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Neurobiology and neurochemistry of Rett syndrome
Brain & Development 23 (2001) S58–S61 www.elsevier.com/locate/braindev
Original article
Neurobiology and neurochemistry of Rett syndrome Toyojiro Matsuishi*, Yushiro Yamashita, Akira Kusaga Department of Pediatrics and Child Health, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan
Abstract The current status of neurobiological and neurochemical research on Rett syndrome is reviewed, and correlations are developed with previously described neurophysiological, neuroimaging, neuropathological, and immunohistochemical changes. We review the abnormalities reported in the biogenic amine neurotransmitters/receptor systems, and of b-phenylethylamine, an endogenous amine synthesized by the decarboxylation of phenylalanine in dopaminergic neurons of the nigrostriatal system. We also discuss the roles of other neurotransmitters, including b-endorphin and substance P, and neurotrophic factors, including nerve growth factors. Recently, DNA mutations in the methyl– CpG binding protein 2, mapped to Xq28, have been identified in some patients with Rett syndrome. The multiple abnormalities in the various neurotransmitters/receptor systems explain the pervasive effects of Rett syndrome. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Neurotransmitters; Neuromodulators; Biogenic amines; Neuropeptides; Nerve growth factors; Methyl–CpG binding protein 2
1. Introduction Rett syndrome (RTT) is a neurodevelopmental disorder of previously unknown etiology, characterized by normal early psychomotor development followed by the loss of psychomotor skills, especially acquired purposeful hand skills, and by the onset of stereotyped movement in the hands and of gait disturbance [1]. Recently, mutations in the gene encoding methyl–CpG binding protein 2 (MECP2) have been discovered in RTT [2]. However, the pathophysiology of RTT is still unknown. This paper discusses the future directions for research in RTT. Research needs to be directed to clarify the link between the discovery of the MECP2 involvement and the alteration in neurobiological, neurochemical, and neurotransmitter/receptor systems. Also, research is needed to develop the experimental model of conditioning MeCP2 knock-out mice to investigate new therapeutic modalities.
2. Neurotransmitters 2.1. Biogenic amines, acetylcholine and gamma aminobutyric acid Nomura and Segawa have suggested that hypoactivity or underdevelopment in the biogenic amines might account for the range of abnormalities found in RTT. They suggested a * Corresponding author. Tel.: 181-942-31-7565; fax: 181-942-38-1792. E-mail address: [email protected] (T. Matsuishi).
impairment in noradrenalin, serotonin, and in dopamine, using clinical and polysomnographic studies [3]. They have proposed that the following points are important in considering the pathophysiology of RTT; the characteristic symptoms and signs appear in sequence within a specific age from infancy. The earliest and pathognomonic manifestations of RTT are the autistic tendency and the decreased rate in head growth. They have suggested that the primary lesions of RTT involve the raphe nuclei and the locus coeruleus [4]. Their report has stimulated cerebrospinal fluid (CSF) biogenic amine studies and immunohistochemical or receptor studies, and neuroimaging studies which have now suggested that various neurotransmitters or neuromodulators, neurotrophic factors and neuronal markers may be involved in RTT. Zoghbi et al. have reported significant reductions in homovanillic acid (HVA) and in 3-methoxy-4-hydroxy-phenylethylene glycol (MHPG) in the CSF levels of children with RTT [5]. However, Perry et al. reported no difference in these levels between RTT and controls [6]. Therefore, the alterations of CSF biogenic amines in RTT remain controversial. Wenk et al. reviewed these studies in 1997 [7]. The biogenic amines, dopamine, serotonin, and noradrenalin, and their respective metabolites, HVA, 5-hydroxyindoleacetic acid, MHPG, were measured in tissues from selected brain regions obtained at postmortem from four patients with RTT. A 50% or greater reduction in each substance, except MHPG, was observed in the substantia nigra from the two older patients at 20 and at 30 years of age, while the youngest patient at 12 years of age had normal levels. These changes
0387-7604/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S03 87- 7604(01)0038 0-1
T. Matsuishi et al. / Brain & Development 23 (2001) S58–S61
