Na(+) -Activated K(+) Channels in Rat Supraoptic Neurones.

The magnocellular neurosecretory cells (MNCs) of the hypothalamus secrete the neurohormones vasopressin and oxytocin. The systemic release of these hormones depends on the rate and pattern of MNC firing and it is therefore important to identify the ion channels that contribute to the electrical behaviour of MNCs. In the present study, we report evidence for the presence of Na(+) -activated K(+) (KN a ) channels in rat MNCs. KN a channels mediate outwardly rectifying K(+) currents activated by the increases in intracellular Na(+) that occur during electrical activity. Although the molecular identity of native KN a channels is unclear, their biophysical properties are consistent with those of expressed Slick (slo 2.1) and Slack (slo 2.2) proteins. Using immunocytochemistry and Western blot experiments, we found that both Slick and Slack proteins are expressed in rat MNCs. Using whole cell voltage clamp techniques on acutely isolated rat MNCs, we found that inhibiting Na(+) influx by the addition of the Na(+) channel blocker tetrodotoxin or the replacement of Na(+) in the external solution with Li(+) caused a significant decrease in sustained outward currents. Furthermore, the evoked outward current density was significantly higher in rat MNCs using patch pipettes containing 60 mm Na(+) than it was when patch pipettes containing 0 mm Na(+) were used. Our data show that functional KN a channels are expressed in rat MNCs. These channels could contribute to the activity-dependent afterhyperpolarisations that have been identified in the MNCs and thereby play a role in the regulation of their electrical behaviour.

Expression of CaV 2.2 and splice variants of CaV 2.1 in oxytocin- and vasopressin-releasing supraoptic neurones.

The magnocellular neurosecretory cells (MNCs) release vasopressin (VP) and oxytocin (OT) from their axon terminals into the circulation and from their somata and dendrites to exert paracrine effects on other MNCs. MNCs express several types of voltage-gated Ca(2+) channels, including Ca(V)2.1 and Ca(V)2.2. These two channels types are similar in structure and function in other cells, but although influx of Ca(2+) through Ca(V)2.2 triggers the release of both OT and VP into the circulation, Ca(V)2.1 is involved in stimulating the release of VP but not OT. Release of OT from MNC somata is also triggered by Ca(V)2.2 but not Ca(V)2.1. These observations could be explained by differences in the level of expression of Ca(V)2.1 in VP and OT MNCs or by differences in the way that the two channels interact with the exocytotic apparatus. We used immunohistochemistry to confirm earlier work suggesting that MNCs express variants of Ca(V)2.1 lacking portions of an internal loop that enables the channels to interact with synaptic proteins. We used an antibody that would recognise both the full-length Ca(V)2.1 and the deletion variants to show that OT MNCs express fewer Ca(V)2.1 channels than do VP MNCs in both somata and axon terminals. We used the reverse transcriptase-polymerase chain reaction and immunocytochemistry to test whether MNCs express similar deletion variants of Ca(V)2.2 and were unable to find any evidence to support this. Our data suggest that the different roles that Ca(V)2.1 and Ca(V)2.2 play in MNC secretion may be a result of the different levels of expression of Ca(V)2.1 in VP and OT MNCs, as well as the expression in MNCs of deletion variants of Ca(V)2.1 that do not interact with exocytotic proteins and therefore may be less likely to mediate exocytotic release.

Vascular endothelial growth factor and angiopoietin production by primate follicles during culture is a function of growth rate, gonadotrophin exposure and oxygen milieu.

