Localization and seizure-regulation of integrin beta 1 mRNA in adult rat brain

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Hippocampus chronic deep brain stimulation induces reversible transcript changes in a macaque model of mesial temporal lobe epilepsy

BACKGROUND

Deep brain stimulation (DBS) has seizure-suppressing effects but the molecular mechanisms underlying its therapeutic action remain unclear. This study aimed to systematically elucidate the mechanisms underlying DBS-induced seizure suppression at a molecular level.

METHODS

We established a macaque model of mesial temporal lobe epilepsy (mTLE), and continuous high-frequency hippocampus DBS (hip-DBS) was applied for 3 months. The effects of hip-DBS on hippocampus gene expression were examined using high-throughput microarray analysis followed by bioinformatics analysis. Moreover, the microarray results were validated using quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot analyses.

RESULTS

The results showed that chronic hip-DBS modulated the hippocampal gene expression. We identified 4119 differentially expressed genes and assigned these genes to 16 model profiles. Series test of cluster analysis showed that profiles 5, 3, and 2 were the predominant expression profiles. Moreover, profile 5 was mainly involved in focal adhesion and extracellular matrix-receptor interaction pathway. Nine dysregulated genes (Arhgap5, Col1a2, Itgb1, Pik3r1, Lama4, Fn1, Col3a1, Itga9, and Shc4) and three genes (Col1a2, Itgb1, and Flna) in these two pathways were further validated by qRT-PCR and Western blot analyses, respectively, which showed a concordance.

CONCLUSION

Our findings suggest that hip-DBS could markedly reverse mTLE-induced abnormal gene expression. Findings from this study establish the basis for further investigation of the underlying regulatory mechanisms of DBS for mTLE.

β1-Integrins in the Developing Orbitofrontal Cortex Are Necessary for Expectancy Updating in Mice

Navigating a changing environment requires associating stimuli and actions with their likely outcomes and modifying these associations when they change. These processes involve the orbitofrontal cortex (OFC). Although some molecular mediators have been identified, developmental factors are virtually unknown. We hypothesized that the cell adhesion factor β1-integrin is essential to OFC function, anticipating developmental windows during which β1-integrins might be more influential than others. We discovered that OFC-selective β1-integrin silencing before adolescence, but not later, impaired the ability of mice to extinguish conditioned fear and select actions based on their likely outcomes. Early-life knock-down also reduced the densities of dendritic spines, the primary sites of excitatory plasticity in the brain, and weakened sensitivity to cortical inputs. Notwithstanding these defects in male mice, females were resilient to OFC (but not hippocampal) β1-integrin loss. Existing literature suggests that resilience may be explained by estradiol-mediated transactivation of β1-integrins and tropomyosin receptor kinase B (trkB). Accordingly, we discovered that a trkB agonist administered during adolescence corrected reward-related decision making in β1-integrin-deficient males. In sum, developmental β1-integrins are indispensable for OFC function later in life.SIGNIFICANCE STATEMENT The orbitofrontal cortex (OFC) is a subregion of the frontal cortex that allows organisms to link behaviors and stimuli with anticipated outcomes, and to make predictions about the consequences of one's behavior. Aspects of OFC development are particularly prolonged, extending well into adolescence, likely optimizing organisms' abilities to prospectively calculate the consequences of their actions and select behaviors appropriately; these decision making strategies improve as young individuals mature into adulthood. Molecular factors are not, however, well understood. Our experiments reveal that a cell adhesion protein termed "β1-integrin" is necessary for OFC neuronal maturation and function. Importantly, β1-integrins operate during a critical period equivalent to early adolescence in humans to optimize the ability of organisms to update expectancies later in life.

Early Synaptic Alterations and Selective Adhesion Signaling in Hippocampal Dendritic Zones Following Organophosphate Exposure

Organophosphates account for many of the world's deadliest poisons. They inhibit acetylcholinesterase causing cholinergic crises that lead to seizures and death, while survivors commonly experience long-term neurological problems. Here, we treated brain explants with the organophosphate compound paraoxon and uncovered a unique mechanism of neurotoxicity. Paraoxon-exposed hippocampal slice cultures exhibited progressive declines in synaptophysin, synapsin II, and PSD-95, whereas reduction in GluR1 was slower and NeuN and Nissl staining showed no indications of neuronal damage. The distinctive synaptotoxicity was observed in dendritic zones of CA1 and dentate gyrus. Interestingly, declines in synapsin II dendritic labeling correlated with increased staining for β1 integrin, a component of adhesion receptors that regulate synapse maintenance and plasticity. The paraoxon-induced β1 integrin response was targeted to synapses, and the two-fold increase in β1 integrin was selective as other synaptic adhesion molecules were unchanged. Additionally, β1 integrin-cofilin signaling was triggered by the exposure and correlations were found between the extent of synaptic decline and the level of β1 integrin responses. These findings identified organophosphate-mediated early and lasting synaptotoxicity which can explain delayed neurological dysfunction later in life. They also suggest that the interplay between synaptotoxic events and compensatory adhesion responses influences neuronal fate in exposed individuals.

Gene Expression Profiling of Two Epilepsy Models Reveals the ECM/Integrin signaling Pathway is Involved in Epiletogenesis

The molecular mechanisms underlying the development of epilepsy, i.e., epileptogenesis, are due to altered expression of a series of genes. Global expression profiling of temporal lobe epilepsy is confounded by a number of factors, including the variability among animal species, animal models, and tissue sampling time-points. In this study, we pooled two microarray datasets of the most used pilocarpine and kainic acid epilepsy models from the Gene Expression Omnibus database. A total of 567 known and novel genes were commonly differentially expressed across the two models. Pathway analyses demonstrated that the dysregulated genes were involved in 46 pathways. Real-time PCR and western blot analysis confirmed the activation of extracellular matrix (ECM)/integrin signaling pathways. Moreover, targeting ECM/integrin signaling inhibits astrocyte activation and promotes neuron injury in the hippocampus of epileptic mice. This study may provide a "gene/pathway database" that with further investigation can determine the mechanisms underlining epileptogenesis and the possible targets for neuron protection in the hippocampus after status epilepticus.

β1 Integrin as a Xenoantigen in Fetal Porcine Mesencephalic Cells Transplanted into the Rat Brain

Xenografts of fetal porcine mesencephalic cells implanted into the rat striatum are generally rejected within several weeks. The fetal donor mesencephalon predominantly consists of neurons, but also contains microglial and endothelial cells, which are more immunogenic. In the present work, we investigated the occurrence of donor endothelial cells in grafts of porcine mesencephalic cells implanted into the rat striatum. Pig endothelial cells were monitored by immunochemical methods, using a monoclonal antibody (mAb) that recognizes a peptidic epitope of the porcine β1 integrin, and isolectin IB4, for the staining of the Galα1,3Gal epitope. The analysis also involved the detection of the pig hyaluronate receptor CD44, and the cell adhesion molecule CD31. The anti-β1 integrin mAb revealed endothelial-like cells in grafts of porcine mesencephalic cells as soon as 1 week after implantation. A similar staining pattern was obtained with the IB4 lectin. Unlike aortic endothelial cells, these pig brain-derived endothelial-like cells were not recognized by the anti-CD44 antibody. They also failed to express the CD31 adhesion molecule, a fact which suggests that they remained poorly mature, even in grafts maintained during 45 days in immunosuppressed rats. Interestingly, a strong expression of β1 integrin immunoreactivity was noticed in a large proportion (80%) of the cells freshly dissociated from the fetal pig mesencephalic tissue. The immunoreactivity decreased progressively after transplantation of the cells into the rat brain. This observation suggests that dissociated neuroblasts are capable of a temporary expression of β1 integrin. This molecule is known to participate in the process of cell sorting and migration in the developing brain. Hence, its expression could be the hallmark of a rescue mechanism triggered by the disruption of the cell/matrix interactions during the dissociation of the fetal mesencephalon. This disruption might account for part of the dramatic cell death process that occurs during the manipulation of the donor tissue.

