RESULTADOS DE LA APLICACIÓN DEL MAS+ CON LAS LICENCIADAS

NAA is a non-essential amino acid and is the most visible metabolite peak of the 1H MRS spectrum. Within the brain NAA is synthesised primarily in neurons being present in concentrations of up to 20mM and representing about 7% of neuronal osmolarity. The majority of MRS studies published to date have found NAA to be the most sensitive MRS visible metabolite marker of pathological states. The role of N-acetyl aspartate in brain tissue, however, has been unclear and although reduced level of NAA is commonly referred to as a marker of “neuronal loss or dysfunction” the biological explanation for this reduction has also been absent. However, evidence has begun to accumulate that NAA synthesis is linked with mitochondrial function(Heales et al., 1995;Bates et al., 1996) and that NAA has an important and dynamic role in removing neuronal water produced as a consequence of increased metabolic activity. NAAG appears to act as a specific transmitter signalling to adjacent astrocyte foot processes to increase regional blood flow and uptake of glucose. [For more detailed discussion of NAA metabolism see (Baslow, 2000;Moreno et al., 2001;Barker, 2001;Baslow et al., 2003;Baslow, 2003b;Baslow et al., 2007;Baslow and Guilfoyle, 2009)]

NAA metabolism appears to span three metabolically linked compartments. Using a combined method of 13C MRS and labelled glucose infusion in humans in vivo, it has been demonstrated that NAA synthesis is directly coupled to glucose metabolism(Moreno et al., 2001). Neurons synthesise NAA in mitochondria and it is calculated that 1mol of NAA is synthesized for every 40 mol of glucose oxidized in the brain. NAAG is formed from NAA and glutamate. However neither NAA nor NAAG can be catabolised within the neuron and must be released to the extra- cellular fluid (ECF). In this space NAAG interacts with astrocyte foot process surface NAAG peptidase producing NAA and glutamate whilst NAA reacts with oligodendrocyte surface membrane aspartoacylase which converts it to acetate and aspartate. The glutamate formed at the astrocyte surface is thought to provide specific signalling to increase blood flow and increase supply of glucose.

Investigations of NAA export from stimulated neurons have estimated that each NAA molecule transports with it 32 molecules of obligated water. This can be achieved using the NAA system at the relatively low energy cost of about 1% of the

brain’s daily energy budget(Baslow and Resnik, 1997). The water removal process from the brain would most likely involve diffusion of liberated water down its gradient via aquaporin-4 (AQP4), the predominant water channel protein that is heavily expressed in astrocytes but not in neurons or oligodendrocytes.

This has led to the theory that NAA probably functions as a molecular water pump (MWP)(Baslow, 2003a) to transport neuronal metabolic water to ECF for its removal from the brain via aquaporin 4 (AQP4) channels present on the surfaces of astrocytes and vascular endothelial cells(Verkman et al., 2006). MWPs are recently discovered entities that can actively pump water against its gradient(Meinild et al., 1998;Zeuthen, 2000;MacAulay et al., 2001). MWP appear to be highly efficient, with 1 molecule of MWP solute able to transport as many as 500 water molecules during its passage.

Baslow has suggested that the metabolism of NAA is via an inter-compartmental shuttle(Baslow, 2002). In humans, NAA turnover in the brain is very rapid, and the efflux of NAA is 0.55 µM/g/h from the neuronal compartment into the extra cellular space. For equilibrium NAA must be removed from the ECS at the same rate. The role of the oligodendrocyte localised amidohydrolase II is to participate in this process by hydrolyzing NAA that has been liberated from neurons(Bhakoo et al., 2001). It follows that the products of NAA hydrolysis, aspartate and acetate, must also be removed, at the same rate as NAA hydrolysis, or the action of this hydrolytic enzyme would also be compromised. NAA-derived acetate is metabolised by oligodendrocytes or astrocytes, and incorporated into many lipid components and thus lost from the NAA cycle. The NAA-derived aspartate diffuses towards the neuron surface, where it is taken up to complete the shuttle action. Aspartate does not easily pass into the brain across the blood-brain barrier, and this conservative step ensures a sufficient supply of aspartate for neuron use in the continuous and rapid formation and inter-compartmental release of NAA. Conservation of acetate is not essential in the brain because it easily passes across the blood brain barrier at about the same rate as glucose.

These findings would propose that NAA function is linked with neuronal metabolic activity both by providing a mechanism for the removal of waste water of metabolism as well as a specific signalling pathway to the astrocyte end processes

leading to increased glucose uptake. Such proposals seem to be supported by the results of functional magnetic resonance spectroscopy (fMRS) studies of the human visual cortex in response to visual stimulation(Baslow et al., 2007) which showed 13% NAA reduction after intense visual activity with complete recovery after a subsequent “rest period” and which represents export of preformed NAA with activation and recovery following re-synthesis during a period of reduced metabolic activity.

The astrocytic surface metabotropic glutamate receptor 3, (GRM3) activation of which initiates intracellular calcium transients(Hashemi et al., 2008) and secondary astrocyte–astrocyte and astrocyte–vascular system signals that increase focal blood flow(Baslow and Guilfoyle, 2007), is the likely target for NAAG(Baslow, 2008;Baslow, 2009)

Disorders of NAA metabolism are known and are associated with significant pathology. Canavan disease is a rare recessive genetic neurodegenerative brain disorder associated with many different mutations in the gene encoding aspartoacylase and leading to accumulation of NAA and the development of a spongiform leukodystrophy. There is also a single human case of apparent lack of NAA(Martin et al., 2001) and relatively preserved neurological function and this observation of apparently normal function without NAA has yet to be assimilated into the theories developed above.

1.7.6.1.2 Creatine plus phosphocreatine (Cr)

Cr is the second major visible metabolite peak in the 1H MRS spectrum at 3.0 ppm. It is the combined peak of Creatine (Creat) and phosphocreatine (PCreat) which together provide an important buffer system of intracellular ATP levels (see section 1.2.2).

1.7.6.1.3 Choline containing compounds

Choline containing compounds (single peak at ~3.22 ppm) include choline, phosphocholine and glycerophosphocholine. In adult human controls choline concentration does not appear to vary in concentration between grey matter and white matter in adult human control frontal and parietal lobes (McLean et al., 2000). In early development Cho is high whilst NAA levels are low. With increasing maturity Cho falls whilst NAA increases (Tkac et al., 2003). The early high levels of

Cho are presumed to be a consequence of rapid cell membrane development(Kreis et al., 1993;Kreis et al., 2002;Kok et al., 2002;Girard et al., 2006a;Girard et al., 2006b).

Elevated Cho levels are now widely recognised as a general marker for neoplastic tissue in the brain as well as in other tissues. Again, the elevation is likely to be due to cell membrane synthesis in rapidly dividing cells(Ross and Michaelis, 1994;Swanson et al., 2001;Yeung et al., 2002;Mueller-Lisse and Scherr, 2007;Papanagiotou et al., 2007). Multiple sclerosis brain lesions are associated with abnormal Cho levels both in acute lesions and in normal appearing white matter(Husted et al., 1994;Rooney et al., 1997) secondary to fluctuations in membrane turnover rates due to inflammatory / demyelinating processes(Arnold et al., 1990).

Cho levels have also been shown to be elevated in malformations of cortical development associated with epilepsy in some studies(Kuzniecky et al., 1997b) although normal findings were observed by other groups(Widjaja et al., 2003). Elevated Cho in the regions of the primitive brain tissue of MCD may be expected from the literature of MRS examinations acquired during early development.