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

Novel insights into the multifaceted and tissue-specific roles of the endocytic receptor LRP1

Receptor-mediated endocytosis provides a mechanism for the selective uptake of specific molecules thereby controlling the composition of the extracellular environment and biological processes. The low-density lipoprotein receptor-related protein 1 (LRP1) is a widely expressed endocytic receptor that regulates cellular events by modulating the levels of numerous extracellular molecules via rapid endocytic removal. LRP1 also participates in signalling pathways through this modulation as well as in the interaction with membrane receptors and cytoplasmic adaptor proteins. LRP1 SNPs are associated with several diseases and conditions such as migraines, aortic aneurysms, cardiopulmonary dysfunction, corneal clouding, and bone dysmorphology and mineral density. Studies using Lrp1 KO mice revealed a critical, nonredundant and tissue-specific role of LRP1 in regulating various physiological events. However, exactly how LRP1 functions to regulate so many distinct and specific processes is still not fully clear. Our recent proteomics studies have identified more than 300 secreted proteins that either directly interact with LRP1 or are modulated by LRP1 in various tissues. This review will highlight the remarkable ability of this receptor to regulate secreted molecules in a tissue-specific manner and discuss potential mechanisms underpinning such specificity. Uncovering the depth of these "hidden" specific interactions modulated by LRP1 will provide novel insights into a dynamic and complex extracellular environment that is involved in diverse biological and pathological processes.

The Role of Impaired Receptor Trafficking in Mediating the Pathological Effects of APOE4 in Alzheimer's Disease

BACKGROUND

Apolipoprotein E4 (APOE4) is the most prevalent genetic risk factor of Alzheimer's disease. Several studies suggest that APOE4 binding to its receptors is associated with their internalization and accumulation in intracellular compartments. Importantly, this phenomenon also occurs with other, non-ApoE receptors. Based on these observations, we hypothesized that APOE4 pathological effects are mediated by impairment in the life cycle of distinct receptors (APOER2, LRP1, IR, VEGFR).

OBJECTIVE

To examine the effects of APOE genotype on receptors protein levels and compartmentalization.

METHODS

Primary mouse neurons were prepared from APOE3 or APOE4 targeted replacement mice, or APOE-KO mice. Specific receptors protein levels were evaluated in these neurons, utilizing immunofluorescent staining. Additionally, surface membrane protein levels of those receptors were assessed by cell surface biotinylation assay and ELISA. Receptors' colocalization with intracellular compartments was assessed by double staining and confocal microscopy, followed by colocalization analysis. Finally, LRP1 or APOER2 were knocked-down with CRISPR/Cas9 system to examine their role in mediating APOE4 effects on the receptors.

RESULTS

Our results revealed lower receptors' levels in APOE4, specifically on the membrane surface. Additionally, APOE4 affects the compartmentation of these receptors in two patterns: the first was observed with LRP1 and was associated with decreased receptor levels in numerous intracellular compartments. The second was obtained with the other receptors and was associated with their accumulation in early endosomes and their decrease in the late endosomes.

CONCLUSIONS

These results provide a unifying mechanism, in which APOE4 drives the down regulation of various receptors, which plays important roles in distinct APOE4 related pathological processes.

ANKS1A regulates LDL receptor-related protein 1 (LRP1)-mediated cerebrovascular clearance in brain endothelial cells

Brain endothelial LDL receptor-related protein 1 (LRP1) is involved in the clearance of Aβ peptides across the blood-brain barrier (BBB). Here we show that endothelial deficiency of ankyrin repeat and SAM domain containing 1 A (ANKS1A) reduces both the cell surface levels of LRP1 and the Aβ clearance across the BBB. Association of ANKS1A with the NPXY motifs of LRP1 facilitates the transport of LRP1 from the endoplasmic reticulum toward the cell surface. ANKS1A deficiency in an Alzheimer's disease mouse model results in exacerbated Aβ pathology followed by cognitive impairments. These deficits are reversible by gene therapy with brain endothelial-specific ANKS1A. In addition, human induced pluripotent stem cell-derived BBBs (iBBBs) were generated from endothelial cells lacking ANKS1A or carrying the rs6930932 variant. Those iBBBs exhibit both reduced cell surface LRP1 and impaired Aβ clearance. Thus, our findings demonstrate that ANKS1A regulates LRP1-mediated Aβ clearance across the BBB.

Low-density lipoprotein receptor-related protein-1 (LRP1) in the glial lineage modulates neuronal excitability

The low-density lipoprotein related protein receptor 1 (LRP1), also known as CD91 or α-Macroglobulin-receptor, is a transmembrane receptor that interacts with more than 40 known ligands. It plays an important biological role as receptor of morphogens, extracellular matrix molecules, cytokines, proteases, protease inhibitors and pathogens. In the CNS, it has primarily been studied as a receptor and clearance agent of pathogenic factors such as Aβ-peptide and, lately, Tau protein that is relevant for tissue homeostasis and protection against neurodegenerative processes. Recently, it was found that LRP1 expresses the Lewis-X (Lex) carbohydrate motif and is expressed in the neural stem cell compartment. The removal of Lrp1 from the cortical radial glia compartment generates a strong phenotype with severe motor deficits, seizures and a reduced life span. The present review discusses approaches that have been taken to address the neurodevelopmental significance of LRP1 by creating novel, lineage-specific constitutive or conditional knockout mouse lines. Deficits in the stem cell compartment may be at the root of severe CNS pathologies.

