A novel and simple fluorescence method for the measurement of presynaptic vesicular zinc release in acute hippocampal slices with a fluorescence plate reader

Microglial reprogramming by Hv1 antagonism protects neurons from inflammatory and glutamate toxicity

Although the precise mechanisms determining the neurotoxic or neuroprotective activation phenotypes in microglia remain poorly characterized, metabolic changes in these cells appear critical for these processes. As cellular metabolism can be tightly regulated by changes in intracellular pH, we tested whether pharmacological targeting of the microglial voltage-gated proton channel 1 (Hv1), an important regulator of intracellular pH, is critical for activated microglial reprogramming. Using a mouse microglial cell line and mouse primary microglia cultures, either alone, or co-cultured with rat cerebrocortical neurons, we characterized in detail the microglial activation profile in the absence and presence of Hv1 inhibition. We observed that activated microglia neurotoxicity was mainly attributable to the release of tumor necrosis factor alpha, reactive oxygen species, and zinc. Strikingly, pharmacological inhibition of Hv1 largely abrogated inflammatory neurotoxicity not only by reducing the production of cytotoxic mediators but also by promoting neurotrophic molecule production and restraining excessive phagocytic activity. Importantly, the Hv1-sensitive change from a pro-inflammatory to a neuroprotective phenotype was associated with metabolic reprogramming, particularly via a boost in NADH availability and a reduction in lactate. Most critically, Hv1 antagonism not only reduced inflammatory neurotoxicity but also promoted microglia-dependent neuroprotection against a separate excitotoxic injury. Our results strongly suggest that Hv1 blockers may provide an important therapeutic tool against a wide range of inflammatory neurodegenerative disorders.

Synaptic Zinc: An Emerging Player in Parkinson's Disease

Alterations of zinc homeostasis have long been implicated in Parkinson's disease (PD). Zinc plays a complex role as both deficiency and excess of intracellular zinc levels have been incriminated in the pathophysiology of the disease. Besides its role in multiple cellular functions, Zn²⁺ also acts as a synaptic transmitter in the brain. In the forebrain, subset of glutamatergic neurons, namely cortical neurons projecting to the striatum, use Zn²⁺ as a messenger alongside glutamate. Overactivation of the cortico-striatal glutamatergic system is a key feature contributing to the development of PD symptoms and dopaminergic neurotoxicity. Here, we will cover recent evidence implicating synaptic Zn²⁺ in the pathophysiology of PD and discuss its potential mechanisms of actions. Emphasis will be placed on the functional interaction between Zn²⁺ and glutamatergic NMDA receptors, the most extensively studied synaptic target of Zn²⁺.

Alzheimer risk factors age and female sex induce cortical Aβ aggregation by raising extracellular zinc

Aging and female sex are the major risk factors for Alzheimer's disease and its associated brain amyloid-β (Aβ) neuropathology, but the mechanisms mediating these risk factors remain uncertain. Evidence indicates that Aβ aggregation by Zn²⁺ released from glutamatergic neurons contributes to amyloid neuropathology, so we tested whether aging and sex adversely influences this neurophysiology. Using acute hippocampal slices, we found that extracellular Zn²⁺-elevation induced by high K⁺ stimulation was significantly greater with older (65 weeks vs 10 weeks old) rats, and was exaggerated in females. This was driven by slower reuptake of extracellular Zn²⁺, which could be recapitulated by mitochondrial intoxication. Zn²⁺:Aβ aggregates were toxic to the slices, but Aβ alone was not. Accordingly, high K⁺ caused synthetic human Aβ added to the slices to form soluble oligomers as detected by bis-ANS, attaching to neurons and inducing toxicity, with older slices being more vulnerable. Age-dependent energy failure impairing Zn²⁺ reuptake, and a higher maximal capacity for Zn²⁺ release by females, could contribute to age and sex being major risk factors for Alzheimer's disease.

How zinc ions shift and enhance the nucleotide's fluorescence spectra

Zinc–nucleotide complexes in alkaline solutions exhibit strong fluorescence enhancement and red shift, thus enabling an optical discrimination between the nucleotides.

Modulation of neuronal signal transduction and memory formation by synaptic zinc

The physiological role of synaptic zinc has remained largely enigmatic since its initial detection in hippocampal mossy fibers over 50 years ago. The past few years have witnessed a number of studies highlighting the ability of zinc ions to regulate ion channels and intracellular signaling pathways implicated in neuroplasticity, and others that shed some light on the elusive role of synaptic zinc in learning and memory. Recent behavioral studies using knock-out mice for the synapse-specific zinc transporter ZnT-3 indicate that vesicular zinc is required for the formation of memories dependent on the hippocampus and the amygdala, two brain centers that are prominently innervated by zinc-rich fibers. A common theme emerging from this research is the activity-dependent regulation of the Erk1/2 mitogen-activated-protein kinase pathway by synaptic zinc through diverse mechanisms in neurons. Here we discuss current knowledge on how synaptic zinc may play a role in cognition through its impact on neuronal signaling.

