Voltage-Gated Proton Channel Hv1 Regulates Neuroinflammation and Dopaminergic Neurodegeneration in Parkinson's Disease Models.

Although the precise mechanisms for neurodegeneration in Parkinson's disease (PD) are unknown, evidence suggests that neuroinflammation is a critical factor in the pathogenic process. Here, we sought to determine whether the voltage-gated proton channel, Hv1 (HVCN1), which is expressed in microglia and regulates NADPH oxidase, is associated with dopaminergic neurodegeneration. We utilized data mining to evaluate the mRNA expression of HVCN1 in the brains of PD patients and controls and uncovered increased expression of the gene encoding Hv1, HVCN1, in the brains of PD patients compared to controls, specifically in male PD patients. In an acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 4 × 16 mg/kg) mouse model of PD, Hvcn1 gene expression was increased 2-fold in the striatum. MPTP administration to wild-type (WT) mice resulted in a ~65% loss of tyrosine hydroxylase positive neurons (TH+) in the substantia nigra (SN), while a ~39% loss was observed in Hv1 knockout (KO) mice. Comparable neuroprotective effects of Hv1 deficiency were found in a repeated-dose LPS model. Neuroprotection was associated with decreased pro-inflammatory cytokine levels and pro-oxidant factors in both neurotoxicant animal models. These in vivo results were confirmed in primary microglial cultures, with LPS treatment increasing Hvcn1 mRNA levels and Hv1 KO microglia failing to exhibit the LPS-mediated inflammatory response. Conditioned media from Hv1 KO microglia treated with LPS resulted in an attenuated loss of cultured dopamine neuron cell viability compared to WT microglia. Taken together, these data suggest that Hv1 is upregulated and mediates microglial pro-inflammatory cytokine production in parkinsonian models and therefore represents a novel target for neuroprotection.

Neonatal hyperoxia impairs adipogenesis of bone marrow-derived mesenchymal stem cells and fat accumulation in adult mice.

Preterm birth is a risk factor for growth failure and development of respiratory disease in children and young adults. Their early exposure to oxygen may contribute to lung disease because adult mice exposed to hyperoxia as neonates display reduced lung function, changes in the host response to respiratory viral infections, and develop pulmonary hypertension and heart failure that shortens their lifespan. Here, we provide new evidence that neonatal hyperoxia also impairs growth by inhibiting fat accumulation. Failure to accumulate fat may reflect a systemic defect in adipogenic potential of stem cells because bone marrow-derived mesenchymal cells (BMSCs) isolated from the mice grew slower and were more oxidized compared to controls. They also displayed reduced capacity to accumulate lipid and differentiate into adipocytes. BMSCs from adult mice exposed to neonatal hyperoxia express lower levels of peroxisome proliferator-activated receptor gamma (PPARγ), a transcription factor that drives adipocyte differentiation. The defect in adipogenesis was rescued by expressing PPARγ in these cells. These findings reveal early life exposure to high levels of oxygen may suppresses fat accumulation and impair adipogenic differentiation upstream of PPARγ signaling, thus potentially contributing to growth failure seen in people born preterm.

Pharmacological inhibition of CSF1R by GW2580 reduces microglial proliferation and is protective against neuroinflammation and dopaminergic neurodegeneration.

Increased pro-inflammatory cytokine levels and proliferation of activated microglia have been found in Parkinson's disease (PD) patients and animal models of PD, suggesting that targeting of the microglial inflammatory response may result in neuroprotection in PD. Microglial proliferation is regulated by many factors, but colony stimulating factor-1 receptor (CSF1R) has emerged as a primary factor. Using data mining techniques on existing microarray data, we found that mRNA expression of the CSF1R ligand, CSF-1, is increased in the brain of PD patients compared to controls. In two different neurotoxic mouse models of PD, acute MPTP and sub-chronic LPS treatment, mRNA and protein levels of CSF1R and CSF-1 were significantly increased. Treatment with the CSF1R inhibitor GW2580 significantly attenuated MPTP-induced CSF1R activation and Iba1-positive cell proliferation, without a reduction of the basal Iba1-positive population in the substantia nigra. GW2580 treatment also significantly decreased mRNA levels of pro-inflammatory factors, without alteration of anti-inflammatory mediators, and significantly attenuated the MPTP-induced loss of dopamine neurons and motor behavioral deficits. Importantly, these effects were observed in the absence of overt microglial depletion, suggesting that targeting CSF1R signaling may be a viable neuroprotective strategy in PD that disrupts pro-inflammatory signaling, but maintains the beneficial effects of microglia.