Differential expression of cytoskeletal regulatory factors in the adolescent prefrontal cortex: Implications for cortical development

The prevalence of depression, anxiety, schizophrenia, and drug and alcohol use disorders peaks during adolescence. Further, up to 50% of "adult" mental health disorders emerge in adolescence. During adolescence, the prefrontal cortex (PFC) undergoes dramatic structural reorganization, in which dendritic spines and synapses are refined, pruned, and stabilized. Understanding the molecular mechanisms that underlie these processes should help to identify factors that influence the development of psychiatric illness. Here we briefly discuss the anatomical connections of the medial and orbital prefrontal cortex (mPFC and OFC, respectively). We then present original findings suggesting that dendritic spines on deep-layer excitatory neurons in the mouse mPFC and OFC prune at different adolescent ages, with later pruning in the OFC. In parallel, we used Western blotting to define levels of several cytoskeletal regulatory proteins during early, mid-, and late adolescence, focusing on tropomyosin-related kinase receptor B (TrkB) and β1-integrin-containing receptors and select signaling partners. We identified regional differences in the levels of several proteins in early and midadolescence that then converged in early adulthood. We also observed age-related differences in TrkB levels, both full-length and truncated isoforms, Rho-kinase 2, and synaptophysin in both PFC subregions. Finally, we identified changes in protein levels in the dorsal and ventral hippocampus that were distinct from those in the PFC. We conclude with a general review of the manner in which TrkB- and β1-integrin-mediated signaling influences neuronal structure in the postnatal brain. Elucidating the role of cytoskeletal regulatory factors throughout adolescence may identify critical mechanisms of PFC development. © 2016 Wiley Periodicals, Inc.

Stage-specific functions of Semaphorin7A during adult hippocampal neurogenesis rely on distinct receptors

The guidance protein Semaphorin7A (Sema7A) is required for the proper development of the immune and nervous systems. Despite strong expression in the mature brain, the role of Sema7A in the adult remains poorly defined. Here we show that Sema7A utilizes different cell surface receptors to control the proliferation and differentiation of neural progenitors in the adult hippocampal dentate gyrus (DG), one of the select regions of the mature brain where neurogenesis occurs. PlexinC1 is selectively expressed in early neural progenitors in the adult mouse DG and mediates the inhibitory effects of Sema7A on progenitor proliferation. Subsequently, during differentiation of adult-born DG granule cells, Sema7A promotes dendrite growth, complexity and spine development through β1-subunit-containing integrin receptors. Our data identify Sema7A as a key regulator of adult hippocampal neurogenesis, providing an example of how differential receptor usage spatiotemporally controls and diversifies the effects of guidance cues in the adult brain.

Activity dependent CAM cleavage and neurotransmission

Spatially localized proteolysis represents an elegant means by which neuronal activity dependent changes in synaptic structure, and thus experience dependent learning and memory, can be achieved. In vitro and in vivo studies suggest that matrix metalloproteinase and adamalysin activity is concentrated at the cell surface, and emerging evidence suggests that increased peri-synaptic expression, release and/or activation of these proteinases occurs with enhanced excitatory neurotransmission. Synaptically expressed cell adhesion molecules (CAMs) could therefore represent important targets for neuronal activity-dependent proteolysis. Several CAM subtypes are expressed at the synapse, and their cleavage can influence the efficacy of synaptic transmission through a variety of non-mutually exclusive mechanisms. In the following review, we discuss mechanisms that regulate neuronal activity-dependent synaptic CAM shedding, including those that may be calcium dependent. We also highlight CAM targets of activity-dependent proteolysis including neuroligin and intercellular adhesion molecule-5 (ICAM-5). We include discussion focused on potential consequences of synaptic CAM shedding, with an emphasis on interactions between soluble CAM cleavage products and specific pre- and post-synaptic receptors.

Intercellular protein-protein interactions at synapses

Chemical synapses are asymmetric intercellular junctions through which neurons send nerve impulses to communicate with other neurons or excitable cells. The appropriate formation of synapses, both spatially and temporally, is essential for brain function and depends on the intercellular protein-protein interactions of cell adhesion molecules (CAMs) at synaptic clefts. The CAM proteins link pre- and post-synaptic sites, and play essential roles in promoting synapse formation and maturation, maintaining synapse number and type, accumulating neurotransmitter receptors and ion channels, controlling neuronal differentiation, and even regulating synaptic plasticity directly. Alteration of the interactions of CAMs leads to structural and functional impairments, which results in many neurological disorders, such as autism, Alzheimer's disease and schizophrenia. Therefore, it is crucial to understand the functions of CAMs during development and in the mature neural system, as well as in the pathogenesis of some neurological disorders. Here, we review the function of the major classes of CAMs, and how dysfunction of CAMs relates to several neurological disorders.

Activity-dependent adaptations in inhibitory axons

Synaptic connections in our brains change continuously and throughout our lifetime. Despite ongoing synaptic changes, a healthy balance between excitation and inhibition is maintained by various forms of homeostatic and activity-dependent adaptations, ensuring stable functioning of neuronal networks. In this review we summarize experimental evidence for activity-dependent changes occurring in inhibitory axons, in cultures as well as in vivo. Axons form many presynaptic terminals, which are dynamic structures sharing presynaptic material along the axonal shaft. We discuss how internal (e.g., vesicle sharing) and external factors (e.g., binding of cell adhesion molecules or secreted factors) may affect the formation and plasticity of inhibitory synapses.

Cell adhesion and homeostatic synaptic plasticity

At synapses, pre- and post-synaptic cells get in direct contact with each other via cell adhesion molecules (CAMs). Several CAMs have been identified at the neuromuscular junction and at central synapses, where they regulate synaptic strength, by recruiting scaffolding proteins, neurotransmitter receptors and synaptic vesicles in response to the binding of counter-receptors across the synaptic cleft. Many synapses are also surrounded by astrocytic processes and embedded in conspicuous extracellular matrix (ECM). It is now widely recognized that astrocytes play a central role in regulating the synaptic machinery by exchanging information with the neuronal elements via diffusible molecules and direct physical interactions; this has lead to the concept of the 'tri-partite synapse'. More recently, the term 'tetra-partite synapse' has been introduced to underlie the importance of ECM in shaping synaptic function by mediating interaction and signaling between neurons and astrocytes. Here, we will review how this integrated view of the synapse can help us understand homeostatic synaptic plasticity at the neuromuscular junction and in the central nervous system. We will explore how synaptic CAMs regulate two forms of homeostatic plasticity: (i) postsynaptic scaling of synaptic currents to counteract changes in neuronal network activity and (ii) the compensatory modulation of presynaptic neurotransmitter release in response to changes in postsynaptic efficacy. We will discuss recent findings on activity-dependent trans-synaptic signaling events and the role of cell adhesion in the feedback control of network activity. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Semaphorin7A and its receptors: pleiotropic regulators of immune cell function, bone homeostasis, and neural development

Semaphorins form a large, evolutionary conserved family of cellular guidance signals. The semaphorin family contains several secreted and transmembrane proteins, but only one GPI-anchored member, Semaphorin7A (Sema7A). Although originally identified in immune cells, as CDw108, Sema7A displays widespread expression outside the immune system. It is therefore not surprising that accumulating evidence supports roles for this protein in a wide variety of biological processes in different organ systems and in disease. Well-characterized biological effects of Sema7A include those during bone and immune cell regulation, neuron migration and neurite growth. These effects are mediated by two receptors, plexinC1 and integrins. However, most of what is known today about Sema7A signaling concerns Sema7A-integrin interactions. Here, we review our current knowledge of Sema7A function and signaling in different organ systems, highlighting commonalities between the cellular effects and signaling pathways activated by Sema7A in different cell types. Furthermore, we discuss a potential role for Sema7A in disease and provide directions for further research.

