RESUMO
Polycystic kidney disease (PKD) is a leading cause of end-stage renal disease. PKD arises from mutations in proteins, one a Ca2+-conducting channel, expressed in the primary cilia of renal epithelial cells. A common hypothesis is that Ca2+ entering through ciliary ion channels may reduce cystogenesis. The cilia have at least two Ca2+-conducting channels: polycystin-2 (PC2) and TRPV4 (transient receptor potential (TRP) cation channel, subfamily V, member 4), but how substantially they can increase intraciliary Ca2+ is unknown. By recording channel activities in isolated cilia, conditions are identified under which the channels can increase free Ca2+ within the cilium by at least 500-fold through regenerative (positive-feedback) signaling. Ca2+ that has entered through a channel can activate the channel internally, which increases the Ca2+ influx, and so on. Regenerative signaling is favored when the concentration of the Ca2+ buffer is reduced or when a slower buffer is used. Under such conditions, the Ca2+ that enters the cilium through a single PC2 channel is sufficient to almost fully activate that same channel. Regenerative signaling is not detectable with reduced external Ca2+. Reduced buffering also allows regenerative signaling through TRPV4 channels, but not through TRPM4 (TRP subfamily M, member 4) channels, which are activated by Ca2+ but do not conduct it. On a larger scale, Ca2+ that enters through TRPV4 channels can cause secondary activation of PC2 channels. I discuss the likelihood of regenerative ciliary Ca2+ signaling in vivo, a possible mechanism for its activation, and how it might relate to cystogenesis.
RESUMO
In 15% of cases, autosomal dominant polycystic kidney disease arises from defects in polycystin-2 (PC2). PC2 is a member of the polycystin transient receptor potential subfamily of cation-conducting channels and is expressed in the endoplasmic reticulum and primary cilium of renal epithelial cells. PC2 opposes a procystogenic influence of the cilium, and it has been proposed that this beneficial effect is mediated in part by a flow of Ca2+ through PC2 channels into the primary cilium. However, previous efforts to determine the permeability of PC2 channels to Ca2+ have yielded widely varying results. Here, we report the mean macroscopic Ca2+ influx through native PC2 channels in the primary cilia of mIMCD-3 cells, which are derived from the murine inner medullary collecting duct. Under conditions designed to isolate inward Ca2+ currents, a small inward Ca2+ current was detected in cilia with active PC2 channels but not in cilia lacking those channels. The current was activated by the addition of 10 µM internal Ca2+, which is known to activate ciliary PC2 channels. It was blocked by 10 µM isosakuranetin, which blocks the same channels. On average, the current amplitude was -1.8 pA at -190 mV; its conductance from -50 to -200 mV averaged 20 pS. Thus, native PC2 channels of renal primary cilia are able to conduct a small but detectable Ca2+ influx under the conditions tested. The possible consequences of this influx are discussed.NEW & NOTEWORTHY In autosomal dominant polycystic kidney disease, it is proposed that Ca2+ entering the primary cilium through polycystin-2 (PC2) channels may limit the formation of cysts. Recent studies predict that any macroscopic Ca2+ influx through these channels should be small. We report that the native PC2 channels in primary cilia of cultured renal epithelial cells can allow a small macroscopic calcium influx. This may allow a significant accumulation of Ca2+ in the cilium in vivo.
Assuntos
Canais de Cálcio/fisiologia , Cálcio/metabolismo , Cílios/fisiologia , Fenômenos Eletrofisiológicos , Canais de Cátion TRPP/metabolismo , Animais , Linhagem Celular , Quelantes/farmacologia , Ácido Egtázico/análogos & derivados , Ácido Egtázico/farmacologia , Células Epiteliais , Túbulos Renais Coletores/citologia , CamundongosRESUMO
Primary cilia of renal epithelial cells express several members of the transient receptor potential (TRP) class of cation-conducting channel, including TRPC1, TRPM3, TRPM4, TRPP2, and TRPV4. Some cases of autosomal dominant polycystic kidney disease (ADPKD) are caused by defects in TRPP2 (also called polycystin-2, PC2, or PKD2). A large-conductance, TRPP2-dependent channel in renal cilia has been well described, but it is not known whether this channel includes any other protein subunits. To study this question, we investigated the pharmacology of the TRPP2-dependent channel through electrical recordings from the cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin. The pharmacology was found to match that of TRPM3 channels. The ciliary TRPP2-dependent channel is known to be activated by depolarization and by increasing cytoplasmic Ca2+. This activation was greatly enhanced by external pregnenolone sulfate, an agonist of TRPM3 channels. Pregnenolone sulfate did not change the single-channel current-voltage relation. The channels were effectively blocked by isosakuranetin, a specific inhibitor of TRPM3 channels. Both pregnenolone sulfate and isosakuranetin were effective at concentrations as low as 1 µM. Knocking out TRPM3 by CRISPR/Cas9 genome editing eliminated the ciliary channel. Thus the channel is both TRPM3-dependent and TRPP2-dependent, suggesting that it may include both types of subunit. Knocking out TRPM3 did not change the level of TRPP2 protein in the cilia, so it is unlikely that the absence of functional ciliary channels results from a failure of trafficking.