therefore appeared to be age related and may be secondary effects [8]. Wenk had previously reported that endogenous levels of dopamine and its metabolites were decreased throughout the neocortex and basal ganglia, and that the number of dopamine D2 receptors was decreased in the putamen [9]. Naidu et al. studied 12 adult patients (at 15–39 years of age) using [ 11C]N-methylspiperone, and found ‘low– normal’ levels of D2 dopamine receptors [10], and those findings were in contrast to those of Chiron et al., whose patients were in the range of 4–15 years of age [11]. The increase in receptors in younger patients with subsequent reductions in the older patients may help to explain the clinical course of RTT. Patients with RTT show severe mental retardation, impaired cognitive function and head growth deceleration. Kitt et al. have reported a 10–30% reduction in the number, and cell size, of the basal forebrain cholinergic neurons compared with controls, which might explain the impaired cognitive function and microcephalus [12]. Wenk et al. reported that the choline acetyltransferase (ChAT) activity in the putamen and thalamus was directly correlated with a decline in [ 3H]vesamicol binding in these regions. Vesamicol binds to the acetylcholine vesicular storage transporter protein and provides an indirect indication of cholinergic function. These results suggested that the cholinergic cells in the RTT brain did not compensate for the loss in the brain of RTT. The number of neurons within the basal forebrain that expressed the 75 kilodaltons P75, the low-affinity receptor for nerve growth factor (NGF), was unchanged [13]. NGF is known as a tropic factor, especially for the cholinergic neurons of the basal forebrain. CSF NGF was markedly decreased in 14 patients (at 1.8–17.4 years of age) with RTT, which may explain the decreased brain size [14]. Blue et al. reported significant changes in specific glutamate receptors, including a decrease in N-methyl-d-asparate (NMDA), a-amino-3-hydroxy-4-isoxazole propionic acid (AMPA), and in metabotropic type glutamate receptors [15]. The results showed a trend for densities of NMDA, AMPA, gamma aminobutyric acid (GABA), and metabotropic glutamate receptors in the superior frontal gyrus to be higher in young patients (aged from 4 to 15 years) than in controls and for densities in older patients (aged from 15 to 39 years) to fall below those of controls. Hamberger et al. have reported an elevation in the glutamate level in the CSF of children with RTT [16]. Elevations in NMDA receptors combined with the increased levels of CSF glutamate have suggested that excitatory neurotransmission was enhanced early in the course of the disease. Immunocytochemical studies on the neocortex in RTT have revealed an abnormality in the expression of the microtubule-associated protein 2 (MAP-2), which is regulated by several neurotransmitter systems in the adult cerebral cortex (particularly dopaminergic and cholinergic afferents): MAP-2 immunoreactivity was reduced throughout the neocortex in patients with RTT. These findings constituted a marked disruption in a major cytoskeletal component in the neocortex in RTT [17].
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Yamashita et al. measured benzodiazepine binding in stage IV RTT using single-photon computed tomography (SPECT) imaging techniques, and noted a significant reduction in the frontotemporal cortex, suggesting a decrease in GABA receptors in RTT [18]. 2.2. Intermediary metabolism Haas et al. have reported elevated CSF lactate and pyruvate [19]. Wakai et al. have reported morphological changes in the mitochondria in sural nerve biopsy specimens from patients with RTT [20]. We have also reported and concluded that the elevation in CSF lactate levels constituted a secondary biochemical change directly related to the abnormalities in respiration [21]. Klushnik et al. have reported that the blood autoantibody to NGF was elevated and that the anti-S100 protein was unchanged. They found higher levels of autoantibodies in milder cases, and suggested a possible involvement of an autoimmune process in RTT [22]. 