Estrogen promotes learning-related plasticity by modifying the synaptic cytoskeleton

Estrogen's acute, facilitatory effects on glutamatergic transmission and long-term potentiation (LTP) provide a potential explanation for the steroid's considerable influence on behavior. Recent work has identified mechanisms underlying these synaptic actions. Brief infusion of 17ß-estradiol (E2) into adult male rat hippocampal slices triggers actin polymerization within dendritic spines via a signaling cascade beginning with the GTPase RhoA and ending with inactivation of the filament-severing protein cofilin. Blocking this sequence, or actin polymerization itself, eliminates E2's effects on synaptic physiology. Notably, the theta burst stimulation used to induce LTP activates the same signaling pathway as E2 plus events that stabilize the reorganization of the sub-synaptic cytoskeleton. These observations suggest that E2 elicits a partial form of LTP, resulting in an increase of fast excitatory postsynaptic potentials (EPSPs) and a reduction in the threshold for lasting synaptic changes. While E2's effects on the cytoskeleton could be direct, results described here indicate that the hormone activates synaptic tropomyosin-related kinase B (TrkB) receptors for brain-derived neurotrophic factor (BDNF), a releasable neurotrophin that stimulates the RhoA to cofilin pathway. It is therefore possible that E2 acts via transactivation of neighboring receptors to modify the composition and structure of excitatory contacts. Finally, there is the question of whether a loss of acute synaptic actions contributes to the memory problems associated with estrogen depletion. Initial tests found that ovariectomy in middle-aged rats disrupts RhoA signaling, actin polymerization, and LTP consolidation. Acute applications of E2 reversed these defects, a result consistent with the idea that disturbances to actin management are one cause of behavioral effects that emerge with reductions in steroid levels.

Interactions between ICAM-5 and β1 integrins regulate neuronal synapse formation

Intercellular adhesion molecule-5 (ICAM-5) is a dendrite-specific adhesion molecule, which functions in both the immune and nervous systems. ICAM-5 is the only negative regulator that has been identified for maturation of dendritic spines so far. Shedding of the ICAM-5 ectodomain promotes spine maturation and enhances synaptic activity. However, the mechanism by which ICAM-5 regulates spine development remains poorly understood. In this study, we found that ablation of ICAM5 expression resulted in a significant increase in the formation of synaptic contacts and the frequency of miniature excitatory post-synaptic currents, an indicator of pre-synaptic release probability. Antibodies against ICAM-5 and β1 integrins altered spine maturation. Furthermore, we found that β1 integrins serve as binding partners for ICAM-5. β1 integrins were immunoprecipitated with ICAM-5 from mouse brain and the binding region in ICAM-5 was localized to the two first Ig domains. β1 integrins were juxtaposed to filopodia tips at the early stage of synaptic formation, but as synapses matured, β1 integrins covered the mushroom spines. Loss of β1 integrins from the pre-synaptic sites affected the morphology of the post-synaptic structures. ICAM-5 ectodomain cleavage decreased or increased when the interaction between ICAM-5 and β1 integrins was potentiated or weakened, respectively, using antibodies. These results suggest that the interaction between ICAM-5 and β1 integrins is important in formation of functional synapses.

Dendritic spine pathology in epilepsy: cause or consequence?

Abnormalities in dendritic spines have commonly been observed in brain specimens from epilepsy patients and animal models of epilepsy. However, the functional implications and clinical consequences of this dendritic pathology for epilepsy are uncertain. Dendritic spine abnormalities may promote hyperexcitable circuits and seizures in some types of epilepsy, especially in specific genetic syndromes with documented dendritic pathology, but in these cases it is difficult to differentiate their effects on seizures versus other comorbidities, such as cognitive deficits and autism. In other situations, seizures themselves may cause damage to dendrites and dendritic spines and this seizure-induced brain injury may then contribute to progressive epileptogenesis, memory problems and other neurological deficits in epilepsy patients. The mechanistic basis of dendritic spine abnormalities in epilepsy has begun to be elucidated and suggests novel therapeutic strategies for treating epilepsy and its complications.

Integrins as receptor targets for neurological disorders

This review focuses on the neurobiology of integrins, pathophysiological roles of integrins in neuroplasticity and nervous system disorders, and therapeutic implications of integrins as potential drug targets and possible delivery pathways. Neuroplasticity is a central phenomenon in many neurological conditions such as seizures, trauma, and traumatic brain injury. During the course of many brain diseases, in addition to intracellular compartment changes, alterations in non-cell compartments such as extracellular matrix (ECM) are recognized as an essential process in forming and reorganizing neural connections. Integrins are heterodimeric transmembrane receptors that mediate cell-ECM and cell-cell adhesion events. Although the mechanisms of neuroplasticity remain unclear, it has been suggested that integrins undergo plasticity including clustering through interactions with ECM proteins, modulating ion channels, intracellular Ca(2+) and protein kinase signaling, and reorganization of cytoskeletal filaments. As cell surface receptors, integrins are central to the pathophysiology of many brain diseases, such as epilepsy, and are potential targets for the development of new drugs for neurological disorders.

Stabilising influence: integrins in regulation of synaptic plasticity

Hebbian synaptic plasticity, such as hippocampal long-term potentiation (LTP), is thought to be important for particular types of learning and memory. It involves changes in the expression and activity of a large array of proteins, including cell adhesion molecules. The integrin class of cell adhesion molecules has been extensively studied in this respect, and appear to have a defined role in consolidating both structural and functional changes brought about by LTP. With the use of integrin inhibitors, it has been possible to identify a critical time window of several minutes after LTP induction for the participation of integrins in LTP. Altering the interactions of integrins with their ligands during this time compromises structural changes involving actin polymerisation and spine enlargement that could be required for accommodating new AMPA receptors (AMPARs). After this critical window of structural remodelling and plasticity, integrins "lock-in" and stabilise the morphological changes, conferring the requisite longevity for LTP. Genetic manipulations targeting integrin subtypes have helped identify the specific integrin subunits involved in LTP and correlate alterations in plasticity with behavioural deficits. Moreover, recent studies have implicated integrins in AMPAR trafficking and glycine receptor lateral diffusion, highlighting their multifaceted functions at the synapse.