Assuntos
Rim/metabolismo , Canais de Cátion TRPM/metabolismo , Canais de Cátion TRPP/metabolismo , Animais , Sinalização do Cálcio , Linhagem Celular , Cílios/efeitos dos fármacos , Cílios/metabolismo , Células Epiteliais/efeitos dos fármacos , Células Epiteliais/metabolismo , Flavonoides/farmacologia , Técnicas de Inativação de Genes , Humanos , Rim/citologia , Camundongos , Pregnenolona/farmacologia , Canais de Cátion TRPM/antagonistas & inibidores , Canais de Cátion TRPM/genética , Canais de Cátion TRPP/antagonistas & inibidores , Canais de Cátion TRPP/genéticaRESUMO
Primary cilia sense environmental conditions, including osmolality, but whether cilia participate in the osmotic response in renal epithelial cells is not known. The transient receptor potential (TRP) channels TRPV4 and TRPM3 are osmoresponsive. TRPV4 localizes to cilia in certain cell types, while renal subcellular localization of TRPM3 is not known. We hypothesized that primary cilia are required for maximal activation of the osmotic response of renal epithelial cells and that ciliary TRPM3 and TRPV4 mediate that response. Ciliated [murine epithelial cells from the renal inner medullary collecting duct (mIMCD-3) and 176-5] and nonciliated (176-5Δ) renal cells expressed Trpv4 and Trpm3 Ciliary expression of TRPM3 was observed in mIMCD-3 and 176-5 cells and in wild-type mouse kidney tissue. TRPV4 was identified in cilia and apical membrane of mIMCD-3 cells by electrophysiology and in the cell body by immunofluorescence. Hyperosmolal stress at 500 mOsm/kg (via NaCl addition) induced the osmotic response genes betaine/GABA transporter (Bgt1) and aldose reductase (Akr1b3) in all ciliated cell lines. This induction was attenuated in nonciliated cells. A TRPV4 agonist abrogated Bgt1 and Akr1b3 induction in ciliated and nonciliated cells. A TRPM3 agonist attenuated Bgt1 and Akr1b3 induction in ciliated cells only. TRPM3 knockout attenuated Akr1b3 induction. Viability under osmotic stress was greater in ciliated than nonciliated cells. Akr1b3 induction was also less in nonciliated than ciliated cells when mannitol was used to induce hyperosmolal stress. These findings suggest that primary cilia are required for the maximal osmotic response in renal epithelial cells and that TRPM3 is involved in this mechanism. TRPV4 appears to modulate the osmotic response independent of cilia.
