Contribution of Adventitial Fibroblasts to Neointima Formation and Vascular Remodeling

The adventitial layer surrounding the blood vessels has long been exclusively considered a supporting tissue the main function of which is to provide adequate nourishment to the muscle layers of tunica media. Although functionally interconnected, the adventitial and medial layers are structurally interfaced at the external elastic lamina level, clearly distinguishable at the maturational phase of vascular morphogenesis. Over the last few years the "passive" role that the adventitia seemed to play in experimental and spontaneous vascular pathologies involving proliferation, migration, differentiation, and apoptosis of vascular smooth muscle cells (VSMCs) has been questioned. It has been demonstrated that fibroblasts from the adventitia display an important partnership with the resident medial VSMCs in terms of phenotypic conversion, proliferation, apoptotic, and migratory properties the result of which is neointima formation and vascular remodeling. This article is an attempt at reviewing the major themes and more recent findings dealing with the phenotypic conversion process that leads adventitial "passive" (static) fibroblasts to become "activated" (mobile) myofibroblasts. This event shows some facets in common with vascular morphogenesis, ie, the process of recruitment, incorporation, and phenotypic conversion of cells surrounding the primitive endothelial tube in the definitive vessel wall. We hypothesize that during the response to vascular injuries in the adult, "activation" of adventitial fibroblasts is, at least in part, reminiscent of a developmental program that also invests, although with distinct spatiotemporal features, medial VSMCs

Fast spiking interneuron control of seizure propagation in a cortical slice model of focal epilepsy

In different animal models of focal epilepsy, seizure-like ictal discharge propagation is transiently opposed by feedforward inhibition. The specific cellular source of this signal and the mechanism by which inhibition ultimately becomes ineffective are, however, undefined. We used a brain slice model to study how focal ictal discharges that were repetitively evoked from the same site, and at precise times, propagate across the cortex. We used Ca(2+) imaging and simultaneous single/dual cell recordings from pyramidal neurons (PyNs) and different classes of interneurons in rodents, including G42 and GIN transgenic mice expressing the green fluorescence protein in parvalbumin (Pv)-fast spiking (FS) and somatostatin (Som) interneurons, respectively. We found that these two classes of interneurons fired intensively shortly after ictal discharge generation at the focus. The inhibitory barrages that were recorded in PyNs occurred in coincidence with Pv-FS, but not with Som interneuron burst discharges. Furthermore, the strength of inhibitory barrages increased or decreased in parallel with increased or decreased firing in Pv-FS interneurons but not in Som interneurons. A firing impairment of Pv-FS interneurons caused by a membrane depolarization was found to precede ictal discharge onset in neighbouring pyramidal neurons. This event may account for the collapse of local inhibition that allows spatially defined clusters of PyNs to be recruited into propagating ictal discharges. Our study demonstrates that Pv-FS interneurons are a major source of the inhibitory barrages that oppose ictal discharge propagation and raises the possibility that targeting Pv-FS interneurons represents a new therapeutic strategy to prevent the generalization of human focal seizures

Astrocytes Modulate Somatostatin Interneuron Signaling in the Visual Cortex

At glutamatergic synapses, astrocytes respond to the neurotransmitter glutamate with intracellular Ca(2+) elevations and the release of gliotransmitters that modulate synaptic transmission. While the functional interactions between neurons and astrocytes have been intensively studied at glutamatergic synapses, the role of astrocytes at GABAergic synapses has been less investigated. In the present study, we combine optogenetics with 2-photon Ca(2+) imaging experiments and patch-clamp recording techniques to investigate the signaling between Somatostatin (SST)-releasing GABAergic interneurons and astrocytes in brain slice preparations from the visual cortex (VCx). We found that an intense stimulation of SST interneurons evokes Ca(2+) elevations in astrocytes that fundamentally depend on GABA(B) receptor (GABA(B)R) activation, and that this astrocyte response is modulated by the neuropeptide somatostatin. After episodes of SST interneuron hyperactivity, we also observed a long-lasting reduction of the inhibitory postsynaptic current (IPSC) amplitude onto pyramidal neurons (PNs). This reduction of inhibitory tone (i.e., disinhibition) is counterbalanced by the activation of astrocytes that upregulate SST interneuron-evoked IPSC amplitude by releasing ATP that, after conversion to adenosine, activates A(1)Rs. Our results describe a hitherto unidentified modulatory mechanism of inhibitory transmission to VCx layer II/III PNs that involves the functional recruitment of astrocytes by SST interneuron signaling

