Melatonin May Be Beneficial in Treatment of Neurodegenerative Diseases
Melatonin (N-acetyl-5-methoxytryptamine) is a natural hormone secreted by the pineal gland. In clinical use for many years, melatonin is safe and well-tolerated even at high doses  and easily crosses the blood–brain barrier. Besides being used to increase sleep efficiency, treat jet lag, improve the cardiovascular system , and as an antiaging drug [3–5] and a dietary supplement and cancer-protective hormone , intensive research roughly in the past 10 years has indicated melatonin's beneficial effects in experimental models of neurodegenerative disorders. Brain oxidative damage has been implicated as a common link in the pathogenesis of such diseases. This small amphiphilic molecule acts as a free-radical scavenger, and its broad spectrum of antioxidant activities in many central nervous system neurodegenerative diseases  has been well documented and reviewed . There is growing evidence that its antiapoptotic effects play an important role in neurodegeneration as well. This review summarizes the antiapoptotic activities of melatonin via the inhibition of intrinsic apoptotic pathways and the activation of survival signal pathways in stroke, Alzheimer disease (AD), Parkinson disease (PD), Huntington disease (HD), and amyotrophic lateral sclerosis (ALS).
The Intrinsic and Extrinsic Apoptotic Pathways in Neurodegenerative Diseases
Two types of cell death occur in neurodegeneration: apoptosis and necrosis. Apoptosis (also called programmed cell death) occurs naturally under normal physiological conditions and in a variety of diseases, while necrosis is caused by external factors, such as infection, toxins, or trauma. Apoptosis is a feature of both acute and chronic central nervous system neurodegenerative diseases. There are two major apoptotic signaling pathways: extrinsic and intrinsic. The extrinsic apoptotic pathway (death receptor pathway) is initiated by death receptors (e.g., CD95/APO-1/Fas; TNF receptor) on the surface of the cells, involving caspase-8/Bid and caspase-10 activation [9,10]. Since there have been no obvious reports of the involvement of extrinsic pathways in the neuroprotection of melatonin, this review focuses only on the intrinsic pathway (the mitochondrial pathway)  (Fig. 1).
Figure 1. Scheme of neuroprotection of melatonin. The possible inhibition of the intrinsic cell death pathway and activation of the survival pathway by melatonin are schematized.
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Proapoptotic mitochondria molecules, cytochrome c, Smac (second mitochondrion-derived activator of caspase)/Diablo, AIF (apoptosis-inducing factor), and Endo G (endonuclease G), when released into the cytoplasm from mitochondria, induce both caspase-dependent and -independent mitochondrial death pathways in neurodegenerative diseases (Fig. 1) [12–19]. The release of cytochrome c is pivotal in the activation of caspases . During the progression of neurodegenerative diseases, once cytochrome c is released, it binds to Apaf-1 and dATP, which stimulates the activation of caspase-9, and then in turn cleaves the key effector caspase-3 and two other effectors, caspase-6 and -7 [12,14–18,21–24]. In addition, DNA-repairing enzyme poly(ADP-ribose)polymerase (PARP) is cleaved , and transcription factors such as NF-κB [5,25–27], TNF-α-induced activator protein-1 (AP-1) [28,29], and p53 [30,31] are activated. Nuclear condensation and DNA fragmentation are induced, as shown by terminal deoxynucleotidyl transferase-mediated DNA nick-end labeling (TUNEL)-positive cells, Hoechst 33342 stain, PI (propidium iodide), and 4′,6-diamino-2-phenylindole dihydrochloride hydrate (DAPI) staining, as well as DNA ladder. These events ultimately cause neuronal cell death . Other mitochondrial factors include mitochondrial permeability transition pores (mtPTP) and mitochondrial membrane potential (ΔΨm). mtPTP represents a multiprotein complex including inner and outer membrane components. The pores regulate transport of ions and peptides into and out of mitochondria. The activation of the permeability transition and in irreversible opening of mitochondria pores is a major step in the development of neurodegeneration [32–34]. ΔΨm reflects performance of the electron transport chain and can indicate a pathological disorder. The dissipation of ΔΨm and concomitant neuronal death have been reported in experimental models of neurodegeneration [14,18,34–36].
