Albinism and immunity: what's the link?

A small number of inherited diseases show a combination of immunological and pigmentation defects. Chediak-Higashi, Griscellis and Hermansky-Pudlak syndromes are all autosomal recessive diseases with these characteristics. Recent advances in both the identification of the genes giving rise to these diseases and the cell biology of immune cells and melanocytes have begun to reveal the molecular links between immunodeficiencies and albinism. These studies identify key proteins, such as Rab27a, which are critical for secretion of specialised granules found in melanocytes and immune cells. The granules of these cells are modified lysosomes termed 'secretory lysosomes'. These studies reveal that secretory lysosomes use specialised mechanisms of secretion, not found in other cell types, which explains the selective defects in these diseases.     

Actin and microtubules in neurite initiation: are MAPs the missing link?

During neurite initiation microtubules align to form a tight bundle and actin filaments reorganize to produce a growth cone. The mechanisms that underlie these highly coordinated cytoskeletal rearrangements are not yet fully understood. Recently, various levels of coordination between the actin- and microtubule-based cytoskeletons have been observed during cellular migration and morphogenesis, processes that share some similarities to neurite initiation. Direct, physical association between both cytoskeletons has been suggested, because microtubules often preferentially grow along actin bundles and transiently target actin-rich adhesion complexes. We propose that such physical association might be involved in force-based interactions and spatial organization of the two networks during neurite initiation as well. In addition, many signaling cascades that affect actin filaments are also involved in the regulation of microtubule dynamics, and vice versa. Although several candidates for mediating these effects have been identified in non-neuronal cells, the general mechanism is still poorly understood. In neurons certain plakins and neuron-specific microtubule associated proteins (MAPs), like MAP1B and MAP2, which can bind to both microtubules and F-actin, are promising candidates to play key roles in the specific cytoskeletal rearrangements controlling the transition from an undifferentiated state to neurite-bearing morphology. Here we review the effects of MAPs on microtubules and actin, as well as the coordination of both cytoskeletons during neurite initiation.     

Visfatin: the missing link between intra-abdominal obesity and diabetes?

Human obesity-related diabetes and the accompanying metabolic disorders have been specifically linked to increased visceral adipose tissue mass. Understanding the differences in biology of the two human fat depots (visceral and subcutaneous) might hold the key to therapeutic strategies aimed at reducing obesity-induced insulin resistance and alleviating symptoms of the metabolic syndrome. Visfatin (pre-B-cell colony-enhancing factor, PBEF) is a novel adipokine that appears to be preferentially produced by visceral adipose tissue and has insulin-mimetic actions. Could this molecule hold the key to future treatments for type 1 and 2 diabetes? This article discusses the pros and cons of visfatin action and how it might affect future therapeutic strategies.     

Calcium influx: is Homer the missing link?

Calcium signals in cells can arise via release from intracellular stores or influx across the plasma membrane. Recent studies have shed new light on the multi-protein signalling complexes that mediate communication between calcium stores and plasma membrane calcium channels.     

Role of prions in neuroprotection and neurodegeneration: A mechanism involving glutamate receptors?

