RESUMEN
Realizing the full utility of brain organoids to study human development requires understanding whether organoids precisely replicate endogenous cellular and molecular events, particularly since acquisition of cell identity in organoids can be impaired by abnormal metabolic states. We present a comprehensive single-cell transcriptomic, epigenetic, and spatial atlas of human cortical organoid development, comprising over 610,000 cells, from generation of neural progenitors through production of differentiated neuronal and glial subtypes. We show that processes of cellular diversification correlate closely to endogenous ones, irrespective of metabolic state, empowering the use of this atlas to study human fate specification. We define longitudinal molecular trajectories of cortical cell types during organoid development, identify genes with predicted human-specific roles in lineage establishment, and uncover early transcriptional diversity of human callosal neurons. The findings validate this comprehensive atlas of human corticogenesis in vitro as a resource to prime investigation into the mechanisms of human cortical development.
Asunto(s)
Corteza Cerebral , Organoides , Diferenciación Celular , Corteza Cerebral/metabolismo , Humanos , Neurogénesis , Neuronas , Organoides/metabolismoRESUMEN
Genetic risk for autism spectrum disorder (ASD) is associated with hundreds of genes spanning a wide range of biological functions1-6. The alterations in the human brain resulting from mutations in these genes remain unclear. Furthermore, their phenotypic manifestation varies across individuals7,8. Here we used organoid models of the human cerebral cortex to identify cell-type-specific developmental abnormalities that result from haploinsufficiency in three ASD risk genes-SUV420H1 (also known as KMT5B), ARID1B and CHD8-in multiple cell lines from different donors, using single-cell RNA-sequencing (scRNA-seq) analysis of more than 745,000 cells and proteomic analysis of individual organoids, to identify phenotypic convergence. Each of the three mutations confers asynchronous development of two main cortical neuronal lineages-γ-aminobutyric-acid-releasing (GABAergic) neurons and deep-layer excitatory projection neurons-but acts through largely distinct molecular pathways. Although these phenotypes are consistent across cell lines, their expressivity is influenced by the individual genomic context, in a manner that is dependent on both the risk gene and the developmental defect. Calcium imaging in intact organoids shows that these early-stage developmental changes are followed by abnormal circuit activity. This research uncovers cell-type-specific neurodevelopmental abnormalities that are shared across ASD risk genes and are finely modulated by human genomic context, finding convergence in the neurobiological basis of how different risk genes contribute to ASD pathology.
Asunto(s)
Trastorno del Espectro Autista , Predisposición Genética a la Enfermedad , Neuronas , Trastorno del Espectro Autista/genética , Trastorno del Espectro Autista/metabolismo , Trastorno del Espectro Autista/patología , Corteza Cerebral/citología , Proteínas de Unión al ADN/genética , Neuronas GABAérgicas/metabolismo , Neuronas GABAérgicas/patología , N-Metiltransferasa de Histona-Lisina/genética , Humanos , Neuronas/clasificación , Neuronas/metabolismo , Neuronas/patología , Organoides/citología , Proteómica , RNA-Seq , Análisis de la Célula Individual , Factores de Transcripción/genéticaRESUMEN
De novo heterozygous loss-of-function mutations in phosphatase and tensin homolog (PTEN) are strongly associated with autism spectrum disorders; however, it is unclear how heterozygous mutations in this gene affect different cell types during human brain development and how these effects vary across individuals. Here, we used human cortical organoids from different donors to identify cell-type specific developmental events that are affected by heterozygous mutations in PTEN. We profiled individual organoids by single-cell RNA-seq, proteomics and spatial transcriptomics and revealed abnormalities in developmental timing in human outer radial glia progenitors and deep-layer cortical projection neurons, which varied with the donor genetic background. Calcium imaging in intact organoids showed that both accelerated and delayed neuronal development phenotypes resulted in similar abnormal activity of local circuits, irrespective of genetic background. The work reveals donor-dependent, cell-type specific developmental phenotypes of PTEN heterozygosity that later converge on disrupted neuronal activity.
