**3. TLR and brain disorders**

Toll-like receptor (TLR) is a family composed of multiple pattern recognition members, and these receptors play a crucial role in mediating and modulating innate immunity [51]. This family has an essential role in modulating and maintaining the microglia and microglia translocation protein activity. Histological studies indicated that multiple members of this family are expressed in the brain [52, 53], gut and blood mononuclear cells [54]. Additionally, these receptors are functionally involved in modulating excitatory [55], and inhibitory neuronal populations [56–58]. These modulations include orchestrating different signaling pathways [20]. Also, a couple of TLRs (TLR2 and TLR9) regulate the enteric nervous system. A previous report has shown that both receptors were detected using histological studies in multiple markers of the enteric nervous system. Upon activation of innate immunity by administration LPS, both members were upregulated in the enteric nervous system. Indicating selective disease activation mechanism [55]. Correspondingly, LPS activation of TLR4 leads to stimulation of cytokines-related pathological mechanisms such as dysregulation in oligodendrocytes maintenance, microglial toxicity, and alter myelination [59, 60].

Previous reports linked Alzheimer's disease and polymorphisms in both TLR4 and CD14 genetic codes [61, 62]. Multiple forms of aggregated α-synuclein, a pathological feature of neurodegenerative diseases, can trigger and activate different TLRs. This indicates that TLRs contribute to the pathology of psychiatric and neurodegenerative diseases. Behaviorally they are implicated in regulating impulsivity [63]. A previous study linked TLR4 and the Gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the brain, and the GABAergic inhibitory neurons release it. It was reported that the alpha-2 GABAergic receptor activation of the TLR4 is essential in mediating impulsivity. The co-immunoprecipitation of the alpha-2 GABAergic and TLR4 in the ventral tegmental area leads to Cyclic adenosine monophosphate (cAMP) activation. The cAMP translocation activates the cAMP-response element-binding protein (CREB), subsequently stimulating the tyrosine hydroxylase and the corticotropin-releasing factor. Interestingly, the stereotaxic infusion of alpha-2 GABAergic and TLR4 siRNA in herpes simplex virus vector in the ventral tegmental area prevented alcohol and nicotine seeking. Indicating that TLR4 is involved mechanistically in regulating drug abuse mechanisms [64]. GABAergic synapses are modulated by TLR4 signaling. Stimulation

**143**

*Pharmacological Modulation of Toll-Like Receptors in Brain Disorders*

of TLR4 by the administration of Lipopolysaccharide (LPS) alters both pre and postsynaptic function of the GABAergic system. The study indicated that both the synthesis and the reuptake of GABA are altered. Electrophysiological recordings have shown that Lipopolysaccharide's administration reduces the miniature inhibitory postsynaptic currents in acute slices, and this inhibition is mediated through the microglia [56]. Another study linked the GABAergic system to TLR in their report pharmacological activation of the GABAB receptor (baclofen) reduced TLR3- and TLR4 mediated inflammation in primary glial cell lines. Similar findings were observed in the expression of TLR3 in blood mononuclear cells isolated from multiple sclerosis patients [65]. Indicating the existence of complex interaction

Besides, activation of TLR4 could interfere with addiction and drug abuse through another mechanism. In another report, it was indicated that pharmacological application of opiate antagonists (naloxone and naltrexone) prevented the TLR4 signaling achieved in LPS treated rodents. Both naloxone and naltrexone have been

On the other hand, studies have linked TLR signaling and neurodevelopmental disorders such as Autism spectrum disorders [67, 68]. An impairment identifies these disorders in sociability, communication, and characteristics of repetitive behaviors [69]. Accumulated evidence has linked Autism to neuroinflammation. The peripheral level of different TLRs, including TLR2–5 and TLR9, was elevated significantly in autistic patients in clinical settings [70]. In a previous report, flowcytometric analysis of TLR4/TLR5 and neuregulin 1 - ErbB in the monocytes of schizophrenic and healthy subjects revealed that both TLR4 and TLR5 were elevated

where the level of ErbB is reduced significantly in drug-naïve schizophrenic

patients compared to healthy controls [67]. Neuregulin 1 – ErbB signaling is crucial in modulating brain development [71]. For example, it is involved in axonal growth [72] and maintenance [73], the expression of acetylcholine receptors [74], electrophysiological firing [75], and synaptic wiring [76]. Cytokine-related mechanisms are unified features of schizophrenia and an emerging hypothesis for the pathology

