Biomedical subjects

Gerhard Dahl

Publications and source records attributed to Gerhard Dahl.

9 recordsLinked to original sources

PubMed2006

Pannexin 1 in erythrocytes: function without a gap.

ATP is a widely used extracellular signaling molecule. The mechanism of ATP release from cells is presently unresolved and may be either vesicular or channel-mediated. Erythrocytes release ATP in response to low oxygen or to shear stress. In the absence of vesicles, the release has to be through channels. Erythrocytes do not form gap junctions. Yet, here we show with immunohistochemical and electrophysiological data that erythrocytes express the gap junction protein pannexin 1. This protein, in addition to forming gap junction channels in paired oocytes, can also form a mechanosensitive and ATP-permeable channel in the nonjunctional plasma membrane. Consistent with a role of pannexin 1 as an ATP release channel, ATP release by erythrocytes was attenuated by the gap junction blocker carbenoxolone. Furthermore, under conditions of ATP release, erythrocytes took up fluorescent tracer molecules permeant to gap junction channels.

Adenosine Triphosphate↗

PubMed2006

Pannexin: to gap or not to gap, is that a question?

Vertebrates express two families of gap junction proteins: the well characterized connexins and the recently discovered pannexins. The latter are related to invertebrate innexins. Here we present the hypothesis that pannexins, rather than providing a redundant system to gap junctions formed by connexins, exert a physiological role as nonjunctional membrane channels. Specifically, we propose that pannexins can serve as ATP release channels. This function presumptively is also performed by innexins in invertebrates, in addition to their traditional gap junction role.

Animals↗

PubMed2005

Activation of pannexin 1 channels by ATP through P2Y receptors and by cytoplasmic calcium.

The ability for long-range communication through intercellular calcium waves is inherent to cells of many tissues. A dual propagation mode for these waves includes passage of IP3 through gap junctions as well as an extracellular pathway involving ATP. The wave can be regenerative and include ATP-induced ATP release via an unknown mechanism. Here, we show that pannexin 1 channels can be activated by extracellular ATP acting through purinergic receptors of the P2Y group as well as by cytoplasmic calcium. Based on its properties, including ATP permeability, pannexin 1 may be involved in both initiation and propagation of calcium waves.

Adenosine Triphosphate↗

PubMed2005

Conductance of connexin hemichannels segregates with the first transmembrane segment.

Gap junction channels are intercellular channels that mediate the gated transfer of molecules between adjacent cells. To identify the domain determining channel conductance, the first transmembrane segment (M1) was reciprocally exchanged between Cx46 and Cx32E(1)43. The resulting chimeras exhibited conductances similar to that of the respective M1 donor. Furthermore, a chimera with the carboxy-terminal half of M1 in Cx46 replaced by that of Cx32 exhibited a conductance similar to that of Cx32E(1)43, whereas the chimera with only the amino-terminal half of M1 replaced retained the unitary conductance of wild-type Cx46. Extending the M1 domain swapping to other connexins by replacing the carboxy-terminal half of M1 in Cx46 with that of Cx37 yielded a chimera channel with increased unitary conductance close to that of Cx37. Furthermore, a point mutant of Cx46, with leucine substituted by glycine in position 35, displayed a conductance much larger than that of the wild type. Thus, the M1 segment, especially the second half, contains important determinants of conductance of the connexin channel.

Amino Acid Sequence↗

PubMed2005

Cosegregation of permeability and single-channel conductance in chimeric connexins.

The physiological function of gap junction channels goes well beyond their initially discovered role in electrical synchronization of excitable cells. In most tissues, gap junction cells facilitate the exchange of second messengers and metabolites between cells. To test which parts of the channels formed by connexins determine the exclusion limit for the transit of molecules in the size range of second messengers and metabolites a domain exchange approach was used in combination with an accessibility assay for nonelectrolytes and flux measurements. The experimental results suggest that two open hemichannel forming connexins, Cx46 and Cx32E(1)43, differ in accessibility and permeability. Sucrose is at the exclusion limit for Cx46 channels whereas sorbitol is at the exclusion limit for Cx32E(1)43 channels. In chimeras between these connexins, where the first transmembrane segment M1 is exchanged, the exclusion limits correlate with those of the M1 donor. The same segregation was found in a separate study for the unitary conductance of the channels. Thus, conductance and permeability/accessibility of the channels cosegregate with M1.

