Tangential Migration: Interneurons and the Long Way Round

In a mouse embryo, most of the neurons that will inhibit the cerebral cortex are born outside it.

The cortex’s excitatory neurons, the pyramidal neurons, are made in its own wall and climb straight out to their layers, as you saw in radial migration. The inhibitory neurons start in another part of the forebrain and travel sideways, across territories, before they turn and enter the cortex. This lesson asks how that was found, what keeps the travellers on their route, and where else in the brain neurons take the long way round.

Neurons of the cerebral cortex derive from both intrinsic and extrinsic sources

Start with a map. The telencephalon is the front-most part of the embryonic brain, which becomes the cerebral hemispheres. Its dorsal part, the pallium, forms the cortex. Its ventral part, the subpallium, bulges into the ventricle as three swellings packed with dividing progenitors: the lateral, medial and caudal ganglionic eminences (LGE, MGE and CGE). Despite the name, they are temporary germinal zones, not ganglia. Much of the LGE becomes the striatum.

The two territories switch on different transcription factors and make different neurons. Pallial progenitors make neurons that release glutamate. Subpallial progenitors make neurons that release GABA, the main inhibitory transmitter of the forebrain. GABA is made from glutamate by glutamic acid decarboxylase (GAD), so antibodies to GABA or GAD mark these cells.

That raises a problem. The cortex needs inhibitory interneurons, roughly a fifth of its neurons in rodents, yet its own wall makes excitatory ones. Either the pallium makes a few GABAergic cells anyway, or they come from somewhere else.

Design problem

You suspect that cortical interneurons come from the ventral telencephalon. Design two independent experiments to show it. Then say what result in a mutant lacking the source, or unable to move cells out of it, would confirm your idea.

Both experiments were done. In 1996 J. A. de Carlos, L. López-Mascaraque and F. Valverde injected the dyes DiI or DiA into the LGE region of rat embryos kept alive in culture. Most labelled cells ended up at the surface of the ventrolateral cortex, reaching it along radial glial fibres from the eminence, and others ran tangentially into the preplate of the developing cortex. The paper did not show what kind of neurons they became. In 1997 Nobuaki Tamamaki and colleagues reported that tangentially oriented neurons in the developing cortical intermediate zone were travelling from the ventral side, with the lateral ganglionic eminence as their likely source.

The genetic test came in 1997 from Stewart Anderson, David Eisenstat, Limin Shi and John Rubenstein. Dlx1 and Dlx2 are homeobox transcription factors made in the subpallium. In mouse tissue, cutting the cortex off from the ventral telencephalon reduced its GABA-containing cells. In embryos lacking both genes (a knockout is a mouse in which a gene has been disrupted in every cell), the group detected no migration into the neocortex, and the neocortex had few GABA-containing cells. Later work in mice placed most interneurons in the MGE and CGE, with a smaller share from the preoptic area. MGE identity depends on the transcription factor Nkx2.1, which is switched on by Sonic hedgehog (that lesson covers the patterning). Not every interneuron comes from the MGE.

These are rodent results. In human fetal tissue, a 2002 study by Letinic, Zoncu and Rakic argued that many interneurons arise in the cortex’s own proliferative zones. Studies published in 2013 (Hansen and colleagues; Ma and colleagues) found most human interneuron progenitors in the ganglionic eminences. The human picture was debated, and the weight of later evidence favors a mainly ventral origin there too.

Tangential migration vs radial migration

A pyramidal neuron moves at right angles to the brain surface, from the ventricle outward, mostly by gripping a radial glial fibre. A tangentially migrating interneuron moves parallel to the surface and crosses regional boundaries with no glial fibre to follow. Think of a climber on a fixed rope versus a traveller walking through streets, choosing at each corner.

The interneuron’s leading process branches, and the cell advances by choosing one branch and pulling its nucleus forward into it, with the same motor proteins you met in radial migration. The choices are made by cues along the route. Leaving the MGE, cortex-bound interneurons skirt the striatum. Marín, Rubenstein and colleagues showed in 2001 that these cells carry neuropilin receptors and are repelled by semaphorins made in the striatum, while interneurons that settle in the striatum lack those receptors (see semaphorins).

