Localization of receptors in lipid rafts can inhibit signal transduction

Annexin A6 mitigates neurological deficit in ischemia/reperfusion injury by promoting synaptic plasticity

AIMS

Alleviating neurological dysfunction caused by acute ischemic stroke (AIS) remains intractable. Given Annexin A6 (ANXA6)'s potential in promoting axon branching and repairing cell membranes, the study aimed to explore ANXA6's potential in alleviating AIS-induced neurological dysfunction.

METHODS

A mouse middle cerebral artery occlusion model was established. Brain and plasma ANXA6 levels were detected at different timepoints post ischemia/reperfusion (I/R). We overexpressed and down-regulated brain ANXA6 and evaluated infarction volume, neurological function, and synaptic plasticity-related proteins post I/R. Plasma ANXA6 levels were measured in patients with AIS and healthy controls, investigating ANXA6 expression's clinical significance.

RESULTS

Brain ANXA6 levels initially decreased, gradually returning to normal post I/R; plasma ANXA6 levels showed an opposite trend. ANXA6 overexpression significantly decreased the modified neurological severity score (p = 0.0109) 1 day post I/R and the infarction area at 1 day (p = 0.0008) and 7 day (p = 0.0013) post I/R, and vice versa. ANXA6 positively influenced synaptic plasticity, upregulating synaptophysin (p = 0.006), myelin basic protein (p = 0.010), neuroligin (p = 0.078), and tropomyosin-related kinase B (p = 0.150). Plasma ANXA6 levels were higher in patients with AIS (1.969 [1.228-3.086]) compared to healthy controls (1.249 [0.757-2.226]) (p < 0.001), that served as an independent risk factor for poor AIS outcomes (2.120 [1.563-3.023], p < 0.001).

CONCLUSIONS

This study is the first to suggest that ANXA6 enhances synaptic plasticity and protects against transient cerebral ischemia.

Neuraxial drug delivery in pain management: An overview of past, present, and future

Activation of neuraxial nociceptive linkages leads to a high level of encoding of the message that is transmitted to the brain and that can initiate a pain state with its attendant emotive covariates. As we review here, the encoding of this message is subject to a profound regulation by pharmacological targeting of dorsal root ganglion and dorsal horn systems. Though first shown with the robust and selective modulation by spinal opiates, subsequent work has revealed the pharmacological and biological complexity of these neuraxial systems and points to several regulatory targets. Novel therapeutic delivery platforms, such as viral transfection, antisense and targeted neurotoxins, point to disease-modifying approaches that can selectively address the acute and chronic pain phenotype. Further developments are called for in delivery devices to enhance local distribution and to minimize concentration gradients, as frequently occurs with the poorly mixed intrathecal space. The field has advanced remarkably since the mid-1970s, but these advances must always address the issues of safety and tolerability of neuraxial therapy.

Effect of Spatial Inhomogeneities on the Membrane Surface on Receptor Dimerization and Signal Initiation

Important signal transduction pathways originate on the plasma membrane, where microdomains may transiently entrap diffusing receptors. This results in a non-random distribution of receptors even in the resting state, which can be visualized as “clusters” by high resolution imaging methods. Here, we explore how spatial in-homogeneities in the plasma membrane might influence the dimerization and phosphorylation status of ErbB2 and ErbB3, two receptor tyrosine kinases that preferentially heterodimerize and are often co-expressed in cancer. This theoretical study is based upon spatial stochastic simulations of the two-dimensional membrane landscape, where variables include differential distributions and overlap of transient confinement zones (“domains”) for the two receptor species. The in silico model is parameterized and validated using data from single particle tracking experiments. We report key differences in signaling output based on the degree of overlap between domains and the relative retention of receptors in such domains, expressed as escape probability. Results predict that a high overlap of domains, which favors transient co-confinement of both receptor species, will enhance the rate of hetero-interactions. Where domains do not overlap, simulations confirm expectations that homo-interactions are favored. Since ErbB3 is uniquely dependent on ErbB2 interactions for activation of its catalytic activity, variations in domain overlap or escape probability markedly alter the predicted patterns and time course of ErbB3 and ErbB2 phosphorylation. Taken together, these results implicate membrane domain organization as an important modulator of signal initiation, motivating the design of novel experimental approaches to measure these important parameters across a wider range of receptor systems.

