (p<0.05). Intrathecal application of PPF significantly attenuated the expression of phosphorylated p-38alpha MAPK in superficial dorsal horn neurons (10 mM) and in microglia (1 and 10 mM) in the lumbar spinal cord compared to the hemisection group (p<0.05). In conclusion, our present data demonstrate that activated neuronal and microglial, but not astrocytic, p-38alpha MAPK contributes to the maintenance of neuronal hyperexcitability in caudal regions following spinal cord injury.

The glial activation inhibitor AV411 reduces morphine-induced nucleus accumbens dopamine release

Glial activation has recently been discovered to modulate several effects of morphine, including analgesia, tolerance, and dependence. The present studies extend this line of investigation by exploring whether glial activation may also affect extracellular levels of dopamine (DA) in the nucleus accumbens (NAc) shell, a neurochemical corollary of morphine-induced drug reward, during a challenge dose of morphine in experiments both with and without precipitated withdrawal. Morphine or vehicle was administered s.c. for 4 days (starting at 15 mg/kg/day up to 20 mg/kg/day), and the glial activation inhibitor AV411 (7.5 mg/kg) or vehicle was administered twice daily. A challenge dose of morphine (22.5 mg/kg) or saline was then given during dialysis. In the first experiment, naloxone (10 mg/kg) was administered 1h after morphine during dialysis in AV411- or vehicle-treated rats, and behavioral signs of somatic withdrawal were assessed during microdialysis. In the second experiment, using the same dosing regimen, sampling continued 3 h after morphine or saline in AV411- or vehicle-treated rats. NAc DA increased in vehicle-treated rats significantly more than in AV411-treated rats before naloxone treatment, and withdrawal symptoms were significantly reduced in AV411-treated rats. The decrease in morphine-induced NAc DA by AV411 was persistent, lasting 3+h post-morphine. These results indicate that glial activation contributes to the effects of morphine on NAc DA, which is associated with somatic signs of precipitated withdrawal.

Remote astrocytic and microglial activation modulates neuronal hyperexcitability and below-level neuropathic pain after spinal injury in rat

In this study, we evaluated whether astrocytic and microglial activation mediates below-level neuropathic pain following spinal cord injury. Male Sprague-Dawley (225-250 g) rats were given low thoracic (T13) spinal transverse hemisection and behavioral, electrophysiological and immunohistochemical methods were used to examine the development and maintenance of below-level neuropathic pain. On postoperation day 28, both hind limbs showed significantly decreased paw withdrawal thresholds and thermal latencies as well as hyperexcitability of lumbar (L4-5) spinal wide dynamic range (WDR) neurons on both sides of spinal dorsal horn compared to sham controls (* P<0.05). Intrathecal treatment with propentofylline (PPF, 10 mM) for 7 consecutive days immediately after spinal injury attenuated the development of mechanical allodynia and thermal hyperalgesia in both hind limbs in a dose-related reduction compared to vehicle treatments (* P<0.05). Intrathecal treatment with single injections of PPF at 28 days after spinal injury, attenuated the existing mechanical allodynia and thermal hyperalgesia in both hind limbs in a dose related reduction (* P<0.05). In electrophysiological studies, topical treatment of 10 mM PPF onto the spinal surface attenuated the neuronal hyperexcitability in response to mechanical stimuli. In immunohistochemical studies, astrocytes and microglia in rats with spinal hemisection showed significantly increased GFAP and OX-42 expression in both superficial and deep dorsal horns in the lumbar spinal dorsal horn compared to sham controls (* P<0.05) that was prevented in a dose-related manner by PPF. In conclusion, our present data support astrocytic and microglial activation that contributes to below-level central neuropathic pain following spinal cord injury.

Contribution of TRPV1 to microglia-derived IL-6 and NFkappaB translocation with elevated hydrostatic pressure

PURPOSE

The authors investigated the contributions of the transient receptor potential vanilloid-1 receptor (TRPV1) and Ca(2+) to microglial IL-6 and nuclear factor kappa B (NFkappaB) translocation with elevated hydrostatic pressure.

METHODS

The authors first examined IL-6 colocalization with the microglia marker Iba-1 in the DBA/2 mouse model of glaucoma to establish relevance. They isolated microglia from rat retina and maintained them at ambient or elevated (+70 mm Hg) hydrostatic pressure in vitro and used ELISA and immunocytochemistry to measure changes in the IL-6 concentration and NFkappaB translocation induced by the Ca(2+) chelator EGTA, the broad-spectrum Ca(2+) channel inhibitor ruthenium red, and the TRPV1 antagonist iodo-resiniferatoxin (I-RTX). They applied the Ca(2+) dye Fluo-4 AM to measure changes in intracellular Ca(2+) at elevated pressure induced by I-RTX and confirmed TRPV1 expression in microglia using PCR and immunocytochemistry.

RESULTS

In DBA/2 retina, elevated intraocular pressure increased microglial IL-6 in the ganglion cell layer. Elevated hydrostatic pressure (24 hours) increased microglial IL-6 release, cytosolic NFkappaB, and NFkappaB translocation in vitro. These effects were reduced substantially by EGTA and ruthenium red. Antagonism of TRPV1 in microglia partially inhibited pressure-induced increases in IL-6 release and NFkappaB translocation. Brief elevated pressure (1 hour) induced a significant increase in microglial intracellular Ca(2+) that was partially attenuated by TRPV1 antagonism.

