Clustered γ-Protocadherins Regulate Cortical Interneuron Programmed Cell Death

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 adjusted - during 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.