Taste perception and coding in the periphery

Recent identification of taste receptors and their downstream signaling molecules, expressed in taste receptor cells, led to the understanding of taste coding in the periphery. Ion channels appear to mediate detection of salty and sour taste. The sensations of sweet, umami and bitter taste are initiated by the interaction of sapid molecules with the G-protein-coupled receptors T1Rs and T2Rs. Mice lacking either PLCβ2 or TRPM5 diminish behavioral and nerve responses to sweet, umami and bitter taste stimuli, suggesting that both receptor families converge on a common signaling pathway in the taste receptor cells. Nevertheless, separate populations of taste cells appear to be uniquely tuned to sweet, umami and bitter taste. Since PLCβ2-deficient mice still respond to sour and salty stimuli, sour and salty taste are perceived independent of bitter, umami and sweet taste. In this review, the recent characterization of the cellular mechanisms underlying taste reception and perception, and of taste coding in the periphery will be discussed. Received 5 March 2006; received after revision 2 May 2006; accepted 10 June 2006     

Influence of temperature on taste perception

Daily experience tells us that temperature has a strong influence on how we taste. Despite the longstanding interest of many specialists in this aspect of taste, we are only starting to understand the molecular mechanisms underlying the temperature dependence of different taste modalities. Recent research has led to the identification of some strong thermosensitive molecules in the taste transduction pathway. The cold activation of the epithelial Na⁺ channel and the heat activation of the taste variant of the vanilloid receptor (TRPV1t) may underlie the temperature dependence of salt responses. Heat activation of the transient receptor potential channel TRPM5 explains the enhancement of sweet taste perception by warm temperatures. Current development of methods to study taste cell physiology will help to determine the contribution of other temperature-sensitive events in the taste transduction pathways. Vice versa, the analysis of the thermodynamic properties of these events may assist to unveil the nature of several taste processes. Received 29 August 2006; received after revision 5 October 2006; accepted 20 November 2006     

G proteins as drug targets

The structure and function of heterotrimeric G protein subunits is known in considerable detail. Upon stimulation of a heptahelical receptor by the appropriate agonists, the cognate G proteins undergo a cycle of activation and deactivation; the α-subunits and the βγ-dimers interact sequentially with several reaction partners (receptor, guanine nucleotides and effectors as well as regulatory proteins) by exposing appropriate binding sites. For most of these domains, low molecular weight ligands have been identified that either activate or inhibit signal transduction. These ligands include short peptides derived from receptors, G protein subunits and effectors, mastoparan and related insect venoms, modified guanine nucleotides, suramin analogues and amphiphilic cations. Because compounds that act on G proteins may be endowed with new forms of selectivity, we propose that G protein subunits may therefore be considered as potential drug targets. Received 18 September 1998; received after revision 6 November 1998; accepted 11 November 1998     

Soluble proteins in insect chemical communication

Our understanding of the biochemical mechanisms that mediate chemoreception in insects has been greatly improved after the discovery of olfactory and taste receptor proteins. However, the presence of soluble polypeptides in high concentration around the dendrites of sensory neurons still poses unanswered questions. More than 2 decades after their discovery and despite the wealth of structural information available, the physiological function of odorant-binding proteins is not well understood. More recently, members of a second family of soluble polypeptides, the chemosensory proteins, were also discovered in the lymph of chemosensilla. Here we review the structural properties of both classes of soluble proteins, their affinity to small ligands, and their expression in the different parts of the insect body and subcellular localisation. Finally, we discuss current ideas and models of the role of such proteins in insect chemoreception. Received 27 December 2005; received after revision 6 March 2006; accepted 13 April 2006     

Molecular neurophysiology of taste in <Emphasis Type="Italic">Drosophila</Emphasis>

The recent identification of candidate receptor genes for sweet, umami and bitter taste in mammals has opened a door to elucidate the molecular and neuronal mechanisms of taste. Drosophila provides a suitable system to study the molecular, physiological and behavioral aspects of taste, as sophisticated molecular genetic techniques can be applied. A gene family for putative gustatory receptors has been found in the Drosophila genome. We discuss here current knowledge of the gustatory physiology of Drosophila. Taste cells in insects are primary sensory neurons whereupon each receptor neuron responds to either sugar, salt or water. We found that particular tarsal gustatory sensilla respond to bitter compounds. Electrophysiological studies indicate that gustatory sensilla on the labellum and tarsi are heterogeneous in terms of their taste sensitivity. Determination of the molecular bases for this heterogeneity could lead to an understanding of how the sensory information is processed in the brain and how this in turn is linked to behavior.Received 12 May 2003; received after revision 9 June 2003; accepted 13 June 2003     

