Regeneration of broken tip links and restoration of mechanical transduction in hair cells

A hair cell’s tip links are thought to gate mechanoelectrical transduction channels. The susceptibility of tip links to acoustic trauma raises questions as to whether these fragile structures can be regenerated. We broke tip links with the calcium chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid and found that they can regenerate, albeit imperfectly, over several hours. The time course of tip-link regeneration suggests that this process may underlie recovery from temporary threshold shifts induced by noise exposure. Cycloheximide does not block tip-link regeneration, indicating that new protein synthesis is not required. The calcium ionophore ionomycin prevents regeneration, suggesting regeneration normally may be stimulated by the reduction in stereociliary Ca2+ when gating springs rupture and transduction channels close. Supporting the equivalence of tip links with gating springs, mechanoelectrical transduction returns over the same time period as tip links; strikingly, adaptation is substantially reduced, even 24 hr after breaking tip links.

High-resolution structure of hair-cell tip links

Transduction-channel gating by hair cells apparently requires a gating spring, an elastic element that transmits force to the channels. To determine whether the gating spring is the tip link, a filament interconnecting two stereocilia along the axis of mechanical sensitivity, we examined the tip link's structure at high resolution by using rapid-freeze, deep-etch electron microscopy. We found that the tip link is a right-handed, coiled double filament that usually forks into two branches before contacting a taller stereocilium; at the other end, several short filaments extend to the tip link from the shorter stereocilium. The structure of the tip link suggests that it is either a helical polymer or a braided pair of filamentous macromolecules and is thus likely to be relatively stiff and inextensible. Such behavior is incompatible with the measured elasticity of the gating spring, suggesting that the gating spring instead lies in series with the helical segment of the tip link.

A Calcium-Selective Channel from Root-Tip Endomembranes of Garden Cress1

A Ca2+ channel from root-tip endomembranes of garden cress (Lepidium sativum L.) (LCC1) was characterized using the planar lipid-bilayer technique. Investigation of single-channel recordings revealed that LCC1 is voltage gated and strongly rectifying. In symmetrical 50 mm CaCl2 solutions, the single-channel conductance was 24 picosiemens. LCC1 showed a moderate selectivity for Ca2+ over K+ (9.4:1) and was permeable for a range of divalent cations (Ca2+, Ba2+, and Sr2+). In contrast to Bryonia dioica Ca2+ channel 1, a Ca2+-selective channel from the endoplasmic reticulum of touch-sensitive tendrils, LCC1 showed no bursting channel activity and had a low open probability and mean open time (2.83 ms at 50 mV). Inhibitor studies demonstrated that LCC1 is blocked by micromolar concentrations of erythrosin B (inhibitor concentration for 50% inhibition [IC50] = 1.8 μm) and the trivalent cations La3+ (IC50 = 5 μm) and Gd3+ (IC50 = 10 μm), whereas verapamil showed no blocking effect. LCC1 may play an important role in the regulation of the cytoplasmic free Ca2+ concentration in root-tip and/or root-cap cells. The question of whether this ion channel is part of the gravitropic signal transduction pathway deserves further investigation.

The mammalian fauna on the islands at the northern tip of Sabah, Borneo / Shukor Md. Nor.

n.s. no.83(1996)

TTIP and Public Procurement: Going beyond the tip of the iceberg. CEPS Policy Brief No. 339, March 2016

Trade negotiations involving international public procurement rules are on the rise, stimulating a growing interest in having a clear picture of the economic stakes involved, including the current level of international openness. A recent paper published by the European Centre for International Political Economy (Messerlin, 2016) made an attempt to provide a range of estimates for the EU and the US and found relatively low rates of import penetration. This analytical approach, however, looked only at the ‘tip of the procurement iceberg’, as the data used covered primarily only one modality of international procurement (direct cross-border), which is not the main avenue for international government procurement. Other modalities, such as procurement from foreign subsidiaries established in Europe, account for much more. Such an approach therefore ignores the main modalities through which foreign firms win EU contracts. Once these other main procurement modalities are taken into account, EU openness in procurement is much higher. Comparable data across all modalities do not yet exist for the US, but we do have clear evidence that the US has introduced the largest number of protectionist procurement measures since 2008 affecting all modalities for international procurement. Against this background, this Policy Brief makes four basic points: i. Public procurement is a key area of trade negotiations, and TTIP is no exception to this rule. ii. The existing levels of openness in procurement markets need to be assessed across all three main procurement modalities and not based only on direct cross-border procurement, which is not the main procurement avenue. According to this comprehensive metric, the EU market already has a high foreign participation rate, including by US companies. iii. Unfortunately, similar data do not exist for the US market. But there is growing evidence of discriminatory measures introduced in recent years, which impede the ability of EU firms to compete on a level-playing field in US procurement markets. iv. The importance of procurement as a key negotiating area requires better data and a greater analytical engagement.