Signaling To and From Radial GliaKENNETH CAMPBELL*

Division of Developmental Biology, Children’s Hospital Research Foundation, Cincinnati, Ohio

KEY WORDS radial glia; developing CNS; cell-cell signaling

ABSTRACT Radial glia represent the major glial cell type in the developing CNSand perform many essential functions, which range from acting as neural precursors toproviding physical substrates for newborn neurons to migrate on. Previous work hasshown that cell-cell signaling is important for the development of the radial glialphenotype. In particular, signals from newborn neurons appear to contribute signifi-cantly to the formation of this cell type. In addition, radial glia may be involved inreciprocal signaling roles that contribute to regional patterning and differentiation inthe developing CNS. GLIA 43:44–46, 2003. © 2003 Wiley-Liss, Inc.

The vertebrate nervous system originates as a flatsheet of neuroepithelial cells. Coincident with the onsetof neurogenesis, a specialized cell type appears. Thesecells exhibit radial processes that contact both the ven-tricular and pial surfaces of the developing brain.These radial cells were named radial glia, owinglargely to their astroglial characteristics. Indeed, ra-dial glia contain glycogen granules and certain molec-ular markers typical of astrocytes, such as glial fibril-lary acidic protein (GFAP) (Levitt and Rakic, 1980;Choi, 1981). Most rodent radial glia lack GFAP expres-sion (Bignami and Dahl, 1974; Onteniente et al., 1983;Pixley and de Vellis, 1984); however, they do expressother markers typical of astrocytes, such as the astro-cyte-specific glutamate transporter (GLAST) (Shibataet al., 1997; Hartfuss et al., 2001) and brain lipid bind-ing protein (BLBP) (Feng et al., 1994; Hartfuss et al.,2001). Radial glia also express the neural precursor cellmarker Nestin (Frederiksen and McKay, 1988). In ad-dition, the antigen recognized by the monoclonal anti-body RC2 is expressed in radial glia (Misson et al.,1988), as well as neural precursors (Hartfuss et al.,2001). A recent study suggested that the RC2 antibodymay actually recognize an isoform of nestin (Chanas-Sacre et al., 2000).

The first proposed role for radial glia was in aidingthe migration of newborn neurons. At all levels of thedeveloping CNS, radially migrating neurons are knownto travel out of the germinal zones along the radial glialfibers and into the mantle regions (see, e.g., Rakic,1995). Neurogenesis and neuronal migration arelargely complete at birth, at which time radial glialcells in the mammalian CNS disappear. Studies on

their disappearance led to the finding that these cellstransform into astrocytes in the perinatal brain(Schmechel and Rakic, 1979; Voigt, 1989). Recent stud-ies have shown that, at least a subpopulation of, thesecells are also neuronal precursor cells (Malatesta et al.,2000; Miyata et al., 2001; Noctor et al., 2001). Whilemost of the neurogenic abilities of radial glia are car-ried out via cell division, at least in vitro, these cellscan also transform into neurons (Malatesta et al.,2000), as is the case with astrocytes.

DEVELOPMENTAL SIGNALS TO RADIALGLIA

As mentioned above, radial glial cells appear coinci-dent with the earliest neurons in the forming CNS. Infact, the earliest born neurons have been implicated inthe induction and maintenance of the radial glial phe-notype. The expression of BLBP in radial glia stronglycorrelates with the migration of neurons along the ra-dial fibers (Feng et al., 1994). Moreover, soluble neu-ron-derived signals have been shown to activate thetranscription of BLBP in radial glial cells via a radialglial cell-specific element (Feng and Heintz, 1995), aswell as to induce radial morphologies in cultured as-

*Correspondence to: Kenneth Campbell, Division of Developmental Biology,Children’s Hospital Research Foundation, 3333 Burnet Avenue, Cincinnati, OH45229-3039. E-mail: kenneth.campbell@chmcc.org

Received 14 February 2003; Accepted 3 March 2003

DOI 10.1002/glia.10247

GLIA 43:44–46 (2003)

© 2003 Wiley-Liss, Inc.

trocytes (Hunter and Hatten, 1995). BLBP appears tobe required for radial process elongation, since theaddition of anti-BLBP antibodies inhibit this process(Feng et al., 1994).

The neuron-derived signal(s) that induce radial mor-phologies in cultured astrocytes are restricted to theembryonic period and are lost postnatally (Hunter andHatten, 1995), which correlates well with the transfor-mation of radial glia into astrocytes at postnatalstages. Although the identity of this signal was unclearin the study by Hunter and Hatten (1995), a later study(Anton et al., 1997) identified neuregulin/glial growthfactor (GGF) as a candidate radial glial inducing/main-tenance signal. The addition of recombinant GGF toradial glial cultures promotes the elongation of theradial fibers as well increasing the expression of BLBP(Anton et al., 1997). Furthermore, newborn neurons inthe developing cerebral cortex express GGF, while theradial glia express the GGF receptor ErbB2 (Anton etal., 1997). Thus, GGF represents an important signalfor maintenance of the radial glial phenotype in thecerebral cortex.

