#24 Elucidating the role of bone morphogenetic protein 4 in endogenous cell response and astrocyte reactivity following spinal cord injury
Christopher G Hart, University of Manitoba; Scott M Dyck, University of Manitoba; Hardeep Kataria, University of Manitoba; Kallivalappil T Santhosh, University of Manitoba; Soheila Karimi-Abdolrezaee, University of Manitoba
Thousands of new spinal cord injury (SCI) patients occur in Canada every year. Reactive astrocytes modulate the SCI microenvironment by producing many inhibitory factors that limit functional recovery. Bone morphogenetic protein-4 (BMP4) is robustly activated at the acute stage of injury, and it is an essential morphogen in the developing CNS with an established role in modulating astrocyte differentiation. We hypothesize that BMP4 modulates neural stem/progenitor cell (NPC) differentiation and expression of inhibitory chondroitin sulfate proteoglycans (CSPGs) by reactive astrocytes following injury.
We employed in vitro cultures (primary rodent spinal cord NPCs and cortical astrocytes) and a clinically-relevant model of compressive SCI in rats. Using immunofluorescence, Western and slot blotting, we determined the effect of BMP4 on NPCs and astrocytes using an in vitro model of reactive astrogliosis, and its potential role in modulating the acute SCI.
BMP4 expression is robustly upregulated in the acute phase of SCI between 1 and 3 days post injury. Reactive astrocytes significantly increased their expression of BMP4 by 5-fold in response to IL-1β+TNFα. NPCs treated with BMP4 significantly decreased their proliferation by 60%, while predominantly differentiating into astrocytes (>50% increase). Astrocyte differentiation was inhibited when BMP4 was co-treated with noggin at a ratio of 1:3. BMP4 did not appear to promote pro-inflammatory cytokine or nitric oxide production in reactive astrocytes, but it did increase the production of inhibitory CSPGs by 2.5-fold. Local delivery of noggin to the injured spinal cord reduced astrogliosis (1.89-fold increase in GFAP) compared to injury alone (4.42-fold increase).
Our findings indicate that BMP4 is upregulated following SCI and promotes astrogliosis in mature astrocytes. BMP4 also promotes the differentiation of NPCs into astrocytes, which may contribute to scar formation. Our data has identified a new role for BMP4 in pathophysiology of SCI that requires further elucidation.