Function of muscle lim protein (MLP) in neuroprotection and axon regeneration

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As typical CNS neurons, retinal ganglion cells (RGCs) do not normally regenerate axons in the injured optic nerve and instead undergo apoptosis. However, inflammatory stimulation (IS) in the inner eye, induced by lens injury, is strongly neuroprotective and transforms RGCs into an active regenerative state, enabling them to regrow axons into the injured optic nerve. Based on a microarray study we identified genes differentially expressed in regenerating (induced by IS) vs. solely axotomized and naïve RGCs, one of which was muscle LIM protein (MLP). MLP has been postulated to be muscle-specific (skeletal muscle, but mainly heart) and involved in myogenesis without any known expression in the adult mammalian CNS. Using Western blot analysis and immunohistochemistry, we found profound induction of MLP protein in the cytosol (including nuclei) and axonal growth cones of mature RGCs upon optic nerve injury. Expression was further enhanced when RGCs entered a regenerative state after additional IS. Moreover, we detected MLP in a subpopulation of dorsal root ganglion (DRG) neurons upon sciatic nerve injury. Inhibition of MLP expression by short hairpin RNA (shRNA) compromised neurite growth of cultured RGCs and preliminary data demonstrate that overexpression of MLP further pro-motes IS-induced axon regeneration in the crushed optic nerve. In this proposed research project, we will (i) investigate the role of MLP in neuroprotection of axotomized RGCs, optic nerve as well as spinal cord regeneration, (ii) we will identify the underlying molecular mechanisms by testing the hypothesis that nuclear MLP might regulate gene expression after axon injury and (iii) by identification of functional relevant neuronal interaction partners and downstream signaling cascades of MLP. These experiments will identify the molecular mechanisms underlying the yet unknown role of MLP in CNS axon regeneration and might open novel approaches for CNS repair.