The link between TLRs and depression has been identified in both preclinical [78]

The pharmacological targeting of TLR has emerged as an appealing strategy for many reasons. First, they are an essential part of the innate immune system responsible for the initiation of the immune response [88]. Also, studies indicated that TLRs modulate the homeostasis [89], neuronal morphogenesis [90, 91], and neurogenesis [87]. Additionally, it was reported that TLRs are implicated in the pathology

clinical [79], and postmortem studies [80]. It was further reported that both protein, and mRNA level of TLR2–4, TLR6 and TLR10 was significantly reduced in the prefrontal brain region of depressed suicide subjects compared to the controls [52]. Adult neurogenesis is a physiological process essential for cognitive capacity, learning and memory, synaptic plasticity, modulating mood, and other processes [81, 82]. Dysregulation in adult neurogenesis is linked to schizophrenia [83], Alzheimer's [84], Parkinson's [85], and autism [86]. In TLR2-mutant mice, adult hippocampal neurogenesis was altered. Proliferative cells that are BrdU/doublecortin positive cells were significantly reduced in TLR2-mutant mice. *In Vitro* studies showed that activation of TLR2 enhances the differentiation of neural stem/pro-

*DOI: http://dx.doi.org/10.5772/intechopen.97869*

between microglia, TLR4, and GABAergic system.

shown to non stereoselectively inhibit TLR4 [66].

of schizophrenia [77].

genitor cells [87].

**4. Pharmacological modulation of TLRs**

### *Pharmacological Modulation of Toll-Like Receptors in Brain Disorders DOI: http://dx.doi.org/10.5772/intechopen.97869*

*Therapy Approaches in Neurological Disorders*

amyotrophic lateral sclerosis (sALS) [50].

microglial toxicity, and alter myelination [59, 60].

**3. TLR and brain disorders**

healthy participants exposed to endotoxin systemically was examined. The results suggested that the endotoxin-exposed group exhibited a reduction in cognitive function and reduced capability in processing emotional information compared to the placebo group [43]. Suggesting that short-term exposure to systemic endotoxin has a profound impact on higher cognitive tasks. Disrupted sociability [44], and impaired cognitive capacity are hallmarks of psychiatric disorders [45], mainly schizophrenia, and autism [46, 47]. In another report, a battery of socio-behavioral factors was examined and reported to be functionally linked to the systemic administration of LPS. Indicating a mechanistic link between LPS-inflammation and major depressive disorder [48]. In line with this, the administration of a citalopram, a selective serotonin reuptake inhibitor antidepressant agent, leads to a reduction in fatigue and multiple inflammatory cytokines associated with endotoxins activation [49]. In another clinical setting, the level of circulating endotoxins correlates with the severity of neurodegenerative disorders, including Alzheimer's, sporadic

Toll-like receptor (TLR) is a family composed of multiple pattern recognition members, and these receptors play a crucial role in mediating and modulating innate immunity [51]. This family has an essential role in modulating and maintaining the microglia and microglia translocation protein activity. Histological studies indicated that multiple members of this family are expressed in the brain [52, 53], gut and blood mononuclear cells [54]. Additionally, these receptors are functionally involved in modulating excitatory [55], and inhibitory neuronal populations [56–58]. These modulations include orchestrating different signaling pathways [20]. Also, a couple of TLRs (TLR2 and TLR9) regulate the enteric nervous system. A previous report has shown that both receptors were detected using histological studies in multiple markers of the enteric nervous system. Upon activation of innate immunity by administration LPS, both members were upregulated in the enteric nervous system. Indicating selective disease activation mechanism [55]. Correspondingly, LPS activation of TLR4 leads to stimulation of cytokines-related pathological mechanisms such as dysregulation in oligodendrocytes maintenance,

