BMP-7 and excess glutamate: opposing effects on dendrite growth from cerebral cortical neurons in vitro

Glutamate is an important regulator of dendrite development. During cerebral ischemia, however, there is massive release of glutamate reaching millimolar concentrations in the extracellular space. An early consequence of this excess glutamate is reduced dendrite growth. Bone morphogenetic protein-7 (BMP-7) a member of the transforming growth factor-beta (TGF-beta) superfamily has been demonstrated to enhance dendrite output from cerebral cortical and hippocampal neurons in vitro. However, it is not known whether BMP-7can prevent the reduced dendrite growth associated with excess glutamate or enhance dendrite growth after glutamate exposure. Therefore we quantified axon and primary, secondary, and total dendrite growth from embryonic mouse cortical neurons (E18) grown at low density in vitro in a chemically defined medium and exposed to glutamate (1 or 2 mM) for 48 h. Morphology and double immunolabeling (MAP2, NF-H) were used to identify cortical dendrites and axons after 3 DIV. In these short-term cultures, glutamate did not influence neuron survival. The addition of glutamate to cortical neurons, however, significantly attenuated dendrite output. This effect was mimicked by the addition of NMDA but not AMPA agonists and inhibited by the specific NMDA receptor antagonist MK-801. The reduction in dendrite growth mediated by excess glutamate was ameliorated by the administration of 30 or 100 ng/ml of BMP-7. In addition, when administered in a delayed fashion between 1 and 24 h after the initial glutamate exposure, BMP-7 was able to enhance dendrite growth, including primary dendrite number, primary dendrite length, and secondary dendritic branching. These findings demonstrate that BMP-7 can ameliorate reduced dendrite growth from cerebral cortical neurons associated with excess glutamate in vitro and are important because they may help explain why BMP-7 administration is associated with enhanced functional recovery in models of cerebral ischemia.