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P Ortega-Sáenz

Publications and source records attributed to P Ortega-Sáenz.

4 recordsLinked to original sources
PubMed2001

Cellular mechanism of oxygen sensing.

O2 sensing is a fundamental biological process necessary for adaptation of living organisms to variable habitats and physiological situations. Cellular responses to hypoxia can be acute or chronic. Acute responses rely mainly on O2-regulated ion channels, which mediate adaptive changes in cell excitability, contractility, and secretory activity. Chronic responses depend on the modulation of hypoxia-inducible transcription factors, which determine the expression of numerous genes encoding enzymes, transporters and growth factors. O2-regulated ion channels and transcription factors are part of a widely operating signaling system that helps provide sufficient O2 to the tissues and protect the cells against damage due to O2 deficiency. Despite recent advances in the molecular characterization of O2-regulated ion channels and hypoxia-inducible factors, several unanswered questions remain regarding the nature of the O2 sensor molecules and the mechanisms of interaction between the sensors and the effectors. Current models of O2 sensing are based on either a heme protein capable of reversibly binding O2 or the production of oxygen reactive species by NAD(P)H oxidases and mitochondria. Complete molecular characterization of the hypoxia signaling pathways will help elucidate the differential sensitivity to hypoxia of the various cell types and the gradation of the cellular responses to variable levels of PO2. A deeper understanding of the cellular mechanisms of O2 sensing will facilitate the development of new pharmacological tools effective in the treatment of diseases such as stroke or myocardial ischemia caused by localized deficits of O2.

Adaptation, Physiological↗
PubMed2000

Collapse of conductance is prevented by a glutamate residue conserved in voltage-dependent K(+) channels.

Voltage-dependent K(+) channel gating is influenced by the permeating ions. Extracellular K(+) determines the occupation of sites in the channels where the cation interferes with the motion of the gates. When external [K(+)] decreases, some K(+) channels open too briefly to allow the conduction of measurable current. Given that extracellular K(+) is normally low, we have studied if negatively charged amino acids in the extracellular loops of Shaker K(+) channels contribute to increase the local [K(+)]. Surprisingly, neutralization of the charge of most acidic residues has minor effects on gating. However, a glutamate residue (E418) located at the external end of the membrane spanning segment S5 is absolutely required for keeping channels active at the normal external [K(+)]. E418 is conserved in all families of voltage-dependent K(+) channels. Although the channel mutant E418Q has kinetic properties resembling those produced by removal of K(+) from the pore, it seems that E418 is not simply concentrating cations near the channel mouth, but has a direct and critical role in gating. Our data suggest that E418 contributes to stabilize the S4 voltage sensor in the depolarized position, thus permitting maintenance of the channel open conformation.

Amino Acid Sequence↗
PubMed1998

Pore mutations alter closing and opening kinetics in Shaker K+ channels.

1. We have studied the effects of mutations of amino acids in the pore (positions 447 and 449) and the elevation of extracellular [K+] on the closing and opening kinetics of Shaker B K+ channels transiently expressed in Chinese hamster ovary (CHO) cells. 2. Mutant D447E had closing and C-type inactivation kinetics which were faster than the wild-type channel. These processes were slowed by increasing extracellular [K+] and in these conditions the channels exhibited linear instantaneous current-voltage relationships. Thus, the mutation seems to produce uniform decrease of occupancy by K+ in sites along the channel pore where the cation competes with closing and C-type inactivation. 3. In other mutants also showing K+-dependent fast C-type inactivation, closing was found to be slower than in the wild-type channel and insensitive to variations in external [K+]. These characteristics were particularly apparent in mutant T449K which even in high [K+] has a non-linear instantaneous current-voltage relationship with marked saturation of the inward current recorded at negative membrane potentials. Hence, in this channel type occupation by K+ of the pore appears to be non-uniform with low occupancy of sites near the outer entrance and saturation of the sites accessible from the internal solution. 4. The results show that channel closing is influenced by changes in the pore structure leading to alterations in the occupation of the channels by permeant cations. The differential effects of pore mutations and high external [K+] on closing and C-type inactivation indicate that the respective gates are associated with separate domains of the molecule. 5. Point mutations in the pore sequence can also lead to modifications in channel opening. In general, channels with fast C-type inactivation also show a fast rising phase of activation. However, these effects appear not to be due to primary modifications of the activation process but to arise from the coupling of activation and C-type inactivation. 6. These data, demonstrating that the pore structure influences most of the gating parameters of K+ channels, give further insight into the mechanisms underlying the modulation of K+ channel function by changes in the ionic composition in the extracellular milieu.

Amino Acid Substitution↗