Biomedical subjects

Ivan Quesada

Publications and source records attributed to Ivan Quesada.

8 recordsLinked to original sources

PubMed2004

Secretion in unicellular marine phytoplankton: demonstration of regulated exocytosis in Phaeocystis globosa.

Almost half of the global photosynthetic activity is carried out in the ocean. During blooms, Phaeocystis can fix CO(2) at rates up to 40 g C m(-2) month(-1). Most of this carbon is released as polysaccharides. However, the cellular mechanism whereby this huge amount of organic material is exported into the seawater remains unknown. A vaguely defined process of "exudation" is believed responsible for the release of these biopolymers. Here we report the first demonstration that Phaeocystis globosa does not "exude", but secretes microscopic gels. Secretion is stimulated by blue light (lambda = 470+/-20 nm), and it is transduced by a characteristic intracellular Ca(2+) signal that precedes degranulation. The polysaccharides that form the matrix of these gels remain in condensed phase while stored in secretory vesicles. Upon exocytosis, the exopolymer matrix undergoes a characteristic phase transition accompanied by extensive swelling resulting in the formation of microscopic hydrated gels. Owing to their tangled topology, once released into the seawater, the polymers that make these gels can reptate (axially diffuse), interpenetrate neighboring gels, and anneal them together forming massive mucilage accumulations that are characteristic of Phaeocystis blooms. These gel masses can supply a rich source of microbial substrates, disperse in the seawater, and/or eventually sediment to the ocean floor.

Adhesives↗

PubMed2004

Novel players in pancreatic islet signaling: from membrane receptors to nuclear channels.

Glucose and other nutrients regulate many aspects of pancreatic islet physiology. This includes not only insulin release, but also insulin synthesis and storage and other aspects of beta-cell biology, including cell proliferation, apoptosis, differentiation, and gene expression. This implies that in addition to the well-described signals for insulin release, other intracellular signaling mechanisms are needed. Here we describe the role of global and local Ca(2+) signals in insulin release, the regulation of these signals by new membrane receptors, and the generation of nuclear Ca(2+) signals involved in gene expression. An integrated view of these pathways should improve the present description of the beta-cell biology and provide new targets for novel drugs.

Animals↗

PubMed2004

Nutrients induce different Ca(2+) signals in cytosol and nucleus in pancreatic beta-cells.

Specific activation of Ca(2+)-dependent functions is achieved by the particular dynamics and local restriction of Ca(2+) signals. It has been shown that changes in amplitude, duration, or frequency of Ca(2+) signals modulate gene transcription. Thus, Ca(2+) variations should be finely controlled within the nucleus. Although a variety of mechanisms in the nuclear membrane have been demonstrated to regulate nuclear Ca(2+), the existence of an autonomous Ca(2+) homeostasis within the nucleus is still questioned. In the pancreatic beta-cell, besides their effect on insulin secretion, Ca(2+) messages generated by nutrients also exert their action on gene expression. However, the dynamics of these Ca(2+) signals in relation to nuclear function have been explored little in islet cells. In the current study, Ca(2+) changes both in the nucleoplasm and in the cytosol of INS-1 and pancreatic beta-cells were monitored using spot confocal microscopy. We show that nutrients trigger Ca(2+) signals of higher amplitude in the nucleus than in the cytosol. These amplitude-modulated Ca(2+) signals transmitted to the nucleus might play an important role in the control of gene expression in the pancreatic beta-cell.

ATP-Binding Cassette Transporters↗

PubMed2003

On-line analysis of gap junctions reveals more efficient electrical than dye coupling between islet cells.

Pancreatic beta-cells constitute a well-communicating multicellular network that permits a coordinated and synchronized signal transmission within the islet of Langerhans that is necessary for proper insulin release. Gap junctions are the molecular keys that mediate functional cellular connections, which are responsible for electrical and metabolic coupling in the majority of cell types. Although the role of gap junctions in beta-cell electrical coupling is well documented, metabolic communication is still a matter of discussion. Here, we have addressed this issue by use of a fluorescence recovery after photobleaching (FRAP) approach. This technique has been validated as a reliable and noninvasive approach to monitor functional gap junctions in real time. We show that control pancreatic islet cells did not exchange a gap junction-permeant molecule in either clustered cells or intact islets of Langerhans under conditions that allowed cell-to-cell exchange of current-carrying ions. Conversely, we have detected that the same probe was extensively transferred between islet cells of transgenic mice expressing connexin 32 (Cx32) that have enhanced junctional coupling properties. The results indicate that the electrical coupling of native islet cells is more extensive than dye communication. Dye-coupling domains in islet cells appear more restricted than previously inferred with other methods.

Animals↗

PubMed2003

ATP-independent luminal oscillations and release of Ca2+ and H+ from mast cell secretory granules: implications for signal transduction.

