HMGB1/CXCL12 heterocomplex activates CXCR4 to suppress the ability of Muller glia to become proliferating progenitor-like cells in the chick retina

We investigated how High Mobility Group Box 1 (HMGB1) signaling regulates glial reactivity and Muller glia-derived progenitor cell (MGPC) formation in the chick retina. HMGB1 released from stressed cells is known to form a heterocomplex with C-X-C chemokine ligand 12 (CXCL12) to activate C-X-C chemokine receptor 4 (CXCR4). HMGB1 is broadly expressed in retinal neurons and glia and is rapidly exported from the nuclei of stressed inner retinal neurons after damage, a process blocked by inflachromene. Stressed or damaged amacrine cells also exported different transcription factors out nuclei. CXCL12 is expressed by amacrine, horizontal, and bipolar cells, whereas CXCR4 is expressed by Muller glia and is rapidly upregulated following damage. CXCR4 agonist or recombinant HMGB1/CXCL12 treatment selectively stimulates mTOR and Jak/Stat signaling in Muller glia in undamaged retinas. Inhibition of CXCR4 signaling, HMGB1 nuclear export, or HMGB1/CXCL12 complex formation potently stimulated MGPC proliferation without affecting neuronal death or microglial activation. In retinas treated with insulin and FGF2, to induce MGPC proliferation independent of injury, HMGB1 remains confined to nuclei, CXCR4 is downregulated, and inhibition of HMGB1 or CXCR4 has no effect. We further demonstrate that microglia are required to maintain CXCR4 expression in Muller glia, and blockade of HMGB1/CXCL12-CXCR4 signaling does not restore MGPC proliferation in microglia-depleted retinas. Together, these findings identify HMGB1/CXCL12-CXCR4 signaling as an injury-induced pathway that limits Muller glia reprogramming and MGPC formation in damaged retinas.