The adult CNS contains an abundant population of oligodendrocyte precursor cells (NG2+ cells) that generate oligodendrocytes and repair myelin, but how these ubiquitous progenitors maintain their density is unknown. We generated NG2-mEGFP mice and used in vivo two-photon imaging to study their dynamics in the adult brain. Time-lapse imaging revealed that NG2+ cells in the cortex were highly dynamic; they surveyed their local environment with motile filopodia, extended growth cones and continuously migrated. They maintained unique territories though self-avoidance, and NG2+ cell loss though death, differentiation or ablation triggered rapid migration and proliferation of adjacent cells to restore their density. NG2+ cells recruited to sites of focal CNS injury were similarly replaced by a proliferative burst surrounding the injury site. Thus, homeostatic control of NG2+ cell density through a balance of active growth and self-repulsion ensures that these progenitors are available to replace oligodendrocytes and participate in tissue repair.
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We thank M. Pucak, N. Ye and T. Lee for technical assistance, B. Cudmore (Johns Hopkins University) and S. Wang (Princeton University) for advice on cranial window implantation, W.-B. Gan (New York University) for advice on preparing thinned skull windows, and members of the Bergles laboratory for discussions. E.G.H. was supported by a Kirschstein National Research Service Award grant from the US National Institutes of Health (F32NS076098). Funding was provided by grants from the US National Institutes of Health (NS051509, NS050274) and the Brain Science Institute at Johns Hopkins University.
Density and distribution of NG2+ cells in the somatosensory cortex visualized through in vivo two-photon imaging. In vivo two-photon images of EGFP-expressing cells at increasing depths in the somatosensory cortex of an adult NG2-mEGFP-H mouse implanted with a chronic cranial window. NG2+ cells are evenly distributed throughout the upper layers of the cortex and occupy non-overlapping domains. A subset of perivascular cells that enwrap blood vessels also express EGFP, providing landmarks for locating the same regions during repetitive time-lapse imaging. The autofluoresence at the surface arises from the meninges. (Image width: 300 μm; frame rate: 7 frames per second).
Numerous motile filopodia extend from NG2+ cells. In vivo time-lapse imaging of an individual NG2+ cell located 90–135 μm from the brain surface. Images were acquired every 1.5 minutes for 1 hour. Many thin filopodia can be seen extending and retracting along the processes of this cell on a time scale of minutes (e.g. green arrow). Process tips extend many dynamic filopodia while advancing (magenta arrow). Filopodia retract after making contact with a neighboring NG2+ cell process (yellow arrow). Note the EGFP+ pericyte enwrapping a blood vessel (lower left), which pulsates due to vessel constriction and dilation. (Image width: 112 μm; frame rate: 7 frames per second).
Contact mediated repulsion between NG2+ cell processes. In vivo time-lapse images showing examples of homotypic interactions between processes of an NG2+ cell. Motile filopodia that contact neighboring processes halt their extension and retract. Images were collected 128–132 μm from the cortical surface, and acquired every 3 seconds for 18.5 minutes. (Image width: 14 μm; fame rate: 50 frames per second).
NG2+ cell processes with motile filopodia imaged in a thinned-skull preparation. In vivo time-lapse images from a mouse imaged through a thinned-skull window, showing the presence of motile filopodia along a NG2+ cell process. Images were collected 30–40 μm from the brain surface, and acquired every 1.5 minutes for 15 minutes. Filopodia along the NG2+ cell processes extend and retract on a time scale of minutes, similar to the dynamic behavior of filopodia seen along NG2+ cell processes in mice implanted with chronic cranial windows. (Image width: 118 μm; frame rate: 5 frames per second).
Dynamic reorganization of NG2+ cells within the cortex. In vivo time-lapse images of NG2+ cells located 60–90 μm from the cortical surface. Images were acquired every 2 days for 40 days. NG2+ cells continually reorient their processes, migrate, and change their position within the cortex. An individual NG2+ cell (pseudo-colored green) is highlighted in the movie. It is located below the field of view at the start of the imaging period, and over 40 days migrates into the field of view and divides. Two blood vessels wrapped by EGFP expressing perivascular cells (pseudo-colored magenta) serve as landmarks. (Image width: 326 μm; frame rate: 5 frames per second).
NG2+ cells migrate through the cortex by somatic translocation. In vivo time-lapse images of a NG2+ cell (pseudo-colored green) and a perivascular cell (pseudo-colored red) located 45–114 μm from the cortical surface. During the two week imaging period this NG2+ cell migrated by translocation of the soma after process extension, while the position of the perivascular cell remained stable. Images were acquired every 2 days for 12 days. (Image width: 157 μm; frame rate: 2 frames per second).
NG2+ cells extend processes and encapsulate regions of tissue injury. In vivo time-lapse images of NG2+ cells located 150–165 μm from the cortical surface. Following induction of a laser-induced lesion at 0 hrs, NG2+ cells adjacent to the lesion reorient their processes, extend towards the lesion, encapsulating the site of injury within 24 hours. Images were acquired every 1 hour for 12 hours, and 1 day later. (Image width: 208 μm; frame rate: 3 frames per second).
Tissue injury triggers homeostatic replacement of NG2+ cells. In vivo time-lapse images of NG2+ cells located 100–147 μm from the cortical surface. Following induction of a laser-induced injury (pseudo-colored yellow), NG2+ cells migrate toward the lesion over subsequent days. An individual NG2+ cell (pseudo-colored green) migrated towards the lesion site and proliferated. Note that the movie pauses to highlight reorientation of the processes of this cell and the time of cell proliferation. Images were acquired approximately every 2 days for 41 days. (Image width: 220 μm; frame rate: 3 frames per second).