Abstract
Cerebellar Purkinje neurons (PNs) strongly affect motor coordination and learning. PN study has contributed significantly to fundamental concepts of modern neuroscience. The present investigation defines distinctive molecular signaling pathways through which tissue plasminogen activator/plasmin-based proteolysis regulates postnatal PN dendrite development, synapse formation, mitochondrial morphology and function, and PN survival. These pathways involve differentially acting downstream constituents, including protein kinase Cγ, brain-derived neurotrophic factor, and a voltage-dependent anion channel. The metabolic mechanisms established here may apply to the development and degeneration of PNs and additional types of neurons in many animal and human brain diseases in which PNs are notably vulnerable.
The cerebellar cortex is centrally involved in motor coordination and learning, and its sole output is provided by Purkinje neurons (PNs). Growth of PN dendrites and their major synaptic input from granule cell parallel fiber axons takes place almost entirely in the first several postnatal weeks. PNs are more vulnerable to cell death than most other neurons, but the mechanisms remain unclear. We find that the homozygous nervous (
nr
) mutant mouse’s 10-fold–increased cerebellar tissue plasminogen activator (tPA), a part of the tPA/plasmin proteolytic system, influences several different molecular mechanisms, each regulating a key aspect of postnatal PN development, followed by selective PN necrosis, as follows.
(i
) Excess endogenous or exogenous tPA inhibits dendritic growth in vivo and in vitro by activating protein kinase Cγ and phosphorylation of microtubule-associated protein 2. (
ii
) tPA/plasmin proteolysis impairs parallel fiber-PN synaptogenesis by blocking brain-derived neurotrophic factor/tyrosine kinase receptor B signaling. (
iii
) Voltage-dependent anion channel 1 (a mitochondrial and plasma membrane protein) bound with kringle 5 (a peptide derived from the excess plasminogen) promotes pathological enlargement and rounding of PN mitochondria, reduces mitochondrial membrane potential, and damages plasma membranes. These abnormalities culminate in young
nr
PN necrosis that can be mimicked in wild-type PNs by exogenous tPA injection into cerebellum or prevented by endogenous tPA deletion in
nr:tPA
-knockout double mutants. In sum, excess tPA/plasmin, through separate downstream molecular mechanisms, regulates postnatal PN dendritogenesis, synaptogenesis, mitochondrial structure and function, and selective PN viability.