Asunto(s)
Trastorno del Espectro Autista , Neuronas , Humanos , Neuronas/metabolismo , Diferenciación Celular , Organoides/metabolismo , Trastorno del Espectro Autista/genética , Mutación , Fosfohidrolasa PTEN/genéticaRESUMEN
Seizure protein 6 (SEZ6) is required for the development and maintenance of the nervous system, is a major substrate of the protease BACE1 and is linked to Alzheimer's disease (AD) and psychiatric disorders, but its molecular functions are not well understood. Here, we demonstrate that SEZ6 controls glycosylation and cell surface localization of kainate receptors composed of GluK2/3 subunits. Loss of SEZ6 reduced surface levels of GluK2/3 in primary neurons and reduced kainate-evoked currents in CA1 pyramidal neurons in acute hippocampal slices. Mechanistically, loss of SEZ6 in vitro and in vivo prevented modification of GluK2/3 with the human natural killer-1 (HNK-1) glycan, a modulator of GluK2/3 function. SEZ6 interacted with GluK2 through its ectodomain and promoted post-endoplasmic reticulum transport of GluK2 in the secretory pathway in heterologous cells and primary neurons. Taken together, SEZ6 acts as a new trafficking factor for GluK2/3. This novel function may help to better understand the role of SEZ6 in neurologic and psychiatric diseases.
Asunto(s)
Región CA1 Hipocampal/metabolismo , Proteínas del Tejido Nervioso/metabolismo , Células Piramidales/metabolismo , Receptores de Ácido Kaínico/metabolismo , Animales , Glicosilación , Ratones , Ratones Noqueados , Proteínas del Tejido Nervioso/genética , Transporte de Proteínas , Receptores de Ácido Kaínico/genética , Receptor de Ácido Kaínico GluK2 , Receptor Kainato GluK3RESUMEN
In Alzheimer's disease, BACE1 protease initiates the amyloidogenic processing of amyloid precursor protein (APP) that eventually results in synthesis of ß-amyloid (Aß) peptide. Aß deposition in turn causes accumulation of BACE1 in plaque-associated dystrophic neurites, thereby potentiating progressive Aß deposition once initiated. Since systemic pharmacological BACE inhibition causes adverse effects in humans, it is important to identify strategies that specifically normalize overt BACE1 activity around plaques. The microtubule-associated protein tau regulates axonal transport of proteins, and tau deletion rescues Aß-induced transport deficits in vitro. In the current study, long-term in vivo two-photon microscopy and immunohistochemistry were performed in tau-deficient APPPS1 mice. Tau deletion reduced plaque-associated axonal pathology and BACE1 accumulation without affecting physiological BACE1 expression distant from plaques. Thereby, tau deletion effectively decelerated formation of new plaques and reduced plaque compactness. The data revealed that tau reinforces Aß deposition, presumably by contributing to accumulation of BACE1 in plaque-associated dystrophies. Targeting tau-dependent mechanisms could become a suitable strategy to specifically reduce overt BACE1 activity around plaques, thereby avoiding adverse effects of systemic BACE inhibition.
Asunto(s)
Enfermedad de Alzheimer/prevención & control , Secretasas de la Proteína Precursora del Amiloide/fisiología , Ácido Aspártico Endopeptidasas/antagonistas & inhibidores , Regulación de la Expresión Génica , Placa Amiloide/prevención & control , Proteínas tau/antagonistas & inhibidores , Enfermedad de Alzheimer/etiología , Enfermedad de Alzheimer/metabolismo , Enfermedad de Alzheimer/patología , Secretasas de la Proteína Precursora del Amiloide/antagonistas & inhibidores , Secretasas de la Proteína Precursora del Amiloide/genética , Secretasas de la Proteína Precursora del Amiloide/metabolismo , Animales , Ácido Aspártico Endopeptidasas/genética , Ácido Aspártico Endopeptidasas/metabolismo , Encéfalo/metabolismo , Encéfalo/patología , Modelos Animales de Enfermedad , Femenino , Humanos , Masculino , Ratones , Ratones Transgénicos , Placa Amiloide/etiología , Placa Amiloide/metabolismo , Placa Amiloide/patologíaRESUMEN
Seizure-related gene 6 (Sez6), Sez6-Like (Sez6L), and Sez6-Like 2 (Sez6L2) comprise a family of homologous proteins widely expressed throughout the brain that have been linked to neurodevelopmental and psychiatric disorders. Here, we use Sez6 triple knockout (TKO) mice, which lack all three Sez6 family proteins, to demonstrate that Sez6 family proteins regulate dendritic spine structure and cognitive functions, motor learning, and maintenance of motor functions across the lifespan. Compared to WT controls, we found that Sez6 TKO mice had impaired motor learning and their motor coordination was negatively affected from 6 weeks old and declined more rapidly as they aged. Sez6 TKO mice had reduced spine density in the hippocampus and dendritic spines were shifted to more immature morphologies in the somatosensory cortex. Cognitive testing revealed that they had enhanced stress responsiveness, impaired working, and spatial short-term memory but intact spatial long-term memory in the Morris water maze albeit accompanied by a reversal deficit. Our study demonstrates that the lack of Sez6 family proteins results in phenotypes commonly associated with neuropsychiatric disorders making it likely that Sez6 family proteins contribute to the complex etiologies of these disorders.
