Apolipoprotein D alleviates glucocorticoid-induced osteogenesis suppression in bone marrow mesenchymal stem cells via the PI3K/Akt pathway.

BACKGROUND: To clarify the role of apolipoprotein D (Apod) in alleviating glucocorticoid-induced osteogenesis suppression in bone marrow mesenchymal stem cells (MSCs) via the PI3K/Akt pathway, thus influencing the progression of osteoporosis (OP). METHODS: Osteogenesis in MSCs was induced by dexamethasone (DEX) stimulation. Dynamic expressions of Apod in MSCs undergoing osteogenesis for different time points were determined by qRT-PCR. Relative levels of osteogenesis-associated genes, including ALP, RUNX2, and Osterix, in DEX-induced MSCs overexpressing Apod or not were examined. Moreover, the protein level of RUNX2, ALP, and Osterix; ALP activity; and mineralization ability influenced by Apod in osteogenic MSCs were assessed. At last, the potential influences of Apod on the PI3K/Akt pathway were identified through detecting the expression levels of PI3K and Akt in MSCs by Western blot. RESULTS: Apod was time-dependently upregulated in MSCs undergoing osteogenesis. DEX induction downregulated ALP, RUNX2, and Osterix and attenuated ALP activity and mineralization ability in MSCs undergoing osteogenesis, which were partially reversed by overexpression of Apod. In addition, Apod overexpression upregulated the reduced levels of PI3K and Akt in DEX-induced MSCs. CONCLUSION: Apod alleviates glucocorticoid-induced osteogenesis suppression in MSCs via the PI3K/Akt pathway, thus protecting the progression of OP.

Apolipoprotein D-mediated preservation of lysosomal function promotes cell survival and delays motor impairment in Niemann-Pick type A disease.

Lysosomal Storage Diseases (LSD) are genetic diseases causing systemic and nervous system dysfunction. The glia-derived lipid binding protein Apolipoprotein D (ApoD) is required for lysosomal functional integrity in glial and neuronal cells, ensuring cell survival upon oxidative stress or injury. Here we test whether ApoD counteracts the pathogenic consequences of a LSD, Niemann Pick-type-A disease (NPA), where mutations in the acid sphingomyelinase gene result in sphingomyelin accumulation, lysosomal permeabilization and early-onset neurodegeneration. We performed a multivariable analysis of behavioral, cellular and molecular outputs in 12 and 24 week-old male and female NPA model mice, combined with ApoD loss-of-function mutation. Lack of ApoD in NPA mice accelerates cerebellar-dependent motor deficits, enhancing loss of Purkinje neurons. We studied ApoD expression in brain sections from a NPA patient and age-matched control, and the functional consequences of ApoD supplementation in primary human fibroblasts from two independent NPA patients and two control subjects. Cell viability, lipid peroxidation, and lysosomal functional integrity (pH, Cathepsin B activity, Galectin-3 exclusion) were examined. ApoD is endogenously overexpressed in NPA patients and NPA mouse brains and targeted to lysosomes of NPA patient cells, including Purkinje neurons and cultured fibroblasts. The accelerated lysosomal targeting of ApoD by oxidative stress is hindered in NPA fibroblasts, contributing to NPA lysosomes vulnerability. Exogenously added ApoD reduces NPA-prompted lysosomal permeabilization and alkalinization, reverts lipid peroxides accumulation, and significantly increases NPA cell survival. ApoD administered simultaneously to sphingomyelin overload results in complete rescue of cell survival. Our results reveal that ApoD protection of lysosomal integrity counteracts NPA pathology. ApoD supplementation could significantly delay not only the progression of NPA disease, but also of other LSDs through its beneficial effects in lysosomal functional maintenance.

Protecting cells by protecting their vulnerable lysosomes: Identification of a new mechanism for preserving lysosomal functional integrity upon oxidative stress.

