Forkhead Transcription Factors: Formulating a FOXO Target for Cognitive Loss

Abstract

BACKGROUND

With almost 47 million individuals worldwide suffering from some aspect of dementia, it is clear that cognitive loss impacts a significant proportion of the global population. Unfortunately, definitive treatments to resolve or prevent the onset of cognitive loss are limited. In most cases such care is currently non-existent prompting the need for novel treatment strategies.

METHODS

Mammalian forkhead transcription factors of the O class (FoxO) are one such avenue of investigation that offer an exciting potential to bring new treatments forward for disorders that involve cognitive loss. Here we examine the background, structure, expression, and function of FoxO transcription factors and their role in cognitive loss, programmed cell death in the nervous system with apoptosis and autophagy, and areas to target FoxOs for dementia and specific disorders such as Alzheimer’s disease.

RESULTS

FoxO proteins work in concert with a number of other cell survival pathways that involve growth factors, such as erythropoietin and neurotrophins, silent mating type information regulation 2 homolog 1 (Saccharomyces cerevisiae) (SIRT1), Wnt1 inducible signaling pathway protein 1 (WISP1), Wnt signaling, and cancer-related pathways. FoxO transcription factors oversee pro-inflammatory pathways, affect nervous system amyloid (Aβ) production and toxicity, lead to mitochondrial dysfunction, foster neuronal apoptotic cell death, and accelerate the progression of degenerative disease. However, under some scenarios such as those involving autophagy, FoxOs also can offer protection in the nervous system and reduce toxic intracellular protein accumulations and potentially limit Aβ toxicity.

CONCLUSIONS

Given the ability of FoxOs to not only promote apoptotic cell death in the nervous system, but also through the induction of autophagy offer protection against degenerative disease that can lead to dementia, a fine balance in the activity of FoxOs may be required to target cognitive loss in individuals. Future work should yield exciting new prospects for FoxO proteins as new targets to treat the onset and progression of cognitive loss and dementia.

The Impact of Cognitive Loss in the Global Population

Nervous system disorders affect a significant proportion of the world’s population and result in disability and death for multiple individuals. In particular, cognitive disorders such as Alzheimer’s disease (AD) can impact greater than 5 million individuals in the United States. Familial cases of AD account for less than 2% of all presentations of AD (1, 2). Throughout the world, almost 47 million people suffer from some form of dementia with approximately 60% of these cases resulting from AD (2–5). Unfortunately, the availability of definitive treatments to resolve or prevent the onset of cognitive loss are limited and to the most extent are non-existent (6, 7). For example, alternative treatments provide only some relief to limit the progression of symptoms (8–13). As a result, it is vital to explore novel strategies that can potentially target, treat, and possibly limit the onset and progression of cognitive loss in patients suffering from such disorders.

FoxO and the Family of Forkhead Transcription Factors

Mammalian forkhead transcription factors may be one such avenue that can offer effective therapeutic strategies for dementia and cognitive loss (14–17). Greater than one hundred forkhead genes and 19 human subgroups that range from FOXA to FOXS are known to exist since the original discovery of the Drosophila melanogaster gene forkhead (18). If one focuses upon the mammalian FOXO proteins of the O class, these forkhead box class transcription factors have the members FOXO1, FOXO3, FOXO4, and FOXO6 (19). Previous terminology for forkhead proteins included forkhead in rhabdomyosarcoma (FKHR) (FOXO1), FKHRL1 (forkhead in rhabdomyosarcoma like protein 1) (FOXO3a), the Drosophila gene fork head (fkh), Forkhead RElated ACtivator (FREAC)-1 and -2, and the acute leukemia fusion gene located in chromosome X (AFX) (FOXO4) (20, 21). With the current nomenclature, an Arabic number is provided with the designation of “Fox”, then a subclass or subgroup letter is provided, and finally the member number is listed within the subclasses of the Fox proteins (22). All letters are capitalized for human Fox proteins. For the mouse, only the initial letter is listed as uppercase and for all other chordates the initial and subclass letters are in uppercase (23–25).

