The Hatten Lab, Projects, Mechanism of neuronal migration along radial glial fibers.

The Hatten Lab Projects

Mechanisms of Glial-Guided Neuronal Migration

To study granule cell migration on glial fibers, we developed methods to purify granule neurons and cerebellar astroglia (Hatten, 1985), and then to recombine the purified cell populations in microcultures. By high contrast, Nomarski optics, we imaged the dynamics of neuronal movements along glial guides (Edmondson and Hatten, 1987). Our video recordings provided a detailed view of migration, showing that the nucleus remains in the rear of the cell, that an adhesion junction forms underneath the nucleus and that the neuron extends a leading process in the direction of migration. The leading process differs from a neuronal growth cone because the microtubules within the leading process extend to the tip of the process (unlike growth cones where they extend only into a central core), and because short filopodia rapidly form enwrapping the glial fiber (rather than a single motile ending at the tip of the process). Moreover, quantitation of the dynamics of movement indicate that the leading process does not "pull" the neuron, rather the neuron moves by release and reformation of the adhesion junction beneath the cell soma. The nucleus remains in the posterior, moving with the soma, and does not undergo "nuclear migration", ie. movement through the cytoplasm into the leading process. Thus the granule cell migrates along the glial fiber with a pattern of movement that is similar to that of metazoan cells on a thin fiber. A number of video clips of granule cell migration are shown below:

(MPEG movie, 1.31MB)

(GIF89a movie, 736KB)

Hatten and Edmondson, 1987. (J. Neurosci.)

Migration of cerebellar granule cells along astroglial fibers in vitro. Immature granule neurons, purified from early postnatal cerebellum, extend a specialized, migratory process along the underlying glial fiber. In this sequence, a neuron moves along a glial fiber at approximately 50 microns/h. As the neuron moves, lamellipodia and filopodia extend and retract along the length of the migratory process, enwrapping the glial guide. The neuron forms an interstitial junction along the length of the cell soma, which is released as the cell begins to move.

(MPEG movie, 528KB)

(GIF89a movie, 554KB)

Gasser and Hatten, 1989. (Proc. Natl. Acad. Sci USA)

Migration of neurons from one brain region along astroglial fibers from another. Immature granule neurons, purified from the early postnatal cerebellar cortex, migrate along the processes of glial cells isolated from hippocampus. The dynamics of movement of neurons from one region closely parallel those of neurons from other regions, suggesting that the mode of migration along glial fibers is stereotyped. The movement of the neuron is saltatory. The leading process extends along the glial fiber, after which the cell soma appears to contract and then extend just prior to release of the adhesion junction underneath the cell soma. This sequence is repeated, as the neuron progresses along the glial fiber.

(MPEG movie, 719KB)

(GIF89a movie, 736KB)

Fishell and Hatten, 1991. (Development)
Astrotactin Provides a Neuronal Ligand for Movement along the Glial Fiber. To provide an assay for cell surface receptor systems that function in neuronal migration, low power views are used to track the movement of a large population of cerebelar granule cells. In the first sequence, neurons are seen migrating along glial fibers. The application of antibodies against the neuronal protein astrotactin (flash across the top of the screen) results in rapid arrest of neuronal movement. As migration ceases, the long glial processes "sway" across the field.

With the bioassay above, we isolated and then cloned a neuronal protein Astrotactin (Astn1, a neuron-glial adhesion molecules that mediates neuronal migration on glial fibers. Mice lacking Astn1 have delayed neuronal migration, which in turn affects the development of dendrites in their target cells, the Purkinje neurons (Adams et al, 2002). This has suggested that one function of directed migration is to control the timing of neural circuits in developing brain.

Recently, we have isolated a second Astn gene, Astn2, the structure of which closely resembles Astn1

Astn 2 is heavily expressed in the cerebellum of the postnatal mouse and antibodies specific for Astn2 block neuronal migration in vitro. Experiments are currently in progress to use RNAi methods to knock down Astn2 levels in developing brain, and to generate targeted mutations of the gene (Wilson and Hatten, unpublished).

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