Elizabeth M. Powell

Genetic Disruption of Cortical Interneuron Development Causes Region- and GABA Cell Type-Specific Deficits, Epilepsy, and Behavioral Dysfunction

The generation of properly functioning circuits during brain development requires precise timing of cell migration and differentiation. Disruptions in the developmental plan may lead to neurological and psychiatric disorders. Neocortical circuits rely on inhibitory GABAergic interneurons, the majority of which migrate from subcortical sources. We have shown that the pleiotropic molecule hepatocyte growth factor/scatter factor (HGF/SF) mediates interneuron migration. Mice with a targeted mutation of the gene encoding urokinase plasminogen activator receptor (uPAR), a key component in HGF/SF activation and function, have decreased levels of HGF/SF and a 50% reduction in neocortical GABAergic interneurons at embryonic and perinatal ages. Disruption of interneuron development leads to early lethality in most models. Thus, the long-term consequences of such perturbations are unknown. Mice of theuPAR−/− strain survive until adulthood, and behavior testing demonstrates that they have an increased anxiety state. TheuPAR−/− strain also exhibits spontaneous seizure activity and higher susceptibility to pharmacologically induced convulsions. The neocortex of the adultuPAR−/− mouse exhibits a dramatic region- and subtype-specific decrease in GABA-immunoreactive interneurons. Anterior cingulate and parietal cortical areas contain 50% fewer GABAergic interneurons compared with wild-type littermates. However, interneuron numbers in piriform and visual cortical areas do not differ from those of normal mice. Characterization of interneuron subpopulations reveals a near complete loss of the parvalbumin subtype, with other subclasses remaining intact. These data demonstrate that a single gene mutation can selectively alter the development of cortical interneurons in a region- and cell subtype-specific manner, with deficits leading to long-lasting changes in circuit organization and behavior.

Regulation of neocortical interneuron development and the implications for neurodevelopmental disorders

Neurodevelopmental disorders typically have complex endophenotypes, which can include abnormalities in neuronal excitability, processing of complex information, as well as behaviors such as anxiety and social interactions. Converging experimental and clinical evidence suggests that altered interneuron development may underlie part of the pathophysiological process of such disorders. Consistent with this, mice with abnormal hepatocyte growth factor signaling exhibit disturbances in the development of specific interneuron subclasses that are paralleled by seizure activity and a complex behavioral phenotype. Mutations in molecules that regulate different aspects of interneuron development could provide the heterogeneity in genetic susceptibility that, when combined with environmental disturbances, results in a phenotypic spectrum that serves as the hallmark pathophysiology for autism, mental retardation, schizophrenia and other neurodevelopmental disorders.

Hepatocyte growth factor/scatter factor is a motogen for interneurons migrating from the ventral to dorsal telencephalon.

Cortical interneurons arise from the proliferative zone of the ventral telencephalon, the ganglionic eminence, and migrate into the developing neocortex. The spatial patterns of migratory interneurons reflect the complementary expression of hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, MET, in the forebrain. Scatter assays on forebrain explants demonstrate regionally specific motogenic activity due to HGF/SF. In addition, exogenous ligand disrupts normal cell migration. Mice lacking the urokinase-type plasminogen activator receptor (u-PAR), a key component of HGF/SF activation, exhibit deficient scatter activity in the forebrain, abnormal interneuron migration from the ganglionic eminence, and reduced interneurons in the frontal and parietal cortex. The data suggest that HGF/SF motogenic activity, which is essential for normal development of other organ systems, is a conserved mechanism that regulates trans-telencephalic migration of interneurons.

Double Dissociation of the Effects of Medial and Orbital Prefrontal Cortical Lesions on Attentional and Affective Shifts in Mice

Many neuropsychiatric diseases are associated with cognitive rigidity linked to prefrontal dysfunction. For example, schizophrenia and Parkinsons disease are associated with performance deficits on the Wisconsin Card Sorting Test, which evaluates attentional set shifting. Although the genetic underpinnings of these disorders can be reproduced in mice, there are few models for testing the functional consequences. Here, we demonstrate that an analog of the Wisconsin Card Sorting Test, developed in marmosets and recently adapted to rats, is a behavioral model of prefrontal function in mice. Systematic analysis demonstrated that formation of the attentional set in mice is dependent on the number of problem sets. We found that mice, like rats and primates, exhibit both affective and attentional sets, and these functions are disrupted by neurotoxic damage to orbitofrontal and medial prefrontal cortical areas, respectively. These data are identical to studies in rats and similar to the deficits reported after prefrontal damage in a comparable task in marmosets. These results provide a behavioral model to assess prefrontal function in mice.

Neural structures underlying set-shifting: roles of medial prefrontal cortex and anterior cingulate cortex.

Impaired attentional set-shifting and inflexible decision-making are problems frequently observed during normal aging and in several psychiatric disorders. To understand the neuropathophysiology of underlying inflexible behavior, animal models of attentional set-shifting have been developed to mimic tasks such as the Wisconsin Card Sorting Task (WCST), which tap into a number of cognitive functions including stimulus-response encoding, working memory, attention, error detection, and conflict resolution. Here, we review many of these tasks in several different species and speculate on how prefrontal cortex and anterior cingulate cortex might contribute to normal performance during set-shifting.

