Role of calmodulin in the differentiation/activation of microglial cells

Alzheimer's Disease beyond Calcium Dysregulation: The Complex Interplay between Calmodulin, Calmodulin-Binding Proteins and Amyloid Beta from Disease Onset through Progression

A multifactorial syndrome, Alzheimer's disease is the main cause of dementia, but there is no existing therapy to prevent it or stop its progression. One of the earliest events of Alzheimer's disease is the disruption of calcium homeostasis but that is just a prelude to the disease's devastating impact. Calcium does not work alone but must interact with downstream cellular components of which the small regulatory protein calmodulin is central, if not primary. This review supports the idea that, due to calcium dyshomeostasis, calmodulin is a dominant regulatory protein that functions in all stages of Alzheimer's disease, and these regulatory events are impacted by amyloid beta. Amyloid beta not only binds to and regulates calmodulin but also multiple calmodulin-binding proteins involved in Alzheimer's. Together, they act on the regulation of calcium dyshomeostasis, neuroinflammation, amyloidogenesis, memory formation, neuronal plasticity and more. The complex interactions between calmodulin, its binding proteins and amyloid beta may explain why many therapies have failed or are doomed to failure unless they are considered.

Phytochemical Interactions with Calmodulin and Critical Calmodulin Binding Proteins Involved in Amyloidogenesis in Alzheimer's Disease

An increasing number of plant-based herbal treatments, dietary supplements, medical foods and nutraceuticals and their component phytochemicals are used as alternative treatments to prevent or slow the onset and progression of Alzheimer's disease. Their appeal stems from the fact that no current pharmaceutical or medical treatment can accomplish this. While a handful of pharmaceuticals are approved to treat Alzheimer's, none has been shown to prevent, significantly slow or stop the disease. As a result, many see the appeal of alternative plant-based treatments as an option. Here, we show that many phytochemicals proposed or used as Alzheimer's treatments share a common theme: they work via a calmodulin-mediated mode of action. Some phytochemicals bind to and inhibit calmodulin directly while others bind to and regulate calmodulin-binding proteins, including Aβ monomers and BACE1. Phytochemical binding to Aβ monomers can prevent the formation of Aβ oligomers. A limited number of phytochemicals are also known to stimulate calmodulin gene expression. The significance of these interactions to amyloidogenesis in Alzheimer's disease is reviewed.

Targeting microglia L-type voltage-dependent calcium channels for the treatment of central nervous system disorders

Calcium (Ca2+ ) is a ubiquitous mediator of a multitude of cellular functions in the central nervous system (CNS). Intracellular Ca2+ is tightly regulated by cells, including entry via plasma membrane Ca2+ permeable channels. Of specific interest for this review are L-type voltage-dependent Ca2+ channels (L-VDCCs), due to their pleiotropic role in several CNS disorders. Currently, there are numerous approved drugs that target L-VDCCs, including dihydropyridines. These drugs are safe and effective for the treatment of humans with cardiovascular disease and may also confer neuroprotection. Here, we review the potential of L-VDCCs as a target for the treatment of CNS disorders with a focus on microglia L-VDCCs. Microglia, the resident immune cells of the brain, have attracted recent attention for their emerging inflammatory role in several CNS diseases. Intracellular Ca2+ regulates microglia transition from a resting quiescent state to an "activated" immune-effector state and is thus a valuable target for manipulation of microglia phenotype. We will review the literature on L-VDCC expression and function in the CNS and on microglia in vitro and in vivo and explore the therapeutic landscape of L-VDCC-targeting agents at present and future challenges in the context of Alzheimer's disease, Parkinson's disease, Huntington's disease, neuropsychiatric diseases, and other CNS disorders.

Regulation of microglial process elongation, a featured characteristic of microglial plasticity

Microglia, a type of glia within the brain characterized by a ramified morphology, are essential for removing neuronal debris and restricting the expansion of a lesion site. Upon moderate activation, they undergo a transformation in morphology inducing beneficial responses. However, upon strong stimulation, they mediate neuronal damage via production of pro-inflammatory cytokines. The inhibition of this cascade is considered an effective strategy for neuroinflammation-associated disorder therapy. During this pathological activation microglia also undergo a shortening of process length which contributes to the pathogenesis of such disorders. Thus, microglial plasticity should be considered to have two components: one is the production of inflammatory mediators, and the other is the dynamic changes in their processes. The former role has been well-documented in previous studies, while the latter one remains largely unknown. Recently, we and others have reported that the elongation of microglial process is associated with the transformation of microglia from a pro-inflammatory to an anti-inflammatory state, suggesting that the shortening of process length would make the microglia lose their ability to restrict pathological injury, while the elongation of microglial process would help attenuate neuroinflammation. Compared with the traditional anti-neuroinflammatory strategy, stimulating elongation of microglial process not only reduces the production of pro-inflammatory cytokines, but restores the ability of microglia to scan their surrounding environments, thus rendering their homeostasis regulation more effective. In this review, we provide a discussion of the factors that regulate microglial process elongation in vitro and in vivo, aiming to further drive the understanding of microglial process plasticity.

