Peroxisome proliferator-activated receptor γ ligands regulate neural stem cell proliferation and differentiation in vitro and in vivo

Crosstalk between phosphodiesterase 7 and glycogen synthase kinase-3: two relevant therapeutic targets for neurological disorders

Chronic neuroinflammation has been increasingly recognized as a primary mechanism underlying acute brain injury and neurodegenerative diseases. Enhanced expression of diverse pro-inflammatory agents in glial cells has been shown to contribute to the cell death that takes place in these disorders. Previous data from our group have shown that different inhibitors of the cyclic adenosine monophosphate (cAMP) specific phosphodiesterase 7 (PDE7) and glycogen synthase kinase-3 (GSK-3) enzymes are potent anti-inflammatory agents in different models of brain injury. In this study, we investigated cross-talk between PDE7 and GSK-3, two relevant therapeutic targets for neurological disorders, using a chemical approach. To this end, we compared specific inhibitors of GSK-3 and PDE7 with dual inhibitors of both enzymes with regard to anti-inflammatory effects in primary cultures of glial cells treated with lipopolysaccharide. Our results show that the GSK-3 inhibitors act exclusively by inhibition of this enzyme. By contrast, PDE7 inhibitors exert their effects via inhibition of PDE7 to increase intracellular cAMP levels but also through indirect inhibition of GSK-3. Activation of protein kinase A by cAMP results in phosphorylation of Ser9 of GSK-3 and subsequent inhibition. Our results indicate that the indirect inhibition of GSK-3 by PDE7 inhibitors is an important mechanism that should be considered in the future development of pharmacological treatments.

Repression of integrin-linked kinase by antidiabetes drugs through cross-talk of PPARγ- and AMPKα-dependent signaling: role of AP-2α and Sp1

Nasopharyngeal carcinoma (NPC) is one of the most common cancers of the head and neck, particularly in Southern China and Southeast Asia with high treatment failure due to the development of local recurrence and distant metastasis. The molecular mechanisms related to the progression of NPC have not been fully understood. In this study, we showed that antidiabetes drugs rosiglitazone and metformin inhibit NPC cell growth through reducing the expression of integrin-linked kinase (ILK). Blockade of PPARγ and AMPKα overcame the effects of rosiglitazone and metformin on ILK protein. Importantly, overexpression of ILK abrogated the effect of rosiglitazone and metformin on NPC cell growth. Furthermore, these agents reduced ILK promoter activity, which was not observed in AP-2α, but not Sp1 site mutation in ILK gene promoter. In addition, silencing of AP-2α or overexpression of Sp1 reversed the effect of these agents on ILK protein expression and cell growth. Chromatin immunoprecipitation (ChIP) assay showed that rosiglitazone induced AP-2α, while metformin reduced Sp1 protein binding to the DNA sequences in the ILK gene promoter. Intriguingly, overexpression of Sp1 abolished the effect of rosiglitazone on AP-2α protein expression. Collectively, we show that rosiglitazone and metformin inhibit ILK gene expression through PPARγ- and AMPKα-dependent signaling pathways that are involved in the regulation of AP-2α and Sp1 protein expressions. The effect of combination of rosiglitazone and metformin demonstrates greater extent than single agent alone. The cross-talk of PPARγ and AMPKα signaling enhances the synergistic effects of rosiglitazone and metformin. This study unveils novel mechanisms by which oral antidiabetes drugs inhibit the growth of human NPC cells.

