Mutant Lrp1 Knock-In Mice Generated by Recombinase-Mediated Cassette Exchange Reveal Differential Importance of the NPXY Motifs in the Intracellular Domain of LRP1 for Normal Fetal Development

Lrp1 knock-in mice carrying either a wild-type allele or three different mutated alleles encoding the multifunctional endocytic receptor LRP1 were generated by recombinase-mediated cassette exchange (RMCE). Reinsertion by RMCE of a wild-type allele led to a normal pattern and level of gene expression and a completely normal phenotype, indicating that the RMCE procedure itself is neutral with respect to the function of the gene locus. In contrast, reinsertion of mutated LRP1 alleles carrying either inactivating mutations in the proximal NPXY motif (NPTY→AATA) of the cytoplasmic domain or in the furin cleavage site (RHRR→AHAA) caused distinctive liver phenotypes: respectively, either a late fetal destruction of the organ causing perinatal death or a selective enlargement of von-Kupffer cell lysosomes reminiscent of a mild lysosomal storage without an apparent negative effect on animal survival. Notably, mutation of the distal NPXY motif overlapping with an YXXL motif (NPVYATL→AAVAATL) did not cause any obvious pathological effect. The mutations showed no effect on the LRP1 expression level; however, as expected, the proteolytic maturation of LRP1 into its two subunits was significantly impaired, although not completely abolished, in the furin cleavage mutant. These data demonstrate that RMCE is a reliable and efficient approach to generate multiple mutant knock-in alleles for in vivo functional analysis of individual domains or motifs of large multidomain proteins. Its application in Lrp1 reveals dramatically variant phenotypes, of which further characterization will definitively contribute to our understanding of the biology of this multifunctional receptor

Opposing effects of apolipoprotein m on catabolism of apolipoprotein B-containing lipoproteins and atherosclerosis

Rationale: Plasma apolipoprotein (apo)M is mainly associated with high-density lipoprotein (HDL). HDL-bound apoM is antiatherogenic in vitro. However, plasma apoM is not associated with coronary heart disease in humans, perhaps because of a positive correlation with plasma low-density lipoprotein (LDL). Objective: We explored putative links between apoM and very-low-density (VLDL)/LDL metabolism and the antiatherogenic potential of apoM in vivo. Methods and Results: Plasma apoM was increased approximately 2.1 and approximately 1.5 fold in mice lacking LDL receptors (Ldlr(-/-)) and expressing dysfunctional LDL receptor-related protein 1 (Lrp1(n2/n2)), respectively, but was unaffected in apoE-deficient (ApoE(-/-)) mice. Thus, pathways controlling catabolism of VLDL and LDL affect plasma apoM. Overexpression ( approximately 10-fold) of human apoM increased (50% to 70%) and apoM deficiency decreased ( approximately 25%) plasma VLDL/LDL cholesterol in Ldlr(-/-) mice, whereas apoM did not affect plasma VLDL/LDL in mice with intact LDL receptors. Moreover, plasma clearance of apoM-enriched VLDL/LDL was slower than that of control VLDL/LDL in mice lacking functional LDL receptors and LRP1, suggesting that apoM impairs the catabolism of VLDL/LDL that occurs independently of the LDL receptor and LRP1. ApoM overexpression decreased atherosclerosis in ApoE(-/-) (60%) and cholate/cholesterol-fed wild-type mice (70%). However, in Ldlr(-/-) mice the antiatherogenic effect of apoM was attenuated by its VLDL/LDL-raising effect. Conclusion: The data suggest that defect LDL receptor function leads to increased plasma apoM concentrations, which in turn, impairs the removal of VLDL/LDL from plasma. This mechanism opposes the otherwise antiatherogenic effect of apoM

Lrp1 mediates bidirectional transcytosis of amyloid-beta across the blood-brain barrier

According to the "amyloid hypothesis", the amyloid-beta (A beta) peptide is the toxic intermediate driving Alzheimer's disease (AD) pathogenesis. Recent evidence suggests that the low density lipoprotein receptor-related protein 1 (LRP1) transcytoses A beta out of the brain across the blood-brain barrier (BBB). To provide genetic evidence for LRP1-mediated transcytosis of A beta across the BBB we analyzed A beta transcytosis across primary mouse brain capillary endothelial cells (pMBCECs) derived from wild-type and LRP1 knock-in mice. Here, we show that pMBCECs in vitro express functionally active LRP1. Moreover, we demonstrate that LRP1 mediates transcytosis of [(125)I]-A beta(1-40) across pMBCECs in both directions, whereas no role for LRP1-mediated A beta degradation was detected. Analysis of [(125)I]-A beta(1-40) transport across pMBCECs generated from mice harboring a knock-in mutation in the NPxYxxL endocytosis/sorting domain of endogenous LRP1 revealed a reduced A beta clearance from brain-to-blood and blood-to-brain compared with wild-type derived pMBCECs. Therefore, for the first time, we present genetic evidence that LRP1 modulates the pathogenic actions of soluble A beta in the brain by clearing A beta across the BBB. (C) 2011 Elsevier Inc. All rights reserved

The plasma concentration of HDL-associated apoM is influenced by LDL receptor-mediated clearance of apoB-containing particles