InsP(3) is an important link in the intracellular information network. Previous observations show that activation of InsP(3)-receptor channels on the granular membrane can turn secretory granules into Ca(2+) oscillators that deliver periodic trains of Ca(2+) release to the cytosol (T. Nguyen, W. C. Chin, and P. Verdugo, 1998, Nature, 395:908-912; I. Quesada, W. C. Chin, J. Steed, P. Campos-Bedolla, and P. Verdugo, 2001, BIOPHYS: J. 80:2133-2139). Here we show that InsP(3) can also turn mast cell granules into proton oscillators. InsP(3)-induced intralumenal [H(+)] oscillations are ATP-independent, result from H(+)/K(+) exchange in the heparin matrix, and produce perigranular pH oscillations with the same frequency. These perigranular pH oscillations are in-phase with intralumenal [H(+)] but out-of-phase with the corresponding perigranular [Ca(2+)] oscillations. The low pH of the secretory compartment has critical implications in a broad range of intracellular processes. However, the association of proton release with InsP(3)-induced Ca(2+) signals, their similar periodic nature, and the sensitivity of important exocytic proteins to the joint action of Ca(2+) and pH strongly suggests that granules might encode a combined Ca(2+)/H(+) intracellular signal. A H(+)/Ca(2+) signal could significantly increase the specificity of the information sent by the granule by transmitting two frequency encoded messages targeted exclusively to proteins like calmodulin, annexins, or syncollin that are crucial for exocytosis and require specific combinations of [Ca(2+)] "and" pH for their action.

Adenosine Triphosphate↗

PubMed2002

Low doses of the endocrine disruptor bisphenol-A and the native hormone 17beta-estradiol rapidly activate transcription factor CREB.

Endocrine-disrupting chemicals (EDCs) are hormone-like agents present in the environment that alter the endocrine system of wildlife and humans. Most EDCs have potencies far below those of the natural hormone 17beta-E2 when acting through the classic estrogen receptors (ERs). Here, we show that the environmental estrogen Bisphenol-A and the native hormone 17beta-E2 activate the transcription factor, cAMP-responsive element binding protein (CREB) with the same potency. Phosphorylated CREB (P-CREB) was increased after only a 5-minute application of either BPA or 17beta-E2 in a calcium-dependent manner. The effect was reproduced by the membrane-impermeable molecule E2 conjugated to horseradish peroxidase (E-HRP). The increase in P-CREB was not modified by the anti-estrogen ICI 182,780. Therefore, low-dose of BPA activates the transcription factor CREB via an alternative mechanism, involving a non-classical membrane estrogen receptor. Because these effects are elicited at concentrations as low as 10(-9) M, this observation is of environmental and public health relevance.

Animals↗

PubMed2002

Nuclear KATP channels trigger nuclear Ca(2+) transients that modulate nuclear function.

Glucose, the principal regulator of endocrine pancreas, has several effects on pancreatic beta cells, including the regulation of insulin release, cell proliferation, apoptosis, differentiation, and gene expression. Although the sequence of events linking glycemia with insulin release is well described, the mechanism whereby glucose regulates nuclear function is still largely unknown. Here, we have shown that an ATP-sensitive K(+) channel (K(ATP)) with similar properties to that found on the plasma membrane is also present on the nuclear envelope of pancreatic beta cells. In isolated nuclei, blockade of the K(ATP) channel with tolbutamide or diadenosine polyphosphates triggers nuclear Ca(2+) transients and induces phosphorylation of the transcription factor cAMP response element binding protein. In whole cells, fluorescence in situ hybridization revealed that these Ca(2+) signals may trigger c-myc expression. These results demonstrate a functional K(ATP) channel in nuclei linking glucose metabolism, nuclear Ca(2+) signals, and nuclear function.

Adenosine Triphosphate↗

PubMed2002

Oscillations of pH inside the secretory granule control the gain of Ca2+ release for signal transduction in goblet cell exocytosis.

Although Ca2+ plays a critical function in relaying intracellular messages, the role of subcellular organelles in the dynamics of intracellular Ca2+ still remains largely unexplored. We recently demonstrated that secretory granules can signal their own export from the cell by releasing Ca2+ to the cytosol. Oscillations and release of Ca2+ in/from the granule result from the combined action of a Ca2+/K+ ion exchange process that occurs in the granule's matrix, and the sequential activation of two Ca2+-sensitive ion channels: an inositol 1,4,5-trisphosphate receptor Ca2+ channel (InsP3R) and an apamin-sensitive Ca2+-activated K+ channel (ASK(Ca)). The results reported here from studies using isolated mucin granules indicate that intralumenal granular Ca2+ oscillations ([Ca2+]L) and the corresponding cyclical release of Ca2+ to the cytosol induced by InsP3 are accompanied by corresponding intragranular pH(G) oscillations. Our data show that K+-induced unbinding of Ca2+ from the mucin matrix increases as the pH(G) declines. These observations suggest that oscillations of pH(G) can modulate the gain of the Ca2+/K+ ion exchange process, thereby controlling the amplitude of [Ca2+]L oscillations and the granule-cytosol release gradient of [Ca2+].

Animals↗