Filamin: a family of mechanosensory scaffold proteins

The vertebrate filamin family (A, B, and C) are critically involved in development of brain structure, cardio-vasculature, and skeleton. Filamins are large F-actin cross-linking proteins containing an actin-binding domain, followed by 24 Immunoglobulin(Ig)-like domains. Filamins play critical cellular roles as mechanical and scaffold proteins. As mechanical proteins they cross-link F-actin into gels or fibrils. As scaffold proteins they bind over 70 critical proteins. Filamins have overlapping and distinct roles, however it is not understood why. Nor well understood, are the mechanisms by which filamin act as mechanical proteins. Our work focuses on understanding their function; by analyzing site-specific functional divergence, using the evolutionary trace (ET) method, over vertebrate developmental periods -- Teleostei, Amphibian, and Mammalian; and by analyzing filamin behavior under mechanical stress. We find, isoforms diverge from one gene between urochordate and vertebrate lineages; most divergence occurs in Teleostei; and that filamin C diverged least. In addition, the heterogeneous spatial pattern of functional divergence we observe is not correlated with scaffold protein activity either in binding interfaces or across domains. Our results also suggest isoforms have diverged with regard to specificity for binding partners or regulatory function. To elucidate the structure-function relationship of filamin, we used constant force (0-315 pN) simulations to derive both the critical unfolding force and the unfolding pathways at biological levels of force (35-70 pN). Despite a large heterogeneity in the population of force-induced intermediate states, we find a common initial unfolding intermediate in all the Ig-like domains of filamin, where the N-terminal β strand unfolds. We also study the simulated thermal unfolding of several filamin Ig-like domains. We find that thermally-induced unfolding has an early-stage intermediate state similar to the one observed in force-induced unfolding and characterized by the N-terminal strand being unfurled. We propose that the N-terminal strand may act as a conformational switch that unfolds under physiological forces leading to exposure of cryptic binding sites, removal of native binding sites, and modulating the quaternary structure of domains. This work provides insights into both isoform distinctive and mechanical properties of vertebrate filamin

N-terminal strands of filamin Ig domains act as a conformational switch under biological forces

Conformational changes of filamin A under stress have been postulated to play crucial roles in signaling pathways of cell responses. Direct observation of conformational changes under stress is beyond the resolution of current experimental techniques. On the other hand, computational studies are mainly limited to either traditional molecular dynamics simulations of short durations and high forces or simulations of simplified models. Here we perform all-atom discrete molecular dynamics (DMD) simulations to study thermally and force-induced unfolding of filamin A. The high conformational sampling efficiency of DMD allows us to observe force-induced unfolding of filamin A Ig domains under physiological forces. The computationally identified critical unfolding forces agree well with experimental measurements. Despite a large heterogeneity in the population of force-induced intermediate states, we find a common initial unfolding intermediate in all the Ig domains of filamin, where the N-terminal strand unfolds. We also study the thermal unfolding of several filamin Ig-like domains. We find that thermally induced unfolding features an early-stage intermediate state similar to the one observed in force-induced unfolding and characterized by N-terminal strand being unfurled. We propose that the N-terminal strand may act as a conformational switch that unfolds under physiological forces leading to exposure of cryptic binding sites, removal of native binding sites, and modulating the quaternary structure of domains

High-resolution Xist binding maps reveal 2-step spreading during X-inactivation

The Xist long noncoding RNA (lncRNA) is essential for X-chromosome inactivation (XCI), the process by which mammals compensate for unequal numbers of sex chromosomes1-3. During XCI, Xist coats the future inactive X (Xi)4 and recruits Polycomb Repressive Complex 2 (PRC2) to the X-inactivation center (Xic)5. How Xist spreads silencing on a 150 Mb scale is unclear. Here we generate high-resolution maps of Xist binding on the X chromosome across a developmental time course using CHART-seq. In female cells undergoing XCI de novo, Xist follows a two-step mechanism, initially targeting gene-rich islands before spreading to intervening gene-poor domains. Xist is depleted from genes that escape XCI but may concentrate near escapee boundaries. Xist binding is linearly proportional to PRC2 density and H3 lysine 27 trimethylation (H3K27me3), suggesting co-migration of Xist and PRC2. Interestingly, when the Xi is acutely stripped off Xist in post-XCI cells, Xist recovers quickly within both gene-rich and -poor domains on a time-scale of hours instead of days, suggesting a previously primed Xi chromatin state. We conclude that Xist spreading takes distinct stage-specific forms: During initial establishment, Xist follows a two-step mechanism, but during maintenance, Xist spreads rapidly to both gene-rich and -poor regions

