Journal of Neuroscience, Vol 11, 2249-2258, Copyright © 1991 by Society for Neuroscience
Internalization and processing of basic fibroblast growth factor by neurons and astrocytes
PA Walicke and A Baird
Department of Neuroscience, University of California, San Diego, La Jolla 92037.
The fate of iodinated basic fibroblast growth factor (FGF) after its
binding to cultured astrocytes and hippocampal neurons was studied.
Autoradiography after light and electron microscopy establishes that, if
cells are returned to 37 degrees C, the 125I-basic FGF bound internalizes
into vesicles in the cytoplasm, localizes to the perinuclear cytoplasm, and
is translocated to chromatin structures of the nucleus. The radiolabeled
protein is long-lived, a finding confirmed by biochemical analyses.
Polyacrylamide gel electrophoresis and autoradiography of both hippocampal
neurons and astrocyte extracts reveal that these cells internalize
125I-basic FGF and then metabolize it to three major heparin-binding
peptides with molecular weights of 15.5, 9, and 4 kDa. These peptides are
initially detected 16 hr after binding to neurons and 4 hr after binding to
astrocytes but are still detectable 48 and 16 hr, respectively, after
initial binding (though present at lower levels). Immunoprecipitation with
sequence-specific antisera to basic FGF reveals that the 15.5-kDa fragment
is generated by cleavage at the carboxyl terminus, that the 9-kDa peptide
contains the sequences between residues 30 and 87, and the 4-kDa peptide is
a C- terminus fragment containing the sequence of basic FGF(106-120) but
without basic FGF(139-146) immunoreactivity. The internalization of basic
FGF is required for this processing; the treatment of cells with trypsin
and 2 M NaCl at different times after binding can only prevent the
metabolism of basic FGF if it is performed immediately after binding.
Similarly, WGA, which inhibits basic FGF binding to its high- affinity
receptor, prevents the metabolism of basic FGF. The possible significance
of a metabolic pathway that is responsible for the processing of basic FGF
after its internalization by cells in the CNS is discussed in light of its
potential function as a neurotrophic factor.
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