The inflammatory molecules IL-1\u3b2 and HMGB1 can rapidly enhance focal seizure generation in a brain slice model of temporal lobe epilepsy

Epilepsy is a neurological disorder characterized by a hyperexcitable brain tissue and unpredictable seizures, i.e., aberrant firing discharges in large neuronal populations. It is well established that proinflammatory cytokines, in addition to their canonical involvement in the immune response, have a crucial role in the mechanism of seizure generation. The purpose of the present study was to investigate the role of interleukin-1\u3b2 (IL-1\u3b2) and high mobility group B1 (HMGB1) in the generation of seizure-like discharges using two models of focal epilepsy in a rat entorhinal cortex slice preparation. Seizure like-discharges were evoked by either slice perfusion with low Mg(2+) and picrotoxin or with a double NMDA local stimulation in the presence of the proconvulsant 4-amino-pyridine. The effects of IL-1\u3b2 or HMGB1 were evaluated by monitoring seizure discharge generation through laser scanning microscope imaging of Ca(2+) signals from neurons and astrocytes. In the picrotoxin model, we revealed that both cytokines increased the mean frequency of spontaneous ictal-like discharges, whereas only IL-1\u3b2 reduced the latency and prolonged the duration of the first ictal-like event. In the second model, a single NMDA pulse, per se ineffective, became successful when it was performed after IL-\u3b2 or HMGB1 local applications. These findings demonstrate that both IL-1\u3b2 and HMGB1 can rapidly lower focal ictal event threshold and strengthen the possibility that targeting these inflammatory pathways may represent an effective therapeutic strategy to prevent seizures

RABBIT DUCTUS-ARTERIOSUS DURING DEVELOPMENT - ANATOMICAL STRUCTURE AND SMOOTH-MUSCLE CELL COMPOSITION

The anatomical structure as well as the smooth muscle cell (SMC1) composition of the ductus arteriosus (DA) were studied in rabbits ranging in age from 29 days of gestation to 20 days after birth. Computer-assisted, three-dimensional reconstructions of hematoxylin-eosin stained serial cryosections from ductus arteriosus-aorta (DA-AO) junctures revealed that DA in animals near term is separated from the aorta by a "septumlike" structure that is continuous with the aortic wall. Two days after birth, obliteration of DA is almost complete, and a small "pocketlike" cavity appears in the pre-existing site in which DA merged into the aorta. This small cavity in the aortic arch was still evident in the large majority of animals examined even 20 days after birth, as also demonstrated by scanning electron microscopy. At this time period DA consisted of a central, fibrotic region surrounded by several layers of SMC (the ligamentum arteriosum, LA) and ended within the aortic media just above the small cavity, forming a round "scar." Vascular SMC composition of DA during closure was examined by means of indirect and double immunofluorescence procedures, using a panel of monoclonal antibodies against some cytoskeletal and cytocontractile proteins (vimentin, desmin, smooth muscle (SM), and nonmuscle (NM) myosin isoforms). "Intimal cushions" were particularly evident from 5 hr after birth and were found to be desmin-negative, homogenously reactive for vimentin and NM myosin, and heterogeneously stained with anti-SM myosin antibody. In SMC subjacent to the "intimal cushions," distribution of vimentin and SM myosin was homogeneous, whereas the one of desmin and NM myosin content was heterogeneous. The cytoskeletal and cytocontractile protein content displayed by SMC during the closure of DA is similar to that of "intimal thickening" found in some pathological conditions of the arterial wall in adult rabbits. Completation of DA closure (day 2) was accompanied by the disappearance of cellular heterogeneity in myosin isoform distribution in both the "intimal cushions" and the underlying media. These results give new insights into: (1) the structure of DA-AO juncture, which can be relevant to the physiology of blood circulation in the fetus, and (2) the phenotypic similarity of vascular SMC populations involved in the formation of "intimal cushions" and "intimal thickening.