STUDY QUESTION: What is the time course of production of vascular endothelial growth factor-A (VEGF-A), angiopoietin (ANGPT)-1 and ANGPT-2 by primate follicles during encapsulated three-dimensional culture, and what conditions affect their production? SUMMARY ANSWER: Primate follicles produce VEGF-A and ANGPT-2 in vitro, particularly after developing to the antral stage, with VEGF production influenced by FSH concentration and O(2) tension. WHAT IS KNOWN ALREADY: Folliculogenesis, i.e. the development of primordial follicles into mature, antral follicles, requires the creation of a vascular network in the follicle wall via a process called angiogenesis. Angiogenic factors including VEGFs and ANGPTs have documented roles in angiogenesis. However, direct studies on the production and regulation of angiogenic factors by individual, growing follicles are limited. STUDY DESIGN, SIZE, DURATION: Ovaries (n = 9 pairs) were obtained from rhesus macaques during the early follicular phase of the menstrual cycle (cycle days 1-4). Secondary (125-225 µm) follicles were isolated mechanically, encapsulated into alginate (0.25% w/v) and cultured for 40 days. MATERIALS, SETTING, METHODS: Individual follicles were cultured in a 5 or 20% O(2) environment in alpha minimum essential medium supplemented with recombinant human (h) FSH. Half of the follicles had recombinant hLH added to the media from Days 30 to 40. Follicle diameters were measured weekly. Follicles were categorized at Week 5 as no-grow (NG; <250 µm in diameter), slow-grow (SG; 251-499 µm) and fast-grow (FG; >500 µm). VEGF-A, ANGPT-1 and -2 concentrations in media were measured by ELISA. MAIN RESULTS AND THE ROLE OF CHANCE: VEGF concentrations were low throughout the culture for NG follicles. SG and FG follicles had detectable VEGF concentrations at Week 2, which continued to rise throughout culture. VEGF concentrations were distinct (P < 0.05) among all three follicle categories during Weeks 4 and 5. VEGF concentrations were higher (P < 0.05) in SG follicles in the presence of high/mid-dose FSH at 5% O(2). In contrast, there were no dose-dependent differences in VEGF production for FG follicles based on FSH concentrations or O(2) tension. At Week 5, follicles that produced metaphase II oocytes, following exposure to an ovulatory hCG dose, secreted higher concentrations of VEGF than those containing germinal vesicle-intact oocytes. Media concentrations of ANGPT-1 were low throughout culture for all three follicle categories. ANGPT-2 concentrations were low throughout culture for NG follicles. In contrast, ANGPT-2 concentrations of SG and FG follicles continued to rise from Weeks 1 to 4. During Weeks 2-4, ANGPT-2 concentrations in FG follicles were significantly higher than those of SG and NG follicles (P < 0.05). LIMITATION, REASONS FOR CAUTION: This study reports VEGF-A, ANGPT-1 and -2 production by in vitro-developed individual primate (macaque) follicles, that is limited to the interval from the secondary to small antral stage. After VEGF and ANGPT-1 assays, the limited remaining samples did not allow assessment of the independent effects of gonadotrophin and O(2) on the ANGPT-2 production by cultured follicles. Findings await translation to human follicles. WIDER IMPLICATION OF THE FINDINGS: The above findings provide novel information on the process of primate follicle maturation. We hypothesize that a symbiotic relationship between elevated concentrations of ANGPT-2 and VEGF allows FG antral follicles to excel in follicle maturation, e.g. by promoting its vascularization. Elevated ANGPT-2 may also offer possible insight into future oocyte quality as early as Week 2, compared with Week 4 for VEGF and follicle size. STUDY FUNDING/COMPETING INTEREST(S): The study was funded by the following grants: NIH U54 RR024347/HD058294/PL1-EB008542 (Oncofertility Consortium), NIH U54-HD018185 (SCCPIR), NIH ORWH/NICHD 2K12HD043488 (BIRCWH), NIH FIC TW/HD-00668, ONPRC 8P51OD011092. There are no conflicts of interest to declare.

The expression of voltage-gated ca2+ channels in pituicytes and the up-regulation of L-type ca2+ channels during water deprivation.

The primary components of the neurohypophysis are the neuroendocrine terminals that release vasopressin and oxytocin, and pituicytes, which are astrocytes that normally surround and envelop these terminals. Pituicytes regulate neurohormone release by secreting the inhibitory modulator taurine in an osmotically-regulated fashion and undergo a marked structural reorganisation in response to dehydration as well as during lactation and parturition. Because of these unique functions, and the possibility that Ca2+ influx could regulate their activity, we tested for the expression of voltage-gated Ca2+ channel alpha1 subunits in pituicytes both in situ and in primary culture. Colocalisation studies in neurohypophysial slices show that pituicytes (identified by their expression of the glial marker S100beta), are immunoreactive for antibodies directed against Ca2+ channel alpha1 subunits Ca(V)2.2 and Ca(V)2.3, which mediate N- and R-type Ca2+ currents, respectively. Pituicytes in primary culture express immunoreactivity for Ca(V)1.2, Ca(V)2.1, Ca(V)2.2, Ca(V)2.3 and Ca(V)3.1 (which mediate L-, P/Q-, N-, R- and T-type currents, respectively) and immunoblotting studies confirmed the expression of these Ca2+ channel alpha1 subunits. This increase in Ca2+ channel expression may occur only in pituicytes in culture, or may reflect an inherent capability of pituicytes to initiate the expression of multiple types of Ca2+ channels when stimulated to do so. We therefore performed immunohistochemistry studies on pituitaries obtained from rats that had been deprived of water for 24 h. Pituicytes in these preparations showed a significantly increased immunoreactivity to Ca(V)1.2, suggesting that expression of these channels is up-regulated during the adaptation to long-lasting dehydration. Our results suggest that Ca2+ channels may play important roles in pituicyte function, including a contribution to the adaptation that occurs in pituicytes when the need for hormone release is elevated.

