Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and PACAP4-6 are neuroprotective through inhibition of NADPH oxidase: potent regulators of microglia-mediated oxidative stress

PACAP38 increases vesicular monoamine transporter 2 (VMAT2) expression and attenuates methamphetamine toxicity

Pituitary adenylyl cyclase activating polypeptide, 38 amino acids (PACAP38) is a brain-gut peptide with diverse physiological functions and is neuroprotective in several models of neurological disease. In this study, we show that systemic administration of PACAP38, which is transported across the blood-brain barrier, greatly reduces the neurotoxicity of methamphetamine (METH). Mice treated with PACAP38 exhibited an attenuation of striatal dopamine loss after METH exposure as well as greatly reduced markers of oxidative stress. PACAP38 treatment also prevented striatal neuroinflammation after METH administration as measured by overexpression of glial fibrillary acidic protein (GFAP), an indicator of astrogliosis, and glucose transporter 5 (GLUT5), a marker of microgliosis. In PACAP38 treated mice, the observed protective effects were not due to an altered thermal response to METH. Since the mice were not challenged with METH until 28 days after PACAP38 treatment, this suggests the neuroprotective effects are mediated by regulation of gene expression. At the time of METH administration, PACAP38 treated animals exhibited a preferential increase in the expression and function of the vesicular monoamine transporter (VMAT2). Genetic reduction of VMAT2 has been shown to increase the neurotoxicity of METH, thus we propose that the increased expression of VMAT2 may underlie the protective actions of PACAP38 against METH. The ability of PACAP38 to increase VMAT2 expression suggests that PACAP38 signaling pathways may constitute a novel therapeutic approach to treat and prevent disorders of dopamine storage.

Formyl-methionyl-leucyl-phenylalanine-induced dopaminergic neurotoxicity via microglial activation: a mediator between peripheral infection and neurodegeneration?

BACKGROUND

Parkinson disease (PD), a chronic neurodegenerative disease, has been proposed to be a multifactorial disorder resulting from a combination of environmental mechanisms (chemical, infectious, and traumatic), aging, and genetic deficits. Microglial activation is important in the pathogenesis of PD.

OBJECTIVES

We investigated dopaminergic (DA) neurotoxicity and the underlying mechanisms of formyl-methionyl-leucyl-phenylalanine (fMLP), a bacteria-derived peptide, in relation to PD.

METHODS

We measured DA neurotoxicity using a DA uptake assay and immunocytochemical staining (ICC) in primary mesencephalic cultures from rodents. Microglial activation was observed via ICC, flow cytometry, and superoxide measurement.

RESULTS

fMLP can cause selective DA neuronal loss at concentrations as low as 10(-13) M. Further, fMLP (10(-13) M) led to a significant reduction in DA uptake capacity in neuron/glia (N/G) cultures, but not in microglia-depleted cultures, indicating an indispensable role of microglia in fMLP-induced neurotoxicity. Using ICC of a specific microglial marker, OX42, we observed morphologic changes in activated microglia after fMLP treatment. Microglial activation after fMLP treatment was confirmed by flow cytometry analysis of major histocompatibility antigen class II expression on a microglia HAPI cell line. Mechanistic studies revealed that fMLP (10(-13) M)-induced increase in the production of extracellular superoxide from microglia is critical in mediating fMLP-elicited neurotoxicity. Pharmacologic inhibition of NADPH oxidase (PHOX) with diphenylene-iodonium or apocynin abolished the DA neurotoxicity of fMLP. N/G cultures from PHOX-deficient (gp91PHOX-/ -) mice were also insensitive to fMLP-induced DA neurotoxicity.

CONCLUSION

fMLP (10(-13) M) induces DA neurotoxicity through activation of microglial PHOX and subsequent production of superoxide, suggesting a role of fMLP in the central nervous system inflammatory process.

Curcumin protects dopaminergic neuron against LPS induced neurotoxicity in primary rat neuron/glia culture

Using primary rat mesencephalic neuron-glia cultures as an in vitro model of Parkinson's disease (PD), we tested the effect of curcumin, a natural dietary pigment with well-known anti-inflammation effects, on dopaminergic (DA) degeneration. Curcumin pretreatment mitigated LPS-induced DA neurotoxicity in a concentration-dependent manner and curcumin post-treatment also showed protective effect. Microglia depletion abolished this protective effect of curcumin, indicating that microglia play an important role in this effect. Supportively, observation by immunocytochemistry staining using OX-42 antibody showed that curcumin treatment inhibited LPS-induced morphological change of microglia. Besides, LPS-induced production of many proinflammatory factors and their gene expressions decreased dramatically after curcumin treatment. Results also revealed that curcumin treatment decreased LPS-induced activation of two transcription factors--nuclear factors kappaB (NF-kappaB) and activator protein-1 (AP-1). Taken together, our study implicated that curcumin might be a potential preventive and therapeutic strategy for inflammation-related neurodegenerative diseases.

