Endogenous GDNF Is Unable to Halt Dopaminergic Injury Triggered by Microglial Activation

Overactivation of microglial cells seems to play a crucial role in the degeneration of dopaminergic neurons occurring in Parkinson's disease. We have previously demonstrated that glial cell line-derived neurotrophic factor (GDNF) present in astrocytes secretome modulates microglial responses induced by an inflammatory insult. Therefore, astrocyte-derived soluble factors may include relevant molecular players of therapeutic interest in the control of excessive neuroinflammatory responses. However, in vivo, the control of neuroinflammation is more complex as it depends on the interaction between different types of cells other than microglia and astrocytes. Whether neurons may interfere in the astrocyte-microglia crosstalk, affecting the control of microglial reactivity exerted by astrocytes, is unclear. Therefore, the present work aimed to disclose if the control of microglial responses mediated by astrocyte-derived factors, including GDNF, could be affected by the crosstalk with neurons, impacting GDNF's ability to protect dopaminergic neurons exposed to a pro-inflammatory environment. Also, we aimed to disclose if the protection of dopaminergic neurons by GDNF involves the modulation of microglial cells. Our results show that the neuroprotective effect of GDNF was mediated, at least in part, by a direct action on microglial cells through the GDNF family receptor α-1. However, this protective effect seems to be impaired by other mediators released in response to the neuron-astrocyte crosstalk since neuron-astrocyte secretome, in contrast to astrocytes secretome, was unable to protect dopaminergic neurons from the injury triggered by lipopolysaccharide-activated microglia. Supplementation with exogenous GDNF was needed to afford protection of dopaminergic neurons exposed to the inflammatory environment. In conclusion, our results revealed that dopaminergic protective effects promoted by GDNF involve the control of microglial reactivity. However, endogenous GDNF is insufficient to confer dopaminergic neuron protection against an inflammatory insult. This reinforces the importance of further developing new therapeutic strategies aiming at providing GDNF or enhancing its expression in the brain regions affected by Parkinson's disease.

Development of a bioreactor system for cytotoxic evaluation of pharmacological compounds in living cells using NMR spectroscopy

INTRODUCTION

The evaluation of drug's cytotoxicity is a crucial step in the development of new pharmacological compounds. ³¹P NMR can be a tool for toxicological screening, as it enables the study of drugs' cytotoxicity and their effect on cell energy metabolism in a real-time, in a non- invasive and non-destructive way. This paper details a step-by-step protocol to implement a bioreactor system able to maintain cell viability during NMR acquisitions, at high cell densities and for several hours, enabling toxicological evaluation of pharmacological compounds in living cells.

METHOD

HeLa cells were immobilized in agarose gel threads and continuously perfused with oxygenated medium inside a 5 mm NMR tube. Signals corresponding to intracellular high-energy phosphorous compounds were continuously monitored by ³¹P NMR to assess cell energy levels, intracellular pH and intracellular free Mg²⁺ concentrations ([Mg²⁺]_f) under control and in the presence of two different cytotoxic drugs, calix-NH₂ or 5-fluorouracil (5-FU).

RESULTS

The bioreactor system was effective in maintaining cell energy levels as well as intracellular pH and [Mg²⁺]_f along time, with a good ³¹P NMR signal to noise ratio. Calix-NH₂ and 5-FU decreased cell energy levels by 35% and 39%, respectively, with a negligible increase in intracellular [Mg²⁺]_f, and without affecting intracellular pH.

DISCUSSION

The immobilization and perfusion system here detailed, along with ³¹P NMR, is useful in toxicological evaluation of new pharmacological compounds, enabling the continuous assessment of drugs' effect on energy levels, intracellular pH and [Mg²⁺]_f in intact cells, for several hours without compromising cell viability.

Iodine deficiency a persisting problem: assessment of iodine nutrition and evaluation of thyroid nodular pathology in Portugal

BACKGROUND

The goal of eliminating iodine deficiency (ID) by the year 2000 has still not been achieved in several countries. More than 2 billion people worldwide (over 260 million school age children) remain ID. In Europe, there are still countries, such as Portugal, without national general population data on iodine nutrition (IN). This study aims at evaluating combined complementary data of the IN of the general population through urinary iodine concentration (UIC) and the thyroid histology profile from the inland region of Beira Interior (BI), in Portugal.

