Formyl-methionyl-leucyl-phenylalanine-induced dopaminergic neurotoxicity via microglial activation: a mediator between peripheral infection and neurodegeneration?

Parkinson disease (PD), a chronic neuro-degenerative disease, is characterized by progressive and selective degeneration of dopaminergic (DA) neurons in the substantia nigra (Leenders and Oertel 2001). Although the etiology of PD is still unknown, many environmental toxins and other environmental pathomechanisms (pharmacologic, infectious, and traumatic) have been implicated. Arai et al. (2006) suggested that infection and inflammatory reaction potentially contribute to the pathogenesis of PD. Inflammatory mediators and cellular processes once thought to be restricted to the peripheral immune system are now known to be vital in the progression of PD (Block et al. 2007; Hirsch et al. 1998; McGeer et al. 1988). Microglia, the resident immune cells and major phagocytes in the brain, have recently gained prominence as a key mediator in the inflammatory process of PD. Massive microglial activation in the substantia nigra and striatum has been observed in brains from patients with PD (McGeer et al. 1988). Activated microglia exert cytotoxic effects by releasing inflammatory mediators, such as reactive oxygen species (ROS) and proinflammatory factors (Block et al. 2007). Large amounts of these substances are toxic to adjacent neurons, and the continued production of these mediators by activated microglial cells can lead to chronic degeneration of DA neurons, as seen in progressive PD. Activation of microglia can be regulated by both endogenous immune modulators and exogenous substances (Block et al. 2007). Previously we identified several endogenous neuropeptides, such as substance P, dynorphin, and pituitary adenylate cyclase-activating polypeptide, that can modulate microglial activity and DA neuron survival in sub-picomolar to femtomolar concentrations (Block et al. 2006; Yang et al. 2006) in rodent midbrain neuron/glia (N/G) cultures. We performed pharmacophore perception to search for other naturally occuring peptides that are chemically related to these neuropeptides to determine if they exhibit similar neuro-protective or neurotoxic functions. We found that formylmethionyl-leucyl-phenylalanine (fMLP), a bacterial-derived chemoattractant, has high chemical similarity with substance P. Several chemical features shared between fMLP and substance P, as deduced by the Catalyst/HipHop program (Accelrys, Inc., San Diego, CA), include a hydrogen bond acceptor, a hydrogen bond donor, a positively ionizable region, and a hydrophobic group (Figure 1). Based on this result, fMLP might share similar binding properties to microglia as substance P and hence trigger similar biological pathways. This discovery prompted us to speculate that fMLP might be another peptide that can affect DA neuron toxicity in a microglia-dependent manner. Figure 1 Pharmacophore analysis between fMLP and substance P. Similar chemical features shared by these two peptides are illustrated by three-dimensional relationships with the highest fit values. fMLP resembles the amino acid sequence (Phe-Phe-Gly-Leu-Met) of ... fMLP, an N-formylated peptide, is a potent chemoattractant molecule released from both bacteria and damaged mitochondria, and it plays an important role in host defense against microbial infection or tissue injury by recruiting phagocytes to the site of inflammation. fMLP may exist in the brain in some infectious diseases of the central nervous system (CNS), such as meningitis. Circulating fMLP released from bacteria may also enter the brain through areas with a less tight blood–brain barrier, such as the circumventricular organs (Duvernoy and Risold 2007). Two functional formyl peptide receptors (FPRs), designated FPR and FPRL1R, bind fMLP with high (FPR, in the low nanomolar or picomolar range) and low (FPRL1R, in the micromolar range) affinity, respectively (Hartt et al. 1999). Expression of FPR has been observed in rodent microglia (Lorton et al. 2000). However, microglial cells lack the capacity to migrate in response to fMLP (Yao et al. 1990), suggesting that the expression of FPRs on microglia is either absent or too low to function. Therefore, in spite of all the studies on fMLP in phagocytes during the peripheral inflammatory response, little is known about the function of fMLP in the CNS inflammatory process. In the present study, we investigated the function of fMLP in neurodegeneration, using the well-established primary rat and mouse midbrain culture model of PD. Our results showed that a subpicomolar concentration of fMLP (10−13 M) induces oxidative stress through activation of NADPH oxidase (also called phagocyte oxidase; PHOX) in microglia, resulting in selective DA neurotoxicity. Here, for the first time, we provide evidence for an effect of fMLP in the CNS inflammatory process and propose a new potential mediator between infectious and neurodegenerative diseases in the brain.