The Telomerase Connection of the Brain and Its Implications for Neurodegenerative Diseases

Abstract

Telomerase, consisting of the protein subunit telomerase reverse transcriptase (TERT) and RNA component TERC, is best known for maintaining and extending human telomeres, the ends of linear chromosomes, in tissues, where it is active, such as stem cells, germline cells, lymphocytes and endothelial cells. This function is considered as canonical. However, various non-canonical functions for the protein part TERT have been discovered. There are multiple such roles which can interfere with several signaling pathways, cancer development and many other processes. One of these non-canonical functions includes shuttling of the TERT protein out of the nucleus upon increased oxidative stress into the cytoplasm and organelles such as mitochondria. Mitochondrial TERT is able to protect cells from oxidative stress, DNA damage and apoptosis although the exact mechanisms are incompletely understood. Recently, a protective role for TERT was described in brain neurons. Here TERT is able to counteract effects of toxic neurodegenerative proteins via changes in gene expression, activation of neurotrophic factors as well as activation of protein degrading pathways such as autophagy. Protein degradation processes are prominently involved in degrading toxic proteins in the brain like amyloid-β, pathological tau and α-synuclein that are responsible for various neurodegenerative diseases. These new findings can have implications for the development of novel treatment strategies for neurodegenerative diseases. The current review summarizes our knowledge on the role of the telomerase protein TERT in brain function, in particular, under the aspect of age-related neurodegenerative diseases. It also describes various strategies to increase TERT levels in the brain.

Graphical Abstract

telomerase, TERT, telomerase activator, brain, neurodegenerative diseases

Significance Statement

Telomerase is an enzyme normally maintaining telomeres in tissues where it is active, such as embryonic stem and cancer cells. However, there are non-canonical functions of its protein part telomerase reverse transcriptase. One of these roles recently emerged in the protection of the brain and is thus connected to neurodegenerative diseases. The review discusses the current knowledge on telomerase in brain and for neurodegenerative diseases such as Alzheimer's and Parkinson's including possible protective mechanisms such as changes in gene expression and activation of autophagy which degrades toxic brain-related proteins including beta-amyloid, pathological tau and alpha-synuclein.

Introduction: Telomeres and Telomerase

Originally, the enzyme telomerase has been described in connection to the ends of linear chromosomes: telomeres. Telomeres are nucleoprotein structures at the ends of eukaryotic chromosomes. They consist of tandemly repeated hexanucleotide sequences which in most vertebrates consist of TTAGGG repeats. Telomeric DNA is associated with a number of proteins called shelterin.¹ These proteins also support the formation of higher-order structures known as T-(telomeric) and D-(displacement) loops. The latter sequesters a single-stranded end which forms during normal semiconservative DNA replication due to the “end replication problem”. It arises due to the inability of the DNA polymerase to synthesize both DNA strands directly. While the leading DNA strand is synthesized continuously, the lagging strand is synthesized with the help of an RNA primer and Okasaki-fragments. The latter are then combined by a DNA ligase upon removal of the RNA primer. At the very end, however, the RNA primer is removed and leaves a single-stranded gap-the 3ʹ overhang which is, in cells without telomerase activity, one of the reasons for telomere shortening and the induction of replicative senescence when telomeres reach a critically short length. Another reason for telomere shortening is oxidative stress which can result in DNA damage in telomeres.² This damage is not well repaired and can lead to the activation of cell cycle checkpoints and cellular senescence.

In human cells which possess telomerase activity such as germline, stem-, and cancer cells, the 3ʹ overhang is used as substrate for the de novo addition of telomeric hexanucleotides by the telomerase complex consisting of the hTERT (human TERT) protein and the RNA component hTERC (human TERC) which also contains the template region for telomere synthesis. Thus, for its canonical function these 2 minimal components are essential while in vivo there are more associated proteins, for example dyskerin. Telomere synthesis is performed by telomerase in late S-phase after DNA replication is completed. In contrast, in all other cell cycle phases, the overhang is protected within the D-loop and by the shelterin component Pot1 in order to prevent it from being recognized as a DNA damage signal.³

