Brain Receptor Mosaics and Their Intramembrane Receptor-Receptor Interactions: Molecular Integration in Transmission and Novel Targets for Drug Development

The concept of intramembrane receptor-receptor interactions and evidence for their existence was introduced by Agnati and Fuxe in 1980/81 suggesting the existence of heteromerization of receptors. In 1982, they proposed the existence of aggregates of multiple receptors in the plasma membrane and coined the term receptor mosaics (RM). In this way, cell signaling becomes a branched process beginning at the level of receptor recognition at the plasma membrane where receptors can directly modify the ligand recognition and signaling capacity of the receptors within a RM. Receptor-receptor interactions in RM are classified as operating either with classical cooperativity, when consisting of homomers or heteromers of similar receptor subtypes having the same transmitter, or non-classical cooperativity, when consisting of heteromers. It has been shown that information processing within a RM depends not only on its receptor composition, but also on the topology and the order of receptor activation determined by the concentrations of the ligands and the receptor properties. The general function of RM has also been demonstrated to depend on allosteric regulators (e.g., homocysteine) of the receptor subtypes present. RM as integrative nodes for receptor-receptor interactions in conjunction with membrane associated proteins may form horizontal molecular networks in the plasma membrane coordinating the activity of multiple effector systems modulating the excitability and gene expression of the cells. The key role of electrostatic epitope-epitope interactions will be discussed for the formation of the RM. These interactions probably represent a general molecular mechanism for receptor-receptor interactions and, without a doubt, indicate a role for phosphorylation-dephosphorylation events in these interactions. The novel therapeutic aspects given by the RMs will be discussed in the frame of molecular neurology and psychiatry and combined drug therapy appears as the future way to go.     

国产雷帕霉素洗脱支架治疗急性冠状动脉综合征：安全性和临床有效性

目的:分析国产雷帕霉素洗脱支架治疗急性冠状动脉综合征的安全性和有效性。方法:选择2004-11/2006-02在河北大学附属医院接受冠状动脉介入治疗的急性冠状动脉综合征患者102例,其中ST段抬高型心肌梗死54例,非ST段抬高型心肌梗死28例,不稳定型心绞痛20例。根据血管情况置入国产雷帕霉素药物洗脱支架(Firebird支架),支架选择原则为:支架长度应覆盖病变两端;血管直径:支架直径=1∶1.1。所有患者术前3d均口服阿司匹林100mg,氯吡格雷75mg,术中推注肝素8000￣10000U,手术每延迟1h,补充肝素1000u,术后皮下注射低分子肝素5￣7d;服用氯吡格雷75mg,1次/d,共服用9￣12个月,并长期服用阿司匹林100mg,1次/d。随访情况:术后6个月时随访64例;7个月时随访26例;8个月时随访12例;平均随访6.8个月,患者出院后定期进行门诊随访,记录一般情况及严重心脏不良事件(包括急性、亚急性、迟发支架内血栓形成;再发心肌梗死;急诊冠状动脉旁路移植术;死亡),术后6￣8个月行冠状动脉造影评价支架内再狭窄情况。并观察材料及宿主反应。结果:102例患者经皮冠状动脉介入治疗治疗均获得成功,共治疗靶血管102支,置入Firebird支架116枚,术中3例ST段抬高型心肌梗死患者出现无复流现象,2例发生室颤,电转复恢复窦性心律,3例因分支受压,出现心绞痛症状。术后4例出现穿刺部位血肿,经重新加压压迫后好转。随访6￣8个月所有患者未发生严重心血管事件;42例(41.2%)患者术后6￣8个月行冠状动脉造影复查,无一例发生支架内再狭窄。随访期间所有患者无全身毒性及超敏反应发生,生物相容性好。结论:国产药物洗脱支架治疗急性冠状动脉综合征安全,有效。     

On the cellular localization and distribution of carbonic anhydrase II immunoreactivity in the rat brain

Evidence is provided that carbonic anhydrase-II is localized in the central nervous system to wide spread systems of oligodendrocytes and restricted astroglia populations, involving both fiber bundles and neuropil. It is suggested that CO₂ formed in activated axons may, via carbonic anhydrase-II, give rise to protons controlling the excitability of surrounding neuropil. Thus, CO₂ may represent an important, highly diffusible, signal in brain, involved in the tonic control of neuronal activity.     