Previous reports linked Alzheimer's disease and polymorphisms in both TLR4 and CD14 genetic codes [61, 62]. Multiple forms of aggregated α-synuclein, a pathological feature of neurodegenerative diseases, can trigger and activate different TLRs. This indicates that TLRs contribute to the pathology of psychiatric and neurodegenerative diseases. Behaviorally they are implicated in regulating impulsivity [63]. A previous study linked TLR4 and the Gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the brain, and the GABAergic inhibitory neurons release it. It was reported that the alpha-2 GABAergic receptor activation of the TLR4 is essential in mediating impulsivity. The co-immunoprecipitation of the alpha-2 GABAergic and TLR4 in the ventral tegmental area leads to Cyclic adenosine monophosphate (cAMP) activation. The cAMP translocation activates the cAMP-response element-binding protein (CREB), subsequently stimulating the tyrosine hydroxylase and the corticotropin-releasing factor. Interestingly, the stereotaxic infusion of alpha-2 GABAergic and TLR4 siRNA in herpes simplex virus vector in the ventral tegmental area prevented alcohol and nicotine seeking. Indicating that TLR4 is involved mechanistically in regulating drug abuse mechanisms [64]. GABAergic synapses are modulated by TLR4 signaling. Stimulation

**142**

of TLR4 by the administration of Lipopolysaccharide (LPS) alters both pre and postsynaptic function of the GABAergic system. The study indicated that both the synthesis and the reuptake of GABA are altered. Electrophysiological recordings have shown that Lipopolysaccharide's administration reduces the miniature inhibitory postsynaptic currents in acute slices, and this inhibition is mediated through the microglia [56]. Another study linked the GABAergic system to TLR in their report pharmacological activation of the GABAB receptor (baclofen) reduced TLR3- and TLR4 mediated inflammation in primary glial cell lines. Similar findings were observed in the expression of TLR3 in blood mononuclear cells isolated from multiple sclerosis patients [65]. Indicating the existence of complex interaction between microglia, TLR4, and GABAergic system.

Besides, activation of TLR4 could interfere with addiction and drug abuse through another mechanism. In another report, it was indicated that pharmacological application of opiate antagonists (naloxone and naltrexone) prevented the TLR4 signaling achieved in LPS treated rodents. Both naloxone and naltrexone have been shown to non stereoselectively inhibit TLR4 [66].

On the other hand, studies have linked TLR signaling and neurodevelopmental disorders such as Autism spectrum disorders [67, 68]. An impairment identifies these disorders in sociability, communication, and characteristics of repetitive behaviors [69]. Accumulated evidence has linked Autism to neuroinflammation. The peripheral level of different TLRs, including TLR2–5 and TLR9, was elevated significantly in autistic patients in clinical settings [70]. In a previous report, flowcytometric analysis of TLR4/TLR5 and neuregulin 1 - ErbB in the monocytes of schizophrenic and healthy subjects revealed that both TLR4 and TLR5 were elevated where the level of ErbB is reduced significantly in drug-naïve schizophrenic patients compared to healthy controls [67]. Neuregulin 1 – ErbB signaling is crucial in modulating brain development [71]. For example, it is involved in axonal growth [72] and maintenance [73], the expression of acetylcholine receptors [74], electrophysiological firing [75], and synaptic wiring [76]. Cytokine-related mechanisms are unified features of schizophrenia and an emerging hypothesis for the pathology of schizophrenia [77].

The link between TLRs and depression has been identified in both preclinical [78] clinical [79], and postmortem studies [80]. It was further reported that both protein, and mRNA level of TLR2–4, TLR6 and TLR10 was significantly reduced in the prefrontal brain region of depressed suicide subjects compared to the controls [52].

