Interaction of Lysosomes and Calpain on Neurofilament Steady-state Levels

Abstract Background: An imbalance of protease activity can foster the aberrant accumulation of phosphorylated neurofilaments (NF) within perikarya resembling human neuropathological conditions. Objective: We examined whether or not the inhibition of the lysosomal system increased NF levels. Method: Differentiated NB2a/d1 cells were transfected with GFP-tagged NF-heavy and treated with chlorquine (active against autophagy) and/or a calpain inhibitor. Additional cells were transfected with the cyclin-dependent kinase 5 activator p35. Results: Unexpectedly, chloroquine decreased NF levels, which was prevented by the simultaneous treatment with the calpain inhibitor or expression of p35. Discussion: These findings suggest that compensatory increases in calpain activity can accompany inhibition of autophagy. This reduction was also prevented by the expression of the cdk5 activator p35, consistent with the resistance of extensively phosphorylated NF-H to proteolysis by calpain. Conclusion: These findings highlight the complex interplay among kinase, phosphatase, and protease systems in neuropathology.