A YAC contig in Xq22.3-q23, from DXS287 to DXS8088, spanning the brain-specific genes doublecortin (DCX) and PAK3

Rainer W. Guillery and the genetic analysis of brain development

Ray Guillery had broad research interests that spanned cellular neuroanatomy, but was perhaps best known for his investigation of the connectivity and function of the thalamus, especially the visual pathways. His work on the genetics of abnormal vision in albino mammals served as an early paradigm for genetic approaches for studying brain connectivity of complex species in general, and remains of major relevance today. This work, especially on the Siamese cat, illustrates the complex relationship between genotype and physiology of cerebral cortical circuits, and anticipated many of the issues underlying the imperfect relationship between genes, circuits, and behavior in mammalian species including human. This review also briefly summarizes studies from our own lab inspired by Ray Guillery's legacy that continues to explore the relationship between genes, structure, and behavior in human cerebral cortex.

The PAR polarity complex and cerebellar granule neuron migration

Proper migration of neurons is one of the most important aspects of early brain development. After neuronal progenitors are born in their respective germinal niches, they must migrate to their final locations to form precise neural circuits. A majority of migrating neurons move by associating and disassociating with glial fibers, which serve as scaffolding for the developing brain. Cerebellar granule neurons provide a model system for examination of the mechanisms of neuronal migration in dissociated and slice culture systems; the ability to purify these cells allows migration assays to be paired with genetic, molecular, and biochemical findings. CGNs migrate in a highly polarized fashion along radial glial fibers, using a two-stroke nucleokinesis cycle. The PAR polarity complex of PARD3, PARD6, and an atypical protein kinase C (aPKC) regulate several aspects of neuronal migration. The PAR polarity complex regulates the coordinated movements of the centrosome and soma during nucleokinesis, and also the stability of the microtubule cytoskeleton during migration. PAR proteins coordinate actomyosin dynamics in the leading process of migrating neurons, which are required for migration. The PAR complex also controls the cell-cell adhesions made by migrating neurons along glial cells, and through this mechanism regulates germinal zone exit during prenatal brain development. These findings suggest that the PAR complex coordinates the movement of multiple cellular elements as neurons migrate and that further examination of PAR complex effectors will not only provide novel insights to address fundamental challenges to the field but also expand our understanding of how the PAR complex functions at the molecular level.

The effect of variation in expression of the candidate dyslexia susceptibility gene homolog Kiaa0319 on neuronal migration and dendritic morphology in the rat

We investigated the postnatal effects of embryonic knockdown and overexpression of the candidate dyslexia gene homolog Kiaa0319. We used in utero electroporation to transfect cells in E15/16 rat neocortical ventricular zone with either 1) small hairpin RNA (shRNA) vectors targeting Kiaa0319, 2) a KIAA0319 expression construct, 3) Kiaa0319 shRNA along with KIAA0319 expression construct ("rescue"), or 4) a scrambled version of Kiaa0319 shRNA. Knockdown, but not overexpression, of Kiaa0319 resulted in periventricular heterotopias that contained large numbers of both transfected and non-transfected neurons. This suggested that Kiaa0319 shRNA disrupts neuronal migration by cell autonomous as well as non-cell autonomous mechanisms. Of the Kiaa0319 shRNA-transfected neurons that migrated into the cortical plate, most migrated to their appropriate lamina. In contrast, neurons transfected with the KIAA0319 expression vector attained laminar positions subjacent to their expected positions. Neurons transfected with Kiaa0319 shRNA exhibited apical, but not basal, dendrite hypertrophy, which was rescued by overexpression of KIAA0319. The results provide additional supportive evidence linking candidate dyslexia susceptibility genes to migrational disturbances during brain development, and extends the role of Kiaa0319 to include growth and differentiation of dendrites.

