THE HUMAN-RESTRICTED DUPLICATED FORM OF THE ALPHA7 NICOTINIC ACETYLCHOLINE RECEPTOR, CHRFAM7A: TRANSCRIPTIONAL REGULATION, ROLE IN INFLAMMATION AND POTENTIAL PHARMACOLOGICAL IMPLICATIONS

The α7 nicotine acetylcholine receptor (α7nAChR, CHRNA7) is a homo-pentameric ligand-gated ion channel widely expressed in the Central Nervous System (CNS). Recent evidence has demonstrated its expression also in non-neuronal tissues, including monocytes and macrophages, where it mediates the Cholinergic Anti-Inflammatory Pathway, a neuronal reflex providing the central control of systemic inflammation. Recently, the human-restricted duplicated gene of the α7nAChR, called CHRFAM7A (α7dup) has been discovered. It is the product of the partial duplication and fusion of exon 5-10 of CHRNA7 gene with the novel exons D, C, B and A belonging to the FAM7A gene, of unknown function. The CHRFAM7A gene is expressed in human immune cells and in the CNS and is translated into two proteins, of 45 kDa and 36 kDa, which are the result of alternative splicing. The α7dup protein assembles with the α7 conventional subunits and exerts a dominant negative regulation on the α7nAChR function. The importance of the α7dup protein in the human inflammatory process has been confirmed by the demonstration of its responsiveness to pro-inflammatory stimuli: indeed, the Lipopolysaccharide (LPS) treatment of THP-1 monocytic cells and of human primary monocytes and macrophages down-regulates CHRFAM7A transcript and protein through a transcriptional mechanism reliant on the NF-κB transcription factor. Moreover, unpublished data demonstrated that LPS has the opposite effect on the α7 transcript in monocytes and macrophages, leading to CHRNA7 up-regulation. In this study, we have investigated the transcriptional mechanisms leading to CHRFAM7A expression in the THP-1 monocytic cells and in neuroblastoma SH-SY5Y cells and we demonstrate that the CHRFAM7A gene is endowed with several complex transcriptional mechanisms leading to fine expression modulation, including the presence of alternative and tissue-specific Transcription Start Site (TSS), alternative splicing mechanism, tissue-specific transcriptional elements and intronic silencer elements. Moreover, we demonstrated that the CHRFAM7A down-regulation exerted by LPS involve the chromatin remodeling of CHRFAM7A promoter in THP-1 cells. Increasing evidence has linked CHRNA7 expressional and functional dysregulation to several neurodegenerative disorders, including Alzheimer’s disease. The treatment with the acetylcholinesterase inhibitor Donepezil is effective in temporary ameliorating the cognitive symptoms of AD, by increasing the synaptic levels of ACh and counteracting the cholinergic loss, which is characteristic of AD. However, emerging findings sustained that Donepezil can exert its therapeutic effect also by modulating the immune response and potentiating the Cholinergic Anti-Inflammatory Pathway. So far, the role of CHRFAM7A gene in AD pathogenesis or pharmacological response has not been elucidated. In the present study, we have investigated the expression profile of CHRNA7 and CHRFAM7A genes in human nervous and immune tissues obtained from AD patients, highlighting expressional alterations of both the α7 conventional and duplicated form. Moreover, we have investigated the effect of the AChEI Donepezil on CHRFAM7A and CHRNA7 transcription in THP-1 cells, human primary macrophages and SH-SY5Y cells, collecting new insights about the possible role of the α7dup gene as a pharmacological target in AD therapy.