OCRL1 Deficiency Affects the Intracellular Traffic of ApoER2 and Impairs Reelin-Induced Responses.

Lowe Syndrome (LS) is a rare X-linked disorder characterized by renal dysfunction, cataracts, and several central nervous system (CNS) anomalies. The mechanisms underlying the neurological dysfunction in LS remain unclear, albeit they share some phenotypic characteristics similar to the deficiency or dysfunction of the Reelin signaling, a relevant pathway with roles in CNS development and neuronal functions. In this study, we investigated the role of OCRL1, an inositol polyphosphate 5-phosphatase encoded by the OCRL gene, mutated in LS, focusing on its impact on endosomal trafficking and receptor recycling in human neuronal cells. Specifically, we tested the effects of OCRL1 deficiency in the trafficking and signaling of ApoER2/LRP8, a receptor for the ligand Reelin. We found that loss of OCRL1 impairs ApoER2 intracellular trafficking, leading to reduced receptor expression and decreased levels at the plasma membrane. Additionally, human neurons deficient in OCRL1 showed impairments in ApoER2/Reelin-induced responses. Our findings highlight the critical role of OCRL1 in regulating ApoER2 endosomal recycling and its impact on the ApoER2/Reelin signaling pathway, providing insights into potential mechanisms underlying the neurological manifestations of LS.

Participation of OCRL1, and APPL1, in the expression, proteolysis, phosphorylation and endosomal trafficking of megalin: Implications for Lowe Syndrome.

Megalin/LRP2 is the primary multiligand receptor for the re-absorption of low molecular weight proteins in the proximal renal tubule. Its function is significantly dependent on its endosomal trafficking. Megalin recycling from endosomal compartments is altered in an X-linked disease called Lowe Syndrome (LS), caused by mutations in the gene encoding for the phosphatidylinositol 5-phosphatase OCRL1. LS patients show increased low-molecular-weight proteins with reduced levels of megalin ectodomain in the urine and accumulation of the receptor in endosomal compartments of the proximal tubule cells. To gain insight into the deregulation of megalin in the LS condition, we silenced OCRL1 in different cell lines to evaluate megalin expression finding that it is post-transcriptionally regulated. As an indication of megalin proteolysis, we detect the ectodomain of the receptor in the culture media. Remarkably, in OCRL1 silenced cells, megalin ectodomain secretion appeared significantly reduced, according to the observation in the urine of LS patients. Besides, the silencing of APPL1, a Rab5 effector associated with OCRL1 in endocytic vesicles, also reduced the presence of megalin's ectodomain in the culture media. In both silencing conditions, megalin cell surface levels were significantly decreased. Considering that GSK3ß-mediated megalin phosphorylation reduces receptor recycling, we determined that the endosomal distribution of megalin depends on its phosphorylation status and OCRL1 function. As a physiologic regulator of GSK3ß, we focused on insulin signaling that reduces kinase activity. Accordingly, megalin phosphorylation was significantly reduced by insulin in wild-type cells. Moreover, even though in cells with low activity of OCRL1 the insulin response was reduced, the phosphorylation of megalin was significantly decreased and the receptor at the cell surface increased, suggesting a protective role of insulin in a LS cellular model.

Síndrome de Lowe, enfermedad renal crónica y síndrome de Lennox-Gastaut / Lowe syndrome, chronic kidney disease and Lennox-Gastaut syndrome