There is increasing evidence that cellular prion protein plays important roles in neurodegeneration and neuroprotection. One of the possible mechanism by which this may occur is a functional inhibition of ionotropic glutamate receptors, including N-Methyl-D-Aspartate (NMDA) receptors. Here we review recent evidence implicating a possible interplay between NMDA receptors and prions in the context of neurodegenerative disorders. Such is a functional link between NMDA receptors and normal prion protein, and therefore possibly between these receptors and pathological prion isoforms, raises interesting therapeutic possibilities for prion diseases.Key words: NMDA, NR2D, glutamate, neuroprotection, calciumPrions are most often discussed in the context of transmissible spongiform encephalopathies (TSEs) which encompass a range of neurological disorders that include human Creutzfeldt-Jakob disease (among others), sheep scrapie and bovine spongiform encephalopathy.^1,2 It is well established that these disorders arise from a progressive conversion of the normal, mainly helical form of cellular prion protein (PrP^C) into a different PrP^Sc protein conformation with a high beta sheet content.³ In their PrP^Sc form, prions act as templates that catalyze misfolding of PrP^C to produce increasing levels of PrP^Sc, which likely represents several or even many different conformational states of the same source protein, resulting in diverse clinical phenotypes. This in turn leads to accumulation of PrP^Sc deposits in the brain that can appear as aggregates and amyloid-like plaques⁴ and which disrupt normal neurophysiology.⁵ While the neuropathology of TSE''s has been explored in great detail dating back to the 1920s,⁶ less effort has perhaps been expended on understanding the cellular and physiological function of PrP^C which is ubiquitously expressed, and found even in simple organisms.^5,7,8 A number of mouse lines either lacking PrP^C or overexpressing PrP^C have been created, including the widely used Zurich I PrP^C knockout strain.^9,10 Despite the wide distribution of PrP^C in the mammalian CNS, it perhaps surprisingly has only a relatively mild behavioral phenotype that appears to include some deficits in spatial learning at the behavioral level^11,12 as well as alterations in long term potentiation at the cellular level.^13–17 In addition, it has been shown that these mice show an increased excitability of hippocampal neurons.^13,18–20 In contrast, deletion of certain parts of the PrP^C protein in vivo can have serious physiological consequences. For example, deletion of a stretch of amino acids between just upstream of the octarepeat copper binding motifs produces a lethal phenotype, that can be rescued by overexpression of increasing levels of normal PrP^C.^21,22 Of particular note, these deletion mutants show degeneration of axons and myelin, both in the CNS and in peripheral nerves; indeed some mutants show a predilection for axomyelinic degeneration with little neuronal pathology,²¹ suggesting that certain mutated forms of PrP have a direct toxic effect on oligodendrocytes and/or myelin.²³ Moreover, activation of the Dpl1 gene in mice lacking PrP^C leads to an ataxic phenotype, that is not observed in the presence of PrP^C.²⁴ Collectively, this indicates that PrP^C may act in a protective capacity and in contrast, certain abnormal forms of PrP are “toxic”, promoting much more injury to various elements of the CNS and PNS than outright absence of wild-type PrP^C.This notion is further corroborated by a number of studies in PrP^C knockout mice, both in vivo and in cell culture models. Cultured hippocampal neurons from PrP^C null mice display greater apoptosis during oxidative stress.²⁵ Moreover, overexpression of PrP^C in rats protects them from neuronal damage during ischemic stroke, whereas PrP^C null mice show greater damage.^27–29 When PrP^C null mice are subjected to different types of seizure paradigms, they showed increased mortality and increased numbers of seizures.³⁰ This increased neuronal damage can be diminished by the NMDA receptor blocker MK-801,³¹ potentially implicating glutamate receptors in this process. Finally, it was recently shown that the absence of PrP^C protein protects neurons from the deleterious effects of beta amyloid, a protein involved in Alzheimer disease.³² It is important to note that NMDA receptors have been implicated in seizure disorders and in cell death during ischemic stroke.^33–35 Indeed, our own work has shown that NMDA receptors expressed endogenously in myelin contribute to myelin damage and may be one of the first steps leading to demyelination.