The Ash Twins Blog

Elliott and I are traveling in the Balkans (former Yugoslavia) this August. We went to Dubrovnik, Split, Hvar, and Zadar in Croatia last week. Today we went to Plitvice lakes, which is an incredible series of lakes in the mountains surrounded by rain forest (yeah, it was weird to see rain forests in Europe).

Then our hotel owner Damir drove us across the border to Bihač  in Bosnia. We had to stop for a few minutes at one point to wait for mine sweepers to finish checking a spot by the road. A bit later we drove through a deserted town which Damir told us was an old Serbian village – The townspeople had fled during the 1990s war (it’s very close to Croatia), and now most of them are afraid to come back because there are 12 unexplained Croatian deaths. We took a bus from Bihač to the medieval mountain castle town Jajce, seat of power of the ancient Bosnian kings. One wall of the house we are staying in is a thousand year old stone tower.

Interesting trivia – The medieval Bosnian kings didn’t like Christianity very much, so they converted the entire country to a primitive religion called Bogomil around 600 AD. When the Turks conquered Bosnia in the 1400s, the people didn’t care much about their old-fashioned religion, so thez were easily converted to Islam. Ergo, By the early 1900s, almost the entire country was made up of ethnically European Muslims. Even after being killed in vast numbers in WW1, then again in WW2, and in the mid 1990s, Bosnia and Hercegovina is still the only country in Europe with a Muslim majority.

Forgive the lack of links – it is really tough to make them on this shit keyboard. Wikipedia anything you’re interested in.

In the previous post, I mentioned that our studies in the mouse model for MECP2 duplication syndrome indicate that synapses are being formed and lost at a high, immature rate for the life of the animal, which is associated with the development of stereotyped repetitive behaviors and epileptic seizures.

Achieving a balance between cortical circuit stability and plasticity allows us to adapt to and learn from novel environmental experiences without forgetting useful information from the past. Children and adolescents have little experience to guide their actions, so new information is very important, and plasticity (learning) is favored over stability (memory). Conversely, the elderly have a wealth of past experiences, so incorporating new information into world models is less likely to be beneficial, and stability is favored over plasticity.

One theory for the pathophysiology of developmental brain disorders like Fragile X syndrome, Rett Syndrome, Angelman syndrome, and MECP2 duplication syndrome is that the link between genetic lesions and certain neuropsychiatric phenotypes is an imbalance between stability and plasticity. If the brain’s synapses are too stable, the brain will have trouble adapting to new experiences; Synaptic rigidity of this sort could lead to the permanent infantilism seen in Angelman syndrome, and has indeed been demonstrated Angelman syndrome mouse model. If the brain’s synapses are too labile, on the other hand, extant memories may be continually overturned by new experiences, or even worse synapses will continue to develop past their normal strengths, generating stereotypic behavior and epileptic circuitry. In either case, the cumulative refinement of neural circuitry that is important for brain development cannot occur.

Results in my lab suggest that MECP2 duplication syndrome is an example may be such a syndrome of too much plasticity. In juvenile mice, this bias toward synapse turnover allows relatively rapid acquisition of both adaptive (enhanced motor learning) and maladaptive (enhanced conditioned fear response) environmental adaptations. As the mice reach adulthood, they have sampled a broader range of world states, but the normal developmental deceleration of cortical plasticity which would shift the continuum to stability in cortical circuits does not occur. Instead, circuits oversaturate, leading to hyperexcitable ensembles which continue to self-strengthen by positive feedback. Activity in these dysfunctional cortical circuits produces compulsive, repetitive, stereotyped behaviors as well as runaway resonant excitation and epileptic seizures. Along these lines, it appears that patients steadily acquire developmental milestones until a critical point in which neurodevelopment stagnates and seizures ensue (Karas & Zoghbi, unpublished observations).

In my lab, we are studying the developmental brain disorder MECP2 duplication syndrome. It is caused by genomic duplication of the q28 region on the X chromosome, and common symptoms include severe intellectual disability and autism, progressing to severe epilepsy and early death. Genotype-phenotype analyses of patient samples suggest that the key dosage-sensitive gene in Xq28 duplication is the transcriptional regulator methyl-CpG binding protein 2 (MECP2), mutations of which have also been linked to Rett Syndrome, Angelman Syndrome, and autism. To study the neurobiology of this neurodevelopmental disorder, we use mice which have been engineered to overexpress MECP2 at twice normal levels. Analogous to humans with MECP2 duplication, these MECP2 duplication mice develop a progressive neurological phenotype with repetitive compulsive motor behaviors, abnormal social behavior and learning, anxiety, spasticity, and seizures.

A convincing physiological link between the symptoms of MECP2 duplication syndrome and its underlying genetic lesion has yet to be established, but work in mouse models and neuronal culture indicates that deranged dendritic/synaptic structure and plasticity may be an important locus of disease. The MECP2 duplication mice have a dramatically increased density of excitatory synapses in the hippocampus which is most prominent at young ages. Overexpression of MECP2 in primary slice cultures (to make a primary slice culture – a brain is harvested, sliced into slabs, and placed in a petri dish with oxygen and nutrients. They can survive for weeks and months with appropriate care)  causes a range of structural abnormalities in dendritic arbors and dendritic spines (the small protrusions from dendrites at which most excitatory synapses form). Detailed neuropathology with postmortem MECP2 duplication syndrome patient samples has yet to be performed.

In our lab, we are studying the formation, development, and pruning of these dendritic spines in MECP2 duplication mice. We cross the MECP2-duplication mice to the thy1-GFP (Green Fluorescent Protein form jellyfish) transgenic mice, which have been engineered to express bright fluorescence in a small number of layer  pyramidal neurons. These glow-in-the-dark neurons allows us to image apical dendrite and dendritic spine morphology in intact animals over days, weeks, and months, using two-photon microscopy (basically, a laser scans through the brain tissue identifying points of fluorescence in 3 dimensions). We are imaging the synaptic development from presymptomatic (4 weeks) stages to severe symptomatic (40 weeks) stages.

Our key finding so far is that the process of dendritic spine turnover (formation and pruning of synapses over time) does not undergo the normal developmental deceleration with age, but rather stays abnormally high well into adulthood, and that this elevated spine turnover is correlated with a net decrease in dendritic spine densities.