Alterations in cyclin-dependent protein kinase 5 (CDK5) protein levels, activity and immunocytochemistry in canine motor neuron disease.

Hereditary canine spinal muscular atrophy (HCSMA) is a dominantly inherited motor neuron disease in Brittany spaniels that is clinically characterized by progressive muscle weakness lending to paralysis. Histopathologically, degeneration is confined to motor neurons with accumulation of phosphorylated neurofilaments in axonal internodes. Cyclin-dependent kinase 5 (CDK5), a kinase related to the cell cycle kinase cdc2, phosphorylates neurofilaments and regulates neurofilament dynamics. We examined CDKS activity, protein levels, and cellular immunoreactivity in nervous tissue from dogs with HCSMA, from closely age-matched controls and from dogs with other neurological diseases. On immunoblot analysis, CDKS protein levels were increased in the HCSMA dogs (by similar to 1.5-fold in both the cytosolic and the particulate fractions). CDKS activity was significantly increased (by similar to 3-fold) in the particulate fractions in the HCSMA dogs compared to all controls. The finding that CDKS activity was increased in the young HCSMA homozygotes with the accelerated form of the disease, who do not show axonal swellings histologically, suggests that alterations in CDK5 occurs early in the pathogenesis, prior to the development of significant neurofilament pathology. Immunocytochemically, there was strong CDK5 staining of the nuclei, cytoplasm and axonal processes of the motor neurons in both control dogs and dogs with HCSMA. Further immunocytochemical studies demonstrated CDKS staining where neurofilaments accumulated, in axonal swellings in the dogs with HCSMA. Our observations suggest phosphorylation-dependent events mediated by CDKS occur in canine motor neuron disease.