Species: Mouse
Genes: RAB5A
Modification: RAB5A: Transgenic
Disease Relevance: Alzheimer's Disease
Strain Name: N/A
Genetic Background: C57BL/6J
Availability: Available through Ralph Nixon.

Summary

Rab5 is a small GTPase that regulates endosome trafficking, sorting, and fusion. Enlarged Rab5-positive endosomes are among the earliest neuropathological signs of Alzheimer’s disease and Down’s syndrome (Cataldo et al., 2000), and a body of evidence suggests that endosome enlargement is a consequence of over activation of Rab5, stimulated by β-CTF (reviewed in Colacurcio et al. 2018; Germann et al., 2020).

PA-Rab5 mice (Pensalfini et al., 2020) overexpress human Rab5 in neurons. This model allows investigators to study the effects of overactivation of Rab5 independently of the effects of APP overexpression. Mice exhibit many characteristics of AD, including enlarged endosomes, tau hyperphosphorylation, defective synaptic plasticity, and degeneration of basal forebrain cholinergic neurons.

The following description refers to heterozygous mice.

PA-Rab5 mice outwardly appear healthy: Animals breed normally; have normal lifespans; and body weight, coat quality, and home-cage activity are unremarkable.

Levels of total Rab5—endogenous plus transgenic—in PA-Rab5 cortex and hippocampus are approximately 2.5-fold those in wild-type brains. Transgene expression is greatest in the cortex, hippocampus, amygdala, and midbrain and lower in the thalamus, striatum, and cerebellum. PA-Rab5 mice do not just have more Rab5 in their brains than do their wild-type littermates, they also have a greater proportion of activated Rab5: The ratio of activated Rab5 to total Rab5 in transgenic mice is about double that of their wild-type littermates (measured in the hippocampi of 6-month-old animals).

Levels of β-CTF, Aβ40 and Aβ42, and the Aβ42/Aβ40 ratio do not differ between PA-Rab5 and wild-type mice, as measured in cortical homogenates from 15-month-old animals.

Neuropathology

Enlarged endosomes are found in the brains of the transgenic mice, as in AD brains. Endosomes are also seen to accumulate abnormally in the dendrites of hippocampal neurons.

Levels of total tau are increased in the cortices of PA-Rab5 mice, compared with wild-type animals. Tau hyperphosphorylation, assessed by immunofluorescence using phospho-tau antibodies PHF-1 and CP13, was reported to be greater in the transgenic animals, although the increase in total tau did not appear to be factored into this comparison. PHF-1-immunoreactive neurons are abundant in cortical layers II/III and V; PHF-1 staining is punctate and co-localizes with Rab5-immunoreactive profiles. CP13 immunoreactivity is seen in about half of the neurons in layer II/III; CP13-positive neurons were described as having abnormal morphologies, and CP-13 staining did not appear distinctly punctate and only partially overlapped with Rab5 staining.

Fewer dendritic spines are found in the hippocampi (CA3 and dentate gyrus regions) of 8.5-month-old PA-Rab5 mice than in wild-type animals. While spine density in CA1 does not differ between the genotypes, shorter, stubbier spines are seen in the transgenic animals.

Notably, there is a loss of basal forebrain cholinergic neurons in the PA-Rab5 mice, beginning at 7 months of age. Signs of neurodegeneration, although not outright neuron loss, are seen in the hippocampus, a target of cholinergic projections. In addition to the abnormal accumulation of endosomes mentioned above, autophagic structures are seen in hippocampal dendrites, as well as in presynaptic terminals.

Electrophysiology

Basal synaptic transmission is normal in hippocampal slices from 6-month-old animals. However, LTP at Schaffer collateral-CA1 synapses is slightly impaired in PA-Rab5 slices compared with slices from wild-type mice, and there is a pronounced defect in LTD in the transgenic animals.

Behavior

When tested at 6 months of age, the performance of PA-Rab5 mice differed from wild-type controls in a novel object recognition test.

Modification Details

PA-Rab5 mice over express myc-tagged human Rab5, under the control of a Thy-1 promoter to direct expression to neurons.

Phenotype Characterization

COMMENTS / QUESTIONS

1. • Priyanka Narayan
     National Institutes of Health
   • Posted: 12 Jan 2021
   • Paper: Endosomal Dysfunction Induced by Directly Overactivating Rab5 Recapitulates Prodromal and Neurodegenerative Features of Alzheimer's Disease.