Recent studies have implicated Reelin in the devel-opment of the radial glial phenotype (Forster et al.,2002; Luque et al., 2002). Reelin is the gene that ismutated in the reeler mouse (D’Arcangelo et al., 1995).These mice are well known for the abnormal neuronalmigration of cortical neurons (Caviness and Sidman,1973). Reelin is expressed by Cajal-Retius cells, whichare located in the outer regions of the developing cor-tex. These cells are thought to release Reelin, whichthen signals directly to the migrating neurons (Riceand Curran, 2001). Reelin released from Cajal-Retziuscells could also influence radial glia, since they residein the same location as the radial glial endfeet. Indeed,a recent study showed that reeler mice also exhibitabnormal development of the radial glial scaffold in thedentate gyrus of the hippocampus (Forster et al., 2002).Moreover, both hippocampal and cortical radial gliahave been shown to express components of the Reelinsignaling pathway, including the receptors, the verylow-density-lipoprotein (VLDL) receptor and the apoli-poprotein E (ApoE) receptor, as well as the cytoplasmicadapter protein Dab1 (Forster et al., 2002; Luque et al.,2002). Dab1 mutants display similar defects in thedentate gyrus radial glial scaffold (Forster et al., 2002).Furthermore, Reelin appears to be required for BLBPexpression in cortical radial glia as well as radial pro-cess extension (M. Gotz, personal communication).Taken together, these findings suggest that Reelin reg-ulates the radial glial phenotype and that altered de-velopment of radial glia in the absence of Reelin sig-naling may contribute to the observed alterations inneuronal migration.

Cajal-Retzius cells have been shown to reinduce theradial glial phenotypes in Bergman glia after trans-plantation into the adult cerebellum (Soriano et al.,1997). This study showed that reeler-derived Cajal-Retzius cells were equally efficient at inducing radialglial phenotypes as wild-type cells. This finding sug-gests that Cajal-Retzius cells may provide other fac-

tors, in addition to Reelin, which regulate radial glialphenotypes.

DEVELOPMENTAL SIGNALS FROM RADIALGLIA

The fact that radial glia are long-term residents of allCNS germinal zones makes them good candidates asregulators of developmental events, such as patterningand differentiation. The concept of glial cells acting assignaling centers in the developing CNS is not novel.Indeed, at spinal cord levels, the floor plate is com-prised of glial-like cells that express the glial-enrichedvimentin and contain glycogen granules (Uehara andUeshima, 1984; Flood, 1986; Yang et al., 1993). Thisstructure is a well-established signaling center in-volved in dorsoventral patterning through its produc-tion of the ventralizing factor sonic hedgehog (SHH)(Jessell, 2000).

Despite the fact that radial glia are found throughoutthe developing neuraxis, they do not constitute a ho-mogeneous cell type. In fact, at least three differentsubtypes can be identified based on molecular markers(Hartfuss et al., 2001). Moreover, radial glia are region-ally specified, as evidenced by the differential expres-sion of regional molecular markers. For example, cor-tical radial glia express the paired homeobox proteinPAX6 (Gotz et al., 1998), while their counterparts inthe lateral ganglionic eminence of the ventral telen-cephalon express the cellular retinol binding protein I(RBP1) (Toresson et al., 1999). Since radial glia com-prise a large portion of the precursor cells in the ger-minal zones (Hartfuss et al., 2001), it seems likely thatregionally restricted developmental signals would beproduced by different subsets of these cells. Indeed,subsets of radial glia located in ventral regions of thedeveloping brain appear to express SHH (Campbelland Gotz, 2002).

The fact that radial glia are in contact with migrat-ing neurons until they reach their destination in themantle regions of the brain offers these cells a uniqueopportunity to provide differentiating signals to theseneurons and promote region specific differentiation. Inthis respect, a previous study (Toresson et al., 1999)has shown that glial cells derived from the telence-phalic lateral ganglionic eminence produce retinoicacid unlike glia derived from the immediately adjacentmedial ganglionic eminence. The lateral ganglionic em-inence gives rise to the projection neurons of the stri-atum (or caudate-putamen) (Deacon et al., 1994; Ols-son et al., 1995, 1998). These neurons can be identifiedby the expression of the DARPP-32 phosphoprotein(Anderson and Reiner, 1991). In vitro, retinoic acid canincrease the numbers of DARPP-32-expressing neu-rons (i.e., striatal projection neurons) in lateral gangli-onic eminence cultures (Toresson et al., 1999). Sincethe radial glia in this regions express RBP1, which isoften expressed in cells that produce retinoic acid(Napoli, 1996), it was proposed that radial glia in thelateral ganglionic eminence provide retinoids to mi-

45SIGNALING TO AND FROM RADIAL GLIA

grating neurons and thereby regulate their regionaldifferentiation into striatal projection neurons.

Similarly, glial cells in the embryonic ventral mid-brain may also regulate regional neuronal differentia-tion. Glial cultures derived from the embryonic or peri-natal ventral midbrain have been shown to produce adevelopmental signal that promotes the dopaminergicdifferentiation of neural stem cells engineered to ex-press Nurr1 (Wagner et al., 1999). It is unclearwhether the glial cultures were composed of radialglial-like cells; however, this is likely, as they are thepredominant glial subtypes present at embryonicstages. An earlier study has also implicated embryonicglia from the ventral midbrain in the development ofthe midbrain dopaminergic phenotype (Denis-Donini etal., 1984). In this case, the glial effect was on themorphology of the neurons.

It seems likely, therefore, that reciprocal signalingbetween neurons and radial glia is crucial for the nor-mal development of the CNS. A considerable amount ofdata indicates that neurons promote and maintain theradial glial phenotype through multiple signalingmechanisms. Moreover, subtypes of radial glia withindistinct regions of the developing CNS may producedevelopmental signals, which regulate patterning andsubtype-specific neuronal differentiation.

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