METHODS

UIC from a population sample of 214 volunteers (131 females and 83 males), with ages ranging from 8 to 97 years (mean 51.5 years ± SD 20.74 years), from BI was determined; the thyroid histology pattern in BI (6-year period) was evaluated; and the iodine content of the largest surface water reservoir of BI, never previously reported, was measured.

RESULTS

Median UIC of 62.6 μg/L was measured. Over 92 % of the population had UIC less than 100 μg/L. From 279 histology reports evaluated, the incidence of the different types of thyroid nodular pathology in BI was established. There were 60 histologic diagnoses of malignancy. The observed ratio of papillary to follicular carcinoma relatively close to 1 and the fairly high percentage of anaplastic carcinomas are characteristic of ID areas.

CONCLUSIONS

The findings of this first general population study on IN from the inland region of BI, Portugal, document significant ID. This problem, with its serious public health implications, could be corrected by having affordable iodised salt widely and generally available and by promoting a proactive population attitude generated by ample public information and educational programs as to the negative consequences of ID.

GDNF contributes to oestrogen-mediated protection of midbrain dopaminergic neurones

Parkinson's disease (PD) is characterised by the preferential loss of dopaminergic neurones from the substantia nigra (SN) that leads to the hallmark motor disturbances. Animal and human studies suggest a beneficial effect of oestrogen to the nigrostriatal system, and the regulation of neurotrophic factor expression by oestrogens has been suggested as a possible mechanism contributing to that neuroprotective effect. The present study was designed to investigate whether the neuroprotection exerted by 17β-oestradiol on nigrostriatal dopaminergic neurones is mediated through the regulation of glial cell line-derived neurotrophic factor (GDNF) expression. Using an in vivo rat model of PD, we were able to confirm the relevance of 17β-oestradiol in defending dopaminergic neurones against 6-hydroxydopamine (6-OHDA) toxicity. 17β-oestradiol, released by micro-osmotic pumps, implanted 10 days before intrastriatal 6-OHDA injection, prevented the loss of dopaminergic neurones induced by 6-OHDA. 17β-oestradiol treatment also promoted an increase in GDNF protein levels both in the SN and striatum. To explore the relevance of GDNF increases to 17β-oestradiol neuroprotection, we analysed, in SN neurone-glia cultures, the effect of GDNF antibody neutralisation and RNA interference-mediated GDNF knockdown. The results showed that both GDNF neutralisation and GDNF silencing abolished the dopaminergic protection provided by 17β-oestradiol against 6-OHDA toxicity. Taken together, these results strongly identify GDNF as an important player in 17β-oestradiol-mediated dopaminergic neuroprotection.

13C NMR tracers in neurochemistry: implications for molecular imaging

An overview of 13C nuclear magnetic resonance (NMR) spectroscopy methods and their applications in the study of the metabolism of brain cells in vitro and in the in vivo brain is presented as well as their implications for modern molecular imaging techniques. Various topics will be discussed, such as general properties of the 13C NMR spectrum, 13C NMR spectroscopy acquisition protocols, determination of fractional 13C enrichment, 13C(2H) NMR methodologies, and the use of 13C hyperpolarized substrates for NMR spectroscopy and imaging. Some illustrative applications are described, both in vitro and in vivo.