Assuntos
Células Epiteliais/metabolismo , Túbulos Renais Coletores/metabolismo , Osmorregulação , Pressão Osmótica , Canais de Cátion TRPM/metabolismo , Animais , Sistemas CRISPR-Cas , Linhagem Celular , Cílios/metabolismo , Células Epiteliais/efeitos dos fármacos , Proteínas da Membrana Plasmática de Transporte de GABA/genética , Proteínas da Membrana Plasmática de Transporte de GABA/metabolismo , Edição de Genes , Hidroxiprostaglandina Desidrogenases/genética , Hidroxiprostaglandina Desidrogenases/metabolismo , Túbulos Renais Coletores/efeitos dos fármacos , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Osmorregulação/efeitos dos fármacos , Pressão Osmótica/efeitos dos fármacos , Solução Salina Hipertônica/farmacologia , Transdução de Sinais , Canais de Cátion TRPM/genética , Canais de Cátion TRPV/genética , Canais de Cátion TRPV/metabolismo , TransfecçãoRESUMO
Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening monogenic renal disease. ADPKD results from mutations in either of two proteins: polycystin-1 (also known as PC1 or PKD1) or transient receptor potential cation channel, subfamily P, member 2 (TRPP2, also known as polycystin-2, PC2, or PKD2). Each of these proteins is expressed in the primary cilium that extends from many renal epithelial cells. Existing evidence suggests that the cilium can promote renal cystogenesis, while PC1 and TRPP2 counter this cystogenic effect. To better understand the function of TRPP2, we investigated its electrophysiological properties in the native ciliary membrane. We recorded directly from the cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin. In one-third of cilia examined, a large-conductance channel was observed. The channel was not permeable to Cl¯ but conducted cations with permeability ratios PK:PCa:PNa of 1:0.55:0.14. The single-channel conductance ranged from 97 pS in typical physiological solutions to 189 pS in symmetrical 145 mM KCl. Open probability of the channel was very sensitive to membrane depolarization or increasing cytoplasmic free Ca2+ in the low micromolar range, with the open probability increasing in either case. Knocking out TRPP2 by CRISPR/Cas9 genome editing eliminated the channel current, establishing it as TRPP2 dependent. Possible mechanisms for activating the TRPP2-dependent channel in the renal primary cilium are discussed.
Assuntos
Cílios/metabolismo , Células Epiteliais/metabolismo , Rim/metabolismo , Canais de Cátion TRPP/metabolismo , Animais , Cálcio/metabolismo , Linhagem Celular , Citoplasma/metabolismo , Técnicas de Inativação de Genes/métodos , Camundongos , Rim Policístico Autossômico Dominante/metabolismo , Canais de Cátion TRPP/genéticaRESUMO
Defects in primary cilia lead to a variety of human diseases. One of these, polycystic kidney disease, can be caused by defects in a Ca²âº-gated ion channel (TRPP2) found on the cilium. Other ciliary functions also contribute to cystogenesis, and defects in apical Ca²âº homeostasis have been implicated. By recording directly from the native cilia of mIMCD-3 cells, a murine cell line of renal epithelial origin, we have identified a second Ca²âº-gated channel in the ciliary membrane: the transient receptor potential cation channel, subfamily M, member 4 (TRPM4). In excised primary cilia, TRPM4 was found to have a low sensitivity to Ca²âº, with an EC50 of 646 µM at +100 mV. It was inhibited by MgATP and by 9-phenanthrol. The channel was not permeable to Ca²âº or Clâ» and had a permeability ratio PK/PNa of 1.42. Reducing the expression of Trpm4 mRNA with short hairpin (sh) RNA reduced the TRPM4 current by 87% and shortened primary cilia by 43%. When phospholipase C was inhibited, the sensitivity to cytoplasmic Ca²âº greatly increased (EC50 = 26 µM at +100 mV), which is consistent with previous reports that phosphatidylinositol 4,5-bisphosphate (PIP2) modulates the channel. MgATP did not restore the channel to a preinactivation state, suggesting that the enzyme or substrate necessary for making PIP2 is not abundant in primary cilia of mIMCD-3 cells. The function of TRPM4 in renal primary cilia is not yet known, but it is likely to influence the apical Ca²âº dynamics of the cell, perhaps in tandem with TRPP2.
Assuntos
Rim/metabolismo , Canais de Cátion TRPM/metabolismo , Trifosfato de Adenosina/farmacologia , Animais , Cálcio/metabolismo , Linhagem Celular , Cílios/efeitos dos fármacos , Cílios/metabolismo , Fenômenos Eletrofisiológicos/efeitos dos fármacos , Células Epiteliais/efeitos dos fármacos , Técnicas de Silenciamento de Genes , Ativação do Canal Iônico/efeitos dos fármacos , Rim/efeitos dos fármacos , Camundongos , Fosfoinositídeo Fosfolipase C/farmacologia , Canais de Cátion TRPM/efeitos dos fármacos , Canais de Cátion TRPM/genética , Canais de Cátion TRPP/metabolismo , Fosfolipases Tipo C/antagonistas & inibidoresRESUMO
BACKGROUND: Within the cilia of vertebrate olfactory receptor neurons, Ca(2+) accumulates during odor transduction. Termination of the odor response requires removal of this Ca(2+), and prior evidence suggests that both Na(+)/Ca(2+) exchange and plasma membrane Ca(2+)-ATPase (PMCA) contribute to this removal. PRINCIPAL FINDINGS: In intact mouse olfactory epithelium, we measured the time course of termination of the odor-induced field potential. Replacement of mucosal Na(+) with Li(+), which reduces the ability of Na(+)/Ca(2+) exchange to expel Ca(2+), prolonged the termination as expected. However, treating the epithelium with the specific PMCA inhibitor caloxin 1b1 caused no significant increase in the time course of response termination. CONCLUSIONS: Under these experimental conditions, PMCA does not contribute detectably to the termination of the odor response.