Differential Regulation of Thalamic and Cortical Axonal Growth by Hepatocyte Growth Factor/Scatter Factor

The initial axonal projections between the cerebral cortex and thalamus are established during embryogenesis. Chemoattractants and repellents are thought to provide specific guidance cues for directional growth of these pathways. Hepatocyte growth factor/scatter factor (HGF/SF) serves as an attractant for developing motor neurons, and its distribution in embryonic pallidum, pallium and thalamus suggests a similar role in forebrain development. We examined the effectiveness of HGF/SF in regulating thalamic and cortical neuronal growth using in vitro assays. HGF/SF increased neurite outgrowth of thalamic, but not cortical neurons, grown in dissociated cultures or as explants. HGF/SF also exhibited a chemoattractant property for thalamic axons, promoting the extension of neurites towards an HGF/SF source. These experiments demonstrate HGF/SF has the capacity to selectively direct thalamocortical projections into an intermediate target, the pallidum, and eventually to their final cortical destination.

Reversal learning and attentional set-shifting in mice

Schizophrenia is a complex developmental disorder that presents challenges to modern neuroscience in terms of discovering etiology and aiding in effective treatment of afflicted humans. One approach is to divide the constellation of symptoms of human neuropsychiatric disorders into discrete units for study. Multiple animal models are used to study brain ontogeny, response to psychoactive compounds, substrates of defined behaviors. Frontal cortical areas have been found to have abnormal anatomy and neurotransmitter levels in postmortem brains from schizophrenic patients. The mouse model has the advantage of rather straightforward genetic manipulation and offers numerous genetic variations within the same species. However, until recently, the behavioral analyses in the mice lagged behind the primate and rat, especially with respect to testing of frontal cortical regions. Current reports of mouse prefrontal anatomy and function advocate the mouse as a feasible animal model to study prefrontal cortical function. This review highlights the most recent developments from behavioral paradigms for testing orbital and medial prefrontal cortical function in pharmacological and genetic models of human schizophrenia.

Prefrontal cognitive deficits in mice with altered cerebral cortical GABAergic interneurons.

Alterations of inhibitory GABAergic neurons are implicated in multiple psychiatric and neurological disorders, including schizophrenia, autism and epilepsy. In particular, interneuron deficits in prefrontal areas, along with presumed decreased inhibition, have been reported in several human patients. The majority of forebrain GABAergic interneurons arise from a single subcortical source before migrating to their final regional destination. Factors that govern the interneuron populations have been identified, demonstrating that a single gene mutation may globally affect forebrain structures or a single area. In particular, mice lacking the urokinase plasminogen activator receptor (Plaur) gene have decreased GABAergic interneurons in frontal and parietal, but not caudal, cortical regions. Plaur assists in the activation of hepatocyte growth factor/scatter factor (HGF/SF), and several of the interneuron deficits are correlated with decreased levels of HGF/SF. In some cortical regions, the interneuron deficit can be remediated by endogenous overexpression of HGF/SF. In this study, we demonstrate decreased parvalbumin-expressing interneurons in the medial frontal cortex, but not in the hippocampus or basal lateral amygdala in the Plaur null mouse. The Plaur null mouse demonstrates impaired medial frontal cortical function in extinction of cued fear conditioning and the inability to form attentional sets. Endogenous HGF/SF overexpression increased the number of PV-expressing cells in medial frontal cortical areas to levels greater than found in wildtype mice, but did not remediate the behavioral deficits. These data suggest that proper medial frontal cortical function is dependent upon optimum levels of inhibition and that a deficit or excess of interneuron numbers impairs normal cognition.

Genetic disruption of Met signaling impairs GABAergic striatal development and cognition.

The largest structure of the basal ganglia, the striatum, modulates motor activity and cognitive function and is composed of GABAergic projection neurons and interneurons. To better understand the mechanisms underlying the development of the striatal neurons and their assembly into functional circuits, we used a mouse with a targeted conditional Met mutation in post-mitotic cells of the ventral telencephalon. Characterization of the ontogeny of the striatal neuronal populations demonstrated that disruption of Met signaling specifically altered the GABAergic interneurons. Medium spiny neurons (MSNs) and cholinergic interneurons were largely unaffected. Mice lacking Met signaling have increased numbers of striatal GABAergic interneurons in the lateral sensorimotor areas with distinct behavioral deficits. Motor function and memory formation and consolidation appeared intact, but procedural learning on the cued task of the Morris water maze was delayed. MET is a susceptibility gene in Tourette syndrome and autism, which are human disorders with impaired procedural learning. This study reveals how a striatal targeted disruption in Met signaling after generation of striatal neurons produces behavioral phenotypes shared by Tourette syndrome and autism, linking the human genetics with the mechanism underlying the disorders.

Hepatocyte growth factor (HGF) modulates GABAergic inhibition and seizure susceptibility

Disrupted ontogeny of forebrain inhibitory interneurons leads to neurological disorders, including epilepsy. Adult mice lacking the urokinase plasminogen activator receptor (Plaur) have decreased numbers of neocortical GABAergic interneurons and spontaneous seizures, attributed to a reduction of hepatocyte growth factor/scatter factor (HGF/SF). We report that by increasing endogenous HGF/SF concentration in the postnatal Plaur null mouse brain maintains the interneuron populations in the adult, reverses the seizure behavior and stabilizes the spontaneous electroencephalogram activity. The perinatal intervention provides a pathway to reverse potential birth defects and ameliorate seizures in the adult.