Sortilin-related receptor with A-type repeats (SORLA) is an intracellular sorting receptor that directs target proteins between endocytic and secretory compartments of cells. Mutations in SORL1 , encoding SORLA, are common in individuals suffering from Alzheimer disease (AD) of unknown etiology. Conceptually, characterization of inheritable SORL1 variants associated with AD can provide important new information about functions of this receptor relevant for aging brain health. Here, we focused on elucidation of the AD-associated variant SORLA N1358S, carrying a mutation in the main ligand binding domain of the receptor. Using unbiased quantitative proteome screens, we identified major alterations in the mutant receptor interactome linked to biogenesis and secretion of exosomes. Using advanced biophysical, cell biological, as well as functional studies in stem cell-derived human cell models we corroborated impaired release and loss of neurotrophic action of exosomes from neurons and microglia expressing SORLAN1358S. An impaired neurotrophic potential was attributed to an altered exosomal content of RNA binding proteins and associated microRNAs, known to control neuronal growth and maturation. Our studies identified a so far unknown function for SORLA in controlling the quantity and trophic quality of extracellular vesicles secreted by cells, and they argue for impaired cellular cross talk through exosomes as a pathological trail contributing to the risk of AD seen with carriers of SORL1 variants. ### Competing Interest Statement The authors have declared no competing interest. * ### List of Abbreviations (SORLA) : sortilin-related receptor with A-type repeats (APP) : amyloid precursor protein (Aβ) : amyloid-β peptides (AD) : Alzheimer’s disease (SNPs) : single nucleotide polymorphisms (CR) : complement-type repeats (SPR) : surface plasmon resonance (WT) : wildtype (RU) : response units (SILAC) : stable isotope labeling by amino acids in cell culture (IP) : immunoprecipitated (NTA) : nanoparticle tracking analysis (iMG) : induced human microglia Lundbeck Foundation, https://ror.org/03hz8wd80, (R380-2021-1326 Novo Nordisk Foundation, , NNF18OC0033928 Alzheimer Forschung Initiative, , 18003

Rs11218343 is a non-coding variant of genome-wide significance for sporadic Alzheimer disease (AD) with one of the most protective effects known. It localizes to SORL1, encoding the AD risk factor SORLA. Still, the functional significance of rs11218343 for AD related processes remains unclear. We used iPSC lines from donors, or genome-engineered to carry major and minor rs11218343 alleles, to study the impact of rs11218343 genotype on brain cell activities. We show that rs11218343 uniquely controls functional expression of SORLA in microglia, with incrased receptor expression in the minor protective allele correlating with reduced pro-inflammatory responses. This anti-inflammatory effect is seen in donor iPSC lines but not in SNP-engineered isogenic lines, documenting rs11218343 to be diagnostic but not functional. Our findings corroborate genetically defined expression levels of SORL1 in microglia as a determinant of protection from pro-inflammatory stimulation, a function encoded by a haplotype linked to rs11218343.   ### Competing Interest Statement The authors have declared no competing interest. Novo Nordisk Foundation, https://ror.org/04txyc737, NNF18OC0033928

Sortilin (SORT1) is a lipoprotein receptor that shows genome-wide association with hypercholesterolaemia, explained by its ability to control hepatic output of lipoproteins. Although SORT1 also shows genome-wide association with Alzheimer disease and frontotemporal lobe dementia, the most prevalent forms of age-related dementias, sortilin’s contribution to human brain lipid metabolism and health remains unclear. Here we show that sortilin mediates neuronal uptake of polyunsaturated fatty acids carried by apolipoprotein E (apoE). Using humanized mouse strains and induced pluripotent stem cell-based cell models of brain lipid homeostasis, we demonstrate that internalized lipids are converted into ligands for peroxisome proliferator-activated receptor alpha inducing transcription profiles that enable neurons to use long-chain fatty acids as metabolic fuel when glucose is limited. This pathway works with apoE3 but cannot operate with the Alzheimer disease risk factor apoE4, which disrupts sortilin’s endocytic activity. Our data indicate a role for the lipoprotein receptor sortilin in metabolic fuel choice in neurons, which may be crucial when glucose supply is limited, such as in the ageing brain. Greda et al. show that sortilin and apolipoprotein E3 mediate import and utilization of long-chain fatty acids as a metabolic fuel in neurons after glucose restriction.