However, while telomerase is active in most vertebrates, yeast and many lower eukaryotes, in mammals telomerase and telomere biology is rather heterogeneous. While rodents have mainly long telomeres and active telomerase in most somatic tissues, other species including humans downregulate telomerase activity in the majority of somatic cell types with the exception of T- and B-lymphocytes and endothelial cells early during development.⁴ In contrast, telomerase is highly active in germline tissues and embryonic stem cells (ESC) and can be upregulated in adult stem cells.⁵ Consequently, it is important to keep these differences in telomere and telomerase biology in mind when interpreting data from experiments performed in mice, including many of those for neurodegenerative diseases described below.

Telomerase Activity in Stem Cells and Cancer Cells

While there are high levels of telomerase activity (TA) in human embryonic stem cells, adult stem cells, which are often quiescent, have the potential to upregulate it when activated into progenitor cells.⁵ High TA ensures that human embryonic stem cells (hESC) grow continuously in culture while telomerase activity is downregulated upon differentiation.⁶ In addition, the presence of telomerase also protects hESCs from oxidative stress.⁷ In contrast, human adult stem cells in culture rapidly differentiate and thus do not maintain their stem cell properties in culture and their TA is much lower than in hESCs (see Fig. 1).

Figure 1.

Telomerase activity (TA) measured in cultured human embryonic and various adult stem cell types using a TRAP ELISA (Roche). As a positive control hTERT-transduced MRC-5 fibroblasts and HeLa cells were used and equal amounts of cell protein were loaded for each cell type. TA is measured as absorbance at 450 nm as arbirtary units (a.u.). Error bars show SD from several independent experiments.

In the adult brain, telomerase activity is mainly restricted to areas containing neural stem cells.⁸ The importance of telomerase for the proliferation of adult neural stem cells (NSCs) from the subventricular zone (SVZ) has been demonstrated in models of telomerase knockout mice where lack of telomerase activity compromised the proliferation of NSCs.⁹ In contrast, Jaskelioff and colleagues reactivated somatic telomerase activity using a conditional knock-in mechanism in telomerase negative (late generation TERC−/−) mice resulting in improved telomere maintenance and restored proliferation capacity of neural progenitor cells in the subventricular zone of the brain leading to some amelioration of brain dysfunction caused by telomere dysfunction.¹⁰ Thus, in these stem cells telomerase activity seems to act mainly through its canonical function of telomere maintenance and is downregulated during cell differentiation.

For iPS (induced pluripotency) cells, telomerase activity is essential and upregulated during the reprogramming process of differentiated cells, but secondarily downregulated again upon differentiation.¹¹ Thus, telomerase activity is tightly regulated under physiological conditions. Importantly, iPS cells can be isolated from patients with inherited mutations leading to neurodegenerative diseases, differentiated in vitro and used to model these diseases and to develop therapeutic strategies.^{12, 13}

Telomeres and telomerase activity play an important role in the development of cancer. While critically short telomeres induce a DNA damage response (DDR) followed by senescence or apoptosis in normal somatic cells, the inactivation of critical cell cycle inhibitors such as p53 and p21 can induce further cell proliferation, telomere shortening as well as telo-meric fusions leading to genomic instability. This instability is able to contribute to the activation of oncogenes as well as the inactivation of tumor suppressors. While most of these cells might die of apoptosis, some are able to acquire telomerase activity thereby stabilizing telomeres and conserving the abberations from genomic instability.¹⁴ This is the reason why most cancer cells have rather short telomeres but are still able to proliferate indefinitely. Thus, the activation of telomerase or another telomere maintenance mechanism such as ALT (alternative lengthening of telomeres) is a prerequisite for ongoing proliferation capability. In general, transcriptional activation of TERT is one of the most important mechanisms for the reactivation of telomerase activity in cancer cells. In some cancer types, the activation of telomerase occurs due to mutations in the TERT promoter providing additional binding sites for novel transcription factors.^{15, 16} These non-coding somatic promoter mutations are responsible for more than 50% of melanomas, glioblastomas, hepatocellular, and urothelial carcinomas.¹⁷ Other well-known mechanisms of telomerase re-activation in cancer cells are epigenetic changes of the TERT promoter.^{18, 19} However, additional mechanisms of telomerase activation certainly exist but in general remain poorly understood.

