Inhibition of Astrocytic Histamine N-Methyltransferase as a Possible Target for the Treatment of Alzheimer's Disease.

Alzheimer's disease (AD) represents the principal cause of dementia among the elderly. Great efforts have been established to understand the physiopathology of AD. Changes in neurotransmitter systems in patients with AD, including cholinergic, GABAergic, serotoninergic, noradrenergic, and histaminergic changes have been reported. Interestingly, changes in the histaminergic system have been related to cognitive impairment in AD patients. The principal pathological changes in the brains of AD patients, related to the histaminergic system, are neurofibrillary degeneration of the tuberomammillary nucleus, the main source of histamine in the brain, low histamine levels, and altered signaling of its receptors. The increase of histamine levels can be achieved by inhibiting its degrading enzyme, histamine N-methyltransferase (HNMT), a cytoplasmatic enzyme located in astrocytes. Thus, increasing histamine levels could be employed in AD patients as co-therapy due to their effects on cognitive functions, neuroplasticity, neuronal survival, neurogenesis, and the degradation of amyloid beta (Aß) peptides. In this sense, the evaluation of the impact of HNMT inhibitors on animal models of AD would be interesting, consequently highlighting its relevance.

The naturally occurring mutation Y197C does not affect the expression or signaling of the human histamine H3 receptor.

There is evidence for genetic polymorphism within the human histamine H3 receptor (hH3R), and a Tyr to Cys exchange at position 197 (Y197C), located in the amino terminus of the fifth transmembrane domain, has been reported. In this work we compared the expression and the pharmacological and signaling properties of wild-type (hH3RWT) and mutant (hH3RY197C) receptors transiently expressed in CHO-K1 cells. The hH3RY197C cDNA was created by overlap extension PCR amplification. Receptor expression and affinity were assessed by N-α-[methyl-3H]-histamine binding to cell membranes and intact cells. Receptor function was evaluated by stimulation of [35S]-GTPγS binding to cell membranes and by inhibition of forskolin-induced cAMP accumulation in intact cells. The hH3RWT and hH3RY197C were expressed at similar levels (761±68 and 663±66fmol/mg protein for membranes, and 13,434±1533 and 15,894±1884 receptors per cell, respectively). There were no significant differences in the affinities for H3R agonists or antagonists/inverse agonists between the hH3RWT and hH3RY197C, and the H3R agonist RAMH was similarly efficacious and potent to stimulate [35S]-GTPγS binding and to inhibit forskolin-induced cAMP accumulation. These results indicate that the Y197C mutation does not affect the expression, ligand affinity or signaling of the human H3 receptor.

A single-point mutation (Ala280Val) in the third intracellular loop alters the signalling properties of the human histamine H3 receptor stably expressed in CHO-K1 cells.

BACKGROUND AND PURPOSE: An alanine to valine exchange at amino acid position 280 (A280V) in the third intracellular loop of the human histamine H3 receptor was first identified in a patient suffering from Shy-Drager syndrome and later reported as a risk factor for migraine. Here, we have compared the pharmacological and signalling properties of wild-type (hH3 R(WT)) and A280V mutant (hH3 R(A280V)) receptors stably expressed in CHO-K1 cells. EXPERIMENTAL APPROACH: The hH3 R(A280V) cDNA was created by overlapping extension PCR amplification. Receptor expression and affinity were assessed by radioligand (N-α-[methyl-³H]-histamine) binding to cell membranes, and receptor function by the inhibition of forskolin-induced cAMP accumulation and stimulation of ERK1/2 phosphorylation in intact cells, as well as stimulation of [³5S]-GTPγS binding to cell membranes. KEY RESULTS: Both receptors were expressed at similar levels with no significant differences in their affinities for H3 receptor ligands. Upon activation the hH3 RWT was significantly more efficacious to inhibit forskolin-induced cAMP accumulation and to stimulate [³5S]-GTPγS binding, with no difference in pEC50 estimates. The hH3 RWT was also more efficacious to stimulate ERK1/2 phosphorylation, but this difference was not significant. The inverse agonist ciproxifan was more efficacious at hH3 RWT to reduce [³5S]-GTPγS binding but, for both receptors, failed to enhance forskolin-induced cAMP accumulation. CONCLUSIONS AND IMPLICATIONS: The A280V mutation reduces the signalling efficacy of the human H3 receptor. This effect may be relevant to the pathophysiology of disorders associated with the mutation.