2.3. Membrane lipid composition in the central nervous system The overall lipid composition in the frontal and temporal gray matter and cerebellar white matter was similar to control values. However, the ganglioside patterns differed from normal, with a decrease in GD1a and GT1b, suggesting an alteration or reduction in synaptic connections [23– 25]. One thing in common for GD1a and GT1b is that the same sialyltransferase catalyzes their formation from their immediate precursors GM1 and GD1b. 2.4. Neuropeptides 2.4.1. b -Endorphin, substance P Myer et al. [26] and Budden et al. [27] have reported elevated CSF b-endorphin in RTT. However, elevated bendorphin was not found in the brain, suggesting that the alteration in b-endorphin may be a secondary change. We have reported that the level of substance P was markedly decreased in the CSF in 16 patients (aged from 2.0 to 11.4 years) with RTT, and was only 50% of that of controls. In four adult patients (aged from 24.3 to 35 years) with RTT, the substance P level was further decreased and was 37% of that of adult controls, and this was concluded to reflect the autonomic dysfunction, including constipation, small and cold feet, progressive limb muscle weakness and muscle atrophy in RTT [28]. Substance P — a neuropeptide with 11 amino acids — is a neurotransmitter or neuromodulator in the peripheral as well as the central nervous system (CNS). Immunohistochemical studies have demonstrated that substance P is widely but unevenly distributed in the CNS. Substance P activity is associated with dopaminergic neurons in the substantia nigra and the striatum, the central autonomic nuclei, the dorsal root ganglia, and the peripheral autonomic ganglia [29]. Hedner et al. reported that substance P inter-
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acted with the respiratory control system by at least two different mechanisms; the bulbopontine time setting mechanism, and the inspiratory off-switch mechanism [30]. Recently, Deguchi et al. reported that the substance P immunoreactivity was significantly decreased in brain tissues, especially the dorsal horns, the intermediolateral column of the spinal cord, in the spinal trigeminal tract, solitary tract and nucleus, parvocellular and pontine reticular nuclei, and in the locus coeruleus in patients with RTT. A less significant decrease in substance P immunoreactivity has been found in the substantia nigra, the central gray of the midbrain, frontal cortex, caudate, putamen, globus pallidus, and in the thalamus. Glial fibrillary acidic protein (GFAP)-positive astrocytes were increased in the areas in which SP immunoreactivity was decreased [31]. They suggested that the decreases in substance P were responsible for the clinical signs of autonomic dysfunction (including small and cold feet, constipation, drooling). Substance P may also be related to growth failure and sudden death in RTT. Biological, immunological and immunohistochemical studies have revealed the multiple co-existence of neurotransmitters in various regions [32,33]: substance P co-existed with dopamine neurons in the substantia nigra, with serotonin in the raphe nucleus, and with acetylcholine in the tegmental portion of the pons. In addition to evidence for its function as a putative neurotransmitter, substance P also exerts neurotrophic action based on its stimulation of neurite extension in cultured neuroblastoma cells in vitro. The neurotrophic effects of substance P on the hippocampal neurons have been reported [34]. Deguchi et al. speculated that sleep abnormalities could be influenced by the decreased levels of substance P in the reticular formation and locus coeruleus [31]. They suggested that the sleep abnormalities were not a primary defect of substance P in RTT, but may be secondary to an abnormality in the serotonin system that is co-localized with substance P at some sites.
2.5. b -Phenylethylamine We have recently reported decreased b-phenylethylamine (PEA) levels in patients with RTT (aged from 3.4 to 11.4 years). The range of CSF PEA in RTT varied widely from 24.1 to 978.0 pg/ml, and was 41% of the control value. PEA in epilepsy with mental retardation, and PEA in autistic disorders, have been reported to be not significantly different from PEA in controls [35]. PEA is an endogenous amine synthesized by decarboxylation of phenylalanine in the dopamine neurons of the nigrostriatal system, and plays an important role in both the dopaminergic and noradrenergic systems. Fig. 1 shows the synthesis and catabolism of PEA. PEA has been studied in the peripheral sympathetic nervous system and phenylketon uria, and in psychiatric diseases, including schizophrenia, depression or personality disorder. Decreased levels of
Fig. 1. Synthesis and catabolism of phenylethylamine and dopamine. DOPA, 3,4-dihydroxyphenylalanine; AADC, aromatic amino acid decarboxylase; MAO-B, monoamine oxidase type B; DBH, dopamine 1b hydroxylase.