Animals↗

PubMed2004

Pannexin membrane channels are mechanosensitive conduits for ATP.

Intercellular calcium wave propagation initiated by mechanical stress is a phenomenon found in nearly all cell types. The waves utilize two pathways: transfer of InsP3 directly from cell to cell through gap junction channels and release of ATP onto extracellular purinergic receptors. The conduit for ATP has remained elusive and both a vesicular and a channel mediated release have been considered. Here, we describe the properties of single pannexin 1 channels. They have a wide expression spectrum, they are of large conductance and permeant for ATP, and they are mechanosensitive. Hence, pannexins are candidates for the release of ATP to the extracellular space upon mechanical stress.

Animals↗

PubMed2004

Accessibility of cx46 hemichannels for uncharged molecules and its modulation by voltage.

Excised patches of oocyte membrane containing cx46 hemichannels were used to determine the accessibility of the channels for several uncharged molecules at negative and positive holding potentials. The molecular weights of the test molecules (sugars and polyethylene glycols) ranged from 180 to 666 Daltons, with diameters from 5.8 to 12 A. Activation of the voltage gate (V(j) gate) at positive potentials shifted the accessibility limit for these test molecules to lower sizes indicating that net charge of the test molecule does not determine accessibility. The sugars changed several channel properties: 1), single-channel conductance decreased in an inverse relationship to the size of the test molecule; 2), the apparent open probability of the connexin channels was reduced; 3), the lifetimes of the apparent occupancy states exceeded the expected transit times assuming simple diffusion by orders of magnitude. These results suggest a channel that is not cylindrical and in which test molecules bind or are trapped in the pore or the vestibulum. Furthermore, the effect of sugars and the large difference in transit rates for small ions and larger molecules suggest that they may involve different permeation mechanisms.

Animals↗

PubMed2004

Connexins are mechanosensitive.

Connexins form gap junction channels that provide a hydrophilic path between cell interiors. Some connexins, particularly the lens connexins, Cx46 and Cx50 and their orthologs, can form functional hemichannels in nonjunctional membranes. These hemichannels are a nonselective conduit to the extracellular medium and may jeopardize cell survival. The physiological function of hemichannels has remained elusive, but it has been postulated that hemichannels are involved in ATP-release caused by mechanical stimulation. Here we show with single-channel and whole cell electrophysiological studies that Cx46 hemichannels are mechanosensitive, like other families of ion channels and membrane-bound enzymes. The hemichannel response to mechanical stress is bipolar. At negative potentials stress opens the channel, and at positive potentials stress closes it. Physiologically, Cx46 hemichannels may assist accommodation of the ocular lens by providing a transient path for volume flow as the lens changes shape.

Animals↗

PubMed2002

Function of the voltage gate of gap junction channels: selective exclusion of molecules.

Gap junction channels span the membranes of two adjacent cells and allow the gated transit of molecules as large as second messengers from cell to cell. In vertebrates, gap junctions are composed of proteins from the connexin (cx) gene family. Gap junction channels formed by most connexins are affected by transjunctional voltage. The function of the voltage gate is unclear, because substantial electrical coupling typically remains with activated gates because of the channels dwelling in subconductance rather than closed states. Here, we find in Xenopus oocytes expressing cx43 or cx46 that the activated voltage gate preferentially restricts the passage of larger ions, such as fluorescent tracer molecules and cAMP, while having little effect on the electrical coupling arising from the passage of small electrolytes. Thus, a conceivable physiological role of the voltage gate is to selectively restrict the passage of large molecules between cells while allowing electrical coupling.

Animals↗