Collective dynamics of astrocyte and cytoskeletal systems

Abstract

Advances in imaging and biological sample preparations now allow researchersto study collective behavior in cellular networks with unprecedented detail. Imaging the electrical signaling of neuronal networks at the cellular level has generated exciting insights into the multiscale interactions within the brain. This thesis aims at a complementary view of the general information processing of the brain, focusing on other modes of non-electrical information. The modes discussed are the collective, dynamical characteristics of non-electrically active, non-neuronal brain cells, and mechanical systems. Astrocytes are the studied non-neuronal brain cells, and the cytoskeleton is the studied dynamic, mechanical system consisting of various filamentous networks. The two filamentous networks studied herein are the actin cytoskeleton and the microtubule network. Techniques from calcium imaging and cell mechanics are adapted to measure these often overlooked information channels, which operate at length scales and timescales distinct from electrical information transmission. Structural, astrocyte actin images, microtubule structural image sequences, and the calcium signals of collections of astrocytes are analyzed using computer vision and information theory. Filamentous alignment of actin with nearby boundaries reveals that stellate astrocytes have more perpendicularly oriented actin than undifferentiated astrocytes. Harnessing the larger length scale and slower dynamical time scale of microtubule filaments relative to actin filaments led to the creation of a computer vision tool to measure lateral filamentous fluctuations. Finally, we adapt information theory to the analog calcium (Ca2+) signals within astrocyte networks classified according to subtype. We find that, despite multiple physiological differences between immature and injured astrocytes, stellate (healthy) astrocytes have the same speed of information transport as these other astrocyte subtypes. This uniformity in speed persists when either the cytoskeleton (Latrunculin B) or energy state (ATP) is perturbed. Astrocytes, regardless of physiological subtype, tend to behave similarly when active under normal conditions. However, these healthy astrocytes respond most significantly to energy perturbation, relative to immature and injured astrocytes, as viewed through cross-correlation, mutual information, and partitioned entropy. These results indicate the value of drawing information from structure and dynamics. We developed and adapted tools across scales from nanometer scale alignment of actin filaments to hundreds of microns scale information dynamics in astrocyte networks. Including all potential modalities of information within complex biological systems, such as the collective dynamics of astrocytes and the cytoskeleton in brain networks is a step toward a fuller characterization of brain functioning and cognition.