INTRODUCTION:Mutations in SORL1, encoding the sorting receptor Sortilin-related receptor with A-type repeats (SORLA), are found in individuals with Alzheimer's disease (AD). We studied SORLAN1358S, carrying a mutation in its ligand binding domain, to learn more about receptor functions relevant for human brain health. METHODS:We investigated consequences of SORLAN1358S expression in induced pluripotent stem cell (iPSC)-derived human neurons and microglia, using unbiased proteome screens and functional cell assays. RESULTS:We identified alterations in the SORLAN1358S interactome linked to biogenesis of exosomes. Consequently, the mutant receptor failed to promote release and neurotrophic qualities of exosomes, a defect attributed to altered exosomal content of microRNAs controlling neuronal maturation. DISCUSSION:We identified a role for SORLA in controlling quantity and neurotrophic quality of exosomes secreted by cells, suggesting impaired cellular cross talk through exosomes as a pathological trait contributing to AD pathology in carriers of SORL1 variants. HIGHLIGHTS:Familial Alzheimer's disease mutation in SORL1 changes interactome of mutant Sortilin-related receptor with A-type repeats (SORLA). Mutant SORLA impairs release of exosomes from neurons and microglia. Mutant exosomes lack neurotrophic qualities. Defect linked to alterations in microRNA content.

The importance of astrocytes for Alzheimer disease (AD) pathology is increasingly appreciated, yet the mechanisms whereby this cell type impacts neurodegenerative processes remain elusive. In a genetic mouse model with diminished astrocyte stress response, even low levels of amyloid-beta trigger astrocyte reactivity, resulting in brain inflammation and massive amyloid and tau pathologies. This dysfunctional response of astrocytes to amyloid-beta acts through activation of delta secretase, a stress-induced protease implicated in both amyloid and tau-related proteolytic processing. Our findings identify a failed astrocyte stress response to amyloid-beta as an early inducer of amyloid and tau co-morbidity, a noxious process in AD acting through a unique non-canonical secretase pathway. ### Competing Interest Statement The authors have declared no competing interest.

A subset of neuroendocrine tumors (NETs) can cause an excessive secretion of hormones, neuropeptides, and biogenic amines into the bloodstream. These so-called functional NETs evoke a hormone-related disease and lead to several different syndromes, depending on the factors released. One of the most common functional syndromes, carcinoid syndrome, is characterized mainly by over-secretion of serotonin. However, what distinguishes functional from non-functional tumors on a molecular level remains unknown. Here, we demonstrate that the expression of sortilin, a widely expressed transmembrane receptor involved in intracellular protein sorting, is significantly increased in functional compared to non-functional NETs and thus can be used as a biomarker for functional NETs. Furthermore, using a cell line model of functional NETs, as well as organoids, we demonstrate that inhibition of sortilin reduces cellular serotonin concentrations and may therefore serve as a novel therapeutic target to treat patients with carcinoid syndrome.

The peripheral immune system is important in neurodegenerative diseases, both in protecting and inflaming the brain, but the underlying mechanisms remain elusive. Alzheimer’s Disease is commonly preceded by a prodromal period. Here, we report the presence of large Aβ aggregates in plasma from patients with mild cognitive impairment ( n = 38). The aggregates are associated with low level Alzheimer’s Disease-like brain pathology as observed by 11 C-PiB PET and 18 F-FTP PET and lowered CD18-rich monocytes. We characterize complement receptor 4 as a strong binder of amyloids and show Aβ aggregates are preferentially phagocytosed and stimulate lysosomal activity through this receptor in stem cell-derived microglia. KIM127 integrin activation in monocytes promotes size selective phagocytosis of Aβ. Hydrodynamic calculations suggest Aβ aggregates associate with vessel walls of the cortical capillaries. In turn, we hypothesize aggregates may provide an adhesion substrate for recruiting CD18-rich monocytes into the cortex. Our results support a role for complement receptor 4 in regulating amyloid homeostasis.