CONCLUSIONS

Elevated pressure induces an influx of extracellular Ca(2+) in retinal microglia that precedes the activation of NFkappaB and the subsequent production and release of IL-6 and is at least partially dependent on the activation of TRPV1 and other ruthenium red-sensitive channels.

The neuropathic pain triad: neurons, immune cells and glia

Nociceptive pain results from the detection of intense or noxious stimuli by specialized high-threshold sensory neurons (nociceptors), a transfer of action potentials to the spinal cord, and onward transmission of the warning signal to the brain. In contrast, clinical pain such as pain after nerve injury (neuropathic pain) is characterized by pain in the absence of a stimulus and reduced nociceptive thresholds so that normally innocuous stimuli produce pain. The development of neuropathic pain involves not only neuronal pathways, but also Schwann cells, satellite cells in the dorsal root ganglia, components of the peripheral immune system, spinal microglia and astrocytes. As we increasingly appreciate that neuropathic pain has many features of a neuroimmune disorder, immunosuppression and blockade of the reciprocal signaling pathways between neuronal and non-neuronal cells offer new opportunities for disease modification and more successful management of pain.

Modulation of the cAMP signaling pathway after traumatic brain injury

Traumatic brain injury (TBI) results in both focal and diffuse brain pathologies that are exacerbated by the inflammatory response and progress from hours to days after the initial injury. Using a clinically relevant model of TBI, the parasagittal fluid-percussion brain injury (FPI) model, we found injury-induced impairments in the cyclic AMP (cAMP) signaling pathway. Levels of cAMP were depressed in the ipsilateral parietal cortex and hippocampus, as well as activation of its downstream target, protein kinase A, from 15 min to 48 h after moderate FPI. To determine if preventing hydrolysis of cAMP by administration of a phosphodiesterase (PDE) IV inhibitor would improve outcome after TBI, we treated animals intraperitoneally with rolipram (0.3 or 3.0 mg/kg) 30 min prior to TBI, and then once per day for 3 days. Rolipram treatment restored cAMP to sham levels and significantly reduced cortical contusion volume and improved neuronal cell survival in the parietal cortex and CA3 region of the hippocampus. Traumatic axonal injury, characterized by beta-amyloid precursor protein deposits in the external capsule, was also significantly reduced in rolipram-treated animals. Furthermore, levels of the pro-inflammatory cytokines, interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha), were significantly decreased with rolipram treatment. These results demonstrate that the cAMP-PKA signaling cascade is downregulated after TBI, and that treatment with a PDE IV inhibitor improves histopathological outcome and decreases inflammation after TBI.

The characteristics of neuronal injury in a static compression model of spinal cord injury in adult rats

Studies of spinal cord injury using contusion (impact) injury paradigms have shown that neuronal death is an acute event that is largely over within 24 h. However, much less is known about cell death following compression injury, despite compression being a key component of natural spinal injuries. We have therefore used neuronal nuclei (NeuN) immunostaining to examine the spatiotemporal pattern of neuronal loss after static compression injury in adult rats. 3D reconstruction was used to reveal the full effect of the injury. Neuronal loss at the injury epicentre, assessed by NeuN immunostaining, amounted to 44% at 1 day but increased to 73% at 3 days and 81% at 1 month. Neuronal loss was also seen 5 mm rostral and caudal to the epicentre, but was not significant until 3 days. NeuN loss was greatest in the ventral horns and in the intermediate grey matter, with the lateral dorsal horns relatively spared. Cystic cavities formed after injury, but were not evident until 4 weeks and were small in size. In contrast to the slow profile of neuronal loss, the compression injury also evoked a transient expression of activating transcription factor-3 (ATF3) and activated c-Jun in neurons. ATF3 expression peaked at 3 days and declined at 7 days. Our spatiotemporal analysis of compression injury shows that neuronal loss is much more protracted than in contusion injury, and highlights the potential for neuroprotective strategies. This study is also the first indication of ATF3 involvement in spinal cord injury.

Induction of astrocyte differentiation by propentofylline increases glutamate transporter expression in vitro: heterogeneity of the quiescent phenotype

Reactive astrocytes display decreased glutamate transporters, such as GLT-1, and as a result synaptic glutamate clearance is impaired. In addition, these activated astrocytes are immunocompetent and release algesic mediators that can sensitize neurons in the spinal cord. Currently, we evaluated the effect of propentofylline (PPF), an experimental antiallodynic agent, on the phenotype and glutamate transporter expression of astrocytes. Primary astrocyte cultures, which represent an activated phenotype with a polygonal morphology and low GLT-1 expression, were treated for 3 or 7 days with 10, 100, or 1,000 microM PPF or dibutyryl-cAMP (db-cAMP), a known inducer of GLT-1 expression. PPF dose-dependently induced astrocytes to display a mature phenotype, with elongated processes and a stellate shape, as well as increased GLT-1 and GLAST immunoreactivity, similar to that seen with db-cAMP. Real time RT-PCR and Western blot analysis clearly demonstrated that PPF caused a potent dose-dependent induction of GLT-1 and GLAST mRNA and protein in these astrocytes. Importantly, the observed increase in glutamate transporters was found to have a functional effect, with significantly enhanced glutamate uptake in astrocytes treated with 100 or 1,000 microM PPF that was sensitive to dihydrokainate inhibition, suggesting it is GLT-1 mediated. Finally, the effect of PPF on lipopolysaccharide-induced chemokine release was investigated. Interestingly, PPF was able to dampen both MCP-1 (CCL2) and MIP-2 (CXCL2) release from astrocytes while db-cAMP significantly enhanced this chemokine expression. These findings suggest that PPF is capable of differentiating astrocytes to a homeostatic, mature phenotype, competent for glutamate clearance and distinct from that induced by db-cAMP.

