Neuronal B-binding Factors Consist of Sp1-related Proteins FUNCTIONAL IMPLICATIONS FOR AUTOREGULATION

Neurons contain a protein factor capable of binding DNA elements normally bound by the transcription factor NFB. However, several lines of evidence suggest that this neuronal B-binding factor (NKBF) is not bona fide NFB. We have identified NKBF from cultures of neocortical neurons as a complex containing proteins related to Sp1. This complex was bound by antibodies to Sp1, Sp3, and Sp4 and was competed from binding to an NFB element by an oligonucleotide containing an Sp1binding site. This Sp1 oligonucleotide detected an abundant factor in neuronal nuclei that migrated in electrophoretic mobility shift assays at a position consistent with NKBF. Expression of transfected Sp1 stimulated transcription in a manner dependent upon a B ciselement. Similar to our previous reports for NKBF (Mao, X., Moerman, A. M., Lucas, M. M., and Barger, S. W. (1999) J. Neurochem. 73, 1851–1858 and Moerman, A. M., Mao, X., Lucas, M. M., and Barger, S. W. (1999) Mol. Brain Res. 67, 303–315), the activity of the Sp1-related factor was reduced by activation of ionotropic glutamate receptors, consistent with proteolytic degradation of all three Sp1-related factors. Expression of the N-methyl-D-aspartate receptor-1 (NR1) subunit of glutamate receptors correlated with the activity of the Sp1-related factor, specifically through an Sp1 element in the NR1 promoter. These data provide the first evidence that Sp1 or related family members are responsible for B-binding activity and are involved in a negative feedback for NR1 in central nervous system neurons.