"Skittish" Abca2 knockout mice display tremor, hyperactivity, and abnormal myelin ultrastructure in the central nervous system.

The ATP-binding cassette transporter 2 (ABCA2) is an endolysosomal protein most highly expressed in the central and peripheral nervous system tissues and macrophages. Previous studies indicated its role in cholesterol/steroid (estramustine, estradiol, and progesterone) trafficking/sequestration, oxidative stress response, and Alzheimer's disease. Developmental studies have shown its expression during macrophage and oligodendrocyte differentiation, processes requiring membrane growth. To determine the in vivo function(s) of this transporter, we generated a knockout mouse from a gene-targeted disruption of the murine ABCA2 gene. Knockout males and females are viable and fertile. However, a non-Mendelian inheritance pattern was shown among male progeny of heterozygous crosses. Compared to wild-type and heterozygous littermates, knockout mice displayed a tremor without ataxia, hyperactivity, and reduced body weight; the latter two phenotypes were more marked in females than in males. This sexual disparity suggests a role for ABCA2 in hormone-dependent neurological and/or developmental pathways. Myelin sheath thickness in the spinal cords of knockout mice was greatly increased compared to that in wild-type mice, while a significant reduction in myelin membrane periodicity (compaction) was observed in both spinal cords and cerebra of knockout mice. Loss of ABCA2 function in vivo resulted in abnormal myelin compaction in spinal cord and cerebrum, an ultrastructural defect that we propose to be the cause of the phenotypic tremor.

The ABCA2 transporter: intracellular roles in trafficking and metabolism of LDL-derived cholesterol and sterol-related compounds.

ATP-binding cassette (ABC) transporters comprise a family of critical membrane bound proteins functioning in the translocation of molecules across cellular membranes. Substrates for transport include lipids, cholesterol and pharmacological agents. Mutations in ABC transporter genes cause a variety of human pathologies and elicit drug resistance phenotypes in cancer cells. ABCA2, the second member the A subfamily to be identified, was highly expressed in ovarian carcinoma cells resistant to the anti-cancer agent, estramustine, and more recently, in human vestibular schwannomas. Cells expressing elevated levels of ABCA2 show resistance to variety of compounds, including estradiol, mitoxantrone and a free radical initiator, 2,2'-azobis-(2-amidinopropane). ABCA2 is expressed in a variety of tissues, with greatest abundance in the central nervous system and macrophages. This transporter, along with other proteins that have a high degree of homology to ABCA2, including ABCA1 and ABCA7, are up-regulated in human macrophages during cholesterol import. Recent studies have shown ABCA2 also plays a role in the trafficking of low-density lipoprotein (LDL)-derived free cholesterol and to be coordinately expressed with sterol-responsive genes. A single nucleotide polymorphism in exon 14 of the ABCA2 gene was shown to be linked to early onset Alzheimer disease (AD) in humans, supporting an earlier study showing ABCA2 expression influences levels of APP and beta-amyloid peptide, the primary component of senile plaques. Studies thus far implicate ABCA2 as a sterol transporter, the deregulation of which may affect a cellular phenotype conducive to the pathogenesis of a variety of human diseases including AD, atherosclerosis and cancer.

Ablation of the ATP-binding cassette transporter, Abca2 modifies response to estrogen-based therapies.

The ATP-binding cassette transporter 2 (ABCA2) is an endolysosomal protein expressed in oligodendrocytes and Schwann cells, prostate, ovary and macrophages. In cell cultures, ABCA2 over-expression has been linked with resistance to the anticancer agent, estramustine phosphate (EMP; a nor-nitrogen mustard conjugate of estradiol). The present study shows that Abca2 knockout (KO) mice have greater sensitivity to a variety of side effects induced by EMP treatment. Chronic EMP (12×100 mg/kg body weight) produced mortality in 36% of KO mice, but only 7% of age-matched wild type (WT). Side effects of the drug were also more prevalent in the KO mouse. For example, during the first week of EMP treatments, 67% of KO males (compared to 6% of WT males) responded with episodic erectile events. In WT mice, ABCA2 protein localized within pene corpuscles, (which rely on modified Schwann cells for amplification of tactile signals) suggesting that the transporter may function in the erectile process. Endothelial nitric oxide synthase (eNOS; a source of nitric oxide during erectile response) levels were similar in WT and KO male penile tissue. Treatment with 100 mg/kg EMP (once daily for four days) elevated serum estradiol and estrone in both WT and KO. However, the circulating levels of these estrogens were higher in KO mice implying a reduced plasma clearance of estrogens as a consequence of ABCA2 ablation. Consistent with the pro-convulsant effects of estrogens, KO mice also displayed an increased incidence of seizures following EMP (14% vs. 0%). Taken together, these data indicate that ABCA2 deficiency renders mice more sensitive to EMP treatment-induced effects implying that the transporter has a role in regulating EMP transport and/or metabolism.

Macrophage ABCA2 deletion modulates intracellular cholesterol deposition, affects macrophage apoptosis, and decreases early atherosclerosis in LDL receptor knockout mice.