Identification of a β1 integrin isoform with restricted tissue expression in a teleost fish

The composition and organisation of extracellular matrix (ECM)-related molecules change during development. These components interact with different cell surface receptors to modulate the transduction of signals for cell growth, differentiation, migration, proliferation and apoptosis. Previous findings in the teleost fish gilthead seabream (Sparus aurata L., Teleostei), a marine protandrous hermaphrodite fish, showed that endocrine and immune stimuli are able to modulate the expression of ECM-related molecules, as well as specific correlations between them. In the present study, quantitative reverse transcription–polymerase chain reaction was used to examine the gene expression profile of β1 integrin isoform b (ITGB1b) and its possible role in reproductive physiology, especially in relation to spermatogenesis. Expression profiles were analysed in the context of the reproductive cycle (RC) and in relation with other ECM-related molecules, including matrix metalloproteinase (MMP)-2, MMP-9, MMP-13, tissue-specific inhibitor of metalloproteinase (TIMP)-2a, TIMP-2b, collagen (COL1A1) and ITGB1a. Expression of ITGB1b was found in the testis and brain and, to some extent, in endothelial cells. In contrast, ITGB1a was expressed ubiquitously. In the testis, the ITGB1b expression peaked during spermatogenesis, whereas the expression of the other ECM-related molecules is induced mainly during the post-spawning stage, both stages of marked tissue remodelling during the first and second RC in males. In addition, in fish exposed to the endocrine disruptor 17α-ethynyloestradiol (at 5 and 50 μg g–1 food during 7, 14 and 21 days), ITGB1b expression in the testis was inhibited in a dose- and time-dependent manner and was related to reduced serum levels of testosterone. Together, these results suggest a different functionality for the two ITGB1 isoforms in the gilthead seabream, where ITGB1b is more specifically involved in reproduction. This is the first report of an ITGB1 gene isoform whose expression is restricted to endocrine-related tissues in vertebrates.

Talin 2 concentrations in cerebrospinal fluid in patients with epilepsy

OBJECTIVES

Talin 2 plays an important role in cell adhesion and recycling of synaptic vesicles. To explore the possible role of talin 2 in epilepsy, we designed current study to quantitatively evaluate the changes in talin 2 levels in different epilepsy patients.

DESIGN AND METHODS

We measured talin 2 levels in CSF and the serum of postencephalitic epilepsy (PEE) patients (subdivided into drug-refractory PEE-RPEE, and drug-effective PEE-EPEE groups) and acute encephalitis patients (subdivided into acute encephalitis with secondary seizures-AESS, and acute encephalitis without seizures-AE) by enzyme-linked immunosorbent assay.

RESULTS

We found that, in RPEE patients, interictal CSF-talin 2 concentrations significantly increased, whereas serum-talin 2 significantly decreased when compared with the AE, AESS, and EPEE groups.

CONCLUSIONS

This result suggested that the differential concentration of CSF and serum-talin 2 in the RPEE group is an intractability-related phenomenon that might be involved in the development of RPEE.

New insights into the regulation of ion channels by integrins

By controlling cell adhesion to the extracellular matrix, integrin receptors regulate processes as diverse as cell migration, proliferation, differentiation, apoptosis, and synaptic stability. Because the underlying mechanisms are generally accompanied by changes in transmembrane ion flow, a complex interplay occurs between integrins, ion channels, and other membrane transporters. This reciprocal interaction regulates bidirectional signal transduction across the cell surface and may take place at all levels of control, from transcription to direct conformational coupling. In particular, it is becoming increasingly clear that integrin receptors form macromolecular complexes with ion channels. Besides contributing to the membrane localization of the channel protein, the integrin/channel complex can regulate a variety of downstream signaling pathways, centered on regulatory proteins like tyrosine kinases and small GTPases. In turn, the channel protein usually controls integrin activation and expression. We review some recent advances in the field, with special emphasis on hematology and neuroscience. Some oncological implications are also discussed.

Pivotal role for beta-1 integrin in neurovascular remodelling after ischemic stroke

beta1 integrin is a cell surface molecule that is critical for endothelial cell adhesion, migration and survival during angiogenesis. In the present study we employed in vivo and in vitro models to elucidate the role of beta1 integrin in vascular remodelling and stroke outcomes. At 24 h after cerebral ischemia and reperfusion (I/R), the ischemic cortex (ipsilateral area) exhibited modest beta1 integrin immunoreactivity and a robust increase was observed at 72 h. Double-label immunohistochemical analysis for beta1 integrin with neuronal (NeuN), microglial (Iba-1), astrocyte (GFAP), progenitor cell (Ng2) and blood vessel (collagen 4) markers showed that beta1 integrin expression only localized to blood vessels. In vitro studies using cultured endothelial cells and a beta1 integrin blocking antibody confirmed that beta1 integrin is required for endothelial cell migration, proliferation and blood vessel formation. In vivo studies in the cerebral I/R model using the beta1 integrin blocking antibody further confirmed that beta1 integrin signaling is involved in vascular formation and recovery following ischemic stroke. Finally, we found that beta1 integrin is critically involved in functional deficits and survival after a stroke. These results suggest that beta1 integrin plays important roles in neurovascular remodelling and functional outcomes following stroke, and that targeting the beta1 integrin signalling may provide a novel strategy for modulating angiogenesis in ischemic stroke and other pathological conditions.

Pharmacogenetic analysis reveals a post-developmental role for Rac GTPases in Caenorhabditis elegans GABAergic neurotransmission

The nerve-cell cytoskeleton is essential for the regulation of intrinsic neuronal activity. For example, neuronal migration defects are associated with microtubule regulators, such as LIS1 and dynein, as well as with actin regulators, including Rac GTPases and integrins, and have been thought to underlie epileptic seizures in patients with cortical malformations. However, it is plausible that post-developmental functions of specific cytoskeletal regulators contribute to the more transient nature of aberrant neuronal activity and could be masked by developmental anomalies. Accordingly, our previous results have illuminated functional roles, distinct from developmental contributions, for Caenorhabditis elegans orthologs of LIS1 and dynein in GABAergic synaptic vesicle transport. Here, we report that C. elegans with function-altering mutations in canonical Rac GTPase-signaling-pathway members demonstrated a robust behavioral response to a GABA(A) receptor antagonist, pentylenetetrazole. Rac mutants also exhibited hypersensitivity to an acetylcholinesterase inhibitor, aldicarb, uncovering deficiencies in inhibitory neurotransmission. RNA interference targeting Rac hypomorphs revealed synergistic interactions between the dynein motor complex and some, but not all, members of Rac-signaling pathways. These genetic interactions are consistent with putative Rac-dependent regulation of actin and microtubule networks and suggest that some cytoskeletal regulators cooperate to uniquely govern neuronal synchrony through dynein-mediated GABAergic vesicle transport in C. elegans.

Differential involvement of beta3 integrin in pre- and postsynaptic forms of adaptation to chronic activity deprivation

Neuronal networks can adapt to global changes in activity levels through compensatory modifications in pre- and postsynaptic parameters of synaptic transmission. These forms of synaptic plasticity are known as synaptic homeostasis, and are thought to require specific cellular interactions and signaling across the entire neuronal network. However, the molecular mechanisms underlying synaptic homeostasis have so far been investigated mostly in primary cultures of dissociated neurons, a preparation that lacks the specificity of in vivo circuitry. Here, we show that there are critical differences in the properties of synaptic homeostasis between dissociated neuronal cultures and organotypic slices, a preparation that preserves more precisely in vivo connectivity. Moreover, the cell adhesion molecule beta3 integrin, which regulates excitatory synaptic strength, is specifically required for a postsynaptic form of synaptic homeostasis called synaptic scaling in both dissociated cultures and organotypic slices. Conversely, another form of synaptic homeostasis that involves changes in presynaptic quantal content occurs independently of beta3 integrin. Our findings define the differential involvement of beta3 integrin in two forms of synaptic homeostasis.

