Defects in myosin XVa and the PDZ domain-containing protein, whirlin, underlie deafness in humans and mice. Hair bundles of mutant mice defective for either protein have abnormally short stereocilia. Here, we show that whirlin, like myosin XVa, is present at the very tip of each stereocilium in the developing and mature hair bundles of the cochlear and vestibular system. We found that myosin XVa SH3-MyTH4 region binds to the short isoform of whirlin (PR-PDZ3) that can rescue the stereocilia growth defect in whirlin defective mice. Moreover, the C-terminal MyTH4-FERM region of myosin XVa binds to the PDZ1 and PDZ2 domains of the long whirlin isoform. We conclude that a direct myosin XVa-whirlin interaction at the stereocilia tip is likely to control the elongation of stereocilia. Whirlin, unlike myosin XVa, is also transiently localized in the basal region of developing stereocilia in rat vestibular and cochlear hair cells until P4 and P12, respectively. Notably, whirlin also interacts with myosin VIIa that is present along the entire length of the stereocilia. Finally, we show that the transmembrane netrin-G1 ligand (NGL-1) binds to the PDZ1 and PDZ2 domains of whirlin and has an extracellular region that homophilically self-interacts in a Ca2+-dependent manner. The interaction between whirlin and NGL-1 might be involved in the stabilization of interstereociliar links.

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Almost seventy years ago, chemist Dorothy Jordan noted that "gels are easier to recognize than to define"."] We will attempt a definition nonetheless : Gels are semi-rigid colloidal systems rich in liquid. Protoplasm ranks as the most widespread example of this peculiar state of matter. A gel is formed by a molecular network, often fibrous in nature, that entrains the dispersing medium within its capillary spaces. Most organic gelators are polymeric (for example, gelatin, ['] poIy(acry1ic acid) ,I3] and hyaluronic acidr4]), but small molecules (for example. cholesteryl 4-(2-anthryloxy)b~tanoate, [~~ 5-hexadecyl-2,4,6-triaminopyrimidine,l6] N-octylglucona~nide,~'~ and arborolsr8]) can induce gelation as well.We have long been interested in a little-known but remarkable gelator. dibenzoyl-L-cystine (DBC).19] Although most gels require 1 -1 0 O i O gelator, DBC in water rigidifies at concentrationsas low as 2 mM (0.1 YO). While attempting to learn how such miniscule quantities of DBC manage to self-assemble into a macroscopic network, we performed an X-ray analysis of a gel "fiber". Structural data at the atomic level is virtually unavailable for gel networks, and it is only through good fortune that these data became accessible. But before giving the details, let us summarize the properties of acylcystines in water.DBC forms a firm, translucent gel in a thermally reversible process at a concentration of 4 mM in ethanol (5%)/water. Ethanol was used because the DBC tends to crystallize from pure water. There is, clearly. a delicate balance between gelation and crystallization. The gelating properties are destroyed when the S-S linkage is replaced by CH,CH, or CH=CH groups."'] This implies that the CH,-S-S-CH, dihedral angle["] of about 90 'C plays a key role in the formation of the molecular network. Gels d o not form from esterified DBC or at basic p H values, which suggests the presence of either a favorable -COOH/-COOH attraction or a deleterious -COY /-CO; repulsion. Addition of NaCl (0.1 M) softens the gels, whereas more than 10 YO dioxane induces crystallization.Diacetyl-L-cystine fails to gelate in ethanol ( 5 %)/water. Di-noctanoyl-L-cystine (4 mM) forms only a soft and cloudy gel. The superior quality of the DBC gel leads us to suspect that aromatic K -~T stacking stabilizes the creation of fibers. This conclusion is appealing. because intermolecular hydrogen bonding between DBC molecules in water would seem too unfavorable to serve as the sole adhesive force. Assembly of DBC molecules appears to be a cooperative event much like the micellization of surfactants. This was affirmed by a primitive yet informative experiment in which 2.0 x 2.0 cm2 squares of Parafilm were placed on the surface of aged DBC gels. Known weights were then positioned carefully at the center of a film until the film submerged. A plot of "supportable weight" vs. DBC concentration (not shown) gave a sharp break at 1.6 KIM, a value that might be qualitatively regarded as the "critical gelation concentration".Spin-lattice relaxation stud...