Once in the cortex, the interneurons travel in two streams: a superficial one in the marginal zone just beneath the meninges, the membranes that cover the brain, and a deep one in the subventricular and lower intermediate zones. Between them, the young cortical plate holds relatively few interneurons early on. Some deep-stream cells travel in contact with bundles of descending cortical axons that carry the adhesion molecule TAG-1, a finding from Denaxa and colleagues in 2001. So the guides are axons, other cells and diffusible cues, not radial glia.

Staying in the corridor: CXCL12 and CXCR4

A stream is a picture of movement, not an explanation. Why do cells stay in it instead of diving into the cortical plate at once?

Chemokines are small secreted proteins first known for steering immune cells. CXCL12, also called SDF-1, binds the receptor CXCR4, a G-protein-coupled receptor that signals mainly through Gi proteins (that lesson). Like many chemokines, CXCL12 binds sugar chains on cell surfaces and matrix, which tends to keep it near where it is made, one way a secreted cue can act over short range (that lesson).

In 2003 Ralf Stumm and colleagues found CXCR4 on interneuron precursors travelling from the basal forebrain toward the rat neocortex, and CXCL12 in the meninges. Dissociated striatal precursors from embryonic day 14.5 mouse brain migrated toward CXCL12 in a chamber assay, and did not migrate more when it was added equally on both sides. In mice lacking the ligand or the receptor, fewer GABAergic cells sat in the superficial stream and more appeared deeper in the cortex. A stream still entered the cortex, so the signal positions cells after entry and does not drive the crossing.

The deep stream has its own source. In 2006 Marie-Catherine Tiveron, Harold Cremer and colleagues found CXCL12 made in the subventricular and intermediate zones by dividing precursors of projection neurons, and in 2015 Abe, Stumm and colleagues removed it selectively from intermediate progenitors and reduced the deep stream. The cortex’s own excitatory precursors help hold the inhibitory immigrants in place.

In 2008 two groups, López-Bendito, Marín and colleagues, and Li, Pleasure, Rubenstein and colleagues, blocked or deleted CXCR4 in mice. Interneurons left their streams and entered the cortical plate early, and they spread less widely across the cortex. Many late-born interneurons that entered early ended up in the wrong layer, deep layer V instead of upper layers, though not all did. The chemokine controls retention, timing and spread, and in part final layer.

That points to the switch. To leave a stream, a cell must stop answering CXCL12. Li and colleagues found that interneurons from embryonic mice gathered near beads soaked in CXCL12, while interneurons from newborn mice did not. As responsiveness falls, cells exit the streams and turn radially or obliquely into the plate toward their layer. A second receptor for CXCL12, CXCR7 (also called ACKR3), shapes the response as well. All of this was measured in rodents.

Convergence of migratory patterns

Two populations, born in different places and travelling by different routes, end up interleaved in the same layers. In mice, interneurons from the MGE tend to settle in the same layer as pyramidal neurons born at about the same time, while CGE-derived interneurons settle mostly in superficial layers. Heterochronic transplants, which place MGE cells of one age into a host of another age, indicate that where an interneuron settles depends partly on the environment it meets, not only on its birthday. The projection neurons are part of that environment: when deep-layer projection neurons change identity in Fezf2 mutant mice, the distribution of interneurons shifts with them (Lodato and colleagues, 2011). Interneurons arrive in excess and their numbers are later trimmed by cell death, the subject of naturally occurring death.

The rostral migratory stream: a tangential route for life

Tangential migration is not only an embryonic event. In adult rodents, the subventricular zone lining the lateral ventricles still contains stem cells, which make dividing transit-amplifying “C cells”, which make young neurons. Those neurons travel forward along the rostral migratory stream to the olfactory bulb and become inhibitory interneurons there.

Marion Luskin traced this route from the anterior subventricular zone in newborn rats in 1993. In 1994 Carlos Lois and Arturo Alvarez-Buylla showed that it runs in adult mice: marked subventricular cells travelled to the bulb and became neurons. In 1996, with José Manuel García-Verdugo, they showed the mode of movement. The young neurons move in chains, sliding along one another inside tunnels formed by astrocytes, with no radial glia and no axon scaffold. This is chain migration, a third style next to glia-guided radial movement and the stream migration of cortical interneurons.

In humans, a comparable stream is seen in infants and declines after about 18 months of age and is nearly extinct by adulthood (Sanai and colleagues, 2011). Whether adult humans add neurons at all is taken up in adult neurogenesis.