Persistent spatial working memory deficits in rats with bilateral cortical microgyria

Developmental Alterations in Cortical Organization and Socialization in Adolescents Who Sustained a Traumatic Brain Injury in Early Childhood

This study investigated patterns of cortical organization in adolescents who had sustained a traumatic brain injury (TBI) during early childhood to determine ways in which early head injury may alter typical brain development. Increased gyrification in other patient populations is associated with polymicrogyria and aberrant development, but this has not been investigated in TBI. Seventeen adolescents (mean age = 14.1 ± 2.4) who sustained a TBI between 1-8 years of age, and 17 demographically-matched typically developing children (TDC) underwent a high-resolution, T1-weighted 3-Tesla magnetic resonance imaging (MRI) at 6-15 years post-injury. Cortical white matter volume and organization was measured using FreeSurfer's Local Gyrification Index (LGI). Despite a lack of significant difference in white matter volume, participants with TBI demonstrated significantly increased LGI in several cortical regions that are among those latest to mature in normal development, including left parietal association areas, bilateral dorsolateral and medial frontal areas, and the right posterior temporal gyrus, relative to the TDC group. Additionally, there was no evidence of increased surface area in the regions that demonstrated increased LGI. Higher Vineland-II Socialization scores were associated with decreased LGI in right frontal and temporal regions. The present results suggest an altered pattern of expected development in cortical gyrification in the TBI group, with changes in late-developing frontal and parietal association areas. Such changes in brain structure may underlie cognitive and behavioral deficits associated with pediatric TBI. Alternatively, increased gyrification following TBI may represent a compensatory mechanism that allows for typical development of cortical surface area, despite reduced brain volume.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

Use of an eight-arm radial water maze to assess working and reference memory following neonatal brain injury

Working and reference memory are commonly assessed using the land based radial arm maze. However, this paradigm requires pretraining, food deprivation, and may introduce scent cue confounds. The eight-arm radial water maze is designed to evaluate reference and working memory performance simultaneously by requiring subjects to use extra-maze cues to locate escape platforms and remedies the limitations observed in land based radial arm maze designs. Specifically, subjects are required to avoid the arms previously used for escape during each testing day (working memory) as well as avoid the fixed arms, which never contain escape platforms (reference memory). Re-entries into arms that have already been used for escape during a testing session (and thus the escape platform has been removed) and re-entries into reference memory arms are indicative of working memory deficits. Alternatively, first entries into reference memory arms are indicative of reference memory deficits. We used this maze to compare performance of rats with neonatal brain injury and sham controls following induction of hypoxia-ischemia and show significant deficits in both working and reference memory after eleven days of testing. This protocol could be easily modified to examine many other models of learning impairment.

A behavioral evaluation of sex differences in a mouse model of severe neuronal migration disorder

Disruption of neuronal migration in humans is associated with a wide range of behavioral and cognitive outcomes including severe intellectual disability, language impairment, and social dysfunction. Furthermore, malformations of cortical development have been observed in a number of neurodevelopmental disorders (e.g. autism and dyslexia), where boys are much more commonly diagnosed than girls (estimates around 4 to 1). The use of rodent models provides an excellent means to examine how sex may modulate behavioral outcomes in the presence of comparable abnormal neuroanatomical presentations. Initially characterized by Rosen et al. 2012, the BXD29- Tlr4^lps−2J/J mouse mutant exhibits a highly penetrant neuroanatomical phenotype that consists of bilateral midline subcortical nodular heterotopia with partial callosal agenesis. In the current study, we confirm our initial findings of a severe impairment in rapid auditory processing in affected male mice. We also report that BXD29- Tlr4^lps−2J/J (mutant) female mice show no sparing of rapid auditory processing, and in fact show deficits similar to mutant males. Interestingly, female BXD29- Tlr4^lps−2J/J mice do display superiority in Morris water maze performance as compared to wild type females, an affect not seen in mutant males. Finally, we report new evidence that BXD29- Tlr4^lps−2J/J mice, in general, show evidence of hyper-social behaviors. In closing, the use of the BXD29- Tlr4^lps−2J/J strain of mice – with its strong behavioral and neuroanatomical phenotype – may be highly useful in characterizing sex independent versus dependent mechanisms that interact with neural reorganization, as well as clinically relevant abnormal behavior resulting from aberrant neuronal migration.

