Inhibition of Rac1 in ventral hippocampal excitatory neurons improves social recognition memory and synaptic plasticity.

Rac1 is critically involved in the regulation of the actin cytoskeleton, neuronal structure, synaptic plasticity, and memory. Rac1 overactivation is reported in human patients and animal models of Alzheimer's disease (AD) and contributes to their spatial memory deficits, but whether Rac1 dysregulation is also important in other forms of memory deficits is unknown. In addition, the cell types and synaptic mechanisms involved remain unclear. In this study, we used local injections of AAV virus containing a dominant-negative (DN) Rac1 under the control of CaMKIIα promoter and found that the reduction of Rac1 hyperactivity in ventral hippocampal excitatory neurons improves social recognition memory in APP/PS1 mice. Expression of DN Rac1 also improves long-term potentiation, a key synaptic mechanism for memory formation. Our results suggest that overactivation of Rac1 in hippocampal excitatory neurons contributes to social memory deficits in APP/PS1 mice and that manipulating Rac1 activity may provide a potential therapeutic strategy to treat social deficits in AD.

NGPF2 triggers synaptic scaling up through ALK-LIMK-cofilin-mediated mechanisms.

Synaptic scaling is an extensively studied form of homeostatic plasticity critically involved in various brain functions. Although it is accepted that synaptic scaling is expressed through the postsynaptic accumulation of AMPA receptors (AMPARs), the induction mechanism remains elusive. In this study, we show that TTX treatment induces rapid but transient release of the neurite growth-promoting factor 2 (NGPF2), and this release is necessary and sufficient for TTX-induced scaling up. In addition, we show that inhibition of the anaplastic lymphoma kinase (ALK)-LIMK-cofilin signaling pathway blocks TTX- and NGPF2-induced synaptic scaling up. Furthermore, we show that TTX-induced release of NGPF2 is protein synthesis dependent and requires fragile X mental retardation protein 1 (FMRP1). These results indicate that activity blockade induces NGPF2 synthesis and release to trigger synaptic scaling up through LIMK-cofilin-dependent actin reorganization, spine enlargement, and stabilization of AMPARs at the synapse.

Overexpression of LIMK1 in hippocampal excitatory neurons improves synaptic plasticity and social recognition memory in APP/PS1 mice.

Accumulating evidence indicates that the actin regulator cofilin is overactivated in Alzheimer's Disease (AD), but whether this abnormality contributes to synaptic and cognitive impairments in AD is unclear. In addition, the brain region and cell types involved remain unknown. In this study, we specifically manipulate LIMK1, the key protein kinase that phosphorylates and inactivates cofilin, in the hippocampus of APP/PS1 transgenic mice. Using local injections of the AAV virus containing LIMK1 under the control of the CaMKIIα promoter, we show that expression of LIMK1 in hippocampal excitatory neurons increases cofilin phosphorylation (i.e., decreases cofilin activity), rescues impairments in long-term potentiation, and improves social memory in APP/PS1 mice. Our results suggest that deficits in LIMK1/cofilin signaling in the hippocampal excitatory neurons contribute to AD pathology and that manipulations of LIMK1/cofilin activity provide a potential therapeutic strategy to treat AD.

The Requirement of the C-Terminal Domain of GluA1 in Different Forms of Long-Term Potentiation in the Hippocampus Is Age-Dependent.

Long-term potentiation (LTP) at glutamatergic synapses is an extensively studied form of long-lasting synaptic plasticity widely regarded as the cellular basis for learning and memory. At the CA1 synapse, there are multiple forms of LTP with distinct properties. Although AMPA glutamate receptors (AMPARs) are a key target of LTP expression, whether they are required in all forms of LTP remains unclear. To address this question, we have used our recently developed mouse line, GluA1 C2KI , where the c-terminal domain (CTD) of the endogenous GluA1 is replaced by that of GluA2. Unlike traditional GluA1 global or conditional KO mice, GluA1 C2KI mice have no changes in basal AMPAR properties or synaptic transmission allowing a better assessment of GluA1 in synaptic plasticity. We previously showed that these mice are impaired in LTP induced by high-frequency stimulation (HFS-LTP), but whether other forms of LTP are also affected in these mice is unknown. In this study, we compared various forms of LTP at CA1 synapses between GluA1 C2KI and wild-type littermates by using several induction protocols. We show that HFS-LTP is impaired in both juvenile and adult GluA1 C2KI mice. The LTP induced by theta-burst stimulation (TBS-LTP) is also abolished in juvenile GluA1 C2KI mice. Interestingly, TBS-LTP can still be induced in adult GluA1 C2KI mice, but its mechanisms are altered becoming more sensitive to protein synthesis and the extracellular signal-regulated kinase (ERK) inhibitors compared to wild type (WT) control. The GluA1 C2KI mice are also differentially altered in several forms of LTP induced under whole-cell recording paradigms. These results indicate that the CTD of GluA1 is differentially involved in different forms of LTP at CA1 synapse highlighting the complexity and adaptative potential of LTP expression mechanisms in the hippocampus.

