**4. Amyloid precursor protein (APP) gene**

Tau alternative splicing occurs in exons 2, 3, and 10 and its form is of the (a) type that corre‐

The studies conducted on Tau alternative splicing have been comprehensive, and most of them

Exon 10 displays a splicing pattern of inclusion and it is not present on fetal neurons. It is influenced by exon 9, which promotes its inclusion [18]. Exon 10 codes the second region of the (R) (KXGS) repeats in Tau. Alternative splicing generates Tau isoforms with 3 or 4 repeats that bind to microtubules. In mature brains, the level of 3R and 4R is similar. Exon 10 disruption

Exon 10 is flanked by a long 13.6-kb intron and a short 3.8-kb intron, possessing a weak 5′ splicing site, which is similar to that in 3′. This would allow the inclusion or not of exon 10 in

Exon 2 alternative splicing has been less studied. However, the studies conducted in our laboratory show that when PC12 cells (rat pheochromocytoma) cultures are exposed to the β1 → 42 amyloid peptide; alternative splicing of exons 2 and 3 is affected, as immature forms of Tau mRNA are transcribed in mature PC12 cells (phenotype differentiated into neuron). We observed that processes in these cells begin to retract. In spite the mechanism is still unknown, the effect produced in these cells indicate that immature Tau forms cease to stabilize micro‐ tubules in these cells processes [26] (**Figure 4**). The inclusion of exons 2 and 3 promotes the shift from immature Tau forms to their mature counterparts, stabilizing microtubules.

**Figure 4.** Tau exons 1–9 (modified from [50]), the primers Ex1 and Ex5A, amplify from exons 1 to 5. The exons 2 and 3 are amplified by (Ex1/Ex5A), the exon 6 is amplified by (Ex5/Ex9, Ex6S/Ex6AS) and exon 8 by (Ex7/Ex9). (**A**) Untreated undifferentiated cells and (**B**) NGF-induced differentiated PC12 cells; and from PC12 cells exposed to Aβ(1–42) peptide in (**C**) undifferentiated cells and (**D**) NGF-induced differentiated cells. Differentiation inhibits fetal tau expression. Aβ exposure promotes exclusion of exons 2/3 in undifferentiated and differentiated cells, and exclusion of exon 6 in undif‐

is able and enough to cause neuron degeneration or taupathies [23].

order to generate proteins with or without it [24, 25].

sponds to exon exclusion or inclusion.

have been focused on exon 10.

8 Update on Dementia

ferentiated cells. Scheme taken from [26].

APP is one of the three members of a small gene family coding type I membrane proteins possessing an extracellular domain and a small cytoplasmic region. Only APP contains the sequence coding the Aβ domain. APP human gene is located at chromosome 21, which is involved in autosomal dominant inheritance in some families affected by early Alzheimer's disease. This gene contains 18 exons, and it is more than 170 kb long [33]. More than 25 mutations have been identified to cause the familial type of AD. All of these mutations substitute amino acids near or within the Aβ domain [34]. Aβ is derived from APP by proteolytic cleavage due to an alternative splicing process (generating three isoforms com‐ posed by 695, 751, and 770 residues, respectively) [35], of exons 7, 8, and 15 (**Figure 5**). The APP form without the residues coded by exon 15 is called L-APP, and this isoform is found in most tissues [36]. The APP695 isoform is predominantly expressed in neurons, whereas

**Figure 5.** APP isoforms, showing the different APP proteins from alternative splicing of exons 7 and 8.

APP751 is expressed in all tissues and it includes exon 7, codifying a domain similar to that of the Kunitz protease inhibitor [37].

In neurons, APP is found on terminal vesicles in axons and it can be transported in an anterograde or retrograde manner [38]. Other brain cells also express APP and release variable amounts of Aβ, including astrocytes and microglia.

APP may be subjected to protelytic cleavage during and after its transit through the secretory pathway. The first of them is carried out by the α-secretase enzyme resulting in the release of a large and soluble ectodomain fragment (α-APP) [39] in the extracellular space, while retaining a 83-residue C-terminal fragment (CTF) in the membrane. Alternatively, some APP molecules that were not cleaved by α-secretase may be processed by the activity of an enzyme named β-secretase, generating a β-APP ectodomain that retains one residue from the 99 CTF [6].

The main β-secretase in neurons is a transmembrane aspartyl protease named BACE1, predominantly located at the transgolgi network (TGN) and also in endosomes [40]. The cleavage mediated by BACE1 generates the N-terminal fragment of Aβ. The high level of neuronal BACE1 expression preferably targets APP to the amyloidogenic processing pathway in the brain [34]. Aβ is constitutively released from cells expressing APP in normal conditions. The cleavage generated by β-secretase is followed by a constitutive trim at the C-terminal of the Aβ region, and it is carried out by the activity of γ-secretase. Simultaneously, a peptide fragment designated as p3 is produced from the sequential activity of both α- and γ-secretases [6]. A substantial amount of α-APP is generated by γ-secretase that acts on the inserted APP in plasma membrane.

The Aβ40 and Aβ42 fragments are generated to a large extent during APP internalization and endosomal processing. Most of the Aβ generated within the cell is destined for secretion.

APP has autocrine and paracrine functions during growth regulation. It has been best characterized as trophic as it has been demonstrated that it stimulates neurite growth. This phenotype is compatible with its increased expression during neuron maturation [41, 42].
