Journal of Neuroscience, Vol 11, 2422-2432, Copyright © 1991 by Society for Neuroscience
Glutamate-stimulated protein phosphorylation in cultured hippocampal pyramidal neurons
WK Scholz and HC Palfrey
University of Chicago, Department of Pharmacological and Physiological Sciences, Illinois 60637.
The regulation of second-messenger production and protein phosphorylation
by glutamate has been investigated in primary cultures of pure hippocampal
pyramidal neurons. Embryonic rat pyramidal neurons were prepared according
to the procedures of Bartlett and Banker (1984) and studied 1-21 d after
plating. Glutamate caused a transient increase in intracellular free
[Ca2+], determined with fura-2, in the presence of 1.26 mM extracellular
Ca2+, but not in 50 nM free Ca(2+)-containing solution. Glutamate also
transiently increased cellular diacylglycerol content in both normal and
low-[Ca2+] media. Neurons were prelabeled with 32P-orthophosphate to label
intracellular ATP, then stimulated with glutamate (100 microM). A rapid
transient incorporation of 32P into primarily three proteins of 120, 87,
and 48 kDa was found by analysis of two-dimensional gels. At 30 sec after
glutamate stimulation, 32P incorporation into the 87-kDa and 48-kDa
proteins peaked (240% and 170% basal levels, respectively), and by 2 min,
phosphorylation of the 87-kDa protein had returned to basal levels, while
that of the 48-kDa protein decreased but remained above control levels. The
phosphorylation of these proteins appeared to be mediated by protein kinase
C (PKC) because all three showed an increase in phosphorylation after
phorbol ester treatment of cultures. Phosphate incorporation was
accompanied by an acidic shift in the isoelectric point of both 87- and
48-kDa proteins. Glutamate stimulation resulted in phosphorylation in the
presence and absence of Ca2+ influx. Antibody recognition and biochemical
characteristics indicated that the 87-kDa phosphoprotein is the PKC
substrate MARCKS (myristoylated, alanine-rich C-kinase substrate). The
48-kDa protein, though very similar to GAP-43, was not recognized by
specific antibodies raised against GAP-43. These results suggest that
glutamate stimulates the transient generation of second messengers that
activate PKC in hippocampal neurons, resulting in a significant increase in
the phosphorylation of three specific proteins.