Adult neurogenesis is a physiological process essential for cognitive capacity, learning and memory, synaptic plasticity, modulating mood, and other processes [81, 82]. Dysregulation in adult neurogenesis is linked to schizophrenia [83], Alzheimer's [84], Parkinson's [85], and autism [86]. In TLR2-mutant mice, adult hippocampal neurogenesis was altered. Proliferative cells that are BrdU/doublecortin positive cells were significantly reduced in TLR2-mutant mice. *In Vitro* studies showed that activation of TLR2 enhances the differentiation of neural stem/progenitor cells [87].

## **4. Pharmacological modulation of TLRs**

The pharmacological targeting of TLR has emerged as an appealing strategy for many reasons. First, they are an essential part of the innate immune system responsible for the initiation of the immune response [88]. Also, studies indicated that TLRs modulate the homeostasis [89], neuronal morphogenesis [90, 91], and neurogenesis [87]. Additionally, it was reported that TLRs are implicated in the pathology of multiple brain disorders such as depression [92], Alzheimer [93], Parkinson [94], and ischemia [95]. Molecularly, it is involved in activating one of the key neuronal signaling pathways [96].

Electrophysiological studies have shown that the administration of immunostimulant results in activation of TLR3 alters the expression of AMPAR, decrease the spontaneous firing, and reduce both the frequency and amplitude of mEPSCs [97]. In line with this, the administration of LPS affect the hippocampal neuronal mEPSC both the frequency and amplitude in hippocampal neurons via modulation of TLR4 [98]. Tlr7 knockout mice showed altered hippocampal LTP, an activitydependent neurophysiological feature, suggesting defects in memory-related functions [99]. Also, Tlr4 mutant mice exhibited an impairment of long-term depression (LTD) in the nucleus accumbens, another activity-dependent neurophysiological feature, suggesting potential alterations in the reward circuitry [100].

Behaviorally, preclinical studies have shown that TLRs' pharmacological modulation is linked to significant phenotypic features of neurological and psychiatric disorders [90]. In a maternal immune activation (MIA) animal model, a valid model for neurodevelopmental psychiatric disorders such as autism and schizophrenia [101], also linked to increased risks for neurodegenerative disorders [102], it was found that the offspring exhibited schizophrenic-like behaviors via modulation of TLR [103], Clinical and preclinical studies have shown that altered TLR pathway is associated with schizophrenic and autism-related behaviors [90, 101, 103–105].

Mice lacking the TLR3 gene exhibited impairment in amygdala-related behaviors and elevated anxiety while performing cued fear-conditioning and elevated plus maze tests [106]. Anatomically, the amygdala is encompassed by a group of subnuclei, more than ten regions [107]. At circuitry level, this brain region receives input from sensory cortical and thalamic areas, which is responsible for the conditioned (CS) and unconditioned stimulus, prefrontal cortex, and hippocampus that mediate the extinction of fear responses and bed nucleus of the stria terminalis (BNST) that coordinate the stress-related responses. Its output is projected to the brainstem, hypothalamic, and cortical areas responsible for emotional responses [108, 109]. The TLR4 mutant mice exhibited altered higher cognitive tasks such as memory retention, acquisition, and contextual fear-learning [110]. The long-term intraventricular infusion with a TLR9 ligand resulted in memory dysfunction and increased risk of neurodegenerative disorders [111].

Prion diseases are a group of progressive neurodegenerative disorders [112], previously it was reported that TLR9 could be involved in the pathology of the progression of prion diseases. A preclinical study has shown that the administration of a TLR 9 ligand, cytosine phosphate guanosine (CpG-ODN) oligodeoxynucleotides, in mice resulted in a significant increase in the survival rate. Suggesting that the activation of TLRs in neurodegenerative diseases could be attributed to neuroprotective mechanisms that involve eliminating of neurotoxic misfolded proteins, which may prove to be a possible therapeutic strategy to the prion diseases [113]. This immunostimulant has been employed and examined in infectious, allergies ad cancer-related studies [114].