Asunto(s)
Cognición/fisiología , Espinas Dendríticas/metabolismo , Locomoción/fisiología , Memoria a Corto Plazo/fisiología , Destreza Motora/fisiología , Proteínas del Tejido Nervioso/deficiencia , Animales , Espinas Dendríticas/patología , Hipocampo/metabolismo , Hipocampo/patología , Masculino , Ratones , Ratones de la Cepa 129 , Ratones Endogámicos C57BL , Ratones Noqueados , Proteínas del Tejido Nervioso/genéticaRESUMEN
The transmembrane protein, ADAM10 (a disintegrin and metalloprotease 10), has key physiologic functions-for example, during embryonic development and in the brain. During transit through the secretory pathway, immature ADAM10 (proADAM10) is converted into its proteolytically active, mature form (mADAM10). Increasing or decreasing the abundance and/or activity of mADAM10 is considered to be a therapeutic approach for the treatment of such diseases as Alzheimer's disease and cancer. Yet biochemical detection and characterization of mADAM10 has been difficult. In contrast, proADAM10 is readily detected-for example, in immunoblots-which suggests that mADAM10 is only a fraction of total cellular ADAM10. Here, we demonstrate that mADAM10, but not proADAM10, unexpectedly undergoes rapid, time-dependent degradation upon biochemical cell lysis in different cell lines and in primary neurons, which prevents the detection of the majority of mADAM10 in immunoblots. This degradation required the catalytic activity of ADAM10, was efficiently prevented by adding active site inhibitors to the lysis buffer, and did not affect proADAM10, which suggests that ADAM10 degradation occurred in an intramolecular and autoproteolytic manner. Inhibition of postlysis autoproteolysis demonstrated efficient cellular ADAM10 maturation with higher levels of mADAM10 than proADAM10. Moreover, a cycloheximide chase experiment revealed that mADAM10 is a long-lived protein with a half-life of approximately 12 h. In summary, our study demonstrates that mADAM10 autoproteolysis must be blocked to allow for the proper detection of mADAM10, which is essential for the correct interpretation of biochemical and cellular studies of ADAM10.-Brummer, T., Pigoni, M., Rossello, A., Wang, H., Noy, P. J., Tomlinson, M. G., Blobel, C. P., Lichtenthaler, S. F. The metalloprotease ADAM10 (a disintegrin and metalloprotease 10) undergoes rapid, postlysis autocatalytic degradation.
Asunto(s)
Proteína ADAM10/metabolismo , Proteolisis , Animales , Línea Celular Tumoral , Células Cultivadas , Células HEK293 , Humanos , Ratones , Ratones Endogámicos C57BL , Neuronas/metabolismoRESUMEN
The ß-secretase ß-site APP cleaving enzyme (BACE1) is a central drug target for Alzheimer's disease. Clinically tested, BACE1-directed inhibitors also block the homologous protease BACE2. Yet little is known about physiological BACE2 substrates and functions in vivo. Here, we identify BACE2 as the protease shedding the lymphangiogenic vascular endothelial growth factor receptor 3 (VEGFR3). Inactivation of BACE2, but not BACE1, inhibited shedding of VEGFR3 from primary human lymphatic endothelial cells (LECs) and reduced release of the shed, soluble VEGFR3 (sVEGFR3) ectodomain into the blood of mice, nonhuman primates, and humans. Functionally, BACE2 inactivation increased full-length VEGFR3 and enhanced VEGFR3 signaling in LECs and also in vivo in zebrafish, where enhanced migration of LECs was observed. Thus, this study identifies BACE2 as a modulator of lymphangiogenic VEGFR3 signaling and demonstrates the utility of sVEGFR3 as a pharmacodynamic plasma marker for BACE2 activity in vivo, a prerequisite for developing BACE1-selective inhibitors for safer prevention of Alzheimer's disease.