The function of Ca(2+) channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release.

By mediating the Ca(2+) influx that triggers exocytotic fusion, Ca(2+) channels play a central role in a wide range of secretory processes. Ca(2+) channels consist of a complex of protein subunits, including an alpha(1) subunit that constitutes the voltage-dependent Ca(2+)-selective membrane pore, and a group of auxiliary subunits, including beta, gamma, and alpha(2)-delta subunits, which modulate channel properties such as inactivation and channel targeting. Subtypes of Ca(2+) channels are constituted by different combinations of alpha(1) subunits (of which 10 have been identified) and auxiliary subunits, particularly beta (of which 4 have been identified). Activity-secretion coupling is determined not only by the biophysical properties of the channels involved, but also by the relationship between channels and the exocytotic apparatus, which may differ between fast and slow types of secretion. Colocalization of Ca(2+) channels at sites of fast release may depend on biochemical interactions between channels and exocytotic proteins. The aim of this article is to review recent work on Ca(2+) channel structure and function in exocytotic secretion. We discuss Ca(2+) channel involvement in selected types of secretion, including central neurotransmission, endocrine and neuroendocrine secretion, and transmission at graded potential synapses. Several different Ca(2+) channel subtypes are involved in these types of secretion, and their function is likely to involve a variety of relationships with the exocytotic apparatus. Elucidating the relationship between Ca(2+) channel structure and function is central to our understanding of the fundamental process of exocytotic secretion.

Mechanical design of proteins studied by single-molecule force spectroscopy and protein engineering.

Mechanical unfolding and refolding may regulate the molecular elasticity of modular proteins with mechanical functions. The development of the atomic force microscopy (AFM) has recently enabled the dynamic measurement of these processes at the single-molecule level. Protein engineering techniques allow the construction of homomeric polyproteins for the precise analysis of the mechanical unfolding of single domains. alpha-Helical domains are mechanically compliant, whereas beta-sandwich domains, particularly those that resist unfolding with backbone hydrogen bonds between strands perpendicular to the applied force, are more stable and appear frequently in proteins subject to mechanical forces. The mechanical stability of a domain seems to be determined by its hydrogen bonding pattern and is correlated with its kinetic stability rather than its thermodynamic stability. Force spectroscopy using AFM promises to elucidate the dynamic mechanical properties of a wide variety of proteins at the single molecule level and provide an important complement to other structural and dynamic techniques (e.g., X-ray crystallography, NMR spectroscopy, patch-clamp).

Intracellular Ca(2+) channel immunoreactivity in neuroendocrine axon terminals.

The concentration of neuroendocrine terminals in the neurohypophysis facilitates the identification and localization of Ca(2+) channel subtypes near neuroendocrine release sites. Immunoblots of rat neurohypophysial tissue identified the alpha(1)1.3, alpha(1)2.1, alpha(1)2.2, and alpha(1)2.3 Ca(2+) channel subunits. Immunofluorescence staining of axon terminal plasma membranes was weak, suggesting that Ca(2+) channels are dispersed. This contrasts with the highly punctate alpha(1)2.2 immunoreactivity in bovine chromaffin cells; the neurohypophysial terminals may therefore lack the specialized release zones found in those cells. Immunofluorescence and immunogold labeling identify dense core granule-like structures in the terminal cytoplasm containing multiple Ca(2+) channel types. Ca(2+) channels in internal membranes may play an important role in channel targeting and distribution in neuroendocrine cells.

Nonpeptide GPIIB/IIIA receptor antagonists. Part 21: C-6 flexibility and amide bond orientation are important factors in determining the affinity of compounds for activated or resting platelet receptors.

Compound affinity for activated and resting GPIIb/IIIa receptors may differ, and comparison of those differences determines selectivity. Structural features that influence selectivity are discussed.

Stretching single molecules into novel conformations using the atomic force microscope.

A dense network of interconnected proteins and carbohydrates forms the complex mechanical scaffold of living tissues. The recently developed technique of single molecule force spectroscopy using the atomic force microscope (AFM) has enabled a detailed analysis of the force-induced conformations of these molecules and the determinants of their mechanical stability. These studies provide some of the basic knowledge required to understand the mechanical interactions that define all biological organisms.

The micro-mechanics of single molecules studied with atomic force microscopy.

The atomic force microscope (AFM) in its force-measuring mode is capable of effecting displacements on an angstrom scale (10 A = 1 nm) and measuring forces of a few piconewtons. Recent experiments have applied AFM techniques to study the mechanical properties of single biological polymers. These properties contribute to the function of many proteins exposed to mechanical strain, including components of the extracellular matrix (ECM). The force-bearing proteins of the ECM typically contain multiple tandem repeats of independently folded domains, a common feature of proteins with structural and mechanical roles. Polysaccharide moieties of adhesion glycoproteins such as the selectins are also subject to strain. Force-induced extension of both types of molecules with the AFM results in conformational changes that could contribute to their mechanical function. The force-extension curve for amylose exhibits a transition in elasticity caused by the conversion of its glucopyranose rings from the chair to the boat conformation. Extension of multi-domain proteins causes sequential unraveling of domains, resulting in a force-extension curve displaying a saw tooth pattern of peaks. The engineering of multimeric proteins consisting of repeats of identical domains has allowed detailed analysis of the mechanical properties of single protein domains. Repetitive extension and relaxation has enabled direct measurement of rates of domain unfolding and refolding. The combination of site-directed mutagenesis with AFM can be used to elucidate the amino acid sequences that determine mechanical stability. The AFM thus offers a novel way to explore the mechanical functions of proteins and will be a useful tool for studying the micro-mechanics of exocytosis.

The study of protein mechanics with the atomic force microscope.

The unfolding and folding of single protein molecules can be studied with an atomic force microscope (AFM). Many proteins with mechanical functions contain multiple, individually folded domains with similar structures. Protein engineering techniques have enabled the construction and expression of recombinant proteins that contain multiple copies of identical domains. Thus, the AFM in combination with protein engineering has enabled the kinetic analysis of the force-induced unfolding and refolding of individual domains as well as the study of the determinants of mechanical stability.

Pulsed laser imaging of Ca(2+) influx in a neuroendocrine terminal.

The surge of Ca(2+) that triggers vesicle fusion is shaped by the distribution of Ca(2+) channels and the physical relationship between those channels and the exocytotic apparatus. Although channels and the release apparatus are thought to be tightly associated at fast synapses, the arrangement at neuroendocrine cells is less clear. The distribution of Ca(2+) influx near release sites is difficult to determine because of spatial and temporal limitations on Ca(2+) imaging techniques. We now present spatially resolved images of Ca(2+) influx into rat neuroendocrine terminals on a millisecond time scale. Images of voltage-dependent Ca(2+) influx into neurohypophysial terminals were captured after excitation of Ca(2+)-sensitive dyes with pulses of laser light lasting a fraction of a microsecond. Submembranous Ca(2+) increases were detected during the first millisecond of an evoked Ca(2+) tail current. Steep gradients of Ca(2+) were evident, with concentrations near the membrane reaching above 1 microM during a 30 msec depolarization. Ca(2+) influx appeared evenly distributed, even when diffusion was restricted with an exogenous Ca(2+) chelator. During longer depolarizations, mean and peak Ca(2+) concentrations reached an asymptote in parallel, suggesting that Ca(2+) binding proteins near the membrane rapidly buffer Ca(2+) and do not become saturated during prolonged influx. These data support the hypothesis that exocytosis is activated in these terminals by the summation of influx through multiple, randomly spaced Ca(2+) channels.

Density of transient K+ current influences excitability in acutely isolated vasopressin and oxytocin neurones of rat hypothalamus.

1. The transient outward K+ current (ITO) was studied using whole-cell recording in immunocytochemically identified oxytocin (OT; n = 23) and vasopressin (VP; n = 67) magnocellular neurosecretory cells (MNCs) acutely isolated from the supraoptic nucleus of adult rats. 2. The peak density of ITO during steps to -10 mV was 26 % smaller in OT-MNCs (355 +/- 23 pA pF-1; mean +/- s.e. m.; n = 18) than in VP-MNCs (478 +/- 17 pA pF-1; n = 52). No differences were observed in the voltage dependence of activation or inactivation. 3. Kinetic analysis revealed two components of ITO inactivation in both OT-MNCs (tau1 = 9.2 +/- 0.4 ms and tau2 = 41.2 +/- 1.6 ms; n = 18) and VP-MNCs (tau1 = 12.4 +/- 0.4 ms and tau2 = 37.1 +/- 1.2 ms; n = 52). Although the density of the rapid component (tau1) was not different (275 +/- 13 versus 265 +/- 16 pA pF-1, respectively), the slow component (tau2) was markedly smaller in OT-MNCs (183 +/- 19 versus 331 +/- 16 pA pF-1 in VP-MNCs). 4. In unidentified MNCs, 0.5 mM 4-aminopyridine reduced ITO amplitude by 29% and decreased the latency to spike discharge by about 70% during depolarization from -70 mV. Latency to discharge from potentials less negative than -60 mV, where ITO is inactivated, was unaffected. 5. Comparison of latency to spike discharge in identified cells showed that OT-MNCs achieve spike threshold twice as fast as VP-MNCs when depolarized from -70 mV. The lower density of ITO in OT-MNCs, therefore, accelerates the rate at which excitation can occur in response to depolarizing stimuli and may facilitate the occurrence of higher frequency discharges in OT-MNCs during physiological activation.

Properties of the transient K+ current in acutely isolated supraoptic neurons from adult rat.

The transient outward current (ITO) in magnocellular neurosecretory cells was studied using whole cell patch clamp recordings made from supraoptic neurons acutely isolated from the adult rat. In the presence of tetrodotoxin, depolarizing steps applied from a negative holding potential evoked a rapidly activating and inactivating outward current followed by a slowly activating and sustained outward current. The ITO was unaffected by tetraethylammonium (TEA), but was blocked by 4-aminopyridine. The ITO evoked by depolarization from negative potentials in the presence of 40 mM TEA was not different from that revealed by digital subtraction of current traces recorded with and without a negative conditioning prepulse in control solutions. Tail current analysis during perfusion of media containing different external [K+] indicated that ITO is selective for K+ ions. Exposure to Ca(2+)-free solutions containing divalent inorganic cations such as Cd2+ and Ni2+ could cause shifts in voltage-dependency and, thereby, reduce the amplitude of ITO recorded at fixed submaximal potentials, The maximal ITO achievable under these conditions, however, was reduced compared to control. Moreover, the ITO was also reduced by application of organic Ca2+ channel antagonists such as nifedipine and omega-conotoxin GVIA, indicating that a component of the ITO is somehow dependent on Ca2+ influx.

Calcium-channel subtypes in the somata and axon terminals of magnocellular neurosecretory cells.

To understand the specific functions of Ca(2+)-channel types it is necessary to know how they are distributed within neurons. The unique structure of the magnocellular neurosecretory cells of the rat supraoptic nucleus has made it possible to obtain whole-cell recordings from individual somata and axon terminals acutely isolated from adult rats. Characterization of elicited Ca2+ currents in these cells has demonstrated that certain types are segregated in somata or axon terminals, and that current types defined pharmacologically can display different kinetic properties in the two loci. Observed biophysical properties correlate with functional requirements in the two compartments and have implications for the roles of specific Ca(2+)-channel subtypes.

Identification and characterization of a Ca(2+)-sensitive nonspecific cation channel underlying prolonged repetitive firing in Aplysia neurons.

