I have blogged before about chromosomal changes in normal brain cells. Normally every cell in your body has the same number of chromosomes, and cases of too few or too many chromosomes (conditions collectively called aneuploidy) can damage the way the cell functions. Scientists who were looking in human brains for copy numbers of chromosome 21 (trisomy of chromosome 21 causes Down's syndrome, which results in cognitive defects) discovered that an appreciable fraction, about 4-5%, of normal human brain cells were aneuploid for this chromosome. They could even identify some cases of cells carrying four copies.
Other work from the same lab had identified mutant mice with greatly elevated numbers of aneuploid cells. The mutations affected the apoptosis pathway, which is responsible for eliminating certain damaged or excess cells. This suggested that aneuploid cells seen in normal brains had escaped a removal process responsible for clearing away most of the others.
Work by Kingsbury et al. in last week's PNAS reports some additional observations about this phenomenon. They wanted to know whether aneuploid cells were neurons, or if they were exclusively nonneuronal. In addition, since the mouse mutant work suggested that aneuploid cells are actively removed, they wanted to know whether the aneuploid cells were neurons in good standing, with functional connections and with regular activity patterns.
Kingsbury et al. tackled the question of neuronal identity and proper connections simultaneously. Since neurons are the only brain cells which make long-distance connections (an introduction to the parts of a neuron is here ), they used a retrograde tracer, a compound which neurons take up from their synapses and transport back (even over long distances) to their cell body. Kingsbury et al. then looked for aneuploid nuclei sitting inside a cell body which was positive for the tracer, thus identifying cells which were aneuploid plus connected. To score for aneuploidy, they used male mice (XY) and looked for cells with more than one copy of X or more than one Y.
They were able to find many such neurons, and in addition they found that different areas of the cortex had different percentages of retrogradely labelled aneuploid neurons. Overall, extra copies of either X or Y were present in about 20% of all the neurons with tracer. (This is much higher than the 5% estimated from the chromosome 21 experiments. In the discussion, Kingsbury et al. cite an estimate of 64% of brain cells with some deviation from normal karyotype.)
To check the activity of the aneuploid neurons, they used immunostaining against two proteins, Egr and c-fos, which are strongly expressed in neurons which have recently been active. (An technical reference to this concept is here. )This ended up being a triple-label experiment, for FluoroGold (connectivity/neuron identity); X or Y aneuploidy; and immunoreactivity. This is an amazing technical accomplishment. Their results in this case are limited to just a few positive observations, including one case in which they stimulated olfactory system activity by presenting the male with a novel smell. I do wish they had pursued this line of experiments a bit farther, to give an estimate of the relative percentages of activation of aneuploid connecting cells versus activation of all. Again, since the apoptosis pathway is apparently actively removing aneuploid brain cells, the activity of aneuploid neurons is an important measure of their health.
What really suprises me about this work is that I would have thought neurons would have stringent DNA requirements for correct function. I like to think of neurons as having the most complicated job to do of any cell, which in turn led me to presume that neurons need intact DNA much more than, say, a skin cell. I guess this presumption is trumped by the general robustness of evolved systems against defects. Neurons might be escaping the "chromosome counter" because they never divide again. Aneuploidy in cells that will never divide is less of a risk to the animal than aneuploidy in, say, a stem cell, that gives rise to many copies, some of which may acquire additional chromosomal defects.