The redox switch/redox coupling hypothesis

We provide an integrative interpretation of neuroglial metabolic coupling including the presence of subcellular compartmentation of pyruvate and monocarboxylate recycling through the plasma membrane of both neurons and glial cells. The subcellular compartmentation of pyruvate allows neurons and astrocytes to select between glucose and lactate as alternative substrates, depending on their relative extracellular concentration and the operation of a redox switch. This mechanism is based on the inhibition of glycolysis at the level of glyceraldehyde 3-phosphate dehydrogenase by NAD(+) limitation, under sufficiently reduced cytosolic NAD(+)/NADH redox conditions. Lactate and pyruvate recycling through the plasma membrane allows the return to the extracellular medium of cytosolic monocarboxylates enabling their transcellular, reversible, exchange between neurons and astrocytes. Together, intracellular pyruvate compartmentation and monocarboxylate recycling result in an effective transcellular coupling between the cytosolic NAD(+)/NADH redox states of both neurons and glial cells. Following glutamatergic neurotransmission, increased glutamate uptake by the astrocytes is proposed to augment glycolysis and tricarboxylic acid cycle activity, balancing to a reduced cytosolic NAD(+)/NADH in the glia. Reducing equivalents are transferred then to the neuron resulting in a reduced neuronal NAD(+)/NADH redox state. This may eventually switch off neuronal glycolysis, favoring the oxidation of extracellular lactate in the lactate dehydrogenase (LDH) equilibrium and in the neuronal tricarboxylic acid cycles. Finally, pyruvate derived from neuronal lactate oxidation, may return to the extracellular space and to the astrocyte, restoring the basal redox state and beginning a new loop of the lactate/pyruvate transcellular coupling cycle. Transcellular redox coupling operates through the plasma membrane transporters of monocarboxylates, similarly to the intracellular redox shuttles coupling the cytosolic and mitochondrial redox states through the transporters of the inner mitochondrial membrane. Finally, transcellular redox coupling mechanisms may couple glycolytic and oxidative zones in other heterogeneous tissues including muscle and tumors.

Tricarboxylic acid cycle inhibition by Li+ in the human neuroblastoma SH-SY5Y cell line: a 13C NMR isotopomer analysis

Li+ effects on glucose metabolism and on the competitive metabolism of glucose and lactate were investigated in the human neuroblastoma SH-SY5Y cell line using 13C NMR spectroscopy. The metabolic model proposed for glucose and lactate metabolism in these cells, based on tcaCALC best fitting solutions, for both control and Li+ conditions, was consistent with: (i) a single pyruvate pool; (ii) anaplerotic flux from endogenous unlabelled substrates; (iii) no cycling between pyruvate and oxaloacetate. Li+ was shown to induce a 38 and 53% decrease, for 1 and 15 mM Li+, respectively, in the rate of glucose conversion into pyruvate, when [U-13C]glucose was present, while no effects on lactate production were observed. Pyruvate oxidation by the tricarboxylic acid cycle and citrate synthase flux were shown to be significantly reduced by 64 and 84% in the presence of 1 and 15 mM Li+, respectively, suggesting a direct inhibitory effect of Li+ on tricarboxylic acid cycle flux. This work also showed that when both glucose and lactate are present as energetic substrates, SH-SY5Y cells preferentially consumed exogenous lactate over glucose, as 62% of the acetyl-CoA was derived from [3-13C]lactate while only 26% was derived from [U-13C]glucose. Li+ did not significantly affect the relative utilisation of these two substrates by the cells or the residual contribution of unlabelled endogenous sources for the acetyl-CoA pool.

Intracellular lithium and cyclic AMP levels are mutually regulated in neuronal cells

In this work, we studied the effect of intracellular 3',5'-cyclic adenosine monophosphate (cAMP) on Li+ transport in SH-SY5Y cells. The cells were stimulated with forskolin, an adenylate cyclase activator, or with the cAMP analogue, dibutyryl-cAMP. It was observed that under forskolin stimulation both the Li+ influx rate constant and the Li+ accumulation in these cells were increased. Dibutyryl-cAMP also increased Li+ uptake and identical results were obtained with cortical and hippocampal neurons. The inhibitor of the Na+/Ca2+ exchanger, KB-R7943, reduced the influx of Li+ under resting conditions, and completely inhibited the effect of forskolin on the accumulation of the cation. Intracellular Ca2+ chelation, or inhibition of N-type voltage-sensitive Ca2+ channels, or inhibition of cAMP-dependent protein kinase (PKA) also abolished the effect of forskolin on Li+ uptake. The involvement of Ca2+ on forskolin-induced Li+ uptake was confirmed by intracellular free Ca2+ measurements using fluorescence spectroscopy. Exposure of SH-SY5Y cells to 1 mm Li+ for 24 h increased basal cAMP levels, but preincubation with Li+, at the same concentration, decreased cAMP production in response to forskolin. To summarize, these results demonstrate that intracellular cAMP levels regulate the uptake of Li+ in a Ca(2+)-dependent manner, and indicate that Li+ plays an important role in the homeostasis of this second messenger in neuronal cells.