Sorting protein-related receptor containing class A repeats (SORLA) is an intracellular trafficking receptor encoded by the Alzheimer's disease (AD) gene SORL1 (sortilin-related receptor 1). Recent findings argue that altered expression in microglia may underlie the genome-wide risk of AD seen with some SORL1 gene variants, however, the functional significance of the receptor in microglia remains poorly explained. Using unbiased omics and targeted functional analyses in iPSC-based human microglia, we identified a crucial role for SORLA in sensitizing microglia to pro-inflammatory stimuli. We show that SORLA acts as a sorting factor for the pattern recognition receptor CD14, directing CD14 exposure on the cell surface and priming microglia to stimulation by pro-inflammatory factors. Loss of SORLA in gene-targeted microglia impairs proper CD14 sorting and blunts pro-inflammatory responses. Our studies indicate an important role for SORLA in shaping the inflammatory brain milieu, a biological process important to local immune responses in AD.

SorLA, encoded by the gene SORL1, is an intracellular sorting receptor of the VPS10P domain receptor gene family. Although SorLA is best recognized for its ability to shuttle target proteins between intracellular compartments in neurons, recent data suggest that also its microglial expression can be of high relevance for the pathogenesis of brain diseases, including glioblastoma (GBM). Here, we interrogated the impact of SorLA on the functional properties of glioma-associated microglia and macrophages (GAMs). In the GBM microenvironment, GAMs are re-programmed and lose the ability to elicit anti-tumor responses. Instead, they acquire a glioma-supporting phenotype, which is a key mechanism promoting glioma progression. Our re-analysis of published scRNA-seq data from GBM patients revealed that functional phenotypes of GAMs are linked to the level of SORL1 expression, which was further confirmed using in vitro models. Moreover, we demonstrate that SorLA restrains secretion of TNFα from microglia to restrict the inflammatory potential of these cells. Finally, we show that loss of SorLA exacerbates the pro-inflammatory response of microglia in the murine model of glioma and suppresses tumor growth.

Motile cilia are protruding organelles on specialized epithelia that beat in a synchronous fashion to propel extracellular fluids. Coordination and orientation of cilia beating on individual cells and across tissues is a complex process dependent on planar cell polarity (PCP) signaling. Asymmetric sorting of PCP pathway components, essential to establish planar polarity, involves trafficking along the endocytic path, but the underlying regulatory processes remain incompletely understood. Here, we identified the endocytic receptor LRP2 as regulator of PCP component trafficking in ependyma, a multi-ciliated cell type that is involved in facilitating flow of the cerebrospinal fluid in the brain ventricular system. Lack of receptor expression in gene-targeted mice results in a failure to sort PCP core proteins to the anterior or posterior cell side and, consequently, in the inability to coordinate cilia arrangement and to aligned beating (loss of rotational and translational polarity). LRP2 deficiency coincides with a failure to sort NHERF1, a cytoplasmic LRP2 adaptor to the anterior cell side. As NHERF1 is essential to translocate PCP core protein Vangl2 to the plasma membrane, these data suggest a molecular mechanism whereby LRP2 interacts with PCP components through NHERF1 to control their asymmetric sorting along the endocytic path. Taken together, our findings identified the endocytic receptor LRP2 as a novel regulator of endosomal trafficking of PCP proteins, ensuring their asymmetric partition and establishment of translational and rotational planar cell polarity in the ependyma.

