Post by Amanda McFarlan
What's the science?
The subgranular zone in the hippocampus regularly generates new dentate granule cells (neural stem cells) in the adult brain. Studies using chemical and transgenic approaches to target these cells have demonstrated that they migrate laterally along the subgranular zone. However, the mechanisms by which these cells migrate and integrate into the existing hippocampal neural circuit remain largely unknown. This week in Nature Communications, Wang and colleagues investigated the developmental stages of newly generated dentate granule cells in the hippocampus using in vivo imaging techniques in awake, behaving mice.
How did they do it?
To characterize the kinetics and critical period of dentate granule cell dispersion, the authors infected newly generated dentate granule cells in adult mice with a virus encoding the green fluorescent protein (GFP) and used immunohistochemistry at days 5, 6, and 7 post-infection to track the movement of these cells relative to the subgranular zone. Then, to assess the dynamics of dentate granule cell dispersion, they performed in vivo imaging of GFP-positive dentate granule cells in freely behaving mice for 2 days. To further investigate this dynamic process, they used a spinning disk confocal microscope to image virally-labelled dentate granule cells in cultured tissue from adult mice every 30 minutes for 3 days.
Next, the authors set out to determine whether newly generated dentate granule cells were electrically coupled, by performing whole-cell paired recordings of adjacent GFP-positive dentate granule cells in acute hippocampal slices. They also explored whether these cells were connected via gap junctions by labelling dentate granule cells with GFP in mice deficient for connexin 43 (a gap junction channel protein) and using in vivo imaging techniques to follow the dispersion of these cells. Finally, the authors investigated the importance of lateral dispersion for the integration of dentate granule cells into existing neural networks by expressing either a dominant-negative variant of the connexin 43 gene (disrupts the formation of gap junctions) or GFP (control) in dentate granule cells. They compared changes in dendritic morphology, membrane properties, and evoked and spontaneous synaptic transmission between the connexin 43-deficient and control dentate granule cells.
What did they find?
The authors found that the proportion of GFP-positive dentate granule cells that remained within ±10° relative to the subgranular zone decreased rapidly between days 5 and 7 post-infection, suggesting that dentate granule cells display a lateral dispersion pattern within this time period. In vivo imaging data collected across 2 days revealed that the average distance travelled by a dentate granule cell was 300 µm and the average speed was 5 µm/h, suggesting that the migration of these cells is highly dynamic. The authors also determined that most dentate granule cells moved in one direction, although a small subset of cells moved both forwards and backwards. When imaged in cultured tissue, they found that 35% of GFP-positive dentate granule cells dispersed in a leap-frog manner, jumping over adjacent cells.
Next, the authors determined that the frequency of electrically coupled cells decreased from day 5 to day 7 post-infection, suggesting that dentate granule cells become uncoupled following the cessation of lateral dispersion. They also found that bath application of carbenoxolone (a connexin blocker) disrupted the electrical coupling of dentate granule cells, suggesting that this coupling occurs through gap junctions. Additionally, in vivo imaging data revealed that lateral dispersion of dentate granule cells between days 5 and 7 post-infection is disrupted in connexin 43-deficient mice compared to controls, suggesting that this dispersion is dependent on electrical coupling via gap junctions. Finally, the authors showed that introducing connexin 43-deficiency in dentate granule cells between days 5 and 7 post-infection resulted in stunted dendritic growth compared to controls, while introducing connexin 43-deficiency in dentate granule cells after day 7 had no effect on dendritic growth compared to controls. Furthermore, they determined that connexin 43-deficient dentate granule cells had reduced evoked excitatory postsynaptic current amplitudes and a decrease in the frequency of spontaneous excitatory postsynaptic currents, suggesting that lateral dispersion of dentate granule cells is important for the integration of these cells into the neural circuit.
What's the impact?
This is the first study to show the developmental stages of newly generated dentate granule cells in the hippocampus of the adult mouse using in vivo imaging techniques. The authors demonstrated that lateral dispersion of the dentate granule cells early on in requires electrical coupling via gap junctions and is necessary for normal dendritic development and circuit integration of these cells. Altogether, these findings provide insight into the migration and integration of new hippocampal cells in the adult brain.
Wang et al. Lateral dispersion is required for circuit integration of newly generated dentate granule cells (2019).Access the original scientific publication here.