Non-Canonical Functions of TERT and its Mitochondrial Localization

In addition to the telomere-related canonical function of the telomerase enzyme, the protein subunit TERT has additional non-telomeric functions.²⁰ There are multiple such functions: interaction with cellular signalling pathways such as Wnt,²¹ inflammatory factors such as NFκB,²² involvement in gene expression as well as intracellular shuttling. The latter was first described by Haendeler et al.²³ who found that phosphorylation of tyrosine 707 of the TERT protein by Src kinase after application of oxidative stress results in TERT leaving the nucleus. Santos and colleagues described a mitochondrial localization of TERT due to a mitochondrial localization signal in the TERT protein.²⁴ Other groups confirmed this localization and its association with a protective cellular effect.^25-28 Mitochondrial localization of hTERT was associated with decreased cellular oxidative stress and ROS (reactive oxygen species) levels, less mitochondrial and nuclear DNA damage as well as a decreased sensitivity against apoptosis.^{25, 26, 28} Various mechanisms for this protective effect have been described: induction of antioxidants ,²⁷ improvement of respiration ²⁷ as well as binding of mitochondrial DNA.²⁹ In addition, hTERT is able to use mitochondrial tRNAs for a reverse transcriptase activity within mitochondria ²⁹ while no biological function has been associated with this new biochemical property yet.

In particular for tumourigenesis, TERT is suggested to play an important role beyond the maintenance of telomeres via its non-canonical functions promoting tumor progression, invasion and processes such as EMT (epithelial–mesenchymal transition), migration and metastasising,³⁰ see ³¹ for review). Others have also described a non-canonical mechanism where TERT promotes cancer cell proliferation by influencing global protein synthesis by augmenting tRNA gene expression.³²

Telomerase in Brain and Neurons

In the brain, telomerase activity is mainly restricted to zones with neural stem cells such as the subventricuar zone (SVZ) and dentate gyrus of the hippocampus. Adult neural stem cells possess telomerase activity which is downregulated during differentiation into neurons and astrocytes.^{8, 33} Microglia cells as resident brain macrophages possess some telomerase activity in the adult brain which can also be transiently upregulated upon brain injury.³⁴ Likewise, vascular endothelial cells, including those in the brain, are also known to be positive for telomerase activity.

The first to recognize and study the importance of telomerase in neurons and for brain function was Mark Mattson’s group from the NIH (USA). More than 20 years ago they described a role for telomerase in cultured mouse neurons for their development as well as protection from amyloid and apoptosis.^35,36 The same group also described telomerase activity in mouse brain in vivo which is still present postnatally but is downregulated around day 10.³⁷ Our own results confirmed this finding (Fig. 2). In contrast, TERT mRNA and protein were shown to persist in adult mouse and human neurons and most likely perform non-canonical functions there.^{37, 38-40}

Figure 2.

Telomerase activity in mouse brain tissue of different ages measured using a TRAP ELISA (Roche). Immortalised mouse 3T3 cells were used as positive control and brain tissue from TERT knockout mice were used as negative control. Equal amounts of cell protein were loaded for each brain sample. TA is measured as absorbance at 450 nm as arbitrary units (a.u.). Error bars show SD from at least 2 independent measurements.

Importantly, there seems to be a fundamentally different telomerase biology in the brain based on a different regulation of TERT and TERC compared with other human somatic tissues where hTERT expression is downregulated early during development ⁴ while hTERC is rather ubiquitously expressed. In contrast, in human brain hTERC gene expression as well as telomerase activity seem to be downregulated early during human brain development while hTERT gene expression persists.⁴¹ Moreover, in mouse brain mTert expression is downregulated during ageing and in some mouse models of neurodegeneration during disease progression.^{38, 42} Others have recently analyzed telomerase activity and TERT protein levels in old rats and found a decrease for both parameters in the cortex and cerebellum.⁴³ However, there are no systematic studies analyzing this parameter during human brain ageing yet.

