Reduced cortical BDNF expression and aberrant memory in Carf knock-out mice.

Transcription factors are a key point of convergence between the cell-intrinsic and extracellular signals that guide synaptic development and brain plasticity. Calcium-response factor (CaRF) is a unique transcription factor first identified as a binding protein for a calcium-response element in the gene encoding brain-derived neurotrophic factor (Bdnf). We have now generated Carf knock-out (KO) mice to characterize the function of this factor in vivo. Intriguingly, Carf KO mice have selectively reduced expression of Bdnf exon IV-containing mRNA transcripts and BDNF protein in the cerebral cortex, whereas BDNF levels in the hippocampus and striatum remain unchanged, implicating CaRF as a brain region-selective regulator of BDNF expression. At the cellular level, Carf KO mice show altered expression of GABAergic proteins at striatal synapses, raising the possibility that CaRF may contribute to aspects of inhibitory synapse development. Carf KO mice show normal spatial learning in the Morris water maze and normal context-dependent fear conditioning. However they have an enhanced ability to find a new platform location on the first day of reversal training in the water maze and they extinguish conditioned fear more slowly than their wild-type littermates. Finally, Carf KO mice show normal short-term (STM) and long-term memory (LTM) in a novel object recognition task, but exhibit impairments during the remote memory phase of testing. Together, these data reveal novel roles for CaRF in the organization and/or function of neural circuits that underlie essential aspects of learning and memory.

Human CHN1 mutations hyperactivate alpha2-chimaerin and cause Duane's retraction syndrome.

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes alpha2-chimaerin, a Rac guanosine triphosphatase-activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase alpha2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance alpha2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant alpha2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that alpha2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

Two pedigrees segregating Duane's retraction syndrome as a dominant trait map to the DURS2 genetic locus.

PURPOSE: The genetic bases of Duane's retraction syndrome (DRS) were investigated to determine its molecular etiologies. In prior studies, the transcription factors SALL4 and HOXA1 were identified as the genes mutated in DRS with radial anomalies, and in DRS with deafness, vascular anomalies, and cognitive deficits, respectively. Less is known, however, about the genetic etiology of DRS when it occurs in isolation, and only one genetic locus for isolated DRS, the DURS2 locus on chromosome 2, has been mapped to date. Toward the goal of identifying the DURS2 gene, two pedigrees have been ascertained that segregate DRS as a dominant trait. METHODS: Members of two large dominant DRS pedigrees were enrolled in an ongoing study of the genetic basis of the congenital cranial dysinnervation disorders, and linkage analysis was conducted to determine whether their DRS phenotype maps to the DURS2 locus. RESULTS: By haplotype analysis, the DRS phenotype in each family cosegregates with markers spanning the DURS2 region. Linkage analysis reveals maximum lod scores >2, establishing that the DRS phenotype in these two pedigrees maps to the DURS2 locus. CONCLUSIONS: These two pedigrees double the published pedigrees known to map to the DURS2 locus and can thus contribute toward the search for the DURS2 gene. The affected members represent a genetically defined population of DURS2-linked DRS individuals, and hence studies of their clinical and structural features can enhance understanding of the DURS2 phenotype, as described in the companion paper.