A pathogenetic hypothesis of temporal lobe epilepsy

Platelet biomarkers in Alzheimer’s disease

The search for diagnostic and prognostic markers in Alzheimer’s disease (AD) has been an area of active research in the last decades. Biochemical markers are correlates of intracerebral changes that can be identified in biological fluids, namely: peripheral blood (total blood, red and white blood cells, platelets, plasma and serum), saliva, urine and cerebrospinal fluid. An important feature of a biomarker is that it can be measured objectively and evaluated as (1) an indicator of disease mechanisms (markers of core pathogenic processes or the expression of downstream effects of these processes), or (2) biochemical responses to pharmacological or therapeutic intervention, which can be indicative of disease modification. Platelets have been used in neuropharmacological models since the mid-fifties, as they share several homeostatic functions with neurons, such as accumulation and release of neurotransmitters, responsiveness to variations in calcium concentration, and expression of membrane-bound compounds. Recent studies have shown that platelets also express several components related to the pathogenesis of AD, in particular to the amyloid cascade and the regulation of oxidative stress: thus they can be used in the search for biomarkers of the disease process. For instance, platelets are the most important source of circulating forms of the amyloid precursor protein and other important proteins such as Tau and glycogen synthase kinase-3B. Moreover, platelets express enzymes involved in membrane homeostasis (e.g., phospholipase A₂), and markers of the inflammatory process and oxidative stress. In this review we summarize the available literature and discuss evidence concerning the potential use of platelet markers in AD.

A radioenzymatic assay to identify three groups of phospholipase A(2) in platelets

Phospholipases A(2) (PLA(2)) are key enzymes in membrane metabolism. The release of fatty acids and lysophospholipids by PLA(2) activates several intra-cellular second messenger cascades that regulate a wide variety of physiological responses. The aim of the present study is to describe a radioenzymatic assay to determine the activity of three main PLA(2) subtypes in platelets, namely extracellular calcium-dependent PLA(2) (sPLA(2)) and intracellular calcium-dependent (cPLA(2)) and calcium-independent PLA(2) (iPLA(2)). The differentiation of these distinct PLA(2) subtypes was based on the enzyme substrate preference (arachdonic acid or palmitoyl acid) and calcium concentration. Our results indicate that this new assay is feasible, precise and specific to measure the activity of the aforementioned subtypes of PLA(2). Therefore, this protocol can be used to investigate modifications of PLA(2) homeostasis in distinct biological models addressing the pathophysiology of many medical and neuropsychiatric disorders such as schizophrenia and Alzheimer's disease.

Increased PLA2 activity in the hippocampus of patients with temporal lobe epilepsy and psychosis

OBJECTIVE

The aim of this work was to investigate whether increased activity of the enzyme phospholipase A(2) (PLA(2)) in the brain, as frequently reported in schizophrenia, is also related to psychosis in epilepsy. Our working hypothesis was based on the increased prevalence of schizophrenia-like psychosis in patients with temporal lobe epilepsy (TLE) secondary to mesial temporal sclerosis (MTS), as compared to patients with other forms of epilepsy.

METHODS

We determined PLA(2) activity in hippocampal tissue from 7 patients with TLE-MTS and psychosis, as compared to 9 TLE-MTS patients without psychosis. Hippocampal tissue was obtained from patients who underwent an anterior temporal lobectomy due to therapy-resistant epilepsy.

RESULTS

We found that patients with TLE-MTS and psychosis had a significantly increased calcium-independent PLA(2) activity as compared to patients without psychosis (p = 0.016).

CONCLUSION

Our finding suggest that an increment in brain PLA(2) activity is not specific for schizophrenia, but rather may be associated to the manifestation of schizophrenia-like psychotic symptoms in general.

Pentylenetetrazol kindling alters adenine and guanine nucleotide catabolism in rat hippocampal slices and cerebrospinal fluid

