Location matters: the endoplasmic reticulum and protein trafficking in dendrites

Neurons are highly polarized, but the trafficking mechanisms that operate in these cells and the topological organization of their secretory organelles are still poorly understood. Particularly incipient is our knowledge of the role of the neuronal endoplasmic reticulum. Here we review the current understanding of the endoplasmic reticulum in neurons, its structure, composition, dendritic distribution and dynamics. We also focus on the trafficking of proteins through the dendritic endoplasmic reticulum, emphasizing the relevance of transport, retention, assembly of multi-subunit protein complexes and export. We additionally discuss the roles of the dendritic endoplasmic reticulum in synaptic plasticity.

The physiological role of the unfolded protein response in plants

Unfolded protein response (UPR) is a signaling mechanism activated by misfolded protein accumulation in the endoplasmic reticulum. It is a widespread process that has been described in organisms ranging from yeasts to mammals. In recent years, our understanding of UPR signaling pathway in plants has advanced. Two transcription factors from Arabidopsis thaliana have been reported to function as the sensor/ transducer of this response (AtbZIP60 and AtbZIP28). They seem to be involved in both heat and biotic stress. Furthermore, overexpression of one of them (AtbZIP60) produces plants with a higher tolerance for salt stress, suggesting that this transcription factor may play a role in abiotic stress. Furthermore, some data suggest that crosstalk between genes involved in abiotic stress and UPR may also exist in plants. On the other hand, UPR is related to programmed cell death (PCD) in plants given that that triggering UPR results in induction of PCD-related genes. This article reviews the latest progress in understanding UPR signaling in plants and analyzes its relationship to key processes in plant physiology.

Aspectos fisiológicos do cálcio e sua disfunção em patologias neurodegenerativas / Physiologic aspects of calcium and your dysfunction in neurodegenerative pathologies

O íon cálcio funciona como um segundo mensageiro que regula um amplo espectro de processos celulares. A diminuição ou perda do controle dos mecanismos que regulam a concentração intracelular desse íon está associada, respectivamente, ao envelhecimento dos neurônios e a doenças neurodegenerativas. A gênese dessas modificações é desconhecida. Entretanto, estudos recentes apontam para uma provável correlação entre expressão gênica alterada, estresse do retículo endoplasmático e os processos patológicos associados à disfunção na concentração intracelular do cálcio. O esclarecimento dessas questões poderá trazer novos alvos terapêuticos capazes de frear ou reverter tais alterações, combatendo, dessa forma, tanto o envelhecimento neuronal quanto as doenças neurodegenerativas.

Calcium is a second messenger that regulates a lot of cellular functions. The following mechanisms regulate the intracellular concentrations of the ion: influx, release, extrusion and storage. Decrease or loss in control of these mechanisms is related to aging of neurons and neurodegenerative diseases, respectively. The genesis of these alterations is unknown. However, recent studies point to a correlation between calcium dysfunction and altered gene expression. There is also a correlation between endoplasmic reticulum stress and pathological processes. Further investigations may reveal new therapeutical targets that can block or revert these alterations.

Mediators and mechanisms of pancreatic beta-cell death in type 1 diabetes

Type 1 diabetes mellitus (T1D) is characterized by severe insulin deficiency resulting from chronic and progressive destruction of pancreatic beta-cells by the immune system. The triggering of autoimmunity against the beta-cells is probably caused by environmental agent(s) acting in the context of a predisposing genetic background. Once activated, the immune cells invade the islets and mediate their deleterious effects on beta-cells via mechanisms such as Fas/FasL, perforin/granzyme, reactive oxygen and nitrogen species and pro-inflammatory cytokines. Binding of cytokines to their receptors on the beta-cells activates MAP-kinases and the transcription factors STAT-1 and NFkappa-B, provoking functional impairment, endoplasmic reticulum stress and ultimately apoptosis. This review discusses the potential mediators and mechanisms leading to beta-cell destruction in T1D.

O diabetes melito tipo 1 (DM1) tem como característica uma grave deficiência de insulina que resulta da destruição da célula-beta, crônica e progressiva, pelo sistema imune. O desencadeamento da autoimunidade contra a célula-beta é causado, provavelmente, por agentes ambientais que atuam quando existe predisposição genética. Uma vez ativadas, células imunes invadem as ilhotas, e os efeitos deletérios sobre as células-beta são mediados por mecanismos relacionados a Fas/FasL, perforina/granzima, espécies reativas de oxigênio e nitrogênio, e a citocinas pró-inflamatórias. A ligação de citocinas a seus receptores na célula-beta ativa MAP-quinase e fatores de transcrição STAT-1 e NFkapaB, provocando prejuízo funcional, estresse de retículo endoplasmático e, por fim, apoptose. Esta revisão discute os mecanismos e os mediadores potenciais que levam à destruição da célula-beta no DM1.

