Polarity of microtubule assemblies during neuronal cell migration.

Abstract

The active migration of neurons from their sites of origin to their final destinations requires the unidirectional translocation of the nuclei and somatic cytoplasm within the growing leading processes. To explore the cellular machinery underlying this translocation, we determined the polarity of microtubules situated within the leading and trailing processes of migrating cerebellar granule cells in situ. Our analysis reveals that the newly assembled positive ends of the microtubules in the leading process uniformly face the growing tip, while their disintegrating negative ends face the nucleus. In the trailing process, by contrast, microtubule arrays are of mixed polarity. We suggest that the dynamics of slow polymerization in combination with fast disintegration of oriented microtubules create "push" and "pull" forces that contribute to the piston-like saltatory displacement of the nucleus and cytoplasm within the membrane cylinder of the leading process of the migrating neuron.The cerebellar granule cell has fascinated developmental neurobiologists since the classical description by Ramon y Cajal (1) of its orderly and sequential migration across the molecular layer of the cerebellar cortex. Migrating neurons initially extend a cell extension called the leading process, which is followed by the translocation of the cell's nucleus within the extended membrane envelope (2-4). Although many aspects of neuronal cell migration have been elucidated in recent years (reviewed in refs. 5-8), the molecular machinery that translocates the cell nucleus itself remains unknown. It seems reasonable that a considerable mechanical force including a rearrangement of cytoskeletal scaffolding is an essential prerequisite for this translocation since the entire cell body is propelled across the developing molecular layer, an area that is densely packed with the cellular processes of earlier-generated neurons. Electron microscopic analyses have revealed parallel arrays of microtubules in both the leading and trailing processes of granule cells as they begin to move across the molecular layer (2). The large number of microtubules and their longitudinal deployment in the leading process have implicated them in the translocation of a cell's soma (9, 10), but beyond this there is little known about how microtubule proteins become assembled and how they function in migrating neurons. Therefore, knowledge about the polarity of the assemblies, sites of nucleation and posttranslational modifications of microtubule components in the cytoplasm of premigratory granule cells may provide new insights into the mechanism of the movement of cells during their migration.Microtubules in mammalian neurons consist of a and f3 tubulin molecules polymerized in the form of a hollow tube (13). In general, new tubulin oligomeres are added to each microtubule at one pole, the positive end, and are deleted at the other pole, the negative end (14). The recently developed "hook assay" technique has permitted determination of the polarity of polymeriza...

Translocation of Synaptically Connected Interneurons across the Dentate Gyrus of the Early Postnatal Rat Hippocampus

“…In general, translocation of the nucleus and perinuclear cytoplasm during migration of immature neurons is associated with rearrangement of the cytoskeleton (Rivas and Hatten, 1995;Rakic et al, 1996). Furthermore, the rate of nuclear translocation depends of Ca 2ϩ influx through N-type Ca 2ϩ channels and the NMDA subtype of the glutamate receptor Rakic, 1992, 1993) that affects depolarization of microtubules , as shown in migrating cerebellar granule cells.…”

“…Although migrating projection neurons and interneurons use different molecular cues and guidance substrates (Polleux et al, 2002;Ang et al, 2003;Stumm et al, 2003;Tanaka et al, 2003;Flames et al, 2004), both share some fundamental mechanisms (Rakic, 1990;Rakic et al, 1996;Hatten, 1999;Marin and Rubenstein, 2003). For example, subclasses of GABAergic interneurons in the human, which are generated in the neocortical proliferative zones, migrate along radial glial shafts, similar to the projection neurons (Letinic et al, 2002).…”

Prenatal exposure to ultrasound waves impacts neuronal migration in mice

“…These mechanical effects could interfere with the delicate adhesion between the migratory neurons and the surface of migratory substrates, such as radial glial shafts, which serve as guides (5,6,14). The USW may also disturb exocytosis, essential for the extension of the leading tip of migrating neurons or disrupt cytoskeletal rearrangement essential for the translocation of the nucleus within its leading process (18,20,21,37). Based on the present results, the effect on other forms of cell motility, such as tangential neuronal migration (13,15,17) or spinning of the mitotic spindle (40), cannot be excluded.…”

“…Contact interaction between migrating neurons and the surfaces of neighboring cells plays a decisive role in selecting the migratory pathway and determining their final position (18,19). Neuronal migration involves translocation of the nucleus and the surrounding cytoplasm with the leading process, which requires rearrangement of the cytoskeleton (20,21). As a consequence of these complex cellular and molecular interactions, the process of neuronal migration is highly sensitive to a variety of biological, physical, and chemical agents, as well as to specific genetic mutations (7)(8)(9)(10)(11)(12)(13).…”

The Suppression of CRMP2 Expression by Bone Morphogenetic Protein (BMP)-SMAD Gradient Signaling Controls Multiple Stages of Neuronal Development

“…Microtubule assembly and reorganization of actin filaments has been shown recently to be essential for neuronal radial migration (25)(26)(27)(28)(29). Based on evidence from primary culture systems, CRMP2 can regulate microtubule assembly, reorganization of actin filaments, and protein trafficking during neurite elongation and axon specification (15,16,18).…”