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Strength, Timing, and Proper Neuronal Development

1 February 2009. Like the flying trapeze, brain development requires a delicate balance of strength and timing. If either is off, connections fail and the whole act can falter. Two recent Nature papers emphasize as much, and one of them, surprisingly, suggests that getting disrupted development back into the swing of things may be easier than it seems. Both papers may have implications for schizophrenia research.

In the 11 January Nature Genetics online, researchers led by Orly Reiner, The Weizmann Institute of Science, Rehovot, Israel, and James Lupinski, Baylor College of Medicine, Houston, Texas, report that overexpression of the gene LIS1 causes brain development problems in both mice and humans. Mutations in LIS1 are known to cause lissencephaly, a developmental brain disorder, but this is the first time researchers have shown that having too much of the lissencephaly 1 protein (the product of the LIS1 gene) can be just as problematic as having too little. LIS1 is of particular interest to schizophrenia researchers because it is a DISC1 binding partner (see SRF related news story). DISC1 mutations have been linked to aberrant neuronal migration and neuronal development (see Kamiya et al., 2005 and SRF related news story), and the DISC1 gene is a strong schizophrenia risk gene candidate (see SRF related news story). The second paper also has a lissencephaly connection. Joseph LoTurco and colleagues at the University of Connecticut in Storrs report that restoring expression of the doublecortin gene (DCX) after birth helps rescue failed neuronal development that occurs in the absence of Dcx. In fact, loss of DCX also causes lissencephaly and leads to epileptic seizures in animals. The researchers show that turning on DCX after birth reduces seizure activity and restores proper neuronal migration. The work, described in the 21 December Nature Medicine online, hints that it may be possible to treat neuronal migration disorders by rebooting neuronal development, even after birth.

Too much of a good thing
Reiner and colleagues made the connection between LIS1 overexpression and development when they identified seven unrelated individuals with duplications of a small region of chromosome 17 (17p13.3) that houses the LIS1 gene. Joint first authors Weimin Bi, Tamar Sapir, Oleg Shchelochkov, and colleagues found that four of those patients carried a duplication of YWHAE, a gene that encodes the protein 14-3-3ε, and that has been associated with schizophrenia (see comment by A. Kamiya). Two patients had a duplication of PAFAH1B1, the gene encoding LIS1, while one patient had a duplication that spanned both genes. Patients carrying the additional PAFAH1B1 gene had more severe abnormalities, including moderate to severely delayed development, reduced cerebral volume, malformation of the corpus callosum, and significant atrophy of the cerebellum.

To test if the extra gene copies are related to the patients’ disabilities, the researchers took two approaches. They first explored whether the extra genes were functional, and found increased expression of LIS1and 14-3-3ε in patients with an extra PAFAH1B1 and YWHAE gene, respectively. They also tested the effect of increasing the copy number in mice. Animals with an extra LIS1 gene also showed developmental problems, with a smaller and anatomically more disorganized brain. These animals had an increase in the number of mitotic cells in the ventricular zone and also an increased number of cells undergoing apoptosis, or programmed cell death. Cells had also lost their polarity, or ability to navigate, a major detriment to proper cell migration. Using time lapse microscopy, the authors found that cell motility was significantly reduced in mice with an extra LIS1 copy. In all, the findings indicate that overexpression of LIS1 causes major setbacks for proper brain development.

Double trouble
Doublecortin may also be essential for cell polarity (see Cardoso et al., 2003), which could explain why it is needed for proper neuronal migration and development. Mutations in DCX lead to a condition called, not surprisingly, double cortex, in which an extra band of gray matter, comprising abnormally migrated neurons, appears between the ventricles and the cortex proper. In humans this causes mild to moderate mental retardation. Pockets of wayward neurons are also found in other human conditions, such as some severe forms of epilepsy that are usually not responsive to pharmacologic intervention, and in a variety of animal models. Many of these conditions, referred to as subcortical band heterotopias in reference to the misplaced neurons, are accompanied by seizures, suggesting that the wayward neuronal migration alters connectivity in such a way that neural networks become hyperexcitable.

LoTurco and colleagues tested whether reactivating cell migration might mitigate disabilities, including seizure, that accompany subcortical band heterotopia (SBH). First author Jean-Bernard Manent and colleagues knocked down Dcx expression in utero in rat pups using RNAi, then conditionally re-expressed Dcx at birth using a construct that lacks the 3’UTR targeted by the RNAi knockdown. The result was pups that had no Dcx between embryonic day 14 and birth, but then had Dcx re-expressed.

The RNAi approach led to pronounced SBH at birth. Neuronal malformations were significantly reduced, however, when Dcx re-expression was induced at birth with neurons actually migrating into the upper layers of the cortex, as in control pups, by P20. Those neurons that migrated to the correct position after postnatal induction of Dcx also appeared morphologically normal, expressing upper cortical layer markers and having a normal looking dendritic tree. The reduction in SBH was accompanied by a dramatic improvement in susceptibility to seizure. At P30, Dcx-re-expressing rats showed the same sensitivity to seizure induction as normal rats of the same age.

The study indicates that it may be possible to correct diseases caused by neuronal migration deficits even after birth. “To our knowledge, this is the first study to demonstrate that a molecular intervention can reduce the size and functional effects of a pre-existing disruption in neuronal migration,” write the authors. Their strategy, electroporating DNA constructs into the brain in utero, could not be used in humans, however, but the authors suggest possible alternative approaches, including viral transduction of cells with appropriate DNAs, and/or pharmacological intervention that boosts Dcx signaling pathways. Whether either strategy would work in humans is not clear, and it will probably be a long time before either would even be considered. There is also another drawback—the intervention would most likely have to be given before birth because the authors found that if they waited until P5 the strategy was less successful, while at P10 (the equivalent of full-term infants in humans) it failed to mitigate SBH at all. Nevertheless, the finding opens up the possibility that developmental problems stemming from abnormal neural migration could be treated after the fact. It is not clear whether this strategy could ever apply to conditions that emerge in adolescence, such as schizophrenia, which may, at least in part, have a neurodevelopmental etiology. “In general, our study raises the possibility that, in some contexts, neuronal migration is a form of neuronal plasticity that may be engaged to induce neural repair,” write the authors.—Tom Fagan.

References:
Bi W, Sapir T, Shchelochkov OA, Zhang F, Withers MA, Hunter JV, Levy T, Shinder V, Peiffer DA, Gunderson KL, Nezarati MM, Shotts VA, Amato SS, Savage SK, Harris DJ, Day-Salvatore D-L, Horner M, Lu X-Y, Sahoo T, Yanagawa Y, Beaudet AL, Cheung SW, Martinez S, Lupski JR, Reiner R. Increased LIS1 expression affects human and mouse brain development. Nature Genetics advanced online publication. 2009 January 11. Abstract

Manent J-B, Wang Y, Chang YJ, Paramasivam M, LoTurco JJ. Dcx reexpression reduces subcortical band heterotopia and seizure threshold in an animal model of neuronal migration disorder. Nature Medicine advanced online publication. 2008, December 21. Abstract

 
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