Rescue of Neurons from Ischemic Injury by Peroxisome Proliferator-Activated Receptor-γ Requires a Novel Essential Cofactor LMO4

Abstract

Activation of peroxisome proliferator-activated receptor-␥ (PPAR␥) signaling after stroke may reduce brain injury, but this effect will depend on the levels of receptor and cofactors. Here, we showed that the direct effect of PPAR␥ signaling to protect neurons from ischemic injury requires a novel cofactor LMO4, because this effect was lost in LMO4-null cortical neurons. PPAR␥ agonist also failed to reduce cerebral infarction after transient focal ischemia in CaMKII␣Cre/LMO4loxP mice with LMO4 ablated in neurons of the forebrain. Expressing LMO4 in LMO4-null cortical neurons rescued the PPAR␥-protective effect. PPAR␥ signaling activates the promoter of the antioxidant gene SOD2 and this process requires LMO4. Addition of a superoxide dismutase mimetic MnTBAP [manganese(III)tetrakis(4-benzoic acid)porphyrin] bypassed the deficiency in PPAR␥ signaling and was able to directly rescue LMO4-null cortical neurons from ischemic injury. Like LMO4, PPAR␥ and PGC1␣ (PPAR␥ coactivator 1␣) levels in neurons are elevated by hypoxic stress, and absence of LMO4 impairs their upregulation. Coimmunoprecipitation and mammalian two-hybrid assays revealed that LMO4 interacts in a ligand-dependent manner with PPAR␥. LMO4 augments PPAR␥-dependent gene activation, in part, by promoting RXR␣ (retinoid X receptor-␣) binding to PPAR␥ and by increasing PPAR␥ binding to its target DNA sequence. Together, our results identify LMO4 as an essential hypoxia-inducible cofactor required for PPAR␥ signaling in neurons. Thus, upregulation of LMO4 expression after stroke is likely to be an important determinant of neuron survival.

LIM Domain Only 4 (LMO4) Regulates Calcium-Induced Calcium Release and Synaptic Plasticity in the Hippocampus

“…LMO4 is a small protein (165 aa) that contains two protein-interacting LIM domains. LMO4 serves not only as a cofactor of many transcription factors (Manetopoulos et al, 2003;Kashani et al, 2006;Schock et al, 2008), but also interacts with transmembrane receptors to modulate their signaling (Novotny-Diermayr et al, 2005;Bong et al, 2007). Whether LMO4 couples signals from membrane receptors to changes in gene expression is not known, although we showed that LMO4 is present in the cytoplasm and nucleus, and translocates from the cytoplasm to the nucleus in response to extracellular stimuli (Chen et al, 2007a).…”

Prevention of JNK Phosphorylation as a Mechanism for Rosiglitazone in Neuroprotection after Transient Cerebral Ischemia: Activation of Dual Specificity Phosphatase

“…Recent studies have shown that both rosiglitazone and pioglitazone are neuroprotective in animal models of acute focal ischemia. 3 Studies have also shown that neuroprotection by TZDs following ischemia may involve: (a) a reduction in the inflammatory response, such as by upregulation of nuclear factor-kB, intercellular adhesion molecule-1, or cyclooxygenase; [16][17][18][19] (b) activation of an anti-inflammatory mediator, such as synthesis of lipoxin A 4 by 5-lipoxygenase expression; 20 (c) activation of superoxide dismutase; 21,22 or (d) prevention of harmful neuronal responses to ischemic insult, such as enhanced 14-3-3e expression. 23 There is significant evidence from these studies that PPARg activation is involved in neuroprotection.…”

LIM domain only 4 protein promotes granulocyte colony-stimulating factor-induced signaling in neurons

“…We showed previously that signals like extracellular ATP, hypoxia, and depolarization stabilize LMO4 mRNA and protein [11,18,19]. Ablation of LMO4 increases neuron susceptibility to ischemic injury, resulting in a larger infarction in an experimental model of stroke [11].…”

“…LMO4 interacts with transcription factors, including LIM homeodomain proteins, basic HelixLoop-Helix (bHLH) proteins, peroxisome proliferator-activated receptor gamma (PPARγ), and cAMP response element-binding protein (CREB), to activate gene expression in the nervous system [11][12][13][14][15]. Germline deletion of LMO4 leads to embryonic lethality and failure of the cranial neural tube to close, resulting in exencephaly [16,17].…”

“…Ablation of LMO4 increases neuron susceptibility to ischemic injury, resulting in a larger infarction in an experimental model of stroke [11]. Hypoxia-induced expression of LMO4 promotes protein expression of the superoxide dismutase SOD2, an antioxidant gene that scavenges free radicals and reduces oxidative stress [11].…”