Caspase-1 activation is an early event in neurodegenerative diseases [24,37]. Caspase-1 activator receptor interacting protein-2 (Rip2) stimulates caspase-1 to activate IL-1β by truncating the proinflammatory cytokine. The release of mature IL-1β indicates caspase-1 activation . The inhibition of pro-IL-1β cleavage and mature IL-1β secretion are associated with inhibition of apoptosis in neurodegeneration [16,18]. Rip2 upregulation has already been reported in AD , HD , and stroke .
Bcl-2 family members include proapoptotic molecules (Bax, Bak, Bok, Bad, Bid, Bik, Blk, Hrk, BNIP3, and BimL) and antiapoptotic molecules (Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and A1). Bcl-2 family proteins participate in the modulation and execution of cell death  and can preserve or disrupt mitochondrial integrity by regulating the release of cytochrome c/Smac/AIF/endonuclease G [41,42]. Cytosolic Bax translocates to mitochondria on death stimulus [23,43], promoting cytochrome c release . Besides the involvement of the Fas/caspase-8/Bid cascade, Bid also mediates cytochrome c release while binding to both proapoptotic members (e.g., Bax) and antiapoptotic members Bcl-2 and Bcl-xL ; moreover, cleavage of Bid by caspase-8 and caspase-1 mediates the mitochondrial damage [45,46]. Bax mediates cell death relates with mitochondrial permeability transition . Bcl-2 and Bcl-xL bind to Apaf-1, inhibiting the association of caspase-9 with Apaf-1 .
Prostate apoptosis response-4 (Par-4) induces mitochondrial membrane permeability changes and promotes mitochondrial dysfunction . Par-4 increases the secretion of β-amyloid (Aβ) and neuronal degeneration . Par-4 levels are augmented in AD patients  and in models of stroke . RNAi knockdown of Par-4 inhibits neurosynaptic degeneration in ALS-linked mice . Par-4 interacts with Bcl-2, caspase-8, and PKCζ, thus inhibiting NF-κB-dependent survival signaling .
The MAPK family includes three members: extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (p38 MAPK), and c-Jun NH(2)-terminal kinase (JNK). Another kinase is MAP kinase kinase (MEK). JNK pathway has been observed in neurodegenerative diseases mostly by activating apoptosis [54,55] and partly by inhibiting cell death . DNA damage causes the JNK activation, which contributes to the mitochondrial transduction of Bax [57,58]. The absence of JNK causes a defect in the mitochondrial death signaling pathway, including the failure to release cytochrome c. Moreover, SP600125, a JNK inhibitor, enhances the activation of JNK pathway and attenuation of apoptosis through protection of mitochondrial dysfunction and reduction of caspase-9 activity in PC12 cells .
The Survival Signaling Pathways in Neurodegenerative Diseases
During the progression of neurodegenerative diseases, the survival signaling cascades are activated by neuroprotective agents  including the phosphoinositol-3 kinase (PI3K)/Akt pathway, the Bcl-2 pathway, the NF-κB pathway, as well as the MAPK pathway (Fig. 1). AKT (v-Akt murine thymoma viral oncogene)/PKB (protein kinase-B) has been identified as an important mediator of neuronal cell survival that helps counteract apoptotic stimuli. PI3K/Akt pathways play essential roles in neuronal cell survival. PI3K is activated and the membrane phospholipid phosphatidylinositol-3,4,5-trisphosphate is generated, which in turn recruits Akt to the membrane, where it becomes phosphorylated. Once Akt is activated, it phosphorylates survival-mediated targets including Bcl-2 family members, thereby promoting cell survival and inhibiting apoptosis . The antiapoptotic Bcl-2 family encodes Bcl-2, Bcl-xL, and BfI-1 (A1) . These antiapoptotic proteins repress mitochondrial death pathways through heterodimerization . Depletion of the endogenous neuroprotective Bcl-2 family signals directly contributes to neuronal loss in neurodegenerative diseases . NF-κB (nuclear factor kappa B) is an inducible transcription factor that exists in several dimeric forms, with the p50/p65 heterodimer predominant . The NF-κB pathway induces the expression of stress proteins, antioxidant enzymes, and calcium-regulating proteins. The activation of NF-κB not only induces apoptotic signaling [5,25,26,64] but also has been known to activate survival signals in neurodegeneration . Additionally, the phosphorylation of Raf-1, MEK1/2, and ERK1/2 has been reported in neurodegeneration . The JNK pathway is also involved in neurodegenerative diseases by inhibiting cell death .