Environmental insults such as oxidative stress can damage cell membranes. Lysosomes are particularly sensitive to membrane permeabilization since their function depends on intraluminal acidic pH and requires stable membrane-dependent proton gradients. Among the catalog of oxidative stress-responsive genes is the Lipocalin Apolipoprotein D (ApoD), an extracellular lipid binding protein endowed with antioxidant capacity. Within the nervous system, cell types in the defense frontline, such as astrocytes, secrete ApoD to help neurons cope with the challenge. The protecting role of ApoD is known from cellular to organism level, and many of its downstream effects, including optimization of autophagy upon neurodegeneration, have been described. However, we still cannot assign a cellular mechanism to ApoD gene that explains how this protection is accomplished. Here we perform a comprehensive analysis of ApoD intracellular traffic and demonstrate its role in lysosomal pH homeostasis upon paraquat-induced oxidative stress. By combining single-lysosome in vivo pH measurements with immunodetection, we demonstrate that ApoD is endocytosed and targeted to a subset of vulnerable lysosomes in a stress-dependent manner. ApoD is functionally stable in this acidic environment, and its presence is sufficient and necessary for lysosomes to recover from oxidation-induced alkalinization, both in astrocytes and neurons. This function is accomplished by preventing lysosomal membrane permeabilization. Two lysosomal-dependent biological processes, myelin phagocytosis by astrocytes and optimization of neurodegeneration-triggered autophagy in a Drosophila in vivo model, require ApoD-related Lipocalins. Our results uncover a previously unknown biological function of ApoD, member of the finely regulated and evolutionary conserved gene family of extracellular Lipocalins. They set a lipoprotein-mediated regulation of lysosomal membrane integrity as a new mechanism at the hub of many cellular functions, critical for the outcome of a wide variety of neurodegenerative diseases. These results open therapeutic opportunities by providing a route of entry and a repair mechanism for lysosomes in pathological situations.

Apolipoprotein D takes center stage in the stress response of the aging and degenerative brain.

Apolipoprotein D (ApoD) is an ancient member of the lipocalin family with a high degree of sequence conservation from insects to mammals. It is not structurally related to other major apolipoproteins and has been known as a small, soluble carrier protein of lipophilic molecules that is mostly expressed in neurons and glial cells within the central and peripheral nervous system. Recent data indicate that ApoD not only supplies cells with lipophilic molecules, but also controls the fate of these ligands by modulating their stability and oxidation status. Of particular interest is the binding of ApoD to arachidonic acid and its derivatives, which play a central role in healthy brain function. ApoD has been shown to act as a catalyst in the reduction of peroxidized eicosanoids and to attenuate lipid peroxidation in the brain. Manipulating its expression level in fruit flies and mice has demonstrated that ApoD has a favorable effect on both stress resistance and life span. The APOD gene is the gene that is upregulated the most in the aging human brain. Furthermore, ApoD levels in the nervous system are elevated in a large number of neurologic disorders including Alzheimer's disease, schizophrenia, and stroke. There is increasing evidence for a prominent neuroprotective role of ApoD because of its antioxidant and anti-inflammatory activity. ApoD emerges as an evolutionarily conserved anti-stress protein that is induced by oxidative stress and inflammation and may prove to be an effective therapeutic agent against a variety of neuropathologies, and even against aging.

Amyloid-ß25-35 induces apolipoprotein D Synthesis and growth arrest in HT22 hippocampal cells.

Apolipoprotein D (ApoD) is a secreted glycoprotein that is markedly induced in several pathological and stressful conditions in the nervous system. In the central nervous system, ApoD expression is upregulated during aging, after traumatic brain injury, and in several human neuropathologies such as Alzheimer's disease (AD), where it is found associated with amyloid-ß (Aß) plaques. Recent studies have indicated that ApoD has an important function as a neuroprotective and antioxidant protein. The aim of this work is to study the effect of the peptide fragment Aß25-35, which is believed to play a major role in the neurodegenerative process of AD, in ApoD expression in a mouse hippocampal cell line. In addition, we studied whether direct addition of exogenous human recombinant ApoD protein has neuroprotective effect against Aß25-35 treatment on neuronal cells. Our results demonstrate that Aß25-35 induces ApoD expression in hippocampal cells in response to stress-induced growth arrest. This observed relationship between Aß and ApoD expression could explain the elevated levels of ApoD found in AD brain, where it may be a neuroprotective molecule in the course of AD, probably related to its lipid transport function or a direct antioxidant property. However, the addition of exogenous human recombinant ApoD does not exert any protective effect, most likely due to its major structural modifications.

Apolipoprotein D mediates autocrine protection of astrocytes and controls their reactivity level, contributing to the functional maintenance of paraquat-challenged dopaminergic systems.

