Investigating amyloid precursor protein derivatives as novel therapeutic agents following traumatic brain injury

Abstract

Despite the significant health and economic burden that traumatic brain injury (TBI) places on society, the development of successful therapeutic agents to reduce its burden has not been successful. The Amyloid Precursor Protein (APP) is an ideal therapeutic candidate, as its acute upregulation following TBI has been shown to serve a number of neuroprotective roles following TBI. Recently, the APP derivative APP96 110, a 15 amino acid length peptide derivative, has continued to display these neuroprotective and neurotrophic functions. While the mechanisms for this remain unclear, it is hypothesized that these neuroprotective properties are linked to its ability to bind to heparin, and that these regions are responsible for the protection against neuronal injury and the improvement in neurological outcome following TBI. In order to further develop APP96 110 as a novel and clinically relevant therapeutic agent following TBI, it was essential to determine the optimal dose, route of and timepoint for administration, as well as examining ways in which the neuroprotective response of this peptide could be further enhanced. In order to investigate these, a dose response study was carried out in male Sprague Dawley rats that was the first to assess the efficacy of intravenous (IV) APP96 110 acutely at 30 minutes post TBI, followed by the more clinically relevant 5 hour timepoint. IV administration of APP96 110 was shown to be neuroprotective for up to 5 hours post TBI, with animals demonstrating improvements in motor outcome and reductions in axonal injury (AI) and neuroinflammation when treated with the highest dose. Studies also assessed the ability to generate APP analogues with alterations to the amino acid sequence, to enhance the heparin binding affinity of the peptide, and examine whether this translated to increased neuroprotection following TBI. Mutation of the APP96 110 peptide to enhance its heparin binding affinity resulted in a peptide with a stronger affinity for heparin (3+APP96 110), and in vivo, resulted in smaller doses conferring equal neuroprotective action to higher doses of wildtype (WT) APP96 110 post TBI. In addition, the long term efficacy of these APP96 110 derivatives was examined, with emphasis placed on assessing long term functional outcome, and beginning to elucidate the long term neuroinflammatory and neurodegenerative changes. Assessment of the long term efficacy demonstrated that, whilst WT and 3+APP96 110 produced similar protection for neuroinflammation, axonal structure, myelination, synaptogenesis and neurodegeneration, treatment with 3+APP96 110 significantly outperformed WT APP96 110 in its ability to improve functional outcome. Together, results demonstrate that the neuroprotective benefits of APP96 110 may relate to its heparin binding ability, with this binding playing a crucial role in the mechanisms though which APP96 110 can exert their neuroprotective actions post TBI. Overall, APP96 110 shows promise as a novel and clinically relevant treatment option, that could overcome many of the challenges that have stalled development of efficacious treatments, by offering substantial neuroprotective and neurotrophic effects that reduce secondary injury and functional deficits associated with acute TBI.