hTERT protein was detected outside the nucleus in hippocampal neurons from brains from rather old donors as well as in microglia cells, but was absent in astrocytes.⁴⁰ Intriguingly, Iannilli and co-workers described that TERT protein from mice and rats forms a complex with RNA particles and mRNA of the cell cycle inhibitor p15 in the cytoplasm of cultured neurons which is dissolved upon oxidative stress allowing the different components to perform various functions.³⁹

The localization of TERT outside the nucleus in neurons can be explained by the fact that these cells are postmitotic and thus do not need to maintain telomeres as those only shorten upon cell division.⁴⁴ However, no mechanism has been described yet how TERT protein is excluded from the nucleus of neurons under normal physiological conditions. In contrast, Eitan et al. described differential changes of nuclear and mitochondrial mTERT protein levels upon stress treatment with X-irradiation and ecxitotoxic glutamate stress, respectively.⁴⁵ In general, there is no final consensus on the localization of TERT in different brain regions and intracellular localization as there exist some contradictory data for different neuron types such as mouse Purkinje neurons were mTert expression and protein were identified in the nucleus, mitochondria and cytoplasm.^{45, 46} Consequently, more research is required in this important novel area in order to better understand and harness the protective effect of telomerase in the brain, in particular, as it might impact therapeutic strategies to modify and ameliorate neurodegenerative diseases in the future.

TERT and Neurodegenerative Diseases

Esther Priel’s group was the first to describe a decrease of mTert expression and protein level in hSOD1 transgenic mice (a model for amyotrophic lateral sclerosis (ALS)) during disease progression.⁴² This study will be described in more detail in the next heading. The same group also described that human mesenchymal stem cells (hMSCs) which normally do not express telomerase activity, obtained this property at a rather low level when isolated from the bone marrow of patients with the neurodegenerative disease ALS.⁴⁷ Based on the finding of elevated TERT protein levels in the cytoplasm in hMSCs derived from patients with ALS, the authors suggested a non-canonical role of telomerase in the disease.

The Saretzki group characterized the role of hTERT protein for Alzheimer’s disease (AD). Despite the initial hypothesis that hTERT protein might also decrease during human brain ageing as shown previously in mice ³⁸ and neurodegeneration the study did not find any decrease of hTERT protein in hippocampal tissue from AD cases compared with age-matched healthy brains.⁴⁰ Instead, the group found an increased mitochondrial localization of the hTERT protein in Braak stage 6 AD brain tissue compared with healthy controls in hippocampal neurons from areas CA1, 2, and 3. However, it is not entirely clear how to interpret this finding: whether it is a result of increased oxidative stress known to be enhanced in AD brains ⁴⁸ or whether it suggests a protective role of hTERT or both, as had been demonstrated previously in cell culture models.^25-28 Analyzing hTERT staining together with pathological tau using an AT8 antibody, the study found that both proteins were mutually exclusive. While in healthy controls there was only hTERT staining in hippocampal neurons, higher Braak stages also showed neurofibrillary tangles (NFT) or neuropil threads (NT). However, neurons positive for hTERT never showed any pathological tau staining while neurons with the latter were never positive for hTERT. Again, it is not clear whether neurons with hTERT protein do not develop NFT and NT or those with the latter displace hTERT from neurons. In order to get a better inside into the relation between TERT and pathological tau, the authors transduced mouse embryonic neurons from either wild-type (WT) or TERT knockout (TERT−/−) mice with a P301L mutated tau and found higher levels of oxidative stress (ROS) in neuronal processes as well as more lipid peroxidation products in the soma of neurons derived from TERT−/− mice.⁴⁰ Increased oxidative stress is known to be associated with AD.⁴⁸ These results suggest that the presence of TERT protein in WT neurons is protective against the effects of pathological tau in decreasing oxidative stress-related parameters.

Recently, Shim and colleagues from the de Pinho group demonstrated a decreased TERT expression in a triple transgenic (tg) mouse model of AD as well as in neurons derived from human induced pluripotency cells (iPSC) characterized by a duplication of the APP gene from AD donors.⁴⁹

The demonstration of a protective role of the telomerase protein TERT against neurodegenerative factors^36,40 prompted various groups to increase telomerase genetically or to apply telomerase activators on models of neurodegenerative diseases for therapeutic purposes.

