Effects of phospholipases, proteases and neuraminidase on gamma-hydroxybutyrate binding sites

Heterodimerization of the sialidase NEU1 with the chaperone protective protein/cathepsin A prevents its premature oligomerization

Lysosomal neuraminidase-1 (NEU1) forms a multienzyme complex with beta-galactosidase and protective protein/cathepsin A (PPCA). Because of its association with PPCA, which acts as a molecular chaperone, NEU1 is transported to the lysosomal compartment, catalytically activated, and stabilized. However, the mode(s) of association between these two proteins both en route to the lysosome and in the multienzyme complex has remained elusive. Here, we have analyzed the hydrodynamic properties of PPCA, NEU1, and a complex of the two proteins and identified multiple binding sites on both proteins. One of these sites on NEU1 that is involved in binding to PPCA can also bind to other NEU1 molecules, albeit with lower affinity. Therefore, in the absence of PPCA, as in the lysosomal storage disease galactosialidosis, NEU1 self-associates into chain-like oligomers. Binding of PPCA can reverse self-association of NEU1 by causing the disassembly of NEU1-oligomers and the formation of a PPCA-NEU1 heterodimeric complex. The identification of binding sites between the two proteins allowed us to create innovative structural models of the NEU1 oligomer and the PPCA-NEU1 heterodimeric complex. The proposed mechanism of interaction between NEU1 and its accessory protein PPCA provides a rationale for the secondary deficiency of NEU1 in galactosialidosis.

Gamma-hydroxybutyric acid as a signaling molecule in brain

Gamma-hydroxybutyric acid was synthesized 35 years ago to obtain a GABAergic substance that penetrates the brain freely. Since then, gamma-hydroxybutyric acid has been used in human beings for its sedative and anesthetic properties when administered at high doses, and most of the studies on gamma-hydroxybutyric acid have focused on its pharmacological effects. However, gamma-hydroxybutyric acid is also an endogenous substance, which is synthesized and released in the brain by specific neuronal pathways, implicated in the control of the GABAergic, dopaminergic, and opioid systems. This control is mediated by specific gamma-hydroxybutyric acid receptors with a unique distribution in brain and a specific ontogenesis and pharmacology. Stimulation of these receptors induces specific cellular responses. Taken together, these results suggest that gamma-hydroxybutyric acid possesses most of the properties required of a neurotransmitter/neuromodulator in the brain.

The gamma-hydroxybutyrate signalling system in brain: organization and functional implications

gamma-Hydroxybutyrate is a metabolite of GABA which is synthesized and accumulated by neurons in brain. This substance is present in micromolar quantities in all brain regions investigated as well as in several peripheral organs. Neuronal depolarization releases gamma-hydroxybutyrate into the extracellular space in a Ca(2+)-dependent manner. Gamma-hydroxybutyrate high-affinity receptors are present only in neurons, with a restricted specific distribution in the hippocampus, cortex and dopaminergic structures of rat brain (the striatum in general, olfactory bulbs and tubercles, frontal cortex, dopaminergic nuclei A9, A10 and A12). Stimulation of these receptors with low amounts of gamma-hydroxybutyrate induces in general hyperpolarizations in dopaminergic structures with a reduction of dopamine release. However, in the hippocampus and the frontal cortex, it seems that gamma-hydroxybutyrate induces depolarization with an accumulation of cGMP and an increase in inositol phosphate turnover. Some of the electrophysiological effects of GHB are blocked by NCS-382, a gamma-hydroxybutyrate receptor antagonist while some others are strongly attenuated by GABAB receptors antagonists. Gamma-hydroxybutyrate penetrates freely into the brain when administered intravenously or intraperitoneally. This is a unique situation for a molecule with signalling properties in the brain. Thus, the gamma-hydroxybutyrate concentration in brain easily can be increased more than 100 times. Under these conditions, gamma-hydroxybutyrate receptors are saturated and probably desensitized and down-regulated. It is unlikely that GABAB receptors could be stimulated directly by GHB. Most probably, GABA is released in part under the control of GHB receptors in specific pathways expressing GABAB receptors. Alternatively, GABAB receptors might be specifically stimulated by the GABA formed via the metabolism of gamma-hydroxybutyrate in brain. In animals and man, these GHBergic and GABAergic potentiations induce dopaminergic hyperactivity (which follows the first phase of dopaminergic terminal hyperpolarization), a strong sedation with anaesthesia and some EEG changes with epileptic spikes. It is presumed that, under pathological conditions (hepatic failure, alcoholic intoxication, succinic semialdehyde dehydrogenase defects), the rate of GHB synthesis or degradation in the peripheral organ is modified and induces increased GHB levels which could interfere with the normal brain mechanisms. This pathological status could benefit from treatments with gamma-hydroxybutyric and/or GABAB receptors antagonists. Nevertheless, the regulating properties of the endogenous gamma-hydroxybutyrate system on the dopaminergic pathways are a cause for the recent interest in synthetic ligands acting specifically at gamma-hydroxybutyrate receptors and devoid of any role as metabolic precursor of GABA in brain.

