What is N-Acetylaspartic Acid (NAA)?

This is a question I've been asking over the last month. What, exactly, is this chemical, NAA, that is low in J's brain?

Simply stated,  N-Acetylaspartic Acid (NAA) is a marker of neuron integrity and viability.  A study in the American Journal of Neuroradiology said the following:  Due to its near-exclusive localization to neurons and their processes, NAA is regarded as a marker for their health and density, and its level has been reported to decline in all neurodegenerative central nervous system disorders in adults.

N-Acetylaspartic Acid even has an entire website dedicated to it.  N-Acetylaspartic Acid.com
NAA is also known as N-acetylaspartate and NAA is the second most  most abundant amino acid in the brain. NAA gives off the largest signal in MRS of the human brain. This along with the fact that it is pretty much exclusive to neurons, makes it a reliable marker.  The levels measured in the brain are decreased in many neuropathological conditions ranging from brain injury, stroke,  Alzheimer's  to Parkinson's. This also makes NAA a reliable diagnostic molecule for doctors treating patients with brain disease.

An article that I found on PubMed says: 
An article I found on PubMed says this:  "MRS studies of human brain disorders have invariably detected decreases in brain NAA concentrations when neuronal loss or dysfunction are involved with one major exception: Canavan Disease... virtually all other neurological disorders involving neuronal loss or dysfunction result in reduction of NAA levels."

A 2003 Medscape article  titled MRI Shows Brain Injury in MS That Precedes Atrophy says the following:  
"Neuronal cell death may be an important marker to measure the progression of MS, and this eventually may be a therapeutic target. "First you lose the neurons, and then the axons atrophy," Dr. Gonen told Medscape. "We don't have a treatment that interrupts this process, but physicians can use these images to encourage their patients to take medications that prevent attacks."

The marker for neuronal cell death is the amino acid derivative N-acetylaspartate (NAA). Because this protein is present almost exclusively in neuronal cells, Dr. Gonen and colleagues theorized that quantifying the level of NAA by proton MR spectroscopy would give an indication of whether the patient was experiencing a loss of these cells. Therefore, they sought to quantify the relationship among NAA levels, brain volume, and disease duration to better understand the early developments in MS."