Sirtuins Modulation: A Promising Strategy For HIV-Associated Neurocognitive Impairments Part 2

Interestingly, Campestrini et al. [23] found that mRNA of Bcl-2 levels were upregulated after 72 h Tat-treated human T lymphocyte cell line (Jurkat). Nevertheless, they used HIVTat clade C, which has a change R57S compared to clade B and interferes with the cell uptake of Tat [50]. 

Lymphocytes are one of the important immune cells in the human body, responsible for fighting the invasion of various viruses and bacteria. Human memory is the ability of the human brain to remember and recall information. Are the two unrelated? There is a certain connection between lymphocytes and memory.

Studies have shown that when we experience certain events or are exposed to certain stimuli, the immune system will initiate a series of reactions, including the activity of lymphocytes. In this process, lymphocytes produce substances called cytokines, which can affect the activity of the nervous system. This means that the activity of lymphocytes can promote the connection between neurons in the human body and enhance the storage and recall of memory.

In addition, T lymphocytes in lymphocytes can also directly participate in the immune defense of the brain to prevent pathogens from invading and causing inflammatory reactions. This protective effect is of great significance to the health of the human brain and the maintenance of memory. After repeated exposure to the same pathogen, T lymphocytes will gradually form memories, so that the next time the pathogen is encountered, the human body can respond more quickly to the immune system, thereby reducing the probability of infection.

Therefore, protecting the health of lymphocytes and maintaining the normal function of the immune system can well maintain the body's immune defense, maintain physical health, and more importantly, improve memory. Eating healthy, getting enough sleep, strengthening exercise, and keeping a happy mood are all ways to enhance lymphocytes and memory. Be sure to maintain a positive attitude and keep your body, brain, and immune system in good condition. It can be seen that we need to improve memory, and Cistanche can significantly improve memory because Cistanche has antioxidant, anti-inflammatory, and anti-aging effects, which can help reduce oxidation and inflammatory reactions in the brain, thereby protecting the health of the nervous system. In addition, Cistanche can also promote the growth and repair of nerve cells, thereby enhancing the connectivity and function of neural networks. These effects can help improve memory, learning ability, and thinking speed, and can also prevent the occurrence of cognitive dysfunction and neurodegenerative diseases.

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In addition, several reports suggested differences in cell effects between clade B and C, with the last being less neurotoxic [72]. Therefore, partial mitochondrial damage is suggested, or an incomplete MOMP where the undamaged mitochondria show high levels of BCL-2 [54], but further studies are required to confirm this hypothesis. 

Conversely, ER stress is observed in the intrinsic apoptotic pathway, which derives into a UPR [23]. The ER is largely known for its functions of protein traffic, folding, and maturation. 

ER stress is acquired when a stimulus causes an alteration in the functional protein process, leading to unfolded or misfolded proteins. To counter malfunction, ER triggers the UPR where signaling cascades are activated to reconfigure upstream protein production, protein translocation to ER, and autophagy, to restore proteostasis or induce apoptosis if ER homeostasis is not reached. 

The pathways involved in UPR are conducted by the membrane ER proteins PERK, IRE1a, and ATF6a. PERK phosphorylates eukaryotic translation initiation factor 2 subunit-α (eIF2α) so protein synthesis and unfolded protein influx can be reduced. 

eIF2α induces translation of ATF4 mRNA, which stimulates gene expression of proteins required for reduction-oxidation balance; autophagy; and apoptosis such as CCAAT/enhancer-binding protein homologous protein (CHOP), growth arrest, and DNA damage-inducible protein (GADD34). 

CHOP regulates GADD34, which at the same time modulates death receptor 5 (DR5), which recruits caspase 8 for cleavage-mediated activation of BID, one of the pro-apoptotic proteins in the mitochondrial pathway discussed above [73]. 

Another signaling pathway is the one regulated by the IRE1 dissociation from GRP78 and its RNase activity activation. Then, IRE1 targets the mRNA of the X-box-binding protein 1 (XBP1), removing a 26-nucleotide intron that allows for the expression of the activated form of XBP1. 

This protein is an upregulation of other genes related to the folding, secretion, and elimination of misfolded proteins. In addition, IRE1 can undergo a process called regulated IRE1-dependent decay (RIDD), where mRNAs are degraded, including the DR5, an anti-apoptotic mechanism. However, conditions of prolonged stress reduce RIDD effectiveness, causing cell death. 