Regulation of TrkB receptor translocation to lipid rafts by adenosine A(2A) receptors and its functional implications for BDNF-induced regulation of synaptic plasticity

Brain-derived neurotrophic factor (BDNF) signalling is critical for neuronal development and transmission. Recruitment of TrkB receptors to lipid rafts has been shown to be necessary for the activation of specific signalling pathways and modulation of neurotransmitter release by BDNF. Since TrkB receptors are known to be modulated by adenosine A(2A) receptor activation, we hypothesized that activation of A(2A) receptors could influence TrkB receptor localization among different membrane microdomains. We found that adenosine A(2A) receptor agonists increased the levels of TrkB receptors in the lipid raft fraction of cortical membranes and potentiated BDNF-induced augmentation of phosphorylated TrkB levels in lipid rafts. Blockade of the clathrin-mediated endocytosis with monodansyl cadaverine (100 μM) did not modify the effects of the A(2A) receptor agonists, but significantly impaired BDNF effects on TrkB recruitment to lipid rafts. The effect of A(2A) receptor activation in TrkB localization was mimicked by 5 μM forskolin, an adenylyl cyclase activator. Also, it was blocked by the PKA inhibitors RpcAMPs and PKI-(14-22) and by the Src-family kinase inhibitor PP2. Moreover, removal of endogenous adenosine or disruption of lipid rafts reduced BDNF stimulatory effects on glutamate release from cortical synaptosomes. Lipid raft integrity was also required for the effects of BDNF upon hippocampal long-term potentiation at CA1 synapses. Our data demonstrate, for the first time, a BDNF-independent recruitment of TrkB receptors to lipid rafts, induced by the activation of adenosine A(2A) receptors, with functional consequences for TrkB phosphorylation and BDNF-induced modulation of neurotransmitter release and hippocampal plasticity.

Regulation of TrkB receptor translocation to lipid rafts by adenosine A(2A) receptors and its functional implications for BDNF-induced regulation of synaptic plasticity

Brain-derived neurotrophic factor (BDNF) signalling is critical for neuronal development and transmission. Recruitment of TrkB receptors to lipid rafts has been shown to be necessary for the activation of specific signalling pathways and modulation of neurotransmitter release by BDNF. Since TrkB receptors are known to be modulated by adenosine A(2A) receptor activation, we hypothesized that activation of A(2A) receptors could influence TrkB receptor localization among different membrane microdomains. We found that adenosine A(2A) receptor agonists increased the levels of TrkB receptors in the lipid raft fraction of cortical membranes and potentiated BDNF-induced augmentation of phosphorylated TrkB levels in lipid rafts. Blockade of the clathrin-mediated endocytosis with monodansyl cadaverine (100 μM) did not modify the effects of the A(2A) receptor agonists, but significantly impaired BDNF effects on TrkB recruitment to lipid rafts. The effect of A(2A) receptor activation in TrkB localization was mimicked by 5 μM forskolin, an adenylyl cyclase activator. Also, it was blocked by the PKA inhibitors RpcAMPs and PKI-(14-22) and by the Src-family kinase inhibitor PP2. Moreover, removal of endogenous adenosine or disruption of lipid rafts reduced BDNF stimulatory effects on glutamate release from cortical synaptosomes. Lipid raft integrity was also required for the effects of BDNF upon hippocampal long-term potentiation at CA1 synapses. Our data demonstrate, for the first time, a BDNF-independent recruitment of TrkB receptors to lipid rafts, induced by the activation of adenosine A(2A) receptors, with functional consequences for TrkB phosphorylation and BDNF-induced modulation of neurotransmitter release and hippocampal plasticity.

