**2. Presenilin genetic structure and transcriptional regulation network**

tau protein deposited into neurofibrillary tangles (NFTs). Aβ is processed by the γ-secretase complex, where the most important component is presenilin [1]. There are two major types of AD: early-onset AD (EOAD), often linked with familial AD (FAD), and late-onset AD (LOAD), linked with sporadic AD (SAD). Familial EOAD represents 5–10% of all cases of AD and is associated with mutations in *PSEN1* encoding presenilin (PS1), *PSEN2* encoding presenilin 2 (PS2), and *APP* encoding amyloid β protein precursor (APP) [2, 3]. Overall, presenilins and APP mutations directly cause a production of toxic assemblies of oligomerized Aβ, followed by a formation of senile plaques [4]. Toxic Aβ forms induce apoptosis, oxidative stress, unfolded protein stress response, inflammation, or disturbances in calcium signalling, of which many

Normal ageing results from natural maturational processes, whereas pathological ageing is related to non-normative factors such as disease or trauma to the brain. Ageing disproportionately affects frontal lobes [5]. Substantial overlap between ageing and neurodegeneration was demonstrated in several brain autopsy studies of aged people with no record of neurological diseases. These reports showed the presence of amyloid plaques, neurofibrillary tangles, Lewy bodies, inclusions of TAR DNA-binding protein 43 (TDP-43), synaptic dystrophy, and loss of neurons in most of ageing brains [6, 7]. However, unlike AD, pathological ageing usually lacks cognitive impairment despite similar senile plaque [8]. It was found that oxidative stress, commonly accompanying both ageing and AD, causes pathogenic conformational change of PS1 in neurons in vitro, which was followed by an increased ratio of Aβ42/40. It was further concluded that this conformational shift and deregulation of PS1 precedes Aβ deposition in pathological ageing [9]. These data demonstrated a direct connection between presenilins and PA. Presenilins contribute to brain pathology not only by deposition of toxic Aβ. Both PS1 and PS2 have been found to be involved in the regulation of apoptosis in neurons induced by trophic withdrawal or Aβ and via Jun Kinase pathway, respectively [10]. What is more, the role of presenilins in the progression of AD and PA is underlined by their numerous functions in the adult cerebral cortex functions, including maintenance of synaptic plasticity, long-term memory, and neuronal survival, which are critical for normal ageing, healthy brain,

Summarizing, presenilin functions can be controlled at different cellular levels, that is, (1) gene architecture, together with the influence of damaging genetic variants, in *PSEN1* and *PSEN2*, (2) gene expression, together with corresponding regulatory protein networks, (3) protein structure with its enzymatic activity, controlled by the assembly of the γ-secretase complex with accompanying partners and by post-translational modifications (phosphorylation and ubiquitination), (4) quantity, quality and availability of numerous substrates of presenilins and finally (5) by the interaction with molecular partners involved in numerous biological processes. Hereby, we highlighted that presenilins can determine different physiological and pathological processes by the interplay with diverse signal transduction pathways and by processing of various substrates. Generally, presenilins form a signalling network, which is critical for both AD and PA. Therefore, we present below molecular players that might affect biological functions of presenilins forming together so-called presenilin

are present in pathological ageing or in Alzheimer's disease.

and cognitive ability [11].

96 Senescence - Physiology or Pathology

interactome.

Presenilins 1 and 2 are encoded by homologous genes *PSEN1* and *PSEN2*, located at chromosomes 14q24.3 and 1q42.1, respectively [12, 13]. The genomic sizes of *PSEN1* and *PSEN2* are largely different, and it is 70 kb for *PSEN1* and 24 kb for *PSEN2. PSEN1* contains 13 exons and three first exons are located in the 5′ untranslated region (5′UTR) [14]. The first two exons and exon 9 of *PSEN1* could be alternatively spliced, causing structural changes to the protein [15]. *PSEN2* contains 12 exons and two first are located in the 5′ UTR [16]. The alternatively spliced products in *PSEN2* include in-frame omissions of exon 8 and simultaneous omissions of exons 3 and 4 [17]. Moreover, it has been found that splicing of exon 5 in *PSEN2* occurred under hypoxic stress conditions [18]. The transcription of *PSEN1* depends on two promoters producing two mRNA transcripts of 2.7 and 7.5 kb, with different 5´ UTRs [15]. *PSEN2* is also transcribed into two different transcripts of 2.4 and 2.8 kb [16].

