B. S. Shankaranarayana Rao

Correction of fragile X syndrome in mice

Fragile X syndrome (FXS) is the most common form of heritable mental retardation and the leading identified cause of autism. FXS is caused by transcriptional silencing of the FMR1 gene that encodes the fragile X mental retardation protein (FMRP), but the pathogenesis of the disease is unknown. According to one proposal, many psychiatric and neurological symptoms of FXS result from unchecked activation of mGluR5, a metabotropic glutamate receptor. To test this idea we generated Fmr1 mutant mice with a 50% reduction in mGluR5 expression and studied a range of phenotypes with relevance to the human disorder. Our results demonstrate that mGluR5 contributes significantly to the pathogenesis of the disease, a finding that has significant therapeutic implications for fragile X and related developmental disorders.

Loss of Presenilin Function Causes Impairments of Memory and Synaptic Plasticity Followed by Age-Dependent Neurodegeneration

Mutations in presenilins are the major cause of familial Alzheimers disease, but the pathogenic mechanism by which presenilin mutations cause memory loss and neurodegeneration remains unclear. Here we demonstrate that conditional double knockout mice lacking both presenilins in the postnatal forebrain exhibit impairments in hippocampal memory and synaptic plasticity. These deficits are associated with specific reductions in NMDA receptor-mediated responses and synaptic levels of NMDA receptors and alphaCaMKII. Furthermore, loss of presenilins causes reduced expression of CBP and CREB/CBP target genes, such as c-fos and BDNF. With increasing age, mutant mice develop striking neurodegeneration of the cerebral cortex and worsening impairments of memory and synaptic function. Neurodegeneration is accompanied by increased levels of the Cdk5 activator p25 and hyperphosphorylated tau. These results define essential roles and molecular targets of presenilins in synaptic plasticity, learning and memory, and neuronal survival in the adult cerebral cortex.

Altered Cortical Synaptic Morphology and Impaired Memory Consolidation in Forebrain- Specific Dominant-Negative PAK Transgenic Mice

Molecular and cellular mechanisms for memory consolidation in the cortex are poorly known. To study the relationships between synaptic structure and function in the cortex and consolidation of long-term memory, we have generated transgenic mice in which catalytic activity of PAK, a critical regulator of actin remodeling, is inhibited in the postnatal forebrain. Cortical neurons in these mice displayed fewer dendritic spines and an increased proportion of larger synapses compared to wild-type controls. These alterations in basal synaptic morphology correlated with enhanced mean synaptic strength and impaired bidirectional synaptic modifiability (enhanced LTP and reduced LTD) in the cortex. By contrast, spine morphology and synaptic plasticity were normal in the hippocampus of these mice. Importantly, these mice exhibited specific deficits in the consolidation phase of hippocampus-dependent memory. Thus, our results provide evidence for critical relationships between synaptic morphology and bidirectional modifiability of synaptic strength in the cortex and consolidation of long-term memory.

Transgenic brain-derived neurotrophic factor expression causes both anxiogenic and antidepressant effects

Although neurotrophins have been postulated to have antidepressant properties, their effect on anxiety is not clear. We find that transgenic overexpression of the neurotrophin BDNF has an unexpected facilitatory effect on anxiety-like behavior, concomitant with increased spinogenesis in the basolateral amygdala. Moreover, anxiogenesis and amygdalar spinogenesis are also triggered by chronic stress in control mice but are occluded by BDNF overexpression, thereby suggesting a role for BDNF signaling in stress-induced plasticity in the amygdala. BDNF overexpression also causes antidepressant effects, because transgenic mice exhibit improved performance on the Porsolt forced-swim test and an absence of chronic stress-induced hippocampal atrophy. Thus, structural changes in the amygdala and hippocampus, caused by genetic manipulation of the same molecule BDNF, give rise to contrasting effects on anxiety and depressive symptoms, both of which are major behavioral correlates of stress disorders.

