Measurement of cystatin C functional activity in the cerebrospinal fluid of amyotrophic lateral sclerosis and control subjects.

Background: Cystatin C is a constitutively expressed and abundant cysteine protease inhibitor within the cerebrospinal fluid (CSF). Recent studies have reported a significant reduction in cystatin C concentration in the CSF of patients with amyotrophic lateral sclerosis (ALS) and several other neurodegenerative diseases, relative to healthy controls. Cystatin C can exhibit both neuroprotective and neurotoxic properties, suggesting that altered CSF cystatin C concentrations could potentially impact the pathogenesis or progression of these disorders. However, it is unclear if alterations in cystatin C concentration result in physiologically relevant differences in its functional activity within the CSF. Measurements of the cysteine protease inhibitory activity of cystatin C within the CSF have not been reported, and the relationship between CSF cystatin C concentration and activity levels in different disease contexts has not been investigated.

Methods: We used a papain inhibition assay to evaluate the total cystatin C activity in CSF samples from 23 ALS patients, 23 healthy controls, and 23 neurological disease controls. Cystatin C concentrations in these samples were previously measured by ELISA. Correlations between cystatin C concentration and activity were assessed with nonparametric statistics. Activity ratios were compared among diagnostic groups using both one-way ANOVA and repeated measures statistics.

Results: Total cystatin C activity was found to be directly proportional to its protein concentration in all subjects, and cystatin C activity was not altered in ALS patients. In addition, our data suggest that cystatin C is the predominant cysteine protease inhibitor in human CSF.

Conclusions: Our data demonstrate the successful measurement of the functional activity of cystatin C in the CSF, and show that total cystatin C activity can be inferred from its total protein concentration. Our results also suggest that cystatin C is the major cysteine protease inhibitor in human CSF and altered CSF cystatin C concentration may play a role in the pathobiology of ALS and other neurological diseases.