Postnatal analysis of the effect of embryonic knockdown and overexpression of candidate dyslexia susceptibility gene homolog Dcdc2 in the rat

Embryonic knockdown of candidate dyslexia susceptibility gene (CDSG) homologs in cerebral cortical progenitor cells in the rat results in acute disturbances of neocortical migration. In the current report we investigated the effects of embryonic knockdown and overexpression of the homolog of DCDC2, one of the CDSGs, on the postnatal organization of the cerebral cortex. Using a within-litter design, we transfected cells in rat embryo neocortical ventricular zone around embryonic day (E) 15 with either 1) small hairpin RNA (shRNA) vectors targeting Dcdc2, 2) a DCDC2 overexpression construct, 3) Dcdc2 shRNA along with DCDC2 overexpression construct, 4) an overexpression construct composed of the C terminal domain of DCDC2, or 5) an overexpression construct composed of the DCX terminal domain of DCDC2. RNAi of Dcdc2 resulted in pockets of heterotopic neurons in the periventricular region. Approximately 25% of the transfected brains had hippocampal pyramidal cell migration anomalies. Dcdc2 shRNA-transfected neurons migrated in a bimodal pattern, with approximately 7% of the neurons migrating a short distance from the ventricular zone, and another 30% migrating past their expected lamina. Rats transfected with Dcdc2 shRNA along with the DCDC2 overexpression construct rescued the periventricular heterotopia phenotype, but did not affect the percentage of transfected neurons that migrate past their expected laminar location. There were no malformations associated with any of the overexpression constructs, nor was there a significant laminar disruption of migration. These results support the claim that knockdown of Dcdc2 expression results in neuronal migration disorders similar to those seen in the brains of dyslexics.

From genes to behavior in developmental dyslexia

All four genes thus far linked to developmental dyslexia participate in brain development, and abnormalities in brain development are increasingly reported in dyslexia. Comparable abnormalities induced in young rodent brains cause auditory and cognitive deficits, underscoring the potential relevance of these brain changes to dyslexia. Our perspective on dyslexia is that some of the brain changes cause phonological processing abnormalities as well as auditory processing abnormalities; the latter, we speculate, resolve in a proportion of individuals during development, but contribute early on to the phonological disorder in dyslexia. Thus, we propose a tentative pathway between a genetic effect, developmental brain changes, and perceptual and cognitive deficits associated with dyslexia.

A new constitutively active brain PAK3 isoform displays modified specificities toward Rac and Cdc42 GTPases

p21-activated kinases (PAK) are involved in the control of cytoskeleton dynamics and cell cycle progression. Here we report the characterization of a new mammalian PAK3 mRNA that contains a 45-bp alternatively spliced exon. This exon encodes for 15 amino acids that are inserted in the regulatory domain, inside the autoinhibitory domain but outside the Cdc42 and Rac interactive binding domain. The transcript of the 68-kDa new isoform named PAK3b is expressed in various areas of the adult mouse brain. In contrast to PAK3 without the exon b (PAK3a), whose basal kinase activity is weak in resting cells, PAK3b displays a high kinase activity in starved cells that is not further stimulated by active GTPases. Indeed, we demonstrate that the autoinhibitory domain of PAK3b no longer inhibits the kinase activity of PAK3. Moreover, we show that the 15-amino acid insertion within the autoinhibitory domain impedes the ability of PAK3b to bind to the GTPases Rac and Cdc42 and changes its specificity toward the GTPases. Altogether, our results show that the new PAK3b isoform has unique properties and would signal differently from PAK3a in neurons.

Large-insert clone/STS contigs in Xq11-q12, spanning deletions in patients with androgen insensitivity and mental retardation

An integrated large-insert clone map of the region Xq11-q12 is presented. A physical map containing markers within a few hundred kilobases of the centromeric locus DXZ1 to DXS1125 spans nearly 5 Mb in two contigs separated by a gap estimated to be approximately 100-250 kb. The contigs combine 75 yeast artificial chromosome clones, 12 bacterial artificial chromosome clones, and 17 P1-derived artificial chromosome clones with 81 STS or EST markers. Overall marker density across this region is approximately 1 STS/60 kb. Mapped within the contigs are 12 ESTs as well as 5 known genes, moesin (MSN), hephaestin (HEPH), androgen receptor (AR), oligophrenin-1 (OPHN1), and Eph ligand-2 (EPLG2). Orientation of the contigs on the X chromosome, as well as marker order within the contigs, was unambiguously determined by reference to a number of X chromosome breakpoints. In addition, the distal contig spans deletions from chromosomes of three patients exhibiting either complete androgen insensitivity (CAI) or a contiguous gene syndrome that includes CAI, impaired vision, and mental retardation.

