Species: Mouse
Genes: Sorl1
Modification: Sorl1: Knock-Out
Disease Relevance: Alzheimer's Disease
Strain Name: N/A
Genetic Background: Generated on a mixed 129SvEmcTer X C57BL/6N genetic background, subsequently backcrossed to C57BL/6J.
Availability: Available through Thomas Willnow.

In this model of SORLA deficiency, the 5' region of exon 4 of the murine Sorl1 gene was replaced by a neomycin resistance cassette. Although mice homozygous for the disrupted allele do not make full-length SORLA protein (Andersen et al., 2005), an incomplete form of the receptor has been detected at low levels in the brains of these animals (Dodson et al., 2008). Sorl1 mRNA from the disrupted allele is 162 base pairs smaller than wild-type message, and the protein is missing 54 amino acids within the N-terminal region of the VPS10P domain. Levels of the incomplete protein are at least fourfold lower than levels of the full-length protein expressed in wild-type mice. Heterozygous animals generate both full-length and incomplete forms of SORLA.

SORLA-deficient mice are viable and fertile (Andersen et al., 2005). Some physiological abnormalities have been noted in mice homozygous for the disrupted gene, including deficits in salt homeostasis accompanied by lower mean arterial blood pressure (Reiche et al., 2010), and differences in body composition—decreased fat and increased lean body mass—compared with littermate controls (Schmidt et al., 2016). In addition, SORLA-deficient mice were found to have fewer cells in the inner nuclear layer of the retina than wild-type mice (Monti et al., 2020), but it is not known whether this retinal abnormality affects the animals’ vision.

Amyloidosis

Levels of APP and its metabolites were measured in extracts of cerebral cortices from 10-month-old mice. While APP levels did not differ between SORLA-deficient mice and wild-type controls, levels of sAPP and Aβ were elevated—the amounts of both Aβ40 and Aβ42 were about 30 percent higher in the mice with the disrupted Sorl1 allele (Andersen et al., 2005). Amyloid plaques were not seen in the brains of either genotype.

Subsequently, the levels of mature (glycosylated) APP, measured at 6 months of age, were found to be reduced in the hippocampi of SORLA-deficient mice (Rohe et al., 2008).

When SORLA-deficient mice were crossed with the APPswe/PS1ΔE9 model of amyloidosis, levels of APP metabolites were altered—sAPPα and sAPPβ were elevated, while APP CTFs were decreased—and amyloid deposition was accelerated, compared with the parental APPswe/PS1ΔE9 line (Dodson et al., 2008).

Findings from crosses of SORLA-deficient mice with PDAPP mice, another model of amyloidosis, confirmed that SORLA depletion results in increased APP catabolism: Levels of sAPPα, sAPPβ, and Aβ were significantly elevated in primary hippocampal neurons derived from PDAPP mice lacking SORLA, compared with neurons derived from the parental PDAPP line.  Also, amyloid deposition was accelerated, and levels of detergent-insoluble Aβ were elevated, in the brains of 10-month-old PDAPP;Sorl1^-/- mice, compared with littermates heterozygous for the Sorl1 deletion (Rohe et al., 2008).

Behavior

Compared with wild-type mice, 3- to 4-month-old SORLA-deficient mice exhibited more arm entries and more time spent in the open arms of the elevated plus maze—behaviors interpreted as evidence of hyperactivity and reduced anxiety—and were insensitive to amphetamine (Glerup et al., 2013). Hyperactivity was also noticed in the open field test.

Other

SORLA-deficient mice exhibited increased neuronal ERK signaling and enhanced adult neurogenesis; these effects of SORLA deficiency were APP-dependent, possibly stimulated by elevated sAPP in the Sorl1 knockout mice (Rohe et al., 2008).

Early data suggest that nigrostriatal connectivity is disrupted in SORLA-deficient mice: Approximately 25 percent fewer dopaminergic neurons project from the substantia nigra to the striatum in the knockout mice compared with wild-type mice, assessed at 10 weeks of age (Glerup et al., 2013).