³⁶ Furthermore, the NMDA receptor blocker memantine is used to treat Alzheimer disease, implicating NMDA receptors. The observations above suggest that there may be an interplay between NMDA receptor activity and the physiological function of PrP^C. In support of this hypothesis, our recent work has directly identified a common functional and molecular link between NMDA receptors and PrP^C.³⁷ Brain slices obtained from Zurich I PrP^C null mice showed an increased excitability of hippocampal slices, which could be ablated by blocking NMDA receptor activity with amino-5-phosphonovaleric acid. Removal of extracellular magnesium ions to enhance NMDA receptor activity resulted in stronger pro-excitatory effects in slices and cultured neurons from PrP^C null mice compared with those from normal animals. Synaptic recordings indicate that the amplitude and duration of NMDA mediated miniature synaptic currents is increased in PrP^C null mouse neurons, and evoked NMDA receptor currents show a dramatic slowing of deactivation kinetics in PrP^C null mouse neurons. The NMDA current kinetics observed in these neurons were qualitatively consistent with NMDA receptors containing the NR2D subunit.³⁸ Consistent with a possible involvement of NR2D containing receptors, siRNA knockdown of NR2D normalized current kinetics in PrP-null mouse neurons. Furthermore, a selective co-immunoprecipitation between PrP^C and the NR2D, but not NR2B subunits, was observed. This then may suggest the possibility that under normal circumstances, PrP^C serves to suppress NR2D function, but when PrP^C is absent, NR2D containing receptors become active, and because of their slow kinetics, may contribute to calcium overload under circumstances where excessive (or even normal) levels of glutamate are present. This would include conditions such as epileptic seizures, ischemia and Alzheimer disease, thus providing a possible molecular explanation for the link between PrP^C and neuroprotection under pathophysiological conditions. Indeed, NMDA promoted greater toxicity in PrP^C null mouse neurons, and upon injection into brains of PrP^C null mice. It is interesting to note that one of the major NMDA receptor subtypes expressed in myelin is NR2D, thus bridging the observations of Micu et al.³⁶ of NMDA receptor mediated cell death in ischemic white matter, and those of Baumann and colleagues²¹ showing that PrP^C deletion mutants can cause damage to myelin.How might PrP^C deletion mutants affect neuronal survival? One possibility may be that these deletion mutants compete with normal PrP^C for NMDA receptors, but are unable to functionally inhibit them. Alternatively, it is possible that the PrP^C deletion mutants, by virtue of binding to the receptors, may in fact increase receptor activity, thus causing increased cell death. In both cases, increasing the expression of normal PrP^C would be expected to outcompete the deletion variants, thus reestablishing the protective function. A similar mechanism could perhaps apply to TSEs. It is possible that the PrP^Sc form, perhaps in a manner reminiscent of the PrP^C deletion mutants, may be unable to inhibit NMDAR function, or perhaps would even enhance it. Any excess glutamate that may be released as a result of cell damage due to PrP^Sc aggregates, or even normally released amounts glutamate during the course of physiological neuronal signaling, could be sufficient to cause NMDAR mediated cell death and neuronal degeneration. In this context, it is interesting to note that chronic administration of the weakly NR2D selective inhibitor memantine delays death as a consequence of scrapie infection in mice.³⁹ In the context of Alzheimer disease, binding of PrP^C to beta amyloid may prevent the inhibitory action of PrP^C on NMDA receptor function, thus increasing NMDA receptor activity and promoting cell death. This then may perhaps explain the beneficial effects of memantine in the treatment of Alzheimer disease.In summary, despite the fact that PrP^C is one of the most abundantly expressed proteins in the mammalian CNS, its physiological role is uncertain. Recent observations from our labs have established an unequivocal functional link between normal prion protein and the ubiquitous excitatory NMDA receptor. Thus, one of the key physiological roles of PrP^C may be regulation of NMDA receptor activity. The presence of abnormal species of prion protein, whether acquired via “infection”, spontaneous conformational conversion or genetically inherited, may in turn alter normal function and regulation of NMDA receptors, leading to chronic “cytodegeneration” of elements in both gray and white matter regions of the CNS. This key functional link between PrP and glutamate receptors may provide our first opportunity for rational therapeutic design against the devastating spongiform encephalopathies and potentially other neurodegenerative disorders not traditionally considered as TSE''s.     