Vacuolar protein sorting 10 protein (VPS10P) domain receptors are a unique class of intracellular sorting receptors that emerge as major risk factors associated with psychiatric and neurodegenerative diseases, including bipolar disorders, autism, schizophrenia, as well as Alzheimer's disease and frontotemporal dementia. Yet, the lack of suitable experimental models to study receptor functions in the human brain has hampered elucidation of receptor actions in brain disease. Here, we have adapted protocols using human cerebral organoids to the detailed characterization of VPS10P domain receptor expression during neural development and differentiation, including single-cell RNA sequencing. Our studies uncovered spatial and temporal patterns of expression unique to individual receptor species in the human brain. While SORL1 expression is abundant in stem cells and SORCS1 peaks in neural progenitors at onset of neurogenesis, SORT1 and SORCS2 show increasing expression with maturation of neuronal and non-neuronal cell types, arguing for distinct functions in development versus the adult brain. In neurons, subcellular localization also distinguishes between types of receptor species, either mainly localized to the cell soma (SORL1 and SORT1) or also to neuronal projections (SORCS1 and SORCS2), suggesting divergent functions in protein sorting between Golgi and the endo-lysosomal system or along axonal and dendritic tracks. Taken together, our findings provide an important resource on temporal, spatial, and subcellular patterns of VPS10P domain receptor expression in cerebral organoids for further elucidation of receptor (dys) functions causative of behavioral and cognitive defects of the human brain.

ABSTRACT Objective SORCS2 is an intracellular sorting receptor genetically associated with body mass index (BMI) in humans, yet its mode of action remains unknown. Elucidating the receptor function that defines its role in metabolic health is the objective of this work. Methods Combining in vivo metabolic studies in SORCS2-deficient mouse models with ex vivo structural and functional analyses as well as single-cell transcriptomics of murine pancreatic tissues, we studied the pathophysiological consequences of receptor dysfunction for metabolism. Results Our studies identified an important role for SORCS2 in islet stress response essential to sustain glucose-stimulated insulin release. In detail, we show that SORCS2 is predominantly expressed in islet alpha cells. Loss of receptor expression coincides with the inability of these cells to produce osteopontin, a secreted factor that facilitates insulin release from beta cells under stress. In line with diminished osteopontin levels, beta cells in SORCS2- deficient islets show changes in gene expression patterns related to aggravated ER stress, protein misfolding, as well as mitochondrial dysfunction; and they exhibit defects in insulin granule maturation and a blunted response to glucose stimulation in vivo and ex vivo . Impaired glucose tolerance in receptor mutant mice coincides with alterations in body weight and composition. Conclusion Our data identified a novel concept in protective islet stress response involving the alpha cell receptor SORCS2 and provide experimental support for association of SORCS2 with metabolic control in humans.

Vitamin D deficiency is associated with an increased risk of prostate cancer mortality and is hypothesized to contribute to prostate cancer aggressiveness and disparities in African American populations. The prostate epithelium was recently shown to express megalin, an endocytic receptor that internalizes circulating globulin-bound hormones, which suggests regulation of intracellular prostate hormone levels. This contrasts with passive diffusion of hormones that is posited by the free hormone hypothesis. Here, we demonstrate that megalin imports testosterone bound to sex hormone-binding globulin into prostate cells. Prostatic loss of Lrp2 (megalin) in a mouse model resulted in reduced prostate testosterone and dihydrotestosterone levels. Megalin expression was regulated and suppressed by 25-hydroxyvitamin D (25D) in cell lines, patient-derived prostate epithelial cells, and prostate tissue explants. In patients, the relationships between hormones support this regulatory mechanism, as prostatic DHT levels are higher in African American men and are inversely correlated with serum 25D status. Megalin levels are reduced in localized prostate cancer by Gleason grade. Our findings suggest that the free hormone hypothesis should be revisited for testosterone and highlight the impact of vitamin D deficiency on prostate androgen levels, which is a known driver of prostate cancer. Thus, we revealed a mechanistic link between vitamin D and prostate cancer disparities observed in African Americans. Significance: These findings link vitamin D deficiency and the megalin protein to increased levels of prostate androgens, which may underpin the disparity in lethal prostate cancer in African America men.