Lastly, the ATF6 cascade begins with its transport to the Golgi apparatus and cleavage to release a fragment denominated ATFp50, which translocates to the nucleus and induces gene expression in a similar way that XBP1 but with the difference that ATFp50 also modulates Golgi apparatus biogenesis [73]. 

In this context, it has been reported that Tat-induced ER stress leads to unfolded protein response (UPR) activation identified by the gene expression increase of IRE1, PERK, and ATF6, and at the same time, signatures were observed of mitochondrial dysfunction mediated by the loss of mitochondrial membrane potential and caspase 12 and caspase 3 activation [23]. 

Additionally, in the human brain, microvascular endothelial cells Tat-exposure effects include increased protein levels of IRE1, PERK, ATF6, and Bip/GRP78, which resulted in ROS increase [70]. 

Cells suffer from Ca2+ influx increase through Tat interaction with N-methyl-Daspartate receptor (NMDAR) [63]. Nevertheless, strategies to block this receptor have failed to mitigate HAND [74], indicating that Tat uses other pathways to disrupt Ca2+ homeostasis. Ca2+ is stored in ER in normal conditions, and its release is modulated by different stimuli. 

Recently, evidence has indicated that both ER and mitochondria interconnect through a complex known as mitochondria-associated ER membranes, where cellular processes occur, including Ca2+ transport and induction to cell death during long-term cellular stress [75]. 

Ca2+ signaling is the most recognized interaction between these two organelles, with the assistance of IP3 receptors [76]. The ER-mitochondria communication seems to be a feedback loop. 

Increased cytosolic Ca2+ levels due to ER stress-mediated release provoke disruption in mitochondrial membrane potential, causing an increase in ROS production. ER stress can be induced by an impaired protein maturation process triggered by high levels of mtROS. 

Furthermore, mitochondrial Ca2+ uptake induces the opening of permeability transition pore (mPTP) and release of cytochrome c, which drives activation of caspases [75], signifying that Ca2+ signaling between ER and mitochondria regulates cellular respiration. Therefore, an increase in cytoplasmatic Ca2+ concentration could result in cell demise.

Observations in Tat-exposed embryonic rat hippocampal neurons showed an increment in cytosolic Ca2+ and mitochondrial uptake, which cause membrane depolarization and cell death [64]. 

Tat-induced intracellular Ca2++ increase is characterized by an initial transitory stage IP3-mediated and a second prolongated period mediated by NMDARs in fetal human brain neurons [77] and the transient receptor potential canonical channel in rat Nacc neurons [74] that leads to cell death. 

These findings confirm that Tat impacts mitochondria-ER communication through the dysregulation of Ca2+ homeostasis and imbalance of pro and anti-apoptotic proteins, leading to mitochondrial dysfunction, which in turn provokes apoptosis. 

As mentioned before, clinical trials have been performed to tackle Tat-mediated Ca2+ dysregulation using NMDAR antagonists without significant results in mitigating the progression of HAND [74]. The reason behind these results might be the fact that calcium dysregulation is just one of the Tat-mediated effects, which is caused by multiple upstream events. 

Therefore, a strategy could be to tackle an event where most of the disrupted pathways converge and are key to neurodegeneration, such as mitochondrial dysfunction, and to tackle its regulator molecules such as sirtuins.

5. Sirtuins

Sirtuin1–7 are part of a family of proteins class III histones deacetylases. All SIRTs are NAD+-dependent enzymes and participate in a variety of cellular events such as gene expression, DNA repair, and aging [78]. 

They present a diversity of functions and cellular distribution because of structural differences in N- and C-terminal. The conserved catalytic core of SIRTs has two subdomains: Rossmann-fold and a zinc-binding domain. 

Substrates and NAD+ bind to the active cleft between the two subdomains. The interaction between catalytic core, substrate, and NAD+ induces a conformational change that leads to active cleft closing [79]. 

The well-characterized catalytic activity is lysine deacetylation, but several experiments concluded that SIRT5 can also eliminate succinyl and malonyl groups and that SIRT4 is an ADP-ribosyltransferase [80]. SIRT1 and SIRT2 are found in the nucleus and cytoplasm, while SIRT3 is mainly located in mitochondria. Indeed, this helps to restrict their substrates since SIRTs showed no preference in protein sequence. 

However, acetyllysine residue must be within a loop or α-helices to be accessible to deacetylation reaction [80]. Despite the apparent substrate non-specificity of SIRTs, it was revealed that only SIRT1, SIRT2, and SIRT3 can deacetylate Tat [31], which proves a direct interaction that could lead to changes in these SIRTs function towards other substrates. 