Similarly, genetic therapy targeting TLR2 reduces the accumulation of Amyloid β1–42 in the hippocampus of an animal model of Alzheimer's disease and alters the progression of memory loss [115]. Misfolded α-synuclein is a characteristic feature and a leading cause of neurodegenerative diseases. Employment of immunization has gained a lot of attention as an attractive therapeutic option for neurodegenerative disorders. In a transgenic mice model of Parkinson's, it was found that the immunization with human α-synuclein associated with a marked reduction in the accumulated α-Synuclein and overall reduced neurodegeneration. Indicating

**145**

downregulated TLR4 [131].

*Pharmacological Modulation of Toll-Like Receptors in Brain Disorders*

that α-Synuclein vaccination could be efficient in reducing neurodegeneration

A recent study has reported that treating Parkinson's mice model with a natural compound, Juglanin, lead to enhanced memory function, reduced amyloid-beta accumulation, reversed α-synuclein accumulation and overall anti-inflammatory, and antioxidant effects through the modulation of TLR4/nuclear factor (NF)-κB pathway in the hippocampus [117]. In a clinical setting, treatment with vinpocetine, an alkaloid derivative and a phosphodiesterase type 1 inhibitor, compared to traditional treatment with levodopa, resulted in a significant reduction of TLR 2,4 mRNA level along with reduced the level of serum inflammatory mediators. Interestingly these alterations were associated with a marked elevation while performing the Mini-Mental State Examination score [118]. Although this study did not elucidate the link between TLR2,4 and the enhanced cognitive capacity, it was reported previously that in a dementia model, vinpocetine modulates long-term potentiation [119], Additionally, vinpocetine was found learning and memory while performing Morris maze tasks in fetal alcohol spectrum disorders mice model [120]. Although this study did not elucidate the link between TLR2,4 and the enhanced cognitive capacity, it was reported previously that in a dementia model, vinpocetine modulates long-term potentiation [119]. Additionally, vinpocetine was found learning and memory while performing Morris maze tasks in fetal alcohol spectrum disorders mice model [120]. Interestingly, previously it was found that the inhibition of Cyclic Nucleotide Phosphodiesterase is associated with an alteration of TLRs signaling, apoptotic pathway, and in chronic lymphocytic leukemia cells [121].

Transgenic animal studies have demonstrated that genetic manipulation of TLRs is associated with increased aggravated of Aβ [122]. Treatment with an anti-TLR2 antibody has found to be an effective strategy in providing significant protection preclinically against sepsis-associated death [123], stroke [124], Alzheimer's [123], and its safety, tolerability, along with pharmacokinetic profiling have been conducted clinically in healthy subjects [125]. A 7-month administration of anti-TLR2 antibody to an Alzheimer mice model, APP/PS1 Mice, resulted in an overall reduction in the activation of both microglial and astroglia. This reduction was detected by quantifying immunoreactive MHCII, CD68 (microglial markers), and GFAP (astroglia marker) positive cells. Along with a marked reduction in Ab plaque burden in the hippocampal brain region. Behaviorally, the chronic treatment with TLR2 antibody has improved their performance in water maze test, and the latency

was reduced significantly, and the time spent in the platform zone [126].

Studies have linked vitamin D deficiency and increased risk of neurodegenerative diseases [127, 128]. In MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)- Parkinson's induced mouse model treatment with vitamin D has shown notable attenuated nigrostriatal neurodegeneration. Additionally, it increased the tyrosine hydrolase neuronal cells, altered the expression of Iba1 positive cells (microglial activation marker), and TLR-4 [129]. In another study, the same model has employed and treated with Rosmarinus acid, a phenolic compound with antioxidant, anti-apoptotic, and anti-inflammatory effects [130]. In a dose-dependent manner, Rosmarinus acid treatment led to a significant improvement of motor dysfunction, elevated the number of tyrosine hydroxylase-positive cells, and

In a rat model of subarachnoid hemorrhage, pharmacological application of a natural flavonoid (Fisetin) minimizes the brain edema, improved modulate neurological scores, and modulate apoptosis, mainly through the regulation of TLR 4/ NF-κB signaling [132]. Taken together, the TLR pathway is an attractive candidate

for the development of future neurodegenerative therapies.