Microglia induce neural cell death via a proximity-dependent mechanism involving nitric oxide

Microglial cells play a major role in the pathogenesis of many neurological diseases by exacerbating neuronal and non-neuronal cell death, but the mechanisms involved are unclear. To investigate the microglial-neuronal interactions, we used the murine BV-2 microglial cell line and the human neuronal-like SK-N-SH neuroblastoma cell line in a co-culture system that enabled proximity-dependent interaction and communication, a trans-well system that allowed proximity-independent communication through diffusible molecules only, and a conditioned media system through which no proximity-dependent interactions or cell-to-cell communication is possible. Activation of BV-2 cells with lipopolysaccharide and interferon-gamma (LPS/IFN-gamma) decreased viability of the BV-2 cells alone and in co-cultures with SK-N-SH cells, but not SK-N-SH cells grown alone. In contrast, activation of BV-2 cells in the trans-well and conditioned media system did not have any effect on the viability of SK-N-SH cells, suggesting that microglia must be in close proximity to the neural cells to elicit cytotoxicity. To determine the molecules involved in proximity-dependent cell death, inhibitors of microglial activation were investigated. Only the specific inducible nitric oxide synthase (iNOS) inhibitor S-methylisothiourea, and hypothermia, which is known to suppress microglial iNOS expression, prevented cell death after LPS/IFN-gamma activation. These results suggest that activated microglia release nitric oxide that is, at least partially, responsible for proximity-dependent microglial-mediated neural toxicity.

Intrathecal minocycline attenuates peripheral inflammation-induced hyperalgesia by inhibiting p38 MAPK in spinal microglia

Activation of p38 mitogen-activated protein kinase (p38) in spinal microglia is implicated in spinal nociceptive processing. Minocycline, a tetracycline derivative, displays selective inhibition of microglial activation, a function that is distinct from its antibiotic activity. In the present study we examined antinociceptive effects of intrathecal (IT) administration of minocycline in experimental models of inflammation-evoked hyperalgesia in addition to the effect of minocycline on stimulation-induced activation of p38 in spinal microglia. Intrathecal minocycline produced a dose-dependent reduction of formalin-evoked second-phase flinching behaviour in rats, and prevented thermal hyperalgesia induced by carrageenan injection into the paw. In contrast, systemic delivery (intraperitoneally) of minocycline inhibited the first but not the second phase of formalin-induced flinching, and it had no effect on carrageenan-induced hyperalgesia. Centrally mediated hyperalgesia induced by IT delivery of N-methyl-d-aspartate was completely blocked by IT minocycline. An increase in phosphorylation (activation) of p38 (P-p38) was observed in the dorsal spinal cord after carrageenan paw injection, assessed by both Western blotting and immunohistochemistry. The increased P-p38 immunoreactivity was seen primarily in microglia but also in a small population of neurons. Minocycline, at the IT dose that blocked carrageenan-induced hyperalgesia, also attenuated the increased P-p38 in microglia. In addition, minocycline suppressed lipopolysaccharide-evoked P-p38 in cultured spinal microglial cells. Taken together, these findings show that minocycline given IT produces a potent and consistent antinociception in models of tissue injury and inflammation-evoked pain, and they provide strong support for the idea that this effect is mediated by direct inhibition of spinal microglia and subsequent activation of p38 in these cells.

Cytokine production by a human microglial cell line: effects of beta-amyloid and alpha-melanocyte-stimulating hormone

Senile plaques in the Alzheimer's disease (AD) are formed by aggregation of beta-amyloid (Abeta) peptide. Abeta peptide has been shown to activate microglia and stimulate their production of inflammatory factors, such as cytokines. In the AD brain, the continued presence of amyloid plaques may keep microglia persistently activated, leading to chronic inflammation in the CNS. It is well established that alpha-melanocyte-stimulating hormone (alpha-MSH) gives rise to anti-inflammatory and anti-pyretic effects. The biological activities of alpha-MSH are mediated by one or more of the melanocortin receptor (MCR) subtypes, i.e. MCR1 - MCR5. The aim of the present study was to determine the effect of alpha-MSH alone and on Abeta-activated microglial cells with regard to the secretion of inflammatory cytokines, such as interleukin-6 (IL-6), and to determine which receptor subtype mediates the effects of alpha-MSH. The human microglial cell line, CHME3, was incubated for 24 h with freshly dissolved Abeta(1-40), interferon-gamma (IFN-gamma) and/or alpha-MSH. Freshly dissolved Abeta(1-40) (5-60 microM) resulted in a dose-dependent decrease in cell viability, along with a dose-dependent increase in IL-6 release. Neither IFN-gamma nor alpha-MSH affected the Abeta-induced secretion of IL-6, but resulted in a dose-dependent increase in basal IL-6 release. Agouti, the endogenous antagonist of MCR1 and 4, further increased the alpha-MSH-induced secretion of IL-6. RT-PCR showed the expression of MCR1, MCR3, MCR4 and MCR5 mRNA. The combined data suggest that the effect of alpha-MSH in increasing IL-6 release from the human microglial cell line is mediated by MCR3 or MCR5.