urinary PEA have been reported in patients with attentiondeficit hyperactivity disorders [36]. We have reported reduced levels of PEA in the CSF of patients with Parkinson’s disease. We also found that the CSF HVA level was unchanged in Parkinson’s disease, and concluded that this was because CSF HVA was influenced by dopaminergic therapy. However, the PEA level in CSF was not influenced, and was correlated with decreased dopamine neurons. The PEA level was also negatively correlated with the severity of the Parkinson’s disease (the Hohen and Yahr stage) [37]. Future studies should be undertaken on these neurochemical and neurotransmitter abnormalities and on MECP2. Amir et al. reported clinical and laboratory features versus genotype of MECP2. They reported that the CSF HVA was significantly decreased in patients with truncating mutations than those in patients with missense mutations [38]. We should study the relationship of MECP2 and expand our neurochemical studies. The male knock-out mouse with MECP2 showed short stature and could not survive for a long time, and female knock-out mice with MECP2 do not show any clinical phenotype [39]. Therefore, we should establish a better conditional knock-out mouse animal model and investigate the above correlation. We should explore new therapeutic modalities including NGF, and other neurotrophic factors, methylphe-
Table 1 Future directions in Rett syndrome Research needed
Future direction
MECP2 mutation type and X chromosome inactivation Developed conditioning knockout animal model of MECP2
Link between MECP2 and neurochemical studies Gene therapy for RTT Explore new therapeutic modalities NGF, PEA, substance P
T. Matsuishi et al. / Brain & Development 23 (2001) S58–S61
nidate (Ritalin) for which the chemical structure is similar to that of PEA, and other drugs (Table 1).
[18]
Acknowledgements
[19]
This research was supported in part by Grant-in-Aid for Scientific Research (C) and grant 8A-8B-16 from the National Center of Neurology and Psychiatry of the Ministry of Health and Welfare, Japan. The authors would like to thank the many valuable contributions from colleagues in Japan and in USA, especially Drs Shinichiro Nagamitsu, Mika Satoi, and Shigeto Yamada for their established methodology of CSF substance P, and b-phenylethylamine, and Professor Alan K. Percy who kindly inspired and advised our studies.
[20]
[21]
[22]
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Original article
Neurobiology and neurochemistry of Rett syndrome Toyojiro Matsuishi*, Yushiro Yamashita, Akira Kusaga Department of Pediatrics and Child Health, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan
Abstract The current status of neurobiological and neurochemical research on Rett syndrome is reviewed, and correlations are developed with previously described neurophysiological, neuroimaging, neuropathological, and immunohistochemical changes. We review the abnormalities reported in the biogenic amine neurotransmitters/receptor systems, and of b-phenylethylamine, an endogenous amine synthesized by the decarboxylation of phenylalanine in dopaminergic neurons of the nigrostriatal system. We also discuss the roles of other neurotransmitters, including b-endorphin and substance P, and neurotrophic factors, including nerve growth factors. Recently, DNA mutations in the methyl– CpG binding protein 2, mapped to Xq28, have been identified in some patients with Rett syndrome. The multiple abnormalities in the various neurotransmitters/receptor systems explain the pervasive effects of Rett syndrome. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Neurotransmitters; Neuromodulators; Biogenic amines; Neuropeptides; Nerve growth factors; Methyl–CpG binding protein 2
1. Introduction Rett syndrome (RTT) is a neurodevelopmental disorder of previously unknown etiology, characterized by normal early psychomotor development followed by the loss of psychomotor skills, especially acquired purposeful hand skills, and by the onset of stereotyped movement in the hands and of gait disturbance [1]. Recently, mutations in the gene encoding methyl–CpG binding protein 2 (MECP2) have been discovered in RTT [2]. However, the pathophysiology of RTT is still unknown. This paper discusses the future directions for research in RTT. Research needs to be directed to clarify the link between the discovery of the MECP2 involvement and the alteration in neurobiological, neurochemical, and neurotransmitter/receptor systems. Also, research is needed to develop the experimental model of conditioning MeCP2 knock-out mice to investigate new therapeutic modalities.
