EGFRs mediate chemotactic migration in the developing telencephalon

Damira Caric 0 Heather Raphael 0 Jane Viti 0 Angela Feathers 0 Debbie Wancio 0 Laura Lillien ) 0 0 Department of Neurobiology and Pittsburgh Cancer Institute, University of Pittsburgh School of Medicine , W1454 Biomedical Science Tower, Pittsburgh, PA 15261 , USA - Epidermal growth factor receptors (EGFRs) have been implicated in the control of migration in the telencephalon, but the mechanism underlying their contribution is unclear. We show that expression of a threshold level of EGFRs confers chemotactic competence in stem cells, neurons and astrocytes in cortical explants. This level of receptor expression is normally achieved by a subpopulation of cells during mid-embryonic development. Cells that express high levels of EGFR are located in migration pathways, including the tangential pathway to the olfactory bulb via the rostral migratory stream (RMS), the lateral cortical stream (LCS) leading to ventrolateral cortex and the radial pathway from proliferative zones to cortical plate. The targets of these pathways express the ligands HB-EGF and/or TGFa . To test the idea that EGFRs Construction of the telencephalon involves the migration of neurons and glia from proliferative zones to specific sites in the cerebral wall. Different types of neurons and glial cells leave the ventricular zone (VZ) and subventricular zone (SVZ) at specific times during development, follow characteristic routes during their migration, and settle in distinct locations (Berry et al., 1964; Hicks and DAmato, 1968; Rakic, 1974; Raedler and Raedler, 1978; Smart and Smart, 1982; Bayer and Altman, 1991a). Some cells migrate radially to the cortical plate, using glial cells for guidance, while others migrate tangentially (Rakic, 1995; Walsh and Cepko, 1988; Misson et al., 1988; ORourke et al., 1992; Tan et al., 1998; Goldman and Luskin, 1998; de Carlos et al., 1996; Anderson et al., 1997; Lavdas et al., 1999; Parnavelas, 2000). Tangential migration occurs in ventral-to-dorsal, dorsal-to-ventral and caudal-torostral directions (deCarlos et al., 1996; Anderson et al., 1997; Lavdas et al., 1999; Tomioka et al., 2000; Luskin, 1993; Reid et al., 1995). The complexity of migration in the telencephalon is reflected in the variety of intrinsic and extrinsic molecules that play a role in its regulation (Reiner, 2000; Trommsdorff et al., 1999; Osterhout et al., 1997; Anton et al., 1997; Rio et al., 1997; Colamarino and Tessier-Lavigne, 1995; Alcantara et al., 2000; Hu, 1999; Wu et al., 1999; Zhu et al., 1999; Jacques et al., 1998). Although some of the molecules that control migration have been identified, little is known about the mediate chemotactic migration these pathways, we increased the size of the population of cells expressing threshold levels of EGFRs in vivo by viral transduction. Our results suggest that EGFRs mediate migration radially to the cortical plate and ventrolaterally in the LCS, but not tangentially in the RMS. Within the bulb, however, EGFRs also mediate radial migration. Our findings suggest that developmental changes in EGFR expression, together with changes in ligand expression regulate the migration of specific populations of cells in the telencephalon by a chemoattractive mechanism. mechanisms that cause different types of cells to leave proliferative zones at specific times, use distinct modes of migration and stop in appropriate locations. Epidermal growth factor receptors (EGFRs) have been shown to regulate migration in a variety of cells through direct and indirect mechanisms (Blay and Brown, 1985; Bailly et al., 2000; Chen et al., 1994; Duchek and Rorth, 2001; Miettinen et al., 2000; Hoschuetzky et al., 1994; Chan et al., 2000; Li et al., 1999; Li et al., 2000; Tokumaru et al., 2000; Xie et al., 1998). In the telencephalon, EGFRs are expressed in a precise temporal pattern. At early embryonic stages (before E14 in mice or E16 in rats), low levels of EGFRs can be detected; however, at mid-late embryonic stages, higher levels of EGFR mRNA and protein are expressed by a small heterogeneous population of cells that includes astrocytes, neurons and multipotent stem cells (Morshead et al., 1994; Weikert and Blum, 1995; Seroogy et al., 1995; Weikert and Blum, 1995; Eagleson et al., 1996; Kornblum et al., 1995; Kornblum et al., 1997, Burrows et al., 1997; Lillien and Raphael, 2000). Many of the cells that express a high level of EGFRs are found in migration pathways, including the radial pathway from proliferative zones to cortical plate through the intermediate zone, the tangential pathway from anterior SVZ to olfactory bulb via the rostral migratory stream (RMS), and the ventrolateral pathway from the corner between the cortex and striatum (corticostriatal sulcus) to the lateral and ventral cortex via the lateral cortical stream (LCS) (Bayer et al., 1991). In EGFR-null mice, progenitor cells appear to accumulate in proliferative zones of the late embryonic telencephalon rather than migrating to the cortical plate or olfactory bulb (Threadgill et al., 1995), suggesting that EGFR-mediated signaling is involved in both radial and tangential modes of migration. Consistent with this phenotype, mis-expressing EGFRs in the embryonic rat telencephalon promoted radial migration to the cortical plate (Burrows et al., 1997), and infusing EGF into the lateral ventricle diverted cells from the rostral migratory stream to the cortex (Craig et al., 1996). Several ligands that activate the EGFR are expressed in the developing telencephalon, including transforming growth factor a (TGFa) and heparin binding epidermal growth factor (HB-EGF) (Lazar and Blum, 1992; Weikert and Blum, 1995; Kornblum et al., 1997; Kornblum et al., 1999; Nakagawa et al., 1998; Opanshuk et al., 1999). TGFa is expressed in the olfactory bulb, striatum, and choroid plexus, but not in the cerebral cortex, with the exception of ventrolateral cortex (Lazar and Blum, 1992; Weikert and Blum, 1995; Kornblum et al., 1997). By contrast, HB-EGF is expressed in the embryonic cortex, particularly the cortical plate (Nakagawa et al., 1998; Kornblum et al., 1999). Its expression increases during embryonic development, and it continues to be expressed in ventrolateral cortex postnatally (Opanshuk et al., 1999; Kornblum et al., 1999). Previous studies demonstrated that the level of EGFR expression affects the timing of migration and the settling patterns of cells (Burrows et al., 1997), but did not resolve the underlying mechanism. Migration associated with a high level of EGFR expression could reflect chemoattraction or chemorepulsion. To clarify the underlying mechanism, we have used explants to alter the location and concentration of ligand while monitoring the migratory behavior of cells. These studies demonstrate that a high level of EGFRs characteristic of late embryonic cells mediates chemoattraction. EGFRs and ligands for the EGFR, particularly HB-EGF, are expressed in several migration pathways. To test the idea that EGFR (...truncated)