Inhibition of p21-activated kinase rescues symptoms of fragile X syndrome in mice

Fragile X syndrome (FXS), the most commonly inherited form of mental retardation and autism, is caused by transcriptional silencing of the fragile X mental retardation 1 (FMR1) gene and consequent loss of the fragile X mental retardation protein. Despite growing evidence suggesting a role of specific receptors and biochemical pathways in FXS pathogenesis, an effective therapeutic method has not been developed. Here, we report that abnormalities in FMR1 knockout (KO) mice, an animal model of FXS, are ameliorated, at least partially, at both cellular and behavioral levels, by an inhibition of the catalytic activity of p21-activated kinase (PAK), a kinase known to play a critical role in actin polymerization and dendritic spine morphogenesis. Greater spine density and elongated spines in the cortex, morphological synaptic abnormalities commonly observed in FXS, are at least partially restored by postnatal expression of a dominant negative (dn) PAK transgene in the forebrain. Likewise, the deficit in cortical long-term potentiation observed in FMR1 KO mice is fully restored by the dnPAK transgene. Several behavioral abnormalities associated with FMR1 KO mice, including those in locomotor activity, stereotypy, anxiety, and trace fear conditioning are also ameliorated, partially or fully, by the dnPAK transgene. Finally, we demonstrate a direct interaction between PAK and fragile X mental retardation protein in vitro. Overall, our results demonstrate the genetic rescue of phenotypes in a FXS mouse model and suggest that the PAK signaling pathway, including the catalytic activity of PAK, is a novel intervention site for development of an FXS and autism therapy.

Rescue of fragile X syndrome phenotypes in Fmr1 KO mice by the small-molecule PAK inhibitor FRAX486

Fragile X syndrome (FXS) is the most common inherited form of autism and intellectual disability and is caused by the silencing of a single gene, fragile X mental retardation 1 (Fmr1). The Fmr1 KO mouse displays phenotypes similar to symptoms in the human condition—including hyperactivity, repetitive behaviors, and seizures—as well as analogous abnormalities in the density of dendritic spines. Here we take a hypothesis-driven, mechanism-based approach to the search for an effective therapy for FXS. We hypothesize that a treatment that rescues the dendritic spine defect in Fmr1 KO mice may also ameliorate autism-like behavioral symptoms. Thus, we targeted a protein that regulates spines through modulation of actin cytoskeleton dynamics: p21-activated kinase (PAK). Our results demonstrate that a potent small molecule inhibitor of group I PAKs reverses dendritic spine phenotypes in Fmr1 KO mice. Moreover, this PAK inhibitor—which we call FRAX486—also rescues seizures and behavioral abnormalities such as hyperactivity and repetitive movements, thereby supporting the hypothesis that a drug treatment that reverses the spine abnormalities can also treat neurological and behavioral symptoms. Finally, a single administration of FRAX486 is sufficient to rescue all of these phenotypes in adult Fmr1 KO mice, demonstrating the potential for rapid, postdiagnostic therapy in adults with FXS.

Exposure to enriched environment restores the survival and differentiation of new born cells in the hippocampus and ameliorates depressive symptoms in chronically stressed rats

Chronic stress decreases neurogenesis in the adult brain, while exposure to enriched environment (EE) increases it. Recent studies demonstrate the ability of EE to ameliorate stress-induced behavioral deficits. Whether a restored neurogenesis contributes to these effects of EE is unknown. Recently, we demonstrated that EE following restraint stress restores cell proliferation in the dentate gyrus (DG), hippocampal volume and learning. In the current study, we examine the effects of EE following stress on survival and differentiation of the progenitor cells in the DG and behavioral depression using the forced swim test (FST) and sucrose consumption test (SCT). Adult male Wistar rats were subjected to 21 days of restraint stress followed by housing in either standard or enriched conditions (10 days, 6h/day). Survival and differentiation of BrdU-labeled cells were evaluated 31 days post-BrdU administration. Stress decreased the survival and differentiation of progenitor cells, which was ameliorated by EE. Also the percentage of BrdU-ir cells that did not co-localize with NeuN or S100beta was significantly greater in the stressed rats and was restored by EE. Stress increased immobility in FST and decreased sucrose preference in the SCT, and these behaviors were ameliorated by EE. Adult neurogenesis is thought to be linked to learning and memory and in mediating antidepressant effect. Taken together with our earlier report that EE restores stress-induced impairment in learning and cytogenesis, the current results indicate that the reversal of adult neurogenesis could be one of the mechanisms involved in the amelioration of stress-induced deficits.

Enriched environment restores hippocampal cell proliferation and ameliorates cognitive deficits in chronically stressed rats.

