The phosphatidylinositol 3-kinase (PI3K)/AKT serine/threonin protein kinase (AKT)/mammalian target of rapamycin kinase (mTOR) signaling pathway in central nervous system (CNS) has been implicated in metabolism regulation and energy homeostasis [1] and has emerged as an important mediator of neuronal physiology, appearing to serve as a direct link between aspects of learning and memory, neuronal survival, neurogenesis and apoptosis [2,3]. The PI3K is highly conserved through evolution, and has been described in species ranging from insects to mammals [3]. PI3K signaling has been implicated in aging and lifespan regulation, and the proliferation of adult neuronal progenitor cells, as well as in synaptic plasticity, which gives PI3K relevance to cognitive processes in addition to pathological brain aging and neurodegeneration [1-8]. The ability of PI3K to control key functions of the cell provides the incentive for investigations into the effects of the modulation of PI3K signaling pathways in different aspects of neuronal physiology, including its role in neuronal development as well as brain aging and dysfunction.

The PI3Ks are multifaceted enzymes that are primarily responsible for the production of 3′ phosphoinositides from phosphatidylinositol in response to growth factors. The most common PI3K is PI3K IA, a functional heterodimer comprised of two subunits, one catalytic and one regulatory, with molecular weights of 110 kD (p110) and 85 kD (p85), respectively [4,9,10]. Three different genes (p85α, p85β, and p55γ) encode the regulatory unit. In addition, the p85α mRNA can undergo alternative splicing to produce p50α and p50α/AS53. The p110 protein exists as one of four isoforms (α, β, γ, δ) [1].

Most neurons express all the isoforms of the regulatory p85 subunit, but relatively high levels of the catalytic p110α are found in the hippocampus, olfactory bulb, and cerebellum [2,11]. The various isoforms can result in different roles for PI3K in cellular physiology depending upon the sub-cellular location, the metabolic cellular context, and the involvement of upstream signals and receptor signaling pathways [3].

The PI3K/AKT pathway is a key transducer of mitogenic and metabolic signals that drive proliferation and inhibit both the apoptosis and differentiation of adult neuronal progenitors [12]. Activation of PI3K is induced by many growth factors such as insulin and insulin like growth factor 1 (IGF1), Ras proteins, or the diminished activity of the phosphatase and tensin homolog (PTEN). The activation of the catalytic subunit of PI3K leads to the phosphorylation of phosphatydylinositol-4,5-bisphosphate (PIP2) to generate phosphatidylinositol-3,4,5-trisphosphate (PIP3). In turn, PIP3 drives the activation of the phosphoinositide-dependent protein kinase-1 (PDK-1). The serine-threonine kinase, AKT, is recruited to the cell membrane and phosphorylated by PDK-1, which stimulates the catalytic activity of AKT, which in turn phosphorylates and downregulates glycogen synthase kinase 3 β (GSK3β). Once activated, AKT is able to phosphorylate numerous downstream signaling proteins, including members of the Bad-dependent apoptosis machinery, the forkhead transcription factor proteins (FOXOs), the nuclear factor κB transcription factor (NFκB) and GSK3β, as well as mTOR, which modulates protein translation machinery [12]. In addition, signaling proteins that contain pleckstrin homology (PH) domains accumulate at sites of PI3K activation in the inner surface of the plasma membrane via an interaction between the PH domains and the phospholipid products of PI3K [4,5,12].

During brain development, PI3K is involved in a variety of cellular functions that include proliferation, cell migration, and axon guidance [13]. In addition, PI3K activity is essential for microtubule transport during axonal growth cone formation and for the establishment of neuronal polarity [16]. Signaling of PI3K/AKT via the cAMP response element-binding (CREB) transcription factor is involved in the proliferation of hippocampal neuronal cells following fibroblast growth factor 2 (FGF2) stimulus and activation of the FGF receptor 1 [12]. In adult brain, the activation of PI3K promotes the survival of newly-formed dentate gyrus granule cells generated during exercise, which lead to increase in synaptic plasticity [14].

The role of the PI3K/AKT signaling cascade in synaptic plasticity and memory processes in the mature brain deserves a special mention. In the adult brain, evidence has accumulated for a variety of mechanisms by which PI3K signaling modulates long-term potentiation (LTP), a cellular correlate of memory. Intra-cerebro-ventricular (i.c.v.) infusions of the PI3K inhibitor LY294002 blocked LTP maintenance in the in vivo hippocampus and were associated with a transient phosphorylation of the PI3K substrate AKT at Ser473 [15]. Additionally, in vivo infusion of the PI3K inhibitor wortmannin together with LY294002 and the mTOR inhibitor rapamycin into the pre-frontal cortex also inhibited the long-term retention of trace fear memory [16]. PI3K may influence synaptic plasticity via the modulation of receptor trafficking to the synaptic membrane, a mechanism that promotes rapid and long lasting synaptic changes [17,18]. AKT is also involved in the control of synaptic strength, via phosphorylation of the GABA receptor. In this regard, it is possible that insulin stimulation induces a localized activation of PI3K at dendritic synapses, which in turn induces activation of AKT in the post-synaptic vicinity [19]. Interestingly, exposure to neurotrophin 3 (NT-3) or brain derived neurotrophic factor (BDNF) produces synapse maturation at the neuromuscular junction (NMJ) as well as dendritic sprouting in hippocampal neurons, and in both kinds of neurons, PI3K signaling promotes long-lasting changes in synaptic strength that depend upon both presynaptic and postsynaptic protein synthesis mediated via the activity of the eukaryotic translation initiation factor alpha (eIF2-α), a target of mTOR [20,21] (Figure 1).