Regulatory mechanisms involved in modulating RGS function

Regulator of G-Protein Signaling (RGS) refers to a conserved 120–125 amino acid motif that was first identified by its ability to negatively regulate G-Protein-Coupled Receptor (GPCR) signalling. Mechanistically, RGSs were found to regulate GPCR responses by binding to and stimulating the GTPase activity of the receptor-activated GTP-bound G α subunits. There are now over 25 mammalian RGSs containing proteins that are reported to carry out a variety of functions, many of which are unrelated to GPCR signalling. RGS proteins range in size from small proteins that contain little more than an RGS box to very large proteins that contain a variety of domains. The selectivity of function of the RGS proteins is attributable to the divergence of the RGS sequences as well as the presence of a variety of functional motifs, which allow them to interact with other proteins. Here we focus on the RGSs that are involved in modulating GPCR signalling by reviewing the diversity of the mechanisms involved in regulating these RGSs. Received 9 February 2006; received after revision 4 May 2006; accepted 22 May 2006     

Does the urokinase receptor exist in a latent form?

Multiple cellular functions of urokinase and its receptor are associated with the receptor’s capability to interact with a number of ligands at the molecular level. The presence of urokinase is generally needed for the urokinase receptor to acquire this capability. Recent X-ray studies of the structure of the urokinase receptor in complex with either its ligand or peptide inhibitors demonstrate the flexibility of the domain organization of the receptor, suggesting that unliganded urokinase receptor may exist in a latent form that has a conformation different from its ligand-binding form. Received 22 November 2006; received after revision 8 January 2007; accepted 7 February 2007     

Membrane functional organisation and dynamic of μ-opioid receptors

The activation and signalling activity of the membrane μ-opioid receptor (MOP-R) involve interactions among the receptor, G-proteins, effectors and many other membrane or cytosolic proteins. Decades of investigation have led to identification of the main biochemical processes, but the mechanisms governing the successive protein–protein interactions have yet to be established. We will need to unravel the dynamic membrane organisation of this complex and multifaceted molecular machinery if we are to understand these mechanisms. Here, we review and discuss advances in our understanding of the signalling mechanism of MOP-R resulting from biochemical or biophysical studies of the organisation of this receptor in the plasma membrane.     

Cellular uptake of amelogenin,and its localization to CD63, and Lamp1-positive vesicles

Proteins of the developing enamel matrix include amelogenin, ameloblastin and enamelin. Of these three proteins amelogenin predominates. Protein-protein interactions are likely to occur at the ameloblast Tomes’ processes between membrane-bound proteins and secreted enamel matrix proteins. Such protein-protein interactions could be associated with cell signaling or endocytosis. CD63 and Lamp1 are ubiquitously expressed, are lysosomal integral membrane proteins, and localize to the plasma membrane. CD63 and Lamp1 interact with amelogenin in vitro. In this study our objective was to study the molecular events of intercellular trafficking of an exogenous source of amelogenin, and related this movement to the spatiotemporal expression of CD63 and Lamp1 using various cell lineages. Exogenously added amelogenin moves rapidly into the cell into established Lamp1-positive vesicles that subsequently localize to the perinuclear region. These data indicate a possible mechanism by which amelogenin, or degraded amelogenin peptides, are removed from the extracellular matrix during enamel formation and maturation. Received 27 September 2006; received after revision 24 November 2006; accepted 5 December 2006     

Speed and consistency of human decisions to swallow or spit sweet and sour solutions.

Measurements of the frequency and speed of spitting or swallowing citric acid, sodium saccharin, or mixture solutions, using the taste of one of them as the definition of what was to be spit, revealed that 'correct' spits occurred on > or = 70% of trials with equal reliability and latency among the liquids, indicating that recognition-based rejection decisions in adult humans are as rapid and consistent for an arbitrary sweet taste as for a sour or mixed taste.     

Myelin basic protein: a multifunctional protein

Myelin basic protein (MBP), the second most abundant protein in central nervous system myelin, is responsible for adhesion of the cytosolic surfaces of multilayered compact myelin. A member of the ‘intrinsically disordered’ or conformationally adaptable protein family, it also appears to have several other functions. It can interact with a number of polyanionic proteins including actin, tubulin, Ca²⁺-calmodulin, and clathrin, and negatively charged lipids, and acquires structure on binding to them. It may act as a membrane actin-binding protein, which might allow it to participate in transmission of extracellular signals to the cytoskeleton in oligodendrocytes and tight junctions in myelin. Some size isoforms of MBP are transported into the nucleus and thus they may also bind polynucleotides. Extracellular signals received by myelin or cultured oligodendrocytes cause changes in phosphorylation of MBP, suggesting that MBP is also involved in signaling. Further study of this very abundant protein will reveal how it is utilized by the oligodendrocyte and myelin for different purposes. Received 2 March 2006; received after revision 12 April 2006; accepted 16 May 2006     