Neurogenesis in the human hippocampus and its relevance to temporal lobe epilepsies

Ample evidence points to the dentate gyrus as anatomical region for persistent neurogenesis in the adult mammalian brain. This has been confirmed in a variety of animal models under physiological as well as pathophysiological conditions. Notwithstanding, similar experiments are difficult to perform in humans. Postmortem studies demonstrated persisting neurogenesis in the elderly human brain. In addition, neural precursor cells can be isolated from surgical specimens obtained from patients with intractable temporal lobe epilepsy (TLE) and propagated or differentiated into neuronal and glial lineages. It remains a controversial issue, whether epileptic seizures have an effect on or even increase hippocampal neurogenesis in humans. Recent data support the notion that seizures induce neurogenesis in young patients, whereas the capacity of neuronal recruitment and proliferation decreases with age. Animal models of TLE further indicate that these newly generated neurons integrate into epileptogenic networks and contribute to increased seizure susceptibility. However, pathomorphological disturbances within the epileptic hippocampus, such as granule cell dispersion (GCD), may not directly result from compromised neurogenesis. Still, the majority of adult TLE patients present with significant dentate granule cell loss at an end stage of the disease, which relates to severe memory and learning disabilities. In conclusion, surgical specimens obtained from TLE patients represent an important tool to study mechanisms of stem cell recruitment, proliferation and differentiation in the human brain. In addition, increasing availability of surgical specimens opens new avenues to systematically explore disease pathomechanisms in chronic epilepsies.

Integrin regulation of cytoplasmic calcium in excitatory neurons depends upon glutamate receptors and release from intracellular stores

Integrins regulate cytoplasmic calcium levels ([Ca(2+)]i) in various cell types but information on activities in neurons is limited. The issue is of current interest because of the evidence that both integrins and changes in [Ca(2+)]i are required for Long-Term Potentiation. Accordingly, the present studies evaluated integrin ligand effects in cortical neurons. Integrin ligands or alpha5beta1 integrin activating antisera rapidly increased [Ca(2+)]i with effects greater in glutamatergic than GABAergic neurons, absent in astroglia, and blocked by beta1 integrin neutralizing antisera and the tyrosine kinase antagonist genistein. Increases depended upon extracellular calcium and intracellular store release. Ligand-induced effects were reduced by voltage-sensitive calcium channel and NMDA receptor antagonists, but blocked by tetrodotoxin or AMPA receptor antagonists. These results indicate that integrin ligation triggers AMPA receptor/depolarization-dependent calcium influx followed by intracellular store release and suggest the possibility that integrin modulation of activity-induced changes in [Ca(2+)]i contributes importantly to lasting synaptic plasticity in forebrain neurons.

Alpha3-integrins are required for hippocampal long-term potentiation and working memory

Integrins comprise a large family of heterodimeric, transmembrane cell adhesion receptors that mediate diverse neuronal functions in the developing and adult CNS. Recent pharmacological and genetic studies have suggested that beta1-integrins are critical in synaptic plasticity and memory formation. To further define the role of integrins in these processes, we generated a postnatal forebrain and excitatory neuron-specific knockout of alpha3-integrin, one of several binding partners for beta1 subunit. At hippocampal Schaffer collateral-CA1 synapses, deletion of alpha3-integrin resulted in impaired long-term potentiation (LTP). Basal synaptic transmission and paired-pulse facilitation were normal in the absence of alpha3-integrin. Behavioral studies demonstrated that the mutant mice were selectively defective in a hippocampus-dependent, nonmatch-to-place working memory task, but were normal in other hippocampus-dependent spatial tasks. The impairment in LTP and working memory is similar to that observed in beta1-integrin conditional knockout mice, suggesting that alpha3-integrin is the functional binding partner for beta1 for these processes in the forebrain.

Expression patterns of semaphorin7A and plexinC1 during rat neural development suggest roles in axon guidance and neuronal migration

Background

Although originally identified as embryonic axon guidance cues, semaphorins are now known to regulate multiple, distinct, processes crucial for neuronal network formation including axon growth and branching, dendritic morphology, and neuronal migration. Semaphorin7A (Sema7A), the only glycosylphosphatidylinositol-anchored semaphorin, promotes axon growth in vitro and is required for the proper growth of the mouse lateral olfactory tract in vivo. Sema7A has been postulated to signal through two unrelated receptors, an RGD-dependent α1β1-integrin and a member of the plexin family, plexinC1. β1-integrins underlie Sema7A-mediated axon growth and Sema7A function in the immune system. Sema7A-plexinC1 interactions have also been implicated in immune system function, but the neuronal role of this ligand-receptor pair remains to be explored. To gain further insight into the function(s) of Sema7A and plexinC1 during neural development, we present here a detailed analysis of Sema7A and plexinC1 expression in the developing rat nervous system.

Results

In situ hybridization revealed select expression of Sema7A and plexinC1 in multiple neuronal systems including: the olfactory system, the hypothalamo-hypophysial system, the hippocampus, the meso-diencephalic dopamine system, and the spinal cord. Within these systems, Sema7A and plexinC1 are often expressed in specific neuronal subsets. In general, Sema7A transcript levels increase significantly towards adulthood, whereas plexinC1 expression decreases as development proceeds.

PlexinC1, but not Sema7A, is strongly expressed by distinct populations of migrating neurons. In addition to neuronal expression, Sema7A and plexinC1 transcripts were detected in oligodendrocytes and ependymal cells, respectively.

Conclusion

Sema7A and plexinC1 expression patterns are consistent with these proteins serving both cooperative and separate functions during neural development. The prominent expression of plexinC1 in several distinct populations of migrating neurons suggests a novel role for this plexin family member in neuronal migration.

Studies on integrins in the nervous system

Integrins are of interest to neuroscientists because they and many of their ligands are widely expressed in the nervous system and have been shown to have diverse roles in neural development and function (Clegg et al., 2003; Li and Pleasure, 2005; Pinkstaff et al., 1998, 1999; Reichardt and Tomaselli, 1991; Schmid et al., 2005). Integrins have also been implicated in control of pathogenesis in several neurodegenerative diseases, brain tumor pathogenesis, and the aftermath of brain and peripheral nervous system injury (Condic, 2001; Ekstrom et al., 2003; Kloss et al., 1999; Verdier and Penke, 2004; Wallquist et al., 2004). Using integrin antagonists as therapeutic agents in a variety of neurological diseases is of great interest at present (Blackmore and Letourneau, 2006; Mattern et al., 2005; Polman et al., 2006; Wang et al., 2006). In this chapter, we describe methods used in our laboratory to characterize neuronal responses to extracellular matrix proteins, and procedures for assessing integrin roles in neuronal cell attachment and differentiation.

Distinct roles of the beta 1-class integrins at the developing and the mature hippocampal excitatory synapse

Integrins are a large family of cell adhesion receptors involved in a variety of cellular functions. To study their roles at central synapses, we used two cre recombinase lines to delete the Itgb1 beta1 integrin gene in forebrain excitatory neurons at different developmental stages. Removal of the beta1 integrins at an embryonic stage resulted in severe cortical lamination defects without affecting the cellular organization of pyramidal neurons in the CA3 and CA1 regions of the hippocampus. Whereas the hippocampal neurons underwent normal dendritic and synaptic differentiation, the adult synapses exhibited deficits in responses to high-frequency stimulation (HFS), as well as in long-term potentiation (LTP). Deletion of beta1 integrin at a later postnatal stage also impaired LTP but not synaptic responses to HFS. Thus, the beta1-class integrins appear to play distinct roles at different stages of synaptic development, critical for the proper maturation of readily releasable pool of vesicles during early development but essential for LTP throughout adult life.

Beta 1-integrins are required for hippocampal AMPA receptor-dependent synaptic transmission, synaptic plasticity, and working memory

Integrins comprise a large family of cell adhesion receptors that mediate diverse biological events through cell-cell and cell-extracellular matrix interactions. Recent studies have shown that several integrins are localized to synapses with suggested roles in synaptic plasticity and memory formation. We generated a postnatal forebrain and excitatory neuron-specific knock-out of beta1-integrin in the mouse. Electrophysiological studies demonstrated that these mutants have impaired synaptic transmission through AMPA receptors and diminished NMDA receptor-dependent long-term potentiation. Despite the impairment in hippocampal synaptic transmission, the mutants displayed normal hippocampal-dependent spatial and contextual memory but were impaired in a hippocampal-dependent, nonmatching-to-place working memory task. These phenotypes parallel those observed in animals carrying knock-outs of the GluR1 (glutamate receptor subunit 1) subunit of the AMPA receptor. These observations suggest a new function of beta1-integrins as regulators of synaptic glutamate receptor function and working memory.

Integrin α3β1 suppresses long-term potentiation at inhibitory synapses on the cerebellar Purkinje neuron

At the GABAergic synapses between inhibitory interneurons and a Purkinje neuron (PN) in the cerebellum, the postsynaptic depolarization induces the long-term potentiation (called rebound potentiation; RP) of GABAA receptor responsiveness. Here, we show that integrins, a type of cell-adhesion molecules, are involved in the regulation of RP. Integrin activation by Mn2+ impaired the RP induction of GABA responsiveness and mIPSCs in PNs, which was abolished by the function blocking antibody against either integrin alpha3 or beta1 subunit, but not by that against alpha5 or alphaV subunit. Furthermore, overexpression of integrin alpha3 subunit in a PN by itself impaired the RP induction. We also show that Src-family of protein tyrosine kinases mediated the suppressive effect of integrin activity on the RP induction. Thus, the integrin/Src pathway negatively regulates the induction of long-term plasticity at inhibitory synapses on a cerebellar PN.

Integrin-mediated dendrite branch maintenance requires Abelson (Abl) family kinases

Dendrite arbor structure is a critical determinant of nervous system function that must be actively maintained throughout life, but the signaling pathways that regulate dendrite maintenance are essentially unknown. We report that the Abelson (Abl) and Abl-related gene (Arg) nonreceptor tyrosine kinases are required for maintenance of cortical dendrites in the mouse brain. arg-/- cortical dendrites initially develop normally and are indistinguishable from wild-type dendrites at postnatal day 21. Dendrite branches are not efficiently maintained in arg-/- neurons, leading to a reduction in dendrite arbor size by early adulthood. More severe dendrite loss is observed in abl-/-arg-/- neurons. Elevation of Arg kinase activity in primary cortical neurons promotes axon and dendrite branching. Activation of integrin receptors by adhesion to laminin-1 or Semaphorin 7A also promotes neurite branching in cortical neurons, but this response is absent in arg-/- neurons because of the reduced dynamic behavior of mutant neurite branches. These data suggest that integrin signaling through Abl and Arg support cortical dendrite branch maintenance by promoting dendrite branch dynamics in response to adhesive cues.

Activation of integrin alpha5beta1 delays apoptosis of Ntera2 neuronal cells

Integrins are dynamic membrane proteins that mediate adhesion of cells to the extracellular matrix. Integrins initiate signal transduction, alone and cooperatively with growth factor receptors, and regulate many aspects of cell behavior. We report here that alpha5beta1-mediated adhesion of Ntera2 neuronal cells to fibronectin decreased apoptosis in response to serum withdrawal. Adhesion induced phosphorylation of FAK, and strongly increased the AKT phosphorylation induced by growth factors, demonstrating for the first time in neuronal cells that integrin-mediated adhesion and growth factors cooperate to regulate AKT activity. Integrins exist on cells in different activation states, and cell survival on fibronectin was enhanced by the antibody 12G10, that modulates the conformation of beta1 in favor of its active form. The antibody 12G10 specifically delayed loss of phosphorylation of AKT on serine 473, and GSK-3beta on serine 9, induced by serum withdrawal, suggesting that these kinases are critical sensors of integrin activation on neuronal cells.

Integrins are involved in synaptogenesis, cell spreading, and adhesion in the postnatal brain

Integrins are a major family of heterodimeric surface glycoproteins that act as adhesion molecules, have a spectrum of extracellular matrix (ECM) molecules as their ligands, and regulate a variety of cellular functions. Integrins are known to be critical to embryonic brain development, and recent studies have indicated their essential role in adult brain function, although their role in postnatal brain development and function has not been examined. Here, we used the organotypic slice culture system to investigate the role of integrins in postnatal hippocampal development by exposing the tissue to either an integrin competitive antagonist, the peptide GRGDSP containing Arg-Gly-Asp (RGD) attachment site, or to function-blocking beta(1)-integrin antibodies to disrupt integrin interactions. These experiments revealed that beta(1)-integrin antibodies interfered with spreading of the culture, resulting in a rapid and marked diminution of slice area. beta(1)-integrin antibodies and RGD peptide disrupted cell adhesion, causing cell detachment and migration of glial cells from the explant. The majority of the detached cells were of macroglial origin and switched to expression of the intermediate filament proteins vimentin and nestin, suggesting a developmental regression. The organotypic organization of slice cultures was not affected, although exposure to either integrin antagonist or antibody resulted in a statistically significant reduction in the number of synapses measured in the apical dendrites of CA1 pyramidal neurons. The results demonstrate that integrins markedly affect postnatal CNS development, in both ultrastructural construction and organizational processes.

Reelin, Disabled 1, and beta 1 integrins are required for the formation of the radial glial scaffold in the hippocampus

The extracellular matrix molecule Reelin is required for the correct positioning of neurons during the development of the forebrain. However, the mechanism of Reelin action on neuronal migration is poorly understood. Reelin is assumed to act on neurons directly, but it may also affect the differentiation of glial cells necessary for neuronal migration. Here we show that a regular glial scaffold fails to form in vivo in the dentate gyrus of mice deficient of Reelin or Disabled 1, a neuronal adaptor protein in the Reelin signaling pathway. A subset of these defects is observed in mice that lack beta(1)-class integrins, known to bind Reelin. Moreover, recombinant Reelin induced branching of glial processes in vitro. Our data suggest that Reelin affects glial differentiation via Disabled 1 and beta(1)-class integrin-dependent signaling pathways.

Uptake and pathogenic effects of amyloid beta peptide 1-42 are enhanced by integrin antagonists and blocked by NMDA receptor antagonists

Many synapses contain two types of receptors - integrins and N-methyl-D-aspartate (NMDA) receptors - that have been implicated in peptide internalization. The present studies tested if either class is involved in the uptake of the 42-residue form of amyloid beta peptide (Abeta1-42), an event hypothesized to be of importance in the development of Alzheimer's disease. Cultured hippocampal slices were exposed to Abeta1-42 for 6 days in the presence or absence of soluble Gly-Arg-Gly-Asp-Ser-Pro, a peptide antagonist of Arg-Gly-Asp (RGD)-binding integrins, or the disintegrin echistatin. Abeta uptake, as assessed with immunocytochemistry, occurred in 42% of the slices incubated with Abeta peptide alone but in more than 80% of the slices co-treated with integrin antagonists. Uptake was also found in a broader range of hippocampal subfields in RGD-treated slices. Increased sequestration was accompanied by two characteristics of early stage Alzheimer's disease: elevated concentrations of cathepsin D immunoreactivity and activation of microglia. The selective NMDA receptor antagonist D-(-)-2-amino-5-phosphonovalerate completely blocked internalization of Abeta, up-regulation of cathepsin D, and activation of microglia. Our results identify two classes of receptors that cooperatively regulate the internalization of Abeta1-42 and support the hypothesis that characteristic pathologies of Alzheimer's disease occur once critical intraneuronal Abeta concentrations are reached.

Neural (N-) cadherin, a synaptic adhesion molecule, is induced in hippocampal mossy fiber axonal sprouts by seizure

Aberrant mossy fiber sprouting and synaptic reorganization are plastic responses in human temporal lobe epilepsy, and in pilocarpine-induced epilepsy in rodents. Although the morphological features of the hippocampal epileptic reaction have been well documented, the molecular mechanisms underlying these structural changes are not understood. The classic cadherins, calcium-dependent cell adhesion molecules, are known to function in development in neurite outgrowth, synapse formation, and stabilization. In pilocarpine-induced status epilepticus, the expression of N-cadherin mRNA was sharply upregulated and reached a maximum level (1- to 2.5-fold) at 1- to 4 weeks postseizure in the granule cell layer and the pyramidal cell layer of CA3. N-cadherin protein was correspondingly increased and became concentrated in the inner molecular layer of the dentate gyrus, consistent with the position of mossy fiber axonal sprouts. Moreover, N-cadherin labeling was punctate; colocalized with definitive synaptic markers, and partially localized on polysialated forms of neural cell adhesion molecule (PSA-NCAM)-positive dendrites of granule cells in the inner molecular layer. Our findings show that N-cadherin is likely to be a key factor in responsive synaptogenesis following status epilepticus, where it functions as a mediator of de novo synapse formation.

Alpha3 integrin receptors contribute to the consolidation of long-term potentiation

Several lines of evidence suggest that integrin receptors play a pivotal role in consolidation of long-term potentiation (LTP), but which of the many integrin dimers are involved remains to be discovered. The present study used an LTP reversal paradigm to test if alpha3 integrins make an important contribution. Function blocking alpha3 monoclonal antibodies or vehicle were locally infused into recording sites in field CA1 of rat hippocampal slices and LTP induced with theta burst stimulation. Low frequency trains of pulses were applied 30 min after the theta bursts. Previous work indicates that low frequency stimulation reverses LTP when applied immediately after induction but is largely ineffective after 30-45-min delays. If the antibodies were to block consolidation, then they should extend the period over which potentiation is vulnerable to disruption. There was no detectable difference between the two groups in the initial degree of LTP or within slice decay of potentiation 1-10 min after induction; a small but reliable decay occurred from 10 to 30 min with antibody treatment (P<0.01) but not in control slices. Percent potentiation was not statistically different for vehicle (55 +/- 19%, mean +/- S.D.) and anti-alpha3 (43 +/- 21%) slices at 30 min post-theta bursts. Five-Hz stimulation ("theta pulse" stimulation) 30 min after induction caused a reduction of LTP. The percent loss of potentiation after the 1-min trains was greater in the antibody-treated slices than in controls (98 +/- 4% vs. 62 +/- 28%, P<0.01, U-test) and correlated (r=0.84, alpha3 slices) with the percent LTP present prior to low frequency stimulation, as expected if the stimulation reversed potentiation. Recovery occurred in both groups but percent LTP was significantly smaller in experimental slices at 10 min post-theta pulses (5 +/- 11% vs. 36 +/- 15%, P<0.01). Recovery continued for 20 min after theta pulses and, in accordance with earlier work, was nearly complete for the control slices (50 +/- 19% vs 55 +/- 15%, 40 min post- vs. immediately pre-theta pulses). LTP remained depressed after 40 min of recovery in the anti-alpha3 slices (23 +/- 19% vs. 43 +/- 21%) at which point it was substantially less than that found in controls (P<0.01). Western blots with anti-alpha3 antibodies identified a polypeptide with the molecular mass (155 kDa) expected for the alpha3 subunit and further showed that it is broadly distributed in brain. Subcellular fractionation experiments demonstrated that alpha3 is concentrated in synaptic membranes over homogenates to about the same degree as the GluR1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor. From these results we suggest that alpha3-containing integrins are localized to synapses and are needed to stabilize a slowly decaying form of LTP. The findings also show that vulnerability to reversal can be used in place of extended recording sessions in studying consolidation.

Rapid neuromodulatory actions of integrin ligands

Extracellular matrix (ECM) proteins and their receptors, the integrins, actively participate in the control of many fundamental cellular functions in the developing nervous system, including the regulation of cell migration, differentiation, and survival and the control of neurite outgrowth. ECM-integrin interactions in the mature nervous system are commonly considered to be more static in nature and of little importance in the regulation of neuronal function. In contrast, we demonstrate that integrins and their ligands are capable of rapid neuromodulatory actions. Specifically, we show that integrin ligands can alter neuronal pacemaker properties, intracellular free Ca2+ levels, and voltage-gated Ca2+ currents in a matter of minutes. These findings indicate that ECM-integrin interactions play a dynamic role in regulating the physiological status of mature neurons, a process that may contribute to synaptic plasticity, neural regeneration, and neuropathology.

Dentate hilar mossy cells and somatostatin-containing neurons are immunoreactive for the alpha8 integrin subunit: characterization in normal and kainic acid-treated rats

Integrins are heterodimeric cell surface receptors composed of different alpha and beta subunits that mediate cell-cell and cell-extracellular matrix interactions. They have been implicated in the regulation of neuronal migration, differentiation, process outgrowth, and plasticity. The alpha8 integrin subunit associates exclusively with the beta1 subunit to form a receptor (alpha8beta1) for fibronectin, vitronectin, tenascin, and osteopontin. In a previous study, we demonstrated that hippocampal dentate hilar neurons are immunoreactive for alpha8. The present study identifies the major types of alpha8-immunoreactive hilar neurons and characterizes the effects of kainic acid-induced seizures on alpha8-immunoreactivity in these cells. Examination of the hilus in normal rats revealed alpha8-immunoreactivity in the somatodendritic compartments of large hilar neurons identified as mossy cells, including a subset of dendritic thorny excrescences that were contacted by large mossy fiber terminals. alpha8-immunoreactivity also was found in approximately 71% of somatostatin-containing hilar cells. Kainic acid-induced seizures dramatically and rapidly altered the levels and distribution of alpha8-immunoreactivity in hilar neurons. After 1.5 h of seizures, alpha8-immunoreactivity in their dendrites was reduced greatly. One day after kainic acid treatment, labeling was diminished throughout the somatodendritic compartments of most hilar cells. This decrease appeared to be transient, since alpha8 labeling returned to normal levels in surviving hilar neurons within 2 weeks of treatment. In addition, many alpha8-immunoreactive hilar neurons, particularly in caudal dentate regions, were lost 3-5 weeks after kainic acid treatment. Our findings suggest that alpha8beta1 may mediate adhesive interactions of the dendritic processes of mossy cells and somatostatin-containing hilar neurons with other cellular elements or with extracellular matrix components. They also suggest that alpha8 may be susceptible to activity-dependent proteolysis that could modulate its function in the somatodendritic compartment of these cells.