Studies on dopamine turnover in ovariectomized or hypo-physectomized female rats. effects of 17β-estradiol benzoate, ethynodioldiacetate and ovine prolactin

The effects of 17 beta-estradiol benzoate (EB), ethynodioldiacetate and ovine prolactin on dopamine (DA) turnover have been studied. As an index for a change in turnover, differences in DA depletion following tyrosine hydroxylase inhibition with alpha-methyl-tyrosine methylester were observed. DA was measured by means of mass fragmentographical analysis in the rat median eminence, the olfactory tubercle and the striatum. The actions of ethynodioldiacetate and ovine prolactin on DA turnover in various subregions of the rat median eminence were analysed by quantitative microfluorimetry. Repeated injections of EB to ovariectomized rats resulted in a significant increase of DA turnover in the median eminence. Administration of ethynodioldiacetate to ovariectomized rats almost significantly increased DA turnover in the olfactory tubercle. In the median eminence DA turnover was significantly increased only in the lateral palisade zone. In male hypophysectomized rats ovine prolactin increased DA turnover in both the lateral and the medial palisade zone of the median eminence. The results support the involvement of DA neurons in the control of prolactin and luteinizing hormone secretion. It is suggested that the tubero-infundibular DA neurons are involved in mediating the central inhibitory feedback actions of prolactin and gonadal steroids on prolactin and LH secretion, respectively.     

Existence and theoretical aspects of homomeric and heteromeric dopamine receptor complexes and their relevance for neurological diseases

Dopamine (DA) and other receptors physically interact in the plasma membrane of basal ganglia neurons forming receptor mosaics (RMs). Two types of RMs are discussed, homomers formed only by DA-receptor (DA-R) subtypes and heteromers formed by DA-R associated with other receptors, such as A2A, A1, mGluR5, N-methyl-d-aspartate (NMDA), γ-aminobutryic acid (GABA)-A, and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. By being part of horizontal molecular networks, RMs tune multiple effector systems already at membrane level, such as G protein regulated inward rectifying potassium channels and dopamine transporter activity. Also, ligand-gated ion channels such as GABA-A and NMDA receptors are modulated by DA-R, e.g., in the striatal GABA output neurons through the formation of heteromeric complexes with these receptors. Thus, intramembrane DA-R-receptor interactions play an important role in the information handling in the basal ganglia. On this basis, functional implications of DA RM in physiological and pathological conditions are discussed. The effects of temperature on RM are discussed not only because receptor-decoding mechanisms are temperature sensitive, but also in view of the suggestion that possible ordering effects (i.e., changes in the entropy of a receptor complex) induced by a ligand are as a result of alterations in the receptor oligomerization (i.e., are related to rearrangements of the RM). Hence, brain temperature may have profound effects on brain integrative functions not only because its effects on the kinetics of biochemical reactions, but also for its effects on receptor geometry, building up of RM, and alterations in protein expression, as is the case of H-channels following febrile seizures. Note: This article is dedicated to Tullio Giacomini for his 70th birthday.     

Adenosine A2A and dopamine D2 heteromeric receptor complexes and their function

The existence of A2A-D2 heteromeric complexes is based on coimmunoprecipitation studies and on fluorescence resonance energy transfer and bioluminescence resonance energy transfer analyses. It has now become possible to show that A2A and D2 receptors also coimmunoprecipitate in striatal tissue, giving evidence for the existence of A2A-D2 heteromeric receptor complexes also in rat striatal tissue. The analysis gives evidence that these heteromers are constitutive, as they are observed in the absence of A2A and D2 agonists. The A2A-D2 heteromers could either be A2A-D2 heterodimers and/or higher-order A2A -D2 hetero-oligomers. In striatal neurons there are probably A2A-D2 heteromeric complexes, together with A2A-D2 homomeric complexes in the neuronal surface membrane. Their stoichiometry in various microdomains will have a major role in determining A2A and D2 signaling in the striatopallidal GABA neurons. Through the use of D2/D1 chimeras, evidence has been obtained that the fifth transmembrane (TM) domain and/or the I3 of the D2 receptor are part of the A2A-D2 receptor interface, where electrostatic epitope-epitope interactions involving the N-terminal part of I3 of the D2 receptor (arginine-rich epitope) play a major role, interacting with the carboxyl terminus of the A2A receptor. Computerized modeling of A2A-D2 heteromers are in line with these findings. It seems likely that A2A receptor-induced reduction of D2 receptor recognition, G protein coupling, and signaling, as well as the existence of A2A-D2 co-trafficking, are the consequence of the existence of an A2A-D2 receptor heteromer. The relevance of A2A-D2 heteromeric receptor complexes for Parkinson's disease and schizophrenia is emphasized as well as for the treatment of these diseases. Finally, recent evidence for the existence of antagonistic A2A-D3 heteromeric receptor complexes in cotransfected cell lines has been summarized.     