Systemic administration of propentofylline does not attenuate morphine tolerance in non-injured rodents

Propentofylline is a phosphodiesterase inhibitor that has been shown to attenuate the onset of morphine tolerance when administered intrathecally to rats. The present studies examined whether systemic administration could be effective in attenuating morphine tolerance in non-injured rodents using a similar dosing paradigm. Propentofylline at 10, 30, or 50 mg/kg, administered intraperitoneally once daily for 5 days, was unable to attenuate morphine tolerance established by twice daily administration of 10 mg/kg morphine. These results suggest that direct delivery of propentofylline to the central nervous system (CNS) may be required in order to attenuate morphine tolerance.

Propentofylline attenuates tau hyperphosphorylation in Alzheimer's Swedish mutant model Tg2576

Key pathological hallmarks of Alzheimer's disease (AD) are the deposition of amyloid plaques containing Abeta-peptides and the formation of neurofibrillary tangles containing hyperphosphorylated tau. Propentofylline (PPF) is a synthetic xanthine derivative that inhibits phosphodiesterase and adenosine uptake. These effects of PPF influence many cellular functions including stimulating synthesis/release of nerve growth factor. We tested the effects of PPF on disease progression in transgenic mice overexpressing the Swedish mutant human APP (Tg2576). The untreated Tg mice show, together with increased amyloidogenesis, increased levels of tau hyperphosphorylation and increased ratios of the activated to inactivated GSK-3beta, one of the key kinases that can phosphorylate tau. One month of PPF feeding (40 mg/kg per day) reduced the burden of amyloid plaques and the levels of hyperphosphorylated tau and immunoreactive IL-1beta. In parallel with these changes, PPF reduced the activated form of GSK-3beta and increased the inactivated form of GSK-3beta, restoring their ratio almost to normal values. These results demonstrate that PPF can exert multiple protective effects on both amyloidogenesis and tau hyperphosphorylation in an animal model of AD. Our earlier report [Neurochem. Int. 43(3) (2003) 225] demonstrated that Tg2576 animals show decreased levels of mRNA for NGF with increased amyloid burden while feeding of PPF results in a major shift from beta-amyloidogenic to alpha-secretory processing of APP together with increased expression of NGF mRNA. The current new data enlarge our understanding of PPF effects in brain and of tau hyperphosphorylation in Tg animals and are consistent with the hypothesis that GSK-3beta is a nodal point linking amyloid and tau pathology. Therapeutic interventions directed toward multiple pathological processes may be more protective than treatments directed toward a single process. The new results reported here indicate that further testing of PPF as a potential therapy in AD is warranted.

Differential modulation of microglia superoxide anion and thromboxane B2 generation by the marine manzamines

BACKGROUND

Thromboxane B2 (TXB2) and superoxide anion (O2-) are neuroinflammatory mediators that appear to be involved in the pathogenesis of several neurodegenerative diseases. Because activated-microglia are the main source of TXB2 and O2- in these disorders, modulation of their synthesis has been hypothesized as a potential therapeutic approach for neuroinflammatory disorders. Marine natural products have become a source of novel agents that modulate eicosanoids and O2- generation from activated murine and human leukocytes. With the exception of manzamine C, all other manzamines tested are characterized by a complex pentacyclic diamine linked to C-1 of the beta-carboline moiety. These marine-derived alkaloids have been reported to possess a diverse range of bioactivities including anticancer, immunostimulatory, insecticidal, antibacterial, antimalarial and antituberculosis activities. The purpose of this investigation was to conduct a structure-activity relationship study with manzamines (MZ) A, B, C, D, E and F on agonist-stimulated release of TXB2 and O2- from E. coli LPS-activated rat neonatal microglia in vitro.