Fish oil supplementation prevents neointima formation in non- hypercholesterolemic balloon-injured rabbit carotid artery by reducing medial and adventitial cell activation

Abstract —We asked whether balloon-injured neointima formation in the presence of high/low serum cholesterol (CT) levels might be affected by dietary supplementation with fish oil (FO). To test this hypothesis, we examined the differentiation, proliferation, or apoptosis profile of smooth muscle cell (SMC) and adventitial cell response to a mild injury induced via a Fogarty catheter in the carotid artery of adult rabbits that had been fed a standard chow or 0.5% CT-enriched diet starting 4 weeks before the lesion. One week before surgery, animals received FO supplementation. This regimen was continued for the following 3 weeks. The effect of FO on the early proliferative/migratory response of carotid SMCs was also examined in 2- and 7-day–injured normocholesterolemic rabbits. As controls, animals subjected to 3-week endothelial injury and animals kept on a 7-week CT diet were used. Carotid cryosections from the various animal groups were evaluated for morphometry (image analysis), differentiation (immunofluorescence with monoclonal antibodies specific for smooth muscle markers, ie, myosin isoforms, SM22, and fibronectin), proliferation (bromodeoxyuridine incorporation), and apoptosis (terminal deoxynucleotidyl transferase–mediated dUTP nick end-labeling). FO treatment significantly reduced the development of intimal thickening in normocholesterolemic rabbits but had no efficacy in the presence of relatively higher serum CT levels. At day 2 (adventitia) and day 7 (neointima, media, and adventitia), the proliferation index of SMCs in FO-treated injured rabbits was markedly lower than in untreated injured controls. Concomitantly with the antiproliferative effect, FO was able to decrease the size of 2 cell types involved in the cell growth response to endothelial injury, namely, the “fetal-type” medial SMC subpopulation and the fibroblast-derived adventitial myofibroblasts. Thus, in our experimental conditions, a low CT level is a permissive condition for FO to prevent neointima formation to a considerable extent. This event is attributable to the early postinjury effect of FO on SMC/adventitial cell proliferation/differentiation patterns. </jats:p

A brain slice experimental model to study the generation and the propagation of focally-induced epileptiform activity

The early cellular events that in a brain network lead to seizure generation and govern seizure propagation are probably based on different cellular mechanisms. Experimental models in which these events can be separately studied would contribute to improve our understanding of epilepsy. We recently described an in vitro model in entorhinal cortex slices from young rats in which focal seizure-like discharges (SLDs) can be induced in spatially defined regions and at predictable times by local NMDA applications performed in the presence of 4-amimopyridine (4-AP) and low extracellular Mg2+. Through the use of single-dual cell patch-clamp and field potential recordings, and Ca2+ imaging from large ensembles of neurons, interneurons and astrocytes, we here extend this model to entorhinal and temporal cortex slices of rat and mouse brain, providing evidence that multiple SLDs exhibiting the typical tonic-clonic discharge pattern can be also evoked in these cortical regions by successive NMDA applications. Importantly, the temporal cortex is more accessible to viral vector injections than the entorhinal cortex: this makes it feasible in the former region the selective expression in inhibitory interneurons or principal neurons of genetically encoded Ca2+ indicators (GECI) or light-gated opsins. In this model, an optogenetic approach allows to activate specific neuronal types at spatially defined locations, i.e., the focus or the propagating region, and at precise time, i.e., before or during SLD. The NMDA/4-AP model can, therefore, represent a valuable tool to gain insights into the role of specific cell populations in seizure generation, propagation and cessation. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)