Monte Carlo simulations of plasma membrane corral-induced EGFR clustering

Experimental evidence suggests that the cell membrane is a highly organized structure that is compartmentalized by the underlying membrane cytoskeleton (MSK). The interaction between the cell membrane and the cytoskeleton led to the "picket-fence" model, which was proposed to explain certain aspects of membrane compartmentalization. This model assumes that the MSK hinders and confines the motion of receptors and lipids to compartments in the membrane. However, the impact of the MSK on receptor clustering, aggregation, and downstream signaling remains unclear. For example, some evidence suggests that the MSK enhances dimerization, while other evidence suggests decreased dimerization and signaling. Herein, we use computational Monte Carlo simulations to examine the effects of MSK density and receptor concentration on receptor dimerization and clustering. Preliminary results suggest that the MSK may have the potential to induce receptor clustering, which is a function of both picket-fence density and receptor concentration.

Activation of adenosine A2A receptors induces TrkB translocation and increases BDNF-mediated phospho-TrkB localization in lipid rafts: implications for neuromodulation

Brain-derived neurotrophic factor (BDNF) signaling is critical for neuronal development and transmission. Recruitment of TrkB receptors to lipid rafts has been shown to be necessary for the activation of specific signaling pathways and modulation of neurotransmitter release by BDNF. Since TrkB receptors are known to be modulated by adenosine A(2A) receptor activation, we hypothesized that activation of A(2A) receptors could influence TrkB receptor localization among different membrane microdomains. We found that adenosine A(2A) receptor agonists increased the levels of TrkB receptors in the lipid raft fraction of cortical membranes and potentiated BDNF-induced augmentation of phosphorylated TrkB levels in lipid rafts. Blockade of the clathrin-mediated endocytosis with monodansylcadaverine (100 microm) did not modify the effects of the A(2A) receptor agonists but significantly impaired BDNF effects on TrkB recruitment to lipid rafts. The effect of A(2A) receptor activation in TrkB localization was mimicked by 5 microm forskolin, an adenylyl cyclase activator. Also, it was blocked by the PKA inhibitors Rp-cAMPs and PKI-(14-22), and by the Src-family kinase inhibitor PP2. Moreover, removal of endogenous adenosine or disruption of lipid rafts reduced BDNF stimulatory effects on glutamate release from cortical synaptosomes. Lipid raft integrity was also required for the effects of BDNF on hippocampal long-term potentiation at CA1 synapses. Our data demonstrate, for the first time, a BDNF-independent recruitment of TrkB receptors to lipid rafts induced by activation of adenosine A(2A) receptors, with functional consequences for TrkB phosphorylation and BDNF-induced modulation of neurotransmitter release and hippocampal plasticity.

Influence of nonequilibrium lipid transport, membrane compartmentalization, and membrane proteins on the lateral organization of the plasma membrane

Compositional lipid domains (lipid rafts) in plasma membranes are believed to be important components of many cellular processes. The mechanisms by which cells regulate the sizes, lifetimes, and spatial localization of these domains are rather poorly understood at the moment. We propose a robust mechanism for the formation of finite-sized lipid raft domains in plasma membranes, the competition between phase separation in an immiscible lipid system and active cellular lipid transport processes naturally leads to the formation of such domains. Simulations of a continuum model reveal that the raft size distribution is broad and the average raft size is strongly dependent on the rates of cellular and interlayer lipid transport processes. We demonstrate that spatiotemporal variations in the recycling may enable the cell to localize larger raft aggregates at specific parts along the membrane. Moreover, we show that membrane compartmentalization may further facilitate spatial localization of the raft domains. Finally, we demonstrate that local interactions with immobile membrane proteins can spatially localize the rafts and lead to further clustering.

Spatiotemporal control of apical and basal living subcellular chemical environments through vertical phase separation

Molecular distribution within living cells is organized through multiscaled compartmentalization that enables specialized processes to occur with high efficiency. The ability to control the chemical environment at a subcellular level is limited due to deficient positional control over the aqueous stimulant. Here, a multilayered microfluidic system built from polydimethylsiloxane to separate chemical stimulants over single living cells vertically through aqueous-phase separation under laminar flow is demonstrated. Cells are cultured on top of single micrometer-scale channels inside a larger channel, allowing labeling of the apical domain of single cells through the main channel with simultaneous and distinct labeling of the basal domain via the lower microchannels. The system is transparent, which allows the use of optical microscopy to investigate the spatiotemporal response of labeled components. By employing this technique, the examination of localized subcellular domain responses in polarization, lipid bilayer mobility, and apical-to-basal signal transduction can be explored.