Increasing TERT Levels with Telomerase Activators on Models of Neurodegeneration

Maria Blasco’s group used a natural telomerase activator: TA-65, a highly enriched plant extract from Mongolian milkvetch (Astragalus membranaceus) on one and 2 year old mice for 3 months.⁵⁰ The study described a slight increase in length of the shortest telomeres in blood lymphocytes without increasing cancer incidence which was an initial worry with enhancing telomerase activity. Others used TA-65 in human studies as an anti-ageing intervention with the aim to stabilize very short telomeres in blood lymphocytes.^{51, 52}

In the brain however, where there is no or only very little telomerase activity, the main effect of telomerase activators is an increase of TERT levels together with its various other beneficial effects. Moreover, it seems that a modest, transient increase of TERT levels or even telomerase activity is still under a tight physiological control and thus will not promote any oncogenesis which is often associated with genetic or epigenetic events.

The first group to apply a telomerase activator on a mouse model of neurodegeneration was Esther Priel’s group. They used the synthetic aryl compound AGS499 on a mouse model of ALS and demonstrated a slight delay of disease onset in the model.⁴² The same group used a mixed neuronal culture treated with amyloid-β (Aβ) as a model for AD resulting in a number of changes in genes associated with neuronal plasticity as well as various neurotrophic factors.⁵³ As an intervention they used the AGS499 telomerase activator which was able to counteract many of the changes due to Aβ treatment in the cell model and also in a short-term treatment in mice in vivo where similar pathways were identified.

The same group also treated bone marrow-derived human mesenchymal stem cells (hMSCs) with the telomerase activator AGS499 for a prolonged time and found a protective effect against oxidative stress (increased resistance against apoptosis and less DNA damage) as well as a small increase in average telomere length.⁵⁴ Other authors used TERT overexpression in iPS cells derived from Huntington disease (HD) or neural progenitor cells with the aim to either model neurodegenerative diseases or to develop cell-based therapies for regenerative treatments of such diseases.^{55, 56}

Wan and co-authors ⁵⁷ used the plant extract TA-65 as well as the synthetic cycloastragenol GRN510 ⁵⁸ initially on 2 year old wild-type (WT) mice for 3 months and found a significant increase of mTert expression in brain tissue. This increase correlated with an improvement of balance of the treated mice compared to old mice with the activator-treated mice performing almost as well as young mice. In order to confirm that indeed neurons responded specifically to the activators, cultured embryonic neurons were treated with both activators for 2 days and displayed a significant increase in mTert expression.⁵⁷

Finally, the study used a transgenic mouse model of Parkinson’s disease (PD) (line D) which expresses human WT α-synuclein mainly in the hippocampus, neocortex, and olfactory bulb.⁵⁹ Interestingly, the α-synuclein level in this model was significantly increased only after 12 months of age,⁶⁰ identifying the model as a good surrogate of age-dependent spontaneous PD. At the same age, dopamine levels decreased and behavioral deficits appeared.⁶⁰.

Wan and colleagues treated these mice for 14 months starting at 4 months of age with both activators orally at a daily basis. They found a significant increase in mTert expression levels in brain tissue from males and females of TA-65 and GRN510-treated line D mice compared with control mice that just received the diluent DMSO. In order to correlate this increase to behavior associated with PD symptoms, the group used various tests such as rota-rod, stride length, walking speed, and novel object recognition (NOR). Intriguingly, the authors found some strong sex-dependent effects of the 2 telomerase activators in the rota-rod analysis which measures speed, time, and distance travelled. While in females only TA-65 improved balance and locomotion, in males only GRN510 improved these parameters significantly.⁵⁷ This result could be possibly explained by differences in physiology and gene expression between male and female brains. In contrast, for the stride length test which is very similar to the gait analysis used in humans, both activators improved the length of the stride as well as the variation between strides significantly,⁵⁷ (see Fig. 3 for illustration).