Mutation in the distal NPxY motif of LRP1 alleviates dietary cholesterol-induced dyslipidemia and tissue inflammation

The impairment of LDL receptor-related protein-1 (LRP1) in numerous cell types is associated with obesity, diabetes, and fatty liver disease. Here, we compared the metabolic phenotype of C57BL/6J wild-type and LRP1 knock-in mice carrying an inactivating mutation in the distal NPxY motif after feeding a low-fat diet or high-fat (HF) diet with cholesterol supplementation (HFHC) or HF diet without cholesterol supplementation. In response to HF feeding, both groups developed hyperglycemia, hyperinsulinemia, hyperlipidemia, increased adiposity, and adipose tissue inflammation and liver steatosis. However, LRP1 NPxY mutation prevents HFHC diet-induced hypercholesterolemia, reduces adipose tissue and brain inflammation, and limits liver progression to steatohepatitis. Nevertheless, this mutation does not protect against HFHC diet-induced insulin resistance. The selective metabolic improvement observed in HFHC diet-fed LRP1 NPxY mutant mice is due to an apparent increase of hepatic LDL receptor levels, leading to an elevated rate of plasma lipoprotein clearance and lower hepatic cholesterol levels. The unique metabolic phenotypes displayed by LRP1 NPxY mutant mice indicate an LRP1-cholesterol axis in modulating tissue inflammation. The LRP1 NPxY mutant mouse phenotype differs from phenotypes observed in mice with tissue-specific LRP1 inactivation, thus highlighting the importance of an integrative approach to evaluate how global LRP1 dysfunction contributes to metabolic disease development.

LRP1 Has a Predominant Role in Production over Clearance of Aβ in a Mouse Model of Alzheimer's Disease

The low-density lipoprotein receptor-related protein-1 (LRP1) has a dual role in the metabolism of the amyloid precursor protein (APP). In cellular models, LRP1 enhances amyloid-β (Aβ) generation via APP internalization and thus its amyloidogenic processing. However, conditional knock-out studies in mice define LRP1 as an important mediator for the clearance of extracellular Aβ from brain via cellular degradation or transcytosis across the blood-brain barrier (BBB). In order to analyze the net effect of LRP1 on production and clearance of Aβ in vivo, we crossed mice with impaired LRP1 function with a mouse model of Alzheimer's disease (AD). Analysis of Aβ metabolism showed that, despite reduced Aβ clearance due to LRP1 inactivation in vivo, less Aβ was found in cerebrospinal fluid (CSF) and brain interstitial fluid (ISF). Further analysis of APP metabolism revealed that impairment of LRP1 in vivo shifted APP processing from the Aβ-generating amyloidogenic cleavage by beta-secretase to the non-amyloidogenic processing by alpha-secretase as shown by a decrease in extracellular Aβ and an increase of soluble APP-α (sAPP-α). This shift in APP processing resulted in overall lower Aβ levels and a reduction in plaque burden. Here, we present for the first time clear in vivo evidence that global impairment of LRP1's endocytosis function favors non-amyloidogenic processing of APP due to its reduced internalization and subsequently, reduced amyloidogenic processing. By inactivation of LRP1, the inhibitory effect on Aβ generation overrules the simultaneous impaired Aβ clearance, resulting in less extracellular Aβ and reduced plaque deposition in a mouse model of AD.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

The concerted amyloid-beta clearance of LRP1 and ABCB1/P-gp across the blood-brain barrier is linked by PICALM

The accumulation of neurotoxic amyloid-beta (Aβ) in the brain is a characteristic hallmark of Alzheimer's disease (AD). The blood-brain barrier (BBB) provides a large surface area and has been shown to be an important mediator for removal of brain Aβ. Both, the ABC transporter P-glycoprotein (ABCB1/P-gp) and the receptor low-density lipoprotein receptor-related protein 1 (LRP1) have been implicated to play crucial roles in Aβ efflux from brain. Here, with immunoprecipitation experiments, co-immunostainings and dual inhibition of ABCB1/P-gp and LRP1, we show that both proteins are functionally linked, mediating a concerted transcytosis of Aβ through endothelial cells. Late-onset AD risk factor Phosphatidylinositol binding clathrin assembly protein (PICALM) is associated with both ABCB1/P-gp and LRP1 representing a functional link and guiding both proteins through the brain endothelium. Together, our results give more mechanistic insight on Aβ transport across the BBB and show that the functional interplay of different clearance proteins is needed for the rapid removal of Aβ from the brain.

Liver-enriched transcription factor expression relates to chronic hepatic failure in humans

The mechanisms by which the liver fails in end-stage liver disease remain elusive. Disruption of the transcription factor network in hepatocytes has been suggested to mediate terminal liver failure in animals. However, this hypothesis remains unexplored in human subjects. To study the relevance of transcription factor expression in terminal stages of chronic liver failure in humans, we analyzed the expression of liver-enriched transcription factors (LETFs) hepatocyte nuclear factor (HNF)4α, HNF1α, forkhead box protein A2 (FOXA2), CCAAT/enhancer-binding protein (CEBP)α, and CEBPβ. We then selected downstream genes responsible for some hepatic functions (ornithine transcarbamylase [OTC], cytochrome P450 3A4 [CYP3A4], coagulation factor VII [F7], cadherin 1 [CDH1], phospho-ezrin (Thr567)/radixin (Thr564)/moesin (Thr558) [p-ERM], phospho-myosin light chain [p-MLC], low-density lipoprotein receptor-related protein 1 [LRP1]) in liver tissue from patients at different stages of decompensated liver function based upon Child-Pugh classification, Model for End-Stage Liver Disease score, and degree of inflammatory activity/fibrosis. We first examined differential expression of LETF and determined whether a relationship exists between transcript and protein expression, and liver function. We found HNF4α expression was down-regulated and correlated well with the extent of liver dysfunction (P = 0.001), stage of fibrosis (P = 0.0005), and serum levels of total bilirubin (P = 0.009; r = 0.35), albumin (P < 0.001; r = 0.52), and prothrombin time activity (P = 0.002; r = 0.41). HNF4α expression also correlated with CYP3A4, OTC, and F7 as well as CDH1 RNA levels. The Rho/Rho-associated protein kinase pathways, which have been implicated in the regulation of HNF4α, were also differentially expressed, in concert with LRP1, a reported upstream regulator of RhoA function. Conclusion: HNF4α and other members of the LETFs appear to be important regulators of hepatocyte function in patients with chronic hepatic failure. (Hepatology Communications 2018;2:582-594).

Endothelial LRP1 - A Potential Target for the Treatment of Alzheimer's Disease : Theme: Drug Discovery, Development and Delivery in Alzheimer's Disease Guest Editor: Davide Brambilla

The accumulation of the neurotoxin beta-amyloid (Aβ) is a major hallmark in Alzheimer's disease (AD). Aβ homeostasis in the brain is governed by its production and various clearance mechanisms. Both pathways are influenced by the ubiquitously expressed low-density lipoprotein receptor-related protein 1 (LRP1). In cerebral blood vessels, LRP1 is an important mediator for the rapid removal of Aβ from brain via transport across the blood-brain barrier (BBB). Here, we summarize recent findings on LRP1 function and discuss the targeting of LRP1 as a modulator for AD pathology and drug delivery into the brain.

