11. The Regulation of Human Prion Protein and β-Amyloid Genes
The PRNP gene located in chromosome 20 in humans codes for PrPC in the central nervous system and several other tissues [41]. This is a highly conserved housekeeping gene, and it is subject to many transcription factors functioning in its promoter and thus regulating its expression. Amongst many others putative sequences for transcriptional activation by activator protein 1 (AP-1), SP1 and SP2 (members of the SP/KLF family of transcription factors) have been identified as PRNP promoters.
Importantly, a short GC rich region is located upstream from thePRNP gene promoter. These GC-rich regions have the potential to form G-quadruplex (G4) structures and therefore regulate gene disease-related expression, as they are subject to favorable binding by p53. Binding of p53 to GC regions forming G4s has been shown to initiate a series of cellular effects related to disease [95,96]. Furthermore, it has been shown that the PRNP promoter region harbors a sequence matching the binding sequence of p53. p53 binds directly to the suspected sequence, behaving as a potent PrPC transcriptional activator and enhancer of its mRNA expression [97]. In summary, p53 causes an increased expression of PrPC.
RNA translational regulation is considered an important contributor to PrPC conversion to infectious PrPSC. Beyond DNA, it has been shown that the messenger RNA of PrPC contains five naturally existing consecutive regions forming G4s that are susceptible to G4 binding ligands [98]. In this respect, p53 can be regarded as an RNA chaperone that is able to facilitate the folding of G4s and hence stabilize their structure [99]. G4s in 5’-untranslated mRNA regions are found in multiple neurodegenerative diseases and have been shown to inhibit translation and initiate cap-independent translation [100].
The amyloid precursor protein (APP ) gene coding for the APP in humans is located on chromosome 21. Viewing its promoter sequence, it can be designated as a housekeeping gene like the PRNP gene.APP shares some important promoter sequences with PRNPlike AP-1 and Sp1, amongst many others, which however differ from the sequences in the PRNP promoter. This suggests that both genes can be partly transactivated by the activity of common transcription factors [101].
APP mRNA is expressed in a variety of tissues, including muscle, the immune system, and many organs such as the thymus, pancreas, kidneys, the lung and others, in addition to its active expression in the nervous system. However, different variants of APP are cell-type specific in their expression [102].
The variants of APP include APP-like protein-1 (APPL1 gene located on chromosome 21) and APPL2 (APPL2 gene located on chromosome 11), which are both type 1 transmembrane proteins with similar structure and topology. Only APP itself, however, contains the Aβ sequence. The fibrillary form of Aβ (40-42 amino acids), found and constituting the primary source of plaques in brains of patients suffering from AD and Down syndrome, originate only from APP proteolysis. The full length of human APP sustains proteolysis mainly via the α,β,γ-secretases. The derived amino acid sequence of Aβ results from the β-site APP cleaving enzyme 1 (BACE-1) or else β-secretase cleavage yielding APPsβ and APPCTFβ (βAPP) fragments of APP. Thereafter, the cleavage of γ-secretase on βAPP finally yields Aβ and the APP intracellular domain (AIDC) fragments (for details see [103]). Moreover, the AIDC fragment is also produced by α-secretase and subsequent γ-secretase activity.
γ-Secretase is also called presenilin-dependent γ-secretase, since it encompasses presenilin (PS) transmembrane proteins in its catalytic subunit (PS1 or PS2) [104]. In this respect, it has been established that γ-secretase/presenilin-dependent generation of AIDC operates as a transcriptional activator of p53, increasing p53 activity and triggering p53-associated cell death. Moreover, mutations in transcription factor Sp1 increase the p53 activity in vitro and in brains of patients affected with familial Alzheimer’s disease (FAD) [97]. Mutations on Sp1 are considered as a causative factor for FAD.
The tumor suppressor p53, once generated through the γ-secretase/presenilin dependent transcriptional activation of theTP53 gene by AIDC bound to Fe65 and Tip60 cofactors, then acts on the promoter of PrPC and induces the expression of PrPC mRNA. The p53, γ-secretase/presenilin dependent transactivation of PrPC expression is abolished in a p53-deficient environment. Thus, it is ultimately the PSs (PS1 or PS2) which exert rate-limiting control over PrPC expression through their ability to generate AIDC. Finally, βAPP overexpression increases PrPC expression, whereas βAPP depletion results in lower PrPC expression, in both in vitro and in vivo experiments, indicating also the controlling role of BACE-1 activity over PrPC expression [96]. Thus, the metabolism of APP that produces amyloidogenic products also induces increased production of PrPC.