The afterdischarge of Aplysia bag cell neurons has served as a model system for the study of phosphorylation-mediated changes in neuronal excitability. The nature of the depolarization generating the afterdischarge, however, has remained unclear. We now have found that venom from Conus textile triggers a similar prolonged discharge, and we have identified a slow inward current and corresponding channel, the activation of which seems to contribute to the onset of the discharge. The slow inward current is voltage-dependent and Ca(2+)-sensitive, reverses at potentials slightly positive to O mV, exhibits a selectivity of K approximately equal to Na >> Tris > N-methyl-D-glucamine (NMDG), and is blocked by high concentrations of tetrodotoxin. Comparison of these features with those observed in channel recordings provides evidence that a Ca(2+)-sensitive, nonspecific cation channel is responsible for a slow inward current that regulates spontaneous repetitive firing and suggests that modulation of the cation channel underlies prolonged changes in neuronal response properties.

Distinct omega-agatoxin-sensitive calcium currents in somata and axon terminals of rat supraoptic neurones.

1. Voltage-dependent calcium currents were measured at room temperature using whole-cell patch clamp in acutely isolated somata and axon terminals of the magnocellular neurosecretory cells (MNCs) from the rat supraoptic nucleus. 2. Administration of omega-agatoxin IVA (omega-Aga IVA) blocked a high-threshold non-inactivating current. This current has an IC50 for omega-Aga IVA of 3 nM; no other types of currents were blocked at doses of up to 500 nM. 3. In the axon terminals omega-Aga IVA blocked a high-threshold current that inactivates markedly (tau approximately 448 ms), and has a much lower sensitivity to the toxin, with an IC50 of 270 nM. Unlike the somatic current, the effect of omega-Aga IVA in the terminals is largely prevented by omega-conotoxin GVIA (omega-CgTX). 4. These data suggest that MNC somata express a single type of omega-Aga IVA-sensitive calcium current similar to the P-type calcium current described in other cells. However, the omega-Aga IVA-sensitive current in axon terminals differs from both the P-type and the recently identified Q-type current in that it is also sensitive to omega-CgTX. The distinct biophysical properties of the currents in somata and axon terminals may have important physiological implications.

Voltage-gated calcium currents in the magnocellular neurosecretory cells of the rat supraoptic nucleus.

1. Whole-cell patch-clamp techniques were used to analyse voltage-dependent calcium currents in acutely isolated somata of magnocellular neurosecretory cells (MNCs) from the supraoptic nucleus of the hypothalamus of adult rats. Currents were characterized on the basis of their rates of inactivation and their sensitivity to a series of calcium channel blocking agents. 2. Curve fitting analysis of series of long lasting depolarizing voltage steps from a holding potential of -80 mV revealed three current components with different voltage dependences and rates of inactivation (n = 36). These include a low threshold (-60 mV), rapidly inactivating (tau = 42 +/- 3 ms at -10 mV) component, a high threshold (-30 mV), slowly inactivating (tau = 1790 +/- 70 ms) component and a component with an intermediate threshold (-50 mV) and rate of inactivation (tau = 187 +/- 15 ms). There is also a non-inactivating portion of evoked calcium current with a threshold of -50 mV. 3. Based on its voltage dependence, rate of inactivation, greater sensitivity to the divalent cation nickel than to cadmium and insensitivity to omega-conotoxin GVIA (omega-CgTX), the low threshold current appears to be a T-type calcium current. The rate of inactivation, voltage dependence, and sensitivity to omega-CgTX of the slowly inactivating component suggests that it is an N-type current. The characteristics of the intermediate component do not correspond to any identified calcium current type. 4. Portions of the non-inactivating calcium current are sensitive to nifedipine (23 +/- 2% of the total non-inactivating current at -10 mV; n = 10), suggesting the presence of L-type currents, omega-agatoxin-IVA (omega-Aga-IVA; 20 +/- 6% of total; n = 11), suggesting the presence of P-type channels, and omega-CgTX (39 +/- 3% of total; n = 19), suggesting the presence of a non-inactivating N-type current. The non-inactivating component at low potentials (> or = -50 mV) was selectively blocked by nifedipine, suggesting the presence of a novel, low threshold L-type current. 5. We conclude that MNC soma express T-, N-, L-, and P-type calcium currents, as well as a novel low threshold nifedipine-sensitive current and an unidentified inactivating component. This complement of currents is different from that seen in the terminals of these cells, suggesting a spatial and functional segregation of calcium current types in MNCs.