Low-Density Lipoprotein Receptor-Related Proteins in Skeletal Development and Disease

The identification of the low-density lipoprotein receptor (LDLR) provided a foundation for subsequent studies in lipoprotein metabolism, receptor-mediated endocytosis, and many other fundamental biological functions. The importance of the LDLR led to numerous studies that identified homologous molecules and ultimately resulted in the description of the LDL-receptor superfamily, a group of proteins that contain domains also found in the LDLR. Subsequent studies have revealed that members of the LDLR-related protein family play roles in regulating many aspects of signal transduction. This review is focused on the roles of selected members of this protein family in skeletal development and disease. We present background on the identification of this subgroup of receptors, discuss the phenotypes associated with alterations in their function in human patients and mouse models, and describe the current efforts to therapeutically target these proteins to treat human skeletal disease.

Precise genome editing in the silkworm Bombyx mori using TALENs and ds- and ssDNA donors - A practical approach

Engineered nucleases are able to introduce double stranded breaks at desired genomic locations. The breaks can be repaired by an error-prone non-homologous end joining (NHEJ) mechanism, or the repair process can be exploited to introduce precise DNA modifications by homology-directed repair (HDR) when provided with a suitable donor template. We designed a series of DNA donors including long dsDNA plasmids as well as short ssDNA oligonucleotides and compared the effectiveness of their utilization during gene targeting with highly efficient transcription activator-like effector nucleases (TALENs). While the use of long dsDNA donors for the incorporation of larger DNA fragments in Bombyx is still a problem, short single-stranded oligodeoxynucleotides (ssODNs) are incorporated quite efficiently. We show that appropriately designed ssODNs were integrated into germ cells in up to 79% of microinjected individuals and describe in more detail the conditions for the precise genome editing of Bombyx genes. We specify the donor sequence requirements that affected knock-in efficiency, and demonstrate the successful applications of this method of sequence deletion, insertion and replacement in the Bombyx genome.

Low-density lipoprotein receptor-related protein 1 is a novel modulator of radial glia stem cell proliferation, survival, and differentiation

The LDL family of receptors and its member low-density lipoprotein receptor-related protein 1 (LRP1) have classically been associated with a modulation of lipoprotein metabolism. Current studies, however, indicate diverse functions for this receptor in various aspects of cellular activities, including cell proliferation, migration, differentiation, and survival. LRP1 is essential for normal neuronal function in the adult CNS, whereas the role of LRP1 in development remained unclear. Previously, we have observed an upregulation of LewisX (LeX) glycosylated LRP1 in the stem cells of the developing cortex and demonstrated its importance for oligodendrocyte differentiation. In the current study, we show that LeX-glycosylated LRP1 is also expressed in the stem cell compartment of the developing spinal cord and has broader functions in the developing CNS. We have investigated the basic properties of LRP1 conditional knockout on the neural stem/progenitor cells (NSPCs) from the cortex and the spinal cord, created by means of Cre-loxp-mediated recombination in vitro. The functional status of LRP1-deficient cells has been studied using proliferation, differentiation, and apoptosis assays. LRP1 deficient NSPCs from both CNS regions demonstrated altered differentiation profiles. Their differentiation capacity toward oligodendrocyte progenitor cells (OPCs), mature oligodendrocytes and neurons was reduced. In contrast, astrocyte differentiation was promoted. Moreover, LRP1 deletion had a negative effect on NSPCs proliferation and survival. Our observations suggest that LRP1 facilitates NSPCs differentiation via interaction with apolipoprotein E (ApoE). Upon ApoE4 stimulation wild type NSPCs generated more oligodendrocytes, but LRP1 knockout cells showed no response. The effect of ApoE seems to be independent of cholesterol uptake, but is rather mediated by downstream MAPK and Akt activation. GLIA 2016 GLIA 2016;64:1363-1380.

Low density lipoprotein receptor-related protein 1 mediated endocytosis of β1-integrin influences cell adhesion and cell migration

The low density lipoprotein receptor-related protein 1 (LRP1) has been shown to interact with β1-integrin and regulate its surface expression. LRP1 knock-out cells exhibit altered cytoskeleton organization and decreased cell migration. Here we demonstrate coupled endocytosis of LRP1 and β1-integrin and the involvement of the intracellular NPxY2 motif of LRP1 in this process. Mouse embryonic fibroblasts harboring a knock in replacement of the NPxY2 motif of LRP1 by a multiple alanine cassette (AAxA) showed elevated surface expression of β1-integrin and decreased β1-integrin internalization rates. As a consequence, cell spreading was altered and adhesion rates were increased in our cell model. Cells formed more focal adhesion complexes, whereby in vitro cell migration rates were decreased. Similar results could be observed in a corresponding mouse model, the C57Bl6 LRP1 NPxYxxL knock in mice, therefore, the biochemistry of cellular adhesion was altered in primary cortical neurons. In vivo cell migration experiments demonstrated a disturbance of neuroblast cell migration along the rostral migratory stream. In summary, our results indicate that LRP1 interacts with β1-integrin mediating integrin internalization and thus correlates with downstream signaling of β1-integrin such as focal adhesion dynamics. Consequently, the disturbance of this interaction resulted in a dysfunction in in vivo and in vitro cell adhesion and cell migration.

The Matricellular Receptor LRP1 Forms an Interface for Signaling and Endocytosis in Modulation of the Extracellular Tumor Environment

The membrane protein low-density lipoprotein receptor related-protein 1 (LRP1) has been attributed a role in cancer. However, its presumably often indirect involvement is far from understood. LRP1 has both endocytic and signaling activities. As a matricellular receptor it is involved in regulation, mostly by clearing, of various extracellular matrix degrading enzymes including matrix metalloproteinases, serine proteases, protease inhibitor complexes, and the endoglycosidase heparanase. Furthermore, by binding extracellular ligands including growth factors and subsequent intracellular interaction with scaffolding and adaptor proteins it is involved in regulation of various signaling cascades. LRP1 expression levels are often downregulated in cancer and some studies consider low LRP1 levels a poor prognostic factor. On the contrary, upregulation in brain cancers has been noted and clinical trials explore the use of LRP1 as cargo receptor to deliver cytotoxic agents. This mini-review focuses on LRP1's role in tumor growth and metastasis especially by modulation of the extracellular tumor environment. In relation to this role its diagnostic, prognostic and therapeutic potential will be discussed.

