Regulation of dendritic spine stability.

Dendritic spines undergo several types of transformations, ranging from growth to collapse, and from elongation to shortening, and they experience dynamic morphological activity on a rapid time scale. Changes in spine number and morphology occur under pathological conditions like excitotoxicity, but also during normal central nervous system development, during hormonal fluctuations, and in response to neural activity under physiological circumstances. We briefly review evidence for various types of alterations in spines, and discuss the possible molecular basis for changes in spine stability. Filamentous actin appears to be the most important cytoskeletal component of spines, and a growing list of actin-associated and actin-regulatory proteins has been reported to reside within spines. We conclude that spines contain two distinct pools of actin filaments (one stable, the other unstable) that provide the spine with both a stable core structure and a dynamic, complex shape. Finally, we review the current state of knowledge of actin filament regulation, based on studies in nonneuronal cells.

Inhibition of interleukin 7 receptor signaling by antigen receptor assembly.

After the productive rearrangement of immunoglobulin (Ig) heavy chain genes, precursor (pre-)B lymphocytes undergo a limited number of cell divisions in response to interleukin (IL)-7. Here, we present evidence that this phase of IL-7-dependent expansion is constrained by an inhibitory signal initiated by antigen receptor assembly. A line of pre-B cells from normal murine bone marrow that expresses a mu heavy chain with a D-proximal V(H)7183.2 region divides continuously in IL-7. IL-7 responsiveness ceases upon differentiation to the mu(1), kappa(1) stage, despite continuing expression of the IL-7 receptor (IL-7R), suggesting that antigen receptor assembly inhibits IL-7 responsiveness. This is confirmed by introduction of a rearranged lambda light chain gene, which inhibits proliferative signaling through the IL-7R. Inhibition is specific to the IL-7R, because it is overcome by replacement of the IL-7R cytoplasmic domain with corresponding sequences from the closely related IL-2Rbeta chain. Alteration of a single tyrosine residue, Tyr410, in the IL-7R cytoplasmic domain to phenylalanine also prevents the inhibition of proliferation after antigen receptor assembly. Thus, the loss of IL-7 responsiveness after antigen receptor assembly may be mediated through the recruitment of an inhibitory molecule to this residue. Our findings identify a novel mechanism that limits cytokine-dependent proliferation during B lymphopoiesis. This mechanism may be essential for the proper regulation of peripheral B lymphocyte numbers.

The interleukin-7 receptor alpha chain transmits distinct signals for proliferation and differentiation during B lymphopoiesis.

The interleukin 7 receptor (IL7R), which contains a unique alpha chain and a gamma chain shared by other cytokine receptors, is indispensable for normal lymphocyte development. The basis for this role is poorly understood. Here we show that the IL7R alpha chain not only causes progenitors to proliferate, but also has a distinct activity in inducing differentiation. First, we identify a single cytoplasmic tyrosine residue in the IL7R alpha chain that is essential for cell cycle entry and proliferation dependent on phosphatidylinositol 3-kinase. We use a mutant alpha chain in which this residue has been altered to reconstitute B lymphopoiesis by retrovirus-mediated gene transfer in cultures of bone marrow from mice deficient in IL7R alpha chain. The mutation abrogates the proliferation of B-lymphocyte progenitors, but reveals a novel function of the alpha chain in promoting immunoglobulin heavy chain gene rearrangement leading to B-cell differentiation. This function is lost (but proliferation sustained) when the cytoplasmic domain of IL7R alpha is replaced by corresponding sequences from the IL2R, despite the similarity on their signalling mechanisms. Thus, the signals which mediate a differentiative function of the IL7R in B lymphopoiesis are specific and distinct from those causing proliferation.

Parametric stress analysis of bonded "combination-materials" type of orthodontic brackets.

The finite element analysis method and a two-dimensional idealization were used to conduct a detailed parametric study of the stresses in and displacement of models of bonded edgewise "combination-materials" type of orthodontic brackets when subjected to loading and constraint conditions that are deemed to be the same as those that exist in vivo. A "combination-materials" type bracket is herein defined as one in which different materials are used in fabricating the main body and the archwire slot. The present study was conducted in three parts. In Part 1, 16 model cases were analyzed, involving 4 different combinations of materials (for the main body of the bracket and its archwire slot) and 4 different overall bracket configurations (comprising main body, base and slot). The focus of Part 2 was the model bracket configuration that was, on the basis of Part 1 results, deemed to show the "optimum performance". In this part, the parameter investigated was the value of the modulus of elasticity of the adhesive, Eadh. In Part 3, the concept of an efficiency index of the bonded bracket-archwire system, eta, was introduced and explained. The dependence of eta on stated characteristics of a rectangular archwire, for the "optimum performance" model, was explored. Part 1 results led to the conclusion that the "optimum performance" model has equally angulated buccal and lingual edges and the main body and archwire slot are fabricated from glass fiber-reinforced polycarbonate and stainless steel, respectively. Part 2 results showed that, for the aforementioned "optimum performance" model, the longitudinal displacement of the archwire slot is fairly insensitive to Eadh. Part 3 results showed that eta is intimately related to each of the wire characteristics studied (namely cross-sectional dimensions, "interbracket distance" and modulus of rigidity of the material).

Coronary angiography after heart transplantation: should perioperative study be the "gold standard"?

The fact that allograft coronary arteriopathy is frequent and has a poor prognosis means early diagnosis is critical. Furthermore, because of important distinctions between native heart coronary artery disease and allograft arteriopathy, standard noninvasive diagnostic tests seem less sensitive and specific. Assuming that coronary angiography is the optimal method for detection and staging of allograft arteriopathy, one must establish the point at which an initial study should be performed and the incidence of abnormalities in donor hearts. Review of perioperative coronary angiograms in 75 consecutive patients undergoing heart transplantation (within 8 weeks) demonstrated that only six hearts had coronary artery abnormalities: two had focal coronary artery disease, one had an anomalous circumflex coronary artery, and three had nonobstructive calcification of the coronary arteries. To determine if serial quantitative angiography was helpful in detecting progression of coronary disease during a 12-month period, 28 patients underwent baseline and repeat quantification of mean luminal diameter of predetermined segments of the mid and distal portions of the left anterior descending artery. No patient had identifiable disease on the first angiogram, and 12 were studied in the first year of their transplants. During the interval, mean mid left anterior descending coronary diameter was 3.17 +/- 0.6 mm on the first study and 3.06 +/- 0.7 on the second study. Visual assessment of the angiograms, however, identified allograft arteriopathy when two studies were available for inspection in 7 of 28 patients despite no significant diameter reduction of identified coronary segments.(ABSTRACT TRUNCATED AT 250 WORDS)