Clustered γ-Protocadherins Regulate Cortical Interneuron Programmed Cell Death

Abstract

Cortical function critically depends on inhibitory/excitatory balance. Cortical inhibitory interneurons (cINs) are born in the ventral forebrain. After completing their migration into cortex, their final numbers are adjustedduring a period of postnatal development -by programmed cell death. The mechanisms that regulate cIN elimination remains controversial. Here we show that genes in the protocadherin (Pcdh)-γ gene cluster, but not in the Pcdh-α or Pcdh-β clusters, are required for the survival of cINs through a BAX-dependent mechanism. Surprisingly, the physiological and morphological properties of Pcdh-γ deficient and wild type cINs during cIN cell death were indistinguishable. Co-transplantation of wild type and Pcdh-γ deficient interneuron precursor cells demonstrate that: 1) the number of mutant cINs eliminated was much higher than that of wild type cells, but the proportion of mutant or WT cells undergoing cell death was not affected by their density; 2) the presence of mutant cINs increases cell death among wild-type counterparts, and 3) cIN survival is dependent on the expression of Pcdh-γ C3, C4, and C5. We conclude that Pcdh-γ, and specifically γC3, γC4, and γC5, play a critical role in regulating cIN survival during the endogenous period of programmed cIN death. SignificanceInhibitory cortical interneurons (cIN) in the cerebral cortex originate from the ventral embryonic forebrain. After a long migration, they come together with local excitatory neurons to form cortical circuits. These circuits are responsible for higher brain functions, and the improper balance of excitation/inhibition in the cortex can result in mental diseases. Therefore, an understanding of how the final number of cINs is determined is both biologically and, likely, therapeutically significant. Here we show that cell surface homophilic binding proteins belonging to the clustered protocadherin gene family, specifically three isoforms in the Pcdh-γ cluster, play a key role in the regulation cIN programmed cell death. Co-transplantation of mutant and wild-type cINs shows that Pcdh-γ genes have cell-autonomous and non-cell autonomous roles in the regulation of cIN cell death. This work will help identify the molecular mechanisms and cell-cell interactions that determine how the proper ratio of excitatory to inhibitory neurons is determined in the cerebral cortex.

Protocadherins at the Crossroad of Signaling Pathways

“…Members of the γ-Pcdh cluster are known to prevent neuronal apoptosis of spinal cord interneurons, retinal cells, and cortical interneurons (cINs) (Wang et al, 2002b;Lefebvre et al, 2008;Prasad et al, 2008;Chen W. V. et al, 2012;Garrett et al, 2012;Hasegawa et al, 2016Hasegawa et al, , 2017Carriere et al, 2020;Leon et al, 2020). Transplantation studies and knockout mice phenotyping showed that loss of C-type γ-Pcdhs leads to increased cell death in spinal cord interneurons and cINs (Chen W. V. et al, 2012;Leon et al, 2020). Remarkably, within the C-type γ-Pcdhs, only one isoform (PcdhγC4) seems to be necessary for neuronal survival of several cell populations (Garrett et al, 2019).…”