Disruption of CCTβ2 Expression Leads to Gonadal Dysfunction

Abstract

There are two mammalian genes that encode isoforms of CTP:phosphocholine cytidylyltransferase (CCT), a key rate-controlling step in membrane phospholipid biogenesis. Quantitative determination of the CCT transcripts reveals that CCT␣ is ubiquitously expressed and is found at the highest levels in the testis and lung, with lower levels in the liver and ovary. CCT␤2 is a very minor isoform in most tissues but is significantly expressed in the brain, lung, and gonads. CCT␤3 is the third isoform recently discovered in mice and is expressed in the same tissues as CCT␤2, with its highest level in testes. We investigated the role(s) of CCT␤2 by generating knockout mice. The brains and lungs of mice lacking CCT␤2 expression did not exhibit any overt defects. On the other hand, a large percentage of the CCT␤2 ؊/؊ females were sterile and their ovaries exhibited defective ovarian follicle development. The proportion of female CCT␤2 ؊/؊ mice with defective ovaries increased as the animals aged. The rare litters born from CCT␤2 ؊/؊ ؋ CCT␤2 ؊/0 matings had the normal number of pups. The abnormal ovarian histopathology was characterized by disorganization of the tissue in young adult mice and absence of follicles and ova in older mice, along with interstitial stromal cell hyperplasia which culminated in the emergence of tubulostromal ovarian tumors by 16 months of age. Grossly defective CCT␤2 ؊/؊ ovaries were associated with high follicle-stimulating (FSH) and luteinizing (LH) hormone levels. Male CCT␤2 ؊/0 mice exhibited progressive multifocal testicular degeneration and reduced fertility but had normal FSH and LH levels. Thus, the most notable phenotype of CCT␤2 knockout mice was gonad degeneration and reproductive deficiency. The results indicate that although CCT␤2 is expressed at very low levels compared to the ␣-isoform, loss of CCT␤2 expression causes a breakdown in the gonadal response to hormonal stimulation.Phosphatidylcholine (PtdCho) is a major component of biological membranes in higher eukaryotes and is also secreted by specialized tissues for important extracellular tasks. CTP: phosphocholine cytidylyltransferase (CCT) is a key rate-controlling step in the major biosynthetic pathway leading to PtdCho in most tissues (for reviews, see references 15, 17, and 36). In mammals there are two genes, Pcyt1a (formerly Cptct), located on murine chromosome 16, and Pcyt1b, located on the X chromosome, that encode proteins termed CCT␣ and CCT␤, respectively (35, 38). The two genes exhibit tissuespecific expression, with CCT␣ predominating in most tissues and CCT␤ being most abundant in brain tissue (32). Two transcripts arise from the Pcyt1a gene that encode the identical CCT␣ protein. Alternate splicing of the X-linked Pcyt1b gene directs the synthesis of two mRNAs that encode the CCT␤2 and CCT␤3 isoforms in mice (32,35,38). CCT␤3 is 28 residues shorter at the amino terminus than CCT␤2 due to transcript initiation at an alternate first exon (32). In humans, intron retention gives rise to a CCT␤1 transcript found in the exp...

Phospholipid Biosynthesis Program Underlying Membrane Expansion during B-lymphocyte Differentiation

“…CCT transcript levels are not elevated in LPS-stimulated CH12 B-cells, although the CK and CPT genes are. Increased CCT expression has been implicated in neuronal differentiation (13,15), but this is not the case in B-cell differentiation. These findings suggest that the constitutive level of CCT␣ transcripts in CH12 cells is sufficient to support ER expansion upon LPS stimulation.…”

“…The rate of membrane PtdCho synthesis increases via stabilization of the transcript encoding CCT␣ when macrophages proliferate in response to colony-stimulating factor-1 (14). Similarly, enhanced expression of CCT accompanies the stimulation of PtdCho synthesis during neuronal differentiation (13,15). