Normal and aberrant functions of integrins in the adult central nervous system

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.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

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.

Developmental regulation and neuronal expression of the cellular disintegrin ADAM11 gene in mouse nervous system

ADAM11 is the prototype member of the predominantly CNS-associated clade of the ADAM metalloprotease-disintegrins that has been implicated in neural adhesion and axon guidance. The present study describes the spatiotemporal expression pattern of the ADAM11 gene in adult and developing mouse, and identifies the cells expressing the gene. In the adult CNS, ADAM11 mRNA was present throughout the forebrain, including different cortical fields and diencephalic nuclei. In brainstem, low to moderate expression was detected in certain midbrain nuclei, while several pontine and medullary nuclei showed a very strong signal. High expression was observed in the cerebellar cortex and spinal cord. In addition, ADAM11 was expressed in ganglia of the peripheral nervous system (PNS), retinae, testes, liver, and at lower levels in epidermal and mucosal epithelia, kidney, and salivary gland. The expression was localized to neurons in all examined CNS and PNS subfields. During pre- and perinatal development, ADAM11 was differentially expressed both in the developing PNS and CNS, as well as in heart, kidney, eyes, and brown fat. The present results suggest a widespread involvement of ADAM11 in neuron-neuron or neuron-glial cell interactions during development as well as in the adult nervous system. They provide novel complementary information to recently accumulated data on CNS integrin gene expression and offer useful clues for further studies of the neural functions of ADAMs and integrins.

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.

Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

Considerable evidence now suggests an interrelationship among long-term potentiation (LTP), extracellular matrix (ECM) reconfiguration, synaptogenesis, and memory consolidation within the mammalian central nervous system. Extracellular matrix molecules provide the scaffolding necessary to permit synaptic remodeling and contribute to the regulation of ionic and nutritional homeostasis of surrounding cells. These molecules also facilitate cellular proliferation, movement, differentiation, and apoptosis. The present review initially focuses on characterizing the ECM and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), in the maintenance and degradation of the ECM. The induction and maintenance of LTP is described. Debate continues over whether LTP results in some form of synaptic strengthening and in turn promotes memory consolidation. Next, the contribution of CAMs and TIMPs to the facilitation of LTP and memory consolidation is discussed. Finally, possible roles for angiotensins, MMPs, and tissue plasminogen activators in the facilitation of LTP and memory consolidation are described. These enzymatic pathways appear to be very important to an understanding of dysfunctional memory diseases such as Alzheimer's disease, multiple sclerosis, brain tumors, and infections.

Sequential upregulation of cell adhesion molecules in degenerating rat basal forebrain cholinergic neurons and in phagocytotic microglial cells

To study the functional role of adhesion molecules in neurodegenerative events in vivo, the basal forebrain cholinergic lesion-induced expression of the intercellular adhesion molecule (ICAM)-1 and leukocyte function-associated antigen (LFA)-1 was studied by double immunocytochemistry and Western blot analysis. A single intracerebroventricular application of the cholinergic immunotoxin, 192IgG-saporin, produced a selective cholinergic cell loss in rat basal forebrain nuclei detectable by gradual loss of choline acetyltransferase (ChAT)-immunoreactive cells starting 3 days but being nearly complete 7 days after injection of the toxin. The degeneration of cholinergic neurons was accompanied by a striking appearance of activated microglial cells in the lesioned areas. Four days following injection of 192IgG-saporin, ICAM-1 immunoreactivity was predominantly observed in ChAT-positive neurons and partly in activated microglia in the basal forebrain nuclei, while LFA-1 expression at this time point was restricted to neurons. However, 7 days after cholinergic lesion, only a few, shrunken neuronal somata were found to be immunoreactive for ICAM-1 and LFA-1, while activated microglial cells demonstrated strong immunoreactivity for ICAM-1 and LFA-1 in the lesioned forebrain areas, persisting up to 14 days after lesion while no immunoreactivity was observed in neurons at this time point. Western blot analysis demonstrated increased ICAM-1 level in the basal forebrain already detectable 4 days after surgery but being more pronounced 7 days post lesion. The data suggest that ICAM-1 and LFA-1 may act as intercellular recognition signals by which degenerating cholinergic neurons actively participate in the sequence of events leading to their targeting and elimination by phagocytotic microglia.

Metalloprotease-disintegrin (ADAM) genes are widely and differentially expressed in the adult CNS

ADAM family of metalloprotease-disintegrins, including enzymes that process TNF-alpha and beta-amyloid precursor protein, has been indicated in neuronal development, but the role of these protease/adhesion/signaling proteins in adult nervous system remains poorly understood. Present study provides a systematic examination of ADAM gene expression in rodent CNS, showing the first quantitative characterization of ADAM mRNA distribution therein. At least 17 ADAM mRNAs were expressed. Individual ADAM mRNAs and their isoforms showed strikingly different expression patterns. Expression of mRNAs for ADAM10, the putative alpha-secretase, and ADAM17 (TACE), also indicated in APP processing, was further characterized using in situ hybridization. Expression of ADAM10 mRNA was widespread, while ADAM17 showed a more restricted pattern. Altogether, the wide and differential expression of ADAM mRNAs suggests versatile roles for ADAMs in the adult CNS.

Integrin family of cell adhesion molecules in the injured brain: regulation and cellular localization in the normal and regenerating mouse facial motor nucleus

Integrins are a large family of heterodimeric glycoproteins that play a crucial role in cell adhesion during development, inflammation, and tissue repair. In the current study, we investigated the localization of different integrin subunits in the mouse facial motor nucleus and their regulation after transection of the facial nerve. In the normal mouse brain, there was clear immunoreactivity for alpha5-, alpha6-, and beta1-integrin subunits on blood vessel endothelia and for alphaM- and beta2-subunits on resting parenchymal microglia. Facial nerve transection led to an up-regulation of the beta1-subunit on the axotomized neurons and an increase in the alpha4-, alpha5-, alpha6-, beta1-, alphaM-, alphaX-, and beta2-subunits on the adjacent, activated microglia. Quantification of the microglial integrins revealed two different expression patterns. The subunits alpha5 and alpha6 showed a monophasic increase with a maximum at day 4, the alphaM-subunit a biphasic regulation, with an early peak at day 1 and an elevated plateau between day 14 and 42. At day 14, there was also an influx of lymphocytes immunoreactive for the alpha4beta1- and alphaLbeta2-integrins, which aggregated at sites of neural debris and phagocytotic microglia. This finding was accompanied by a significant increase of the alpha5beta1-integrin on blood vessel endothelia. In summary, facial axotomy is followed by a strong and cell-type-specific expression of integrins on the affected neurons and on surrounding microglia, lymphocytes, and vascular endothelia. The presence of several, strikingly different temporal patterns suggests a selective involvement of these molecules in the different adhesive events during regeneration in the central nervous system.

Neurobiological problems in long-term deep space flights

Future missions in space may involve long-term travel beyond the magnetic field of the Earth, subjecting astronauts to radiation hazards posed by solar flares and galactic cosmic rays, altered gravitation fields and physiological stress. Thus, it is critical to determine if there will be any reversible or irreversible, detrimental neurological effects from this prolonged exposure to space. A question of particular importance focuses on the long-term effects of the space environment on the central nervous system (CNS) neuroplasticity, with the potential acute and/or delayed effects that such perturbations might entail. Although the short-term effects of microgravity on neural control were studied on previous low earth orbit missions, the late consequences of stress in space, microgravity and space radiation have not been addressed sufficiently at the molecular, cellular and tissue levels. The possibility that space flight factors can interact influencing the neuroplastic response in the CNS looms critical issue not only to understand the ontogeny of the CNS and its functional integrity, but also, ultimately the performance of astronauts in extended space forays. The purpose of this paper is to review the neurobiological modifications that occur in the CNS exposed to the space environment, and its potential consequences for extended deep space flight.