2. Neurotransmitters 2.1. Biogenic amines, acetylcholine and gamma aminobutyric acid Nomura and Segawa have suggested that hypoactivity or underdevelopment in the biogenic amines might account for the range of abnormalities found in RTT. They suggested a * Corresponding author. Tel.: 181-942-31-7565; fax: 181-942-38-1792. E-mail address: [email protected] (T. Matsuishi).
impairment in noradrenalin, serotonin, and in dopamine, using clinical and polysomnographic studies [3]. They have proposed that the following points are important in considering the pathophysiology of RTT; the characteristic symptoms and signs appear in sequence within a specific age from infancy. The earliest and pathognomonic manifestations of RTT are the autistic tendency and the decreased rate in head growth. They have suggested that the primary lesions of RTT involve the raphe nuclei and the locus coeruleus [4]. Their report has stimulated cerebrospinal fluid (CSF) biogenic amine studies and immunohistochemical or receptor studies, and neuroimaging studies which have now suggested that various neurotransmitters or neuromodulators, neurotrophic factors and neuronal markers may be involved in RTT. Zoghbi et al. have reported significant reductions in homovanillic acid (HVA) and in 3-methoxy-4-hydroxy-phenylethylene glycol (MHPG) in the CSF levels of children with RTT [5]. However, Perry et al. reported no difference in these levels between RTT and controls [6]. Therefore, the alterations of CSF biogenic amines in RTT remain controversial. Wenk et al. reviewed these studies in 1997 [7]. The biogenic amines, dopamine, serotonin, and noradrenalin, and their respective metabolites, HVA, 5-hydroxyindoleacetic acid, MHPG, were measured in tissues from selected brain regions obtained at postmortem from four patients with RTT. A 50% or greater reduction in each substance, except MHPG, was observed in the substantia nigra from the two older patients at 20 and at 30 years of age, while the youngest patient at 12 years of age had normal levels. These changes
0387-7604/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S03 87- 7604(01)0038 0-1
T. Matsuishi et al. / Brain & Development 23 (2001) S58–S61
therefore appeared to be age related and may be secondary effects [8]. Wenk had previously reported that endogenous levels of dopamine and its metabolites were decreased throughout the neocortex and basal ganglia, and that the number of dopamine D2 receptors was decreased in the putamen [9]. Naidu et al. studied 12 adult patients (at 15–39 years of age) using [ 11C]N-methylspiperone, and found ‘low– normal’ levels of D2 dopamine receptors [10], and those findings were in contrast to those of Chiron et al., whose patients were in the range of 4–15 years of age [11]. The increase in receptors in younger patients with subsequent reductions in the older patients may help to explain the clinical course of RTT. Patients with RTT show severe mental retardation, impaired cognitive function and head growth deceleration. Kitt et al. have reported a 10–30% reduction in the number, and cell size, of the basal forebrain cholinergic neurons compared with controls, which might explain the impaired cognitive function and microcephalus [12]. Wenk et al. reported that the choline acetyltransferase (ChAT) activity in the putamen and thalamus was directly correlated with a decline in [ 3H]vesamicol binding in these regions. Vesamicol binds to the acetylcholine vesicular storage transporter protein and provides an indirect indication of cholinergic function. These results suggested that the cholinergic cells in the RTT brain did not compensate for the loss in the brain of RTT. The number of neurons within the basal forebrain that expressed the 75 kilodaltons P75, the low-affinity receptor for nerve growth factor (NGF), was unchanged [13]. NGF is known as a tropic factor, especially for the cholinergic neurons of the basal forebrain. CSF NGF was markedly decreased in 14 patients (at 1.8–17.4 years of age) with RTT, which may explain the decreased brain size [14]. Blue et al. reported significant changes in specific glutamate receptors, including a decrease in N-methyl-d-asparate (NMDA), a-amino-3-hydroxy-4-isoxazole propionic acid (AMPA), and in metabotropic type glutamate receptors [15]. The results showed a trend for densities of NMDA, AMPA, gamma aminobutyric acid (GABA), and metabotropic glutamate receptors in the superior frontal gyrus to be higher in young patients (aged from 4 to 15 years) than in controls and for densities in older patients (aged from 15 to 39 years) to fall below those of controls. Hamberger et al. have reported an elevation in the glutamate level in the CSF of children with RTT [16]. Elevations in NMDA receptors combined with the increased levels of CSF glutamate have suggested that excitatory neurotransmission was enhanced early in the course of the disease. Immunocytochemical studies on the neocortex in RTT have revealed an abnormality in the expression of the microtubule-associated protein 2 (MAP-2), which is regulated by several neurotransmitter systems in the adult cerebral cortex (particularly dopaminergic and cholinergic afferents): MAP-2 immunoreactivity was reduced throughout the neocortex in patients with RTT. These findings constituted a marked disruption in a major cytoskeletal component in the neocortex in RTT [17].
