Distribution of monoamine oxidase proteins in human brain: implications for brain imaging studies

Abstract

Positron emission tomography (PET) imaging of monoamine oxidases (MAO-A: [(11)C]harmine, [(11)C]clorgyline, and [(11)C]befloxatone; MAO-B: [(11)C]deprenyl-D2) has been actively pursued given clinical importance of MAOs in human neuropsychiatric disorders. However, it is unknown how well PET outcome measures for the different radiotracers are quantitatively related to actual MAO protein levels. We measured regional distribution (n=38) and developmental/aging changes (21 hours to 99 years) of both MAOs by quantitative immunoblotting in autopsied normal human brain. MAO-A was more abundant than MAO-B in infants, which was reversed as MAO-B levels increased faster before 1 year and, unlike MAO-A, kept increasing steadily to senescence. In adults, regional protein levels of both MAOs were positively and proportionally correlated with literature postmortem data of MAO activities and binding densities. With the exception of [(11)C]befloxatone (binding potential (BP), r=0.61, P=0.15), correlations between regional PET outcome measures of binding in the literature and MAO protein levels were good (P<0.01) for [(11)C]harmine (distribution volume, r=0.86), [(11)C]clorgyline (λk3, r=0.82), and [(11)C]deprenyl-D2 (λk3 or modified Patlak slope, r=0.78 to 0.87), supporting validity of the latter imaging measures. However, compared with in vitro data, the latter PET measures underestimated regional contrast by ∼2-fold. Further studies are needed to address cause of the in vivo vs. in vitro nonproportionality.

Figure 1

Quantification of monoamine oxidases (MAOs) in autopsied human brain. (A) immunoblots of MAO-A and MAO-B in the pooled striatum tissue standards (0.5 to 15 μg) and in the commercial overexpressed recombinant enzymes (1 to 25 ng). (B) Standard curves for the recombinant MAOs.

Figure 2

Ageing and neonatal developmental changes of levels of monoamine oxidases (MAOs), the ratio of MAO-B vs. MAO-A, and the control protein neuronal-specific enolase (NSE) in autopsied human brain. The insets show enlarged portion below 1 year of age.

Figure 3

Representative immunoblots of the regional distribution of monoamine oxidases (MAO-A and MAO-B) in autopsied human brain. A23, cingulate gyrus posterior; A24, cingulate gyrus anterior; A25, paraolfactory/subgenual gyrus; CCc, corpus callosum caudal; CCr, corpus callosum rostral; cereb, cerebellar cortex; CN, caudate; CNA, hippocampal Ammon's horn; CSTH, subthalamic nucleus; GD, dentate gyrus; GH, hippocampal gyrus; GPe, globus pallidus external; GPi, globus pallidus internal; GUNC, gyrus of uncus; hypothal, hypothalamus; ICr, internal capsule rostral; LGB, lateral geniculate body; MDTH, mediodorsal thalamus; NAM, amygdala; NAV, anterior ventral nucleus of thalamus; N. basalis, nucleus basalis; NL, nucleus lateralis of thalamus; NLV, lateral ventral nucleus of thalamus; NPM, medial pulvinar of thalamus; PUT, putamen; RN, red nucleus; SBI, substantia innominata; SNpc, substantia nigra pars compacta.

Figure 4

Regional distribution (mean±s.e.m.) of monoamine oxidases (MAO-A and MAO-B) in autopsied human brain (n=6 with the exception of n=2 for LGB). A23, cingulate gyrus posterior; A24, cingulate gyrus anterior; A25, paraolfactory/subgenual gyrus; CCc, corpus callosum caudal; CCr, corpus callosum rostral; cereb, cerebellar cortex; CN, caudate; CNA, hippocampal Ammon's horn; CSTH, subthalamic nucleus; GD, dentate gyrus; GH, hippocampal gyrus; GPe, globus pallidus external; GPi, globus pallidus internal; GUNC, gyrus of uncus; hypothal, hypothalamus; ICr, internal capsule rostral; LGB, lateral geniculate body; MDTH, mediodorsal thalamus; NAM, amygdala; NAV, anterior ventral nucleus of thalamus; N. basalis, nucleus basalis; NL, nucleus lateralis of thalamus; NLV, lateral ventral nucleus of thalamus; NPM, medial pulvinar of thalamus; PUT, putamen; RN, red nucleus; SBI, substantia innominata; SNpc, substantia nigra pars compacta.

Figure 5

Correlations (Pearson) between regional protein levels of monoamine oxidases (MAO-A and MAO-B) determined by immunoblotting in this study vs. activities and binding densities of the respective isozymes reported in the literature.

Figure 6

Correlations (Pearson) between regional protein levels of monoamine oxidases (MAO-A and MAO-B) determined in this study vs. outcome measures of density ([¹¹C]harmine distribution volume V_S^, and [¹¹C]befloxatone binding potential k₃/k₄) or activity (λk₃ or modified Patlak slope^, for [¹¹C]clorgyline and/or [¹¹C]deprenyl-D2) of the respective isozymes reported by positron emission tomography (PET) in the literature. For comparison, the correlation between regional protein levels of serotonin transporter (SERT) and [¹¹C]DASB PET measurement of SERT density (BP_ND) is also shown. The dotted lines show expected proportional correlations. Note the poor correlation for [¹¹C]befloxatone, the lack of proportionality for MAO-A PET, and similar binding of MAO-B PET despite twofold differences in MAO-B protein concentrations between cerebellar and cerebral neocortices.