Heterologous, PKC-Mediated Desensitization of Human Histamine H3 Receptors Expressed in CHO-K1 Cells

Histamine H3 Receptor Isoforms: Insights from Alternative Splicing to Functional Complexity

Alternative splicing significantly enhances the diversity of the G protein-coupled receptor (GPCR) family, including the histamine H3 receptor (H3R). This post-transcriptional modification generates multiple H3R isoforms with potentially distinct pharmacological and physiological profiles. H3R is primarily involved in the presynaptic inhibition of neurotransmitter release in the central nervous system. Despite the approval of pitolisant for narcolepsy (Wakix®) and daytime sleepiness in adults with obstructive sleep apnea (Ozawade®) and ongoing clinical trials for other H3R antagonists/inverse agonists, the functional significance of the numerous H3R isoforms remains largely enigmatic. Recent publicly available RNA sequencing data have confirmed the expression of multiple H3R isoforms in the brain, with some isoforms exhibiting unique tissue-specific distribution patterns hinting at isoform-specific functions and interactions within neural circuits. In this review, we discuss the complexity of H3R isoforms with a focus on their potential roles in central nervous system (CNS) function. Comparative analysis across species highlights evolutionary conservation and divergence in H3R splicing, suggesting species-specific regulatory mechanisms. Understanding the functionality of H3R isoforms is crucial for the development of targeted therapeutics. This knowledge will inform the design of more precise pharmacological interventions, potentially enhancing therapeutic efficacy and reducing adverse effects in the treatment of neurological and psychiatric disorders.

Effect of moderate prenatal ethanol exposure on the differential expression of two histamine H3 receptor isoforms in different brain regions of adult rat offspring

We have reported that prenatal alcohol exposure (PAE) elevates histamine H₃ receptor (H3R) agonist-mediated inhibition of glutamatergic neurotransmission in the dentate gyrus. Here, we hypothesized that PAE alters the expression of two prominent H3R isoforms namely, the rH_3A and rH_3C isoforms, which have differing intrinsic activities for H3R agonists, in a manner that may contribute to heightened H3R function in PAE rats. In contrast to our predictions, we found different effects of sex and PAE in various brain regions with significant interactions between sex and PAE in dentate gyrus and entorhinal cortex for both isoforms. Subsequently, to confirm the PAE-and sex-induced differences on H3R isoform mRNA expression, we developed a polyclonal antibody selective for the rH_3A inform. Western blots of rH_3A mRNA-transfected HEK-293 cells identified a  ~ 48 kDa band of binding consistent with the molecular weight of rH_3A, thus confirming antibody sensitivity for rH_3A protein. In parallel, we also established a pan-H3R knockout mice line to confirm antibody specificity in rodent brain membranes. Both qRT-PCR and H3R agonist-stimulated [³⁵S]-GTPγS binding confirmed the absence of mH_3A mRNA and H3 receptor-effector coupling in H3R knockout (KO) mice. Subsequent western blotting studies in both rat and mouse brain membranes were unable to detect rH_3A antibody binding at ~48 kDa. Rather, the H3RA antibody bound to a  ~ 55 kDa band in both rat and mouse membranes, including H3R KO mice, suggesting H3RA binding was not specific for H3Rs in rodent membranes. Subsequent LC/MS analysis of the ~55 kDa band in frontal cortical membranes identified the highly abundant beta subunit of ATPase in both WT and KO mice. Finally, LC/MS analysis of the ~48 kDa band from rH_3A mRNA-transfected HEK-293 cell membranes was able to detect rH_3A protein, but its presence was below the limits of quantitative reliability. We conclude that PAE alters rH_3A and rH_3C mRNA expression in some of the same brain regions where we have previously reported PAE-induced alterations in H3R-effector coupling. However, interpreting the functional consequences of altered H3R isoform expression was limited given the technical challenges of measuring the relatively low abundance of rH_3A protein in native membrane preparations.

Activation of transglutaminase 2 by nerve growth factor in differentiating neuroblastoma cells: A role in cell survival and neurite outgrowth

NGF (nerve growth factor) and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 transamidase activity by NGF in retinoic acid-induced differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. The role of TG2 in NGF-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with NGF in N2a and SH-SY5Y cells. NGF mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON (Z-ZON-Val-Pro-Leu-OMe; Benzyloxycarbonyl-(6-Diazo-5-oxonorleucinyl)-l-valinyl-l-prolinyl-l-leucinmethylester) and R283 (1,3,dimethyl-2[2-oxo-propyl]thio)imidazole chloride) and by pharmacological inhibition of extracellular signal-regulated kinases 1 and 2 (ERK1/2), protein kinase B (PKB) and protein kinase C (PKC), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated NGF induced in situ TG2 activity. TG2 inhibition blocked NGF-induced attenuation of hypoxia-induced cell death and neurite outgrowth in both cell lines. Together, these results demonstrate that NGF stimulates TG2 transamidase activity via a ERK1/2, PKB and PKC-dependent pathway in differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. Furthermore, NGF-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results suggest a novel and important role of TG2 in the cellular functions of NGF.

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 (hH₃R_WT) and mutant (hH₃R_Y197C) receptors transiently expressed in CHO-K1 cells. The hH₃R_Y197C 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 hH₃R_WT and hH₃R_Y197C 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 hH₃R_WT and hH₃R_Y197C, 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.

Role of transglutaminase 2 in PAC1 receptor mediated protection against hypoxia-induced cell death and neurite outgrowth in differentiating N2a neuroblastoma cells

The PAC₁ receptor and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 activity by the PAC₁ receptor in retinoic acid-induced differentiating N2a neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. TG2 phosphorylation was monitored via immunoprecipitation and Western blotting. The role of TG2 in PAC₁ receptor-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27). PACAP-27 mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON and R283 and by pharmacological inhibition of protein kinase A (KT 5720 and Rp-cAMPs), protein kinase C (Ro 31-8220), MEK1/2 (PD 98059), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated PACAP-27 induced in situ TG2 activity. TG2 inhibition blocked PACAP-27 induced attenuation of hypoxia-induced cell death and outgrowth of axon-like processes. TG2 activation and cytoprotection were also observed in human SH-SY5Y cells. Together, these results demonstrate that TG2 activity was stimulated downstream of the PAC₁ receptor via a multi protein kinase dependent pathway. Furthermore, PAC₁ receptor-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results highlight the importance of TG2 in the cellular functions of the PAC₁ receptor.

The Histamine H3 Receptor: Structure, Pharmacology, and Function

Among the four G protein-coupled receptors (H₁-H₄) identified as mediators of the biologic effects of histamine, the H₃ receptor (H₃R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H₃Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H₃R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H₃R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H₃R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction.