Cerebrospinal fluid biochemical studies in patients with Parkinson's disease: toward a potential search for biomarkers for this disease

The blood-brain barrier supplies brain tissues with nutrients and filters certain compounds from the brain back to the bloodstream. In several neurodegenerative diseases, including Parkinson's disease (PD), there are disruptions of the blood-brain barrier. Cerebrospinal fluid (CSF) has been widely investigated in PD and in other parkinsonian syndromes with the aim of establishing useful biomarkers for an accurate differential diagnosis among these syndromes. This review article summarizes the studies reported on CSF levels of many potential biomarkers of PD. The most consistent findings are: (a) the possible role of CSF urate on the progression of the disease; (b) the possible relations of CSF total tau and phospho tau protein with the progression of PD and with the preservation of cognitive function in PD patients; (c) the possible value of CSF beta-amyloid 1-42 as a useful marker of further cognitive decline in PD patients, and (d) the potential usefulness of CSF neurofilament (NFL) protein levels in the differential diagnosis between PD and other parkinsonian syndromes. Future multicentric, longitudinal, prospective studies with long-term follow-up and neuropathological confirmation would be useful in establishing appropriate biomarkers for PD.

References for this review were identified by searching in PubMed from 1966 until June 20, 2014. The term “ Parkinson's disease ” was crossed with “ cerebrospinal fluid ” and “ blood brain barrier ,” and the related references were selected. Table 1 summarizes a classification of the diverse types of compounds which have been analyzed in the CSF of PD patients in accordance with the search. Table 1 Relation and classification of compounds measured in CSF of PD . (A) Neurotransmitters, neuromodulators, and related substances Dopamine (DA) metabolites: dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), 3-orthomethylDOPA (3-OMD) Serotonin (5-hydroxytryptamine or 5-HT) metabolites or precursors: 5-hydroxytryptophan (5-HTP), 5-hydroxyindoleacetic acid (5-HIAA), kynurenine, 3-hydroxykynurenine Noradrenalin (norepinephrine or NE) metabolites or precursors: 3-methoxy-4-hydroxy-phenylethylenglycol (MHPG), dopamine-beta-hydroxylase (DBH) Acetylcholine (Ach) and related substances: choline, acetylcholine-esterase (AchE), butiryl-cholin-esterase (BchE) Neurotransmitter amino acids: gamma-amino butyric acid (GABA), glutamate, aspartate, glycine Neuropeptides: substantia P (SP), cholecystokinin-8 (CCK-8), met-enkephalin (MET-ENK), leu-enkephalin (LEU-ENK), dynorphin A(1-8), somatostatin, neuropeptide Y (NPY), beta-endorphin, arginine-vasopressine (AVP), vasoactive intestinal peptide (VIP), delta sleep-inducing peptide (DSIP), alpha-melanocyte-stimulating hormone-like, diazepam-binding inhibitor, neurokinin A, corticotropin-releasing hormone (CRH), adrenocorticotropin hormone (ACTH), beta-lipotropine, angiotensin, chromogranins A and B, secretogranin II, orexin-A/hypocretin-1 Other neurotransmitters: endogenous cannabinoids, β-phenylethylamine Cyclic nucleotides: cyclic adenosine 3′5′ monophosphate (cAMP), cyclic guanosine 3′5′ monophosphate (cGMP) Biopterin derivatives and other cofactors (B) Endogenous neurotoxins Tetrahydroisoquinolin (TIQ) derivatives: 2-methyl-6,7-dihydroxy1,2,3,4-TIQ (2-MDTIQ), 1-MDTIQ (salsolinol). 1-benzyl-1,2,3,4-TIQ β-carbolinium cations (BC+s) (C) Oxidative stress markers Lipid peroxidation markers: Malonyl-dialdehyde (MDA) (E)-4-hydroxynonenal (HNE) Low density lipoprotein (LDL) oxidation products Schiff bases, conjugated dienes, oxidized proteins, and aldehyde polymers DNA oxidation markers: 8′-hydroxy-2′deoxyguanine (8-OHdG) 8-hydrosyguanosine (8-OHG) 8-OHdG/8-OHG ratio Transition metals and related proteins: iron, ferritin, transferring, copper, cerulopasmin, ferroxidase, manganese, zinc Other metals: selenium, chromium, magnesium, calcium, aluminum, silicon, cobalt, tin, lead, barium, bismuth, cadmium, mercury, molibdenum, nichel, antimony, strontium, thallium, vanadium, wolfram, and zirconium (D) Inflamatory and immunological markers Inteleukins (IL) Tumor necrosis alpha (TNF-α) Other: leukotrienes. α-1-antichymotrypsin (E) Growth and neurotrophic factors Brain-derived neurotrophic factor (BDNF) Transforming Growth Factors: TGF-α, TGF-β1, TGF-β2 Insulin-like growth factor-1 (IGF-1) and IGF-binding proteins (IGFBPs) Neuroregulins (Epidermal Growth Factor or EGF family) (F) Proteins involved in the pathogenesis of PD Microtubular-Associated Protein Tau (MAPT) Alpha-synuclein Amiloyd beta Neurofilament proteins Other proteins: DJ-1, UCH-L1 (G) Other compounds

Several studies have described neuronal loss, and presence of Lewy body in serotonergic raphe nuclei in PD patients (Benito-León et al., 2008 ). Tohgi et al. ( 1993b , c , 1997 ) reported a 15–20% reduction of CSF 5-HT, tryptophan (precursor of 5-HT), kynurenine and 3-hydroxykynurenine (metabolites of tryptophan) levels in PD patients. CSF 5-HT levels showed a negative correlation with the severity of bradykinesia, rigidity and freezing of the gait, and decreased after levodopa therapy. This group also found a correlation between CSF 5-HIAA levels and akinesia and freezing of gait (Tohgi et al., 1993a ). In contrast, Engelborghs et al. ( 2003 ) described increased 5-HT levels. LeWitt et al. ( 2013 ) described increased CSF 3-hydroxykynurenine levels, and Widner et al. ( 2002 ) described an increased CSF kynurenine/tryptophan ratio in PD patients. Several studies have shown reduced CSF levels of 5-hydroxyindoleacetic acid (5-HIAA), the main metabolite of 5-HT, in PD patients (Guldberg et al., 1967 ; Johansson and Roos, 1967 , 1971 ; Olsson and Roos, 1968 ; Gottfries et al., 1969 ; Chase, 1970 ; Rinne and Sonninen, 1972 ; Rinne et al., 1973 ; Davidson et al., 1977 ; Mayeux et al., 1984 , 1986 , 1988 ; Kostić et al., 1987 ; Tohgi et al., 1993c , 1997 ; Mashige et al., 1994 ; Strittmatter et al., 1996 ; Engelborghs et al., 2003 ). Other authors report normal CSF 5-HIAA levels (Papeschi et al., 1970 , 1972 ; Godwin-Austen et al., 1971 ; Granerus et al., 1974 ; Davidson et al., 1977 ; Tabaddor et al., 1978 ; Cramer et al., 1984 ; Burns et al., 1985 ; Chia et al., 1993 ; González-Quevedo et al., 1993 ; Volicer et al., 1985 ; Fukuda et al., 1989 ). Liu et al. ( 1999 ) described lower ventricular CSF 5-HIAA levels in patients with rigid-akinetic PD than in patients with tremoric PD, and a negative correlation between CSF 5-HIAA levels and PD severity. CSF 5-HIAA levels seem to be unchanged by therapy with levodopa (Godwin-Austen et al., 1971 ; Davidson et al., 1977 ), bromocriptine (Gumpert et al., 1973 ), or piribedil (Gumpert et al., 1973 ), or were found decreased by levodopa therapy (Casati et al., 1973 ). Gumpert et al. ( 1973 ) described an association between relatively low pre-treatment CSF 5-HIAA levels with a good response to levodopa, while Davidson et al. ( 1977 ) reported this association with higher CSF 5-HIAA levels, and others found no such relation (Granerus et al., 1974 ). Tetrahydrobiopterin increased (Dissing et al., 1989 ), and L-threo-3,4-dihydroxyphenylserine decreased (Maruyama et al., 1994 ) CSF 5-HIAA levels. Some authors have described decreased CSF 5-HIAA (Mayeux et al., 1984 , 1986 , 1988 ; Mena et al., 1984 ; Kostić et al., 1987 ) and 5-HT levels (Mena et al., 1984 ) in PD patients with depression, while others have described normal CSF 5-HIAA in depressed PD patients (Granerus et al., 1974 ; Kuhn et al., 1996a ), and others still have reported similar CSF 5-HIAA levels in patients with major depression with PD tothose without PD (Pålhagen et al., 2010 ). Moser et al. ( 1996 ) described increased CSF 5-HIAA in PD patients with hallucinations. Iacono et al. ( 1997 ) found higher CSF 5-HT and 5-HIAA and lower 5-HTP levels in PD patients with postural instability and gait disorders than in PD patients without these symptoms. Studies on the correlation of CSF 5-HT metabolite levels and brain 5-HT levels are lacking. The majority of studies report results on CSF 5-HIAA levels, with the controversial results based on short series of cohorts of patients with PD or other parkinsonian syndromes. Current data do not lend support to the role of CSF 5-HIAA as an unequivocal marker of depression linked to PD.

CSF levels of Ach (Duvoisin and Dettbarn, 1967 ; Welch et al., 1976 ; Yamada et al., 1996 ) and its precursor choline (Aquilonius et al., 1972 ; Welch et al., 1976 ; Nasr et al., 1993 ) have been reported to be similar in PD patients to controls with the exception of one study in which lower CSF choline levels were described in PD patients (Manyam et al., 1990 ). CSF activity of acetylcholine-esterase (AchE), the main enzyme involved in Ach degradation, has been reported to be similar in PD patients and controls (Jolkkonen et al., 1986 ; Ruberg et al., 1986 ; Zubenko et al., 1986 ; Sirviö et al., 1987 ; Yoshinaga et al., 1989 ; Manyam et al., 1990 ; Hartikainen et al., 1992 ), although there are studies which have described increased (Ruberg et al., 1986 ), decreased (Konings et al., 1995 ), or normal activity (Zubenko et al., 1986 ; Sirviö et al., 1987 ) in demented patients, and decreased activity only in those patients with the most severe disease (Hartikainen et al., 1992 ). CSF activity of butirylcholine-esterase (BchE) have been found to be similar in PD patients and controls (Ruberg et al., 1986 ; Sirviö et al., 1987 ), but increased in demented PD patients in a single study (Ruberg et al., 1986 ). Data on CSF Ach and related substances are scarce and based on short series of patients, and do not permit valid conclusions.

Neuropeptides modulate neuronal communication by acting on cell surface receptors. Many of them are co-released with classical neurotransmitters. There have been reports of a number of changes in the concentrations of several neuropeptides in PD brain, which are mainly significant decreases in (Jiménez-Jiménez, 1994 ): (a) met-enkephalin (MET-ENK), substantia P (SP), and cholecystokinine 8 (CCK-8) in the substantia nigra; (b) MET-ENK and leu-enkephalin (LEU-ENK) in the putamen and globus pallidus; (c) MET-ENK in the ventral tegmental area; (d) SP, somatostatin and neurotensin in the neocortex, and (e) somatostatin and neurotensin in the hippocampus. It is likely that many of these changes are related with dopaminergic deficit, and the only clear relationship between a neuropeptide and a clinical feature of PD is that of somatostatin with the presence of cognitive impairment (Jiménez-Jiménez, 1994 ). Table 2 summarizes the findings of classical studies on CSF neuropeptide levels in PD patients. Most of these studies enrolled limited series of patients. Table 2 Alterations in CSF neuropeptide levels in PD patients compared with controls . Neuropeptide References PD patients/ Controls Cerebrospinal fluid levels Substantia P (SP) Pezzoli et al., 1984 12/10 Increased 5-fold Cramer et al., 1989 15/9 Normal Cramer et al., 1991 23/9 Decreased by 30% (controls were essential tremor patients) Cholecystokinin-8 (CCK-8) Lotstra et al., 1985 20/68 Decreased by 50% Met-enkephalin (MET-ENK) Pezzoli et al., 1984 12/10 Increased 3-fold in PD patients with slight or moderate disability ( n = 6) Yaksh et al., 1990 8/9 Decreased by 37% Baronti et al., 1991 16/19 Decreased by 31.7% Leu-enkephalin (LEU-ENK) Liu, 1989 22/19 Increased by 122% in untreated PD patients without further modification by levodopa therapy Dynorphin A(1-8) Baronti et al., 1991 16/19 Normal Somatostatin Jolkkonen et al., 1986 35/19 Decreased by 22% ( p < 0.01), especially in demented patients Strittmatter and Cramer, 1992 38/12 Decreased by 27.5% ( p < 0.01) Strittmatter et al., 1996 35/11 Decreased p < 0.05, similar in untreated vs. treatment with levodopa Cramer et al., 1989 15/9 Decreased by 39% Dupont et al., 1982 39/29 Decreased by 40% Christensen et al., 1984 48/32 Decreased by 40% Cramer et al., 1985 50/6 Decreased by 34%(controls were patients with essential tremor) Masson et al., 1990 35/11 Decreased ( p < 0.02), especially in untreated patients and in those with more severe disease Jost et al., 1990 68/6 Decreased by 28% Hartikainen et al., 1992 35/34 Normal Volicer et al., 1986 10/9 Normal Beal et al., 1986 6/84 Normal Poewe et al., 1990 22/11 Normal in PD patients with dementia ( n = 11) and without dementia ( n = 11) Espino et al., 1995 23/26 Increased by 47%, especially in demented patients Neuropeptide Y (NPY) Martignoni et al., 1992 10/20 Decreased by 31% Yaksh et al., 1990 8/9 Normal Beta-endorphin Nappi et al., 1985 24/15 Decreased ( p < 0.005) both in 14 untreated and 10 treated PD patients Jolkkonen et al., 1987 36/35 Normal Arginine-vasopressine (AVP) Sundquist et al., 1983 11/21 Decreased by 68% Olsson et al., 1987 12/32 OND Decreased by 71% Vasoactive intestinal peptide (VIP) Sharpless et al., 1984 19/12 Normal Delta sleep-inducing peptide (DSIP) Ernst et al., 1987 9/20 Decreased by 28.7% (Ferrero et al., 1988 ) Alpha-melanocyte-stimulating hormone-like Rainero et al., 1988 9/12 Increased by 2-fold Diazepam-binding inhibitor Ferrero et al., 1988 25/82 Increased by 42.5% (80% in depressed PD patients and normal in non-depressed PD patients Ferrarese et al., 1990 28/10 Decreased by 50% in PDD ( n = 14), normal in PDND ( n = 14) Neurokinin A Galard et al., 1992 12/11 Decreased by 24% Corticotropin-releasing hormone (CRH) Suemaru et al., 1995 10/5 Normal ACTH Nappi et al., 1985 24/15 Normal Beta-lipotropine Nappi et al., 1985 24/15 Normal Angiotensin converting enzyme (ECA) Konings et al., 1994 88 PDND/18 PDD/20 Increased in PDND patients under levodopa therapy ( p < 0.05). Normal in untreated PDND and in PDD Zubenko et al., 1985 10 PDD/30 Decreased by 27% in demented PD patients Zubenko et al., 1986 15/10 Decreased by 24% Chromogranin A and B and secretogranin II Eder et al., 1998 8/29 Normal OND, other neurological diseases; PDD, Parkinson's disease demented; PDND, Parkinson's disease non-demented. In recent years, there has been increased interest in the possible role of orexin-A/hypocretin-1, a neuropeptide hormone implicated in the pathogenesis of narcolepsia, on the development of excessive daytime sleepiness in PD patients. Since the first report by Drouot et al. ( 2003 ), who described decreased ventricular CSF orexin levels in PD patients, which were related with the severity of the disease, other authors have confirmed decreased CSF orexin in PD (Fronczek et al., 2007 ; Asai et al., 2009 ) and in other neurodegenerative parkinsonisms (Yasui et al., 2006 ), and the relation of CSF orexin with severity of PD (As

These compounds act as intracellular second messengers of neurotransmitters or other compounds such as nitric oxide (NO). The most important are cyclic adenosine 3′5′ monophosphate (cAMP) and cyclic guanosine 3′5′ monophosphate (cGMP). Belmaker et al. ( 1978 ) reported a 40–50% decrease of CSF cAMP and an 80–90% decrease of CSF cGMP levels in PD patients who were not related with levodopa therapy. Decreased CSF cAMP levels in PD have also been reported in another study (Volicer et al., 1986 ), while others found this value to be normal (Cramer et al., 1973 , 1984 ; Covicković-Sternić et al., 1987 ; Oeckl et al., 2012 ), both in PD patients with and without dementia (Oeckl et al., 2012 ). Four further studies described normal CSF cGMP levels (Volicer et al., 1986 ; Covicković-Sternić et al., 1987 ; Ikeda et al., 1995 ; Oeckl et al., 2012 ), while another found a non-significant trend toward higher CSF cGMP levels in PD patients when compared with controls and higher levels in levodopa-treated PD patients compared with those without levodopa treatment (Navarro et al., 1998 ).

One of the classical etiological hypotheses of PD is related with the presence of endogenous substances which share structural similarities with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin that induces a parkinsonism resembling PD. Moser et al. (Moser and Kömpf, 1992 ; Moser et al., 1995 ) identified two tetrahydroisoquinolin (TIQ) derivatives in the CSF of PD patients, but not in healthy controls, 2-methyl and 1-methyl-6,7-dihydroxy1,2,3,4-TIQ (2-MDTIQ and 1-MDTIQ or salsolinol). This group described a relation between high salsolinol levels and the presence of visual hallucinations (Moser et al., 1996 ), and reported an increased HVA/3OMD ratio in PD patients in which 2-MDTIQ was detected when compared with those PD in which it was not detectable. CSF salsolinol levels have been reported to be increased in PD patients compared with controls by other groups (Maruyama et al., 1996 ; Antkiewicz-Michaluk et al., 1997 ; Krygowska-Wajs et al., 1997 ; Naoi and Maruyama, 1999 ), especially in demented PD patients (Antkiewicz-Michaluk et al., 1997 ), and in those patients with more severe parkinsonism (Krygowska-Wajs et al., 1997 ), although other authors have described a trend toward decrease in CSF salsolinol levels with the progression of the disease (Maruyama et al., 1999 ). In contrast, another group reported similar CSF salsolinol (Müller et al., 1999a , b ), but higher levels of harman and norharman β-carbolines (structural analogs of MPTP as well) in PD patients than in controls (Kuhn et al., 1996b ). CSF levels of 1-benzyl-1,2,3,4-TIQ have also been found by another group to be increased (Kotake et al., 1995 ). Matsubara et al. ( 1995 ) measured β-carbolinium cations (BC+s) in the lumbar CSF of 22 PD patients and 11 age-matched controls, and found the 2,9-dimethylnorharmanium cation (2,9-Me2NH+) in 12 PD patients but not in controls. This group described decreased activity of nicotinamide N-methyltranserase (NNMT), an enzyme that catalyzes the N-methylation of nicotinamide and other pyridines in the CSF of younger PD patients compared with younger controls, and a trend toward decrease with aging in PD patients (Aoyama et al., 2001 ). The results of studies on neurotoxins related with the risk for PD are based on small series and are not conclusive.

CSF interleukin (IL) 1-β levels were found to be normal in one study (Pirttila et al., 1994 ) and increased in three (Blum-Degen et al., 1995 ; Mogi et al., 1996a ; Mogi and Nagatsu, 1999 ), CSF IL-2 normal (Blum-Degen et al., 1995 ) or increased (Mogi et al., 1996a ; Mogi and Nagatsu, 1999 ), IL-4 increased (Mogi and Nagatsu, 1999 ), and CSF IL-10, IL-12, and interferon-gamma levels have been reported to be similar in PD patients and controls (Rota et al., 2006 ). CSF IL-6 levels have been found to be decreased in PD patients with major depression in comparison with patients with major depression without PD in one study (Pålhagen et al., 2010 ), while another 4 found higher CSF IL-6 in PD patients than in healthy controls (Blum-Degen et al., 1995 ; Mogi et al., 1996a ; Müller et al., 1998 ; Mogi and Nagatsu, 1999 ), and in one of them CSF IL-6 was correlated with PD severity (Müller et al., 1998 ). CSF tumor necrosis α (TNF-α) levels have been found to be increased (Mogi et al., 1994 ; Mogi and Nagatsu, 1999 ), leukotriene 4 (Irkeç et al., 1989 ), and α-1-antichymotrypsin normal (Pirttila et al., 1994 ), and β-2-microglobuline decreased in PD (Mogi et al., 1989 ; Mogi and Nagatsu, 1999 ). The CSF levels of the cytokine fractalkine have been found to be normal in PD patients and increased in multiple system atrophy (MSA), and Flt3 ligand normal in these two diseases (Shi et al., 2011 ). The presence of certain syalilated isoforms of Serpin A1 in the CSF has been related with the development of dementia in PD patients (Jesse et al., 2012 ). CSF levels of pros-methylimidazol acetic acid, an isomer of the histamine metabolite tele-methylimidazol acetic acid, have been found to be decreased in PD (Prell et al., 1991 ), and were highly positively correlated with the severity of the disease (Prell and Green, 1991 ). CSF complement 3 (C 3 ) and factor H (FH) levels were reported to be normal in one study (Wang et al., 2011 ), while another described a decrease in several isoforms of C 3b, C 4b , FH, and factor B (Finehout et al., 2005 ), and another normal C 4d (Yamada et al., 1994 ). CSF levels of heat shock proteins Hsp65 and Hsp70 have been found to be increased (Fiszer et al., 1996 ), and PD patients have shown higher HLA-DR expression in CSF monocytes in comparison with controls (Fiszer et al., 1994a ). Oligoclonal IgG bands have not been detected in the CSF of PD patients (Chu et al., 1983 ), but antibodies against DA neurons have been detected in 78% of PD patients and in only 3% of controls (Carvey et al., 1991 ), and the CSF of PD patients has shown a higher proportion of gamma-delta-T+ cells than in controls (Fiszer et al., 1994b ). The results of studies on inflammatory and immunological markers in PD have a low number of patients and controls enrolled, and are inconclusive.

Microtubular associated protein Tau (MAPT) Because MAPT gene is one of the main genes involved in the risk for PD (Alonso-Navarro et al., 2014 ), the measurement of CSF protein tau levels are hypothetically useful as a marker of this disease. Tau protein is important for maintaining the stability of axonal microtubules involved in the mediation of fast axonal transport of synaptic constituents. Hyperphosphorylation of tau causes reduces binding affinity for microtubules, leading to their malfunction. Following neuronal damage, tau is released into extracellular space and may be increased in the CSF. Tau is an important component of the neurofibrillary tangles (pairwise, helical protein filaments which are found in the cytoskeleton or neuronal cells in Alzheimer's disease (AD) brains. CSF tau protein levels are increased in AD patients, and so are a useful marker for this disease. The high risk of PD patients of developing cognitive impairment or dementia patients makes measurement of CSF tau reasonable as a possible marker of this disease. Many studies have shown similar CSF total tau and phosphorylated tau (phospho tau ) in PD patients to controls (Blennow et al., 1995 ; Molina et al., 1997c ; Jansen Steur et al., 1998 ; Sjögren et al., 2002 ; Mollenhauer et al., 2006 ; Parnetti et al., 2008 , 2011 , 2014a , b ; Ohrfelt et al., 2009 ; Compta et al., 2009b ; Alves et al., 2010 ; Montine et al., 2010 ; Aerts et al., 2011 ; van Dijk et al., 2013a ; Herbert et al., 2014 ). Several of these studies have shown increased CSF tau in demented PD patients (Mollenhauer et al., 2006 ; Compta et al., 2009b ). The 33 KDa/55 KDa tau isoforms ratio have also been found to be normal in PD (Borroni et al., 2008 , 2009 ), but decreased in progressive supranuclear palsy (PSP), and normal in patients with diffuse Lewy body disease (DLBD), demented PD patients (PDD), AD, and frontotemporal dementia (FTD) (Borroni et al., 2008 , 2009 ). Some authors have found decreased CSF total tau and phospho tau levels when compared with controls (Mollenhauer et al., 2011 ; Shi et al., 2011 ; Kang et al., 2013 ) and similar levels in PD to PSP, DLBD, and MSA (Mollenhauer et al., 2011 ), while others found higher CSF tau in DLBD compared with PDD patients (Andersson et al., 2011 ), and still others higher CSF total tau in MSA than in PD patients (Herbert et al., 2014 ). Hall et al. ( 2012 ) reported decreased CSF total tau and normal phospho tau both in PD and PDD, while total tau was increased in CBD and normal in PSP, DLBD, and MSA, and phospho tau was decreased in PSP and MSA in comparison with controls. Přikrylová Vranová et al. ( 2010 ) found increased CSF tau levels in PD patients with less than 2 years of evolution, and increased CSF tau levels which were higher in patients with PDD than in PD, and in PD than in controls, and similar CSF tau in DLDB than in controls (Vranová et al., 2014 ). This group and others found increased CSF total tau levels in patients with non-tremor variants of PD as compared to tremor-dominant PD and controls (Jellinger, 2012 ; Přikrylová Vranová et al., 2012 ). Compta et al. ( 2011 ) described increased CSF tau levels in PD patients carrying the allele rs242557A. Siderowf et al. ( 2010 ) showed a lack of association between baseline CSF tau levels and cognitive decline in PD patients. Patients with corticobasal degeneration (CBD) and PSP have shown higher CSF total and phospo tau levels (Aerts et al., 2011 ), and patients with DLBD showed similar CSF tau levels to PD patients in one study (Ohrfelt et al., 2009 ), while other authors found higher CSF tau levels in AD than in DLBD, in DLDB higher than in PDD, and in PDD higher than in PD (Parnetti et al., 2008 ). Baseline CSF levels of total and phospho tau in the DATATOP study, involving 403 early PD patients, were negatively correlated with disease progression assessed with the Unified PD Rating Scale (UPDRS) (Zhang et al., 2013 ). Beyer et al. ( 2013 ) reported a lack of correlation between CSF levels of total and phospho tau , and ventricular size in 73 non-demented PD patients and 18 PD patients with mild cognitive impairment. The results of the studies reported on CSF tau levels in PD are summarized in Table 4 . Although these results are not conclusive, CSF tau levels could be related to the progression of the disease (Zhang et al., 2013 ), and to the preservation of cognitive function in PD patients (Stewart et al., 2014 ). Table 4 Results of studies on CSF tau and phosphotau levels in PD, other parkinsonian syndromes and controls . References Cases/Controls Main findings Blennow et al., 1995 44 AD, 31 controls, 17 VAD, 11 FTD, 15 PDND, major depression CSF total tau and phosphorylated tau (phosphotau) higher in AD than in controls, VAD, FTD, PDND, and major depression (PDND similar than controls) Molina et al., 1997c 26 PDND, 25 controls CSF total tau similar in PD and controls Jansen Steur et al., 1998 115 PD (48 with MMSE lower than 25) 15 controls

Amyloid beta (Aβ) are a group of different lengths peptides resulting from the enzymatic cleavage of the amyloid precursor protein (APP). The most common is the 42 amino-acid long Aβ42. These peptides have a differential trend toward aggregation (specially Aβ1-42) to form amyloid plaques, one of the pathological hallmarks of AD and DLBD. The increased risk for developing cognitive impairment and dementia of PD patients in comparison with the general population makes it reasonable to link AD markers such as Aβ42 to PDD. Several studies have shown similar (Holmberg et al., 2003 ; Mollenhauer et al., 2006 ; Ohrfelt et al., 2009 ; Přikrylová Vranová et al., 2010 ; Aerts et al., 2011 ; Parnetti et al., 2011 ; van Dijk et al., 2013a ) or decreased (Sjögren et al., 2002 ; Compta et al., 2009b ; Mollenhauer et al., 2011 ; Shi et al., 2011 ; Kang et al., 2013 ; Nutu et al., 2013a ; Vranová et al., 2014 ) CSF Aβ1-42 (Aβ1-42) in PD patients, with the exception of one study which reports increased levels (Parnetti et al., 2014b ). Other found decreased CSF Aβ-1-42 (Mollenhauer et al., 2006 ; Compta et al., 2009b ; Alves et al., 2010 ; Montine et al., 2010 ; Siderowf et al., 2010 ) and Aβ1-40 (Alves et al., 2010 ) and Aβ1-38 (Alves et al., 2010 ) only in PDD patients or in PD patients with memory impairment. Baseline CSF Aβ levels in the DATATOP study, were negatively correlated with disease progression assessed with UPDRS (Zhang et al., 2013 ). Baseline CSF levels of Aβ1-42 in two studies (Siderowf et al., 2010 ; Parnetti et al., 2014b ); and the combination of lower baseline CSF Aβ, worse verbal learning, semantic fluency and visuoperceptual scores, and thinner superior-frontal/anterior cingulated in precentral regions by 3T-brain-Magnetic Resonance Imaging in another (Compta et al., 2013 ) have been associated with further cognitive decline in PD patients. CSF Aβ1-42 levels have been reported as decreased (Parnetti et al., 2008 ; Andersson et al., 2011 ; Parnetti et al., 2011 ) or similar (Ohrfelt et al., 2009 ; Nutu et al., 2013a ) in DLBD than in PDD and PD patients, decreased in MSA (Holmberg et al., 2003 ; Shi et al., 2011 ), and decreased in DLBD in comparison with PD, PDD (Hall et al., 2012 ; Vranová et al., 2014 ), PSP, MSA, and CBD (Hall et al., 2012 ). Alves et al. ( 2013 ) reported that patients with PD with the postural instability-gait disorders (PIGD) phenotype had significantly reduced CSF Aβ42, Aβ38, Aβ42/40, and Aβ38/40 levels compared with patients with the tremor-dominant phenotype and controls. Nutu et al. ( 2013b ) described lower CSF levels of Aβ1-15/16 in PD, PDD, PSP, and MSA compared to CBD, AD, and controls. Beyer et al. ( 2013 ) reported a correlation between CSF levels of Aβ38, Aβ40, and Aβ42, and ventricular size in 73 non-demented PD patients and 18 PD patients with mild cognitive impairment. The results of the studies reported on CSF Aβ levels in PD are summarized in Table 6 . Many of these studies suggest the potential usefulness of CSF Aβ1-42 levels to predict cognitive impairment in PD patients. Table 6 Results of studies on CSF amiloyd beta (Aβ) levels in PD, other parkinsonian syndromes and controls . References Cases/Controls Main findings Sjögren et al., 2002 19 AD, 14 FTD, 11 ALS, 15 PD, 17 controls CSF Aβ42 markedly decreased in AD = ALS < FTD < PD < controls Holmberg et al., 2003 36 MSA, 48 PD, 15 PSP, 32 controls CSF Aβ42 MSA < PSP = controls = PD Mollenhauer et al., 2006 73 PDD, 23 PDND, 41 controls (non-demented neurological patients) CSF Aβ42 lower in the PDD patients compared to PDND patients and controls. This observation was most marked ( p < 0.05) in a subgroup of patients with PDD carrying the apolipoprotein genotype epsilon3/epsilon3 Parnetti et al., 2008 19 DLBD, 18 PDD, 23 AD, 20 PDND, 20 controls DLBD showed the lowest mean CSF Aβ42 levels, with a negative association to dementia duration. PDD patients had mean CSF Aβ42 similar to those seen in PD patients Ohrfelt et al., 2009 66 AD patients, 15 PD patients, 15 patients with dementia with Lewy bodies (DLBD) and 55 cognitively normal controls CSF Aβ42 AD < DLBD < PD = Controls Compta et al., 2009b 20 PDND, 20 PDD, 30 controls patients CSF Aβ42 ranged from high (controls) to intermediate (PDND) and low (PDD) levels ( P < 0.001). In all PD and PDD patients, in PDND, CSF Aβ42 was related with phonetic fluency Alves et al., 2010 109 PDND, 36 controls, 20 mild AD CSF Aβ42 (19%; p = 0.009), Aβ40 (15.5%; p = 0.008), and Aβ38 (23%; p = 0.004) significantly decreased in PD compared with controls CSF Aβ42 reductions in PD less marked than in AD (53%; p = 0.002) Associations between CSF levels of Aβ42 (β = 0.205; p = 0.019), Aβ40 (β = 0.378; p < 0.001), and Aβ38 (β = 0.288; p = 0.001) and memory impairment, but not executive-attentional or visuospatial dysfunction Montine et al., 2010 150 controls (115 >50 years; 24 amnestic Mild Cognitive Impairment (aMCI), 49 AD, 49 PD, 11 PDD 62 PD-CIND (cognit

Defects in the gene encoding DJ-1 protein cause an autosomal recessive early-onset PD, PARK7 (Alonso-Navarro et al., 2014 ). This protein is also a marker of oxidative stress. CSF levels of DJ-1 protein have been found to be increased in PD in 2 studies (Waragai et al., 2006 ; Herbert et al., 2014 ) and decreased in another 2 (Shi et al., 2011 ; Hong et al., 2010 ). One of these studies described decreased CSF DJ-1 in MSA as well (Shi et al., 2011 ), and other increased DJ-1 in MSA compared with PD and with controls (Herbert et al., 2014 ). Shi et al. ( 2012 ) described a lack of relation between the loss of striatal dopaminergic function and CSF DJ-1 levels in asymptomatic carriers of mutations in the LRRK2 gene (PARK8). The