Asunto(s)
Enfermedad de Alzheimer , Secretasas de la Proteína Precursora del Amiloide , Ácido Aspártico Endopeptidasas , Transducción de Señal , Receptor 3 de Factores de Crecimiento Endotelial Vascular , Pez Cebra , Secretasas de la Proteína Precursora del Amiloide/metabolismo , Secretasas de la Proteína Precursora del Amiloide/genética , Secretasas de la Proteína Precursora del Amiloide/antagonistas & inhibidores , Animales , Ácido Aspártico Endopeptidasas/metabolismo , Ácido Aspártico Endopeptidasas/genética , Ácido Aspártico Endopeptidasas/antagonistas & inhibidores , Humanos , Ratones , Receptor 3 de Factores de Crecimiento Endotelial Vascular/metabolismo , Receptor 3 de Factores de Crecimiento Endotelial Vascular/genética , Pez Cebra/metabolismo , Pez Cebra/genética , Enfermedad de Alzheimer/metabolismo , Enfermedad de Alzheimer/genética , Enfermedad de Alzheimer/patología , Enfermedad de Alzheimer/enzimología , Células Endoteliales/metabolismo , Células Endoteliales/enzimología , Células Endoteliales/patología , Proteínas de Pez Cebra/genética , Proteínas de Pez Cebra/metabolismoRESUMEN
The membrane protein seizure 6-like (SEZ6L) is a neuronal substrate of the Alzheimer's disease protease BACE1, and little is known about its physiological function in the nervous system. Here, we show that SEZ6L constitutive knockout mice display motor phenotypes in adulthood, including changes in gait and decreased motor coordination. Additionally, SEZ6L knockout mice displayed increased anxiety-like behaviour, although spatial learning and memory in the Morris water maze were normal. Analysis of the gross anatomy and proteome of the adult SEZ6L knockout cerebellum did not reveal any major differences compared to wild type, indicating that lack of SEZ6L in other regions of the nervous system may contribute to the phenotypes observed. In summary, our study establishes physiological functions for SEZ6L in regulating motor coordination and curbing anxiety-related behaviour, indicating that aberrant SEZ6L function in the human nervous system may contribute to movement disorders and neuropsychiatric diseases.
Asunto(s)
Secretasas de la Proteína Precursora del Amiloide , Ácido Aspártico Endopeptidasas , Proteínas de la Membrana , Actividad Motora , Secretasas de la Proteína Precursora del Amiloide/metabolismo , Animales , Ácido Aspártico Endopeptidasas/metabolismo , Humanos , Aprendizaje por Laberinto , Proteínas de la Membrana/genética , Proteínas de la Membrana/metabolismo , Ratones , Ratones Endogámicos C57BL , Ratones NoqueadosRESUMEN
The tissue inhibitor of metalloproteinases-3 (TIMP-3) is a major regulator of extracellular matrix turnover and protein shedding by inhibiting different classes of metalloproteinases, including disintegrin metalloproteinases (ADAMs). Tissue bioavailability of TIMP-3 is regulated by the endocytic receptor low-density-lipoprotein receptor-related protein-1 (LRP-1). TIMP-3 plays protective roles in disease. Thus, different approaches have been developed aiming to increase TIMP-3 bioavailability, yet overall effects of increased TIMP-3 in vivo have not been investigated. Herein, by using unbiased mass-spectrometry we demonstrate that TIMP-3-overexpression in HEK293 cells has a dual effect on shedding of transmembrane proteins and turnover of soluble proteins. Several membrane proteins showing reduced shedding are known as ADAM10 substrates, suggesting that exogenous TIMP-3 preferentially inhibits ADAM10 in HEK293 cells. Additionally identified shed membrane proteins may be novel ADAM10 substrate candidates. TIMP-3-overexpression also increased extracellular levels of several soluble proteins, including TIMP-1, MIF and SPARC. Levels of these proteins similarly increased upon LRP-1 inactivation, suggesting that TIMP-3 increases soluble protein levels by competing for their binding to LRP-1 and their subsequent internalization. In conclusion, our study reveals that increased levels of TIMP-3 induce substantial modifications in the cellular secretome and that TIMP-3-based therapies may potentially provoke undesired, dysregulated functions of ADAM10 and LRP-1.
