Biomedical subjects

Martin Paukert

Publications and source records attributed to Martin Paukert.

7 recordsLinked to original sources

PubMed2006

Synaptic communication between neurons and NG2+ cells.

Chemical synaptic transmission provides the basis for much of the rapid signaling that occurs within neuronal networks. However, recent studies have provided compelling evidence that synapses are not used exclusively for communication between neurons. Physiological and anatomical studies indicate that a distinct class of glia known as NG2(+) cells also forms direct synaptic junctions with both glutamatergic and GABAergic neurons. Glutamatergic signaling can influence intracellular Ca(2+) levels in NG2(+) cells by activating Ca(2+) permeable AMPA receptors, and these inputs can be potentiated through high frequency stimulation. Although the significance of this highly differentiated form of communication remains to be established, these neuro-glia synapses might enable neurons to influence rapidly the behavior of this ubiquitous class of glial progenitors.

Animals↗

PubMed2006

Strong modulation by RFamide neuropeptides of the ASIC1b/3 heteromer in competition with extracellular calcium.

Acid-sensing ion channels are excitatory receptors for extracellular H+. Since the extracellular H+ concentration can significantly increase during an inflammation, one of the proposed functions for ASICs is peripheral perception of pain. The ASIC1b and ASIC3 subunits are specifically expressed in sensory ganglia neurons and are candidate sensors of peripheral acidosis. However, the function of these ASIC subunits is limited by their steady-state desensitization during a small but persistent increase of the H+ concentration and by their desensitization after stronger H+ stimuli. Here we show that ASIC1b and ASIC3 form a heteromeric channel that, at steady-state, desensitizes at more acidic values than either homomeric ASIC1b or homomeric ASIC3 alone. Moreover, we show that RFamide neuropeptides, putative modulators of ASIC activity during inflammation, drastically slow down the desensitization of the ASIC1b/3 heteromer with an apparent dissociation constant of approximately 24microM. The apparent affinity for RFamide-induced effects was about 3-fold higher at low extracellular calcium concentrations. Our results suggest that the ASIC1b/3 heteromer is a possible target for RFamide neuropeptides in the peripheral nervous system.

Acid Sensing Ion Channels↗

PubMed2005

Xenopus TRPN1 (NOMPC) localizes to microtubule-based cilia in epithelial cells, including inner-ear hair cells.

In vertebrates, the senses of hearing and balance depend on hair cells, which transduce sounds with their hair bundles, containing actin-based stereocilia and microtubule-based kinocilia. A longstanding question in auditory science is the identity of the mechanically sensitive transduction channel of hair cells, thought to be localized at the tips of their stereocilia. Experiments in zebrafish implicated the transient receptor potential (TRP) channel NOMPC (drTRPN1) in this role; TRPN1 is absent from the genomes of higher vertebrates, however, and has not been localized in hair cells. Another candidate for the transduction channel, TRPA1, apparently is required for transduction in mammalian and nonmammalian vertebrates. This discrepancy raises the question of the relative contribution of TRPN1 and TRPA1 to transduction in nonmammalian vertebrates. To address this question, we cloned the TRPN1 ortholog from the amphibian Xenopus laevis, generated an antibody against the protein, and determined the protein's cellular and subcellular localization. We found that TRPN1 is prominently located in lateral-line hair cells, auditory hair cells, and ciliated epidermal cells of developing Xenopus embryos. In ciliated epidermal cells TRPN1 staining was enriched at the tips and bases of the cilia. In saccular hair cells, TRPN1 was located prominently in the kinocilial bulb, a component of the mechanosensory hair bundles. Moreover, we observed redistribution of TRPN1 upon treatment of hair cells with calcium chelators, which disrupts the transduction apparatus. This result suggests that although TRPN1 is unlikely to be the transduction channel of stereocilia, it plays an essential role, functionally related to transduction, in the kinocilium.

Animals↗

PubMed2004

A family of acid-sensing ion channels from the zebrafish: widespread expression in the central nervous system suggests a conserved role in neuronal communication.