Alternative microglial activation is associated with cessation of progressive dopamine neuron loss in mice systemically administered lipopolysaccharide.

Inflammation arising from central and/or peripheral sources contributes to the pathogenesis of multiple neurodegenerative diseases including Parkinson's disease (PD). Emerging data suggest that differential activation of glia could lead to the pathogenesis and progression of PD. Here, we sought to determine the relationship between lipopolysaccharide (LPS) treatment, loss of dopaminergic neurons and differential activation of glia. Using a model of repeated injections with LPS (1mg/kg, i.p. for 4days), we found that LPS induced a 34% loss of dopamine neurons in the substantia nigra 19days after initiation of treatment, but no further cell loss was observed at 36days. LPS induced a strong pro-inflammatory response with increased mRNA expression of pro-inflammatory markers, including tumor necrosis factor-α (4.8-fold), inducible nitric oxide synthase (2.0-fold), interleukin-1 beta (8.9-fold), interleukin-6 (10.7-fold), and robust glial activation were observed at 1day after final dose of LPS. These pro-inflammatory genes were then reduced at 19days after treatment, when there was a rise in the anti-inflammatory genes Ym1 (1.8-fold) and arginase-1 (2.6-fold). Additionally, 36days after the last LPS injection there was a significant increase in interleukin-10 (2.1-fold) expression. The qPCR data results were supported by protein data, including cytokine measurements, western blotting, and immunofluorescence in brain microglia. Taken together, these data demonstrate that progressive neurodegeneration in the substantia nigra following LPS is likely arrested by microglia shifting to an anti-inflammatory phenotype. Thus, strategies to promote resolution of neuroinflammation may be a promising avenue to slow the progressive loss of dopamine neurons in PD.

Sclerostin activity plays a key role in the negative effect of glucocorticoid signaling on osteoblast function in mice.

Stress during prenatal development is correlated with detrimental cognitive and behavioral outcomes in offspring. However, the long-term impact of prenatal stress (PS) and disrupted glucocorticoid signaling on bone mass and strength is not understood. In contrast, the detrimental effect of lead (Pb) on skeletal health is well documented. As stress and Pb act on common biological targets via glucocorticoid signaling pathways and co-occur in the environment, this study first sought to assess the combined effect of stress and Pb on bone quality in association with alterations in glucocorticoid signaling. Bone parameters were evaluated using microCT, histomorphometry, and strength determination in 8-month-old male mouse offspring subjected to PS on gestational days 16 and 17, lifetime Pb exposure (100 p.p.m. Pb in drinking water), or to both. Pb reduced trabecular bone mass and, when combined with PS, Pb unmasked an exaggerated decrement in bone mass and tensile strength. Next, to characterize a mechanism of glucocorticoid effect on bone, prednisolone was implanted subcutaneously (controlled-release pellet, 5 mg·kg-1 per day) in 5-month-old mice that decreased osteoblastic activity and increased sclerostin and leptin levels. Furthermore, the synthetic glucocorticoid dexamethasone alters the anabolic Wnt signaling pathway. The Wnt pathway inhibitor sclerostin has several glucocorticoid response elements, and dexamethasone administration to osteoblastic cells induces sclerostin expression. Dexamethasone treatment of isolated bone marrow cells decreased bone nodule formation, whereas removal of sclerostin protected against this decrement in mineralization. Collectively, these findings suggest that bone loss associated with steroid-induced osteoporosis is a consequence of sclerostin-mediated restriction of Wnt signaling, which may mechanistically facilitate glucocorticoid toxicity in bone.

