Biology subjects

Brack, A. S.

Publications and source records attributed to Brack, A. S..

5 recordsLinked to original sources

bioRxiv2025

The Integrated Stress Response Pathway Improves Aged Murine Muscle Stem Cell Activation and in vivo Regeneration.

For efficient regeneration, muscle stem cells (MuSCs) transition out of quiescence through a series of progressively more activated states. During MuSC aging, transition through the earliest steps is the slowest and delayed, with the molecular regulators that govern this transition not well characterized. By analyzing the dynamic changes of MuSCs at the molecular (scRNA-Seq and Cell Painting) and phenotypic (heteromotility) level at single cell resolution we found that the Integrated Stress Response (ISR) Pathway is a critical regulator of MuSC transition states. Aged MuSCs have increased baseline ISR activity in quiescence that does not increase during activation to levels observed in adult MuSCs. Rapid and transient pharmacological ISR activation in vitro was sufficient to increase aged MuSC activation rate and migratory behavior as well as alter the transcriptional states toward a younger phenotype. ISR activation also improved aged MuSC potency and aged mouse muscle regeneration in vivo. Therefore, pharmacological activation of the ISR has therapeutic potential to improve MuSC function and skeletal muscle repair during aging.

cell biology↗

bioRxiv2020

Spatial Heterogeneity of Delta-Like 4 Within a Multinucleated Niche Cell Maintains Muscle Stem Cell Diversity

The quiescent muscle stem cell (QSC) pool is heterogeneous and generally characterized by the presence and levels of intrinsic myogenic transcription factors. Whether extrinsic factors maintain the diversity of states across the QSC pool remains unknown. The muscle fiber is a multinucleated syncytium that serves as a niche to QSCs, raising the possibility that the muscle fiber regulates the diversity of states across the QSC pool. Here we show that the muscle fiber maintains a continuum of quiescent states, through a gradient of Notch ligand, Dll4, produced by the fiber and captured by QSCs. The abundance of Dll4 captured by the QSC correlates with levels of the SC identity gene, Pax7. Niche-specific loss of Dll4 decreases QSC diversity and shifts the continuum, towards more proliferative and committed states. We reveal that fiber-derived Mindbomb1 (Mib1), an E3 ubiquitin ligase activates Dll4 and controls the spatial localization of Dll4. In response to injury, with a Dll4-replenished niche, the normal continuum and diversity of SC pool is restored, demonstrating bi-directionality within the SC continuum. Our data shows that a post-translational mechanism controls spatial heterogeneity of Notch ligands in a multinucleated niche cell to maintain a continuum of diverse states within the SC pool during tissue homeostasis.

cell biology↗

bioRxiv2020

PDH Mediated Mitochondrial Respiration Controls the Speed of Muscle Stem Cell Activation in Muscle Repair and Aging

Decline in the skeletal muscle stem cell (MuSC) function is a major contributor to age-associated impairments in muscle regeneration and function. The ability of MuSCs to activate (i.e. exit quiescence, enter the cell cycle, and divide) following injury is a critical step that initiates muscle regeneration. However, the mechanisms that regulate MuSC activation function are poorly understood. Here, we show that the activation function, specifically the speed by which cells progress through G0-G1, declines tremendously with age in mouse MuSCs. Using a number of in vivo models and ex vivo assays of MuSC activation and muscle regenerative functions, live cell metabolic flux analyses, and metabolomics we present data indicating that changes in MuSC mitochondrial flux underlie age-associated changes in MuSC activation. We show that, in the course of MuSC activation, there is a profound,16-fold, increase in ATP production rates, which is fueled largely by increases in pyruvate flux into mitochondria. We found that MuSCs from aged mice display progressive defects in the ability to increase mitochondrial flux during activation and that this correlates with higher levels of phosphorylated, inactivated, pyruvate dehydrogenase (PDH). Importantly, we demonstrate that pharmacologic and physiologic methods to induce dephosphorylation and activation of PDH in MuSCs are sufficient to rescue the activation and muscle regenerative functions of MuSCs in aged mice. Collectively the data presented show that MuSC mitochondrial function is a central regulator of MuSC activation and muscle regenerative functions. Moreover, our results suggest that approaches to increase MuSC pyruvate oxidation may have therapeutic potential to promote muscle repair and regeneration.

cell biology↗

bioRxiv2019

Aging induces aberrant state transition kinetics in murine muscle stem cells

Murine muscle stem cells (MuSCs) experience a transition from quiescence to activation that is required for regeneration, but it remains unclear if the transition states and rates of activation are uniform across cells, or how features of this process may change with age. Here, we use timelapse imaging and single cell RNA-seq to measure activation trajectories and rates in young and aged MuSCs. We find that the activation trajectory is conserved in aged cells, and develop effective machine learning classifiers for cell age. Using cell behavior analysis and RNA velocity, we find that activation kinetics are delayed in aged MuSCs, suggesting that changes in stem cell dynamics may contribute to impaired stem cell function with age. Intriguingly, we also find that stem cell activation appears to be a random walk like process, with frequent reversals, rather than a continuous, linear progression. These results support a view of the aged stem cell phenotype as a combination of differences in the location of stable cell states and differences in transition rates between them.\n\nSummary StatementWe find that aged muscle stem cells display delayed activation dynamics, but retain a youthful activation trajectory, suggesting that changes to cell state dynamics may contribute to aging pathology.

developmental biology↗

bioRxiv2017

Inferring cell state by quantitative motility analysis reveals a dynamic state system and broken detailed balance

Cell populations display heterogeneous phenotypic states at multiple scales. Similar to molecular features commonly used to explore cell heterogeneity, cell behavior is a rich phenotypic space that may allow for identification of relevant cell states. Inference of cell state from cell behavior across a time course may enable the investigation of dynamics of transitions between heterogeneous cell states, a task difficult to perform with destructive molecular observations. Cell motility is one such easily observed cell behavior with known biomedical relevance. To investigate cell heterogeneity through the lens of cell behavior, we developed Heteromotility, a software tool to extract quantitative motility features from timelapse cell images. In mouse embryonic fibroblasts (MEFs), myoblasts, and muscle stem cells (MuSCs), Heteromotility analysis identifies multiple motility phenotypes within the population. In all three systems, the motility state identity of individual cells is dynamic. Quantification of state transitions reveals that MuSCs undergoing activation transition through progressive motility states toward the myoblast phenotype. By probability flux analysis, we find that this MuSC motility state system breaks detailed balance, while the MEF and myoblast systems do not. Our data indicate that the system regulating cell behavior can be decomposed into a set of attractor states which depend on the identity of the cell, together with a set of transitions between states governed by inputs from signaling pathways such as oncogenes and growth factors. Within one state, equilibrium formalisms can capture variation in behavior, while switching between states violates equilibrium conditions and would require an external driving force. These results support a conceptual view of the cell as a non-deterministic state automaton, responding to inputs from signaling pathways and generating outputs in the form of observable motile behaviors.

cell biology↗