Walnut extract modulates activation of microglia through alteration in intracellular calcium concentration

Diets supplemented with walnuts have shown to protect brain against oxidative and inflammatory cytotoxicity and promote protective cellular and cognitive function. The current study was undertaken to test the hypothesize that whole walnut extract (WNE) inhibits lipopolysaccharide (LPS)-induced microglial activation by regulating calmodulin (CaM) expression through [Ca²⁺]_i. To test this hypothesis, we used an in vitro model the highly aggressively proliferating immortalized cells, a rat microglial cell line, treated with various concentrations of WNEs. Treatment with WNE (1.5%, 3%, or 6%) induced a slow rise in intracellular calcium in a concentration- and time-dependent manner, and this rise became exaggerated when cells were depolarized with potassium chloride (100 mmol/L). Cells treated with WNE (1%, 3%, or 6%) upregulated CaM protein levels, with 1 hour posttreatment being the peak time, regardless of WNE concentration. Interestingly, this WNE-induced upregulation of CaM was blocked by pretreatment with thapsigargin. Additionally, treatment with WNE (1%, 3%, or 6%) 1 hour prior to LPS treatment was found to be effective in preventing LPS-induced upregulation of inducible nitric oxide synthase expression, upregulation of ionized Ca²⁺-binding adaptor-1, and downregulation of CaM. These findings suggest that bioactive compounds in walnut are capable of modulating microglial activation through regulation of intracellular calcium and CaM expression. Nutritional interventions using walnuts may be effective in the amelioration of chronic inflammation and neurodegeneration.

Calmodulin inhibition regulates morphological and functional changes related to the actin cytoskeleton in pure microglial cells

The roles of calmodulin (CaM), a multifunctional intracellular calcium receptor protein, as concerns selected morphological and functional characteristics of pure microglial cells derived from mixed primary cultures from embryonal forebrains of rats, were investigated through use of the CaM antagonists calmidazolium (CALMID) and trifluoperazine (TFP). The intracellular localization of the CaM protein relative to phalloidin, a bicyclic heptapeptide that binds only to filamentous actin, and the ionized calcium-binding adaptor molecule 1 (Iba1), a microglia-specific actin-binding protein, was determined by immunocytochemistry, with quantitative analysis by immunoblotting. In unchallenged and untreated (control) microglia, high concentrations of CaM protein were found mainly perinuclearly in ameboid microglia, while the cell cortex had a smaller CaM content that diminished progressively deeper into the branches in the ramified microglia. The amounts and intracellular distributions of both Iba1 and CaM proteins were altered after lipopolysaccharide (LPS) challenge in activated microglia. CALMID and TFP exerted different, sometimes opposing, effects on many morphological, cytoskeletal and functional characteristics of the microglial cells. They affected the CaM and Iba1 protein expressions and their intracellular localizations differently, inhibited cell proliferation, viability and fluid-phase phagocytosis to different degrees both in unchallenged and in LPS-treated (immunologically challenged) cells, and differentially affected the reorganization of the actin cytoskeleton in the microglial cell cortex, influencing lamellipodia, filopodia and podosome formation. In summary, these CaM antagonists altered different aspects of filamentous actin-based cell morphology and related functions with variable efficacy, which could be important in deciphering the roles of CaM in regulating microglial functions in health and disease.

Inhibitory effects of glucosamine on lipopolysaccharide‐induced activation in microglial cells

The aim of the present study was to investigate the effects of glucosamine on lipopolysaccharide (LPS)-induced cellular activation in microglia and to evaluate the inhibitory mechanisms involved. Lipopolysaccharide (100 ng/mL) was used for the activation of primary cultured rat microglial or BV2 microglial cells. Changes in intracellular Ca2+ levels and outward K+ currents were measured using fura-2/AM and whole-cell patch-clamp methods, respectively. Lipopolysaccharide-induced expression of tumour necrosis factor (TNF)-alpha mRNA was analysed by reverse transcription-polymerase chain reaction. Lipopolysaccharide transformed cell morphology into an amoeboid shape in vitro and induced microglial activation in vivo, as measured by immunohistochemical staining, but glucosamine inhibited this activation. Glucosamine also inhibited LPS-induced Ca2+ influx, outward K+ currents and TNF-alpha mRNA expression, which are typically representative of microglial activation. 4. The results suggest that the inhibitory mechanisms of glucosamine on LPS-induced microglial activation include inhibition of Ca2+ influx and outward K+ currents, as well as downregulation of the microglial activator gene TNF-alpha.