N-Docosahexaenoylethanolamine is a potent neurogenic factor for neural stem cell differentiation

Docosahexaenoic acid (DHA) has been shown to promote neuronal differentiation of neural stem cells (NSCs) in vivo and in vitro. Previously, we found that N-docosahexaenoylethanolamine (synaptamide), an endogenous DHA metabolite with an endocannabinoid-like structure, promotes neurite growth, synaptogenesis, and synaptic function. In this study, we demonstrate that synaptamide potently induces neuronal differentiation of NSCs. Differentiating NSCs were capable of synthesizing synaptamide from DHA. Treatment of NSCs with synaptamide at low nanomolar concentrations significantly increased the number of MAP2 and Tuj-1-positive neurons with concomitant induction of protein kinase A (PKA)/cAMP response element binding protein (CREB) phosphorylation. Conversely, PKA inhibitors or PKA knockdown abolished the synaptamide-induced neuronal differentiation of NSCs. URB597, a fatty acid amide hydrolase (FAAH) inhibitor, elevated the level of DHA-derived synaptamide and further potentiated the DHA- or synaptamide-induced neuronal differentiation of NSCs. Similarly, NSCs obtained from FAAH KO mice exhibited greater capacity to induce neuronal differentiation in response to DHA or synaptamide compared to the wild type NSCs. Neither synaptamide nor DHA affected NSC differentiation into GFAP-positive glia cells. These results suggest that endogenously produced synaptamide is a potent mediator for neurogenic differentiation of NSCs acting through PKA/CREB activation.

Perfluorooctane sulfonate induces neuronal and oligodendrocytic differentiation in neural stem cells and alters the expression of PPARγ in vitro and in vivo

Perfluorinated compounds are ubiquitous chemicals of major concern for their potential adverse effects on the human population. We have used primary rat embryonic neural stem cells (NSCs) to study the effects of perfluorooctane sulfonate (PFOS) on the process of NSC spontaneous differentiation. Upon removal of basic fibroblast growth factor, NSCs were exposed to nanomolar concentrations of PFOS for 48 h, and then allowed to differentiate for additional 5 days. Exposure to 25 or 50 nM concentration resulted in a lower number of proliferating cells and a higher number of neurite-bearing TuJ1-positive cells, indicating an increase in neuronal differentiation. Exposure to 50 nM also significantly increased the number of CNPase-positive cells, pointing to facilitation of oligodendrocytic differentiation. PPAR genes have been shown to be involved in PFOS toxicity. By q-PCR we detected an upregulation of PPARγ with no changes in PPARα or PPARδ genes. One of the downstream targets of PPARs, the mitochondrial uncoupling protein 2 (UCP2) was also upregulated. The number of TuJ1- and CNPase-positive cells increased after exposure to PPARγ agonist rosiglitazone (RGZ, 3 μM) and decreased after pre-incubation with the PPARγ antagonist GW9662 (5 μM). RGZ also upregulated the expression of PPARγ and UCP2 genes. Meanwhile GW9662 abolished the UCP2 upregulation and decreased Ca²⁺ activity induced by PFOS. Interestingly, a significantly higher expression of PPARγ and UCP3 genes was also detected in mouse neonatal brain after prenatal exposure to PFOS. These data suggest that PPARγ plays a role in the alteration of spontaneous differentiation of NSCs induced by nanomolar concentrations of PFOS.

Hypoxia modulation of peroxisome proliferator-activated receptors (PPARs) in human glioblastoma stem cells. Implications for therapy

Gliobastoma (GB), the most common adult brain tumor, infiltrates normal brain area rendering impossible the complete surgical resection, resulting in a poor median survival (14-15 months), despite the aggressive multimodality treatments post-surgery, such as radiation and chemo-therapy. GB is characterized by hypoxic and necrotic regions due to a poorly organized tumor vascularization, leading to inadequate blood supply and consequently to hypoxic and necrotic areas. We have previously shown that, under hypoxia GB primary cells increased the expression of stemness markers as well as the expression of the nuclear receptor peroxisome proliferator-activated receptor α (PPARα) and also the crucial role played by PPARs in mouse neural stem cells maintenance and differentiation. Due to the importance of lipid signaling in cell proliferation and differentiation, in this work, we analyzed the expression of PPARs in GB neurospheres both in normoxic and hypoxic conditions. The results obtained suggest a differential regulation of the three PPARs by hypoxia, thus indicating a possible therapeutic strategy to counteract GB recurrencies.