G-quadruplexes within prion mRNA: the missing link in prion disease?

Cellular ribonucleic acid (RNA) plays a crucial role in the initial conversion of cellular prion protein PrP^C to infectious PrP^Sc or scrapie. The nature of this RNA remains elusive. Previously, RNA aptamers against PrP^C have been isolated and found to form G-quadruplexes (G4s). PrP^C binding to G4 RNAs destabilizes its structure and is thought to trigger its conversion to PrP^Sc. Here it is shown that PrP messenger RNA (mRNA) itself contains several G4 motifs, located in the octarepeat region. Investigation of the RNA structure in one of these repeats by circular dichroism, nuclear magnetic resonance and ultraviolet melting studies shows evidence of G4 formation. In vitro translation of full-length PrP mRNA, naturally harboring five consecutive G4 motifs, was specifically affected by G4-binding ligands, lending support to G4 formation in PrP mRNA. A possible role of PrP binding to its own mRNA and the role of anti-prion drugs, many of which are G4-binding ligands, in prion disease are discussed.     

Hyperinsulinemia: the missing link among oxidative stress and age-related diseases?

Mounting evidence supports Harman’s hypothesis that aging is caused by free radicals and oxidative stress. Although it is known that oxidant species are produced during metabolic reactions, it is largely unknown which factor(s), of physiological or pathophysiological significance, modulate their production in vivo. In this hypothesis paper, it is postulated that hyperinsulinemia may have such function and therefore promote aging, independently of elevations of glycemia. Hyperinsulinemia is secondary to impaired insulin stimulated glucose metabolism at the level of skeletal muscle (insulin resistance) and is seen in about one third of glucose tolerant humans following dietary carbohydrate intake. If other insulin-stimulated (or inhibited) pathways retain normal sensitivity to the hormone, hyperinsulinemia could, by its effects on antioxidative enzymes and on free radical generators, enhance oxidative stress. Other proaging effects of insulin involve the inhibition of proteasome and the stimulation of polyunsaturated fatty acid (PUFA) synthesis and of nitric oxide (NO). The hypothesis that hyperinsulinemia accelerates aging also offers a metabolic explanation for the life-prolonging effect of calorie restriction and of mutations decreasing the overall activity of insulin-like receptors in the nematode Caenorhabditis elegans.     

The search for the missing link: a relic plastid in Perkinsus?

Perkinsus marinus (Phylum Perkinsozoa) is a protozoan parasite that has devastated natural and farmed oyster populations in the USA, significantly affecting the shellfish industry and the estuarine environment. The other two genera in the phylum, Parvilucifera and Rastrimonas, are parasites of microeukaryotes. The Perkinsozoa occupies a key position at the base of the dinoflagellate branch, close to its divergence from the Apicomplexa, a clade that includes parasitic protista, many harbouring a relic plastid. Thus, as a taxon that has also evolved toward parasitism, the Perkinsozoa has attracted the attention of biologists interested in the evolution of this organelle, both in its ultrastructure and the conservation, loss or transfer of its genes. A review of the recent literature reveals mounting evidence in support of the presence of a relic plastid in P. marinus, including the presence of multimembrane structures, characteristic metabolic pathways and proteins with a bipartite N-terminal extension. Further, these findings raise intriguing questions regarding the potential functions and unique adaptation of the putative plastid and/or plastid genes in the Perkinsozoa. In this review we analyse the above-mentioned evidence and evaluate the potential future directions and expected benefits of addressing such questions. Given the rapidly expanding molecular/genetic resources and methodological toolbox for Perkinsus spp., these organisms should complement the currently established models for investigating plastid evolution within the Chromalveolata.     

Toll-like receptor signalling in liver disease: ER stress the missing link?

Toll-like receptors induce a complex inflammatory response that can function to alert the body to infection, neutralize pathogens and repair damaged tissues. Toll-like receptors are expressed on kupffer, endothelial, dendritic, biliary epithelial, hepatic stellate cells, and hepatocytes in the liver. The endoplasmic reticulum (ER) is a central organelle of eukaryotic cells that exists as a place of lipid synthesis, protein folding and protein maturation. The ER is a major signal transduction organelle that senses and responds to changes in homeostasis. Conditions interfering with the function of the ER are collectively known as ER stress and can be induced by accumulation of unfolded protein aggregates or by excessive protein traffic as can occur during viral infection. The ability of ER stress to induce an inflammatory response is considered to play a role in disease pathogenesis. Importantly, ER stress is viewed as a contributor to the pathogenesis of liver diseases with evidence linking components of ER homeostasis as requirements for optimal Toll-like receptor function. In this context this review discusses the association of Toll-like receptors with ER stress. This is an emerging paradigm in the understanding of Toll-like receptor signalling which may have an underlying role in the pathogenesis of liver disease.     

Swallowing dynein: a missing link in RNA localization?

Localization of bicoid messenger RNA to the anterior cortex of the developing oocyte is essential for correct anterior-posterior patterning of the Drosophila embryo. It now seems that the Swallow protein functions as an adaptor, bridging bicoid mRNA to dynein, a molecular motor that would transport the complex anteriorly along microtubules.     

Cell polarity: The missing link in skeletal morphogenesis?

Despite extensive genetic analysis of the dynamic multi-phase process that transforms a small population of lateral plate mesoderm into the mature limb skeleton, the mechanisms by which signaling pathways regulate cellular behaviors to generate morphogenetic forces are not known. Recently, a series of papers have offered the intriguing possibility that regulated cell polarity fine-tunes the morphogenetic process via orienting cell axes, division planes and cell movements. Wnt5a-mediated non-canonical signaling, which may include planar cell polarity, has emerged as a common thread in the otherwise distinct signaling networks that regulate morphogenesis in each phase of limb development. These findings position the limb as a key model to elucidate how global tissue patterning pathways direct local differences in cell behavior that, in turn, generate growth and form.