OTX2 stimulates adult retinal ganglion cell regeneration

Phosphorylated S6K1 and 4E-BP1 play different roles in constitutively active Rheb-mediated retinal ganglion cell survival and axon regeneration after optic nerve injury

Ras homolog enriched in brain (Rheb) is a small GTPase that activates mammalian target of rapamycin complex 1 (mTORC1). Previous studies have shown that constitutively active Rheb can enhance the regeneration of sensory axons after spinal cord injury by activating downstream effectors of mTOR. S6K1 and 4E-BP1 are important downstream effectors of mTORC1. In this study, we investigated the role of Rheb/mTOR and its downstream effectors S6K1 and 4E-BP1 in the protection of retinal ganglion cells. We transfected an optic nerve crush mouse model with adeno-associated viral 2-mediated constitutively active Rheb and observed the effects on retinal ganglion cell survival and axon regeneration. We found that overexpression of constitutively active Rheb promoted survival of retinal ganglion cells in the acute (14 days) and chronic (21 and 42 days) stages of injury. We also found that either co-expression of the dominant-negative S6K1 mutant or the constitutively active 4E-BP1 mutant together with constitutively active Rheb markedly inhibited axon regeneration of retinal ganglion cells. This suggests that mTORC1-mediated S6K1 activation and 4E-BP1 inhibition were necessary components for constitutively active Rheb-induced axon regeneration. However, only S6K1 activation, but not 4E-BP1 knockdown, induced axon regeneration when applied alone. Furthermore, S6K1 activation promoted the survival of retinal ganglion cells at 14 days post-injury, whereas 4E-BP1 knockdown unexpectedly slightly decreased the survival of retinal ganglion cells at 14 days post-injury. Overexpression of constitutively active 4E-BP1 increased the survival of retinal ganglion cells at 14 days post-injury. Likewise, co-expressing constitutively active Rheb and constitutively active 4E-BP1 markedly increased the survival of retinal ganglion cells compared with overexpression of constitutively active Rheb alone at 14 days post-injury. These findings indicate that functional 4E-BP1 and S6K1 are neuroprotective and that 4E-BP1 may exert protective effects through a pathway at least partially independent of Rheb/mTOR. Together, our results show that constitutively active Rheb promotes the survival of retinal ganglion cells and axon regeneration through modulating S6K1 and 4E-BP1 activity. Phosphorylated S6K1 and 4E-BP1 promote axon regeneration but play an antagonistic role in the survival of retinal ganglion cells.

Unconventional Secretion, Gate to Homeoprotein Intercellular Transfer

Unconventional secretion allows for the secretion of fully mature and biologically active proteins mostly present in the cytoplasm or nucleus. Besides extra vesicle-driven secretion, non-extravesicular pathways also exist that specifically rely on the ability of the secreted proteins to translocate directly across the plasma membrane. This is the case for several homeoproteins, a family of over 300 transcription factors characterized by the structure of their DNA-binding homeodomain. The latter highly conserved homeodomain is necessary and sufficient for secretion, a process that requires PI(4,5)P2 binding, as is the case for FGF2 and HIV Tat unconventional secretion. An important feature of homeoproteins is their ability to cross membranes in both directions and thus to transfer between cells. This confers to homeoproteins their paracrine activity, an essential facet of their physiological functions.