Calpains I and II are calcium-dependent proteases that have been implicated in several aspects of brain function, including neurofilament turnover, Wallerian degeneration, and excitatory synaptic transmission. In this study, specific affinity-purified antibodies against each of the enzymes were used to determine their cellular distribution in rat brain. Differences between the two were found throughout the brain, with calpain I being located primarily in neurons, whereas calpain II was more prominent in glial cells. In myelinated axons, calpain II was present at low levels but calpain I was not detectable. In all brain areas, both enzymes were concentrated in cell bodies, with lesser amounts in neuronal and glial processes. Calpain I was only detectable proximally in dendrites and was not found in spiny branchlets of either pyramidal or Purkinje cells. These results suggest that calpain II is the likely form of the enzyme involved in calcium-activated proteolytic phenomena in axons. They do not support the existence of a role for calpain at excitatory axospinous synapses.
The developmental distribution patterns of microtubule-associated proteins (MAPs) 1, 2, and 3 were studied using three monoclonal antibodies. Immunochemical staining at the light and electron microscopic levels demonstrated the specific localization of each MAP in different cellular and subcellular compartments. MAP2, which is specifically associated with dendritic microtubules in the adult brain, is strictly associated with growing dendrites from the onset of their formation. MAP3, a recently described MAP of Mr = 180,000, which in the adult brain is associated with neurofilament-rich axons and glial processes, is associated with axons from the beginning of outgrowth. Although MAP3 is not detectable in granule cells and their parallel fiber axons in the mature cerebellum, it does appear transitorily in these axons during development. During neuronal differentiation, MAP1 is found first in axons and only later in dendrites where the highest concentrations are eventually to be found. These results indicate that the combined appearance of MAP1 and MAP2 (dendrites) or MAP1 and MAP3 (axons) correlates with the appearance of morphologically distinct microtubules and provide further evidence that specific MAPs are molecular determinants of dendritic and axonal formation.
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