Multiple calmodulin mRNAs are selectively transported to functionally different neuronal and glial compartments in the rat hippocampus. An electron microscopic in situ hybridization study

The ultrastructural distribution of the calmodulin (CaM) mRNAs transcribed from the three CaM genes was studied in the CA1 region of the adult rat hippocampus by means of electron microscopic in situ hybridization. Digoxigenin-labeled CaM gene-specific riboprobes were detected with nanogold-anti-digoxigenin antibody conjugate. The CaM mRNAs were differentially distributed in both the neuronal and glial cell compartments. The greatest difference in neuronal distribution of the CaM mRNAs was found in the dendrites, where the mRNAs transcribed from the CaM I and III genes were much more abundant than the CaM II mRNA. The neuronal perikarya were heavy labeled for all the CaM mRNAs. Interestingly, the myelinated axons and axon terminals also contained small amounts of nanogold particles for all the CaM mRNAs, which diminished with increasing distance from the soma. Most of the synaptic profiles, however, contained labeling only in the postsynaptic region. The CaM mRNAs were differentially distributed in the glial cells. While the glial cell somata were only lightly labeled, surprisingly concentrated labeling was present in the perisynaptic and perivascular astrocytic processes. In general, the CaM II mRNA was the least represented in the glial processes. Only a very low CaM gene expression was observed in the endothelial and resting microglial cells. These results provide ultrastructural evidence for differential targeting of the multiple CaM mRNA transcripts to the intracellular compartments and suggest their microdomain-specific regulation.

Effects of β-AP peptides on activation of the transcription factor NF-κB and in cell proliferation in glial cell cultures

The effect of two beta amyloid peptides (Abeta 25/35 and Abeta 1/42) on the activation of the transcription factor kappaB (NF-kappaB) in pure astroglial, pure microglial and mixed glial cell cultures was compared by means of single or double immunofluorescence and Western blot techniques. We also studied the effect of both peptides in cell proliferation in mixed glial cultures and pure astrocytes. The Abeta 1/42 peptide induced the activation of NF-kappaB in all studied cell cultures and its effect was potentiated by interferon-gamma (IFN-gamma). Abeta 25/35 alone did not induce NF-kappaB activation, but Abeta 25/35 plus IFN-gamma induced the activation of the transcription factor in the mixed and pure microglial cultures, although not in pure astroglia. The Abeta 1/42 peptide, but not Abeta 25/35, induced proliferation in pure astroglial and mixed glial cell cultures. The results suggest that the state of peptide aggregation is related to their ability to activate glial cells.

High-yield isolation of murine microglia by mild trypsinization

Microglia can be isolated with high purity but low yield by shaking off loosely adherent cells from mixed glial cultures. Here we describe a new technique for isolating microglia with an average yield close to 2,000,000 microglial cells/mouse pup, more than five times higher than that of the shaking method. Confluent mixed glial cultures are subjected to mild trypsinization (0.05-0.12%) in the presence of 0.2-0.5 mM EDTA and 0.5-0.8 mM Ca2+. This results in the detachment of an intact layer of cells containing virtually all the astrocytes, leaving undisturbed a population of firmly attached cells identified as >98% microglia. These almost pure microglial preparations can be kept in culture for weeks and show proliferation and phagocytosis. Treatment with macrophage colony-stimulating factor and lipopolysaccharide, alone or in the presence of interferon gamma, induces typical microglial responses in terms of proliferation, morphological changes, nuclear factor-kappaB translocation, NO, and tumor necrosis alpha release and phagocytosis. This method allows for the preparation of highly enriched mouse or rat microglial cultures with ease and reproducibility. Because of its high yield, it can be especially convenient when high amounts of microglial protein/mRNA are required or in cases in which the starting material is limited, such as microglial cultures from transgenic animals.

Absence of the cell cycle inhibitor p21Cip1 reduces LPS-induced NO release and activation of the transcription factor NF-?B in mixed glial cultures

We have studied possible differences in glial activation between cells from wild-type and p21Cip1-/- mice. We compared the effect of serum mitogenic stimulation on proliferation rate and on the total number of glial cells after 7 days of culture. No differences between wild-type and p21Cip1-/- glial cells were observed. We also compared the effect of lipopolysaccharide (LPS) from Escherichia coli, an agent widely used to induce glial activation. Nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) release, and nuclear factor kappa-B (NF-kappaB) activation were evaluated as indicators of glial activation. We observed an attenuation of NO release and NF-kappaB activation in p21Cip1-/- glial cells when compared with glial cells from wild-type mice. In contrast, TNF-alpha release was enhanced in p21Cip1-/- glia. These results suggest that the cell cycle inhibitor p21Cip1 plays a role in the inflammatory response induced by LPS.