Glycogen synthase kinase 3 inhibition promotes adult hippocampal neurogenesis in vitro and in vivo

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase originally identified as a regulator of glycogen metabolism but it also plays a pivotal role in numerous cellular functions, including differentiation, cell cycle regulation, and proliferation. The dentate gyrus of the hippocampus, together with the subventricular zone of the lateral ventricles, is one of the regions in which neurogenesis takes place in the adult brain. Here, using a chemical genetic approach that involves the use of several diverse inhibitors of GSK-3 as pharmacological tools, we show that inhibition of GSK-3 induces proliferation, migration, and differentiation of neural stem cells toward a neuronal phenotype in in vitro studies. Also, we demonstrate that inhibition of GSK-3 with the small molecule NP03112, called tideglusib, induces neurogenesis in the dentate gyrus of the hippocampus of adult rats. Taken together, our results suggest that GSK-3 should be considered as a new target molecule for modulating the production and integration of new neurons in the hippocampus as a treatment for neurodegenerative diseases or brain injury and, consequently, its inhibitors may represent new potential therapeutic drugs in neuroregenerative medicine.

PPARγ agonists promote oligodendrocyte differentiation of neural stem cells by modulating stemness and differentiation genes

Neural stem cells (NSCs) are a small population of resident cells that can grow, migrate and differentiate into neuro-glial cells in the central nervous system (CNS). Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor transcription factor that regulates cell growth and differentiation. In this study we analyzed the influence of PPARγ agonists on neural stem cell growth and differentiation in culture. We found that in vitro culture of mouse NSCs in neurobasal medium with B27 in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) induced their growth and expansion as neurospheres. Addition of all-trans retinoic acid (ATRA) and PPARγ agonist ciglitazone or 15-Deoxy-Δ^12,14-Prostaglandin J₂ (15d-PGJ2) resulted in a dose-dependent inhibition of cell viability and proliferation of NSCs in culture. Interestingly, NSCs cultured with PPARγ agonists, but not ATRA, showed significant increase in oligodendrocyte precursor-specific O4 and NG2 reactivity with a reduction in NSC marker nestin, in 3–7 days. In vitro treatment with PPARγ agonists and ATRA also induced modest increase in the expression of neuronal β-III tubulin and astrocyte-specific GFAP in NSCs in 3–7 days. Further analyses showed that PPARγ agonists and ATRA induced significant alterations in the expression of many stemness and differentiation genes associated with neuro-glial differentiation in NSCs. These findings highlight the influence of PPARγ agonists in promoting neuro-glial differentiation of NSCs and its significance in the treatment of neurodegenerative diseases.

Control of adult stem cells in vivo by a dynamic physiological environment: diet-dependent systemic factors in Drosophila and beyond

Adult stem cells are inextricably linked to whole-body physiology and nutrient availability through complex systemic signaling networks. A full understanding of how stem cells sense and respond to dietary fluctuations will require identifying key systemic mediators, as well as elucidating how they are regulated and integrated with local and intrinsic factors across multiple tissues. Studies focused on the Drosophila germline have generated valuable insights into how stem cells are controlled by diet-dependent pathways, and increasing evidence suggests that diverse adult stem cell populations respond to nutrients through similar mechanisms. Systemic signals, including nutrients themselves and diet-regulated hormones such as Insulin/Insulin-like growth factor or steroid hormones, can directly or indirectly affect stem cell behavior by modifying local cell-cell communication or intrinsic factors. The physiological regulation of stem cells in response to nutritional status not only is a fascinating biological problem, but also has clinical implications, as research in this field holds the key to noninvasive approaches for manipulating stem cells in vivo. In addition, given the known associations between diet, stem cells, and cancer risk, this research may inspire novel anticancer therapies.