S59
Yamashita et al. measured benzodiazepine binding in stage IV RTT using single-photon computed tomography (SPECT) imaging techniques, and noted a significant reduction in the frontotemporal cortex, suggesting a decrease in GABA receptors in RTT [18]. 2.2. Intermediary metabolism Haas et al. have reported elevated CSF lactate and pyruvate [19]. Wakai et al. have reported morphological changes in the mitochondria in sural nerve biopsy specimens from patients with RTT [20]. We have also reported and concluded that the elevation in CSF lactate levels constituted a secondary biochemical change directly related to the abnormalities in respiration [21]. Klushnik et al. have reported that the blood autoantibody to NGF was elevated and that the anti-S100 protein was unchanged. They found higher levels of autoantibodies in milder cases, and suggested a possible involvement of an autoimmune process in RTT [22]. 2.3. Membrane lipid composition in the central nervous system The overall lipid composition in the frontal and temporal gray matter and cerebellar white matter was similar to control values. However, the ganglioside patterns differed from normal, with a decrease in GD1a and GT1b, suggesting an alteration or reduction in synaptic connections [23– 25]. One thing in common for GD1a and GT1b is that the same sialyltransferase catalyzes their formation from their immediate precursors GM1 and GD1b. 2.4. Neuropeptides 2.4.1. b -Endorphin, substance P Myer et al. [26] and Budden et al. [27] have reported elevated CSF b-endorphin in RTT. However, elevated bendorphin was not found in the brain, suggesting that the alteration in b-endorphin may be a secondary change. We have reported that the level of substance P was markedly decreased in the CSF in 16 patients (aged from 2.0 to 11.4 years) with RTT, and was only 50% of that of controls. In four adult patients (aged from 24.3 to 35 years) with RTT, the substance P level was further decreased and was 37% of that of adult controls, and this was concluded to reflect the autonomic dysfunction, including constipation, small and cold feet, progressive limb muscle weakness and muscle atrophy in RTT [28]. Substance P — a neuropeptide with 11 amino acids — is a neurotransmitter or neuromodulator in the peripheral as well as the central nervous system (CNS). Immunohistochemical studies have demonstrated that substance P is widely but unevenly distributed in the CNS. Substance P activity is associated with dopaminergic neurons in the substantia nigra and the striatum, the central autonomic nuclei, the dorsal root ganglia, and the peripheral autonomic ganglia [29]. Hedner et al. reported that substance P inter-
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acted with the respiratory control system by at least two different mechanisms; the bulbopontine time setting mechanism, and the inspiratory off-switch mechanism [30]. Recently, Deguchi et al. reported that the substance P immunoreactivity was significantly decreased in brain tissues, especially the dorsal horns, the intermediolateral column of the spinal cord, in the spinal trigeminal tract, solitary tract and nucleus, parvocellular and pontine reticular nuclei, and in the locus coeruleus in patients with RTT. A less significant decrease in substance P immunoreactivity has been found in the substantia nigra, the central gray of the midbrain, frontal cortex, caudate, putamen, globus pallidus, and in the thalamus. Glial fibrillary acidic protein (GFAP)-positive astrocytes were increased in the areas in which SP immunoreactivity was decreased [31]. They suggested that the decreases in substance P were responsible for the clinical signs of autonomic dysfunction (including small and cold feet, constipation, drooling). Substance P may also be related to growth failure and sudden death in RTT. Biological, immunological and immunohistochemical studies have revealed the multiple co-existence of neurotransmitters in various regions [32,33]: substance P co-existed with dopamine neurons in the substantia nigra, with serotonin in the raphe nucleus, and with acetylcholine in the tegmental portion of the pons. In addition to evidence for its function as a putative neurotransmitter, substance P also exerts neurotrophic action based on its stimulation of neurite extension in cultured neuroblastoma cells in vitro. The neurotrophic effects of substance P on the hippocampal neurons have been reported [34]. Deguchi et al. speculated that sleep abnormalities could be influenced by the decreased levels of substance P in the reticular formation and locus coeruleus [31]. They suggested that the sleep abnormalities were not a primary defect of substance P in RTT, but may be secondary to an abnormality in the serotonin system that is co-localized with substance P at some sites.
