Layer-specific spine remodeling and synaptopodin-linked spine resilience in the mouse dentate gyrus following entorhinal denervation.

Following brain injury, neurons receiving afferent input from the site of injury become denervated. An established model for studying denervation-induced changes of neurons is the denervation of dentate granule cells following unilateral transection of the perforant pathway. In response to denervation, granule cells show a characteristic sequence of spine loss followed by spine recovery. Notably, some spines are more resistant to denervation than others, and the basis of this resilience is unknown. As in vitro data suggest that the actin-modulating protein synaptopodin stabilizes denervated spines, we hypothesized that the resilient granule cell spines contain this protein. To test this hypothesis in vivo, we performed entorhinal denervation in mice (3, 7, 14, and 28 days), intracellularly injected granule cells in fixed slices with Alexa 568 -hydrazide dye, stained for synaptopodin and subsequently analyzed granule cell spines in the denervated outer and non-denervated inner molecular layer of the denervated dentate gyrus. In the denervated zone, synaptopodin-positive spines exhibited resilience to denervation, whereas synaptopodin-negative spines showed a transient ∼40% drop in spine density. Furthermore, surviving synaptopodin-negative spines exhibited an increased head size. Within the non-denervated zone, spine density was maintained. However, the subpopulation of synaptopodin-negative spines also showed larger spine heads. Together, our morphological data are in line with (i) a spine-stabilizing role of synaptopodin, (ii) functional data indicating strengthening of surviving spines in the denervated outer molecular layer, and (iii) a functional reorganization of the network within the non-denervated inner molecular layer of the dentate gyrus.