LIMK1 and LIMK2 regulate cortical development through affecting neural progenitor cell proliferation and migration.

LIMK1 and LIMK2 are key downstream targets to mediate the effects of the Rho family small GTPases and p21-activated kinases (PAK) in the regulation of the actin cytoskeleton. LIMKs are also critical for synaptic transmission, plasticity and memory formation. Changes in LIMK signaling are associated with several neurodevelopmental and neurodegenerative diseases, including autism, intellectual disability and Alzheimer's disease. However, the role of LIMK signaling in brain development remains unknown. In this study, we used LIMK1 KO and LIMK2 KO mice to investigate the role of LIMK signaling in the cerebral cortical development. We found that these KO mice are reduced in the number of pyramidal neurons in upper cortical layers and this reduction is accompanied by a smaller pool of neural progenitor cells and impaired neuronal migration. These results are similar to those found in PAK1 KO mice and suggest that LIMK-dependent actin regulation may play a key role in mediating the effects of PAK1 and Rho signaling in the regulation of cortical development.

Publisher Correction: The C-terminal tails of endogenous GluA1 and GluA2 differentially contribute to hippocampal synaptic plasticity and learning.

In the version of this article initially published, the wrong version of Supplementary Fig. 10 was posted and the city for affiliation 4, the Co-innovation Center of Neuroregeneration, Nantong University, was given as Nanjing instead of Nantong. The errors have been corrected in the HTML and PDF versions of the article.

The C-terminal tails of endogenous GluA1 and GluA2 differentially contribute to hippocampal synaptic plasticity and learning.

Long-term potentiation (LTP) and depression (LTD) at glutamatergic synapses are intensively investigated processes for understanding the synaptic basis for learning and memory, but the underlying molecular mechanisms remain poorly understood. We have made three mouse lines where the C-terminal domains (CTDs) of endogenous AMPA receptors (AMPARs), the principal mediators of fast excitatory synaptic transmission, are specifically exchanged. These mice display profound deficits in synaptic plasticity without any effects on basal synaptic transmission. Our study reveals that the CTDs of GluA1 and GluA2, the key subunits of AMPARs, are necessary and sufficient to drive NMDA receptor-dependent LTP and LTD, respectively. In addition, these domains exert differential effects on spatial and contextual learning and memory. These results establish dominant roles of AMPARs in governing bidirectional synaptic and behavioral plasticity in the CNS.

Regulation of hippocampal long-term potentiation by p21-activated protein kinase 1 (PAK1).

The Rho family small GTPases are critically involved in the regulation of spine and synaptic properties, but the underlying mechanisms are poorly defined. We took genetic approaches to create and analyze knockout mice deficient in the expression of the protein kinase PAK1 that is directly associated with and activated by the Rho GTPases. We demonstrated that while these knockout mice were normal in both basal and presynaptic function, they were selectively impaired in long-term potentiation (LTP) at hippocampal CA1 synapses. Consistent with the electrophysiological deficits, the PAK1 knockout mice showed changes in the actin cytoskeleton and the actin binding protein cofilin. These results indicate that PAK1 is critical in hippocampal synaptic plasticity via regulating cofilin activity and the actin cytoskeleton.

A critical role of Rho-kinase ROCK2 in the regulation of spine and synaptic function.

The actin cytoskeleton is critically involved in the regulation of the dendritic spine and synaptic properties, but the molecular mechanisms underlying actin dynamics in neurons are poorly defined. We took genetic approaches to create and analyze knockout mice specifically lacking ROCK2, a protein kinase that directly interacts with and is activated by the Rho GTPases, the central mediator of actin reorganization. We demonstrated that while these knockout mice were normal in gross brain anatomy, they were impaired in both basal synaptic transmission and hippocampal long-term potentiation (LTP). Consistent with the electrophysiological deficits, the ROCK2 knockout neurons showed deficits in spine properties, synapse density, the actin cytoskeleton, and the actin-binding protein cofilin. These results indicate that ROCK2/cofilin signaling is critical in the regulation of neuronal actin, spine morphology and synaptic function.

Abnormal long-lasting synaptic plasticity and cognition in mice lacking the mental retardation gene Pak3.

Mutations in the Pak3 gene lead to nonsyndromic mental retardation characterized by selective deficits in cognition. However, the underlying mechanisms are yet to be elucidated. We report here that the knock-out mice deficient in the expression of p21-activated kinase 3 (PAK3) exhibit significant abnormalities in synaptic plasticity, specifically hippocampal late-phase long-term potentiation, and deficiencies in learning and memory. A dramatic reduction in the active form of transcription factor cAMP-responsive element-binding protein in the knock-out mice implicates a novel signaling mechanism by which PAK3 and Rho signaling regulate synaptic function and cognition.