2.5. b -Phenylethylamine We have recently reported decreased b-phenylethylamine (PEA) levels in patients with RTT (aged from 3.4 to 11.4 years). The range of CSF PEA in RTT varied widely from 24.1 to 978.0 pg/ml, and was 41% of the control value. PEA in epilepsy with mental retardation, and PEA in autistic disorders, have been reported to be not significantly different from PEA in controls [35]. PEA is an endogenous amine synthesized by decarboxylation of phenylalanine in the dopamine neurons of the nigrostriatal system, and plays an important role in both the dopaminergic and noradrenergic systems. Fig. 1 shows the synthesis and catabolism of PEA. PEA has been studied in the peripheral sympathetic nervous system and phenylketon uria, and in psychiatric diseases, including schizophrenia, depression or personality disorder. Decreased levels of
Fig. 1. Synthesis and catabolism of phenylethylamine and dopamine. DOPA, 3,4-dihydroxyphenylalanine; AADC, aromatic amino acid decarboxylase; MAO-B, monoamine oxidase type B; DBH, dopamine 1b hydroxylase.
urinary PEA have been reported in patients with attentiondeficit hyperactivity disorders [36]. We have reported reduced levels of PEA in the CSF of patients with Parkinson’s disease. We also found that the CSF HVA level was unchanged in Parkinson’s disease, and concluded that this was because CSF HVA was influenced by dopaminergic therapy. However, the PEA level in CSF was not influenced, and was correlated with decreased dopamine neurons. The PEA level was also negatively correlated with the severity of the Parkinson’s disease (the Hohen and Yahr stage) [37]. Future studies should be undertaken on these neurochemical and neurotransmitter abnormalities and on MECP2. Amir et al. reported clinical and laboratory features versus genotype of MECP2. They reported that the CSF HVA was significantly decreased in patients with truncating mutations than those in patients with missense mutations [38]. We should study the relationship of MECP2 and expand our neurochemical studies. The male knock-out mouse with MECP2 showed short stature and could not survive for a long time, and female knock-out mice with MECP2 do not show any clinical phenotype [39]. Therefore, we should establish a better conditional knock-out mouse animal model and investigate the above correlation. We should explore new therapeutic modalities including NGF, and other neurotrophic factors, methylphe-
Table 1 Future directions in Rett syndrome Research needed
Future direction
MECP2 mutation type and X chromosome inactivation Developed conditioning knockout animal model of MECP2
Link between MECP2 and neurochemical studies Gene therapy for RTT Explore new therapeutic modalities NGF, PEA, substance P
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nidate (Ritalin) for which the chemical structure is similar to that of PEA, and other drugs (Table 1).
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Acknowledgements
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This research was supported in part by Grant-in-Aid for Scientific Research (C) and grant 8A-8B-16 from the National Center of Neurology and Psychiatry of the Ministry of Health and Welfare, Japan. The authors would like to thank the many valuable contributions from colleagues in Japan and in USA, especially Drs Shinichiro Nagamitsu, Mika Satoi, and Shigeto Yamada for their established methodology of CSF substance P, and b-phenylethylamine, and Professor Alan K. Percy who kindly inspired and advised our studies.
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