TRF2-mediated telomere protection is dispensable in pluripotent stem cells.

In mammals, telomere protection is mediated by the essential protein TRF2, which binds chromosome ends and ensures genome integrity1,2. TRF2 depletion results in end-to-end chromosome fusions in all cell types that have been tested so far. Here we find that TRF2 is dispensable for the proliferation and survival of mouse embryonic stem (ES) cells. Trf2-/- (also known as Terf2) ES cells do not exhibit telomere fusions and can be expanded indefinitely. In response to the deletion of TRF2, ES cells exhibit a muted DNA damage response that is characterized by the recruitment of γH2AX-but not 53BP1-to telomeres. To define the mechanisms that control this unique DNA damage response in ES cells, we performed a CRISPR-Cas9-knockout screen. We found a strong dependency of TRF2-null ES cells on the telomere-associated protein POT1B and on the chromatin remodelling factor BRD2. Co-depletion of POT1B or BRD2 with TRF2 restores a canonical DNA damage response at telomeres, resulting in frequent telomere fusions. We found that TRF2 depletion in ES cells activates a totipotent-like two-cell-stage transcriptional program that includes high levels of ZSCAN4. We show that the upregulation of ZSCAN4 contributes to telomere protection in the absence of TRF2. Together, our results uncover a unique response to telomere deprotection during early development.

Different requirements of functional telomeres in neural stem cells and terminally differentiated neurons.

Telomeres have been studied extensively in peripheral tissues, but their relevance in the nervous system remains poorly understood. Here, we examine the roles of telomeres at distinct stages of murine brain development by using lineage-specific genetic ablation of TRF2, an essential component of the shelterin complex that protects chromosome ends from the DNA damage response machinery. We found that functional telomeres are required for embryonic and adult neurogenesis, but their uncapping has surprisingly no detectable consequences on terminally differentiated neurons. Conditional knockout of TRF2 in post-mitotic immature neurons had virtually no detectable effect on circuit assembly, neuronal gene expression, and the behavior of adult animals despite triggering massive end-to-end chromosome fusions across the brain. These results suggest that telomeres are dispensable in terminally differentiated neurons and provide mechanistic insight into cognitive abnormalities associated with aberrant telomere length in humans.

"Interstellar Wanderers:" Digital Support for Teaching Place Value Within the Activity Approach Framework.

Background: The majority of mistakes students make while using "well-learned" decimal counting, can be attributed to their miscomprehension of the structure and arrangement of the "positional" numeral system. Digital support may merely serve an illustrative and training function, or it may provide the special environment for locating the problem of positional counting as a part of meaningful actions by the students. Following the Activity approach, we aimed to scaffold the students' own learning actions, in such a way as to reveal the origin of the multi-digit number concept. Thus, we used counting in other-base systems as a way for students to reconsider the reasoning behind familiar operations in the most common base-ten system. Objective: The purpose of this paper is to present the approach to computer support which we have designed, based on our analysis of the activity content related to the multi-digit number concept, and to discuss some preliminary results of the first training series. Design: The approach to educational environment design developed within the Learning Activity theory defines the ways in which a computer becomes essential. The computer should provide a transparent interface which allows students to perform transformations with objects which will react accordingly. The additional opportunities to perform concept-mediated orientation procedures should also be scaffolded by digital means. For the purposes of our study, the computer-based educational module "Interstellar wanderers" was designed. Four groups of students from 2nd to 5th grade (8-12 years of age, 20 children in total) participated in the experimental computer-based lessons (over 30 hours); classroom observations, videotaped discussions, and logs of students' individual work in the computer simulation were used for analysis. Results: The preliminary results of the experimental teaching showed that the computer support which we developed may scaffold students' progress toward acquisition of the multi-digit number concept through a reflective re-thinking of the well-learned decimal system. Yet further research is needed to get a quantitative analysis of students' performance. Conclusion: The general principles of computer support design based on the Activity approach in education (Galperin, Davydov, & Talyzina) demand a thorough analysis of the origin of the concept being studied, as well as the design of appropriate content and means of students' actions and corresponding contexts and tasks. The digital means which we designed to support students' learning activity, are in demand and bring promising results.

The Action "Wanted!": The Concept of Valency during the Early Steps of Mastering Chemistry.

Background: We apply the theory of step-by-step concept formation (Galperin) and the theory of learning activity (Davydov) to the practice of education and teacher training. Objective: This paper describes a feasible way to teach the basics of Galperin's theory to students studying pedagogical psychology, by involving them in an exemplary educational module on combining chemical formulas according to the elements' valency values. Design: We suggested that our students participate in an educational module which was designed as an example of how to materialize orientation components of action as the basis of concept formation. The "practical" action for mastering the valency concept was to combine the correct formula for a pair of elements, whose valency was provided, and correct the formulas made by someone else. However, the core "orientation" required an extended procedure of building a "molecule" structure with a special construction kit. The key challenge for the students was to coordinate their calculation of the number of bonds needed for the molecule, and name the exact total before they would receive their atom-tokens for constructing the model. Results: Our workshop participants took on the role of students facing their first encounter with chemistry, and embarked on the formation sequence. At the same time they analyzed the mistakes they had made by ignoring some procedural steps. Considered through the lens of Galperin's theory, these "adult" mistakes proved how vital his theoretical principles are for educational design. Conclusion: Our workshop thus illustrated that the search for the proper action for concept formation within Galperin's theory framework is a challenging task. The difficulties that our participants experienced while they worked as pupils revealed the divergence of didactic approaches. The effectiveness of the concept formation approach, even within our small exemplary educational module, once again confirmed the practical value of pedagogical psychology in general, and Galperin's theory in particular.

The "Moon Test": A Step Towards Evaluating Comprehension of Educational Text through Model Mediation.

Background: This paper addresses the issue of educational text comprehension, which is one of the major problems in secondary schools, especially when such texts are introduced in the natural sciences. Studies on text comprehension often regard reading as a standalone skill: its mechanisms are discussed from leading theoretical approaches (cognitivism, constructivism, etc.), and variables are distinguished and evaluated. Most of the researchers consider text comprehension to be active reconstruction of the meaning which the text delivers, and regard the application of the information retrieved from the text to problem-solving as the indicator for a deep comprehension level. Since we work within the framework of Cultural-Historical Activity Theory (CHAT), we consider educational text comprehension to be mediated through special content-related models which students have to acquire. Unfortunately, there are no studies which have directly linked reading, corresponding problem-solving, and working with content-related models (symbolic means, schemes); hence, with this research, we are seeking to fill in the gap. Objective: Our goal is to elaborate the perspective on educational text comprehension as mediated through mastering special modeling (symbolic) means. In this article we illustrate this approach with the "Moon test"- an assessment procedure which we designed to materialize the components of orientation of students' action as they succeed or fail to solve problems by relying on the educational text provided. Design: We conducted the "Moon test" among the fifth graders (10-12 years old). The text, which told the students how to use the moon's visual transformations as a calendar, was followed by 12 tasks on the topic. The tasks required using the text to master the model provided, and then solve challenging tasks which only referred to the model implicitly. Results: To analyze the results, we grouped the tasks in four blocks: 1) model acquisition; 2) mastering; 3) application; and 4) experience. The results showed a statistically significant decrease in the students' performance on tasks of the third and the fourth blocks, which required reasonable application of the models. Further analysis of individual patterns of performance allowed us to distinguish clusters of students with different levels of success in each block. Conclusion: Our results attest to the importance of model mediation for reading comprehension and the development of scientific literacy.