Regulation of ADF/cofilin phosphorylation and synaptic function by LIM-kinase.

To investigate the role of the LIM-kinase (LIMK) family in the regulation of ADF/cofilin phosphorylation and synaptic function in the mammalian central nervous system (CNS), we conducted biochemical and electrophysiological analysis using mice that were genetically altered in the expression of LIMK-1 and LIMK-2. We showed here that while LIMK-2 knockout mice exhibited minimal abnormalities, the LIMK-1/2 double knockout mice were more severely impaired in both ADF/cofilin phosphorylation and excitatory synaptic function in the CA1 region of the hippocampus. These results indicate a critical role for the LIMK family in the regulation of ADF/cofilin and synaptic function in the brain.

Influence of experimental chronic hypertension on the cerebral and renal arteries of wild cats.

The influence of chronic hypertension (HT) on the cerebral and renal arteries was examined pathologically and morphometrically in wild cats without a specific genetic background. Chronic HT for 8-15 months was induced by uninephrectomy and salt-loading, and the blood pressure was monitored for a maximum of 5 months. The grade of systolic blood pressure elevation in each cat during the monitoring period was 21-51 mmHg. Histologically, the cerebral arachnoid and medullary arteries of all hypertensive cats showed a well-preserved medial layer, and neither loss of medial smooth muscle cells, adventitial fibrosis or fibrinoid exudation was detected. This experimental model of chronic HT in wild cats for 8-15 months induced segmental intimal elastofibrosis of the arachnoid and renal arteries, but spared the cerebral medullary artery. The parenchymal changes in the brain were negligible. Morphometrically, the arachnoid artery in control cats had a significantly thinner media than the renal artery, and the medial hypertrophy of the arachnoid artery resulting from HT occurred significantly less frequently than that of the renal artery. These findings suggest that the arachnoid and medullary arteries are relatively well protected from HT, and that this may be characteristic of cerebral arteries in general and ascribed to autoregulation.

Regulation of spine morphology and synaptic function by LIMK and the actin cytoskeleton.

Filamentous actin (F-actin) is highly enriched in the dendritic spine, a specialized postsynaptic structure on which the great majority of the excitatory synapses are formed in the mammalian central nervous system (CNS). The protein kinases of the Lim-kinase (LIMK) family are potent regulators of actin dynamics in many cell types and they are abundantly expressed in the CNS, including the hippocampus. Using a combination of genetic manipulations and electrophysiological recordings in mice, we have demonstrated that LIMK-1 signaling is important in vivo in the regulation of the actin cytoskeleton, spine morphology, and synaptic function, including hippocampal long-term potentiation (LTP), a prominent form of long lasting synaptic plasticity thought to be critical to memory formation. Our results provide strong genetic evidence that LIMK and its substrate ADF/cofilin are involved in spine morphology and synaptic properties and are consistent with the notion that the Rho family small GTPases and the actin cytoskeleton are critical to spine structure and synaptic regulation.

Synaptic transmission and plasticity in the absence of AMPA glutamate receptor GluR2 and GluR3.

The AMPA glutamate receptor (AMPAR) subunits GluR2 and GluR3 are thought to be important for synaptic targeting/stabilization of AMPARs and the expression of hippocampal long-term depression (LTD). In order to address this hypothesis genetically, we generated and analyzed knockout mice deficient in the expression of both GluR2 and GluR3. We show here that the double knockout mice are severely impaired in basal synaptic transmission, demonstrating that GluR2/3 are essential to maintain adequate synaptic transmission in vivo. However, these mutant mice are competent in establishing several forms of long-lasting synaptic changes in the CA1 region of the hippocampus, including LTD, long-term potentiation (LTP), depotentiation, and dedepression, indicating the presence of GluR2/3-independent mechanisms of LTD expression and suggesting that AMPA receptor GluR1 alone is capable of various forms of synaptic plasticity.

Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice.

In vitro studies indicate a role for the LIM kinase family in the regulation of cofilin phosphorylation and actin dynamics. In addition, abnormal expression of LIMK-1 is associated with Williams syndrome, a mental disorder with profound deficits in visuospatial cognition. However, the in vivo function of this family of kinases remains elusive. Using LIMK-1 knockout mice, we demonstrate a significant role for LIMK-1 in vivo in regulating cofilin and the actin cytoskeleton. Furthermore, we show that the knockout mice exhibited significant abnormalities in spine morphology and in synaptic function, including enhanced hippocampal long-term potentiation. The knockout mice also showed altered fear responses and spatial learning. These results indicate that LIMK-1 plays a critical role in dendritic spine morphogenesis and brain function.