Asunto(s)
Proteína ADAM10/metabolismo , Secretasas de la Proteína Precursora del Amiloide/metabolismo , Membrana Celular/metabolismo , Proteína 1 Relacionada con Receptor de Lipoproteína de Baja Densidad/metabolismo , Proteínas de la Membrana/metabolismo , Vías Secretoras/fisiología , Inhibidor Tisular de Metaloproteinasa-3/metabolismo , Matriz Extracelular/metabolismo , Células HEK293 , Humanos , Ligandos , Espectrometría de Masas , Transporte de Proteínas/fisiología , ProteómicaRESUMEN
It is intriguing that a rare, inherited lysosomal storage disorder Niemann-Pick type C (NPC) shares similarities with Alzheimer's disease (AD). We have previously reported an enhanced processing of ß-amyloid precursor protein (APP) by ß-secretase (BACE1), a key enzyme in the pathogenesis of AD, in NPC1-null cells. In this work, we characterized regional and temporal expression and processing of the recently identified BACE1 substrates seizure protein 6 (Sez6) and seizure 6-like protein (Sez6L), and APP, in NPC1-/- (NPC1) and NPC1+/+ (wt) mouse brains. We analysed 4-weeks old brains to detect the earliest changes associated with NPC, and 10-weeks of age to identify changes at terminal disease stage. Sez6 and Sez6L were selected due to their predominant cleavage by BACE1, and their potential role in synaptic function that may contribute to presentation of seizures and/or motor impairments in NPC patients. While an enhanced BACE1-cleavage of all three substrates was detected in NPC1 vs. wt-mouse brains at 4-weeks of age, at 10-weeks increased proteolysis by BACE1 was observed for Sez6L in the cortex, hippocampus and cerebellum of NPC1-mice. Interestingly, both APP and Sez6L were found to be expressed in Purkinje neurons and their immunostaining was lost upon Purkinje cell neurodegeneration in 10-weeks old NPC1 mice. Furthermore, in NPC1- vs. wt-mouse primary cortical neurons, both Sez6 and Sez6L showed increased punctuate staining within the endolysosomal pathway as well as increased Sez6L and BACE1-positive puncta. This indicates that a trafficking defect within the endolysosomal pathway may play a key role in enhanced BACE1-proteolysis in NPC disease. Overall, our findings suggest that enhanced proteolysis by BACE1 could be a part of NPC disease pathogenesis. Understanding the basic biology of BACE1 and the functional impact of cleavage of its substrates is important to better evaluate the therapeutic potential of BACE1 against AD and, possibly, NPC disease.
Asunto(s)
Secretasas de la Proteína Precursora del Amiloide/metabolismo , Ácido Aspártico Endopeptidasas/metabolismo , Encéfalo/metabolismo , Proteínas del Tejido Nervioso/metabolismo , Enfermedad de Niemann-Pick Tipo C/metabolismo , Animales , Encéfalo/crecimiento & desarrollo , Encéfalo/patología , Modelos Animales de Enfermedad , Péptidos y Proteínas de Señalización Intracelular , Ratones Endogámicos BALB C , Ratones Noqueados , Neuronas/metabolismo , Neuronas/patología , Proteína Niemann-Pick C1 , Enfermedad de Niemann-Pick Tipo C/patología , Proteínas/genética , Proteínas/metabolismo , ProteolisisRESUMEN
Tissue inhibitor of metalloproteinases 3 (TIMP-3) is a key regulator of extracellular matrix turnover for its ability to inhibit matrix metalloproteinases (MMPs), adamalysin-like metalloproteinases (ADAMs) and ADAMs with thrombospondin motifs (ADAMTSs). TIMP-3 is a secreted protein whose extracellular levels are regulated by endocytosis via the low-density-lipoprotein receptor-related protein-1 (LRP-1). In this study we developed a molecule able to "trap" TIMP-3 extracellularly, thereby increasing its tissue bioavailability. LRP-1 contains four ligand-binding clusters. In order to investigate the TIMP-3 binding site on LRP-1, we generated soluble minireceptors (sLRPs) containing the four distinct binding clusters or part of each cluster. We used an array of biochemical methods to investigate the binding of TIMP-3 to different sLRPs. We found that TIMP-3 binds to the ligand-binding cluster II of the receptor with the highest affinity and a soluble minireceptor containing the N-terminal half of cluster II specifically blocked TIMP-3 internalization, without affecting the turnover of metalloproteinases. Mass spectrometry-based secretome analysis showed that this minireceptor, named T3TRAP, selectively increased TIMP-3 levels in the extracellular space and inhibited constitutive shedding of a number of cell surface proteins. In conclusion, T3TRAP represents a biological tool that can be used to modulate TIMP-3 levels in the tissue and could be potentially developed as a therapy for diseases characterized by a deficit of TIMP-3, including arthritis.