A study using sodium (²³Na) magnetic resonance imaging found evidence of metabolic dysfunction in the brains of SORLA-deficient mice as young as 8 weeks of age (Bøgh et al., 2024). This method measures tissue Na⁺ concentrations—it is postulated that Na⁺ imbalances reflect metabolic disturbances, as maintenance of the Na⁺ gradient across cell membranes is energy intensive. Compared with wild-type mice, heterozygous and homozygous SORL1-null mice showed 11 and 23 percent reductions, respectively, in brain sodium concentrations.

Modification Details

The 5' region of exon 4 of the murine Sorl1 gene was replaced by a neomycin resistance cassette, through homologous recombination.

Phenotype Characterization

Complementary Models

SORL1-deficient human iPSCs

Human induced pluripotent cell lines (iPSCs) are complementary models for the study of SORL1 biology at the cellular level.

An isogenic set of iPSCs homozygous or heterozygous for a SORL1-null allele was generated from a parental cell line expressing wild-type SORL1 (Hung et al., 2021). SORLA-deficient neurons derived from these iPSCs—either heterozygous or homozygous for the SORL1-null allele—showed enlarged early endosomes, compared with the parental cell line. Additionally, homozygous SORL1-null neurons showed enlargement of late endosomes/lysosomes and a time-dependent decrease in levels of full-length APP and increases in levels of CTF-β/APP, Aβ40 and Aβ42. These iPSC lines may be requested from the lead investigator.

Additional SORL1-null iPSC lines have been generated and differentiated into various neural cell types (Knupp et al., 2020; Lee et al., 2023). These lines also may be requested from the lead investigators.

Confirming the finding cited above (Hung et al., 2021), SORL1-null iPSC-derived neurons exhibited enlarged early endosomes (Knupp et al., 2020). APP trafficking was also affected by loss of SORL1: There was increased colocalization of APP with a marker of early endosomes and decreased colocalization with a marker for the trans-Golgi network (Knupp et al., 2020). SORL1-null neurons produced more Aβ40 and Aβ42 than the parental lines, although levels of APP did not differ between genotypes (Knupp et al., 2020; Lee et al., 2023). Expression and release of ApoE and clusterin were reduced and levels of AT8-immunoreactive tau were increased in SORL1-null neurons, compared with the parental cell lines (Lee et al., 2023).  Transcriptomic and Gene Ontology analyses revealed upregulation of genes encoding synaptic proteins and downregulation of genes involved in extracellular matrix organization, SMAD signaling, transmembrane serine/threonine kinase signaling, lysosome localization, endocytosis, lipid biosynthetic processes and lipid-mediated signaling, protein secretion, and calcium-dependent phospholipid binding (Lee et al., 2023). Lipidomic analysis indicated differences between SORL1-null neurons and neurons derived from their parental lines, as well as more and larger lipid droplets in the cells lacking SORL1 (Lee et al., 2023).

Contrary to the findings in iPSC-derived neurons, loss of SORL1 did not affect endosome size in microglia-like cells derived from these iPSCs (Knupp et al., 2020) or levels of ApoE or clusterin (Lee et al., 2023).

Loss of SORL1 led to elevated levels of APOE and CLU mRNA in iPSC-derived astrocytes, while levels of ApoE and clusterin proteins did not depend on SORL1 (Lee et al., 2023).

SORL1 haploinsufficient minipigs

A minipig model of SORL1 haploinsufficiency is commercially available. Animals are heterozygous for the wild-type porcine SORL1 allele and a CRISPR/Cas9-modified allele with a 609-base pair deletion encompassing exon 1. The initial characterization of the model focused on neuropathology and AD-related fluid and imaging biomarkers in a relatively small number of animals ranging in age from 5 to 38 months (Andersen et al., 2022).

Levels of SORL1 mRNA were found to be reduced by approximately half in the cortices and cerebella of 24- to 30-month-old heterozygotes (hereafter referred to as “SORLA-deficient pigs”), compared with wild-type minipigs. While levels of SORLA protein were also reduced in the cortex (by approximately 70 percent), protein levels in the cerebellum did not differ between the SORLA-deficient and wild-type pigs. CSF concentrations of sSORLA, a soluble fragment cleaved from the cell surface, were also reduced in the SORLA-deficient pigs. Levels of APP in the cortex and cerebellum did not differ between genotypes.