Effects of test experience and neocortical microgyria on spatial and non-spatial learning in rats

Neocortical neuronal migration anomalies such as microgyria and heterotopia have been associated with developmental language learning impairments in humans, and rapid auditory processing deficits in rodent models. Similar processing impairments have been suggested to play a causal role in human language impairment. Recent data from our group has shown spatial working memory deficits associated with neocortical microgyria in rats. Similar deficits have also been identified in humans with language learning impairments. To further explore the extent of learning deficits associated with cortical neuronal migration anomalies, we evaluated the effects of neocortical microgyria and test order experience using spatial (Morris water maze) and non-spatial water maze learning paradigms. Two independent groups were employed (G1 or G2) incorporating both microgyria and sham conditions. G1 received spatial testing for five days followed by non-spatial testing, while the reverse order was followed for G2. Initial analysis, including both test groups and both maze conditions, revealed a main effect of treatment, with microgyric rats performing significantly worse than shams. Overall analysis also revealed a task by order interaction, indicating that each group performed better on the second task as compared to the first, regardless of which task was presented first. Independent analyses of each task revealed a significant effect of treatment (microgyria worse than sham) only for the spatial water maze condition. Results indicate that prior maze experience (regardless of task type) leads to better subsequent performance. Results suggest that behavioral abnormalities associated with microgyria extend beyond auditory and working memory deficits seen in previous studies, to include spatial but not non-spatial learning impairments and that non-specific test experience may improve behavioral performance.

Neocortical disruption and behavioral impairments in rats following in utero RNAi of candidate dyslexia risk gene Kiaa0319

Within the last decade several genes have been identified as candidate risk genes for developmental dyslexia. Recent research using animal models and embryonic RNA interference (RNAi) has shown that a subset of the candidate dyslexia risk genes--DYX1C1, ROBO1, DCDC2, KIAA0319--regulate critical parameters of neocortical development, such as neuronal migration. For example, embryonic disruption of the rodent homolog of DYX1C1 disrupts neuronal migration and produces deficits in rapid auditory processing (RAP) and working memory--phenotypes that have been reported to be associated with developmental dyslexia. In the current study we used a modified prepulse inhibition paradigm to assess acoustic discrimination abilities of male Wistar rats following in utero RNA interference targeting Kiaa0319. We also assessed spatial learning and working memory using a Morris water maze (MWM) and a radial arm water maze. We found that embryonic interference with this gene resulted in disrupted migration of neocortical neurons leading to formation of heterotopia in white matter, and to formation of hippocampal dysplasia in a subset of animals. These animals displayed deficits in processing complex acoustic stimuli, and those with hippocampal malformations exhibited impaired spatial learning abilities. No significant impairment in working memory was detected in the Kiaa0319 RNAi treated animals. Taken together, these results suggest that Kiaa0319 plays a role in neuronal migration during embryonic development, and that early interference with this gene results in an array of behavioral deficits including impairments in rapid auditory processing and simple spatial learning.

Persistent spatial working memory deficits in rats following in utero RNAi of Dyx1c1

Disruptions in the development of the neocortex are associated with cognitive deficits in humans and other mammals. Several genes contribute to neocortical development, and research into the behavioral phenotype associated with specific gene manipulations is advancing rapidly. Findings include evidence that variants in the human gene DYX1C1 may be associated with an increased risk of developmental dyslexia. Concurrent research has shown that the rat homolog for this gene modulates critical parameters of early cortical development, including neuronal migration. Moreover, recent studies have shown auditory processing and spatial learning deficits in rats following in utero transfection of an RNA interference (RNAi) vector of the rat homolog Dyx1c1 gene. The current study examined the effects of in utero RNAi of Dyx1c1 on working memory performance in Sprague-Dawley rats. This task was chosen based on the evidence of short-term memory deficits in dyslexic populations, as well as more recent evidence of an association between memory deficits and DYX1C1 anomalies in humans. Working memory performance was assessed using a novel match-to-place radial water maze task that allows the evaluation of memory for a single brief (∼4-10 seconds) swim to a new goal location each day. A 10-min retention interval was used, followed by a test trial. Histology revealed migrational abnormalities and laminar disruption in Dyx1c1 RNAi-treated rats. Dyx1c1 RNAi-treated rats exhibited a subtle, but significant and persistent impairment in working memory as compared to Shams. These results provide further support for the role of Dyx1c1 in neuronal migration and working memory.