Molecular cloning and characterization of TRPC5 (HTRP5), the human homologue of a mouse brain receptor-activated capacitative Ca2+ entry channel

A novel human gene, TRPC5, was cloned from the region of Xq23 that contains loci for nonsyndromic mental retardation (MRX47 and MRX35) and two genes, DCX and HPAK3, implicated in two X-linked disorders (LISX and MRX30). Within a single YAC, we have determined the order cen-HPAK3(5'-3')-DCX(3'-5')-DXS7012E-TRPC5(3'-5' )-ter. TRPC5 encodes a 974-residue novel human protein (111.5 kDa predicted mass) and displays 99% homology with mouse TRP5, (MGD-approved symbol Trrp5) a novel member of a family of receptor-activated Ca2+ channels. It contains eight transmembrane domains, including a putative pore region. A transcript larger than 9.5 kb is observed only in fetal and adult human brain, with a relatively higher level in the adult human cerebellum. We devised an efficient method, Incorporation PCR SSCP (IPS), for detection of gene alterations. Five single-nucleotide variations in the TRPC5 gene were identified in males with mental retardation. However, these were found to be polymorphic variants. Exclusive expression of the TRPC5 gene in developing and adult brain suggests a possible role during development and provides a candidate gene for instances of mental retardation and other developmental defects.

Genes that regulate neuronal migration in the cerebral cortex

Malformations of cortical development are increasingly recognized as causes of mental retardation and epilepsy. However, little is known about the molecular and biochemical signals that control the proliferation, migration, and organization of the cells involved in normal cerebral cortical development. Analysis of genes required for cortical development will help elucidate the pathogenesis of some epilepsies. In humans, two striking examples of abnormal cortical development, with varying degrees of epilepsy and mental retardation, are 'double cortex' and lissencephaly. Double cortex (DC), also known as subcortical band heterotopia, shows an abnormal band of neurons in the white matter underlying a relatively normal cortex. In pedigrees, DC often occurs in females, whereas affected males show more severe lissencephaly (XLIS), i.e. an abnormally thick cortex with decreased or absent surface convolutions. We and others have identified a novel brain specific gene, doublecortin, that is mutated in Double Cortex/X-linked lissencephaly (DC/XLIS) patients. Although the cellular function of doublecortin (DCX) is unknown, sequence analysis reveals a cytoplasmic protein with potential MAP kinase phosphorylation sites, as well as a site that is likely to be phosphorylated by c-Abl, suggesting that doublecortin functions as an intracellular signaling molecule critical for the migration of developing neurons. Interestingly, the scrambler mouse mutant demonstrates abnormal lamination with some similarity to lissencephaly and reflects a mutation in the murine homolog of the Drosophila disabled gene, mdab1, which binds c-Abl. Although a direct interaction between doublecortin and mDab1 has not been demonstrated, it is plausible that these two proteins may be part of a common signaling pathway. Therefore, abnormalities in signal transduction may be an underlying mechanism for the neuronal migration defects in DC/XLIS and the scrambler mouse, but further research is necessary to determine how such abnormalities give rise to cortical malformations and epilepsy.

Integrated STS/YAC physical, genetic, and transcript map of human Xq21.3 to q23/q24 (DXS1203-DXS1059)

A map has been assembled that extends from the XY homology region in Xq21.3 to proximal Xq24, approximately 20 Mb, formatted with 200 STSs that include 25 dinucleotide repeat polymorphic markers and more than 80 expressed sequences including 30 genes. New genes HTRP5, CAPN6, STPK, 14-3-3PKR, and CALM1 and previously known genes including BTK, DDP, GLA, PLP, COL4A5, COL4A6, PAK3, and DCX are localized; candidate loci for other disorders for which genes have not yet been identified, including DFN-2, POF, megalocornea, and syndromic and nonsyndromic mental retardation, are also mapped in the region. The telomeric end of the contig overlaps a yeast artificial chromosome (YAC) contig from Xq24-q26 and with other previously published contigs provides complete sequence-tagged site (STS)/YAC-based coverage of the long arm of the X chromosome. The order of published landmark loci in genetic and radiation hybrid maps is in general agreement. Combined with high-density STS landmarks, the multiple YAC clone coverage and integrated genetic, radiation hybrid, and transcript map provide resources to further disease gene searches and sequencing.