Role Of Cholinergic System Burden On Malfunction Of Learning And Memory In Mice

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the progressive loss of cognitive ability, neuropathological senile plaque, neurofibrillary tangles, and neuronal loss in selective brain regions. Much evidence indicates that the memory and cognitive deficits of patients with AD are closely associated with the dysfunction of the central cholinergic system. In AD brain, the activity of choline acetyltransferase (acetyl CoA: choline O-acetyltransferase, EC 2.3.1.6, ChAT) and the level of neurotransmitter acetylcholine (ACh) obviously reduced. ChAT is the key enzyme responsible for catalyzing synthesis of ACh in cholinergic system, and ACh is the one of the importamt neurotansmitters concerning learning and memory. The level of ACh is significantly correlated with the severity of dementia or cognitive impairments observed in AD. Thus we suppose that supplementation of exogenous ChAT in the source should improve the function of the cholinergic nerve system and promote the ability of learning and memory in vivo.However, the delivery of therapeutic protein into brain across the blood-brain barrier is severely limited by the proteins' size and biochemical properties. ChAT, a large molecule with a molecular weight of 69 kDa, cannot pass through the biological membrane. However, recently an approach to deliver therapeutic peptides to the brain is the application of fusion proteins linked to so-called protein transduction domain (PTD), derived from the human immunodeficiency virus trans-activator transcription (TAT) protein, capable of passing through the blood-brain barrier and cell membranes. To produce genetic in-frame PTD-ChAT fusion protein, we constructed a bacterial expression vector, pGEX4T-PTD-ChAT, and PTD-ChAT was obtained by genetic engineering expression. The results showed that the PTD-ChAT could pass through the blood-brain barrier and enter the neurons in mice, increasing the levels of ChAT and neurotransmitter ACh. The recombination PTD-ChAT fusion protein injected intravenously improved the memory and cognitive dysfunction in AD model mice induced by P-amyloid peptide, and in the aged mice with naturally occurring cognitive deficits. In the behavior tests, the behavioral changes of the PTD-ChAT-treated mice almost completely reversed. The mice showed a significant smaller number of error counts and longer memory retention in the step-through test (an index of long-term memory), and a shorter swimming time in water maze test (an index of spatical...