The recent study by Olson and colleagues (Olson et al., 2006) touches on an important aspect of embryonic brain development that is the poorly understood link between neuronal migration and process maturation during the formation of cortical layers. In the embryonic neocortex principal neurons are generated near the ventricle from neuronal progenitors (radial glia) undergoing their final cell division. The newborn neurons then migrate radially towards the marginal zone and stop near the surface of the cortex. This area is rich in the extracellular protein Reelin, which is well known to be required for cellular layer formation and neurite extension in many areas of the developing brain (D'Arcangelo, 2006). Reelin signaling in cortical neurons is mediated by Dab1, an adaptor protein that is rapidly phosphorylated on tyrosine residues upon exposure to this protein. As neurogenesis proceeds, each cohort of newborn neurons...  Read more

The recent study by Olson and colleagues (Olson et al., 2006) touches on an important aspect of embryonic brain development that is the poorly understood link between neuronal migration and process maturation during the formation of cortical layers. In the embryonic neocortex principal neurons are generated near the ventricle from neuronal progenitors (radial glia) undergoing their final cell division. The newborn neurons then migrate radially towards the marginal zone and stop near the surface of the cortex. This area is rich in the extracellular protein Reelin, which is well known to be required for cellular layer formation and neurite extension in many areas of the developing brain (D'Arcangelo, 2006). Reelin signaling in cortical neurons is mediated by Dab1, an adaptor protein that is rapidly phosphorylated on tyrosine residues upon exposure to this protein. As neurogenesis proceeds, each cohort of newborn neurons bypasses its predecessor and becomes positioned at the boundary between the marginal zone and the developing cortical plate, while the cell bodies of previously arrived neurons descend into deeper layers. This mode of corticogenesis is known as inside-out layer formation (Angevine and Sidman, 1961). In reeler mutant mice lacking Reelin, or mutant mice lacking Dab1 (scrambler, yotari, or Dab1 knockout), layer formation appears to be somewhat inverted, with early-born neurons occupying superficial layers and late-born neurons ectopically accumulating underneath their predecessors (Caviness and Rakic, 1978; Lambert de Rouvroit and Goffinet, 1998). Reelin and Dab1 mutant neurons not only occupy ectopic positions, but also develop stunted dendritic arborizations both in vivo and in vitro (Niu et al., 2004). Thus, Reelin appears to control not just cell body movement but also neuronal maturation. How are these two processes coordinated during cortical development and exactly what role does the Reelin-Dab1 pathway play in this complex process?

Olson and colleagues used the in utero electroporation technique pioneered by the lab of Kazunori Nakajima (Tabata and Nakajima, 2001) for the in vivo analysis of neuronal migration and maturation in the embryonic mouse cortex. When a plasmid encoding GFP was electroporated in the wild-type mouse cortex at embryonic day 16, and the mice analyzed on the day of birth, many labeled neurons were found to have migrated into the upper cortical layer. Many of these neurons also could be seen to contact the Reelin-rich marginal zone by their leading edge, which formed a branch point at the boundary between the cortical plate and the marginal zone. Secondary processes also entered and further branched in this cell-poor zone. The formation of a leading edge branch point specifically at the marginal zone boundary is significant, because previous studies have shown that neurons can translocate their cell body towards a leading edge branch point in a mode of rapid radial migration that is independent of radial glia support (Nadarajah et al., 2001). Thus, the branch point appears to demark the precise position where radial migration must terminate and neuronal maturation must commence with the development of apical dendritic arborizations. The apical dendrites can be seen to develop from a transformation of the leading edge, as initially postulated in 1979 (Pinto Lord and Caviness, 1979).

To address the function of Reelin and Dab1 in this complex migration/maturation process, Olson et al. electroporated specific RNAi to suppress Dab1 expression in normal cortical neurons in vivo, together with GFP to visualize co-transfected neurons. They observed that neurons in which Dab1 was knocked down mostly failed to position themselves in the upper cortical layers. Their leading edge also often failed to reach the marginal zone boundary and to branch into this area. As a result, the complexity of the apical dendrites was significantly reduced. These observations agree very well with previous studies that demonstrated impaired neuronal migration and reduced dendrite development in reeler or Dab1-deficient mice (Niu et al., 2004; Tabata and Nakajima, 2002). In addition, this study demonstrates that loss of Dab1 activity affects these neuronal properties in a cell autonomous fashion. This is an important point because other studies have shown that Reelin also promotes branching of radial glia cells (Hartfuss et al., 2003; Pinto-Lord et al., 1982). Since these cells provide a scaffold for radial neuronal migration (Rakic, 1971), it was conceivable that the defects observed in mutant mice resulted from a radial glia abnormality. This study, together with a previous study in which mutant Dab1 expression plasmids were electroporated in the embryonic neocortex (Sanada et al., 2004), clearly demonstrates that Dab1 activity is required in neurons themselves to achieve proper migration and maturation. Specifically, it seems that Dab1 is required in order for neurons to undergo their final stage of radial migration, which consists of somal translocation towards the leading edge branch point. This Reelin-dependent cellular mechanism thus allows migrating neurons to bypass their predecessor already positioned in the cortical plate and to establish the inside-out pattern of layer formation.