Figure 3.

Representative images of stride length from a control (DMSO) and telomerase activator (GRN510) treated mouse. The arrows indicate stride length as well as the variation in length between strides from left hind legs (red).

Similar results were obtained from the walking speed test which is a read-out for bradykinesia in PD patients. Interestingly, the novel object recognition (NOR) test again showed some sex-specificity with both activators improving NOR-performance in females, while in males exclusively GRN510 improved it. Similarly, the analysis of oxidative stress from isolated brain mitochondria only found a significant effect of TA-65 but not GRN510 on forward and reverse electron flow while the effect of sex was not analyzed in this application.⁵⁷ From this results, the conclusion can be drawn that a possible mitochondrial TERT localization and decrease in the levels of reactive oxygen species, as had been shown previously by the same group ,^{25, 26, 40} were most likely not the underlying mechanism for the improvement in PD symptoms in the PD mouse model since in general both activators improved most PD symptoms.

Surprisingly, the histological characterization of different α-synuclein species in hippocampal areas and the neocortex from treated mice showed a striking decrease in total, phosphorylated as well as aggregated α-synuclein following the treatment with the 2 telomerase activators,⁵⁷ (see Fig. 4 for representative images). It has been demonstrated in human cellular models previously that hTERT overexpression was able to promote both proteasomal activity as well as autophagy.^{61, 62} Recently, also Yao and co-workers ⁶³ reported an activation of autophagy by astragalus-polysaccharides in vivo in mouse liver tissue without analyzing telomerase. It is known that autophagy is an important mechanism in the brain that is responsible for degrading oligomeric and aggregated species of toxic proteins such as amyloid, pathological tau, and a-synuclein.^{64, 65} Wan and co-authors demonstrated a decrease in the adaptor protein p62 as well as the autophagosome component LC3B which suggests an activated autophagy mechanism due to the treatment with the 2 telomerase activators.⁵⁷ A physiological link between the presence of mTERT and mTOR protein (an upstream signaling molecule of autophagy) levels for the decrease of ROS levels in mouse brains, but not liver tissue had been demonstrated by the same group previously.³⁸ Mitochondrial dysfunction and oxidative stress are known to be associated with most neurodegenerative diseases.⁴⁸ Moreover, a direct physical association of the hTERT and mTOR proteins in a larger complex had been identified in cellular models previously^{66, 67} while its existence has not been described in brain in vivo yet. Future research has to establish a direct correlation between increased telomerase levels and activated autophagy in brain tissue which might inform future therapies for neurodegenerative diseases.

Figure 4.

Representative immuno-fluorescence images of hippocampal CA1 in a control and telomerase-activator treated mouse brain at equal microscope setting demonstrating decreased fluorescence for total (green) and phosphorylated (red) a-synuclein (for colour figure refer to online version).⁵⁷ Scale bars = 50 μm.

It is important to mention that the study from Wan and colleagues ⁵⁷ as many other studies on mTERT in mice and mouse brain in particular, are hampered for a direct analysis of mTERT protein levels and localization due to the lack of specific commercially available anti-TERT antibodies ⁶⁸ while some of these antibodies work well on human brain tissue and in cultured mouse brain cells ⁴⁰ as well as in mouse Purkinje cells.⁴⁵

Recently, another natural telomerase activator, ReverseTM, consisting of a Centella asiatica extract, vitamin C, zinc and vitamin D3 was used for 3 months to increase TERT expression and telomerase activity in rat brain cortex and cerebellum from 18-month-old rats in order to combat brain ageing.⁴³ The authors also described a decrease of telomerase activity and TERT protein level in old (21 months) compared with young (6 months) rats.

Increase of TERT Levels Using Genetic Tools

Maria Blasco’s group used telomerase knockout mice with short telomeres and showed that these contribute to phenotypes similar to neurodegeneration.⁶⁹ Using adenoviral mTert overexpression was able to counteract a neurodegenerative phenotype. However, most of these effects were mainly dependent on telomerase activity and its role in telomere maintenance while non-canonical functions of mTERT were not analyzed in detail in this study. However, the decrease in DNA damage in brain tissue found could also stem from such non-telomeric functions of TERT in the brain.