Therefore, in this review, we pay attention to only the abovementioned three molecules, and the SIRTs-regulated pathways are shown in Figure 2.

5.1. Sirtuins in Neurodegenerative Diseases

Sirtuins 1–7 are expressed in the brain at different degrees. SIRT1 and SIRT2 are the most frequent in this tissue, but SIRT1 is highly expressed in neurons, while SIRT2 is abundant in oligodendrocytes. 

The sirtuins 3–5 is the second group to be found in the brain, and the least common are the SIRT6-7. SIRT1 is predominantly located at the nucleus, and SIRT2 is distributed in the cytoplasm, while SIRT3 is by preference abundant in mitochondria but with some isoforms being in the cytoplasm and nucleus [81]. SIRT expression differs depending on brain areas, age, and pathological conditions and has been related to neurodegenerative disorders such as PD and AD. 

PD pathogenesis is characterized by aggregation of α-synuclein, which can be reduced in mouse brain PD models due to SIRT1 overexpression [82]. However, inhibition of SIRT2 mitigates α-synuclein toxicity in human neuroglioma cells [83]. 

On the other hand, SIRT1, SIRT3, and SIRT6 expression are reduced in the brain of patients with AD [84]. Moreover, SIRT1 [85] and SIRT3 [86] protein concentrations are reduced in the cerebral cortex of individuals with AD. Both SIRT1 and SIRT3 dysregulation have been related to cognition performance [85,87].

5.2. SIRT1

SIRT1 participates in several cellular processes, including cell metabolism, DNA repair, mitochondrial biogenesis, apoptosis [88,89], circadian timing system [90], gene silencing through the interaction with DNA methyltransferase 1 [91], heterochromatin formation [92], cell cycle progression [93], and oxidative stress response [94]. 

Due to the interactions with numerous molecules, SIRT1 has drawn attention for its role in many diseases, including cancer [95], type 2 diabetes [96], and neurodegenerative disorders [97,98].

5.2.1. SIRT1 in ER Stress and UPR

Recently, findings have revealed that SIRT1 plays an intrinsic role in ER stress mitochondria apoptotic pathway regulation [27,99,100]. In primary chondrocytes, pharmacological and genetic inhibition of SIRT1 leads to phosphorylated PERK increase and downstream proteins such as eIF-2α and CHOP [27]. Similarly, in cardiac cells, depletion of SIRT1 increases protein levels of phosphorylated eIF-2α, ATF4, GADD34, and CHOP [99]. 

However, overexpression or activation of this sirtuin causes abrogation of cell death in both studies. Additionally, an increase in acetylated forms of both PERK [27] and eIF-2α [99] was observed. Nevertheless, Ghosh et al. [100] reported that even without ER stress stimulus, SIRT1 depletion is involved in PERK UPR branch activation by regulating levels of phosphorylated eIF-2α; in addition, they found evidence of physical interaction between SIRT1, CHOP, and GADD34 [100] that was later confirmed by evidence showing formation of an arsenite-induced Sirt1/GADD34/PP1/eIF-2α complex and nuclear SIRT1 translocation to cytoplasm, which leads to GADD34-mediated dephosphorylation/deacetylation of eIF-2α and dephosphorylation of SIRT1 [101]. Collectively, this information indicates that SIRT1 can act as a regulator of apoptosis; hence, the complex GADD34/PP1 is a repressor of eIF-2α, leading to the restoration of ER function when cellular stress has been solved [73]. 

Moreover, it has been suggested that the role of SIRT1 in cell proliferation is mediated by its location [102], which is also dependent on the cell type [103]. However, it is needed to elude the mechanisms that cell types use against different types of stressors and the compensatory responses that are activated.

5.2.2. SIRT1 and Mitochondrial Dysfunction

Mitochondria dysfunction emerges from the disruption in their metabolic and energetic functions, as well as dysregulation in their quality control events, including organelle fission-fusion, mitochondrial biogenesis, and mitophagy. 

The dynamics between fusion and fission are essential for cell adaptation and organelle distribution upon several conditions, such as mitophagy and mitochondria biogenesis [104]. SIRT1 regulates mitochondrial biogenesis through several deacetylation reactions. 