Pilot data on telmisartan short-term effects on glucose metabolism in the olfactory tract in Alzheimer's disease

The possible effect of antihypertensive therapy on Alzheimer's disease (AD) has been studied, and angiotensin II receptor blockers (ARBs) have been suggested to exert an effect on cognitive decline. The purpose of this study is to clarify the functional effects of telmisartan, a long-acting ARB, on AD brain using prospective longitudinal (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) studies. For this purpose, brain glucose metabolism of four hypertensive patients with AD was examined with FDG-PET before and after administration of telmisartan. Studied subjects underwent three FDG-PET studies at intervals of 12 weeks. Antihypertensive treatment except for telmisartan was started after the first FDG-PET and continued for 24 weeks. Then 40-80 mg of telmisartan was added after the second FDG-PET and continued for 12 weeks.Glucose metabolism was significantly decreased during the first 12 weeks without telmisartan use at an area (-10, 21, -22, x, y, z; Z = 3.56) caudal to the left rectal gyrus and the olfactory sulcus corresponding to the left olfactory tract. In contrast, the introduction of telmisartan during the following 12 weeks preserved glucose metabolism at areas (5, 19, -20, x, y, z; Z = 3.09; 6, 19, -22, x, y, z; Z = 2.88) caudal to the bilateral rectal gyri and olfactory sulci corresponding to the bilateral olfactory tracts. No areas showed decreased glucose metabolism after the introduction of telmisartan. In AD, amyloid-β deposition is observed in the anterior olfactory nucleus (AON) of the olfactory tract. Glucose metabolism in AON may be progressively decreased and preserved by telmisartan.

Protection of neurons and microglia against ethanol in a mouse model of fetal alcohol spectrum disorders by peroxisome proliferator-activated receptor-γ agonists

Fetal alcohol spectrum disorders (FASD) result from ethanol exposure to the developing fetus and are the most common cause of mental retardation in the United States. These disorders are characterized by a variety of neurodevelopmental and neurodegenerative anomalies which result in significant lifetime disabilities. Thus, novel therapies are required to limit the devastating consequences of FASD. Neuropathology associated with FASD can occur throughout the central nervous system (CNS), but is particularly well characterized in the developing cerebellum. Rodent models of FASD have previously demonstrated that both Purkinje cells and granule cells, which are the two major types of neurons in the cerebellum, are highly susceptible to the toxic effects of ethanol. The current studies demonstrate that ethanol decreases the viability of cultured cerebellar granule cells and microglial cells. Interestingly, microglia have dual functionality in the CNS. They provide trophic and protective support to neurons. However, they may also become pathologically activated and produce inflammatory molecules toxic to parenchymal cells including neurons. The findings in this study demonstrate that the peroxisome proliferator-activated receptor-γ agonists 15-deoxy-Δ12,15 prostaglandin J2 and pioglitazone protect cultured granule cells and microglia from the toxic effects of ethanol. Furthermore, investigations using a newly developed mouse model of FASD and stereological cell counting methods in the cerebellum elucidate that ethanol administration to neonates is toxic to both Purkinje cell neurons as well as microglia, and that in vivo administration of PPAR-γ agonists protects these cells. In composite, these studies suggest that PPAR-γ agonists may be effective in limiting ethanol-induced toxicity to the developing CNS.

Broad-spectrum neuroprotection against traumatic brain injury by agonism of peroxisome proliferator-activated receptors

In a recent issue of Experimental Neurology, Sauerbeck and colleagues demonstrated that treatment with the peroxisome proliferator-activated receptor (PPAR) agonist Pioglitazone after experimental traumatic brain injury (TBI) in rats was protective against mitochondrial dysfunction, cognitive impairment, cortical tissue loss and microglial activation. In this commentary, we review the key findings of this work and their relevance to previous and future neurotrauma research. More broadly, we speculate about their significance in the context of developing therapeutic strategies for a wide range of neuroinflammatory conditions.