Site-specific recombination in the chicken genome using Flipase recombinase-mediated cassette exchange

Targeted genome recombination has been applied in diverse research fields and has a wide range of possible applications. In particular, the discovery of specific loci in the genome that support robust and ubiquitous expression of integrated genes and the development of genome-editing technology have facilitated rapid advances in various scientific areas. In this study, we produced transgenic (TG) chickens that can induce recombinase-mediated gene cassette exchange (RMCE), one of the site-specific recombination technologies, and confirmed RMCE in TG chicken-derived cells. As a result, we established TG chicken lines that have, Flipase (Flp) recognition target (FRT) pairs in the chicken genome, mediated by piggyBac transposition. The transgene integration patterns were diverse in each TG chicken line, and the integration diversity resulted in diverse levels of expression of exogenous genes in each tissue of the TG chickens. In addition, the replaced gene cassette was expressed successfully and maintained by RMCE in the FRT predominant loci of TG chicken-derived cells. These results indicate that targeted genome recombination technology with RMCE could be adaptable to TG chicken models and that the technology would be applicable to specific gene regulation by cis-element insertion and customized expression of functional proteins at predicted levels without epigenetic influence.

Increased uptake of dietary retinoids at the maternal-fetal barrier in the nitrofen model of congenital diaphragmatic hernia

BACKGROUND/PURPOSE

The retinol signaling pathway is disrupted in congenital diaphragmatic hernia (CDH). Since there is no fetal retinol synthesis, maternal retinol has to cross the placenta. Nitrofen interferes with the retinol-binding protein (RBP) transfer pathway in CDH. However, in RBP knockout mice, retinol has been shown to be present. In this model, increased uptake of maternal dietary retinyl ester (RE) bounded in low-dense-lipoprotein (LDL) through low-density-lipoprotein-receptor 1 (LRP1) and increased activity of RE hydrolysis by lipoprotein-lipase (LPL) have been found. The aim of this study was to investigate the RE transfer pathway in the nitrofen CDH model.

METHODS

Pregnant rats were treated with nitrofen or vehicle on gestational day (D9) and sacrificed on D21. Immunohistochemistry was performed to evaluate LRP1 and LPL protein expression. Serum LDL levels were measured by ELISA. Pulmonary and serum retinoid levels were measured using HPLC.

RESULTS

Markedly increased trophoblastic and pulmonary LRP1 and LPL immunoreactivity were observed in CDH compared to controls. Significantly increased serum LDL and RE levels were observed in CDH compared to controls.

CONCLUSIONS

The increased uptake of dietary retinoids at the maternal-fetal barrier in the nitrofen CDH model suggests that the RE transfer pathway may be the main source of retinol in this model.

Generation of an allelic series of knock-in mice using recombinase-mediated cassette exchange (RMCE)

Molecular genetic strategies applying embryonic stem cell (ES cell) technologies to study the function of a gene in mice or to generate a mouse model for a human disease are continuously under development. Next to (conditional) inactivation of genes the application and importance of approaches to generate knock-in mutations are increasing. In this chapter the principle and application of recombinase-mediated cassette exchange (RMCE) are discussed as being a new emerging knock-in strategy, which enables easy generation of a series of different knock-in mutations within one gene. An RMCE protocol, which was used to generate a series of different knock-in mutations in the Lrp1 gene of ES cells, is described in detail as an example of how RMCE can be used to generate highly efficiently an allelic series of differently modified ES cell clones from a parental modified ES cell clone. Subsequently the differently modified ES cell clones can be used to generate an allelic series of mutant knock-in mice.

Low-density lipoprotein receptors in liver: old acquaintances and a newcomer

The lipoprotein receptors low-density lipoprotein receptor (LDLR), the low-density lipoprotein receptor-related protein 1 (LRP1) and megalin/LRP2 share characteristic structural elements. In addition to their well-known roles in endocytosis of lipoproteins and systemic lipid homeostasis, it has been established that LRP1 mediates the endocytotic clearance of a multitude of extracellular ligands and regulates diverse signaling processes such as growth factor signaling, inflammatory signaling pathways, apoptosis, and phagocytosis in liver. Here, possible functions of LRP1 expression in hepatocytes and non-parenchymal cells in healthy and injured liver are discussed. Recent studies indicate the expression of megalin (LRP2) by hepatic stellate cells, myofibroblasts and Kupffer cells and hypothesize that LRP2 might represent another potential regulator of hepatic inflammatory processes. These observations provide the experimental framework for the systematic and dynamic analysis of the LDLR family during chronic liver injury and fibrogenesis.

LRP1 is critical for the surface distribution and internalization of the NR2B NMDA receptor subtype

Background

The N-methyl-D-aspartate receptors are key mediators of excitatory transmission and are implicated in many forms of synaptic plasticity. These receptors are heterotetrameres consisting of two obligatory NR1 and two regulatory subunits, usually NR2A or NR2B. The NR2B subunits are abundant in the early postnatal brain, while the NR2A/NR2B ratio increases during early postnatal development. This shift is driven by NMDA receptor activity. A functional interplay of the Low Density Lipoprotein Receptor Related Protein 1 (LRP1) NMDA receptor has already been reported. Such abilities as interaction of LRP1 with NMDA receptor subunits or its important role in tPa-mediated NMDA receptor signaling were already demonstrated. Moreover, mice harboring a conditional neuronal knock-out mutation of the entire Lrp1 gene display NMDA-associated behavioral changes. However, the exact role of LRP1 on NMDA receptor function remains still elusive.

Results

To provide a mechanistic explanation for such effects we investigated whether an inactivating knock-in mutation into the NPxY2 motif of LRP1 might influence the cell surface expression of LRP1 and NMDA receptors in primary cortical neurons. Here we demonstrate that a knock-in into the NPxY2 motif of LRP1 results in an increased surface expression of LRP1 and NR2B NMDA receptor subunit due to reduced endocytosis rates of LRP1 and the NR2B subunit in primary neurons derived from LRP1ΔNPxY2 animals. Furthermore, we demonstrate an altered phosphorylation pattern of S1480 and Y1472 in the NR2B subunit at the surface of LRP1ΔNPxY2 neurons, while the respective kinases Fyn and casein kinase II are not differently regulated compared with wild type controls. Performing co-immunoprecipitation experiments we demonstrate that binding of LRP1 to NR2B might be linked by PSD95, is phosphorylation dependent and this regulation mechanism is impaired in LRP1ΔNPxY2 neurons. Finally, we demonstrate hyperactivity and changes in spatial and reversal learning in LRP1ΔNPxY2 mice, confirming the mechanistic interaction in a physiological readout.

Conclusions

In summary, our data demonstrate that LRP1 plays a critical role in the regulation of NR2B expression at the cell surface and may provide a mechanistic explanation for the behavioral abnormalities detected in neuronal LRP1 knock-out animals reported earlier.

Endocytic receptor-mediated control of morphogen signaling

Receptor-mediated endocytosis provides a mechanism by which cells take up signaling molecules from the extracellular space. Recent studies have shown that one class of endocytic receptors, the low-density lipoprotein receptor-related proteins (LRPs), is of particular relevance for embryonic development. In this Primer, we describe how LRPs constitute central pathways that modulate morphogen presentation to target tissues and cellular signal reception, and how LRP dysfunction leads to developmental disturbances in many species.

Impaired LDL receptor-related protein 1 translocation correlates with improved dyslipidemia and atherosclerosis in apoE-deficient mice

Objective

Determination of the in vivo significance of LDL receptor-related protein 1 (LRP1) dysfunction on lipid metabolism and atherosclerosis development in absence of its main ligand apoE.

Methods and Results

LRP1 knock-in mice carrying an inactivating mutation in the NPxYxxL motif were crossed with apoE-deficient mice. In the absence of apoE, relative to LRP1 wild-type animals, LRP1 mutated mice showed an increased clearance of postprandial lipids despite a compromised LRP1 endocytosis rate and inefficient insulin-mediated translocation of the receptor to the plasma membrane, likely due to inefficient slow recycling of the mutated receptor. Postprandial lipoprotein improvement was explained by increased hepatic clearance of triglyceride-rich remnant lipoproteins and accompanied by a compensatory 1.6-fold upregulation of LDLR expression in hepatocytes. One year-old apoE-deficient mice having the dysfunctional LRP1 revealed a 3-fold decrease in spontaneous atherosclerosis development and a 2-fold reduction in LDL-cholesterol levels.

Conclusion

These findings demonstrate that the NPxYxxL motif in LRP1 is important for insulin-mediated translocation and slow perinuclear endosomal recycling. These LRP1 impairments correlated with reduced atherogenesis and cholesterol levels in apoE-deficient mice, likely via compensatory LDLR upregulation.

A novel pathway combining calreticulin exposure and ATP secretion in immunogenic cancer cell death

Surface-exposed calreticulin (ecto-CRT) and secreted ATP are crucial damage-associated molecular patterns (DAMPs) for immunogenic apoptosis. Inducers of immunogenic apoptosis rely on an endoplasmic reticulum (ER)-based (reactive oxygen species (ROS)-regulated) pathway for ecto-CRT induction, but the ATP secretion pathway is unknown. We found that after photodynamic therapy (PDT), which generates ROS-mediated ER stress, dying cancer cells undergo immunogenic apoptosis characterized by phenotypic maturation (CD80(high), CD83(high), CD86(high), MHC-II(high)) and functional stimulation (NO(high), IL-10(absent), IL-1β(high)) of dendritic cells as well as induction of a protective antitumour immune response. Intriguingly, early after PDT the cancer cells displayed ecto-CRT and secreted ATP before exhibiting biochemical signatures of apoptosis, through overlapping PERK-orchestrated pathways that require a functional secretory pathway and phosphoinositide 3-kinase (PI3K)-mediated plasma membrane/extracellular trafficking. Interestingly, eIF2α phosphorylation and caspase-8 signalling are dispensable for this ecto-CRT exposure. We also identified LRP1/CD91 as the surface docking site for ecto-CRT and found that depletion of PERK, PI3K p110α and LRP1 but not caspase-8 reduced the immunogenicity of the cancer cells. These results unravel a novel PERK-dependent subroutine for the early and simultaneous emission of two critical DAMPs following ROS-mediated ER stress.

Generation of a series of knock-in alleles using RMCE in ES cells

Recombinase-mediated cassette exchange (RMCE) is a powerful tool to generate a series of knock-in mutations into a particular gene of the mouse. It uses standard ES cell technologies to introduce an exchangeable cassette into the gene of interest by homologous recombination. Because the introduced exchangeable cassette is flanked by heterotypic specific recombination target sequences, site-specific recombinases can be used in a subsequent RMCE step to exchange the cassette by other sequences. Here, an experimental procedure for the application of RMCE in E14 ES cells using heterotypic FRT recombination target sequences and the site-specific recombinase Flp is elaborated.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.

Knock-in approaches

Molecular genetic strategies to study gene function in mice or to generate a mouse model for a human disease are continuously under development. The application and importance of knock-in approaches are increasing. This chapter elaborates on novel developments for the generation of knock-in mice. Special emphasis is given to recombinase-mediated cassette exchange, a new emerging knock-in strategy that enables easy generation of a series of different knock-in mutations within one gene.

Signaling through LRP1: Protection from atherosclerosis and beyond

The low-density lipoprotein receptor-related protein (LRP1) is a multifunctional cell surface receptor that belongs to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 consists of an 85-kDa membrane-bound carboxyl fragment (β chain) and a non-covalently attached 515-kDa (α chain) amino-terminal fragment. Through its extracellular domain, LRP1 binds at least 40 different ligands ranging from lipoprotein and protease inhibitor complex to growth factors and extracellular matrix proteins. LRP-1 has also been shown to interact with scaffolding and signaling proteins via its intracellular domain in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. LRP-1 is thus implicated in two major physiological processes: endocytosis and regulation of signaling pathways, which are both involved in diverse biological roles including lipid metabolism, cell growth/differentiation processes, degradation of proteases, and tissue invasion. The embryonic lethal phenotype obtained after target disruption of the LRP-1 gene in the mouse highlights the biological importance of this receptor and revealed a critical, but yet undefined role in development. Tissue-specific gene deletion studies also reveal an important contribution of LRP1 in vascular remodeling, foam cell biology, the central nervous system, and in the molecular mechanisms of atherosclerosis.

LRP1 mediates bidirectional transcytosis of amyloid-β across the blood-brain barrier

According to the "amyloid hypothesis", the amyloid-β (Aβ) 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 Aβ out of the brain across the blood-brain barrier (BBB). To provide genetic evidence for LRP1-mediated transcytosis of Aβ across the BBB we analyzed Aβ 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]-Aβ(1-40) across pMBCECs in both directions, whereas no role for LRP1-mediated Aβ degradation was detected. Analysis of [(125)I]-Aβ(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 Aβ 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 Aβ in the brain by clearing Aβ across the BBB.

Efficient conditional and promoter-specific in vivo expression of cDNAs of choice by taking advantage of recombinase-mediated cassette exchange using FlEx gene traps

Recombinase-mediated cassette exchange (RMCE) exploits the possibility to unidirectionally exchange any genetic material flanked by heterotypic recombinase recognition sites (RRS) with target sites in the genome. Due to a limited number of available pre-fabricated target sites, RMCE in mouse embryonic stem (ES) cells has not been tapped to its full potential to date. Here, we introduce a universal system, which allows the targeted insertion of any given transcriptional unit into 85 742 previously annotated retroviral conditional gene trap insertions, representing 7013 independent genes in mouse ES cells, by RMCE. This system can be used to express any given cDNA under the control of endogenous trapped promoters in vivo, as well as for the generation of transposon ‘launch pads’ for chromosomal region-specific ‘Sleeping Beauty’ insertional mutagenesis. Moreover, transcription of the gene-of-interest is only activated upon Cre-recombinase activity, a feature that adds conditionality to this expression system, which is demonstrated in vivo. The use of the RMCE system presented in this work requires one single-cloning step followed by one overnight gateway clonase reaction and subsequent cassette exchange in ES cells with efficiencies of 40% in average.

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.

Inactivation of the LRP1 intracellular NPxYxxL motif in LDLR-deficient mice enhances postprandial dyslipidemia and atherosclerosis

OBJECTIVE

The purpose of this study was to determine the significance of the intracellular NPxYxxL motif of LRP1 for the atheroprotective role of this multifunctional receptor.

METHODS AND RESULTS

LRP1 knock-in mice carrying an inactivating mutation in the NPxYxxL motif were crossed with LDLR-deficient mice, a model for atherosclerosis. In this LDLR(-/-) background the mutated mice showed a more atherogenic lipoprotein profile, which was associated with a decreased clearance of postprandial lipids because of a compromised endocytosis rate and reduced lipase activity. On an atherogenic diet LRP1 mutant mice revealed a 50% increased development of atherosclerosis. This aggravation was accompanied by an increase in smooth muscle cell (SMC) and collagen content and apoptotic cells in the lesions. The mutation showed, however, a limited impact on basal PDGFR-beta expression and signaling and the antimigratory property of apoE on PDGF-BB-stimulated SMCs. Additionally, levels of LRP1 atherogenic ligands, like MMP2, t-PA, FVIII, and the inflammatory ligand TNF-alpha showed to be significantly elevated.

CONCLUSIONS

These findings demonstrate that the NPxYxxL motif is essential for the atheroprotective role of LRP1. This motif is relevant for normal control of lipid metabolism and of atherogenic and inflammatory ligands, but has no pronounced effect on regulating PDGF-BB/PDGFR-beta signaling in SMCs.

Efficient generation of long-distance conditional alleles using recombineering and a dual selection strategy in replicate plates

BACKGROUND

Conditional knockout mice are a useful tool to study the function of gene products in a tissue-specific or inducible manner. Classical approaches to generate targeting vectors for conditional alleles are often limited by the availability of suitable restriction sites. Furthermore, plasmid-based targeting vectors can only cover a few kB of DNA which precludes the generation of targeting vectors where the two loxP sites are placed far apart. These limitations have been overcome in the recent past by using homologous recombination of bacterial artificial chromosomes (BACs) in Escherichia coli to produce large targeting vector containing two different loxP-flanked selection cassettes so that a single targeting event is sufficient to introduce loxP-sites a great distances into the mouse genome. However, the final targeted allele should be free of selection cassettes and screening for correct removal of selection cassettes can be a laborious task. Therefore, we developed a new strategy to rapidly identify ES cells containing the desired allele.

RESULTS

Using BAC recombineering we generated a single targeting vector which contained two different selection cassettes that were flanked by loxP-loxP sites or by FRT-FRT/loxP sites so that they could be deleted sequentially by Cre- and FLPe-recombinases, respectively. Transfected ES cells were first selected in the presence of both antibiotics in vitro before correctly targeted clones were identified by Southern blot. After transfection of a Cre recombinase expression plasmid ES cell clones were selected on replicate plates to identify those clones which maintained the FRT-FRT/loxP flanked cassette and lost the loxP-loxP flanked cassette. Using this strategy facilitated the identification of ES cell clones containing the desired allele before blastocyst injection.

CONCLUSION

The strategy of ES cell cultures in replicate plates proved to be very efficient in identifying ES cells that had undergone the correct recombination event. This approach facilitates the generation of conditional knock-out mice when large parts of the genome are intended to be flanked by loxP sites.