Microglial apolipoprotein E and astroglial apolipoprotein J expression in vitro: opposite effects of lipopolysaccharide

Apolipoprotein E (apoE) and apoJ are lipid carriers produced in the brain primarily by glial cells. A variety of glial-activating stimuli induce a parallel upregulation of both apolipoproteins expression in vivo and in vitro. To further characterize the cell type and mechanisms by which apoE and apoJ expression are upregulated in activated glia, mixed glial cultures from neonatal rat cortex were treated with the endotoxin lipopolysaccharide (LPS). LPS induced dose-dependent increases in apoJ and decreases in apoE expression and secretion with maximum effects at 1-10 ng/mL and 0.1-1 microg/mL, respectively. Experiments with enriched astroglial and microglial cultures demonstrated that apoE and apoJ expression are predominantly microglial and astroglial, respectively. Given the pivotal role that nuclear factor-kappa B (NF-kappa B) plays in glial activation, we assessed its possible role in mediating apoE and apoJ expression by activated glia. LPS robustly increased NF-kappa B activation in mixed glial cultures. Two NF-kappa B inhibitors, aspirin (10 mM) and MG-132 (0.1 microM), blocked basal apoE and apoJ secretion as well as LPS-induced apoJ secretion. These data demonstrate that glial apoE and apoJ expression are independently regulated by LPS in microglia and astroglia, respectively, and that activated microglia are the predominant source of apoE in mixed glial cultures. The transcription factor NF-kappa B appears to be a critical mediator of LPS-stimulated apoJ expression from astroglia.

Astrocytes enhance lipopolysaccharide-induced nitric oxide production by microglial cells

Several stimuli result in glial activation and induce nitric oxide (NO) production in microglial and astroglial cells. The bacterial endotoxin lipopolysaccharide (LPS) has been widely used to achieve glial activation in vitro, and several studies show that both microglial and, to a lesser extent, astroglial cell cultures produce NO after LPS treatment. However, NO production in endotoxin-treated astrocyte cultures is controversial. We characterized NO production in microglial, astroglial and mixed glial cell cultures treated with lipopolysaccharide, measured as nitrite accumulation in the culture media. We also identified the NO-producing cells by immunocytochemistry, using specific markers for the inducible NO synthase (iNOS) isoform, microglial and astroglial cells. Only microglial cells showed iNOS immunoreactivity. Thus, contaminating microglial cells were responsible for NO production in the secondary astrocyte cultures. We then analysed the effect of astrocytes on NO production by microglial cells using microglial-astroglial cocultures, and we observed that this production was clearly enhanced in the presence of astroglial cells. Soluble factors released by astrocytes did not appear to be directly responsible for such an effect, whereas nonsoluble factors present in the cell membrane of LPS-treated astrocytes could account, at least in part, for this enhancement.

Differential expression of multiple calmodulin genes in cells of the white matter of the rat spinal cord

Calmodulin (CaM) displays complex cytoplasmic and synaptic functions in the nervous system. However, the very little information that is available on the gene expression of the multiple CaM genes in different glial cell types are from brain tissues of rodents, and no data have been published on their CaM gene expression in the spinal cord. Therefore, we have modified and tested a color in situ hybridization method sensitive enough to detect mRNA populations in cells with low CaM mRNA abundances in the white matter of the rat lumbar spinal cord. Morphologically, two distinct cell types expressing CaM mRNAs were detected. Differential CaM gene expression was demonstrated in medium-sized astrocyte-like cells that reside predominantly in the dorsal column of the spinal cord, where CaM I mRNA was most abundant, followed by the CaM III and CaM II mRNA populations. The oligodendrocytes displayed a less differential CaM gene expression in both the dorsal and the lateral columns, but the CaM I gene had a slightly higher expression level than those of the other CaM genes. The results indicate that the CaM gene expression profile of the spinal cord is richer and more complex than previously thought on the basis of conventional radioactive in situ hybridization techniques. Thus, when a method that is sufficiently sensitive was used, more cell types could be demonstrated to express CaM mRNAs; hence, in spite of their lower CaM expression, glial cells could also be visualized.

Relationship between beta-AP peptide aggregation and microglial activation

We compared the relationship between the state of aggregation of two peptides (beta-AP 25-35 and beta-AP 1-42) and microglial activation. After 7 days at 37 degrees C beta-AP 25-35 was in an amorphous state and did not activate microglial cells. In the same conditions, aggregated beta-AP 1-42 activated these cells and caused changes in microglial ramification, increasing the proliferation index and inducing tumor necrosis factor alpha (TNF alpha) release. Neither peptide induced a release of nitric oxide (NO). As the toxicity of beta-AP peptides in cell culture is associated with the formation of amyloid fibrils, we also examined the toxicity of both peptides in microglial cell cultures and in PC 12 cell cultures. The results suggest that the two beta-AP fragments studied have similar neurotoxic effects but different pro-inflammatory activities.