Theta Rhythms Coordinate Hippocampal–Prefrontal Interactions in a Spatial Memory Task

Decision-making requires the coordinated activity of diverse brain structures. For example, in maze-based tasks, the prefrontal cortex must integrate spatial information encoded in the hippocampus with mnemonic information concerning route and task rules in order to direct behavior appropriately. Using simultaneous tetrode recordings from CA1 of the rat hippocampus and medial prefrontal cortex, we show that correlated firing in the two structures is selectively enhanced during behavior that recruits spatial working memory, allowing the integration of hippocampal spatial information into a broader, decision-making network. The increased correlations are paralleled by enhanced coupling of the two structures in the 4- to 12-Hz theta-frequency range. Thus the coordination of theta rhythms may constitute a general mechanism through which the relative timing of disparate neural activities can be controlled, allowing specialized brain structures to both encode information independently and to interact selectively according to current behavioral demands

CEO succession and the CEO’s commitment to the status quo

Chief executive officer (CEO) commitment to the status quo (CSQ) is expected to play an important role in any firm’s strategic adaptation. CSQ is used often as an explanation for strategic change occurring after CEO succession: new CEOs are expected to reveal a lower CSQ than established CEOs. Although widely accepted in the literature, this relationship remains imputed but unobserved. We address this research gap and analyze whether new CEOs reveal lower CSQ than established CEOs. By analyzing the letters to the shareholders of German HDAX firms, we find empirical support for our hypothesis of a lower CSQ of newly appointed CEOs compared to established CEOs. However, our detailed analyses provide a differentiated picture. We find support for a lower CSQ of successors after a forced CEO turnover compared to successors after a voluntary turnover, which indicates an influence of the mandate for change on the CEO’s CSQ. However, against the widespread assumption, we do not find support for a lower CSQ of outside successors compared to inside successors, which calls for deeper analyses of the insiderness of new CEOs. Further, our supplementary analyses propose a revised tenure effect: the widely assumed relationship of an increase in CSQ when CEO tenure increases might be driven mainly by the event of CEO succession and may not universally and continuously increase over time, pointing to a “window of opportunity” to initiate strategic change shortly after the succession event. By analyzing the relationship between CEO succession and CEO CSQ, our results contribute to the CSQ literature and provide fruitful impulses for the CEO succession literature

A Typed Pattern Calculus

Abstract. The pure pattern calculus generalises the pure lambda-calculus by basing computation on pattern-matching instead of beta-reduction. The simplicity and power of the calculus derive from allowing any term to be a pattern. As well as supporting a uniform approach to functions, it supports a uniform approach to data structures which underpins two new forms of polymorphism. Path polymorphism supports searches or queries along all paths through an arbitrary data structure. Pattern polymorphism supports the dynamic creation and evaluation of patterns, so that queries can be customised in reaction to new information about the structures to be encountered. In combination, these features provide a natural account of tasks such as programming with XML paths. As the variables used in matching can now be eliminated by reduction it is necessary to separate them from the binding variables used to control scope. Then standard techniques suffice to ensure that reduction progresses and to establish confluence of reduction.

PAR-TERRA directs homologous sex chromosome pairing

In mammals, homologous chromosomes rarely pair outside of meiosis. An exception is the X-chromosome, which transiently pairs during X-chromosome inactivation (XCI). How two chromosomes find each other in 3D space is not known. Here, we reveal a required interaction between the X-inactivation center (Xic) and the telomere in mouse embryonic stem cells. The sub-telomeric, pseudoautosomal region (PAR) of both sex chromosomes (X,Y) also undergoes pairing. PAR transcribes a class of telomeric RNA, dubbed “PAR-TERRA”, which accounts for a vast majority of all TERRA transcripts. PAR-TERRA binds throughout the genome, including PAR and Xic. PAR-TERRA anchors the Xic to PAR, creating a “tetrad” of pairwise homologous interactions (Xic:Xic, PAR:PAR, Xic:PAR). Xic pairing occurs within the tetrad. Depleting PAR-TERRA abrogates pairing and blocks initiation of XCI, whereas autosomal PAR-TERRA induces ectopic pairing. We proposed a Constrained Diffusion Model in which PAR-TERRA creates an interaction hub to guide Xic homology searching during XCI