Altered Balance of Reelin Proteolytic Fragments in the Cerebrospinal Fluid of Alzheimer’s Disease Patients

Abstract

Reelin binds to the apolipoprotein E receptor apoER2 to activate an intracellular signaling cascade. The proteolytic cleavage of reelin follows receptor binding but can also occur independently of its binding to receptors. This study assesses whether reelin proteolytic fragments are differentially affected in the cerebrospinal fluid (CSF) of Alzheimer’s disease (AD) subjects. CSF reelin species were analyzed by Western blotting, employing antibodies against the N- and C-terminal domains. In AD patients, we found a decrease in the 420 kDa full-length reelin compared with controls. In these patients, we also found an increase in the N-terminal 310 kDa fragment resulting from the cleavage at the so-called C-t site, whereas the 180 kDa fragment originated from the N-t site remained unchanged. Regarding the C-terminal proteolytic fragments, the 100 kDa fragment resulting from the cleavage at the C-t site also displayed increased levels, whilst the one resulting from the N-t site, the 250 kDa fragment, decreased. We also detected the presence of an aberrant reelin species with a molecular mass of around 500 kDa present in AD samples (34 of 43 cases), while it was absent in the 14 control cases analyzed. These 500 kDa species were only immunoreactive to N-terminal antibodies. We validated the occurrence of these aberrant reelin species in an Aβ42-treated reelin-overexpressing cell model. When we compared the AD samples from APOE genotype subgroups, we only found minor differences in the levels of reelin fragments associated to the APOE genotype, but interestingly, the levels of fragments of apoER2 were lower in APOE ε4 carriers with regards to APOE ε3/ε3. The altered proportion of reelin/apoER2 fragments and the occurrence of reelin aberrant species suggest a complex regulation of the reelin signaling pathway, which results impaired in AD subjects.

Morphological and biomolecular targets in retina and vitreous from Reelin-deficient mice (Reeler): Potential implications for age-related macular degeneration in Alzheimer’s dementia

“…This cytokine expression, most probably the result of Reelin deprivation and consequent neurodegeneration, could be the trigger of AD markers’ deposition/expression. The localization of APP/Aβ and tau in these Reelin-deprived retinas might sustain the hypothesis of a link between Reelin and AD markers, as recently reported ( Lopez-Font et al, 2022 ).…”

“…Neuroinflammation, neurodegeneration and neuroprotection remain important aspects to investigate in human AD progression ( Fuster-Matanzo et al, 2013 ; Cai et al, 2014 ; Ransohoff, 2016 ). It remains unclear whether Reelin-deprivation or the consequent inflammation/neurodegeneration might take part in Aβ plaque accumulation in human retina, as observed in the central nervous system ( Lopez-Font et al, 2022 ). Cross-studies of Reelin-deficient and Reelin-overexpressing transgenic mice might sustain AD-genetic mice in understanding how to reduce amyloid plaque formation and counteract the neurodegenerative events occurring in nervous and visual systems ( Pujadas et al, 2014 ).…”

Sex differences in basal reelin levels in the paraventricular hypothalamus and in response to chronic stress induced by repeated corticosterone in rats

“…with different biological functions, and alterations in proteolytic cleavage of reelin have previously been detected in the CSF of subjects with different neurodegenerative diseases (Botella-López et al, 2006;Lopez-Font et al, 2022). Moreover cleavage at the N-terminal (N-t) site is thought to provide better diffusion of the central and NR6 (N-terminal with central) reelin fragments throughout the brain and promote more efficient reelin signalling (Koie et al, 2014) and the N-t fragment mediates the non-canonical actions of reelin (Wasser & Herz, 2017).…”

“…An altered balance of reelin proteolytic fragments has been observed in various reelin-related psychiatric disorders. The NR2 fragment is 166 increased in AD and frontotemporal dementia patients (Sáez-Valero et al, 2003;Botella-López et al, 2006) while it is decreased in blood samples of depression and bipolar disorder compared to healthy subjects (Lopez-Font et al, 2022). Moreover, in schizophrenia patients, the NR6 fragment is increased (Fatemi et al, 2001).…”

Reelin Signaling in Neurodevelopmental Disorders and Neurodegenerative Diseases

“…Studies of AD cerebrospinal fluid suggest a potential alteration in Reelin processing as well; however, the fragment analysis data are conflicting. One group observed an increase in C-terminal cleavage but no change in N-terminal cleavage [197], while others observed an increase in total Reelin and fragments indicative of increased N-terminal cleavage [100]. Some of these problems with analysis could depend on sample handling and preparation for analysis, due to Reelin's instability and labile nature rending quantitative analysis difficult.…”