RESULTS

The manzamines differentially attenuated PMA (phorbol 12-myristate 13-acetate)-stimulated TXB2 generation in the following order of decreasing potency: MZA (IC50 < 0.016 microM) > MZD (IC50 = 0.23 microM) > MZB (IC50 = 1.6 microM) > MZC (IC50 = 2.98 microM) > MZE and F (IC50 > 10 microM). In contrast, there was less effect on OPZ (opsonized zymosan)-stimulated TXB2 generation: MZB (IC50 = 1.44 microM) > MZA (IC50 = 3.16 microM) > MZC (IC50 = 3.34 microM) > MZD, MZE and MZF (IC50 > 10 microM). Similarly, PMA-stimulated O2- generation was affected differentially as follows: MZD (apparent IC50 < 0.1 microM) > MZA (IC50 = 0.1 microM) > MZB (IC50 = 3.16 microM) > MZC (IC50 = 3.43 microM) > MZE and MZF (IC50 > 10 microM). In contrast, OPZ-stimulated O2- generation was minimally affected: MZB (IC50 = 4.17 microM) > MZC (IC50 = 9.3 microM) > MZA, MZD, MZE and MZF (IC50 > 10 microM). From the structure-activity relationship perspective, contributing factors to the observed differential bioactivity on TXB2 and O2- generation are the solubility or ionic forms of MZA and D as well as changes such as saturation or oxidation of the beta carboline or 8-membered amine ring. In contrast, the fused 13-membered macrocyclic and isoquinoline ring system, and any substitutions in these rings would not appear to be factors contributing to bioactivity.

CONCLUSION

To our knowledge, this is the first experimental study that demonstrates that MZA, at in vitro concentrations that are non toxic to E. coli LPS-activated rat neonatal microglia, potently modulates PMA-stimulated TXB2 and O2- generation. MZA may thus be a lead candidate for the development of novel therapeutic agents for the modulation of TXB2 and O2- release in neuroinflammatory diseases. Marine natural products provide a novel and rich source of chemical diversity that can contribute to the design and development of new and potentially useful anti-inflammatory agents to treat neurodegenerative diseases.

Aspirin modulates LPS-induced nitric oxide release in rat glial cells

Nitric oxide and prostaglandins are among the numerous substances released by activated glial cells. The aim of this study was to evaluate the effect of high-level aspirin on iNOS expression in cultured rat glial cells treated with lipopolysaccharide (LPS) as pathological stimulator. Using Western Blotting, we verified that aspirin enhanced LPS-induced iNOS expression and the presence of 15-deoxy-Delta(12,14)-prostaglandin (15d-PGJ2) suppressed this aspirin effect. However, the exposure of LPS-treated glial cells to aspirin resulted in a decrease of NO production. These results suggest that aspirin interferes with the cross-talk of prostaglandins and NO, blocking the endogenous negative control exerted by COX products on iNOS expression. On the other side, aspirin seems to act directly on iNOS reducing its activity, even if it does not completely block NO release by LPS-stimulated glial cells. Then aspirin could maintain homeostatic functions of NO, while it prevents toxic effects, corresponding to high NO concentrations.

Dual role of cAMP in iNOS expression in glial cells and macrophages is mediated by differential regulation of p38-MAPK/ATF-2 activation and iNOS stability

We reported previously that cAMP analogues or cAMP synthesis activator (forskolin; FSK) inhibit lipopolysaccharide (LPS)-induced inducible nitric-oxide systase (iNOS) gene expression in astrocytes, while they enhance that in macrophages. Here, we report that the FSK-mediated inhibition of iNOS expression in C6 glial cells is due to its reduced transcriptional activity, while the FSK-mediated enhancement of iNOS expression in RAW264.7 macrophages is a result of increased stability of iNOS protein without transcriptional enhancement. The LPS/interferon-gamma (IFN)-induced iNOS transcription was inhibited by FSK via inhibition of p38-MAPK/ATF-2 activity in glial cells while it was not affected in macrophages. In both cell types, proteasome activities were required for the spontaneous degradation of iNOS protein, and the inhibition of proteasome activity by MG132 after maximum increase of iNOS protein levels further enhanced iNOS protein induction by LPS/IFN, suggesting the involvement of proteasome in iNOS degradation. More importantly, the iNOS protein levels were equalized by the MG132 posttreatment in macrophages treated with LPS/IFN alone and along with FSK, and ubiquitinated iNOS protein levels were reduced by FSK posttreatment, suggesting that the FSK-mediated inhibition of ubiquitination of iNOS protein and the following increased stability of iNOS protein are one of the mechanisms of cAMP-pathway-mediated enhancement of iNOS gene expression in macrophages. To our knowledge, this is the first evidence that cAMP regulates iNOS expression at the posttranslational level in macrophages.

A new anti-inflammatory agent KL-1037 represses proinflammatory cytokine and inducible nitric oxide synthase (iNOS) gene expression in activated microglia

Excessive proinflammatory cytokine and NO production by activated microglia play a role in neurodegenerative disorders. In this study, we found that a new compound KL-1037 suppressed LPS-induced NO release/inducible nitric oxide synthase expression in BV2 mouse microglial cells. In addition, KL-1037 prominently diminished LPS-induced production of pro-inflammatory cytokines such as TNF-alpha, IL-1 beta and IL-6, while it increased anti-inflammatory IL-10 and TGF-beta 1 production. By RNase protection assay and RT-PCR, we showed that KL-1037 regulated iNOS and cytokines at transcriptional or post-transcriptional level. Further analysis of molecular mechanisms revealed that KL-1037 prominently increased intracellular cAMP levels and potentiated LPS-induced pCREB expression. However, LPS-induced MAP kinase or NF-kappa B activities were slightly or little changed by KL-1037. Treatment with cAMP antagonist or IL-10 neutralizing antibody completely reversed upregulation of IL-10 and partially repression of TNF-alpha or NO induced by KL-1037. These data suggest that microglial inactivation by KL-1037 is at least in part due to activation of PKA pathway and/or upregulation of IL-10. Thus, repressing proinflammatory cytokines and iNOS gene expression in activated microglia by KL-1037 may provide potential therapeutic strategies for various neurodegenerative diseases including ischemic cerebral disease.