It has been suggested that signaling pathways that play a relevant role during development may also be implicated in aging [22]. Accordingly, mutations that inactivate certain signaling pathways, such as insulin/PI3K signaling, slow senescence and increases lifespan up to several folds in mouse models [23].

Brain aging is a highly complex process involving multiple systems and cellular pathways. Since more than a decade it is known that mutation in some elements of the insulin/PI3K pathway significantly impact longevity [7,24]. A constant feature of aging is the induction of stress response pathways controlled at the molecular level by a number of highly conserved molecules and transcriptional regulators, including proteins of the insulin and insulin like growth factor 1 (IGF1), mTOR, sirtuin system, and AMP activated protein kinase (AMPK) pathways [26-28].

The search for longevity-related genes has led to the increased understanding of signaling pathways involved in the regulation of lifespan in some model organisms. Among these, insulin/PI3K signaling has been demonstrated to be a key player in the control of longevity. Insulin, as well as insulin like growth factor 1 (IGF1), binds to the insulin receptor (IR). Adapter proteins, such as insulin receptor substrate proteins (IRS1-4), bind to tyrosine residues and, in turn, activate PI3K and AKT, which then act upon different target proteins such as GSK3β and mTOR [7,8].

Mutations in some genes that participate in PI3K signaling can lead to increased life span. In Caenorhabditis elegans (C. elegans), mutations in Age-1, a PI3K catalytic subunit homologue, produce an elevated lifespan [29-31]. Another example is Daf-2, an IR homologue, a temperature-dependent mutation of which nearly doubles lifespan in C. elegans. Daf-2 produces stress resistance and longevity via the inhibition of the FOXO transcription factor homologue Daf-16. In addition, loss of the Drosophila melanogaster IRS homologue, CHICO, has shown to increase longevity [31].

In mammals, the role of the insulin/IGF1/PI3K pathway in longevity seems to be more complex. For example, in older humans, a compensatory hyperinsulinemia is developed to maintain glucose homeostasis and prevent type 2 diabetes mellitus (T2DM). Nevertheless, elevated levels of circulating insulin have negative effects on the brain and can diminish lifespan. It has been proposed, therefore, that attenuating insulin signaling in aged or in overweight people may halt the damaging effects of hyperinsulinemia [32]. In support of this idea, mice with IR mutations were found to have diminished adipose tissue and increased longevity [33]. Interestingly, the selective mutation of brain IRS2 is sufficient for increased longevity in mice, suggesting that targeted inhibition of this protein may also impact longevity in humans [32]. Although the role of insulin signaling in human longevity is controversial, it has been shown that centenarian individuals show an increase in peripheral sensitivity to insulin and reduced levels of circulating insulin [34]. Findings from studies of nonagenarians likewise suggest that longevity may be associated with differences in insulin sensitivity [35]. Two external factors that impact aging are known to be physical exercise and caloric restriction; this last factor, in particular, appears to have important effects on longevity-related processes. Caloric restriction in mice reduces the intensity and duration of insulin secretion required for glucose homeostasis, leading to diminished insulin levels and promoting the expression of antioxidant enzymes [36].

The mTOR kinase function, a downstream participant of the insulin/PI3K pathway, may be particularly important for longevity. Numerous cellular signals regulate mTOR activity; these include low oxygen pressure, reduced nutrient concentration, oxidative stress, and DNA damage. Once activated, mTOR participates in nutrient importation, translation of messenger RNA, and the biogenesis of ribosomes. mTOR activates the eukaryotic translation elongation factor 2 (eEF2), eEF2 kinase (eEF2K), ribosomal protein S6 kinase (S6K), which regulates protein synthesis and gene transcription [37,38]. Diminished activation of the PI3K/AKT/mTOR pathway significantly augments longevity in mice [39]. Likewise, centenarian individuals that show increased sensitivity to insulin also have decreased mTOR activity. Thus, longevity seems to be associated with the reduced activity of the insulin or IGF-mediated PI3K/AKT/mTOR pathways, implicating these signaling cascades as important targets for pharmacological manipulation [22]. Importantly, mTOR activates S6K, which in turn phosphorylates IRS proteins in serine residues, inhibiting insulin signaling, by a negative feedback loop of regulation [40]. The PI3K/mTOR pathway also plays an important role in autophagy, a catabolic process that helps to maintain cellular homeostasis. The regulation of autophagy is intimately associated with the control of cell growth, cell proliferation, cell survival, and cell death. Impairments of autophagy have also been demonstrated in neurodegenerative diseases, such as Alzheimer's disease (AD) [40,41].