results on DJ-1 are, therefore, inconsistent and should not be considered as a marker of PD. Defects in the gene encoding ubiquitin carboxy-terminal hydrolase 1 (UCH-L1) cause familial PD, PARK5. A recent study found decreased CSF UCH-L1 levels in PD, MSA, and PSP compared with controls (Mondello et al., 2014 ). Among proteins related with apoptosis, Bcl-2 protein has not been detected in the CSF of PD patients (Mogi et al., 1996b ; Mogi and Nagatsu, 1999 ). CSF levels of clusterin have been reported to be increased (Přikrylová Vranová et al., 2010 ; Vranová et al., 2014 ) or normal (van Dijk et al., 2013a ), tissue transglutaminase (Vermes et al., 2004 ) increased in PD, and cystatin C normal in PD (Přikrylová Vranová et al., 2010 ; Yamamoto-Watanabe et al., 2010 ) and decreased in MSA (Yamamoto-Watanabe et al., 2010 ). Studies measuring CSF levels of lysosomal hydrolases (involved in the α-Syn degradation) found decreased (Balducci et al., 2007 ), normal (van Dijk et al., 2013b ), or increased (Parnetti et al., 2014a ) β-hexosaminidase, increased cathepsin E (van Dijk et al., 2013b ), decreased α-mannosidase (Balducci et al., 2007 ), decreased (Balducci et al., 2007 ) or normal β-mannosidase (Mollenhauer et al., 2011 ; van Dijk et al., 2013b ), decreased α-fucosidase (van Dijk et al., 2013b ), β-glucocerebrosidase decreased (Balducci et al., 2007 ; Parnetti et al., 2014a ) or normal (van Dijk et al., 2013b ), β-galactosidase increased (van Dijk et al., 2013b ) or normal (Balducci et al., 2007 ; Parnetti et al., 2014a ), and cathepsin D normal (van Dijk et al., 2013b ) in PD patients compared with controls. CSF Prion protein (PrP) (Meyne et al., 2009 ) and tetranectin (involved in tissue remodeling) (Hong et al., 2010 ) levels have been found to be decreased, and apolipoprotein A-1 normal (Wang et al., 2010 ) in PD patients. CSF levels of transthyretin (TTR, a clearance protein produced in the choroid plexus) have been found to be increased in Lewy body diseases, including PD, PDD, and DLBD in relation with controls (Maetzler et al., 2012 ). CSF levels of the soluble proteoglycan NG2 (sNG2), involved in proliferation, migration, and differentiation of perycites and NG2 cells in the brain, have been found to be similar in PD patients and controls, and decreased in DLBD (Nielsen et al., 2014 ). In PD patients there are reports of decreased CSF post-proline cleaving enzyme (Hagihara and Nagatsu, 1987 ), increased dipeptidyl-aminopeptidase II (Hagihara et al., 1987 ), normal dipeptidyl-aminopeptidase IV (Hagihara et al., 1987 ), and normal glutamic oxaloacetic transaminase (GOT) (Steen and Thomas, 1962 ; Weiss et al., 1975 ; Qureshi et al., 1995 ) and glutamic pyruvic transaminase (GPT) (Weiss et al., 1975 ) levels.

The majority of classical biochemical studies on neurotransmitter and related substances have described decreased CSF HVA, and normal NE, MHPG, ACh, AChE, glutamate, aspartate, and glycine levels in patients with PD. Results on CSF GABA and 5-HIAA levels are controversial. Many of these classical studies included patients with different types of Parkinsonism and had a limited number of patients and controls. Studies on the possible value of endogenous neurotoxins, oxidative stress markers, inflammatory and immunological markers, and growth and neurotrophic factors as biological markers of PD should be considered as inconclusive. The most consistent finding related with these issues is the possible role of CSF urate on the progression of the disease (Ascherio et al., 2009 ). Data regarding the role of CSF total tau and phospho tau as biological markers for PD are inconsistent. The most interesting findings are the possible relations of these markers with the progression of the disease (Zhang et al., 2013 ), and with the preservation of cognitive function in PD patients (Stewart et al., 2014 ). CSF α-synuclein levels have been found to be decreased in most, but not all, studies in PD patients compared with controls. This marker should be useful for the differential diagnosis between synucleopathies and other parkinsonian syndromes, but its usefulness to differentiate among synucleopathies (PD, PDD, DLBD, and MSA), remains to be elucidated. CSF Aβ1-42 levels could be considered as a useful marker of the presence of further cognitive decline in PD patients. CSF NFL protein levels should be useful for the differential diagnosis of PSP, MSA, CBD, and PDD from PD, but not to discriminate between PD and healthy controls.