Signaling through ERBB receptors: Multiple layers of diversity and control

The four known ERBB receptors in humans are involved in a broad range of cellular responses, and their deregulation is a significant aspect in a large number of disease states. However, their mechanism of action and modes of control are still poorly understood. This is largely due to the fact that the control of ERBB activity is a multilayered process with significant differences between the various ERBB members. In contrast to other receptor tyrosine kinases, the kinase domain of EGFR (ERBB1) does not require phosphorylation for activation. Consequently, the overall activation state of the receptor is controlled by constant balancing of activity favoring and activity suppressing actions within the receptor molecule. Influences of the membrane microenvironment and context dependent interactions with varying sets of signaling partners are superimposed on this system of intramolecular checks and balances. We will discuss current models of the control of ERBB signaling with an emphasis on the multilayered nature of activation control and aspects that give rise to diversity between ERBB receptors.

Dynamic tradeoffs in the raft-mediated entry of human immunodeficiency virus type 1 into cells

To initiate an infection human immunodeficiency virus type 1 (HIV-1) particles must first bind to receptors on the surface of their host cells, a process that eventually leads to fusion of viral and cellular membranes and release of the viral genome into the cytoplasm. Understanding the molecular mechanisms of these processes may enable the development of new anti-HIV strategies. Disagreement currently prevails on the role in virus entry of microdomains within the cellular plasma membrane known as lipid rafts. Experiments have suggested that lipid rafts, in their interactions with cellular receptors and viral particles, either promote or have minimal effect on viral entry. Here we develop a dynamic model for HIV-1 entry that enables us to identify and quantitatively assess tradeoffs that can arise from the clustering of receptors in rafts. Specifically, receptor clustering can be detrimental to the initiation of viral infection by reducing the probability that a virus particle finds its primary receptor, CD4. However, receptor clustering can also enable a virus particle, once bound, to rapidly form multivalent interactions with receptors and co-receptors that are required for virus-cell membrane fusion. We show how the resolution of such tradeoffs hinges on the level and spatial distribution of receptors and co-receptors on the cell surface, and we discuss implications of these effects for the design of therapeutics that inhibit HIV-1 entry.

Inositol phospholipids localized to caveolae in rat heart are regulated by alpha1-adrenergic receptors and by ischemia-reperfusion

Postischemic reperfusion of rat or mouse hearts causes generation of inositol (1,4,5)trisphosphate [Ins(1,4,5)P3] and the initiation of arrhythmias. In the current study we investigated the possibility that the enhanced Ins(1,4,5)P3 generation in postischemic reperfusion was associated with an increased availability of the precursor lipid phosphatidylinositol(4,5)bisphosphate (PIP2) for alpha1-adrenergic receptor-activated phospholipase C (PLC). Isolated-perfused rat hearts were labeled with [3H]inositol and subjected to ischemia-reperfusion or stimulation with norepinephrine under normoxic conditions. Caveolar fractions were prepared by buoyant density sucrose gradient centrifugation. [3H]PIP2 was concentrated in caveolae, along with Galphaq and PLCbeta1b. Caveolae contained only 27.3 +/- 6.9% (means +/- SE, n = 6) of the total alpha1-adrenergic receptor complement of the heart. These did not migrate to PIP2-containing caveolar fractions with norepinephrine stimulation under normoxic conditions, even though caveolar PIP2 was depleted. In contrast, [3H]PIP2 in caveolae increased during 2 min of reperfusion, independently of norepinephrine release and thus of alpha1-adrenergic receptor activation. The increased PIP2 in the caveolar fractions where signaling proteins are concentrated may be critical for the heightened generation of Ins(1,4,5)P3 in early reperfusion.