ASTROCYTIC MITOCHONDRIA FORM COMPLEX STRUCTURES WITH ROBUST PLASTICITY, REPAIR CAPABILITIES AND CALCIUM CONTROL

Cortical astrocytes host a strikingly heterogeneous mitochondrial network, from small spheres to large, highly branched structures. In vivo 2-photon microscopy in GLAST-mito-eGFP mice in the somatosensory cortex, validated by focused ion beam–scanning electron microscopy, revealed a distinct subnetwork of oversized, intricately branched “giant” mitochondria in cortical astrocytes. While most mitochondria are small and simple, the largest particles account for almost half of total mitochondrial volume. Astrocytic mitochondria exhibit constrained, Brownian-like motion with limited net displacement, which is not affected by increase in cytoplasmic calcium concentration. Despite their restricted movement, astrocytic mitochondria remain highly interactive through prominent fusion/fission-like events, enabling local remodeling without long-range transport across the cell. Chronic intravital multiphoton imaging revealed striking structural stability of astrocytic mitochondria within a cell persisting for weeks. Although stable, the complex astrocytic mitochondria could be disorganized by laser-induced photoablation, which triggers severe mitochondrial fragmentation and depletion, followed by robust structural restoration within 1-2 weeks, indicating strong intrinsic repair capability in vivo. Functionally, mitochondrial subtypes (small vs large) are not equivalent. While the entire mitochondrial population collectively engages in calcium signaling, large and complex mitochondria are selectively recruited during periods of high cytosolic calcium load and show minimal engagement during brief spontaneous calcium transients. Together, our data suggests that giant astrocytic mitochondria represent a specialized, resilient subnetwork optimized to manage complex calcium signaling mechanism in astrocytes, with potential relevance to cellular stress and senescence.