Specific inhibition of the phosphorylation of protein I, a synaptic protein, by affinity-purified anti-protein I antibody

Glycolytic lactate production supports status epilepticus in experimental animals

OBJECTIVE

Status epilepticus (SE) requires rapid intervention to prevent cerebral injury and mortality. The ketogenic diet, which bypasses glycolysis, is a promising remedy for patients with refractory SE. We tested the role of glycolytic lactate production in sustaining SE.

METHODS

Extracellular lactate and glucose concentration during a seizure and SE in vivo was measured using lactate and glucose biosensors. A lactate dehydrogenase inhibitor, oxamate, blocked pyruvate to lactate conversion during SE. Video-EEG recordings evaluated seizure duration, severity, and immunohistochemistry was used to determine neuronal loss. Genetically encoded calcium indicator GCaMP7 was used to study the effect of oxamate on CA1 pyramidal neurons in vitro. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from CA1 neurons to study oxamate's impact on neurotransmission.

RESULTS

The extracellular glucose concentration dropped rapidly during seizures, and lactate accumulated in the extracellular space. Inhibition of pyruvate to lactate conversion with oxamate terminated SE in mice. There was less neuronal loss in treated compared to control mice. Oxamate perfusion decreased tonic and phasic neuronal activity of GCaMP7-expressing CA1 pyramidal neurons in vitro. Oxamate application reduced the frequency, but not amplitude of sEPSCs recorded from CA1 neurons, suggesting an effect on the presynaptic glutamatergic neurotransmission.

INTERPRETATION

A single seizure and SE stimulate lactate production. Diminishing pyruvate to lactate conversion with oxamate terminated SE and reduced associated neuronal death. Oxamate reduced neuronal excitability and excitatory neurotransmission at the presynaptic terminal. Glycolytic lactate production sustains SE and is an attractive therapeutic target.

Glycolysis regulates neuronal excitability via lactate receptor, HCA1R

Repetitively firing neurons during seizures accelerate glycolysis to meet energy demand, which leads to the accumulation of extracellular glycolytic by-product lactate. Here, we demonstrate that lactate rapidly modulates neuronal excitability in times of metabolic stress via the hydroxycarboxylic acid receptor type 1 (HCA1R) to modify seizure activity. The extracellular lactate concentration, measured by a biosensor, rose quickly during brief and prolonged seizures. In two epilepsy models, mice lacking HCA1R (lactate receptor) were more susceptible to developing seizures. Moreover, HCA1R deficient (knockout) mice developed longer and more severe seizures than wild-type littermates. Lactate perfusion decreased tonic and phasic activity of CA1 pyramidal neurons in genetically encoded calcium indicator 7 imaging experiments. HCA1R agonist 3-chloro-5-hydroxybenzoic acid (3CL-HBA) reduced the activity of CA1 neurons in HCA1R WT but not in knockout mice. In patch-clamp recordings, both lactate and 3CL-HBA hyperpolarized CA1 pyramidal neurons. HCA1R activation reduced the spontaneous excitatory postsynaptic current frequency and altered the paired-pulse ratio of evoked excitatory postsynaptic currents in HCA1R wild-type but not in knockout mice, suggesting it diminished presynaptic release of excitatory neurotransmitters. Overall, our studies demonstrate that excessive neuronal activity accelerates glycolysis to generate lactate, which translocates to the extracellular space to slow neuronal firing and inhibit excitatory transmission via HCA1R. These studies may identify novel anticonvulsant target and seizure termination mechanisms.

Synapsin I is associated with cholinergic nerve terminals in the electric organs of Torpedo, Electrophorus, and Malapterurus and copurifies with Torpedo synaptic vesicles

Using an affinity-purified monospecific polyclonal antibody against bovine brain synapsin I, the distribution of antigenically related proteins was investigated in the electric organs of the three strongly electric fish Torpedo marmorata, Electrophorus electricus, Malapterurus electricus and in the rat diaphragm. On application of indirect fluorescein isothiocyanate-immunofluorescence and using alpha-bungarotoxin for identification of synaptic sites, intense and very selective staining of nerve terminals was found in all of these tissues. Immunotransfer blots of tissue homogenates revealed specific bands whose molecular weights are similar to those of synapsin Ia and synapsin Ib. Moreover, synapsin I-like proteins are still attached to the synaptic vesicles that were isolated in isotonic glycine solution from Torpedo electric organ by density gradient centrifugation and chromatography on Sephacryl-1000. Our results suggest that synapsin I-like proteins are also associated with cholinergic synaptic vesicles of electric organs and that the electric organ may be an ideal source for studying further the functional and molecular properties of synapsin.

Characterization of glutamate uptake into synaptic vesicles

Recent evidence indicates that L-glutamate is taken up into synaptic vesicles in an ATP-dependent manner, supporting the notion that synaptic vesicles may be involved in glutamate synaptic transmission. In this study, we further characterized the ATP-dependent vesicular uptake of glutamate. Evidence is provided that a Mg-ATPase, not Ca-ATPase, is responsible for the ATP hydrolysis coupled to the glutamate uptake. The ATP-dependent glutamate uptake was inhibited by agents known to dissipate the electrochemical proton gradient across the membrane of chromaffin granules. Hence, it is suggested that the vesicular uptake of glutamate is driven by electrochemical proton gradients generated by the Mg-ATPase. Of particular interest is the finding that the ATP-dependent glutamate uptake is markedly stimulated by chloride over a physiologically relevant, millimolar concentration range, suggesting an important role of intranerve terminal chloride in the accumulation of glutamate in synaptic vesicles. The vesicular glutamate translocator is highly specific for L-glutamate, and failed to interact with aspartate, its related agents, and most of the glutamate analogs tested. It is proposed that this vesicular translocator plays a crucial role in determining the fate of glutamate as a neurotransmitter.