Transcriptional regulation of presenilins might have an implication in AD and PA pathogenesis. Promoters of *PSENs* lack a TATA box but contain transcriptionally active GC. *PSEN1* promoter contains GC boxes corresponding to Sp1-like transcriptional factor, and the most active region is located between −22 and −6 bp. Transcriptional co-activator p300 with histone acetyl-transferase (HAT) activates *PSEN1* transcription. In particular in neuronal system, enhanced transcription of *PSEN1* was observed upon stimulation by N-methyl-D-aspartate (NMDA) or brain-derived neurotrophic factor (BDNF), under control of cAMP-responsive element binding (CREB). *PSEN1* expression and risk of AD and premature PA are also influenced by *PSEN1* promoter polymorphisms, found at −22C/T and −48C/T positions. Another suppressor of presenilin 1 is p53 protein that recruits other proteins to occupy *PSEN1* promoter [19]. Relatively little is known on the transcriptional regulation of *PSEN2*, where the promoter is located in a CpG island and is regulated by early growth response gene-1 (Egr-1) transcription factor, involved in learning and memory processes [20]. In addition, *PSEN2* promoter has been found to be regulated by nerve growth factor (NGF), with an NGF-responsive element localized between −403 and +13 [19]. Interestingly, parkin protein, known to be associated with Parkinson's disease, was found to act as a transcriptional factor modulating trans-activation of *PSEN1* and *PSEN2* promoters via RING1-IBR-RING2 domain and to influence γ-secretase activity [21]. The expression of both *PSEN1* and *PSEN2* was also described to be under thigh control of inflammatory cytokines, including tumour necrosis factor (TNF)-α, interferon (IFN)-γ, interleukin (IL)-1 β, IL-10 or TGF-β1 [22]. Generally, transcriptional regulation of presenilins is based on the complex signalling cascades controlling promoter's activation and requires a large variety of transcriptional factors. The dense network of signalling pathways related to the regulation of the promoters of *PSEN1* and *PSEN2* indicates numerous cellular processes that may contribute to the incidence and progression of AD and PA.

### **3. Presenilin structure and expression patterns**

Structurally, PS1 and PS2 are integral membrane proteins of 467 and 448 amino acids, respectively [14, 15]. The homology between PS1 and PS2 is about 67%, with the highest similarity in transmembrane domains (TMDs). PS1 and PS2 comprise nine TM, among them TM1-6 are located at N′-terminal and TM7-9 at the C′-terminal. The catalytic centre with aspartate residues is located at the cytoplasmic side of TM6 and TM7, forming large hydrophilic loop (HL) [14]. Presenilins are activated by endoproteolysis yielding N′-terminal and C′-terminal portions. Endoproteolytical cleavage of PS1 occurs at HL, with the predominant cleavage site between amino acids 291 and 292, generating 28 kDa N′-terminal and 17 kDa C′-terminal fragments [23]. Similarly, PS2 is endoproteolytically cleaved into 35 kDa N′-terminal and 20 kDa C′-terminal fragment [24]. The most common mutations of presenilins occur in gene portion encoding C′-terminal, containing proline, alanine and leucin residues, and are usually loss of function for presenilins [25]. Due to protein structure complexity, presenilins interact with different partners, which will be *described in detail in Section 6*.

Presenilins are ubiquitously expressed, with some tissue-specific differences. Generally, *PSEN1* transcript is expressed at higher levels than *PSEN2*. The expression pattern of *PSEN1* and *PSEN2* in the brain is similar and present in different brain cells, such as cortical neurons, hippocampal neurons, granule cells or neurons of amygdala [26], and different types of glial cells [27]. In neurons, presenilins are expressed in the cell body and dendrites [28] and are localized in several subcellular compartments, that is, rough endoplasmic reticulum, Golgi complex, mitochondria, and at plasma membrane [29]. Moreover, presenilins were found to be expressed in several non-nervous cells and tissues, including lymphoblasts, fibroblasts, liver, spleen, and kidney [15].