The study of glial derived factors induced by injury and degeneration is important to understand the nervous system response to deteriorating conditions. We focus on Apolipoprotein D (ApoD), a Lipocalin expressed by glia and strongly induced upon aging, injury or neurodegeneration. Here we study ApoD function in the brain of wild type and ApoD-KO mice by combining in vivo experiments with astrocyte cultures. Locomotor performance, dopamine concentration, and gene expression levels in the substantia nigra were assayed in mice treated with paraquat (PQ). The regulation of ApoD transcription, a molecular screening of oxidative stress (OS)-related genes, cell viability and oxidation status, and the effects of adding human ApoD were tested in astrocyte cultures. We demonstrate that (1) ApoD is required for an adequate locomotor performance, modifies the gene expression profile of PQ-challenged nigrostriatal system, and contributes to its functional maintenance; (2) ApoD expression in astrocytes is controlled by the OS-responsive JNK pathway; (3) ApoD contributes to an autocrine protecting mechanism in astrocytes, avoiding peroxidated lipids accumulation and altering the PQ transcriptional response of genes involved in ROS managing and the inflammatory response to OS; (4) Addition of human ApoD to ApoD-KO astrocytes promotes survival through a mechanism accompanied by protein internalization and modulation of astroglial reactivity. Our data support that ApoD contributes to the endurance of astrocytes and decreases their reactivity level in vitro and in vivo. ApoD function as a maintenance factor for astrocytes would suffice to explain the observed protection by ApoD of OS-vulnerable dopaminergic circuits in vivo.

Apolipoproteins D and E3 exert neurotrophic and synaptogenic effects in dorsal root ganglion cell cultures.

Co-cultures of 3T3-L1 adipocytes with neurons from the rat dorsal root ganglia (DRG) showed enhanced neuritogenesis and synaptogenesis. Microarray analysis for upregulated genes in adipocyte/DRG co-cultures currently points to apolipoproteins D and E (ApoD, ApoE) as influential proteins. We therefore tested adipocyte-secreted cholesterol and the carrier proteins ApoD and ApoE3. Cholesterol, ApoD, and ApoE3 each increased neurite outgrowth and upregulated the expression of presynaptic synaptophysin and synaptotagmin, as well as the postsynaptic density protein 95. The neurotrophic effects of ApoD and ApoE3 were associated with an increased expression of the low-density lipoprotein receptor and apolipoprotein E receptor 2. Simultaneous treatment with receptor-associated protein, an apolipoprotein receptor antagonist, inhibited the neurotrophic function of both apolipoproteins. The application of ApoD, ApoE3, and cholesterol to DRG cell cultures corresponded with increased expression of the chemokine stromal cell-derived factor 1 and its receptor CXC chemokine receptor 4 (CXCR4). Surprisingly, the inhibition of CXCR4 by the antagonistic drug AMD3100 decreased the apolipoprotein/cholesterol dependent neurotrophic effects. We thus assume that apolipoprotein-induced neuritogenesis in DRG cells interferes with CXCR4 signaling, and that adipocyte-derived apolipoproteins might be helpful in nerve repair.

Apolipoprotein D modulates F2-isoprostane and 7-ketocholesterol formation and has a neuroprotective effect on organotypic hippocampal cultures after kainate-induced excitotoxic injury.

Apolipoprotein D (apoD), a member of the lipocalin family of transporter proteins binds a number of small lipophilic molecules including arachidonic acid and cholesterol. Recent studies showed a protective function of mammalian apoD as well as its insect and plant homologs against oxidative stress. In this study we investigated the effect of direct addition of exogenous human apoD protein purified from breast cystic fluid to rat hippocampal slice cultures after excitotoxic injury induced by the glutamate analog kainate. ApoD at a concentration of 10 microg/ml partially prevented loss of MAP2 immunostaining and LDH release from injured hippocampal neurons after kainate injury. ApoD also attenuated the increase in oxidative products of arachidonic acid and cholesterol, F(2)-isoprostanes and 7-ketocholesterol, respectively, after kainate treatment. In view of the molecular structure of apoD which consists of an eight stranded beta barrel that forms a binding pocket for a number of small hydrophobic molecules, we propose that apoD promotes its neuroprotective effects by binding to arachidonic acid and cholesterol thus preventing their oxidation to neurotoxic products such as 4-hydroxynonenal (4-HNE) and 7-ketocholesterol.