Along PI3K/AKT/mTOR pathway the sirtuin system, found from bacteria to mammals, has shown to play an important role in controlling longevity, oxidative stress, insulin resistance, metabolism and neuroprotection. Sirtuins belongs to a family of class III histone deacetylases that transfers acetyl groups from lysine residues to ADP-ribose moiety of NAD⁺ producing a deacetylated protein, nicotinamide and a 2′-O-acetyl-ADP-ribose. SIRT1 and SIRT2 are highly expressed in brain [27,42-44]. It has been probed that SIRT1 interacts directly with the p85 subunit of PI3K forming a complex that, after insulin stimulation, binds to IRS1/2 and activates this pathway. Moreover, SIRT1 levels correlate positively with AKT phosphorylation at serine 473. Other target proteins of SIRT1 are AMPK, acetyl CoA synthetase, glutamate dehydrogenase, IRS, histones, and other molecules involved in the modulation of energy metabolism, stress responses, and cell survival [27,42-45].

Biological processes linked to normal aging also include systemic inflammatory and immune responses, oxidative stress, and altered calcium regulation. Among these, oxidative stress leads to mitochondrial dysfunction resulting in reduced respiratory metabolism and the increased generation of reactive oxygen species (ROS). Also, accumulation of DNA damage may results from both increased oxidative damage and the reduced efficiency of DNA repair, predisposing the cell to apoptosis, senescence, and inflammation. Aging is also associated with protein misfolding and subsequent aggregation in the cytoplasm, nucleus, and endoplasmic reticulum. Age-related cellular damage, as well as stress and physiological decline, contribute to the pathogenesis of age-related diseases including metabolic syndromes, inflammatory disorders, cancer, and neurodegenerative diseases [46-49] (Figure 2).

In this review, we have described how aging is associated with changes in several transduction pathways. In particular, we have reviewed how insulin/IGF1-dependent PI3K/AKT/mTOR dysregulation is associated with abnormal cellular functions that lead to cellular stress, cellular senescence, altered neuronal plasticity, apoptosis, and cell death. The hippocampus is a nodal region in the control of neuronal reactions to stress, and is highly plastic region implicated in spatial learning and memory, which make the hippocampal circuit a particularly vulnerable brain site. Therefore, research involving PI3K modulation in this specific brain region may help to retard senescence and/or aging-related brain damage and cognitive impairments. However, the pleiotropic nature of the PI3K/AKT pathway makes its selective inhibition/activation difficult. The analysis of specific PI3K isoforms and the development of specific molecular modulators are necessary to address this problem. In this regard, there is currently work being conducted into the structural analysis of oncogenic PI3Kα mutations as a basis for the molecular design of isozyme-specific and mutation-specific inhibitors for individualized cancer therapies [108].

There is the possibility of ameliorating some of the molecular hallmarks of AD. For example, it has been reported that the PI3K inhibitor LY294002 induces the accumulation of a highly ubiquitinated form of presenilin-1. Although the biological significance of this is not known, possible interventions into PI3K-related Aβ generation therefore exist [109]. Interestingly, a recent trial with the insulin sensitizer, rosiglitazone, showed positive trends in mild-to-moderate AD patients [110].

Another important candidate for the modulation of PI3K signaling is mTOR, the downstream PI3K target. For example, inhibition of mTOR activity regulates longevity by stimulating cellular autophagy and the removal of misfolded proteins and damaged organelles, thus restoring cellular function [37,38,111]. It is interesting to note that hyperactivation of mTOR is associated with multiple degenerative diseases, autoimmune diseases, and cancer, therefore its inhibition is a reasonable strategy to combat such disorders [37].

The main mTOR inhibitor, rapamycin, is well-tolerated by humans and is interesting as an anti-aging drug. Nevertheless, it is necessary to clearly understand the role of mTOR action in normal cell functioning [37,112,113]. Rapamycin has been proposed for the treatment of neurodegenerative disorders, as it is a hydrophobic molecule that readily crosses the blood-brain barrier. Rapamycin promotes autophagy, and therefore may combat protein aggregates like tau or Aβ [37]. Recently, novel catalytic inhibitors of mTOR have been designed that are synthetic small molecules that function as competitive ATP-binding inhibitors [114,115]. In addition, newly developed dual mTOR-PI3K inhibitors have been described that counteracts the PI3K/AKT overstimulation that occurs when mTOR alone is inhibited [116,117].

Another candidate to be regulated is the sirtuin system implicated in insulin/PI3K/AKT/mTOR pathway modulation. Several conditions such as caloric restriction, and exercise as well as the compound resveratrol improve insulin/PI3K/AKT/mTOR signaling pathway trough modulation of sirtuin content [27,42-45].

The modulation of molecules involved in signal transduction pathways is an emerging therapeutic option for a number of human diseases. The PI3K/AKT/mTOR signaling pathway coordinates a variety of complex events that lead to changes in cell metabolism, cell growth, cell movement, and cell survival. Further studies of this pathway should be directed towards the design of new small-molecule modulators of PI3K isoforms implicated in neurodegeneration.