It is believed that gene therapy employing NAMPT will lead to axon cell toxicity, albeit elevating the NAD+ levels in the cells. NAMPT functions to convert NAM to NMN, which is then converted to NAD+. Intracellular NAD+ is a key molecule associated with axon degeneration. When NAD+ levels are low, axons rapidly degenerate. It is also established that following axonal injury in sciatic nerve axons, NMN accumulates rapidly. In this model axon degeneration, NMN levels rise within 12 hrs, followed by neural injury occurs at 36-hr following injury. Blocking the NMN producing enzyme NAMPT using FK866 potentially inhibits this axon degeneration, hinting that NMN is toxic to neurons. Axon degeneration appears to be NMN-dependent, and inhibiting the increase of NMN using FK866 protects against axon degeneration. In a zebrafish model of axon degeneration (two-photon- laser axotomy), FK866 potently delayed axon degeneration. In a cell culture model of neurite degeneration (superia cervical ganglion; SCG), rapidly clearing NMN (by overexpression of the bacterial enzyme NMN deamidase which converts NMN to NAMN) robustly protects from axon degeneration.Accordingly, the present invention represents an unexpected finding that gene therapy employing NMNAT, unlike that of NAMPT, is effective in protecting ocular neurodegeneration. Thus we chose Nmnatl over Nampt, as Nampt overexpression would drive the production of NMN, which, without proper clearance, is toxic to neurons....Mice with the Wlds gene have delayed Wallerian [axonal] degeneration. The Wlds mutation is an autosomal-dominant mutation occurring in the mouse chromosome 4. The gene mutation is a naturally occurring 85-kb tandem triplication, resulting in a mutated region containing two associated genes: nicotinamide mononucleotide adenylyl transferase 1 (Nmnat-1) and ubiquitination factor e4b (Ube4b), and a linker region encoding 18 amino acids. The protein created localizes within the nucleus and is undetectable in axons.The mutation appears to cause no harm to the mouse. The only known effect is that the Wallerian degeneration is delayed by up to three weeks on average after injury of a nerve. Recent studies suggest that the mutation protects axons by a poorly understood mechanism. While not wishing to be bound by any particular theory, it is likely that the Wld mutation leads to overexpression and/or improved localization of the Nmnat {e.g., Nmnat-1) protein and increased NAD synthesis.
Edited by Michael, 15 November 2017 - 10:22 PM.