Inhibition of microglial inflammatory responses by norepinephrine: effects on nitric oxide and interleukin-1beta production

BACKGROUND: Under pathological conditions, microglia produce proinflammatory mediators which contribute to neurologic damage, and whose levels can be modulated by endogenous factors including neurotransmitters such as norepinephrine (NE). We investigated the ability of NE to suppress microglial activation, in particular its effects on induction and activity of the inducible form of nitric oxide synthase (NOS2) and the possible role that IL-1beta plays in that response. METHODS: Rat cortical microglia were stimulated with bacterial lipopolysaccharide (LPS) to induce NOS2 expression (assessed by nitrite and nitrate accumulation, NO production, and NOS2 mRNA levels) and IL-1beta release (assessed by ELISA). Effects of NE were examined by co-incubating cells with different concentrations of NE, adrenergic receptor agonists and antagonists, cAMP analogs, and protein kinase (PK) A and adenylate cyclase (AC) inhibitors. Effects on the NFkappaB:IkappaB pathway were examined by using selective a NFkappaB inhibitor and measuring IkappaBalpha protein levels by western blots. A role for IL-1beta in NOS2 induction was tested by examining effects of caspase-1 inhibitors and using caspase-1 deficient cells. RESULTS: LPS caused a time-dependent increase in NOS2 mRNA levels and NO production; which was blocked by a selective NFkappaB inhibitor. NE dose-dependently reduced NOS2 expression and NO generation, via activation of beta2-adrenergic receptors (beta2-ARs), and reduced loss of inhibitory IkBalpha protein. NE effects were replicated by dibutyryl-cyclic AMP. However, co-incubation with either PKA or AC inhibitors did not reverse suppressive effects of NE, but instead reduced nitrite production. A role for IL-1beta was suggested since NE potently blocked microglial IL-1beta production. However, incubation with a caspase-1 inhibitor, which reduced IL-1beta levels, had no effect on NO production; incubation with IL-receptor antagonist had biphasic effects on nitrite production; and NE inhibited nitrite production in caspase-1 deficient microglia. CONCLUSIONS: NE reduces microglial NOS2 expression and IL-1beta production, however IL-1beta does not play a critical role in NOS2 induction nor in mediating NE suppressive effects. Changes in magnitude or kinetics of cAMP may modulate NOS2 induction as well as suppression by NE. These results suggest that dysregulation of the central cathecolaminergic system may contribute to detrimental inflammatory responses and brain damage in neurological disease or trauma.

Attenuation of morphine tolerance, withdrawal-induced hyperalgesia, and associated spinal inflammatory immune responses by propentofylline in rats

The activation of glial cells and enhanced proinflammatory cytokine expression at the spinal cord has been implicated in the development of morphine tolerance, and morphine withdrawal-induced hyperalgesia. The present study investigated the effect of propentofylline, a glial modulator, on the expression of analgesic tolerance and withdrawal-induced hyperalgesia in chronic morphine-treated rats. Chronic morphine administration through repeated subcutaneous injection induced glial activation and enhanced proinflammatory cytokine levels at the lumbar spinal cord. Moreover, glial activation and enhanced proinflammatory cytokine levels exhibited a temporal correlation with the expression of morphine tolerance and hyperalgesia. Consistently, propentofylline attenuated the development of hyperalgesia and the expression of spinal analgesic tolerance to morphine. The administration of propentofylline during the induction of morphine tolerance also attenuated glial activation and proinflammatory cytokines at the L5 lumbar spinal cord. These results further support the hypothesis that spinal glia and proinflammatory cytokines contribute to the mechanisms of morphine tolerance and associated abnormal pain sensitivity.

Adenosine triphosphate inhibits cytokine release from lipopolysaccharide-activated microglia via P2y receptors

Microglial proliferation and activation have been reported to occur after several central nervous system injuries. In this study, we tested the effects of adenosine triphosphate (ATP) on cultured microglia obtained from the spinal cord of rat embryos. The amounts of tumor necrosis factor alpha (TNF-alpha), interleukin 1beta and interleukin 6 released from the microglia, which were stimulated by lipopolysaccharide (LPS; 100 ng/ml), were inhibited by the simultaneous addition of ATP in a dose dependent manner (10-300 microM). We examined the effect of several endogenous purines (ATP, ADP, CTP, UDP, UTP) and P(2)y receptor agonists (ADPbetaS and 2-methylthio-ATP) on LPS-induced TNF-alpha release. The rank order of inhibitory potency of endogenous purines on TNF-alpha release was: ATP>ADP>>UTP>UDP>CTP. The latter three were much less potent than the former two. The addition of 10 microM 2-methylthio-ATP showed a potency similar to 100 microM ATP. The addition of ADPbetaS, however, showed less effect. These endogenous purines and selective ATP receptor agonists showed a similar inhibitory effect in their rank order on LPS-induced interleukin 6 release. We demonstrate that ATP inhibits cytokine release from LPS-activated microglia via metabotropic receptors. The application of P(2)y receptor agonists might be considered as a pharmacological treatment of several pathological conditions of the spinal cord, including toxic immunoreactions.