*DOI: http://dx.doi.org/10.5772/intechopen.97869*

associated with accumulated α-Synuclein [116].

### *Pharmacological Modulation of Toll-Like Receptors in Brain Disorders DOI: http://dx.doi.org/10.5772/intechopen.97869*

*Therapy Approaches in Neurological Disorders*

signaling pathways [96].

of multiple brain disorders such as depression [92], Alzheimer [93], Parkinson [94], and ischemia [95]. Molecularly, it is involved in activating one of the key neuronal

Electrophysiological studies have shown that the administration of immunostimulant results in activation of TLR3 alters the expression of AMPAR, decrease the spontaneous firing, and reduce both the frequency and amplitude of mEPSCs [97]. In line with this, the administration of LPS affect the hippocampal neuronal mEPSC both the frequency and amplitude in hippocampal neurons via modulation of TLR4 [98]. Tlr7 knockout mice showed altered hippocampal LTP, an activitydependent neurophysiological feature, suggesting defects in memory-related functions [99]. Also, Tlr4 mutant mice exhibited an impairment of long-term depression (LTD) in the nucleus accumbens, another activity-dependent neurophysiological

Behaviorally, preclinical studies have shown that TLRs' pharmacological modulation is linked to significant phenotypic features of neurological and psychiatric disorders [90]. In a maternal immune activation (MIA) animal model, a valid model for neurodevelopmental psychiatric disorders such as autism and schizophrenia [101], also linked to increased risks for neurodegenerative disorders [102], it was found that the offspring exhibited schizophrenic-like behaviors via modulation of TLR [103], Clinical and preclinical studies have shown that altered TLR pathway is associated with schizophrenic and autism-related behaviors [90, 101, 103–105]. Mice lacking the TLR3 gene exhibited impairment in amygdala-related behaviors and elevated anxiety while performing cued fear-conditioning and elevated plus maze tests [106]. Anatomically, the amygdala is encompassed by a group of subnuclei, more than ten regions [107]. At circuitry level, this brain region receives input from sensory cortical and thalamic areas, which is responsible for the conditioned (CS) and unconditioned stimulus, prefrontal cortex, and hippocampus that mediate the extinction of fear responses and bed nucleus of the stria terminalis (BNST) that coordinate the stress-related responses. Its output is projected to the brainstem, hypothalamic, and cortical areas responsible for emotional responses [108, 109]. The TLR4 mutant mice exhibited altered higher cognitive tasks such as memory retention, acquisition, and contextual fear-learning [110]. The long-term intraventricular infusion with a TLR9 ligand resulted in memory dysfunction and

Prion diseases are a group of progressive neurodegenerative disorders [112], previously it was reported that TLR9 could be involved in the pathology of the progression of prion diseases. A preclinical study has shown that the administration of a TLR 9 ligand, cytosine phosphate guanosine (CpG-ODN) oligodeoxynucleotides, in mice resulted in a significant increase in the survival rate. Suggesting that the activation of TLRs in neurodegenerative diseases could be attributed to neuroprotective mechanisms that involve eliminating of neurotoxic misfolded proteins, which may prove to be a possible therapeutic strategy to the prion diseases [113]. This immunostimulant has been employed and examined in infectious, allergies ad

Similarly, genetic therapy targeting TLR2 reduces the accumulation of Amyloid β1–42 in the hippocampus of an animal model of Alzheimer's disease and alters the progression of memory loss [115]. Misfolded α-synuclein is a characteristic feature and a leading cause of neurodegenerative diseases. Employment of immunization has gained a lot of attention as an attractive therapeutic option for neurodegenerative disorders. In a transgenic mice model of Parkinson's, it was found that the immunization with human α-synuclein associated with a marked reduction in the accumulated α-Synuclein and overall reduced neurodegeneration. Indicating

feature, suggesting potential alterations in the reward circuitry [100].

increased risk of neurodegenerative disorders [111].

cancer-related studies [114].