Effects of Li+ transport and intracellular binding on Li+/Mg2+ competition in bovine chromaffin cells

Li(+) transport, intracellular immobilisation and Li(+)/Mg(2+) competition were studied in Li(+)-loaded bovine chromaffin cells. Li(+) influx rate constants, k(i), obtained by atomic absorption (AA) spectrophotometry, in control (without and with ouabain) and depolarising (without and with nitrendipine) conditions, showed that L-type voltage-sensitive Ca(2+) channels have an important role in Li(+) uptake under depolarising conditions. The Li(+) influx apparent rate constant, k(iapp), determined under control conditions by (7)Li NMR spectroscopy with the cells immobilised and perfused, was much lower than the AA-determined value for the cells in suspension. Loading of cell suspensions with 15 mmol l(-1) LiCl led, within 90 min, to a AA-measured total intracellular Li(+) concentration, [Li(+)](iT)=11.39+/-0.56 mmol (l cells)(-1), very close to the steady state value. The intracellular Li(+) T(1)/T(2) ratio of (7)Li NMR relaxation times of the Li(+)-loaded cells reflected a high degree of Li(+) immobilisation in bovine chromaffin cells, similar to neuroblastoma, but larger than for lymphoblastoma and erythrocyte cells. A 52% increase in the intracellular free Mg(2+) concentration, Delta[Mg(2+)](f)=0.27+/-0.05 mmol (l cells)(-1) was measured for chromaffin cells loaded with the Mg(2+)-specific fluorescent probe furaptra, after 90-min loading with 15 mmol l(-1) LiCl, using fluorescence spectroscopy, indicating significant displacement of Mg(2+) by Li(+) from its intracellular binding sites. Comparison with other cell types showed that the extent of intracellular Li(+)/Mg(2+) competition at the same Li(+) loading level depends on intracellular Li(+) transport and immobilisation in a cell-specific manner, being maximal for neuroblastoma cells.

Effects of Li+ transport and Li+ immobilization on Li+/Mg2+ competition in cells: implications for bipolar disorder

Li(+)/Mg(2+) competition has been implicated in the therapeutic action of Li(+) treatment in bipolar illness. We hypothesized that this competition depended on cell-specific properties. To test this hypothesis, we determined the degree of Li(+) transport, immobilization, and Li(+)/Mg(2+) competition in lymphoblastomas, neuroblastomas, and erythrocytes. During a 50 mM/L Li(+)-loading incubation, Li(+) accumulation at 30 min (mmoles Li(+)/L cells) was the greatest in lymphoblastomas (11.1+/-0.3), followed by neuroblastomas (9.3+/-0.5), and then erythrocytes (4.0+/-0.5). Li(+) binding affinities to the plasma membrane in all three cell types were of the same order of magnitude; however, Li(+) immobilization in intact cells was greatest in neuroblastomas and least in erythrocytes. When cells were loaded for 30 min in a 50 mM/L Li(+)-containing medium, the percentage increase in free intracellular [Mg(2+)] in neuroblastoma and lymphoblastoma cells ( approximately 55 and approximately 52%, respectively) was similar, but erythrocytes did not exhibit any substantial increase ( approximately 6%). With the intracellular [Li(+)] at 15 mM/L, the free intracellular [Mg(2+)] increased by the greatest amount in neuroblastomas ( approximately 158%), followed by lymphoblastomas ( approximately 75%), and then erythrocytes ( approximately 50%). We conclude that Li(+) immobilization and transport are related to free intracellular [Mg(2+)] and to the extent of Li(+)/Mg(2+) competition in a cell-specific manner.