Anti-hyperalgesic and morphine-sparing actions of propentofylline following peripheral nerve injury in rats: mechanistic implications of spinal glia and proinflammatory cytokines

Injury to peripheral nerves often produces non-physiological, long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to standard treatment and often insensitive to opioids, such as morphine. Recent studies demonstrate spinal glial activation and increased proinflammatory cytokines in animal models of neuropathic pain. When these data are considered together, a unifying hypothesis emerges which implicates a role of central neuroimmune processes in the etiology of neuronal and behavioral hypersensitivity. The present investigation assessed the influence of propentofylline, a glial modulating and anti-inflammatory agent, on the development of L5 spinal nerve transection-induced hyperalgesia and associated enhancement of spinal neuroimmune responses using real-time reverse transcription-polymerase chain reaction, RNase protection assay, enzyme-linked immunosorbent assay, and immunocytochemistry in rats. The results show that chronic propentofylline treatment attenuated the development of hyperalgesia and restored the analgesic activity of acute morphine in neuropathic rats. These findings directly correlated with the ability of propentofylline to inhibit glial activation and enhanced spinal proinflammatory cytokines following peripheral nerve injury. These findings along with our earlier observations of an anti-allodynic activity of propentofylline using the identical animal model of mononeuropathy supports the concept that modulation of glial and neuroimmune activation may be potential therapeutic targets to treat or prevent neuropathic pain. Further, restoration of the analgesic activity of morphine by propentofylline treatment suggests that increased glial activity and proinflammatory cytokine responses may account for the decreased analgesic efficacy of morphine observed in the treatment of neuropathic pain.

Delayed neuronal damage related to microglia proliferation after mild spinal cord compression injury

In order to investigate the mechanism of delayed progressive or secondary neuronal damage after the spinal cord injury, we developed a mild-compression injury model in the rat thoracic spinal cord. Our compression device consists of a soft silicone point of contact to the dura, in order to prevent violent injury that may cause axonal tears or hemorrhages in the spinal cord. Since rats often assume a 'standing' posture, i.e. raising head with lifting their fore-limbs, damage to the thoracic spinal cord was evaluated by measuring the frequency of 'standing', which effectively indicates hind limb function. Twenty-four hours after compression by a 20 g weight for 10 or 20 min, the standing frequency of the injured rat was almost the same as that of sham animals that underwent laminectomy without compression. However, the standing frequency decreased with time; the frequency of standing at 72 h was approximately 30-50% that of sham animals. In the compressed spinal cord tissue, microglial cells, detected by lectin staining, proliferated with time. An enormous amount of microglia was observed at 48 and 72 h after compression, although only a small amount of cells were positive to lectin staining at 24 h after the compression. These results suggest that our mild-compression spinal cord injury model showed late-onset or delayed neuronal damage that may be related to pathological microglia proliferation.

Selective modulation of lipopolysaccharide-stimulated cytokine expression and mitogen-activated protein kinase pathways by dibutyryl-cAMP in BV2 microglial cells

Cyclic AMP is a very important regulator in a wide range of biological processes, including inflammatory reactions. To investigate the role of cAMP in microglia, we examined the effect of dibutyryl-cAMP (dbcAMP) on lipopolysaccharide (LPS)-stimulated cytokine expression and signaling pathways in murine BV2 microglial cells. DbcAMP strongly suppressed LPS-induced TNF-alpha expression, without affecting NO, IL-6 or TGF-beta1 expression. In contrast, LPS-induced IL-1beta or IL-10 expressions were dramatically increased by dbcAMP. We further examined the effect of elevated cAMP on signaling molecules such as MAP kinases (p38 MAPK, ERK and JNK), NF-kappaB and AP1, which are involved in the regulation of inflammatory responses. DbcAMP decreased the LPS-induced phosphorylation of p38 MAPK, while it modestly enhanced the ERK activity. JNK phosphorylation was slightly reduced by dbcAMP only at the later time point. Electrophoretic mobility shift assay revealed that the elevated cAMP potentiated AP-1 binding activity by enhancing c-fos binding. On the other hand, dbcAMP repressed NF-kappaB-mediated transcription without affecting NF-kappaB binding. Treatment with H89, a selective inhibitor of protein kinase A, completely reversed cAMP-induced IL-10 and IL-1beta upregulation but only partially reversed the cAMP-induced repression of TNF-alpha. Thus, the effect of dbcAMP in BV2 cells appears to be mediated through both protein kinase A-dependent and -independent pathways. Taken together, our results demonstrate that cAMP modulates microglia activation in a diverse and complex manner.