Elevated Lifetime Lead Exposure Impedes Osteoclast Activity and Produces an Increase in Bone Mass in Adolescent Mice.

The heavy metal lead (Pb) has a deleterious effect on skeletal health. Because bone mass is maintained through a balance of bone formation and resorption, it is important to understand the effect of Pb levels on osteoblastic and osteoclastic activity. Pb exposure is associated with low bone mass in animal models and human populations; however, the correlation between Pb dosing and corresponding bone mass has been poorly explored. Thus, mice were exposed to increasing Pb and at higher levels (500 ppm), there was unexpectedly an increase in femur-tibial bone mass by 3 months of age. This is contrary to several studies alluded to earlier. Increased bone volume (BV) was accompanied by a significant increase in cortical thickness of the femur and trabecular bone that extended beyond the epiphyseal area into the marrow cavity. Subsequent evaluations revealed an increase in osteoclast numbers with high Pb exposure, but a deficiency in osteoclastic activity. These findings were substantiated by observed increases in levels of the resorption-altering hormones calcitonin and estrogen. In addition we found that pro-osteoclastic nuclear factor-kappa beta (NF-κB) pathway activity was dose dependently elevated with Pb, both in vivo and in vitro. However, the ability of osteoclasts to resorb bone was depressed in the presence of Pb in media and within test bone wafers. These findings indicate that exposure to high Pb levels disrupts early life bone accrual that may involve a disruption of osteoclast activity. This study accentuates the dose dependent variation in Pb exposure and consequent effects on skeletal health.

Heavy Metal Ion Regulation of Gene Expression: MECHANISMS BY WHICH LEAD INHIBITS OSTEOBLASTIC BONE-FORMING ACTIVITY THROUGH MODULATION OF THE Wnt/ß-CATENIN SIGNALING PATHWAY.

Exposure to lead (Pb) from environmental sources remains an overlooked and serious public health risk. Starting in childhood, Pb in the skeleton can disrupt epiphyseal plate function, constrain the growth of long bones, and prevent attainment of a high peak bone mass, all of which will increase susceptibility to osteoporosis later in life. We hypothesize that the effects of Pb on bone mass, in part, come from depression of Wnt/ß-catenin signaling, a critical anabolic pathway for osteoblastic bone formation. In this study, we show that depression of Wnt signaling by Pb is due to increased sclerostin levels in vitro and in vivo. Downstream activation of the ß-catenin pathway using a pharmacological inhibitor of GSK-3ß ameliorates the Pb inhibition of Wnt signaling activity in the TOPGAL reporter mouse. The effect of Pb was determined to be dependent on sclerostin expression through use of the SOST gene knock-out mice, which are resistant to Pb-induced trabecular bone loss and maintain their mechanical bone strength. Moreover, isolated bone marrow cells from the sclerostin null mice show improved bone formation potential even after exposure to Pb. Also, our data suggest that the TGFß canonical signaling pathway is the mechanism by which Pb controls sclerostin production. Taken together these results support our hypothesis that the osteoporotic-like phenotype observed after Pb exposure is, in part, regulated through modulation of the Wnt/ß-catenin pathway.

Effects of Combined Exposure to Lead and High-Fat Diet on Bone Quality in Juvenile Male Mice.

