RESEARCH
Early patterning of cortical primordium
The first step in creating a properly patterned cortical primordium is the creation of discrete signaling centers at the borders of the cortical neuroepithelium, which work to lay down a blueprint for the cortex to form. We discovered that the hippocampus forms in its precise position in the embryonic brain due to inductive signals from an organizer, the hem. We identified that 3 fundamental developmental regulators, LHX2, FOXG1 and PAX6, control the positioning of the hem itself. We are currently engaged in exploring the different lineages that arise from the dorsal midline and examining how the hem itself responds to WNT signaling.
Multiple hems and Multiple hippocampi: (left) schematic (Chou and Tole 2019), (right) IHC on sections from chimaera brains (Mangale et al, 2008)
Regulation of the neuron-astrocyte cell fate switch in the embryonic hippocampus
A characteristic feature of central nervous system development, conserved across all vertebrate species, is that neurogenesis precedes gliogenesis. Control of this transition has direct consequences on the number of neurons versus glia produced. This process is likely to be one of the critical steps shaped by evolution to bring about the unique nervous systems and subsystems seen across vertebrate species. We uncovered an interconnected cross-regulatory network of transcription factors that regulates the neuron-glia cell fate switch, and are currently exploring how this mechanism operates in different brain regions.
Molecular control of cell fate in the developing hippocampus. From Subramanian et al (author summary, 2011).
(Left) Progenitor cells (blue circles) first give rise to neurons (green triangles), then glia (yellow stars). This change in cell fate is paralleled by decreasing levels of transcription factor Lhx2. Removing Lhx2 from early progenitors causes them to produce glia prematurely. Supplementing Lhx2 levels in late progenitors is sufficient to prolong their neuron-production phase. Therefore, Lhx2 is necessary and sufficient to suppress glia production and enhance neuron production in hippocampal progenitors.
(Right) In vitro organotypic explant culture system to recapitulate in utero electroporation findings.
Regulation of cell fate in the neocortex: examining epigenetic regulation by Lhx2
The progenitors that produce different subtypes of neurons have a significant role in controlling their identities. In the neocortex, we discovered a mechanism that controls the specification of layer 6 versus layer 5 neuronal subtype identity from common progenitors. Loss of Lhx2 in neocortical progenitors results in a decrease in layer 6 (Tbr1) expressing neurons, and an increase in layer 5 (Fezf2) expressing neurons. LHX2 binds to proteins in the NuRD-HDAC chromatin regulatory complex. In the absence of LHX2, there is an increase in activatory marks on the TSS and the LHX2 binding regions of Fezf2 and Sox11. Ongoing studies focus on examining how LHX2 affects genome-wide regulation of chromatin accessibility.
Cortex-specific loss of Lhx2 alters the expression of neuronal subtype markers in layers 5 and 6. From Muralidharan et al, 2017.
Thalamocortical connectivity
Given the wide-ranging functions of LHX2 described thus far, it is perhaps not surprising that LHX2 is also critical for the development of major axonal tracts in the forebrain. In Shetty et al. (2013), we reported that the thalamocortical tract does not innervate the cortex normally upon cortex-specific loss of Lhx2.
In Pal et al., (2021), we discovered that LHX2 is required in the progenitors of the subplate, where thalamocortical axons “wait” for several days before they innervate the cortex. When Lhx2 is lost from subplate progenitors, the resulting subplate cells are electrically deficient, and thalamocortical axons overshoot the subplate, entering the cortex prematurely. This premature ingrowth is followed by atrophy of the sensory VB thalamic nucleus at embryonic stages leading to eventual loss of thalamocortical innervation and loss of barrels in cortical-progenitor Lhx2 mutants. No premature ingrowth of thalamocortical axons is seen when Lhx2 is lost in postmitotic neurons of the cortex, including the subplate. Therefore, the function of LHX2 in progenitors regulates the properties of the subplate neurons, and also the regulation of thalamocortical ingrowth.
(Left) When Lhx2 is deleted from E11.5 in cortical progenitors, the thalamocortical innervation of the barrel cortex is barely detectable, indicating that the function of LHX2 in the cortex is critical for the barrel cortex development (Shetty et al., 2013).
(Right) Loss of Lhx2 from E11.5 in cortical progentors using Emx1Cre, this results in exuberant, premature ingrowth of thalamocortical axons into the cortex. This defect is not seen when Lhx2 is lost exclusively in postmitotic neurons using NexCre. From Pal et al., 2021.
Many thanks to the labs we collaborated with over the years :
Sanjeev Galande Lab, IISER Pune, India
Orly Reiner Lab, Weizmann Institute of Science, Israel
Kathleen J. Millen lab, Seattle Children’s Research Institute, USA
Denis Jabaudon Lab, University of Geneva, Switzerland
Ed Monuki Lab, University of California Irvine, USA
Upinder S. Bhalla Lab, NCBS , India
Ullas Kolthur Lab, TIFR Mumbai, India
Vidita Vaidya Lab, TIFR Mumbai, India
Gord Fishell, Harvard Medical School, USA
Takuji Iwasato group, National Institute of Genetics, Japan
Alessandra Pierani Lab, Institut Imagine, Paris
Gundela Meyer lab, University of La Laguna, Spain
Jeffrey Macklis Lab, Harvard Department of Stem Cell and Regenerative Biology, USA
Bin Chen lab, University of California, Santa Cruz, USA
With Orly Reiner and "Flat Stanley", Jan 2020
With Denis Jabaudon, 2018