Assuntos
Mucosa Olfatória/efeitos dos fármacos , Mucosa Olfatória/fisiologia , Neurônios Receptores Olfatórios/efeitos dos fármacos , Neurônios Receptores Olfatórios/fisiologia , ATPases Transportadoras de Cálcio da Membrana Plasmática/antagonistas & inibidores , Animais , Cálcio/metabolismo , Cílios/efeitos dos fármacos , Cílios/metabolismo , Cílios/fisiologia , Lítio/metabolismo , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Camundongos , Odorantes , Mucosa Olfatória/metabolismo , Neurônios Receptores Olfatórios/metabolismo , Peptídeos/farmacologia , ATPases Transportadoras de Cálcio da Membrana Plasmática/metabolismo , Transdução de Sinais/efeitos dos fármacos , Sódio/metabolismo , Trocador de Sódio e Cálcio/metabolismoRESUMO
BACKGROUND: In vertebrate olfactory receptor neurons, sensory cilia transduce odor stimuli into changes in neuronal membrane potential. The voltage changes are primarily caused by the sequential openings of two types of channel: a cyclic-nucleotide-gated (CNG) cationic channel and a calcium-gated chloride channel. In frog, the cilia are 25 to 200 µm in length, so the spatial distributions of the channels may be an important determinant of odor sensitivity. PRINCIPAL FINDINGS: To determine the spatial distribution of the chloride channels, we recorded from single cilia as calcium was allowed to diffuse down the length of the cilium and activate the channels. A computational model of this experiment allowed an estimate of the spatial distribution of the chloride channels. On average, the channels were concentrated in a narrow band centered at a distance of 29% of the ciliary length, measured from the base of the cilium. This matches the location of the CNG channels determined previously. This non-uniform distribution of transduction proteins is consistent with similar findings in other cilia. CONCLUSIONS: On average, the two types of olfactory transduction channel are concentrated in the same region of the cilium. This may contribute to the efficient detection of weak stimuli.
Assuntos
Cloretos/química , Cílios/metabolismo , Condutos Olfatórios/fisiologia , Animais , Biofísica/métodos , Cálcio/química , Cátions , AMP Cíclico/metabolismo , Difusão , Eletrofisiologia/métodos , Ativação do Canal Iônico/fisiologia , Modelos Teóricos , Ranidae , Software , Fatores de TempoRESUMO
BACKGROUND: In any fine sensory organelle, a small influx of Ca(2+) can quickly elevate cytoplasmic Ca(2+). Mechanisms must exist to clear the ciliary Ca(2+) before it reaches toxic levels. One such organelle has been well studied: the vertebrate olfactory cilium. Recent studies have suggested that clearance from the olfactory cilium is mediated in part by plasma membrane Ca(2+)-ATPase (PMCA). PRINCIPAL FINDINGS: In the present study, electrophysiological assays were devised to monitor cytoplasmic free Ca(2+) in single frog olfactory cilia. Ca(2+) was allowed to enter isolated cilia, either through the detached end or through membrane channels. Intraciliary Ca(2+) was monitored via the activity of ciliary Ca(2+)-gated Cl(-) channels, which are sensitive to free Ca(2+) from about 2 to 10 microM. No significant effect of MgATP on intraciliary free Ca(2+) could be found. Carboxyeosin, which has been used to inhibit PMCA, was found to substantially increase a ciliary transduction current activated by cyclic AMP. This increase was ATP-independent. CONCLUSIONS: Alternative explanations are suggested for two previous experiments taken to support a role for PMCA in ciliary Ca(2+) clearance. It is concluded that PMCA in the cilium plays a very limited role in clearing the micromolar levels of intraciliary Ca(2+) produced during the odor response.