Endoplasmic reticulum (ER) stress & diabetes.

The endoplasmic reticulum (ER) is a central organelle entrusted with lipid synthesis, protein folding and protein maturation. It is endowed with a quality control system that facilitates the recognition and targeting of aberrant proteins for degradation. When the capacity of this quality control system is exceeded, a stress response (ER stress) is switched on. Prolonged stress leads to apoptosis and may thus be an important factor in the pathogenesis of many diseases. A complex homeostatic signaling pathway, known as the unfolded protein response (UPR), has evolved to maintain a balance between the load of newly synthesized proteins and the capacity of the ER to aid in their maturation. Dysfunction of the UPR plays an important role in certain diseases, especially those involving tissues dedicated to extracellular protein synthesis. Diabetes is an example of such a disease, since pancreatic beta-cells depend on efficient UPR signaling to meet the demands for constantly varying levels of insulin synthesis. Recent studies have indicated that the importance of the UPR in diabetes is not restricted to the beta-cell but also to tissues of peripheral insulin resistance such as liver and adipose tissue. Better understanding of the basic mechanisms of ER stress and development of insulin resistance/type 2 diabetes is pivotal for the identification of newer molecular targets for therapeutic interventions.

Evaluation of the role of the Endoplasmic Reticulum-Golgi transit in the biogenesis of peroxisomal membrane proteins in wild type and peroxisome biogenesis mutant CHO cells

Peroxisomes are thought to be formed by division of pre-existing peroxisomes after the import of newly synthesized proteins. However, it has been recently suggested that the endoplasmic reticulum (ER) provides an alternative de novo mechanism for peroxisome biogenesis in some cells. To test a possible role of the ER-Golgi transit in peroxisome biogenesis in mammalian cells, we evaluated the biogenesis of three peroxisomal membrane proteins (PMPs): ALDRP (adrenoleukodystrophy related protein), PMP70 and Pex3p in CHO cells. We constructed chimeric genes encoding these PMPs and green fluorescent protein (GFP), and transiently transfected them to wild type and mutant CHO cells, in which normal peroxisomes were replaced by peroxisomal membrane ghosts. The expressed proteins were targeted to peroxisomes and peroxisomal ghosts correctly in the presence or absence of Brefeldin A (BFA), a drug known to block the ER-Golgi transit. Furthermore, low temperature did not disturb the targeting of Pex3p-GFP to peroxisomes. We also constructed two chimeric proteins of PMPs containing an ER retention signal "DEKKMP": GFP-ALDRP-DEKKMP and myc- Pex3p-DEKKMP. These proteins were mostly targeted to peroxisomes. No colocalization with an ER maker was found. These results suggest that the classical ER-Golgi pathway does not play a major role in the biogenesis of mammalian PMPs.

Interplay between ER Ca2+ uptake and release fluxes in neurons and its impact on [Ca2+] dynamics

In neurons, depolarizing stimuli open voltage-gated Ca2+ channels, leading to Ca2+ entry and a rise in the cytoplasmic free Ca2+ concentration ([Ca2+]i). While such [Ca2+]i elevations are initiated by Ca2+ entry, they are also influenced by Ca2+ transporting organelles such as mitochondria and the endoplasmic reticulum (ER). This review summarizes contributions from the ER to depolarization-evoked [Ca2+]i responses in sympathetic neurons. As in other neurons, ER Ca2+ uptake depends on SERCAs, while passive Ca2+ release depends on ryanodine receptors (RyRs). RyRs are Ca2+ permeable channels that open in response to increases in [Ca2+]i, thereby permitting [Ca2+]i elevations to trigger Ca2+ release through Ca2+-induced Ca2+ release (CICR). However, whether this leads to net Ca2+ release from the ER critically depends upon the relative rates of Ca2+ uptake and release. We found that when RyRs are sensitized with caffeine, small evoked [Ca2+]i elevations do trigger net Ca2+ release, but in the absence of caffeine, net Ca2+ uptake occurs, indicating that Ca2+ uptake is stronger than Ca2+ release under these conditions. Nevertheless, by increasing ER Ca2+ permeability, RyRs reduce the strength of Ca2+ buffering by the ER in a [Ca2+]I-dependent manner, providing a novel mechanism for [Ca2+]i response acceleration. Analysis of the underlying Ca2+ fluxes provides an explanation of this and two other modes of CICR that are revealed as [Ca2+]i elevations become progressively larger.