OBJECTIVE: The ABCA2 transporter shares high structural homology to ABCA1, which is crucial for the removal of excess cholesterol from macrophages and, by extension, in atherosclerosis. It has been suggested that ABCA2 sequesters cholesterol inside the lysosomes, however, little is known of the macrophage-specific role of ABCA2 in regulating lipid homeostasis in vivo and in modulating susceptibility to atherosclerosis. METHODS: Chimeras with dysfunctional macrophage ABCA2 were generated by transplantation of bone marrow from ABCA2 knockout (KO) mice into irradiated LDL receptor (LDLr) KO mice. RESULTS: Interestingly, lack of ABCA2 in macrophages resulted in a diminished lesion size in the aortic root (-24.5%) and descending thoracic aorta (-36.6%) associated with a 3-fold increase in apoptotic cells, as measured by both caspase 3 and TUNEL. Upon oxidized LDL exposure, macrophages from wildtype (WT) transplanted animals developed filipin-positive droplets in lysosomal-like compartments, corresponding to free cholesterol (FC) accumulation. In contrast, ABCA2-deficient macrophages displayed an abnormal diffuse distribution of FC over peripheral regions. The accumulation of neutral sterols in lipid droplets was increased in ABCA2-deficient macrophages, but primarily in cytoplasmic clusters and not in lysosomes. Importantly, apoptosis of oxLDL loaded macrophages lacking ABCA2 was increased 2.7-fold, probably as a consequence of the broad cellular distribution of FC. CONCLUSIONS: Lack of functional ABCA2 generates abnormalities in intracellular lipid distribution/trafficking in macrophages consistent with its lysosomal sequestering role, leading to an increased susceptibility to apoptosis in response to oxidized lipids and reduced atherosclerotic lesion development.

ABCA2 transporter deficiency reduces incidence of TRAMP prostate tumor metastasis and cellular chemotactic migration.

In order to study the effects of ATP-binding cassette transporter 2 (ABCA2) deficiency on the progression of prostate cancer, congenic Abca2 knockout (KO) mice were crossed to the transgenic adenocarcinoma of the mouse prostate (TRAMP) model. ABCA2 expression was elevated in wild-type/TRAMP (WT/Tg) dorsal prostate, a region comprising the most aggressive tumors in this model, compared to non-transgenic WT mice. Primary prostate tumor progression was similar in KO/Tg and WT/Tg mice with respect to pathological score, prostate tumor growth, as calculated using MRI volumetry, and proliferative index, as determined by PCNA immunostaining. Vimentin, a marker of the epithelial-mesenchymal transition, was expressed at similar levels in prostate, but elevated in histologically normal seminal vesicles (SV) in KO/Tg mice (P < 0.02), concomitant with an increased SV volume (P < 0.01). These changes in the SV did not exacerbate the metastatic phenotype of this disease model; rather, KO/Tg mice aged 20-25 weeks had no detectable metastases while 38% of WT/Tg developed metastases to lung and/or lymph nodes. The absence of a metastatic phenotype in KO/Tg mice was reprised in stable ABCA2 knockdown (KD) cells where chemotactic, but not random, migration was impaired (P = 0.0004). Expression levels of sphingolipid biosynthetic enzymes were examined due to the established link of the transporter with sphingolipid homeostasis. Galactosylceramide synthase (GalCerS) mRNA levels were over 8-fold higher in KD cells (P = 0.001), while lactosylceramide synthase (LacCerS) and CTP:choline cytidylyltransferase (CCT) were significantly reduced (P < 0.0001 and 0.03, respectively). Overall, we demonstrate that ABCA2-deficiency inhibits prostate tumor metastasis in vivo and decreases chemotactic potential of cells, conceivably due to altered sphingolipid metabolism.

ABCA2 as a therapeutic target in cancer and nervous system disorders.

BACKGROUND: Overexpression of ATP-binding cassette (ABC) transporters is a major adaptive advantage used by tumor cells to evade the accumulation of cytotoxic agents. ABCA2, a transporter highly expressed in the cells of the nervous and haematopoetic systems, is associated with lipid transport and drug resistance in cancer cells, including tumor stem cells. Recently, a single nucleotide polymorphism (SNP) in Abca2 was linked to early onset Alzheimer's disease (AD). The characterization of two independent knockout mouse models has shed light on putative in vivo functions of this transporter in the development and maintenance of myelin membrane lipids in the CNS. OBJECTIVE: The objective of this review is to guide the reader through the existing scope of literature on the ABCA2 transporter, focusing on its potential as a future target in human pathologies, specifically cancer and neurological disease. METHODS: An NCBI PubMed literature search was conducted to address the growing body of ABCA2 literature that, at the time of publication, included 39 reports. From these, we focused on papers that provided insight into the functional importance of this transporter in tumor stem cells, cancer, drug resistance, Alzheimer's disease and myelination. RESULTS/CONCLUSION: These studies have implicated ABCA2 as a therapeutic target in modulating the drug resistance phenotype prevalent in human cancers and in the treatment of neuropathies, including Alzheimer's disease and myelin-related disorders.