Polarized distribution of alpha5 integrin in dendrites of hippocampal and cortical neurons

The distribution of immunoreactivity for the alpha5 subunit of the fibronectin receptor was evaluated in adult rat brain with particular interest in the cellular localization of immunostaining in the hippocampal formation and neocortex. Beyond localization to neuronal perikarya and short dendritic fragments within most brain areas, alpha5 immunoreactivity (-ir) was particularly dense within primary apical dendrites of pyramidal cells in both hippocampus and neocortex and within the dendritic arbors of cerebellar Purkinje cells. In hippocampal and cortical pyramidal cells, immunostaining was clearly polarized: alpha5-ir was not detectable in basal dendrites in hippocampal neurons and was limited to proximal arbors or absent from basal dendrites in pyramidal cells in superficial and deep layers of neocortex. Beyond this, alpha5-ir was distributed within the dendritic ramifications of the dentate gyrus granule cells and within perikarya and dendrites of occasional nonpyramidal neurons. Developmental studies demonstrated that, in both hippocampus and neocortex, alpha5-ir appears first within perikarya and is distributed to dendrites during the second postnatal week. These results are in accord with the broad hypothesis that integrins contribute to apical-basal differences in dendrites and that the integrin fibronectin (alpha5beta1) receptor, in particular, contributes to some late developing features of dendritic structure or function.

Differential immunoreactivity for alpha-actinin-2, an N-methyl-D-aspartate-receptor/actin binding protein, in hippocampal interneurons

Recent studies have demonstrated that hippocampal interneurons possess distinct cytoskeletal and cell-signaling proteins in comparison to hippocampal principal cells; however, little is known about the differences in the actin cytoskeleton between these two populations. This study examined the immunoreactivity of alpha-actinin-2, an actin binding/N-methyl-D-aspartate-receptor linking protein, in the rat hippocampal formation using double-labelling immunofluorescence. Alpha-actinin-2 immunoreactivity is seen throughout the hippocampus with heavy labeling observed in the dendrites of granule cells, in CA2 pyramidal cells and in presumed interneuronal somata throughout the dentate gyrus and CA1. All the cells with heavy somatic alpha-actinin-2 immunoreactivity in the dentate gyrus and CA1 were GABAergic interneurons labeled by glutamate decarboxylase (99%). Examination of the neurochemical marker content of the alpha-actinin-2 immunoreactive interneurons revealed that the majority of this population was neuropeptide-Y-positive and a minority was positive for calretinin. Fluid percussion head trauma did not result in significant alterations of alpha-actinin-2 immunoreactivity in hippocampal interneurons. The developmental profile of alpha-actinin-2 immunoreactivity showed the presence of alpha-actinin-2 in the hippocampus at P1, labeling of interneurons by P7 and the adult staining pattern seen by P21. This study demonstrates that principal cells and interneurons are differentially immunoreactive for alpha-actinin-2, and that alpha-actinin-2 staining is restricted to a subpopulation of interneurons. Each of the three classes of cytoskeletal elements have been shown to be differentially expressed in hippocampal interneurons and principal cells, suggesting that the cytoskeleton is a defining feature of neuronal populations. Additionally, the limited expression of alpha-actinin-2 could have important functional implications in N-methyl-D-aspartate receptor localization and modulation.

Internal initiation of translation of five dendritically localized neuronal mRNAs

In neurons, translation of dendritically localized mRNAs is thought to play a role in affecting synaptic efficacy. Inasmuch as components of the translation machinery may be limiting in dendrites, we investigated the mechanisms by which translation of five dendritically localized mRNAs is initiated. The 5' leader sequences of mRNAs encoding the activity-regulated cytoskeletal protein, the alpha subunit of calcium-calmodulin-dependent kinase II, dendrin, the microtubule-associated protein 2, and neurogranin (RC3) were evaluated for their ability to affect translation in the 5' untranslated region of a monocistronic reporter mRNA. In both neural and nonneural cell lines, the activity-regulated cytoskeletal protein, microtubule-associated protein 2, and alpha-CaM Kinase II leader sequences enhanced translation, whereas the dendrin and RC3 5' untranslated regions slightly inhibited translation as compared with controls. When cap-dependent translation of these constructs was suppressed by overexpression of a protein that binds the cap-binding protein eIF4E, it was revealed that translation of these mRNAs had both cap-dependent and cap-independent components. The cap-independent component was further analyzed by inserting the 5' leader sequences into the intercistronic region of dicistronic mRNAs. All five leader sequences mediated internal initiation via internal ribosome entry sites (IRESes). The RC3 IRES was most active and was further characterized after transfection in primary neurons. Although translation mediated by this IRES occurred throughout the cell, it was relatively more efficient in dendrites. These data suggest that IRESes may increase translation efficiency at postsynaptic sites after synaptic activation.

Regulation of neurite outgrowth by integrin activation

During late-embryonic development, retinal neurons lose the ability to attach and extend neurites on the extracellular matrix molecule laminin-1 (LN-1), despite the fact that they retain expression of integrin receptors for LN-1. Here we show that the developmental loss of responsiveness to LN-1 can be reversed by treatments that increase the activation state of integrins. Both extracellular application of Mn(2+) (at micromolar concentrations) and viral-mediated neuronal expression of a constitutively active form of the ras-related GTPase R-ras (R-ras(38V)) potently promoted late-embryonic retinal neurite outgrowth on LN-1 substrata. In both cases, outgrowth was mediated by integrin alpha6beta1 and not alpha3beta1, even though these neurons express alpha3beta1 and use it for outgrowth on other laminin isoforms, as well as on LN-1 that has been proteolytically or conformationally activated (Ivins et al., 1998). Mn(2+)-and to a much lesser extent R-ras(38V)-also reversed the developmental loss of retinal neuron responsiveness to type IV collagen, by promoting the function of integrin alpha1beta1. Interestingly, the responses of other late-embryonic CNS neurons to LN-1 were also enhanced by treatments that activate integrin function, but those of peripheral nervous system neurons (dorsal root ganglion neurons) were either not enhanced (embryonic neurons) or only modestly improved (adult neurons). These results suggest that a developmental decline occurs in the activation state of neuronal integrins, particularly among CNS neurons. Such a decline may underlie some of the intrinsic loss of regenerative ability sustained by CNS neurons during development and may be a valid target for therapeutic intervention.

Active zones on motor nerve terminals contain alpha 3beta 1 integrin

Active zones are the sites along nerve terminals where synaptic vesicles dock and undergo calcium-dependent exocytosis during synaptic transmission. Here we show, by immunofluorescent staining with antibodies generated against Xenopus laevis integrins, that alpha3beta1 integrin is concentrated at the active zones of Xenopus motor nerve terminals. Because integrins can link extracellular matrix molecules to cytoskeletal elements and participate in the formation of signaling complexes, the localization of integrin at active zones suggests that it may play a role in the adhesion of the nerve terminals to the synaptic basal lamina, in the formation and maintenance of active zones, and in some of the events associated with calcium-dependent exocytosis of neurotransmitter. Our findings also indicate that the integrin composition of the terminal Schwann cells differs from that of the motor nerve terminals, and this may account at least in part for differences in their adhesiveness to the synaptic basal lamina.