Hearing molecules: contributions from genetic deafness

Considerable progress has been made over the past decade identifying many genes associated with deafness. With the identification of these hereditary deafness genes and the proteins they encode, molecular elements of basic hearing mechanisms emerge. As functional studies of these molecular elements become available, we can put together the pieces of the puzzle and begin to reach an understanding of the molecular mechanisms of hearing. The goal of this review is to discuss studies over the past decade that address the function of the proteins implicated in genetic deafness and to place them in the context of basic molecular mechanisms in hearing. The first part of this review highlights structural and functional features of the cochlea and auditory nerve. This background will provide a context for the second part, which addresses the molecular mechanisms underlying cochlear function as elucidated by genetic causes of deafness. Received 20 September 2006; received after revision 24 October 2006; accepted 5 December 2006     

Speed and consistency of human decisions to swallow or spit sweet and sour solutions

Measurements of the frequency and speed of spitting or swallowing citric acid, sodium saccharin, or mixture solutions, using the taste of one of them as the definition of what was to be spit, revealed that ‘correct’ spits occurred on ≥70% of trials with equal reliability and latency among the liquids, indicating that recognition-based rejection decisions in adult humans are as rapid and consistent for an arbitrary sweet taste as for a sour or mixed taste.     

nDsbD: a redox interaction hub in the Escherichia coli periplasm

DsbD is a redox-active protein of the inner Escherichia coli membrane possessing an N-terminal (nDsbD) and a C-terminal (cDsbD) periplasmic domain. nDsbD interacts with four different redox proteins involved in the periplasmic disulfide isomerization and in the cytochrome c maturation systems. We review here the studies that led to the structural characterization of all soluble DsbD domains involved and, most importantly, of trapped disulfide intermediate complexes of nDsbD with three of its four redox partners. These results revealed the structural features enabling nDsbD, a ‘redox hub’ with an immunoglobulin-like fold, to interact efficiently with its different thioredoxin-like partners. Received 3 February 2006; received after revision 1 March 2006; accepted 5 April 2006     

The small heat shock proteins and their clients

Small heat shock proteins are ubiquitous proteins found throughout all kingdoms. One of the most notable features is their large oligomeric structures with conserved structural organization. It is well documented that small heat shock proteins can capture unfolding proteins to form stable complexes and prevent their irreversible aggregation. In addition, small heat shock proteins coaggregate with aggregation-prone proteins for subsequent, efficient disaggregation of the protein aggregates. The release of substrate proteins from the transient reservoirs, i.e. complexes and aggregates with small heat shock proteins, and their refolding require cooperation with ATP-dependent chaperone systems. The amphitropic small heat shock proteins were shown to associate with membranes, although they do not contain transmembrane domains or signal sequences. Recent studies indicate that small heat shock proteins play an important role in membrane quality control and thereby potentially contribute to the maintenance of membrane integrity especially under stress conditions. Received 11 July 2006; received after revision 4 October 2006; accepted 10 November 2006     

Inflammasomes: current understanding and open questions

The innate immune system relies on its capability to detect invading microbes, tissue damage, or stress via evolutionarily conserved receptors. The nucleotide-binding domain leucine-rich repeat (NLR)-containing family of pattern recognition receptors includes several proteins that drive inflammation in response to a wide variety of molecular patterns. In particular, the NLRs that participate in the formation of a molecular scaffold termed the “inflammasome” have been intensively studied in past years. Inflammasome activation by multiple types of tissue damage or by pathogen-associated signatures results in the autocatalytic cleavage of caspase-1 and ultimately leads to the processing and thus secretion of pro-inflammatory cytokines, most importantly interleukin (IL)-1β and IL-18. Here, we review the current knowledge of mechanisms leading to the activation of inflammasomes. In particular, we focus on the controversial molecular mechanisms that regulate NLRP3 signaling and highlight recent advancements in DNA sensing by the inflammasome receptor AIM2.     