Corticosterone strongly increases the affinity of dorsal raphe 5-HT1A receptors

The effects of corticosterone (10 mg/kg, s.c., 6 h) on dorsal raphe 5-HT1A autoreceptors have been studied in adrenalectomized rats with or without porcine galanin modulation. Adrenalectomy diminishes 5-HT1A autoreceptors affinity. Corticosterone increases 5-HT1A autoreceptor agonist affinity (+90%, p<0.001) in adrenalectomized rats. Galanin (10 nM) increases dorsal raphe 5-HT1A autoreceptor density (+65%, p<0.05) and its Kd value (+248%, p<0.05) only in adrenalectomized rats treated with corticosterone. Dorsal raphe glucocorticoid receptors activation by corticosterone may therefore lead to an increased signalling of 5-HT1A autoreceptors that may become counteracted by galanin receptor activation. Glucocorticoids, by enhancing dorsal raphe 5-HT1A autoreceptor function, may therefore cause reduced 5-HT neuronal activity and thus lead to a depressive state.     

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.     

Possible role of intramembrane receptor-receptor interactions in memory and learning via formation of long-lived heteromeric complexes: focus on motor learning in the basal ganglia

Learning in neuronal networks occurs by instructions to the neurons to change their synaptic weights (i.e., efficacies). According to the present model a molecular mechanism that can contribute to change synaptic weights may be represented by multiple interactions between membrane receptors forming aggregates (receptor mosaics) via oligomerization at both pre- and post-synaptic level. These assemblies of receptors together with inter alia single receptors, adapter proteins, G-proteins and ion channels form the membrane bound part of a complex three-dimensional (3D) molecular circuit, the cytoplasmic part of which consists especially of protein kinases, protein phosphatases and phosphoproteins. It is suggested that this molecular circuit has the capability to learn and store information. Thus, engram formation will depend on the resetting of 3D molecular circuits via the formation of new receptor mosaics capable of addressing the transduction of the chemical messages impinging on the cell membrane to certain sets of G-proteins. Short-term memory occurs by a transient stabilization of the receptor mosaics producing the appropriate change in the synaptic weight. Engram consolidation (long-term memory) may involve intracellular signals that translocate to the nucleus to cause the activation of immediate early genes and subsequent formation of postulated adapter proteins which stabilize the receptor mosaics with the formation of long-lived heteromeric receptor complexes. The receptor mosaic hypothesis of the engram formation has been formulated in agreement with the Hebbian rule and gives a novel molecular basis for it by postulating that the pre-synaptic activity change in transmitter and modulator release reorganizes the receptor mosaics at post-synaptic level and subsequently at pre-synaptic level with the formation of novel 3D molecular circuits leading to a different integration of chemical signals impinging on pre- and post-synaptic membranes hence leading to a new value of the synaptic weight. Engram retrieval is brought about by the scanning of the target networks by the highly divergent arousal systems. Hence, a continuous reverberating process occurs both at the level of the neural networks as well as at the level of the 3D molecular circuits within each neuron of the network until the appropriate tuning of the synaptic weights is obtained and, subsequently, the reappearance of the engram occurs. Learning and memory in the basal ganglia is discussed in the frame of the present hypothesis. It is proposed that formation of long-term memories (consolidated receptor mosaics) in the plasma membranes of the striosomal GABA neurons may play a major role in the motivational learning of motor skills of relevance for survival. In conclusion, long-lived heteromeric receptor complexes of high order may be crucial for learning, memory and retrieval processes, where extensive reciprocal feedback loops give rise to coherent synchronized neural activity (binding) essential for a sophisticated information handling by the central nervous system.