Some impressive and comprehensive results were published recently by Shim and coworkers⁴⁹ using genetic means to decrease and overexpress mTert in various mouse models. The authors describe changes in the expression of various AD-relevant genes in mTert-haploinsufficient mice including the upregulation of the genes for the amyloid precursor protein (APP) and for the APP-processing enzyme Bace2. The reduction of the gene dosage for the telomerase protein mTERT led to the suppression of different neuronal pathways responsible for neuronal differentiation, axon extension, transmission of neuronal pulses and regulation of neuronal action potential.⁴⁹ Similar to the findings of Baruch-Eliyahu and colleagues,⁵³ the authors described a decrease of brain-derived neurotrophic factor (BDNF) in their mTERT-haploinsufficient mouse model. Importantly, in a triple transgenic (tg) mouse model of AD as well as in neurons that were derived from human induced pluripotency cells (iPSC) harboring a duplication of the APP gene from AD donors with this mutation, the authors identified decreased hTERT expression.⁴⁹ These results differ from the findings of Spilsbury et al. ⁴⁰ who did not find any changes of TERT protein amounts in the hippocampus of spontaneous AD patients compared to healthy control brains while gene expression was not analyzed in that study. In contrast to patients with non-genetic spontaneous AD from that latter study, one could speculate that the presence of AD-related mutations in the models of Shim and co-workers ⁴⁹ exacerbated changes in TERT levels. Thus, the results from the latter study ⁴⁹ convincingly demonstrated that a reduction in TERT expression results in important changes of brain-related and AD-relevant phenotypes and signaling pathways that are functionally involved in AD pathogenesis. In order to directly confirm this conclusion, the authors overexpressed an inducible mTertin vivo in mouse neurons and crossed those tg mice with AD model mice. Indeed, the induction of mTert was associated with a decrease of pathological amyloid as well as neuro-inflammation in non-neuronal cells such as astrocytes and microglia.⁴⁹ At the same time, it improved learning and cognition capabilities of these novel mouse crosses. As described previously in other mouse models, changes in mTert expression levels resulted in a modified gene expression of synaptic signaling pathways, neuronal transmission and plasticity as well as neuronal projections in the opposite direction from that which was found in Tert-haploinsufficient mice.⁴⁹ Importantly, an in vitro experiment in the iPSC-derived neurons employing a catalytically inactive hTERT showed very similar changes to those obtained in the above mouse model. Consequently, this approach confirmed and emphasized the non-canonical function of TERT for the improvement of AD-related symptoms and phenotype. In summary, this new study from the de Pinho group convincingly demonstrated that activation of TERT in brain and neurons of different models counteracted various features of AD such as synaptic dysfunction and amyloid-related pathology. It thus also confirms the results from other studies described above which increased physiological TERT levels using either natural or synthetic telomerase activators which was associated with the improvement of AD- or PD symptoms.

Conclusion

In the last decade, there is an increasing interest in and evidence for a brain-specific role of the telomerase protein TERT adding to its non-canonical functions. Recent studies demonstrated a protective function of the telomerase protein TERT for physiological brain function as well as neurodegenerative diseases. In this brain-specific, non-canonical function, increased TERT levels were able to counteract some adverse effects of different pathological proteins and change neuronal genes and pathways which are known to be associated with different neurodegenerative diseases such as AD and PD. AD-related toxic proteins such as amyloid-β and pathological tau were shown to change the expression of brain- and disease-related genes and to decrease oxidative stress in neurons, respectively.^{40, 49, 53}

Importantly, the successful use of various available telomerase activators on in vitro and in vivo models of neurodegenerative diseases such as AD and PD was able to boost TERT expression levels and to decrease disease-related symptoms and pathological parameters in different experimental settings.^{40, 42, 53, 57} These results are encouraging and will hopefully be translated into novel therapeutic approaches in order to delay and ameliorate various neurodegenerative diseases in the future.

Funding

None declared.

Conflict of Interest

The author indicated no financial relationships.

Data Availability

No new data were generated or analyzed in support of this review.