Endocannabinoids in adipocytes during differentiation and their role in glucose uptake

The molecular basis for the control of energy balance by the endocannabinoid anandamide (AEA) is still unclear. Here, we show that murine 3T3-L1 fibroblasts have the machinery to bind, synthesize and degrade AEA, and that their differentiation into adipocytes increases by approximately twofold the binding efficiency of cannabinoid receptors (CBR), and by approximately twofold and approximately threefold, respectively, the catalytic efficiency of the AEA transporter and AEA hydrolase. In contrast, the activity of the AEA synthetase and the binding efficiency of vanilloid receptor were not affected by the differentiation process. In addition, we demonstrate that AEA increases by approximately twofold insulin-stimulated glucose uptake in differentiated adipocytes, according to a CB1R-dependent mechanism that involves nitric oxide synthase, but not lipoxygenase or cyclooxygenase. We also show that AEA binding to peroxisome proliferator-activated receptor-γ, known to induce differentiation of 3T3-L1 fibroblasts into adipocytes, is not involved in the stimulation of glucose uptake. Received 11 October 2006; received after revision 9 November 2006; accepted 28 November 2006 V.Gasperi and F. Fezza equally contributed to the study.     

Giant axonal neuropathy

Giant axonal neuropathy (GAN) is a rare autosomal recessive disorder affecting both the central and peripheral nervous systems. Cytopathologically, the disorder is characterized by giant axons with derangements of cytoskeletal components. Geneticists refined the chromosomal interval containing the locus, culminating in the cloning of the defective gene, GAN. To date, many distinct mutations scattered throughout the coding region of the locus have been reported by researchers from different groups around the world. GAN encodes the protein, gigaxonin. Recently, a genetic mouse model of the disease was generated by targeted disruption of the locus. Over the years, the molecular mechanisms underlying GAN have attracted much interest. Studies have revealed that gigaxonin appears to play an important role in cytoskeletal functions and dynamics by directing ubiquitin-mediated degradations of cytoskeletal proteins. Aberrant accumulations of cytoskeletal-associated proteins caused by a defect in the ubiquitinproteasome system (UPS) have been shown to be responsible for neurodegeneration occurring in GAN-null neurons, providing strong support for the notion that UPS plays crucial roles in cytoskeletal functions and dynamics. However, many key questions about the disease remain unanswered. Received 6 September 2006; received after revision 11 October 2006; accepted 5 December 2006 Y. Yang, E. Allen The authors contributed equally to this work.     

BH3-only proteins in tumorigenesis and malignant melanoma

BH3-only proteins are a subset of the Bcl-2 family of apoptotic regulators. BH3-only proteins function as ‘damage sensors’ in the cell; they are activated in response to cellular stress or DNA damage, whereupon they initiate apoptosis. Apoptosis is the primary mechanism by which the body rids itself of genetically defective cells and is critical for preventing the accumulation of cells with tumorigenic potential. Therefore, dysregulation of BH3-only proteins may promote tumorigenesis. Furthermore, functional apoptosis pathways are required for the success of most cancer treatments, including chemotherapy. Resistance to chemotherapy, as seen with malignant melanoma, often reflects an inability of tumor cells to undergo apoptosis. By deciphering the roles of BH3-only proteins in tumorigenesis, we may learn how to manipulate cell death pathways to overcome apoptotic resistance. This review summarizes the current knowledge of BH3-only proteins and how they contribute to tumorigenesis, with particular attention given to studies involving melanoma. Received: 12 August 2006; received after revision: 2 October 2006; accepted 13 November 2006     

A VEGF-A splice variant defective for heparan sulfate and neuropilin-1 binding shows attenuated signaling through VEGFR-2

The development of functional blood and lymphatic vessels requires spatio-temporal coordination of the production and release of growth factors such as vascular endothelial growth factors (VEGFs). VEGF family proteins are produced in multiple isoforms with distinct biological properties and bind to three types of VEGF receptors. A VEGF-A splice variant, VEGF-A₁₆₅b, has recently been isolated from kidney epithelial cells. This variant is identical to VEGF-A₁₆₅ except for the last six amino acids encoded by an alternative exon. VEGF-A₁₆₅b and VEGF-A₁₆₅ bind VEGF receptors 1 and 2 with similar affinity. VEGF-A₁₆₅b elicits drastically reduced activity in angiogenesis assays and even counteracts signaling by VEGF-A₁₆₅. VEGF-A₁₆₅b weakly binds to heparan sulfate and does not interact with neuropilin-1, a coreceptor for VEGF receptor 2. To determine the molecular basis for altered signaling by VEGF-A₁₆₅b we measured VEGF receptor 2 and ERK kinase activity in endothelial cells in culture. VEGF-A₁₆₅ induced strong and sustained activation of VEGF receptor 2 and ERK-1 and −2, while activation by VEGF-A₁₆₅b was only weak and transient. Taken together these data show that VEGF-A₁₆₅b has attenuated signaling potential through VEGF receptor 2 defining this new member of the VEGF family as a partial receptor agonist. Received 31 May 2006; received after revision 26 June 2006; accepted 14 July 2006