ApoM is mainly associated with HDL. Nevertheless, we have consistently observed positive correlations of apoM with plasma LDL cholesterol in humans. Moreover, LDL receptor deficiency is associated with increased plasma apoM in mice. Here, we tested the idea that plasma apoM concentrations are affected by the rate of LDL receptor-mediated clearance of apoB-containing particles. We measured apoM in humans each carrying one of three different LDL receptor mutations (n = 9) or the apoB3500 mutation (n = 12). These carriers had increased plasma apoM (1.34 +/- 0.13 mu M, P = 0.003, and 1.23 +/- 0.10 mu M, P = 0.02, respectively) as compared with noncarriers (0.93 +/- 0.04 mu M). When we injected human apoM-containing HDL into Wt (n = 6) or LDL receptor-deficient mice (n = 6), the removal of HDL-associated human apoM was delayed in the LDL receptor-deficient mice. After 2 h, 54 +/- 5% versus 90 +/- 8% (P < 0.005) of the initial amounts of human apoM remained in the plasma of Wt and LDL receptor-deficient mice, respectively. Finally, we compared the turnover of radio-iodinated LDL and plasma apoM concentrations in 45 normocholesterolemic humans. There was a negative correlation between plasma apoM and the fractional catabolic rate of LDL (r = -0.38, P = 0.009). These data suggest that the plasma clearance of apoM, despite apoM primarily being associated with HDL, is influenced by LDL receptor-mediated clearance of apoB-containing particles.-Christoffersen, C., M. Benn, P. M. Christensen, P. L. S. M. Gordts, A. J. M. Roebroek, R. Frikke-Schmidt, A. Tybjaerg-Hansen, B. Dahlback, and L. B. Nielsen. The plasma concentration of HDL-associated apoM is influenced by LDL receptor-mediated clearance of apoB-containing particles. J. Lipid Res. 2012. 53: 2198-2204

Inactivation of the proximal NPXY motif impairs early steps in LRP1 biosynthesis

The proximal NPXY and distal NPXYXXL motifs in the intracellular domain of LRP1 play an important role in regulation of the function of the receptor. The impact of single and double inactivating knock-in mutations of these motifs on receptor maturation, cell surface expression, and ligand internalization was analyzed in mutant and control wild-type mice and MEFs. Single inactivation of the proximal NPXY or in combination with inactivation of the distal NPXYXXL motif are both shown to be associated with an impaired maturation and premature proteasomal degradation of full-length LRP1. Therefore, only a small mature LRP1 pool is able to reach the cell surface resulting indirectly in severe impairment of ligand internalization. Single inactivation of the NPXYXXL motif revealed normal maturation, but direct impairment of ligand internalization. In conclusion, the proximal NPXY motif proves to be essential for early steps in the LRP1 biosynthesis, whereas NPXYXXL appears rather relevant for internalization

The functional role of the second npxy motif of the lrp1 beta-chain in tissue-type plasminogen activator-mediated activation of N-methyl-d-aspartate receptors

The low density lipoprotein receptor-related protein 1 (LRP1) emerges to play fundamental roles in cellular signaling pathways in the brain. One of its prominent ligands is the serine proteinase tissue-type plasminogen activator (tPA), which has been shown to act as a key activator of neuronal mitogen-activated protein kinase pathways via the N-methyl-D-aspartate (NMDA) receptor. However, here we set out to examine whether LRP1 and the NMDA receptor might eventually act in a combined fashion to mediate tPA downstream signaling. By blocking tPA from binding to LRP1 using the receptor-associated protein, we were able to completely inhibit NMDA receptor activation. Additionally, inhibition of NMDA receptor calcium influx with MK-801 resulted in dramatic reduction of tPA-mediated downstream signaling. This indicates a functional interaction between the two receptors, since both experimental approaches resulted in strongly reduced calcium influx and Erk1/2 phosphorylation. Additionally, we were able to inhibit Erk1/2 activation by competing for the LRP1 C-terminal binding motif with a truncated PSD95 construct resembling its PDZ III domain. Furthermore, we identified the distal NPXY amino acid motif in the C terminus of LRP1 as the crucial element for LRP1-NMDA receptor interaction via the adaptor protein PSD95. These results provide new insights into the mechanism of a tPA-induced, LRP1-mediated gating mechanism for NMDA receptors

Generation and Characterization of an <i>Nxf7</i> Knockout Mouse to Study <i>NXF5</i> Deficiency in a Patient with Intellectual Disability

<div><p>Members of the Nuclear eXport Factor (NXF) family are involved in the export of mRNA from the nucleus to the cytoplasm, or hypothesized to play a role in transport of cytoplasmic mRNA. We previously reported on the loss of <i>NXF5</i> in a male patient with a syndromic form of intellectual disability. To study the functional role of NXF5 we identified the mouse counterpart. Based on synteny, mouse <i>Nxf2</i> is the ortholog of human <i>NXF5</i>. However, we provide several lines of evidence that mouse Nxf7 is the actual functional equivalent of NXF5. Both Nxf7 and NXF5 are predominantly expressed in the brain, show cytoplasmic localization, and present as granules in neuronal dendrites suggesting a role in cytoplasmic mRNA metabolism in neurons. Nxf7 was primarily detected in the pyramidal cells of the hippocampus and in layer V of the cortex. Similar to human <i>NXF2</i>, mouse <i>Nxf2</i> is highly expressed in testis and shows a nuclear localization. Interestingly, these findings point to a different evolutionary path for both <i>NXF</i> genes in human and mouse. We thus generated and validated <i>Nxf7</i> knockout mice, which were fertile and did not present any gross anatomical or morphological abnormalities. Expression profiling in the hippocampus and the cortex did not reveal significant changes between wild-type and <i>Nxf7</i> knockout mice. However, impaired spatial memory was observed in these KO mice when evaluated in the Morris water maze test. In conclusion, our findings provide strong evidence that mouse Nxf7 is the functional counterpart of human <i>NXF5</i>, which might play a critical role in mRNA metabolism in the brain.</p></div