Acute decrease in alkaline phosphatase after brain injury: A potential mechanism for tauopathy

Dephosphorylation of phosphorylated Tau (pTau) protein, which is essential for the preservation of neuronal microtubule assemblies and for protection against trauma-induced tauopathy and chronic traumatic encephalopathy (CTE), is primarily achieved in brain by tissue non-specific alkaline phosphatase (TNAP). Paired helical filaments (PHFs) and Tau isolated from Alzheimer's disease (AD) patients' brains have been shown to form microtubule assemblies with tubulin only after treatment with TNAP or protein phosphatase-2A, 2B and -1, suggesting that Tau protein in the PHFs of neurons in AD brain is hyperphosphorylated, which prevents microtubule assembly. Using blast or weight drop models of traumatic brain injury (TBI) in rats, we observed pTau accumulation in the brain as early as 6 h post-injury and further accumulation which varied regionally by 24 h post-injury. The pTau accumulation was accompanied by reduced TNAP expression and activity in these brain regions and a significantly decreased plasma total alkaline phosphatase activity after the weight drop. These results reveal that both blast- and impact acceleration-induced head injuries cause an acute decrease in the level/activity of TNAP in the brain, which potentially contributes to trauma-induced accumulation of pTau and the resultant tauopathy. The regional changes in the level/activity of TNAP or accumulation of pTau after these injuries did not correlate with the accumulation of amyloid precursor protein, suggesting that the basic mechanism underlying tauopathy in TBI might be distinct from that associated with AD.