We investigated how environmental factors initiate Heterosigma akashiwo and Skeletonema costatum blooms from resting stages in bottom sediments in a shallow port over 2 yr. Using field-collected sediments, we also conducted laboratory experiments on how light intensity affects germination of resting stages and growth of the germinated cells. Both phytoplankton species bloomed only in summer, when water temperature and solar radiation were high enough for growth. All three blooms of H. akashiwo and the earliest bloom of S. costatum in a year occurred right after transmission of strong light (.200 mmol quanta m 22 s 21 ) to the bottom layer and a peak occurred in dissolved inorganic phosphorus (DIP). In the laboratory, resting stages of H. akashiwo and S. costatum germinated even in dim light (20 and 65 mmol quanta m 22 s 21 , respectively), but germinated cells required stronger light of .130 and 280 mmol quanta m 22 s 21 , respectively, for rapid growth. This value is much higher than the threshold for survival, and is higher than the half-saturating light intensity for growth of vegetative cells. Abundance of the resting stages of both species in the sediments rapidly increased during blooms and logarithmically decreased during nonbloom periods, suggesting that resting stages are continuously consumed. For both species, our results suggest that blooms initiate when transmission of sufficient light permits: first, germination of cells from the sediment; second, rapid growth of these germinated cells. Temperature and DIP must also exceed a facilitating threshold.

We investigated growth interactions between Skeletonema costatum (Greville) Cleve and Heterosigma akashiwo (Hada) Hada ex Hara et Chihara using bi-algal cultures under axenic conditions. When inoculated at high cell densities, growth of both species was coincidentally suppressed. In other combinations of inoculation density, the species first reaching stationary phase substantially reduced maximum cell densities of the other species. When cultured together under conditions without cell contact, growth of S. costatum and H. akashiwo were both suppressed. Furthermore, despite re-enrichment with nutrients, filtrates from dense cultures of S. costatum and H. akashiwo reciprocally reduced their maximum cell densities. In additional experiments, growth of Chaetoceros muelleri was also suppressed with filtrates from the above cultures, but growth of Prorocentrum minimum was not. Therefore, growth interactions between these species strongly suggest the involvement of allelopathic substances secreted by both species. Finally, growth and interaction of S. costatum and H. akashiwo in bi-algal cultures were simulated using a mathematical model. This model indicated that S. costatum and H. akashiwo steadily approach a stable equilibrium point of about 3.4 × 10 5 cells ml -1 and 4.8 × 10 5 cells ml -1 , respectively, when the 2 species coexist.KEY WORDS: Allelopathy · Bacillariophyceae · Skeletonema costatum · Raphidophyceae · Heterosigma akashiwo · Bi-algal culture · Growth inhibition Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 339: [83][84][85][86][87][88][89][90][91][92] 2007 Honjo (1994) and Smayda (1998) indicated that most pelagic blooms attributed to O. luteus were almost certainly those of H. akashiwo. Therefore, we will treat O. luteus described by Pratt (1966) and Honjo & Tabata (1985) as H. akashiwo.In in situ and in vitro experiments, high concentrations of Olisthodiscus luteus inhibit the growth of Skeletonema costatum, while lower concentrations stimulate the growth of S. costatum (Pratt 1966). Similarly, Honjo et al. (1978) reported that Heterosigma akashiwo and S. costatum alternated in forming red tide blooms in the fishing port of Hakozaki. They found that filtrate from dense cultures of H. akashiwo, reenriched with nutrients, suppressed the growth of S. costatum. Furthermore, Honjo & Tabata (1985) reported an apparent and reciprocal codominance between O. luteus and diatoms in a 70 m 3 outdoor tank with flowing coastal water.Unfortunately, none of the in vitro experiments mentioned above used axenic strains. In this study, we used axenic strains of Heterosigma akashiwo and Skeletonema costatum. First, we conducted bi-algal culture experiments with several combinations of initial cell densities of the 2 species. Second, we examined allelopathic interactions between H. akashiwo and S. costatum by way of both growth experiments using culture filtrates and of bi-algal culture experiments under noncontact conditions. Finally, we simulated the...