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Abstract
The weaver mouse mutation is a genetic defect of unknown origin that leads to impairment
of cerebellar granule neuronal migration and to neuronal cell death. We investigated
laminin expression and proteolytic enzyme activity in this migration-deficient mouse
mutant in vivo and in vitro to search for a molecular basis for the weaver defect.
The weaver cerebellum showed a general increase in immunoreactivity for laminin, for
a neurite outgrowth domain of the B2 chain of laminin, and for tissue plasminogen
activator compared to the normal animals. Zymographic assays and immunocytochemistry
confirmed that tissue plasminogen activator was the proteolytic enzyme synthesized
in excess in the weaver mouse cerebellum in vivo. When placed in culture, the weaver
granule neurons survived poorly on a laminin substratum, and failed to extend long
neurites, unlike the normal cerebellar granule neurons. The cultured weaver granule
neurons were proteolytically overactive and secreted excessive amounts of tissue plasminogen
activator, which was likely to interfere with their neurite outgrowth potential on
a laminin substratum. Indeed, the weaver granule neurons but not the normal neurons
degraded laminin from their culture substratum and deposited a neurite outgrowth domain
of the B2 chain of laminin onto their surfaces. Electrophysiology showed that the
weaver granule neurons had poor resting membrane potentials (-38 V), whereas the normal
neurons had normal resting membrane potentials of (-61 V). The resting membrane potentials
of the weaver granule neurons were restored to near normal (-59 V) by a protease inhibitor,
aprotinin. Aprotinin also rescued the weaver granule neurons from death on a laminin
substratum and promoted their neurite outgrowth to the level of the normal animals.
These results indicate that increased proteolytic activity accompanied with increased
synthesis of laminin, and its B2 chain, distinguish the weaver mutation from the normal
animals. These molecular changes may contribute to the impairment of granule neuronal
migration and to the neuronal death, characteristic of the weaver mutation.