Assuntos
Cálcio/metabolismo , Neurônios Receptores Olfatórios/enzimologia , ATPases Transportadoras de Cálcio da Membrana Plasmática/metabolismo , Trifosfato de Adenosina/metabolismo , Animais , Sinalização do Cálcio , Cílios/enzimologia , AMP Cíclico/metabolismo , Condutos Olfatórios , Rana pipiens/metabolismoRESUMO
Identification of detailed features of neuronal systems is an important challenge in the biosciences today. Cilia are long thin structures that extend from the olfactory receptor neurons into the nasal mucus. Transduction of an odor into an electrical signal occurs in the membranes of the cilia. The cyclic-nucleotide-gated (CNG) channels which reside in the ciliary membrane and are activated by adenosine 3',5'-cyclic monophosphate (cAMP) allow a depolarizing influx of Ca(2+) and Na(+) and are thought to initiate the electrical signal.In this paper, a mathematical model consisting of two nonlinear differential equations and a constrained Fredholm integral equation of the first kind is developed to model experiments involving the diffusion of cAMP into cilia and the resulting electrical activity. The unknowns in the problem are the concentration of cAMP, the membrane potential and, the quantity of most interest in this work, the distribution of CNG channels along the length of a cilium. A simple numerical method is derived that can be used to obtain estimates of the spatial distribution of CNG ion channels along the length of a cilium. Certain computations indicate that this mathematical problem is ill-conditioned.
RESUMO
The basal conductance of unstimulated frog olfactory receptor neurons was investigated using whole-cell and perforated-patch recording. The input conductance, measured between -80 mV and -60 mV, averaged 0.25 nS in physiological saline. Studies were conducted to determine whether part of the input conductance is due to gating of neuronal cyclic-nucleotide-gated (CNG) channels. In support of this idea, the neuronal resting conductance was reduced by each of five treatments that reduce current through CNG channels: external application of divalent cations or amiloride; treatment with either of two adenylate cyclase inhibitors; and application of AMP-PNP, a competitive substrate for adenylate cyclase. The current blocked by divalent cations or by a cyclase inhibitor reversed near 0 mV, as expected for a CNG current. Under physiological conditions, gating of CNG channels contributes approximately 0.06 nS to the resting neuronal conductance. This implies a resting cAMP concentration of 0.1-0.3 micro M. A theoretical model suggests that a neuron containing 0.1-0.3 micro M cAMP is poised to give the largest possible depolarization in response to a very small olfactory stimulus. Although having CNG channels open at rest decreases the voltage change resulting from a given receptor current, it more substantially increases the receptor current resulting from a given increase in [cAMP].
Assuntos
Condutividade Elétrica , Canais Iônicos/fisiologia , Modelos Neurológicos , Neurônios Receptores Olfatórios/fisiologia , Adenilil Imidodifosfato/farmacologia , Amilorida/farmacologia , Animais , Células Cultivadas , Simulação por Computador , Canais de Cátion Regulados por Nucleotídeos Cíclicos , Canais Iônicos/efeitos dos fármacos , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Neurônios Receptores Olfatórios/efeitos dos fármacos , Rana pipiensRESUMO
We studied the outward currents elicited by an odorous compound, isoamyl acetate, in isolated olfactory receptor neurons of the grass frog under whole-cell perforated-patch voltage-clamp recording. Odorant-induced outward currents were relatively rare, occurring in about 16% of the responding cells. Responses had smaller amplitudes and shorter time courses when compared to the more commonly found odorant-induced inward currents. There was a high correlation between odorant-induced outward current and responses evoked by either 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate, a membrane-permeant cyclic adenosine monophosphate analog, or 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor. The outward current responses to all three substances increased in amplitude when the membrane potential was more negative than -60 mV and decreased in amplitude when the membrane potential was more positive. Responses were still present when the potential was held at -100 mV, indicating that the responses are not the result of a potassium conductance. Removal of external calcium from the perfusion medium abolished the outward currents. Our results indicate that the odorant-induced outward current is a calcium-dependent event that may be mediated by cyclic adenosine monophosphate.