No gross or histological abnormalities were observed in the brains of SORLA-deficient pigs at 5 months of age. Immunohistochemical staining of the hippocampi of 30-month-old pigs using polyclonal antibodies directed against Aβ42 or tau phosphorylated at threonine-231 did not show differences between genotypes. Enlarged endosomes were observed in cortical neurons of SORLA-deficient pigs 24 to 30 months of age. Endosomal enlargement was confirmed in fibroblasts cultured from the SORLA-deficient pigs compared with fibroblasts from wild-type pigs.

Levels of Aβ38, Aβ40, and Aβ42 were elevated in the cerebrospinal fluid of SORLA-deficient pigs 5 to 38 months of age, compared with age-matched wild-type pigs, while levels of  neurofilament light chain (NfL) did not differ between genotypes. SORLA deficiency led to elevated levels of CSF tau in 18- to 30-month-old animals.

In the initial characterization (Andersen et al., 2022), no differences were observed between the SORLA-deficient pigs and wild-type animals studied using PiB-PET to visualize amyloid load and FDG-PET to visualize glucose uptake in 21-month-old pigs, and structural and diffusion tensor imaging MRI in 27-month-old pigs. However, a subsequent exploratory study using two other imaging markers—MRI with hyperpolarized [1-¹³C]pyruvate and sodium (²³Na) MRI—found evidence of metabolic dysfunction in SORLA-deficient animals 22 to 27 months of age (Bøgh et al., 2024). The former method measures metabolism through glycolytic pathways, while the latter measures tissue Na⁺ concentrations (maintenance of the Na⁺ gradient across cell membranes is energy intensive, and it is postulated that metabolic disturbances lead to Na⁺ imbalances).

