The human brain has some 100 billion neurons. That sounds like a lot, but I’m still keen on keeping ALL of mine healthy and in good working order. One way that cells protect themselves from damage and untimely death is by protecting their DNA – by wrapping it up and coiling it tightly – using chromatin proteins – which keeps it away from chemical and viral damage. This is especially important in the brain, since – unlike the skin or gut – we can’t really re-grow brain tissue once its damaged. We have to protect the neurons we have!
Here’s the problem. In order to USE the BRAIN (to learn and remember stuff) we have to also USE the GENOME (to encode the proteins that synapses use in the process of memory formation). When we’re thinking, we have to take our precious DNA out of its protective supercoiled, proteinaceous shell and allow the double helix to melt into single strands and expose their naked A’s, G’s, T’s and C’s to the chemical milieu (to the start the transcription process). This is risky business damage to DNA can lead to cell death!
One might imaging that its best to carry out this precarious act quickly and in proximity to DNA repair enzymes (I’d think). A very important job that includes: uncoiling chromatin superstructures, transcribing DNA (that encodes proteinaceous building blocks that synapses use to strengthen and weaken themselves) – and then – making sure there was no damage incurred along the way. A BIG job that MUST get done each and every time my cells engage in learning. Wow! I didn’t realize that learning new stuff means I’m exposing my DNA to damage? Hmm … I wonder if that PhD was worth it?
To perform this important job, it seems there is an amazing handyman of a molecule named poly(ADP-ribose) polymerase-1 (PARP-1). Amazing, because it – itself – can function in many of the steps involved in uncoiling chromatin structures, transcription initiation and DNA repair. The protein that can “do it all” … get the job done quickly and even fix any errors made along the way! It is known to function in the so-called base excision repair (BER) pathway and is also known have a role in transcription through remodeling of chromatin by ADP-ribosylating histones and relaxing chromatin structure, thus allowing transcription to occur (click here for a great open review of PARP-1). Nice!
According to OMIM, earlier studies by Cohen-Armon et al. (2004) found that poly(ADP-ribose) polymerase-1 is activated in neurons that mediate several forms of long-term memory in Aplysia. Because poly(ADP-ribosyl)ation of nuclear proteins is a response to DNA damage in virtually all eukaryotic cells (indeed, PARP-1 knock-out mice are more sensitive to DNA damage), it was surprising that activation of the polymerase occurred during learning and was required for long-term memory. Cohen-Armon et al. (2004) suggested that the fast and transient decondensation of chromatin structure by poly(ADP-ribosyl)ation enables the transcription needed to form long-term memory without strand breaks in DNA.
A recent article in Journal of Neuroscience seems to confirm this function – now in the mouse brain. Histone H1 Poly[ADP]-Ribosylation Regulates the Chromatin Alterations Required for Learning Consolidation [doi:10.1523/JNEUROSCI.3010-10.2010] by Fontán-Lozano et al., examined cells in the hippocampus at different times during the learning of an object recognition paradigm. They confirm (using a PARP-1 antagonist) that PARP-1 is needed to establish object memory and also that PARP-1 seems to contribute during the paradigm and up to 2 hours after the training session. They suggest that the poly(ADP-ribosyl)ation of histone H1 influences whether H1 is bound or unbound and thus helps regulate the opening and closing of the chromatin so that transcription can take place.
Nice to know that PARP-1 is on the job! Still am wondering if the PhD was worth all the learning. Are there trade-offs at play here? MORE learning vs. LESS something? Perhaps. Check out the paper by Grube and Bürkle (1992) – Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span. This gene may influence life span!