The serine/threonine-protein kinase STK11(LKB1) is activated by SIRT1, leading to the phosphorylation of AMP-activated protein kinase (AMPK). Subsequently, FOXO3 is phosphorylated and deacetylated by SIRT1 to promote transcription of PGC1α, which induces transcription of nuclear transcription factors (Nrf1-2) and the transcription factor A (TFAM), which is related to mtDNA replication and transcription [28]. 

Paradoxically, SIRT1 has been related to mitophagy, but the detailed mechanisms are still being determined. Some studies have revealed that activators such as resveratrol (RV) and SRT1720 induce mitophagy by transcriptional induction of PINK1 through FOXO3 SIRT1-mediated acetylation [105,106]. 

Additionally, SIRT1 is required for mitochondrial fragmentation by acting upon the cytoskeleton in the context of Ca2+ dysregulation [107]. These findings underpin the fact that SIRT1 is a key modulator in mitochondrial dysfunction.

5.2.3. SIRT1 and Tat

In HIV infection, SIRT1 is required for Tat Lys50 deacetylation since Tat-trans-acting responsive element (TAR) complex formation only takes place in its unacetylated form, but Tat acetylation is required to proceed with the transcription. 

Once the process is completed, the same Tat molecules can be recycled through SIRT1-deacetylation to initiate another TAR complex [31,108]. Interestingly, the SIRT1-Tat interaction is independent of the Tat acetylation state [31], which suggests that Tat-induced SIRT1 inhibition takes place when Tat recycling is no longer needed [108]. 

However, there are some questions about the inhibition mechanisms because it is unclear if allosteric mechanisms are taking place and if there is a concentration dependence on the inhibition. 

However, it is notable that Tat-induced SIRT1 inhibition leads to hyperactivation of T cells [30]. As a central player in several cellular processes, Tat-mediated SIRT1 inhibition might be involved in cell dysregulation that in turn causes neuronal loss. Therefore, the activation of this sirtuin should be considered to tackle HAND.

5.3. SIRT2

Although SIRT2 concentrations are higher in the cytosol, during the cell cycle G2/M transition they are translocated to the nucleus, where they regulate H4K20 methylation by deacetylation of histone H4K16, which is key in chromosome compaction [109]. 

Paradoxically, there is evidence of cell proliferation disruption and cell cycle arrest in overexpressing SIRT2 lung cancer cells [110]. In the cytoplasm, SIRT2 interacts with α-tubulin, which is part of the cytoskeleton. Acetylation levels correlate with microtubule stability, and in SIRT2-overexpressed cerebellar granule cells of slow Wallerian degeneration mice, disruption of hyperacetylation and resistance to axonal degeneration was observed [111]. 

However, it was found that SIRT2-/- mice cause axonal damage accompanied by low glutathione levels reduced ATP, reduced mtDNA concentration, and resulted in high expression of SIRT1 compared to wild type [112], signifying disruption of mitochondria homeostasis. 

In line with these results, Liu et al. [113]. observed that in striatum SIRT2-/- mice cells, several mitochondrial proteins have higher acetylation levels than WT, including those involved in energy production. The possibility that SIRT2 acts in the mitochondria was also confirmed in the same group since SIRT2 could be localized in the mitochondria of Wt mice brains and murine embryonic fibroblasts [113]. 

Furthermore, SIRT2-/- mice cortex revealed that depletion of this sirtuin causes mitochondrial size change, increased levels of acetylated PGC1α, and the downregulation of expression of mitochondrial fusion-related genes Mfn1, Mnf2, and Opa1 [113]. Conversely, under stress conditions, HepG2 SIRT2-Wt cells decreased DRP1; meanwhile, SIRT2-catalytic inactivated cells did not [114]. 

In the context of oxidative stress, SIRT2 overexpression increases cell viability and induces FOXO3a-mediated SOD2 expression in neuroblastoma cells [115]. On the other hand, SIRT2-/- MFs showed an increment in ROS and mitophagy key regulator proteins, PINK1, and parkin, as well as LC3B, a maker for impairment of damaged mitochondria clearance [113]. 

The neuroprotective effects discrepancies of SIRT2, which might be the consequence of the several compensatory responses the cell takes under different stimuli. Nonetheless, the role of SIRT2 in mitochondrial homeostasis is notable. 

The fact that Tat physically interacts with this sirtuin and that a marker of Tat-induced neurotoxicity is mitochondrial dysfunction can lead us to hypothesize a possible modulation in the same manner as SIRT1. Therefore, future investigations are necessary to decipher the relationship between SIRT2 and Tat.

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