Mouse model of OPRM1 (A118G) polymorphism has sex-specific effects on drug-mediated behavior

A single nucleotide polymorphism (SNP) in the human mu-opioid receptor gene (OPRM1 A118G) has been widely studied for its association in a variety of drug addiction and pain sensitivity phenotypes; however, the extent of these adaptations and the mechanisms underlying these associations remain elusive. To clarify the functional mechanisms linking the OPRM1 A118G SNP to addiction and analgesia phenotypes, we derived a mouse model possessing the equivalent nucleotide/amino acid substitution in the Oprm1 gene. Mice harboring this SNP (A112G) demonstrated several phenotypic similarities to humans carrying the A118G SNP, including reduced mRNA expression and morphine-mediated antinociception. We found additional phenotypes associated with this SNP including significant reductions of receptor protein levels, morphine-mediated hyperactivity, and the development of locomotor sensitization in mice harboring the G112 allele. In addition, we found sex-specific reductions in the rewarding properties of morphine and the aversive components of naloxone-precipitated morphine withdrawal. Further cross-species analysis will allow us to investigate mechanisms and adaptations present in humans carrying this SNP.

Polarized traffic of LRP1 involves AP1B and SNX17 operating on Y-dependent sorting motifs in different pathways

Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic recycling receptor with two cytoplasmic tyrosine-based basolateral sorting signals. Here we show that during biosynthetic trafficking LRP1 uses AP1B adaptor complex to move from a post-TGN recycling endosome (RE) to the basolateral membrane. Then it recycles basolaterally from the basolateral sorting endosome (BSE) involving recognition by sorting nexin 17 (SNX17). In the biosynthetic pathway, Y(29) but not N(26) from a proximal NPXY directs LRP1 basolateral sorting from the TGN. A N(26)A mutant revealed that this NPXY motif recognized by SNX17 is required for the receptor's exit from BSE. An endocytic Y(63)ATL(66) motif also functions in basolateral recycling, in concert with an additional endocytic motif (LL(86,87)), by preventing LRP1 entry into the transcytotic apical pathway. All this sorting information operates similarly in hippocampal neurons to mediate LRP1 somatodendritic distribution regardless of the absence of AP1B in neurons. LRP1 basolateral distribution results then from spatially and temporally segregation steps mediated by recognition of distinct tyrosine-based motifs. We also demonstrate a novel function of SNX17 in basolateral/somatodendritic recycling from a different compartment than AP1B endosomes.

Structural and functional consequences of tyrosine phosphorylation in the LRP1 cytoplasmic domain

The cytoplasmic domain of LRP1 contains two NPXY motifs that have been shown to interact with signaling proteins. In previous work, we showed that Tyr(4507) in the distal NPXY motif is phosphorylated by v-Src, whereas denaturation of the protein was required for phosphorylation of Tyr(4473) in the membraneproximal NPXY motif. Amide H/D exchange studies reveal that the distal NPXY motif is fully solvent-exposed, whereas the proximal one is not. Phosphopeptide mapping combined with in vitro and in vivo kinase experiments show that Tyr(4473) can be phosphorylated, but only if Tyr(4507) is phosphorylated or substituted with glutamic acid. Amide H/D exchange experiments indicate that solvent accessibility increases across the entire LRP1 cytoplasmic region upon phosphorylation at Tyr(4507); in particular the NPXY(4473) motif becomes much more exposed. This differential phosphorylation is functionally relevant: binding of Snx17, which is known to bind at the proximal NPXY motif, is inhibited by phosphorylation at Tyr(4473). Conversely, Shp2 binds most strongly when both of the NPXY motifs in LRP1 are phosphorylated.

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.

Recombinase-mediated cassette exchange reveals the selective use of Gq/G11-dependent and -independent endothelin 1/endothelin type A receptor signaling in pharyngeal arch development

The endothelin (Edn) system comprises three ligands (Edn1, Edn2 and Edn3) and their G-protein-coupled type A (Ednra) and type B (Ednrb) receptors. During embryogenesis, the Edn1/Ednra signaling is thought to regulate the dorsoventral axis patterning of pharyngeal arches via Dlx5/Dlx6 upregulation. To further clarify the underlying mechanism, we have established mice in which gene cassettes can be efficiently knocked-in into the Ednra locus using recombinase-mediated cassette exchange (RMCE) based on the Cre-lox system. The first homologous recombination introducing mutant lox-flanked Neo resulted in homeotic transformation of the lower jaw to an upper jaw, as expected. Subsequent RMCE-mediated knock-in of lacZ targeted its expression to the cranial/cardiac neural crest derivatives as well as in mesoderm-derived head mesenchyme. Knock-in of Ednra cDNA resulted in a complete rescue of craniofacial defects of Ednra-null mutants. By contrast, Ednrb cDNA could not rescue them except for the most distal pharyngeal structures. At early stages, the expression of Dlx5, Dlx6 and their downstream genes was downregulated and apoptotic cells distributed distally in the mandible of Ednrb-knock-in embryos. These results, together with similarity in craniofacial defects between Ednrb-knock-in mice and neural-crest-specific Galpha(q)/Galpha(11)-deficient mice, indicate that the dorsoventral axis patterning of pharyngeal arches is regulated by the Ednra-selective, G(q)/G(11)-dependent signaling, while the formation of the distal pharyngeal region is under the control of a G(q)/G(11)-independent signaling, which can be substituted by Ednrb. This RMCE-mediated knock-in system can serve as a useful tool for studies on gene functions in craniofacial development.

Mutational analysis of the FXNPXY motif within LDL receptor-related protein 1 (LRP1) reveals the functional importance of the tyrosine residues in cell growth regulation and signal transduction

LRP1 [LDL (low-density lipoprotein) receptor-related protein 1]-null CHO cells (Chinese-hamster ovary cells) (13-5-1 cells) exhibited accelerated cell growth and severe tumour progression after they were xenografted into nude mice. Reconstitution of LRP1 expression in these cells, either with the full-length protein or with a minireceptor, reduced growth rate as well as suppressed tumour development. We tested the role of the tyrosine residue in the FXNPXY63 motif within the LRP1 cytoplasmic domain in signal transduction and cell growth inhibition by site-specific mutagenesis. The LRP1 minireceptors harbouring Tyr63 to alanine or Tyr63 to phenylalanine substitution had diametrically opposite effects on cell growth, cell morphology and tumour development in mice. The Y63F-expressing cells showed suppressed cell growth and tumour development, which were associated with decreased beta-catenin and cadherin concentrations in the cells. On the other hand, the Y63A-expressing cells lacked inhibition on cell growth and tumour development, which were associated with hyperactivation of ERKs (extracellular-signal-regulated kinases), FAK (focal adhesion kinase) and cyclin D1 in the cells. The mutant Y63A minireceptor also exhibited reduced capacity in binding to the Dab2 (disabled 2) adaptor protein. In addition, the Y63A mutant showed increased caveolar localization, and cells expressing Y63A had altered caveolae architecture. However, tyrosine to alanine substitution at the other NPXY29 motif had no effect on cell growth or tumorigenesis. These results suggest that the FXNPXY63 motif of LRP1 not only governs cellular localization of the receptor but also exerts multiple functional effects on signalling pathways involved in cell growth regulation.

