**2. Amyloid precursor protein**

Amyloid precursor protein (APP) is an integral membrane glycoprotein that is expressed in the brain and the central nervous system (CNS). APP can be cleaved by specific proteases in two different pathways: α-path and β-path [5]. In most cases, APP is cleaved in the α-path with the participation of enzymes α- and γ-secretases. The cleavage of APP by α-secretase proceeds in the way, which can be described as non-amyloidogenic one, while the cleavage in the β-way leads to formation of the toxic fragments of Aβ. In the case the non-amyloidogenic path, APP

**Figure 1.** Scheme of the amyloidogenic processing of APP.

is cleaved by α-secretase to form a soluble extracellular fragment of sAPP-*α* and C83 fragment, which is split by *γ*-secretase, similar to the case of C99 [6]. However, the α-path does not release Aβ, but it leads to splitting out a short protein fragment p3. The exact physiological function of the fragment p3 has not been completely clarified yet [7]. In the course of the β-path, APP is first cleaved by the enzyme β-secretase (BACE-1) providing the *C*-terminal fragment of the length of 99 amino acids (C99) and a chain, which is transferred to the extracellular space. This remaining protein chain can be found in the literature under the acronym sAPP-β. Subsequently, C99 is cleaved by the activity of γ-secretase to short-length peptides consisting of 38–43 amino acids (referred to as Aβ) and the intracellular *C*-terminal domain (AICD). In most cases, formation of Aβ1–40 mainly occurs, although a longer and more toxic form Aβ1–42 sometimes can be also produced. However, recent findings also point to the fact that the production of Aβ can take place even within the proteolytic cleavage of APP along the α-path (**Figure 1**) [8].