Introducción: El síndrome de Lowe o síndrome oculocerebrorenal es un trastorno multisistémico. Es una enfermedad rara e infrecuente. Objetivo: Exponer un caso clínico típico, con fracaso renal controlado sin método dialítico y que de manera tardía en su adolescencia se diagnostica con síndrome de Lennox-Gastaut. Presentación caso: Adolescente masculino de 18 años nacido de un embarazo sin riesgo y parto por cesárea, macrofeto. Desde el nacimiento, con diagnóstico perinatal de catarata congénita y evolutivamente glaucoma bilateral con tratamiento conservador. A partir del primer año de vida padece de acidosis tubular y síndrome de Fanconi con evolución a una enfermedad renal crónica. Relacionado todo con retraso del desarrollo psicomotor, discapacidad intelectual, estereotipias, síntomas obsesivos compulsivos y depresión. De manera tardía, a los 10 años tuvo inicio de crisis epilépticas de varios tipos con predominio de las tónicas durante el sueño y en vigilia, frecuentes, con electroencefalograma característico de síndrome Lennox-Gastaut. Tuvo atención interdisciplinaria y evolución clínica estable hasta la edad adulta cumplida. El estudio molecular de ADN materno y del niño confirmaron la mutación c2224_2226 del GTA (exón 19), delección de valina en la posición 742 del cromosoma X del OCRL 1, que ratifica el síndrome de Lowe. Conclusiones: El Síndrome de Lowe, es un trastorno multisistémico, ligado al cromosoma X y frecuente en varones. Clínicamente se caracteriza por alteraciones oftalmológicas, renales y neurológicas(AU)

Introduction: Lowe syndrome or oculocerebrorenal syndrome is a multisystemic disorder. It's a rare and uncommon disease. Objective: Show a typical clinical case, with controlled renal failure without dialytic method and that late in the adolescence is diagnosed with Lennox-Gastaut syndrome. Case presentation: 18-year-old male adolescent born of a risk-free pregnancy and cesarean delivery, with macrosomia. From birth, he had a perinatal diagnosis of congenital cataract and evolutionarily bilateral glaucoma with conservative treatment. From the first year of life he suffers from tubular acidosis and Fanconi syndrome with an evolution to chronic kidney disease. All of these were related to delayed psychomotor development, intellectual disability, stereotypies, obsessive-compulsive symptoms and depression. Lately, at 10 years old, epileptic seizures of various types with predominance of tonic ones during sleep and wakefulness began; they were frequent, with characteristic electroencephalogram of Lennox-Gastaut syndrome. He had interdisciplinary care and stable clinical evolution into adulthood. The molecular study of the mother and child DNA confirmed the c2224_2226 mutation of GTA (exon 19), valine deletion at position 742 of the X chromosome of OCRL 1, which ratifies Lowe syndrome. Conclusion: Lowe Syndrome is a multisystemic disorder, linked to the X chromosome and common in males. Clinically, it is characterized by ophthalmological, renal and neurological alterations(AU)

New Insights into the Roles of Megalin/LRP2 and the Regulation of its Functional Expression

Since the discovery of the low-density lipoprotein receptor (LDLR) and its association with familial hypercholesterolemia in the early 1980s, a family of structurally related proteins has been discovered that has apolipoprotein E as a common ligand, and the broad functions of its members have been described. LRP2, or megalin, is a member of the LDLR family and was initially called gp330. Megalin is an endocytic receptor expressed on the apical surface of several epithelial cells that internalizes a variety of ligands including nutrients, hormones and their carrier proteins, signaling molecules, morphogens, and extracellular matrix proteins. Once internalized, these ligands are directed to the lysosomal degradation pathway or transported by transcytosis from one side of the cell to the opposite membrane. The availability of megalin at the cell surface is controlled by several regulatory mechanisms, including the phosphorylation of its cytoplasmic domain by GSK3, the proteolysis of the extracellular domain at the cell surface (shedding), the subsequent intramembrane proteolysis of the transmembrane domain by the gamma-secretase complex, and exosome secretion. Based on the important roles of its ligands and its tissue expression pattern, megalin has been recognized as an important component of many pathological conditions, including diabetic nephropathy, Lowe syndrome, Dent disease, Alzheimer's disease (AD) and gallstone disease. In addition, the expression of megalin and some of its ligands in the central and peripheral nervous system suggests a role for this receptor in neural regeneration processes. Despite its obvious importance, the regulation of megalin expression is poorly understood. In this review, we describe the functions of megalin and its association with certain pathological conditions as well as the current understanding of the mechanisms that underlie the control of megalin expression.