Mosaic complementation demonstrates a regulatory role for myosin VIIa in actin dynamics of stereocilia

We have developed a bacterial artificial chromosome transgenesis approach that allowed the expression of myosin VIIa from the mouse X chromosome. We demonstrated the complementation of the Myo7a null mutant phenotype producing a fine mosaic of two types of sensory hair cells within inner ear epithelia of hemizygous transgenic females due to X inactivation. Direct comparisons between neighboring auditory hair cells that were different only with respect to myosin VIIa expression revealed that mutant stereocilia are significantly longer than those of their complemented counterparts. Myosin VIIa-deficient hair cells showed an abnormally persistent tip localization of whirlin, a protein directly linked to elongation of stereocilia, in stereocilia. Furthermore, myosin VIIa localized at the tips of all abnormally short stereocilia of mice deficient for either myosin XVa or whirlin. Our results strongly suggest that myosin VIIa regulates the establishment of a setpoint for stereocilium heights, and this novel role may influence their normal staircase-like arrangement within a bundle.

Lipoproteins and their receptors in embryonic development: more than cholesterol clearance

Previously, the relevance of lipoproteins and their receptors has mainly been discussed in terms of cholesterol clearance in the adult organism. Now, findings from nematodes to fruit flies to mammals all point towards novel and unexpected roles for lipoprotein metabolism in the control of key regulatory pathways in the developing embryo, including signaling through steroid hormones and throughout the hedgehog and Wnt signaling pathways. Here, we discuss the emerging view of how lipoproteins and their receptors regulate embryogenesis.

Road to precision: recombinase-based targeting technologies for genome engineering

In the past years, recombinase-based approaches for integrating transgenes into defined chromosomal loci of mammalian cells have gained increasing attention. This method is attractive since it enables to precisely integrate transgenes of interest into pre-defined integration sites, thereby allowing to predict the expression properties of a genetically manipulated cell. This review focuses on the current state of targeting strategies including RMCE employing site-specific recombinases such as Cre, Flp and PhiC31. In particular, applications for protein expression, virus production, transgenic animals and chromosome engineering are described.

Recombinase-mediated cassette exchange to rapidly and efficiently generate mice with human cardiac sodium channels

SCN5A encodes the predominant voltage-gated sodium channel isoform in human heart and nearly 100 variants have now been described and studied in vitro. However, development of animal models to analyze function of such large numbers of human gene variants represents a continuing challenge in translational medicine. Here, we describe the implementation of a two stage procedure, recombinase-mediated cassette exchange (RMCE), to efficiently and rapidly generate mice in which a full-length human cDNA replaces expression of the murine ortholog. In the first step of RMCE, conventional homologous recombination in mouse ES cells was used to replace scn5a exon 2 (that contains the translation start site) with a cassette acceptor that includes the thymidine kinase gene, flanked by loxP/inverted loxP sites. In the second step, the cassette acceptor site was replaced by the full-length wild-type human SCN5A cDNA by Cre/loxP-mediated recombination. The exchange event occurred in 7/29 (24%) colonies, and the time from electroporation to first homozygotes was only 8 months. PCR-restriction fragment length polymorphism (RFLP) showed that the murine isoform was replaced by the human one, and functional studies indicated that mice with human cardiac sodium channels have wild-type sodium current density, action potential durations, heart rates, and QRS durations. These data demonstrate that RMCE can be used to generate mice in which a targeted allele can be rapidly and efficiently replaced by variants of choice, and thereby can serve as an enabling approach for the functional characterization of ion channel and other DNA variants.

The LDL receptor-related protein (LRP) family: an old family of proteins with new physiological functions

The low-density lipoprotein (LDL) receptor is the founding member of a family of seven structurally closely related transmembrane proteins (LRP1, LRP1b, megalin/LRP2, LDL receptor, very low-density lipoprotein receptor, MEGF7/LRP4, LRP8/apolipoprotein E receptor2). These proteins participate in a wide range of physiological processes, including the regulation of lipid metabolism, protection against atherosclerosis, neurodevelopment, and transport of nutrients and vitamins. While currently available data suggest that the role of the LDL receptor is limited to the regulation of cholesterol homeostasis by receptor-mediated endocytosis of lipoprotein particles, there is growing experimental evidence that the other members of the gene family have additional physiological functions as signal transducers. In this review, we focus on the latest discovered functions of two major members of this family, LRP1 and megalin/LRP2, and on the newly elucidated physiological role of a third member of the family, MEGF7/LRP4, which can also function as a modulator of diverse signaling pathways during development.

Functional role of the low-density lipoprotein receptor-related protein in Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder, characterized by neuronal loss, neurofibrillary tangle formation and the extracellular deposition of amyloid-beta (Abeta) plaques. The amyloid precursor protein (APP) and the enzymes responsible for Abeta generation seem to be the base elements triggering the destructive processes. Initially, the low-density lipoprotein receptor-related protein (LRP) was genetically linked to AD and later it emerged to impact on many fundamental events related to this disease. LRP is not only involved in Abeta clearance but is also the major receptor of several AD-associated ligands, e.g. apolipoprotein E and alpha2-macroglobulin. APP processing is mediated by LRP on many levels. Enhanced APP internalization through LRP decreases cell surface APP levels and thereby reduces APP shedding. As a consequence of increased APP internalization LRP enhances Abeta secretion. These effects could be attributed to the cytoplasmic tails of LRP and APP. The receptors bind via their NPXY motifs to the two PID domains of FE65 and form a tripartite complex. However, it appears that the second NPVY motif of LRP is the one responsible for the observed influence over APP metabolism. A more in-depth knowledge of the mechanisms regulating APP cleavage may offer additional targets for therapeutic intervention.