**144**

that α-Synuclein vaccination could be efficient in reducing neurodegeneration associated with accumulated α-Synuclein [116].

A recent study has reported that treating Parkinson's mice model with a natural compound, Juglanin, lead to enhanced memory function, reduced amyloid-beta accumulation, reversed α-synuclein accumulation and overall anti-inflammatory, and antioxidant effects through the modulation of TLR4/nuclear factor (NF)-κB pathway in the hippocampus [117]. In a clinical setting, treatment with vinpocetine, an alkaloid derivative and a phosphodiesterase type 1 inhibitor, compared to traditional treatment with levodopa, resulted in a significant reduction of TLR 2,4 mRNA level along with reduced the level of serum inflammatory mediators. Interestingly these alterations were associated with a marked elevation while performing the Mini-Mental State Examination score [118]. Although this study did not elucidate the link between TLR2,4 and the enhanced cognitive capacity, it was reported previously that in a dementia model, vinpocetine modulates long-term potentiation [119], Additionally, vinpocetine was found learning and memory while performing Morris maze tasks in fetal alcohol spectrum disorders mice model [120]. Although this study did not elucidate the link between TLR2,4 and the enhanced cognitive capacity, it was reported previously that in a dementia model, vinpocetine modulates long-term potentiation [119]. Additionally, vinpocetine was found learning and memory while performing Morris maze tasks in fetal alcohol spectrum disorders mice model [120]. Interestingly, previously it was found that the inhibition of Cyclic Nucleotide Phosphodiesterase is associated with an alteration of TLRs signaling, apoptotic pathway, and in chronic lymphocytic leukemia cells [121].

Transgenic animal studies have demonstrated that genetic manipulation of TLRs is associated with increased aggravated of Aβ [122]. Treatment with an anti-TLR2 antibody has found to be an effective strategy in providing significant protection preclinically against sepsis-associated death [123], stroke [124], Alzheimer's [123], and its safety, tolerability, along with pharmacokinetic profiling have been conducted clinically in healthy subjects [125]. A 7-month administration of anti-TLR2 antibody to an Alzheimer mice model, APP/PS1 Mice, resulted in an overall reduction in the activation of both microglial and astroglia. This reduction was detected by quantifying immunoreactive MHCII, CD68 (microglial markers), and GFAP (astroglia marker) positive cells. Along with a marked reduction in Ab plaque burden in the hippocampal brain region. Behaviorally, the chronic treatment with TLR2 antibody has improved their performance in water maze test, and the latency was reduced significantly, and the time spent in the platform zone [126].

Studies have linked vitamin D deficiency and increased risk of neurodegenerative diseases [127, 128]. In MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)- Parkinson's induced mouse model treatment with vitamin D has shown notable attenuated nigrostriatal neurodegeneration. Additionally, it increased the tyrosine hydrolase neuronal cells, altered the expression of Iba1 positive cells (microglial activation marker), and TLR-4 [129]. In another study, the same model has employed and treated with Rosmarinus acid, a phenolic compound with antioxidant, anti-apoptotic, and anti-inflammatory effects [130]. In a dose-dependent manner, Rosmarinus acid treatment led to a significant improvement of motor dysfunction, elevated the number of tyrosine hydroxylase-positive cells, and downregulated TLR4 [131].

In a rat model of subarachnoid hemorrhage, pharmacological application of a natural flavonoid (Fisetin) minimizes the brain edema, improved modulate neurological scores, and modulate apoptosis, mainly through the regulation of TLR 4/ NF-κB signaling [132]. Taken together, the TLR pathway is an attractive candidate for the development of future neurodegenerative therapies.