What is the role of the gamma-hydroxybutyrate receptor?

A possible functional role for endogenous gamma-hydroxybutyrate has been disputed. However, there are receptor sites for this molecule, which are highly enriched in the synaptosomal membrane fraction in the rat brain and are functionally linked to a guanosine triphosphate-binding protein. These data suggest that they play a neurological role. The binding sites recognize some drug molecules that bear no structural similarity to gamma-hydroxybutyrate. Recent experimental evidence indicates the existence of endogenous hydrophobic ligands. As a minor brain metabolite directly or indirectly involved in scavenging oxygen-derived free radicals, gamma-hydroxybutyrate demonstrates similarities with melatonin. The gamma-hydroxybutyrate receptor is compared to the cannabis receptor, for which an endogenous hydrophobic ligand has been identified. Structurally similar molecules to this ligand are believed to be implicated in the sleep process. As gamma-hydroxybutyrate itself can induce sleep, a search amongst these molecules as possible ligands for the gamma-hydroxybutyrate receptor might be enlightening.

Gamma hydroxybutyrate is not a GABA agonist

Gamma hydroxybutyrate (GHB) is primarily known and used as a relatively specific inhibitor of central DA release. However, it is also widely assumed to be an agonist or prodrug of gamma-aminobutyric acid (GABA) and its central activity has been attributed to an action exerted at GABA receptors. Nevertheless, there is compelling evidence that: (1) GHB formation may occur independently of GABA; (2) GHB is behaviorally, biochemically and physiologically distinct from GABA in many ways, and does not consistently effect GABAA or GABAB agonist induced responses; (3) GHB has little effect on either GABAA or GABAB receptors at less than millimolar concentrations. Consequently, GHB does not appear to be either a GABA prodrug or a GABA agonist. However, the GHB metabolite gamma butyrolactone (GBL) may possess some limited GABA agonist activity.

gamma-Hydroxybutyrate receptor binding in rat brain is inhibited by guanyl nucleotides and pertussis toxin

gamma-Hydroxybutyrate is an endogenous substance of mammalian brain which is primarily derived from GABA. This compound exhibits neuromodulatory influences on dopamine and serotonin synthesis and release in rat brain. These effects are mediated by specific brain receptors which are mainly distributed in the hippocampus, cortex and striatum. In order to characterize this type of receptor, we have studied the possibility that guanosine triphosphate (GTP) and/or pertussis toxin mediated modification of the affinity for gamma-hydroxybutyrate binding to the receptor. Results presented in this paper favor the presence of guanine nucleotide binding proteins (G0 or Gi family), which are coupled to the gamma-hydroxybutyrate receptor, modifying the high-affinity gamma-hydroxybutyrate binding. We conclude that the gamma-hydroxybutyrate receptor in brain belongs to the G-protein family of receptors.

Selective distribution pattern of gamma-hydroxybutyrate receptors in the rat forebrain and midbrain as revealed by quantitative autoradiography

Using quantitative autoradiography to study the precise distribution of gamma-hydroxybutyrate high-affinity binding sites, the present results showed the heterogeneous localization of these sites in cortical and hippocampal layers and also in some diencephalic and mesencephalic nuclei. In frontal, parietal and temporal cortex, GHB binding sites are generally distributed in three distinct layers. The olfactory system, the amygdala, septum, basal ganglia and substantia nigra also exhibited significant amounts of GHB receptors. In thalamus, the radioactivity was heterogeneously distributed, the highest amounts being in the lateral posterior nucleus. Hypothalamus, cerebellum, colliculi and pons-medulla were apparently devoid of binding sites. This more accurate mapping of GHB high-affinity receptors in rat brain is due to some technical improvements and the use of [3H]GHB of higher specific activity.