FoxO proteins are transcription factors and bind to deoxyribonucleic acid (DNA) through the FoxO-recognized element in the C-terminal basic region of the forkhead DNA binding domain (26, 27). Following forkhead binding to DNA, target gene expression is repressed or activated through fourteen protein-DNA contacts with the primary recognition site located at α-helix H3 (28). Phosphorylation or acetylation that can block FoxO activity may alter the binding of the C-terminal basic region to DNA to prevent transcriptional activity (29). In addition, multiple mechanisms may contribute to forkhead DNA binding that involve variations in the N-terminal region of the recognition helix, changes in electrostatic distribution, and nuclear translocation of FoxO proteins (30–33).

FoxO Transcription Factors in the Nervous System

FoxO proteins are expressed in all tissues of the body, but serve to have multiple roles in the nervous system (27, 34). Interestingly, individual FoxO proteins can have selective expression in the nervous system (27, 34). For example, FoxO6 is present in several regions of the brain, such as the hippocampus, the amygdala, and the nucleus accumbens (35, 36) and may control memory consolidation and emotion (37). FoxO6 also is involved in other regions of the body and may control hepatic gluconeogenesis and cellular metabolism (38). FoxO3 may be involved in a number of pathways that involve in auditory synaptic transmission (39), cerebral endothelial vascular cell survival (40, 41), oxidative stress injury in mouse cerebellar granule neurons (42), erythroid cell growth (43), hippocampal neuronal injury (44, 45), and neonatal hypoxic-ischemic encephalopathy (46). In relation to FoxO1, this transcription factor may impact astrocyte survival (47), embryonic endothelial stem cell survival (48), ischemic brain injury (49), vascular disease (50), and memory pathways in the striatum and sub-regions of the hippocampus (35).

FoxO Transcription Factors and Cell Death

FoxO transcription factors may impact cognitive loss through the modulation of pathways that involve programmed cell death pathways of autophagy and apoptosis (51, 52). In regards to autophagy, FoxO proteins may be protective. Some studies suggest that FoxO activity, such as FoxO1, can function to increase basal autophagy and reduce atherogenesis (53, 54). Ectopic expression of FoxO1 enhances autophagy and toxic mHtt protein clearance in neuronal cell cultures (55). In addition, a loss of FoxO activity with autophagy inhibition during aging may contribute to neuronal dysfunction and the induction of β-amyloid (Aβ) production (56).

In regards to apoptosis, loss of FoxO activity usually improves cell survival. Loss of FoxO transcription factors can protect against microglial cell demise during oxidative stress (57) and Aβ exposure (58), promotes the protective effects of metabotropic glutamate receptors (59), increases neuronal cell survival through nicotinamide adenine dinucleotide (NAD⁺) precursors (60), enhances survival with growth factors (61), such as erythropoietin (EPO) (30, 40, 43, 62) and neurotrophins (63–65), and can lessen metabolic and vascular disease (66). Work has suggested that some antipsychotics, such as clozapine, may function through FoxO inhibition to protect against apoptotic neuronal cell loss (67).

In some scenarios, pathways involving silent mating type information regulation 2 homolog 1 (Saccharomyces cerevisiae) (SIRT1) and Wnt signaling may affect FoxO modulation of apoptotic cell death. In cortical bone formation, loss of SIRT1 activity can lead to depletion of Wnt activity with a corresponding increase in FoxO activity and the loss of bone formation (68). SIRT1 can result in enhanced neuronal survival through modulation of FoxO activity (69–73). SIRT1-mediated deacetylation of FoxO1 also results in starvation-induced increases in autophagic flux that can maintain vascular and cardiac left ventricular function during periods of starvation (74). Under some conditions, sirtuins and FoxO transcription factors may function synergistically to increase neuronal cell survival (4, 71). FoxO proteins in conjunction with SIRT1 pathways may offer protection against Aβ toxicity (75) and forkhead transcription factors, such as FoxO3a, may be dependent upon SIRT1 to reduce oxidative stress and cell injury during exposure to Aβ (76). In models of Drosophila, loss of FoxO and SIRT1 activity with a reduction in autophagy activity can lead to neuronal accumulation of Aβ (56).

In addition, Wnt signaling pathways (77) with Wnt1 in microglial cells of the central nervous system can block apoptosis through the post-translational phosphorylation and sequestration of FoxO3a in the cytoplasm to prevent caspase activation (78). Wnt1 inducible signaling pathway protein 1 (WISP1), a target of Wnt signaling (79, 80), prevents apoptotic neuronal death through the post-translational phosphorylation of FoxO3a, sequestration of FoxO3a in the cytoplasm with protein 14-3-3, and limiting the deacytelation of FoxO3a (44). Neuroprotective trophic factors and cytokines, such as EPO (22, 30, 40), rely upon Wnt signaling to offer cellular protection through the inhibition of FoxO proteins.

