origin and migration of cortical interneurons

10

Calbindin!

In utero homotopic transplantation of!MGE lacZ labelled neuroblasts!

experimental proofs of telencephalic ventro-dorsal tangential migration

between 1995 and 2000 several groups reported 3 substantial proofs that cortico-cerebral interneurons are generated in ventral telencephalon:

(1) reduced number of interneurons upon cortico-striatal severing of living slices

(2) migrating DiI-labelled interneurons on living slices

(3) migrating lacZ-labelled interneurons upon in vivo homochronic homotopic transplantation

11

(1)

(2)

(3) *

migrating gabaergic neurons leaving ganglionic eminence are all post-mitotic.

within the cortex, they point to the VZ and spend there some time; after that, they radially migrate to their final laminar destination.

12

general articulation of interneuronogenesis in rodents

GABAergic interneurons are generated in the lateral halves of both MGE and LGE/CGE, whereas the medial halves of these structures give rise to local projection neurons

13

interneuronogenic sectors of ganglionic eminences

(to cx)

(to cx)

origin, morphology and connections of distinctive interneuronal subtypes

the generation of distinctive cortico-cerebral inter-neuron types is spatio-temporally compartimentalized

interneurons expressing different molecular markers display: - distinct morphologies - different firing patterns - diverse synaptic connections

14

early………….…………late

impact of interneurons on cortical excitability

at first approximation, the analysis of LOF mutants suggests that:

- earlier, MGE-generated PV+ and SST+ interneurons mainly limit cortical excitability*

- later, CGE-generated CR+ and VIP+ ones may enhance it**

- the prevailing function of POA/CGE-generated NPY ones is more difficult to decipher

15 * except – maybe – Chandelier cells projecting to the axonal hillock ** this especially applies to CR+ cells projecting to other inhibitory GABAergic elements

16

genetic control of generation of distinctive interneuron types

17

tangential interneuronal migration (I)

1

2 3

4

the Cxcl12 ligand released by meninges and basal progenitors (BPs) of the SVZ, by acting on the Cxcr4 receptor expressed by interneurons, causes their clustering and attrction to the MZ and the SVZ/IZ, as well their competitive exclusion from the CP

Tbr2 is necessary and sufficient for BP expression of Cxcl12 by BPs, but not for its pial expression

attraction of interneurons by BPs allows to proportionate the GABAergic to the glutamaterc population of the cortex in placentals (in the absence of Tbr2, interneurons massively persist within ventral telencephalon, where they massively die)

18

coupling tangential interneuronal migration to glutamatergic neuronogenesis [Broccoli and coll (2010) Genes Dev. (16):1816-26]

only 1/3 of human cortical interneurons originate from basal ganglia, as originally found in rodents. such ventrally born interneurons are mainly generated at early ages, starting from about E30

like in rodents, human immature interneurons coming from the GE and located in the developing Cx express Dlx and GAD genes, but not Mash1, and are already postmitotic

as many as 2/3 of human cortical interneurons derive from proliferating precursors located within the cortical SVZ. these precursors co-express Dlx and GAD genes, but also Mash1. they can be distinguished in living slices from the previous ones by M-phase and S-phase markers as well as by infection with pTa1-EGFP-encoding gammaretroviruses

human, dorsally born neurons are generated prevalently late, from E85 onward. 19

human vs rodent cortico-cerebral interneuronogenesis [Rakic and coll (2002) Nature

(6889):645-9; Zecevic and coll (2011) J

Neurosci (7):2413-20. Zecevic and coll (2011) Dev

Neurobiol (1):18-33]

postnatal interneuronogenesis in the rodent cortex

this is an example of the 3-4 studies reporting neonatal generation of interneurons in the murine cortical SVZ

GABAergic interneurons are also produced in the murine peri- and postnatal neocortical SVZ, from which they diffuse to the olfactory bulb (OB, via RMS), the hippocampus & occipital cortex (H & OCx, via DMS), striatum & lateral cortex (via LMS). many of these cells leave these streams and migrate towards the overlying neocortex (NCx).

postnatally-born interneurons were discovered by a transgenic line expressing EGFP under the control of the HT3A promoter [this promoter selectively fires in a subset of GABAergic neurons, expressing calretinin (CR) or cholecystokinin (CCK), but not parvalbumin (PV) or somatostatin (SST). it is not active in glial cells or glutamatergic neurons].

postnatal-interneuronogenesis was characterized by (1) time course fluorescent profiling, (2) videolapse microscopy (3) BrdU birthdating (timed labelling followed by end-point analysis)

[Monyer & coll (2008) doi/10.1073/pnas.0807059105] (1)

(2)

(3)

OB

OCx

H

NCx

this process is pronounced in the first 3 postnatal weeks and then it declines. however it is still active at P90.

in mice as many as 1/6 of all CR+/HT3A+ interneurons are apparently born after birth (!!!)

21

[Yu & Zecevic (2011): J.Neurosci: 31(7):2413–2420] human, not murine,late neuronogenic pallial RGCs are able to generate astrocytes

murine RGCs isolated from the E16 cortex via immunopanning and labelled via irreversible activation of EGFP driven by the blbp promoter, do not give rise to GABAergic interneurons after prolonged in vitro differentiation

conversely, RGC cells originating from both murine E16 basal ganglia and human E125 cortex are able to do that

all that suggests that peri- and postnatally generated, murineCR+ interneurons diffusing from the cortical SVZ to the overlying neocortex should have their RGC ancestors outside of the pallium or, alternatively, in the early pallium