Pentylenetetrazol (PTZ) is commonly used as a convulsant drug. The enhanced seizure susceptibility induced by kindling is probably attributable to plastic changes in the synaptic efficacy. Adenosine and guanosine act both as important neuromodulators and neuroprotectors with mostly inhibitory effects on neuronal activity. Adenosine and guanosine can be released per se or generated from released nucleotides (ATP, ADP, AMP, GTP, GDP, and GMP) that are metabolized and rapidly converted to adenosine and guanosine. The aim of this study was to evaluate nucleotide hydrolysis by ecto- and soluble nucleotidases (hippocampal slices and CSF, respectively) after PTZ-kindling (stages 3, 4, or 5 seizures) or saline treatment in rats. Additionally, the levels of purines in rat cerebrospinal fluid (CSF), as well as ecto-NTPDases (1, 2, 3, 5, 6 and 8) and ecto- 5'-nucleotidase expression were determined. Ecto-enzyme assays demonstrated that ATP, AMP, GDP, and GMP hydrolysis enhanced when compared with controls. In addition, there was an increase of ADP, GDP, and GMP hydrolysis by soluble nucleotidases in PTZ-kindling rats compared to control group. The HPLC analysis showed a marked increase in PTZ-kindled CSF concentrations of GTP, ADP, and uric acid, but GDP, AMP, and hypoxanthine concentrations were decreased. Such alterations indicate that the modulatory role of purines in CNS could be affected by PTZ-kindling. However, the physiological significance of these findings remains to be elucidated.

Inhibition of phospholipase A2 reduces neurite outgrowth and neuronal viability

Phospholipase A2 (PLA(2)) has been implicated in neurodevelopmental processes and in the early development of the nervous system. We investigated the effects of the inhibition of calcium-dependent and calcium-independent subtypes of cytosolic PLA2 (cPLA2 and iPLA2) on the development and viability of primary cultures of cortical and hippocampal neurons. PLA2 in these cultures was continuously inhibited with methylarachidonyl-fluorophosphonate (MAFP), an irreversible inhibitor of cPLA2 and iPLA2, or with bromoenol lactone (BEL), an irreversible selective iPLA2 inhibitor. The effect of PLA2 inhibitors on the development of neuronal cultures was ascertained by total cell count and morphological characterisation. Neuronal viability was quantified with MTT assays. Inhibition of PLA2 resulted in reduction of neuritogenesis and neuronal viability, disrupting neuronal homeostasis and leading to neuronal death. We conclude that the functional integrity of both calcium-dependent and calcium-independent cytosolic PLA2 is necessary for the in vitro development of cortical and hippocampal neurons.

Diacylglycerol kinase epsilon modulates rapid kindling epileptogenesis

PURPOSE

Diacylglycerol kinase epsilon (DGKepsilon) regulates seizure susceptibility and long-term potentiation through arachidonoyl-inositol lipid signaling. We studied the significance of arachidonoyl-diacylglycerol (20:4 DAG) in epileptogenesis in DGKepsilon-deficient mice undergoing rapid kindling epileptogenesis.

METHODS

Tripolar electrode units were implanted in right dorsal hippocampi of male DGKepsilon(+/+) and DGKepsilon(-/-) mice. Ten days after surgery, kindling was achieved by stimulating 6 times daily for 4 days with a subconvulsive electrical stimulation (10-s train of 50-Hz biphasic pulses, 75-200 muA amplitude) at 30-min intervals. After 1 week, mice were rekindled. EEGs were recorded and analyzed to characterize epileptogenic events as spikes, sharp waves, or abnormal amplitudes and rhythms. Right hippocampi were analyzed by histology [Timm's staining, neuropeptide Y (NPY) and glial fibrillary acidic protein immunoreactivity], and for DNA fragmentation (TUNEL).

RESULTS

DGKepsilon(-/-) mice had significantly fewer motor seizure and epileptic events compared with DGKepsilon(+/+) mice from the second day of stimulation. These differences were maintained during rekindling. DGKepsilon(-/-) mice also exhibited low-amplitude spike-wave complexes, short spreading depression, and predominant lower-frequency (1-4 Hz) bands throughout stimulation, whereas DGKepsilon(+/+) mice exhibited increased high-frequency bands (4-8 Hz; 8-15 Hz) from the second day of stimulation, as determined by power spectral analysis. DGKepsilon(-/-) mice displayed no sprouting in the supragranular area or NPY inmunoreactivity in the hilus and had weak astrocyte reactivation in all hippocampal areas. No TUNEL-positive cells were detected in any group of mice.

CONCLUSIONS

DGKepsilon modulates kindling epileptogenesis through inositol lipid signaling. Because arachidonate-containing diacylglycerol phosphorylation to phosphatidic acid is selectively blocked in DGKepsilon(-/-) mice, we postulate that the shortage of arachidonoyl-moiety inositol lipids and/or the messengers derived thereof is a key signaling event in epileptogenesis.

