**3.1 Distribution of PON2 in the nervous system**

PON2 is the only PON to be expressed in the brain. Distribution in the spinal cord has not been described adequately. Most information of the probable role of PON2 in brain is extrapolated from studies on mice [69].

PON2 is an intracellular protein [70]. It is postulated to be involved in neuroprotection by the virtue of its anti-inflammatory and antioxidant properties, and exclusive intracellular location. Measurement of its lactonase activity has been used to study the regional distribution and sex differences in mice and the findings have been extrapolated to humans. Higher levels are seen in [73].


These differences in quantification correspond to the comparative levels of neuroprotection seen in the said groups [69]. The major mechanism by which PON2 is said to reduce oxidative neuroinflammation is by regulating mitochondrial CoQ. The mechanism is applicable in all cells PON2 is present. During transport of electrons in the electron transport chain of the inner mitochondrial membrane, CoQ accepts electrons and becomes unstable. It regains its stable form by transfer of electrons.

*Paraoxonase in Nervous System DOI: http://dx.doi.org/10.5772/intechopen.110843*

Most electrons are transferred to Complex III and energy is generated. However, some electrons may be transferred by CoQ to oxygen molecules when reduced CoQ comes in contact with them, given rise to reactive oxygen species. PON2 has been postulated to reduce this transfer of electrons to oxygen, thereby reduced oxidative stress and consequent neuroinflammation [70].

The role of PON2 in the cytoplasm is postulated to be different than that in the mitochondria in the nervous system; however, research on this topic is next to none [73]. Further, the role of PON2 in the endoplasmic reticula of the nerves has not been studied [69]. In the brain, the loss of PON2 in both neurons and astrocytes impairs their ability to recover from toxic levels of oxidative stress generated by oxidants hydrogen peroxide (H2O2) or 2,3-dimethoxy-1,4-naphthoquinone (DMNQ ) [73].

## **3.2 Modulation of PON2**

Quercetin has been found to increase the levels of astrocyte PON2 in mice approximately two times. It is postulated that since quercetin is a phytoestrogen, it may activate estrogen-α receptors like oestradiol, leading to increased PON2 synthesis. Alternatively, the JNK/AP-1 pathway has also been suggested to be the central underlying mechanism—it was shown that inhibition of this pathway lead to antagonization of the effect of quercetin on PON2 levels [71].

### **3.3 Impact on motor behavior**

Mice deficient in PON2 showed slightly more activity during the dark cycle than mice having PON2. There were no differences in food consumption. During the rotarod experiment, PON2 deficient mice had significant shorter latency to fall, suggesting impaired motor co-ordination [73].

#### **3.4 Probable role in dopaminergic system**

Oxidation of dopamine during its metabolism generates free radicals and reactive quinones in the dopaminergic neurons. Due to this additional oxidative burden, the dopaminergic neurons are more susceptible to additional oxidative stress, neuroinflammation and cellular death. PON2 reduces the oxidative stress here by inhibition of superoxide formation by mitochondrial CoQ [74]. PON2 was also found to be modulated by dopamine receptor activity in a receptor-specific manner, with protein and transcript upregulated in neurons upon exposure to a dopamine receptor 2 agonist, but not an agonist for dopamine receptor 1 [73].

PON2 has been shown to interact with PARK7 in vivo. The protective antioxidant effects of PARK7 are partly mediated by expression of PON2 [74]. *In vitro*, PON2 has been shown to reduce the increased susceptibility of striatal neurons to oxidative stress in cases of PARK7 deficiency [73].

In mice, in PON2 deficient striatum, the levels of tyrosine hydroxylase protein were found to be lower and that of tyrosine hydroxylase transcript were found to be higher than in the wild type (with PON2 present). The levels of Vesicular Amine Transporter 2 (VMAT2) transcript were found to be increased but there was no change in protein levels. The levels of Dopamine Transporter, which is involved in the reuptake of dopamine form the synaptic cleft, were found to be unchanged at both the transcript and protein level. Dopamine receptors DRD-1, DRD-2, and DRD-5 were found to be upregulated at the transcript level but not

at the protein level in PON2 deficiency. Thus, PON2 deficiency significantly upregulates the transcript of multiple dopaminergic related genes in the striatum of mice.

Transcript levels of antioxidant enzymes, heme-oxygeneases-1 and -2 and MADPH-oxidase 2, were increased in PON2 deficient mice, although the protein levels were not altered. This implied that in absence of PON2, the oxidative stress in these neurons increased, due to which levels of antioxidant enzymes were increased [74].

#### **3.5 PON2 and Parkinson disease**

As said above, PON2 interacts with the DJ-1 (PARK7) gene. Of all cases of familial Parkinson's disease (PD), loss-of-function mutations in DJ-1 (PARK7) gene account for about 1%. The actions of this gene are said to reduce the oxidative stress-mediated damage; however, the mechanisms for the same are unknown. It was shown that *in vivo*, PON2 associates with DJ-1.

MPTP is one of the causative agents of Parkinson Disease, and it, along with its metabolite MPP is used for research regarding Parkinson disease. MPP is a complex I inhibitor which leads to oxidative stress and death of a number of different neurons. Exposure to MPP lead to increase in the PON2 lactonase activity in mice. It was found that in deficiency of DJ-1 both the basal and MPP-induced lactonase activity of PON2 was blocked. Loss of DJ-1 thus impairs PON2 activity. However, it was also noted that DJ-1 does not alter PON2 levels in neurons, implying that it increases PON2 activity by increasing the rate of enzyme action, and not the amount of enzyme synthesized. The exact mechanism is unclear.

Absence of either of PON2 or DJ-1 leads to increased sensitivity of the neuron towards oxidative stress by MPP. Interestingly, PON2 expression effectively rescues DJ-1 deficiency-mediated hypersensitivity to oxidative stress, although DJ-1 expression cannot do so for PON2. This suggests PON2 to be a downstream target of DJ-1. Thus, PON2 expression protects neurons against MPP and can also reverse the hypersensitivity observed with DJ-1 loss [75].

#### **3.6 PON2 and amyotrophic lateral sclerosis**

ALS is a neurodegenerative disorder of spinal tract. It is a multifactorial disease characterized by cerebral cell dysfunction and mitochondrial alteration. It is associated with the progressive increase in neuro-inflammation, generalized oxidative stress and metabolic alterations. The C allele of the C311S PON2 has been associated with sporadic ALS. In addition, the expression of messenger RNA of the PON2 gene was decreased in spinal cord and trunk tissue of patients with ALS [76]. ALSassociated variant in PON2 is present as a homozygous defect.

However, since there are multiple substrates for PONs, it is difficult to ascertain the exact role of PONs in the pathogenesis of AML. ALS-related mutations have been found in all the three forms of PON, implying a higher probability that the property of the PONs which plays a role in pathogenesis is likely to be a common property/ feature. One of the theories says that loss of antioxidant functions of the PONs leads to the inability of nervous tissue to detoxify the abnormal oxidative stresses the motor neurons and spinal cord are exposed to, leading to neurotoxicity. Another theory says that mutations in PONs may lead to failure to inhibit some unknown exotoxin, which leads to ALS [72].