Progress towards a cellular neurobiology of reading disability

Reading Disability (RD) is a significant impairment in reading accuracy, speed and/or comprehension despite adequate intelligence and educational opportunity. RD affects 5-12% of readers, has a well-established genetic risk, and is of unknown neurobiological cause or causes. In this review we discuss recent findings that revealed neuroanatomic anomalies in RD, studies that identified 3 candidate genes (KIAA0319, DYX1C1, and DCDC2), and compelling evidence that potentially link the function of candidate genes to the neuroanatomic anomalies. A hypothesis has emerged in which impaired neuronal migration is a cellular neurobiological antecedent to RD. We critically evaluate the evidence for this hypothesis, highlight missing evidence, and outline future research efforts that will be required to develop a more complete cellular neurobiology of RD.

Early acoustic discrimination experience ameliorates auditory processing deficits in male rats with cortical developmental disruption

Auditory temporal processing deficits have been suggested to play a causal role in language learning impairments, and evidence of cortical developmental anomalies (microgyria (MG), ectopia) has been reported for language-impaired populations. Rodent models have linked these features, by showing deficits in auditory temporal discrimination for rats with neuronal migration anomalies (MG, ectopia). Since evidence from human studies suggests that training with both speech and non-speech acoustic stimuli may improve language performance in developmentally language-disabled populations, we were interested in whether/how maturation and early experience might influence auditory processing deficits seen in male rats with induced focal cortical MG. Results showed that for both simple (Normal single tone), as well as increasingly complex auditory discrimination tasks (silent gap in white noise and FM sweep), prior experience significantly improved acoustic discrimination performance--in fact, beyond improvements seen with maturation only. Further, we replicated evidence that young adult rats with MG were significantly impaired at discriminating FM sweeps compared to shams. However, these MG effects were no longer seen when experienced subjects were retested in adulthood (even though deficits in short duration FM sweep detection were seen for adult MG rats with no early experience). Thus while some improvements in auditory processing were seen with normal maturation, the effects of early experience were even more profound, in fact resulting in amelioration of MG effects seen at earlier ages. These findings support the clinical view that early training intervention with appropriate acoustic stimuli could similarly ameliorate long-term processing impairments seen in some language-impaired children.

COG1410 improves cognitive performance and reduces cortical neuronal loss in the traumatically injured brain

We have previously shown that a single dose of COG1410, a small molecule ApoE-mimetic peptide derived from the apolipoprotein E (ApoE) receptor binding region, improves sensorimotor and motor outcome following cortical contusion injury (CCI). The present study evaluated a regimen of COG1410 following frontal CCI in order to examine its preclinical efficacy on cognitive recovery. Animals were prepared with a bilateral CCI of the frontal cortex. A regimen of COG1410 (0.8mg/kg intravenously [IV]) was administered twice, at 30min and again at 24h post-CCI. Starting on day 11, the animals were tested for their acquisition of a reference memory task in the Morris water maze (MWM), followed by a working memory task in the MWM on day 15. Following CCI, the animals were also tested on the bilateral tactile adhesive removal test to measure sensorimotor dysfunction. On all of the behavioral tests the COG1410 group was no different from the uninjured sham group. Administration of the regimen of COG1410 significantly improved recovery on the reference and working memory tests, as well as on the sensorimotor test. Lesion analysis revealed that COG1410 significantly reduced the size of the injury cavity. Administration of COG1410 also reduced the number of degenerating neurons, as measured by Fluoro-Jade C staining, in the frontal cortex at 48h post-CCI. These results suggest that a regimen of COG1410 appeared to block the development of significant behavioral deficits and reduced tissue loss. These combined findings suggest that COG1410 appears to have strong preclinical efficacy when administered following traumatic brain injury (TBI).