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References

Research Models Citations

1. APPswe/PSEN1dE9 (line 85)
2. PDAPP(line109)

AlzAntibodies Citations

1. Tau (AT8); Phospho Tau (Ser 202, Thr 205) 18 May 2022

Paper Citations

1. Andersen OM, Reiche J, Schmidt V, Gotthardt M, Spoelgen R, Behlke J, von Arnim CA, Breiderhoff T, Jansen P, Wu X, Bales KR, Cappai R, Masters CL, Gliemann J, Mufson EJ, Hyman BT, Paul SM, Nykjaer A, Willnow TE. Neuronal sorting protein-related receptor sorLA/LR11 regulates processing of the amyloid precursor protein. Proc Natl Acad Sci U S A. 2005 Sep 20;102(38):13461-6. PubMed.
2. Dodson SE, Andersen OM, Karmali V, Fritz JJ, Cheng D, Peng J, Levey AI, Willnow TE, Lah JJ. Loss of LR11/SORLA enhances early pathology in a mouse model of amyloidosis: evidence for a proximal role in Alzheimer's disease. J Neurosci. 2008 Nov 26;28(48):12877-86. PubMed.
3. Reiche J, Theilig F, Rafiqi FH, Carlo AS, Militz D, Mutig K, Todiras M, Christensen EI, Ellison DH, Bader M, Nykjaer A, Bachmann S, Alessi D, Willnow TE. SORLA/SORL1 functionally interacts with SPAK to control renal activation of Na(+)-K(+)-Cl(-) cotransporter 2. Mol Cell Biol. 2010 Jun;30(12):3027-37. Epub 2010 Apr 12 PubMed.
4. Schmidt V, Schulz N, Yan X, Schürmann A, Kempa S, Kern M, Blüher M, Poy MN, Olivecrona G, Willnow TE. SORLA facilitates insulin receptor signaling in adipocytes and exacerbates obesity. J Clin Invest. 2016 Jul 1;126(7):2706-20. Epub 2016 Jun 20 PubMed.
5. Monti G, Jensen ML, Mehmedbasic A, Jørgensen MM, Holm IE, Barkholt P, Zole E, Vægter CB, Vorum H, Nyengaard JR, Andersen OM. SORLA Expression in Synaptic Plexiform Layers of Mouse Retina. Mol Neurobiol. 2020 Jul;57(7):3106-3117. Epub 2020 May 29 PubMed.
6. Rohe M, Carlo AS, Breyhan H, Sporbert A, Militz D, Schmidt V, Wozny C, Harmeier A, Erdmann B, Bales KR, Wolf S, Kempermann G, Paul SM, Schmitz D, Bayer TA, Willnow TE, Andersen OM. Sortilin-related receptor with A-type repeats (SORLA) affects the amyloid precursor protein-dependent stimulation of ERK signaling and adult neurogenesis. J Biol Chem. 2008 May 23;283(21):14826-34. PubMed.
7. Glerup S, Lume M, Olsen D, Nyengaard JR, Vaegter CB, Gustafsen C, Christensen EI, Kjolby M, Hay-Schmidt A, Bender D, Madsen P, Saarma M, Nykjaer A, Petersen CM. SorLA controls neurotrophic activity by sorting of GDNF and its receptors GFRα1 and RET. Cell Rep. 2013 Jan 31;3(1):186-99. Epub 2013 Jan 17 PubMed.
8. Bøgh N, Sørensen CB, Alstrup AK, Hansen ES, Andersen OM, Laustsen C. Mice and minipigs with compromised expression of the Alzheimer's disease gene SORL1 show cerebral metabolic disturbances on hyperpolarized [1-13C]pyruvate and sodium MRI. Brain Commun. 2024;6(2):fcae114. Epub 2024 Mar 31 PubMed.
9. Hung C, Tuck E, Stubbs V, van der Lee SJ, Aalfs C, van Spaendonk R, Scheltens P, Hardy J, Holstege H, Livesey FJ. SORL1 deficiency in human excitatory neurons causes APP-dependent defects in the endolysosome-autophagy network. Cell Rep. 2021 Jun 15;35(11):109259. PubMed.
10. Knupp A, Mishra S, Martinez R, Braggin JE, Szabo M, Kinoshita C, Hailey DW, Small SA, Jayadev S, Young JE. Depletion of the AD Risk Gene SORL1 Selectively Impairs Neuronal Endosomal Traffic Independent of Amyloidogenic APP Processing. Cell Rep. 2020 Jun 2;31(9):107719. PubMed.
11. Lee H, Aylward AJ, Pearse RV 2nd, Lish AM, Hsieh YC, Augur ZM, Benoit CR, Chou V, Knupp A, Pan C, Goberdhan S, Duong DM, Seyfried NT, Bennett DA, Taga MF, Huynh K, Arnold M, Meikle PJ, De Jager PL, Menon V, Young JE, Young-Pearse TL. Cell-type-specific regulation of APOE and CLU levels in human neurons by the Alzheimer's disease risk gene SORL1. Cell Rep. 2023 Aug 29;42(8):112994. Epub 2023 Aug 22 PubMed.
12. Andersen OM, Bøgh N, Landau AM, Pløen GG, Jensen AM, Monti G, Ulhøi BP, Nyengaard JR, Jacobsen KR, Jørgensen MM, Holm IE, Kristensen ML, Alstrup AK, Hansen ES, Teunissen CE, Breidenbach L, Droescher M, Liu Y, Pedersen HS, Callesen H, Luo Y, Bolund L, Brooks DJ, Laustsen C, Small SA, Mikkelsen LF, Sørensen CB. A genetically modified minipig model for Alzheimer's disease with SORL1 haploinsufficiency. Cell Rep Med. 2022 Sep 20;3(9):100740. Epub 2022 Sep 12 PubMed.
13. Bøgh N, Sørensen CB, Alstrup AK, Hansen ES, Andersen OM, Laustsen C. Mice and minipigs with compromised expression of the Alzheimer's disease gene SORL1 show cerebral metabolic disturbances on hyperpolarized [1-13C]pyruvate and sodium MRI. Brain Commun. 2024;6(2):fcae114. Epub 2024 Mar 31 PubMed.

Other Citations

1. Thomas Willnow

External Citations

1. commercially available

Further Reading