Sinomenine, a natural dextrorotatory morphinan analog, is anti-inflammatory and neuroprotective through inhibition of microglial NADPH oxidase

Background

The mechanisms involved in the induction and regulation of inflammation resulting in dopaminergic (DA) neurotoxicity in Parkinson's disease (PD) are complex and incompletely understood. Microglia-mediated inflammation has recently been implicated as a critical mechanism responsible for progressive neurodegeneration.

Methods

Mesencephalic neuron-glia cultures and reconstituted cultures were used to investigate the molecular mechanisms of sinomenine (SN)-mediated anti-inflammatory and neuroprotective effects in both the lipopolysaccharide (LPS)- and the 1-methyl-4-phenylpyridinium (MPP⁺)-mediated models of PD.

Results

SN showed equivalent efficacy in protecting against DA neuron death in rat midbrain neuron-glial cultures at both micro- and sub-picomolar concentrations, but no protection was seen at nanomolar concentrations. The neuroprotective effect of SN was attributed to inhibition of microglial activation, since SN significantly decreased tumor necrosis factor-α (TNF-α, prostaglandin E₂ (PGE₂) and reactive oxygen species (ROS) production by microglia. In addition, from the therapeutic point of view, we focused on sub-picomolar concentration of SN for further mechanistic studies. We found that 10^-14 M of SN failed to protect DA neurons against MPP⁺-induced toxicity in the absence of microglia. More importantly, SN failed to show a protective effect in neuron-glia cultures from mice lacking functional NADPH oxidase (PHOX), a key enzyme for extracellular superoxide production in immune cells. Furthermore, we demonstrated that SN reduced LPS-induced extracellular ROS production through the inhibition of the PHOX cytosolic subunit p47^phoxtranslocation to the cell membrane.

Conclusion

Our findings strongly suggest that the protective effects of SN are most likely mediated through the inhibition of microglial PHOX activity. These findings suggest a novel therapy to treat inflammation-mediated neurodegenerative diseases.

Neuroprotection: a comparative view of vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide

The neuroprotective properties of vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) place these peptides in a special category of ligands that have implications for our understanding of pathological conditions as well as a potential basis for therapeutic intervention. It is remarkable that these peptides have a protective impact against such a wide variety of clinical relevant toxic substances. This protective diversity is consistent with the multiple pathways that are activated or inhibited by the action of these peptides. Although knowledge is emerging on the neuroprotective mechanisms of VIP and PACAP, it is already evident that these two peptides are not identical in their action and each peptide has multiple mechanisms that allow for neuroprotective diversity. The multiple intracellular signaling pathways and differing extracellular mediators of neuroprotection contribute to this diversity of action. In this review, examples of neuroprotective actions will be presented that serve to demonstrate the remarkable breadth of neuroprotective processes produced by VIP and PACAP.

Cadmium-induced toxicity in rat primary mid-brain neuroglia cultures: role of oxidative stress from microglia

This study examined the role of oxidative stress in neurotoxic effects of cadmium chloride (Cd) in rat primary mid-brain neuron-glia cultures. Cd accumulated in neuron-glia cultures and produced cytotoxicity in a dose-dependent manner, with IC(50) of 2.5microM 24 h after exposure. (3)H-dopamine uptake into neuron-glia cultures was decreased 7 days after Cd exposure, with IC(50) of 0.9microM, indicative of the sensitivity of dopaminergic neurons to Cd toxicity. To investigate the role of microglia in Cd-induced toxicity to neurons, microglia-enriched cultures were prepared. Cd significantly increased intracellular reactive oxygen species production in microglia-enriched cultures, as evidenced by threefold increases in 2',7'-dichlorofluorescein signals. Using 5,5-dimethyl-1-pyrroline N-oxide as a spin-trapping agent, Cd increased electron spin resonance signals by 3.5-fold in microglia-enriched cultures. Cd-induced oxidative stress to microglia-enriched cultures was further evidenced by activation of redox-sensitive transcription factor nuclear factor kappa B and activator protein-1 (AP-1), and the increased expression of oxidative stress-related genes, such as metallothionein, heme oxygenase-1, glutathione S-transferase pi, and metal transport protein-1, as determined by gel-shift assays and real-time reverse transcription-PCR, respectively, in microglia-enriched cultures. In conclusion, Cd is toxic to neuron-glia cultures, and the oxidative stress from microglia may play important roles in Cd-induced damage to dopaminergic neurons.