Adult neurogenesis, particularly in the subgranular zone, is thought to be linked with learning and memory. Chronic stress inhibits adult hippocampal neurogenesis and also impairs learning and memory. On the other hand, exposure to enriched environment (EE) is reported to enhance the survival of new neurons and improve cognition. Accordingly, in the present study, we examined whether short‐term EE after stress could ameliorate the stress‐induced decrease in hippocampal cell proliferation and impairment in radial arm maze learning. After restraint stress (6 hr/day, 21 days) adult rats were exposed to EE (6 hr/day, 10 days). We observed that chronic restraint stress severely affected formation of new cells and learning. Stressed rats showed a significant decrease (70%) in the number of BrdU (5‐bromo‐2′‐deoxyuridine)‐immunoreactive cells and impairment in the performance of the partially baited radial arm maze task. Interestingly, EE after stress completely restored the hippocampal cell proliferation. On par with the restoration of hippocampal cytogenesis, short‐term EE after stress resulted in a significant increase in percentage correct choices and a decrease in the number of reference memory errors compared with the stressed animals. Also, EE per se significantly increased the cell proliferation compared with controls. Furthermore, stress significantly reduced the hippocampal volume that was reversed after EE. Our observations demonstrate that short‐term EE completely ameliorates the stress‐induced decrease in cell proliferation and learning deficit, thus demonstrating the efficiency of rehabilitation in reversal of stress‐induced deficits and suggesting a probable role of newly formed cells in the effects of EE.

Stress-induced spine loss in the medial amygdala is mediated by tissue-plasminogen activator.

The amygdala, which exerts a regulatory influence on the stress response, is itself affected by stress. It has been reported that the serine protease tissue-plasminogen activator (tPA), a key mediator of spine plasticity, is required for stress-induced facilitation of anxiety-like behavior. Importantly, tPA is also involved in stress-induced activation of molecular signals that have the potential to contribute to neuronal remodeling in the medial amygdala (MeA). However, little is known about the precise nature of, and specific role played by tPA in, stress-induced structural plasticity in the MeA. Hence, we compared the impact of chronic restraint stress on spine density of medium spiny stellate neurons in MeA in wild-type mice with mice in which the tPA gene is disrupted (tPA-/-). In wild-type mice, chronic stress caused significant reduction in MeA spine density, which was in contrast to enhanced spine density in the neighboring basolateral amygdala (BLA). Strikingly, tPA-/- mice exhibited significant attenuation of stress-induced spine retraction in the MeA, but BLA spinogenesis was not affected. Therefore, tPA-dependence of stress-induced modulation in spine density was restricted to the MeA. Further, MeA neurons in tPA-/- mice, even when challenged with repeated stress, were able to maintain levels of spine density that were comparable to that of wild-type mice without stress. Our findings provide novel evidence for a permissive role for tPA in amygdalar spine plasticity elicited by behavioral stress.

HYPOBARIC HYPOXIA-INDUCED DENDRITIC ATROPHY OF HIPPOCAMPAL NEURONS IS ASSOCIATED WITH COGNITIVE IMPAIRMENT IN ADULT RATS

Simulated hypobaric hypoxia (HBH), resembling high altitude hypoxia severely affects the CNS and results in several physiological changes. The hippocampus is closely associated with learning and memory and an insult to this region affects cognition. Previous studies suggest that rapid or prolonged exposures to HBH are associated with psychomotor and cognitive impairments. The defense personnel, mountain climbers and rescue teams are exposed to such harsh environment and thus it demands a systematic study emphasizing the subtle effects of such extreme environments on cognitive function. Accordingly, this study evaluated the effect of hypobaric hypoxia on structural changes in the principal neurons of the hippocampus and learning in eight-arm radial maze. Adult male Wistar rats, subjected to simulated hypobaric hypoxia equivalent to an altitude of 6000 m for a period of 2 or 7 days, in a hypoxic chamber served as hypoxic group (HY). Rats housed in a similar chamber for the same period of time, without hypoxic exposure served as sham control (SC), while normal control (NC) group of rats were housed in standard laboratory conditions. The dendritic morphology of neurons in cornu ammonis region 1 (CA1) and cornu ammonis region 3 (CA3) was studied in Golgi-impregnated hippocampal sections. Exposure for 2 days to hypobaric hypoxia had minimal deleterious effects on the CA1 pyramidal neurons, while exposure for 7 days resulted in a significant decrease in the number of branching points, intersections and dendritic length. Unlike the CA1 pyramidal neurons, the CA3 neurons exhibited dendritic atrophy following both 2 and 7 days of hypoxic exposure. Further, hippocampal-dependent spatial learning was affected marginally following 2 day exposure, while 7 day exposure severely affected learning of the partially baited radial arm maze task. Our study suggests that dendritic atrophy in the hippocampus on exposure to HBH could be one of the bases for the cognitive deficits exhibited under such conditions.