Diadenosine tetraphosphate (Ap4A) inhibits ATP-induced excitotoxicity: a neuroprotective strategy for traumatic spinal cord injury treatment

Abstract

Reducing cell death during the secondary injury is a major priority in the development of a cure for traumatic spinal cord injury (SCI). One of the earliest processes that follow SCI is the excitotoxicity resulting from the massive release of excitotoxicity mediators, including ATP, which induce an excessive and/or prolonged activation of their receptors and a deregulation of the calcium homeostasis. Diadenosine tetraphosphate (Ap 4 A) is an endogenous purinergic agonist, present in both extracellular and intracellular fluids, with promising cytoprotective effects in different diseases including neurodegenerative processes. In a search for efficient neuroprotective strategies for SCI, we have tested the capability of Ap 4 A to reduce the excitotoxic death mediated by the ATP-induced deregulation of calcium homeostasis and its consequences on tissue preservation and functional recovery in a mouse model of moderate contusive SCI. Our analyses with the murine neural cell line Neuro2a demonstrate that treatment with Ap 4 A reduces ATP-dependent excitotoxic death by both lowering the intracellular calcium response and decreasing the expression of specific purinergic receptors. Follow-up analyses in a mouse model of contusive SCI showed that acute administration of Ap 4 A following SCI reduces tissue damage and improves motor function recovery. These results suggest that Ap 4 A cytoprotection results from a decrease of the purinergic tone preventing the effects of a massive release of ATP after SCI, probably together with a direct induction of anti-apoptotic and pro-survival pathways via activation of P2Y 2 proposed in previous studies. In conclusion, Ap 4 A may be a good candidate for an SCI therapy, particularly to reduce excitotoxicity in combination with other modulators and/or inhibitors of the excitotoxic process that are being tested.

Roles of Nicotinamide Adenine Dinucleotide (NAD+) in Biological Systems

“…NUDIX homology domains (NHD) in enzymes and the Nudix type 2 gene product, Ap4A hydrolase, have been shown responsible for Ap4A degradation. The human fragile histidine triad (FHIT) is a tumor suppressor, whose Ap4A hydrolase activity is not required for antioncogene activity, while Ap4A binding has a positive effect [42][43][44][45][46][47]. The Microphthalmia transcription factor (MITF) is a basic helix-loop-helix leucine zipper (bHLH-Zip) DNA-binding protein.…”

“…A pharmacological use of Ap4A in various disorders has been envisaged, such as in ocular tissue pathologies and retinal detachment, as well as in inhibition of ATP-induced excitotoxicity, as neuroprotection for traumatic spinal cord injury treatment [45][46][47]. A phosphodiesterase, NUDT16, can hydrolyze protein bound ADP-ribose, in vertebrate systems [35].…”

Spinal Cord Injury: Pathophysiology, Multimolecular Interactions, and Underlying Recovery Mechanisms

“…After SCI, the disrupted calcium homeostasis leads to neuronal dysfunction. Hence, the modulation of calcium within damaged tissues helps prevent neuro-degeneration [ 66 ]. The voltage-gated calcium channel blockers (VGCCs) play a vital role in calcium load regulation during SCI.…”

“…VGCCs have six subtypes, i.e., L-, N-, P-, Q-, R- and T-type channels [ 67 ]. T-type calcium channels are present on neuron surface and their blockage results in long-term neuroprotection and maintenance of homeostasis by improving neuronal microcirculation [ 66 ]. L-type VGCCs include dihydropyridines such as nimodipine.…”

“…T-type VGCCs can be blocked by mibefradil with 10–30 times higher potency than nimodipine [ 67 ]. Some Na + and Ca 2+ channel blockers possessing neuroprotective activity [ 66 , 67 , 68 , 69 , 70 , 71 , 72 ] are listed in Table 3 .…”