Modulation of phospholipase A2 activity in primary cultures of rat cortical neurons

In neurons, phospholipase A2 (PLA2) plays a central role in the regulation of membrane phospholipid metabolism. We have addressed the pharmacological modulation of PLA2 in primary cultures of rat cortical neurons. Inhibition curves were obtained in 4 day-in-culture neurons treated for 30 minutes with either the dual PLA2 inhibitor methyl arachidonyl fluorophosphonate (MAFP), or the iPLA2 inhibitor bromoenol lactone (BEL). Full inhibition was achieved with 100 and 250 microM of MAFP, or 10 and 20 microM of BEL. Conversely, a dose-dependent activation of PLA2 was obtained with 10-20 microg/ml of melitin. PLA2 inhibition with MAFP or BEL was not acutely toxic for cultured neurons. However, sustained inhibition of the enzyme precluded the development of neurites, and resulted in long-term loss of neuronal viability. We present a model of pharmacological challenge of PLA2 in vitro, which can be further used to address the involvement of the enzyme in neurodevelopment and neurodegeneration models.

Nonobligate role of early or sustained expression of immediate-early gene proteins c-fos, c-jun, and Zif/268 in hippocampal mossy fiber sprouting

Axon sprouting in dentate granule cells is an important model of structural plasticity in the hippocampus. Although the process can be triggered by deafferentation, intense activation of glutamate receptors, and other convulsant stimuli, the specific molecular steps required to initiate and sustain mossy fiber (MF) reorganization are unknown. The cellular immediate early genes (IEGs) c-fos, c-jun, and zif/268 are major candidates for the initial steps of this plasticity, because they encode transcription factors that may trigger cascades of activity-dependent neuronal gene expression and are strongly induced in all experimental models of MF sprouting. The mutant mouse stargazer offers an important opportunity to test the specific role of IEGs, because it displays generalized nonconvulsive epilepsy and intense MF sprouting in the absence of regional cell injury. Here we report that stargazer mice show no detectable elevations in c-Fos, c-Jun, or Zif/268 immediate early gene proteins (IEGPs) before or during MF growth. Experimental results in stargazer, including (1) a strong IEGP response to kainate-induced convulsive seizures, (2) no IEGP response after prolongation of spike-wave synchronization, (3) no IEGP increase at the developmental onset of seizures or after prolonged seizure suppression, and (4) unaltered levels of the intracellular Ca2+-buffering proteins calbindin-D28k or parvalbumin, exclude the possibility that absence of an IEGP response in stargazer is either gene-linked or suppressed by known refractory mechanisms. These data demonstrate that increased levels of these IEGPs are not an obligatory step in MF-reactive sprouting and differentiate the early downstream molecular cascades of two major seizure types.

Phospholipase A2-induced neurotoxicity in vitro and in vivo in rats

The present study evaluated the neurotoxic potential of phospholipase A2 (PLA2) in in vitro (primary neuronal cultures) and in vivo (EEG and behavior) rat models of CNS excitability. In vitro, PLA2 (0.0038-5.8 nM) or melittin (a potent activator of endogenous PLA2; 100-5000 nM), were highly neurotoxic, causing approximately 500 units/ml LDH release. The neurotoxic EC50s for PLA2 and melittin were 1.8 (1.4-2.3) and 848 (501-1280) nM, respectively. Neurotoxic concentrations of PLA2 stimulated neuronal release of [3H]AA. Preliminary in vitro experiments evaluating changes in neuronal calcium flux indicated that PLA2 caused transient, and melittin sustained, increases in [Ca2+]i. In vivo, PLA2 (0.5-5 micrograms i.c.v.) or melittin (2.5-20 micrograms i.c.v.) produced nonconvulsive EEG seizures, which generalized to status epilepticus. While the onset of seizure development was markedly delayed for PLA2 (1.5-4.5 h), the seizure inducing effects of melittin were evident within 3.5 +/- 0.2 min and more severe. Both PLA2 and melittin were lethal, exhibiting LD50s of 0.62 micrograms and 8.4 micrograms, respectively. Pretreatment with (+)-MK801 (5 micrograms, i.c.v.) significantly attenuated melittin, but not PLA2, in vivo neurotoxicity. PLA2 induced neuropathology in surviving rats revealed extensive cortical and subcortical injury to forebrain neurons and fibre pathways. Collectively, these results demonstrate the potent neurotoxic potential of PLA2, the delayed clinical nature of its in vivo neurotoxicity and the applicability of these model systems to future studies on mechanisms of PLA2 neurotoxicity and the development of potential PLA2 antagonists.