Sorting receptor SORCS2 is a stress-response factor protecting neurons from acute insults, such as during epilepsy. SORCS2 is also expressed in the pancreas, yet its action in this tissue remains unknown. Combining metabolic studies in SORCS2-deficient mice with ex vivo functional analyses and single-cell transcriptomics of pancreatic tissues, we identified a role for SORCS2 in protective stress response in pancreatic islets, essential to sustain insulin release. We show that SORCS2 is predominantly expressed in islet alpha cells. Loss of expression coincides with inability of these cells to produce osteopontin, a secreted factor that facilitates insulin release from stressed beta cells. In line with diminished osteopontin levels, beta cells in SORCS2-deficient islets show gene expression patterns indicative of aggravated cell stress, and exhibit defects in insulin granule maturation and a blunted glucose response. These findings corroborate a function for SORCS2 in protective stress response that extends to metabolism.

S1. Accompanies Figure 1 and 2. S2. Accompanies Figure 2. No differences in bitransgenic mouse prostate histology, weights and fertility. S3. Relates to Figure 3. Vitamin D receptor and androgen receptor response elements in LRP2 promoter. S4. Relates to Figure 3. Prostate slices express hormone response components and respond to 25D and T. S5. Relates to Figure 4. Androgen levels by Gleason and Age. S6. Relates to Figure 6. Expression of LRP2 by age and BCR in DFKZ and TCGA cohorts. Table S1. Cell and tissue characteristics Table S2. Primer sequences

All species continuously evolve short open reading frames (sORFs) that can be templated for protein synthesis and may provide raw materials for evolutionary adaptation. We analyzed the evolutionary origins of 7,264 recently cataloged human sORFs and found that most were evolutionarily young and had emerged de novo. We additionally identified 221 previously missed sORFs potentially translated into peptides of up to 15 amino acids-all of which are smaller than the smallest human microprotein annotated to date. To investigate the bioactivity of sORF-encoded small peptides and young microproteins, we subjected 266 candidates to a mass-spectrometry-based interactome screen with motif resolution. Based on these interactomes and additional cellular assays, we can associate several candidates with mRNA splicing, translational regulation, and endocytosis. Our work provides insights into the evolutionary origins and interaction potential of young and small proteins, thereby helping to elucidate this underexplored territory of the human proteome.

Department of Pathology, University of Illinois at Chicago, Chicago, IL 60612, USA University of Illinois Cancer Center, Chicago, IL 60612, USA Department of Systems Pharmacology & Translational Therapeutics, University of Pennsylvania Perelman School of Medicine, Philadelphia, PA 19104, USA Center of Excellence in Environmental Toxicology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA Max-DelbrückCenter for Molecular Medicine, 13125 Berlin, Germany Department of Population Sciences, City of Hope, Duarte, CA 91010, USA Departments of Urology, Physiology and Biophysics, University of Illinois at Chicago, Chicago, IL 60612, USA

Aims/ hypothesis Sorting-related receptor with type A repeats (SORLA) is a neuronal sorting receptor that prevents accumulation of amyloid-beta peptides, the main constituent of senile plaques in Alzheimer disease. Recent transcriptomic studies show that SORLA transcripts are also found in pancreatic islet beta cells, yet the role of SORLA in islets is unclear so far. Based on its protective role in reducing amyloid burden in the brain, we hypothesized that SORLA may have a similar function in the pancreas, regulating islet amyloid plaque formation from islet amyloid polypeptide (IAPP). Methods We generated human IAPP transgenic mice lacking SORLA (hIAPP:SORLA KO) to assess the consequences of receptor deficiency for islet histopathology and function in vivo. Using both primary islet cells and established cell lines, we further investigated the molecular mechanisms whereby SORLA controls the cellular metabolism and accumulation of IAPP. Results Loss of SORLA activity in hIAPP:SORLA KO resulted in a significant increase in islet amyloid deposits and associated islet cell death as compared to hIAPP:SORLA WT animals expressing the receptor. Aggravated islet amyloid deposition was observed in mice fed a normal chow diet, not requiring high-fat diet feeding typically needed to induce islet amyloidosis in mouse models. Further in vitro studies showed that SORLA binds to and mediates the endocytic uptake of proIAPP, but not mature IAPP, delivering the propeptide to an endolysosomal fate. Conclusions/interpretation SORLA functions as a clearance receptor specific for proIAPP, protecting against islet amyloid deposition and associated cell death caused by IAPP.