FoxO Transcription Factors and Cognitive Loss

FoxO transcription factors can affect multiple pathways in the body and influence a number of disease entities (34, 81). As an example, FoxO proteins are considered as therapeutic targets for cancer (23, 82–84), diabetes mellitus (30, 85, 86), and pathways involving oxidative stress (82, 87, 88). Interestingly, FoxO transcription factors also play a significant role in inflammation. The FoxO pathway can affect renal inflammation (89), vascular inflammatory pathways (21), and cardiac injury (21).

Some of these very same pathways involving FoxOs including cancer-related signaling pathways (90, 91) also impact the nervous system and cognition. Calcineurin and FoxO3 can interact in astrocytes during Aβ exposure that results in pro-inflammatory cytokines and injury to neurons (92). FoxO transcription factors could be considered a target to block Aβ production and possibly suppress the onset and progression of AD. Nuclear translocation of FoxO3 is tied to apoptotic neuronal DNA damage (15, 45, 93). Histone deacetylase 2 (HDAC2) can form a physical complex with FoxO3a that can lead to oxidative stress-induced cerebellar granule neuron apoptosis (42). In some circumstances, Aβ can result in the dephosphorylation and mitochondrial translocation of FoxO3a that leads to mitochondrial dysfunction (17). Blockade of FoxO activity can protect against oxidative stress and Aβ toxicity (58, 94). Increased FoxO activity can function in concert with tribbles pseudokinase 3 to result in apoptotic and autophagic Aβ induced neuronal cell death (95).

Yet, as previously noted, some studies suggest that Aβ toxicity may be attenuated through SIRT1 and FoxO3a antioxidant dependent pathways (76) as well as those involving autophagy. In models of full-length mutant Huntingtin (mHtt) transgenic mice, ectopic expression of FoxO1 enhances autophagy and toxic mHtt protein clearance in neuronal cell cultures (55). SIRT1 and FoxO proteins can function synergistically to promote cell survival. As an example, in differentiated chrondrocytes exposed to oxidative stress, forkhead transcription factors FoxO1 and FoxO3 in combination with SIRT1 activity are protective with the production of autophagic related proteins (87). In other systems such as the maternal decidua, FoxO proteins may function independently during oxidative stress with FOXO1 preventing oxidative stress damage and FOXO3a promoting oxidative cell death (96).

Considerations for the Future

Cognitive disorders affect a significant proportion of the world’s population. Yet, the availability of treatments that can resolve the loss of cognition are severely limited. Mammalian forkhead transcriptions of the O class (FoxOs) offer an exciting pathway to develop novel treatments for cognitive loss. These transcription factors can target gene expression in the nervous system and are found in regions that affect memory, such as the hippocampus. FoxOs oversee the programmed cell death pathways of autophagy and apoptosis. Under some scenarios such as with autophagy, FoxOs can offer protection in the nervous system and reduce toxic intracellular protein accumulations and potentially limit Aβ toxicity. FoxOs have been shown to reduce oxidative stress and protect neurons through the induction of autophagy. Yet, FoxOs also contribute to apoptotic cellular death and therefore by limiting the activity of FoxOs, neuronal protection can ensue when apoptotic cellular death is prominent. Recent work suggests that some antipsychotics, such as clozapine, may function through FoxO inhibition to protect against apoptotic neuronal cell loss. Importantly, FoxOs function with a number of other cell survival pathways that involve growth factors, such as EPO and neurotrophins, SIRT1, WISP1, Wnt signaling, and cancer-related pathways. Ultimately, FoxOs can lead to the activation of pro-inflammatory pathways, alter Aβ production, result in mitochondrial dysfunction, foster neuronal injury, and accelerate the progression of degenerative Aβ toxicity. Yet, as suggested above, a fine balance that oversees the activity of FoxOs may be required to offset cognitive loss since under some circumstances FoxOs can be protective. Future studies of FoxO transcription factors can provide exciting areas of investigation and discovery to uncover some of the hidden opportunities for new drug development to treat cognitive loss.