How does neurite outgrowth relate to neuronal migration? The authors suggest that abnormalities associated with the leading process and dendrites are secondary to a migration defect (Olson et al., 2006), and that is certainly a possibility. However, in my opinion, it is neuronal migration that is secondary to at least one early aspect of neurite maturation, that is, the branching of the leading edge at the marginal zone boundary. A branch point is, in fact, required for glia-independent somal translocation, the final step in radial migration. Further development of the branched leading edge into an apical dendrite tree may be dispensable for somal translocation, but it could contribute to the inside-out development of cortical layers. Extension of apical dendrites, particularly the elongation of primary dendrites of principal neurons that retain an anchorage with the marginal zone, could, in fact, enable the descent of older neuronal cell bodies into the deep layers of the cortical plate as new neurons complete somal translocation towards the upper layers. For cortical plate development it seems that what goes up must indeed come down, and early neuronal processes are likely to play an important role in this bidirectional movement.

References:

Angevine JG, and Sidman RL. (1961). Autoradiographic study of cell migration during histogenesis of cerebral cortex in the mouse. Nature 192, 766-768.

Caviness VS Jr, Rakic P. Mechanisms of cortical development: a view from mutations in mice. Annu Rev Neurosci. 1978;1:297-326. Review. No abstract available. Abstract

D'Arcangelo G. Reelin mouse mutants as models of cortical development disorders. Epilepsy Behav. 2006 Feb;8(1):81-90. Epub 2005 Nov 2. Abstract

Hartfuss E, Forster E, Bock HH, Hack MA, Leprince P, Luque JM, Herz J, Frotscher M, Gotz M. Reelin signaling directly affects radial glia morphology and biochemical maturation. Development. 2003 Oct;130(19):4597-609. Erratum in: Development. 2003 Nov;130(21):5291. Abstract

Lambert de Rouvroit C, Goffinet AM. The reeler mouse as a model of brain development. Adv Anat Embryol Cell Biol. 1998;150:1-106. Review. No abstract available. Abstract

Nadarajah B, Brunstrom JE, Grutzendler J, Wong RO, Pearlman AL. Two modes of radial migration in early development of the cerebral cortex. Nat Neurosci. 2001 Feb;4(2):143-50. Abstract

Niu S, Renfro A, Quattrocchi CC, Sheldon M, D'Arcangelo G. Reelin promotes hippocampal dendrite development through the VLDLR/ApoER2-Dab1 pathway. Neuron. 2004 Jan 8;41(1):71-84. Abstract

Olson EC, Kim S, Walsh CA. Impaired neuronal positioning and dendritogenesis in the neocortex after cell-autonomous Dab1 suppression. J Neurosci. 2006 Feb 8;26(6):1767-75. Abstract

Pinto Lord MC, Caviness VS Jr. Determinants of cell shape and orientation: a comparative Golgi analysis of cell-axon interrelationships in the developing neocortex of normal and reeler mice. J Comp Neurol. 1979 Sep 1;187(1):49-69. Abstract

Pinto-Lord MC, Evrard P, Caviness VS Jr. Obstructed neuronal migration along radial glial fibers in the neocortex of the reeler mouse: a Golgi-EM analysis. Brain Res. 1982 Aug;256(4):379-93. Abstract

Rakic P. Guidance of neurons migrating to the fetal monkey neocortex. Brain Res. 1971 Oct 29;33(2):471-6. No abstract available. Abstract

Sanada K, Gupta A, Tsai LH. Disabled-1-regulated adhesion of migrating neurons to radial glial fiber contributes to neuronal positioning during early corticogenesis. Neuron. 2004 Apr 22;42(2):197-211. Abstract

Tabata H, Nakajima K. Efficient in utero gene transfer system to the developing mouse brain using electroporation: visualization of neuronal migration in the developing cortex. Neuroscience. 2001;103(4):865-72. Abstract

Tabata H, Nakajima K. Neurons tend to stop migration and differentiate along the cortical internal plexiform zones in the Reelin signal-deficient mice. J Neurosci Res. 2002 Sep 15;69(6):723-30. Abstract