Involvement of protein kinase C in glutamate release from cultured microglia

Glutamate release from microglial cells may cause neuronal damage. To elucidate the mechanism of glutamate release, we examined the possible regulation by nitric oxide and protein kinase C. Cultured microglia prepared from the whole brains of newborn rats released glutamate by the stimulation with lipopolysaccharide (LPS) dose dependently. The time course study revealed that glutamate release showed a long lag time about 6 h after LPS stimulation, whereas about 3 h lag time was observed in LPS-induced NO production. An inhibitor for NO synthase, N(G)-nitro-L-arginine, could effectively inhibit the glutamate release. Glutamate release induced by LPS was enhanced by 1 nM phorbol myristate acetate (PMA). Furthermore, high concentrations of PMA (>10 nM) induced glutamate release even without LPS stimulation. Glutamate release stimulated either by 100 ng/ml LPS or 100 nM PMA was inhibited by staurosporine, and also by alpha-aminoadipate. These results provide insight into the pathways regulating microglial pathological activation by protein kinase C and may be a base for the protection against microglia-evoked neurotoxicity.

Neuroinflammation and anti-inflammatory therapy for Alzheimer's disease

Neuroinflammation is now recognized as a prominent feature in Alzheimer's pathology and a potential target for therapy aimed at treatment and prevention of disease. This review provides a synopsis of current information about cellular and molecular mediators involved in Alzheimer's neuroinflammation as well as interactions between these mediators that influence pathology. Anti-inflammatory therapies, particularly nonsteroidal anti-inflammatory drugs, are considered from experimental and clinical perspectives and potential mechanisms underlying their apparent benefits are discussed. Finally, possible protective effects of the inflammatory response in Alzheimer's are described. Taken all together, evidence presented in this review suggests a scheme for Alzheimer's pathogenesis, with neuroinflammation playing a crucial role influencing and linking beta-amyloid deposition to neuronal damage and clinical disease.

Differential effects of lowering culture temperature on mediator release from lipopolysaccharide-stimulated neonatal rat microglia

OBJECTIVE

Therapeutic moderate hypothermia has the potential for neuronal protection against brain injury. Microglia, a type of immune-related cell in the brain, may play a certain role in neuronal damage subsequent to injury. We examined the effects of culture temperature changes from 37 degrees C to 33 degrees C or 30 degrees C on mediator release, including nitric oxide, interleukin-6, and tumor necrosis factor-alpha from lipopolysaccharide-stimulated microglia harvested from neonatal rats.

DESIGN

Laboratory study.

SETTING

University medical school.

SUBJECTS

Microglial cells isolated from primary cultures of rat brains.

INTERVENTIONS

The production of nitric oxide was measured by a nitrite accumulation method in a culture medium, whereas cytokines, interleukin-6, and tumor necrosis factor-alpha were measured by enzyme-linked immunosorbent assay.

MEASUREMENT AND MAIN RESULTS

At 30 degrees C and 33 degrees C, nitric oxide production stimulated by lipopolysaccharide decreased to 10 and 30% of control (37 degrees C), respectively, 24 hrs after the stimulation, and the decrease was sustained for 48 hrs. Interleukin-6 production at 30 degrees C and 33 degrees C was also reduced to 30% of control 6 hrs after the activation. Such responses lasted throughout the study. However, tumor necrosis factor-alpha release at 30 degrees C and 33 degrees C was depressed for only 6 hrs after stimulation, followed by subsequent elevation to concentrations similar to those at 37 degrees C. Microglial morphologic activation, showing changes from round to bipolar, reached a peak at 6 hrs in the 37 degrees C group, returning to round 12 hrs after lipopolysaccharide application. In 30 degrees C and 33 degrees C, the zenith was detected at 6 hrs, with activation remaining even 12 hrs after the stimulation, suggesting prolongation of the microglial response to lipopolysaccharide, which was inconsistent with changes in tumor necrosis factor release.

CONCLUSIONS

Decreasing culture temperature inhibits the production of nitric oxide and interleukin-6 from activated microglia. Differences were found in the degree or time course change between tumor necrosis factor-alpha and the other mediators. Also, the time course of morphologic changes in microglia was dependent on culture temperature. Further studies are required to define the mechanisms for such differences in mediator release from cooled microglia and also to clarify the inconsistency between morphologic change and its function in the cell.

Prostaglandin E1 analog inhibits the microglia function: suppression of lipopolysaccharide-induced nitric oxide and TNF-alpha release

Release of nitric oxide and TNF-alpha, a toxic cytokine, have been reported to accelerate neuronal damage under several pathological conditions, such as trauma or ischemia in the central nervous system. In the present study, we tested the effect of alprostadil alfadex, a prostaglandin E1 analog, on cultured microglia from the rat spinal cord. The cultured microglia were exposed to lipopolysaccharide (LPS) (100 ng/ml), an endotoxin, for 24 h, then the released nitric oxide and TNF-alpha in the culture media was analyzed. The released nitric oxide was detected by the Griess reaction and released TNF-alpha was measured using ELISA method. The LPS-induced nitric oxide release was inhibited by the simultaneous addition of alprostadil alfadex in a dose-dependent manner (0.1-100 microM). The LPS-induced TNF-alpha release was also inhibited by alprostadil alfadex addition (0.1-100 microM). The IC50 values of alprostadil alfadex on nitric oxide and TNF-alpha release were about 1 and 10 microM, respectively. These results suggest that prostaglandin E1 possibly protects spinal cord neurons from several types of neurodegenerative damage, not only via increased blood supply, but also via inhibition of pathological immunoreactions of activated microglia.