Acid-sensing ion channels (ASICs) are excitatory receptors for extracellular H(+). Proposed functions include synaptic transmission, peripheral perception of pain, and mechanosensation. Despite the physiological importance of these functions, the precise role of ASICs has not yet been established. In order to increase our understanding of the physiological role and basic structure-function relationships of ASICs, we report here the cloning of six new ASICs from the zebrafish (zASICs). zASICs possess the basic functional properties of mammalian ASICs: activation by extracellular H(+), Na(+) selectivity, and block by micromolar concentrations of amiloride. The zasic genes are broadly expressed in the central nervous system, whereas expression in the peripheral nervous system is scarce. This pattern suggests a predominant role for zASICs in neuronal communication. Our results suggest a conserved function for receptors of extracellular H(+) in the central nervous system of vertebrates.

Acid Sensing Ion Channels↗

PubMed2004

Identification of the Ca2+ blocking site of acid-sensing ion channel (ASIC) 1: implications for channel gating.

Acid-sensing ion channels ASIC1a and ASIC1b are ligand-gated ion channels that are activated by H+ in the physiological range of pH. The apparent affinity for H+ of ASIC1a and 1b is modulated by extracellular Ca2+ through a competition between Ca2+ and H+. Here we show that, in addition to modulating the apparent H+ affinity, Ca2+ blocks ASIC1a in the open state (IC50 approximately 3.9 mM at pH 5.5), whereas ASIC1b is blocked with reduced affinity (IC50 > 10 mM at pH 4.7). Moreover, we report the identification of the site that mediates this open channel block by Ca2+. ASICs have two transmembrane domains. The second transmembrane domain M2 has been shown to form the ion pore of the related epithelial Na+ channel. Conserved topology and high homology in M2 suggests that M2 forms the ion pore also of ASICs. Combined substitution of an aspartate and a glutamate residue at the beginning of M2 completely abolished block by Ca2+ of ASIC1a, showing that these two amino acids (E425 and D432) are crucial for Ca2+ block. It has previously been suggested that relief of Ca2+ block opens ASIC3 channels. However, substitutions of E425 or D432 individually or in combination did not open channels constitutively and did not abolish gating by H+ and modulation of H+ affinity by Ca2+. These results show that channel block by Ca2+ and H+ gating are not intrinsically linked.

Acid Sensing Ion Channels↗

PubMed2002

Alternative splicing and interaction with di- and polyvalent cations control the dynamic range of acid-sensing ion channel 1 (ASIC1).

Homomeric acid-sensing ion channel 1 (ASIC1) can be activated by extracellular H(+) in the physiological pH range and may, therefore, contribute to neurotransmission and peripheral pain perception. ASIC1a and ASIC1b are alternative splice products of the ASIC1 gene. Here we show that both splice variants show steady-state inactivation when exposed to slightly decreased pH, limiting their operational range. Compared with ASIC1a, steady-state inactivation and pH activation of ASIC1b are shifted to more acidic values by 0.25 and 0.7 pH units, respectively, extending the dynamic range of ASIC1. Shifts of inactivation and activation are intimately linked; only two amino acids in the ectodomain, which are exchanged by alternative splicing, control both properties. Moreover, we show that extracellular, divalent cations like Ca(2+) and Mg(2+) as well as the polyvalent cation spermine shift the steady-state inactivation of ASIC1a and ASIC1b to more acidic values. This leads to a potentiation of the channel response and is due to a stabilization of the resting state. Our results indicate that ASIC1b is an effective sensor of transient H(+) signals during slight acidosis and that, in addition to alternative splicing, interaction with di- and polyvalent cations extends the dynamic range of ASIC H(+) sensors.

Acid Sensing Ion Channels↗

PubMed2002

The P2X(7) receptor from Xenopus laevis: formation of a large pore in Xenopus oocytes.

The purinergic P2X(7) receptor is an ATP-receptor channel predominantly expressed in immune cells. P2X(7) has been cloned from human, rat and mouse. Here we report cloning of the Xenopus laevis P2X(7) receptor (xP2X(7)). xP2X(7) is only about 50% identical to the mammalian homologues, shows a broad tissue expression pattern, and has the electrophysiological characteristics typical of a P2X(7) receptor: low agonist affinity (EC(50) about 2.6 mM) and a non-desensitizing current. Moreover, expression of xP2X(7) in Xenopus oocytes is sufficient to induce the formation of a large pore, which is permeable to large cations such as NMDG(+). Identification of a non-mammalian P2X(7) receptor may help to identify functionally important parts of the protein.

Amino Acid Sequence↗