The distribution and function of GDE2, a regulator of spinal motor neuron survival, are disrupted in Amyotrophic Lateral Sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that affects the viability of upper and lower motor neurons. Current options for treatment are limited, necessitating deeper understanding of the mechanisms underlying ALS pathogenesis. Glycerophosphodiester phosphodiesterase 2 (GDE2 or GDPD5) is a six-transmembrane protein that acts on the cell surface to cleave the glycosylphosphatidylinositol (GPI)-anchor that tethers some proteins to the membrane. GDE2 is required for the survival of spinal motor neurons but whether GDE2 neuroprotective activity is disrupted in ALS is not known. We utilized a combination of mouse models and patient post-mortem samples to evaluate GDE2 functionality in ALS. Haplogenetic reduction of GDE2 exacerbated motor neuron degeneration and loss in SOD1G93A mice but not in control SOD1WT transgenic animals, indicating that GDE2 neuroprotective function is diminished in the context of SOD1G93A. In tissue samples from patients with ALS, total levels of GDE2 protein were equivalent to healthy controls; however, membrane levels of GDE2 were substantially reduced. Indeed, GDE2 was found to aberrantly accumulate in intracellular compartments of ALS motor cortex, consistent with a disruption of GDE2 function at the cell surface. Supporting the impairment of GDE2 activity in ALS, tandem-mass-tag mass spectrometry revealed a pronounced reduction of GPI-anchored proteins released into the CSF of patients with ALS compared with control patients. Taken together, this study provides cellular and biochemical evidence that GDE2 distribution and activity is disrupted in ALS, supporting the notion that the failure of GDE2-dependent neuroprotective pathways contributes to neurodegeneration and motor neuron loss in disease. These observations highlight the dysregulation of GPI-anchored protein pathways as candidate mediators of disease onset and progression and accordingly, provide new insight into the mechanisms underlying ALS pathogenesis.

Loss of GDE2 leads to complex behavioral changes including memory impairment

“…Impairments in many of these brain regions can result in behavioral changes associated with disease. Further, GDE2 is aberrantly mis-localized in AD, ALS, and ALS/FTD post-mortem brain and appears dysfunctional in these diseases [ 21 , 22 ]. Accordingly, these collective observations motivated us to investigate the consequences of GDE2 ablation in a range of behavioral tests, some of which evaluate behaviors known to be affected in disease.…”

“…In the mature nervous system, GDE2 is required for the survival of spinal motor neurons and, in the brain, for the activation of ADAM10 (A Disintegrin and metalloprotease domain-containing protein 10) α-secretase processing of APP via cleavage and inactivation of the GPI-anchored metalloprotease inhibitor RECK (Reversion inducing cysteine rich protein with Kazal motifs) [ 20 , 21 ]. Notably, GDE2 abnormally accumulates in intracellular compartments in the brains of patients with AD, ALS, and ALS/FTD [ 21 , 22 ]. Supporting GDE2 dysfunction in disease, amounts of membrane RECK are increased in AD brain, and proteomic studies show a disproportionate reduction of released GPI-anchored proteins in the cerebrospinal fluid (CSF) of patients with ALS compared with controls [ 21 , 22 ].…”

“…Notably, GDE2 abnormally accumulates in intracellular compartments in the brains of patients with AD, ALS, and ALS/FTD [ 21 , 22 ]. Supporting GDE2 dysfunction in disease, amounts of membrane RECK are increased in AD brain, and proteomic studies show a disproportionate reduction of released GPI-anchored proteins in the cerebrospinal fluid (CSF) of patients with ALS compared with controls [ 21 , 22 ]. These observations raise the possibility that the erosion of GDE2 function may contribute to disease-related cellular pathologies.…”

“…Behavioral changes can occur early in ADRDs and progress over time with worsening cellular and molecular pathological changes. Since Gde2 KO mice exhibit neurodegenerative changes by 19 months, we chose to start our behavioral tests prior to this time point to capture behavioral changes that may develop earlier [ 21 , 22 ]. We conducted several behavioral tasks in WT and Gde2 KO mice at 7 months of age and explored more complex social and memory phenotypes at 11 and 16 months.…”

Discovery of Biomarkers for Amyotrophic Lateral Sclerosis from Human Cerebrospinal Fluid Using Mass-Spectrometry-Based Proteomics

“…Out of ≈3000 proteins identified in the TMT-based discovery experiment of this study, the data for 78 glycosylphosphatidylinositol-anchored proteins (GPI-APs) were published previously to show the differential release of GPI-APs by differential glycerophosphodiester phosphodiesterase 2 activity in ALS patients compared to control individuals [38].…”