A novel application of the fluorescent dye bis-ANS for labeling neurons in acute brain slices

The cell-impermeant oligomer-(e.g. beta-amyloid-, or tubulin-) specific fluorescent dye, bis-ANS (4,4'-bis-1-anilinonaphtalene-8-sulfonate), was successfully used for labeling mechanically damaged but still viable neuron bodies, neurites and neurite cross sections in acute brain slices. Acute hippocampal brain slices of rats were co-stained with bis-ANS and the cell-impermeant, DNA-specific dye propidium iodide (PI) and were then analyzed using fluorescence and confocal microscopes. Both the neuron bodies and the neurites were found to exhibit increased fluorescence intensities, suggesting that using this method they can be detected more easily. In addition, bis-ANS showed good region - but not cell specific co-localization with the neuron-specific fluorescent dye Dil (1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). These two dyes label different neuronal structures: Dil binds specifically to intact cell membranes while bis-ANS can enter cells with compromised cell membranes and then stain the microtubules in the cytoplasm. For a quick (10min) staining of acute brain slices with bis-ANS both HEPES and NaHCO(3) were needed in order to achieve high signal intensity. Labeling with bis-ANS fluorescent dye is an easy method for imaging the neuronal structures on the surface of acute brain slices.

ZnT3 mRNA levels are reduced in Alzheimer's disease post-mortem brain

Background

ZnT3 is a membrane Zn²⁺ transporter that is responsible for concentrating Zn²⁺ into neuronal presynaptic vesicles. Zn²⁺ homeostasis in the brain is relevant to Alzheimer's disease (AD) because Zn²⁺ released during neurotransmission may bind to Aβ peptides, accelerating the assembly of Aβ into oligomers which have been shown to impair synaptic function.

Results

We quantified ZnT3 mRNA levels in Braak-staged human post mortem (pm) brain tissue from medial temporal gyrus, superior occipital gyrus, superior parietal gyrus, superior frontal gyrus and cerebellum from individuals with AD (n = 28), and matched controls (n = 5) using quantitative real-time PCR. ZnT3 mRNA levels were significantly decreased in all four cortical regions examined in the AD patients, to 45-60% of control levels. This reduction was already apparent at Braak stage 4 in most cortical regions examined. Quantification of neuronal and glial-specific markers in the same samples (neuron-specific enolase, NSE; and glial fibrillary acidic protein, GFAP) indicated that loss of cortical ZnT3 expression was more pronounced, and occurred prior to, significant loss of NSE expression in the tissue. Significant increases in cortical GFAP expression were apparent as the disease progressed. No gene expression changes were observed in the cerebellum, which is relatively spared of AD neuropathology.

Conclusions

This first study to quantify ZnT3 mRNA levels in human pm brain tissue from individuals with AD and controls has revealed a significant loss of ZnT3 expression in cortical regions, suggesting that neuronal cells in particular show reduced expression of ZnT3 mRNA in the disease. This suggests that altered neuronal Zn²⁺ handling may be an early event in AD pathogenesis.

Contributions of Ca2+ and Zn2+ to spreading depression-like events and neuronal injury

The phenomenon of spreading depression (SD) involves waves of profound neuronal and glial depolarization that spread throughout brain tissue. Under many conditions, tissue recovers full function after SD has occurred, but SD-like events are also associated with spread of injury following ischemia or trauma. Initial large cytosolic Ca2+ increases accompany all forms of SD, but persistently elevated Ca2+ loading is likely responsible for neuronal injury following SD in tissues where metabolic capacity is insufficient to restore ionic gradients. Ca2+ channels are also involved in the propagation of SD, but the channel subtypes and cation fluxes differ significantly when SD is triggered by different types of stimuli. Ca2+ influx via P/Q type channels is important for SD generated by localized application of high K+ solutions. In contrast, SD-like events recorded in in vitro ischemia models are not usually prevented by Ca2+ removal, but under some conditions, Zn2+ influx via L-type channels contributes to SD initiation. This review addresses different roles of Ca2+ in the initiation and consequences of SD, and discusses recent evidence that selective chelation of Zn2+ can be sufficient to prevent SD under circumstances that may have relevance for ischemic injury.