   This work points to the central role of early endocytosis in neurodegeneration. Studies of convergence of both GWAS hits and functional studies on early endocytosis, including this one, make it clear that disrupted early endocytosis is a key feature of neurodegeneration. Early endocytic defects have functional consequences beyond just their impact on APP or APP cleavage products.

   This study reminds me of work reporting that APOE4 homozygous iPSC-derived neurons also have enlarged early endosomes (Lin et al., 2018), like those observed upon increasing Rab5 expression. Perhaps APOE4 here is having a similar phenotypic effect as increased Rab5 expression.

   Interestingly, Lin et al. and our work (Narayan et al., 2020) has shown that, in the context of astrocytes, APOE4 results in a lower number of Rab5+ early endosomes. Overexpression of early endocytic factors like PICALM increases the efficiency of early endocytosis but, surprisingly, leads to compromised early endocytosis in a healthy APOE3 homozygous background. These and other studies make me very curious (1) whether different cell types respond the same way to increased Rab5 expression, and (2) whether increased expression of other early endocytic factors (in otherwise healthy contexts) may lead to similar detrimental consequences?

   It is very possible that increasing Rab5 expression, as the authors did in this study, phenocopies the action of other risk factors aside from APOE4. This could suggest a further convergence of risk phenotypes for AD. There is clearly lots to look at in the future! 

   References:

   Lin YT, Seo J, Gao F, Feldman HM, Wen HL, Penney J, Cam HP, Gjoneska E, Raja WK, Cheng J, Rueda R, Kritskiy O, Abdurrob F, Peng Z, Milo B, Yu CJ, Elmsaouri S, Dey D, Ko T, Yankner BA, Tsai LH. APOE4 Causes Widespread Molecular and Cellular Alterations Associated with Alzheimer's Disease Phenotypes in Human iPSC-Derived Brain Cell Types. Neuron. 2018 Jun 27;98(6):1141-1154.e7. Epub 2018 May 31 PubMed.

   Narayan P, Sienski G, Bonner JM, Lin YT, Seo J, Baru V, Haque A, Milo B, Akay LA, Graziosi A, Freyzon Y, Landgraf D, Hesse WR, Valastyan J, Barrasa MI, Tsai LH, Lindquist S. PICALM Rescues Endocytic Defects Caused by the Alzheimer's Disease Risk Factor APOE4. Cell Rep. 2020 Oct 6;33(1):108224. PubMed.

   View all comments by Priyanka Narayan

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References

Antibody Citations

1. Tau phos Ser396/Ser404 (PHF-1)
2. Tau phos Ser202 (CP13)

Paper Citations

1. Cataldo AM, Peterhoff CM, Troncoso JC, Gomez-Isla T, Hyman BT, Nixon RA. Endocytic pathway abnormalities precede amyloid beta deposition in sporadic Alzheimer's disease and Down syndrome: differential effects of APOE genotype and presenilin mutations. Am J Pathol. 2000 Jul;157(1):277-86. PubMed.
2. Colacurcio DJ, Pensalfini A, Jiang Y, Nixon RA. Dysfunction of autophagy and endosomal-lysosomal pathways: Roles in pathogenesis of Down syndrome and Alzheimer's Disease. Free Radic Biol Med. 2018 Jan;114:40-51. Epub 2017 Oct 6 PubMed.
3. Germann UA, Alam JJ. P38α MAPK Signaling-A Robust Therapeutic Target for Rab5-Mediated Neurodegenerative Disease. Int J Mol Sci. 2020 Jul 31;21(15) PubMed.
4. Pensalfini A, Kim S, Subbanna S, Bleiwas C, Goulbourne CN, Stavrides PH, Jiang Y, Lee JH, Darji S, Pawlik M, Huo C, Peddy J, Berg MJ, Smiley JF, Basavarajappa BS, Nixon RA. Endosomal Dysfunction Induced by Directly Overactivating Rab5 Recapitulates Prodromal and Neurodegenerative Features of Alzheimer's Disease. Cell Rep. 2020 Nov 24;33(8):108420. PubMed.

Other Citations

1. Ralph Nixon

Further Reading