Krauss, R. | bioRxiv Science

bioRxiv2025

Regulation of Muscle Stem Cell Dynamics and Quiescence by Netrin-1 Cytoskeletal Signaling

The hallmark property of muscle stem cells (MuSCs) at homeostasis is quiescence. However, MuSCs have cellular protrusions that are heterogeneous, complex, and tipped with filopodia, all signs of motile structures. Such protrusions may serve as sensors of the MuSC niche. Through development of a novel ex vivo live imaging assay for MuSC protrusion dynamics, we report here the identification of a regulatory pathway in this process. The axon guidance cue Netrin-1 promotes MuSC protrusion outgrowth ex vivo in a manner dependent on its receptors Dcc and Neogenin, the small GTPase Rac1, and the actin-branching factor Arp2/3; this pathway is also required for Netrin-dependent axonal growth cone motility. Adult MuSC-specific genetic removal of Netrin-1 receptors, Rac1, and Arp2/3 each result in failure to maintain homeostatic protrusion lengths, spontaneous quiescence exit, and MuSC attrition in uninjured mice. These findings reveal an unanticipated level of morphological dynamism by a quiescent stem cell at homeostasis and link this phenomenon to preservation of the quiescent state.

cell biology↗

bioRxiv2025

Evidence for the Transient Presence of Atypical Astrocytes in Mice Following a Single, Closed-Head Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) affects roughly 42 million people each year, causes a variety of physical, behavioral, and cognitive symptoms, and increases the risk for developing neurological disorders, including post-traumatic headache (PTH) and Alzheimers disease (AD). Multiple molecular and cellular changes occur following mTBI; here we focus on astrocytes - cells that respond to brain injury and are critical to maintaining neuronal and circuit homeostasis. While some astrocytes become reactive after mTBI, others adopt an atypical state characterized by the loss of multiple functional astrocyte proteins, including glutamate transporters (GLT-1, GLAST) and ion channels (Kir4.1), without upregulation of prototypical reactive astrocyte markers (glial fibrillary acidic protein [GFAP]). Previous studies have shown that repeated mTBI causes atypical astrocytes (AtAs) that can persist for months, but we know much less about whether a single mTBI causes similar astrocyte phenotypes. To address this, we employed a closed-head mild traumatic brain injury (chmTBI) model in male and female mice and quantified the abundance of AtAs both acutely (3-days) and chronically (1-month) after a single injury. We found that 3-days after chmTBI, AtAs were present in areas subject to blunt force trauma (BFT), consistent with previous reports, as well as in other brain regions presumably affected by diffuse injury. One month after chmTBI, however, the proportion of AtAs was similar between chmTBI and sham injured mice, thereby suggesting AtAs do not persist long term in this model. Consistent with previous studies, this chmTBI model did not induce significant GFAP-positive reactive astrocytes as assayed using immunohistochemistry, at either timepoint. Overall, we show an increase in AtAs 3-days after a single chmTBI that returns to sham levels when examined 1-month after injury. This suggests that after a single mTBI, AtAs are present but do not persist long term, unlike in repeated mTBI where AtAs persist for months after injury.

neuroscience↗