Asunto(s)
Células Epiteliales/metabolismo , Matriz Extracelular/química , Proteína 1 Relacionada con Receptor de Lipoproteína de Baja Densidad/metabolismo , Receptores Artificiales/metabolismo , Inhibidor Tisular de Metaloproteinasa-3/metabolismo , Animales , Sitios de Unión , Células COS , Línea Celular Tumoral , Chlorocebus aethiops , Endocitosis , Células Epiteliales/citología , Matriz Extracelular/metabolismo , Regulación de la Expresión Génica , Células HEK293 , Humanos , Cinética , Proteína 1 Relacionada con Receptor de Lipoproteína de Baja Densidad/genética , Anotación de Secuencia Molecular , Neuroglía/citología , Neuroglía/metabolismo , Unión Proteica , Dominios y Motivos de Interacción de Proteínas , Mapeo de Interacción de Proteínas , Transporte de Proteínas , Receptores Artificiales/genética , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Transducción de Señal , Solubilidad , Inhibidor Tisular de Metaloproteinasa-3/genética , TransfecciónRESUMEN
Inhibition of the protease ß-site amyloid precursor protein-cleaving enzyme 1 (BACE1) is a promising treatment strategy for Alzheimer's disease, and a number of BACE inhibitors are currently progressing through clinical trials. The strategy aims to decrease production of amyloid-ß (Aß) peptide from the amyloid precursor protein (APP), thus reducing or preventing Aß toxicity. Over the last decade, it has become clear that BACE1 proteolytically cleaves a number of substrates in addition to APP. These substrates are not known to be involved in the pathogenesis of Alzheimer's disease but have other roles in the developing and/or mature central nervous system. Consequently, BACE inhibition and knockout in mice results in synaptic and other neuronal dysfunctions and the key substrates responsible for these deficits are still being elucidated. Of the BACE1 substrates that have been validated to date, a number may contribute to the synaptic deficits seen with BACE blockade, including neuregulin 1, close homologue of L1 and seizure-related gene 6. It is important to understand the impact that BACE blockade may have on these substrates and other proteins detected in substrate screens and, if necessary, develop substrate-selective BACE inhibitors.
Asunto(s)
Enfermedad de Alzheimer/metabolismo , Secretasas de la Proteína Precursora del Amiloide/metabolismo , Precursor de Proteína beta-Amiloide/metabolismo , Ácido Aspártico Endopeptidasas/metabolismo , Sinapsis/metabolismo , Secretasas de la Proteína Precursora del Amiloide/genética , Animales , Ácido Aspártico Endopeptidasas/genética , Encéfalo/metabolismo , Humanos , Proteolisis , Sinapsis/fisiologíaRESUMEN
BACKGROUND: The protease BACE1 (beta-site APP cleaving enzyme) is a major drug target in Alzheimer's disease. However, BACE1 therapeutic inhibition may cause unwanted adverse effects due to its additional functions in the nervous system, such as in myelination and neuronal connectivity. Additionally, recent proteomic studies investigating BACE1 inhibition in cell lines and cultured murine neurons identified a wider range of neuronal membrane proteins as potential BACE1 substrates, including seizure protein 6 (SEZ6) and its homolog SEZ6L. METHODS AND RESULTS: We generated antibodies against SEZ6 and SEZ6L and validated these proteins as BACE1 substrates in vitro and in vivo. Levels of the soluble, BACE1-cleaved ectodomain of both proteins (sSEZ6, sSEZ6L) were strongly reduced upon BACE1 inhibition in primary neurons and also in vivo in brains of BACE1-deficient mice. BACE1 inhibition increased neuronal surface levels of SEZ6 and SEZ6L as shown by cell surface biotinylation, demonstrating that BACE1 controls surface expression of both proteins. Moreover, mass spectrometric analysis revealed that the BACE1 cleavage site in SEZ6 is located in close proximity to the membrane, similar to the corresponding cleavage site in SEZ6L. Finally, an improved method was developed for the proteomic analysis of murine cerebrospinal fluid (CSF) and was applied to CSF from BACE-deficient mice. Hereby, SEZ6 and SEZ6L were validated as BACE1 substrates in vivo by strongly reduced levels in the CSF of BACE1-deficient mice. CONCLUSIONS: This study demonstrates that SEZ6 and SEZ6L are physiological BACE1 substrates in the murine brain and suggests that sSEZ6 and sSEZ6L levels in CSF are suitable markers to monitor BACE1 inhibition in mice.