BACKGROUND: Lead (Pb) exposure and obesity are co-occurring risk factors for decreased bone mass in the young, particularly in low socioeconomic communities. OBJECTIVES: The goal of this study was to determine whether the comorbidities of Pb exposure and high-fat diet-induced obesity amplify skeletal deficits independently associated with each of these risk factors, and to explore associated mechanisms of the observed deficiencies. METHODS: Five-week-old male C57BL/6J mice were placed on low-fat (10% kcal, LFD) or high-fat (60% kcal, HFD) diets for 12 weeks. Mice were exposed to lifetime Pb (50 ppm) through drinking water. RESULTS: HFD was associated with increased body mass and glucose intolerance. Both HFD and Pb increased fasting glucose and serum leptin levels. Pb and HFD each reduced trabecular bone quality and together had a further detrimental effect on these bone parameters. Mechanical bone properties of strength were depressed in Pb-exposed bones, but HFD had no significant effect. Both Pb and HFD altered progenitor cell differentiation, promoting osteoclastogenesis and increasing adipogenesis while suppressing osteoblastogenesis. In support of this lineage shift being mediated through altered Wnt signaling, Pb and non-esterified fatty acids in MC3T3 cells increased in vitro PPAR-γ activity and inhibited ß-catenin activity. Combining Pb and non-esterified fatty acids enhanced these effects. CONCLUSIONS: Pb and HFD produced selective deficits in bone accrual that were associated with alterations in progenitor cell activity that may involve reduced Wnt signaling. This study emphasizes the need to assess toxicants together with other risk factors relevant to human health and disease.

Inhibition of beta-catenin signaling by Pb leads to incomplete fracture healing.

There is strong evidence in the clinical literature to suggest that elevated lead (Pb) exposure impairs fracture healing. Since Pb has been demonstrated to inhibit bone formation, and Wnt signaling is an important anabolic pathway in chondrocyte maturation and endochondral ossification, we investigated the impact of Wnt therapy on Pb-exposed mice undergoing bone repair in a mouse tibial fracture model. We established that tibial fracture calluses from Pb-treated mice were smaller and contained less mineralized tissue than vehicle controls. This resulted in the persistence of immature cartilage in the callus and decreased ß-catenin levels. Reduction of ß-catenin protein was concurrent with systemic elevation of LRP5/6 antagonists DKK1 and sclerostin in Pb-exposed mice throughout fracture healing. ß-catenin stimulation by the GSK3 inhibitor BIO reversed these molecular changes and restored the amount of mineralized callus. Overall, Pb is identified as a potent inhibitor of endochondral ossification in vivo with correlated effects on bone healing with noted deficits in ß-catenin signaling, suggesting the Wnt/ß-catenin as a pivotal pathway in the influence of Pb on fracture repair.

Heavy metal lead exposure, osteoporotic-like phenotype in an animal model, and depression of Wnt signaling.

BACKGROUND: Exposure to lead (Pb) from environmental and industrial sources remains an overlooked serious public health risk. Elucidating the effect of Pb on bone cell function is therefore critical for understanding its risk associated with diseases of low bone mass. OBJECTIVES: We tested the hypothesis that Pb negatively affects bone mass. We also assessed the underlying mechanisms of Pb on bone signaling pathways. METHODS: We used a model of low-level Pb exposure in a rodent beginning before conception and continuing over 18 months. We characterized the effect of Pb on bone quality using dual-energy X-ray absorptiometry (DXA), micro-computed tomography, Raman spectroscopy, and histology. We assessed the effect of Pb on bone and adipocyte formation by mineral deposition, lipid droplet formation, and Western blot and RNA analysis. RESULTS: Pb-exposed animals had decreased bone mass that resulted in bones that were more susceptible to fracture. Pb decreased osteoblastic cell number leading to a depression of bone formation. Accompanying this, Pb exposure elevated sclerostin protein levels in the skeleton, and correspondingly reduced levels of ß-catenin and Runx2 in stromal precursor cells. Pb also increased skeletal expression of peroxisome proliferator-activated receptor-γ (PPAR-γ). These results indicate a shift in mesenchymal differentiation wherein Pb promoted enhanced adipogenesis and decreased osteoblastogenesis. Substantial differences in bone marrow composition were observed, highlighted by an increase in adipocytes. CONCLUSIONS: The disruption Pb has on bone mass and bone homeostasis is principally explained by inhibition of the Wnt/ß-catenin pathway, which may provide a molecular basis for novel therapeutic strategies to combat Pb-induced bone pathologies.