DNA damage responses in neural cells: Focus on the telomere

Abstract

Postmitotic neurons must survive for the entire life of the organism and be able to respond adaptively to adverse conditions of oxidative and genotoxic stress. Unrepaired DNA damage can trigger apoptosis of neurons which is typically mediated by the ataxia telangiectasia mutated (ATM)-p53 pathway. As in all mammalian cells, telomeres in neurons consist of TTAGGG DNA repeats and several associated proteins that form a nucleoprotein complex that prevents chromosome ends from being recognized as double strand breaks. Proteins that stabilize telomeres include TRF1 and TRF2, and proteins known to play important roles in DNA damage responses and DNA repair including ATM, Werner and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). We have been performing studies of developing and adult neurons aimed at understanding the effects of global and telomere-directed DNA damage responses in neuronal plasticity and survival in the contexts of aging and neurodegenerative disorders. Deficits in specific DNA repair proteins, including DNA-PKcs and uracil DNA glycosylase (UDG), render neurons vulnerable to adverse conditions of relevance to the pathogenesis of neurodegenerative disorders such as Alzheimer's disease and stroke. Similarly, early postmitotic neurons with reduced telomerase activity exhibit accentuated responses to DNA damage and are prone to apoptosis demonstrating a pivotal role for telomere maintenance in both mitotic cells and postmitotic neurons. Our recent findings suggest key roles for TRF2 in regulating the differentiation and survival of neurons. TRF2 affects cell survival and differentiation by modulating DNA damage pathways, and gene expression. A better understanding of the molecular mechanisms by which neurons respond to global and telomere-specific DNA damage may reveal novel strategies for prevention and treatment of neurodegenerative disorders. Indeed, work in this and other laboratories has shown that dietary folic acid can protect neurons against Alzheimer's disease by keeping homocysteine levels low and thereby minimizing the misincorporation of uracil into DNA in neurons.

Figure 1

Telomeres are located in the constitutive heterochromatin region at the ends of chromosomes. They consist of G-rich noncoding repeating sequences and non-histone architectural proteins which form a complex t-loop structure also known as a telosome. The telosome prevents chromosome ends from being recognized as a double strand breaks (DSB). It also plays active roles in DNA replication, DNA repair and chromatin reorganization through telomere associated proteins (A). Telomere length can be increased by telomerase, which consists of telomerase reverse transcriptase (TERT), the telomerase RNA component (hTR) and heat-shock protein 90. Telomere tertiary structure is stabilized by telomere repeating factors (TRF1 and TRF2), TRF1 interacting nuclear protein 2(TIN2) and Rap1. In addition to maintenance of telomeres, several telomere associated proteins such as TERT and TRF2 cross-talk with DNA repair and chromatin remodeling protein complexes (B, black arrow). ATM and hTERT interact with chromatin silencing factors HDAC1 and HP1, respectively.

Figure 2

Differential effect of telomere dysfunction/damage on mitotic neural cells and postmitotic neurons. The activation of ATM and phosphorylated H2AX (γH2AX) (insert photograph on top shows the presence of γH2AX foci) is an early response to telomeric DNA damage observed in both glia and neurons after introduction of dominant negative TRF2 (DN-TRF2) mutant. However, several events downstream of the initial DNA damage response including cell cycle checkpoint activation (insert photograph in the middle shows ATM-mediated check point activation detected using an antibody that recognizes ATM substrate proteins phosphoryated on common core motif SQ or TQ, such as p53-S15 and Chk2-T68). The insert photograph at the bottom shows negative and positive (SA-β-gal) staining in neurons and astrocytes, respectively, indicating that senescence-like changes were only observed in cycling glial cells, but not in non-cycling neurons. However, telomere damage-induced neuronal differentiation was observed in both mitotic and post-mitotic neural cells regardless the activation of cell cycle checkpoint (bottom left panel, arrow and insert box shows the co-existence of γH2AX activation and differentiation in postmitotic neurons).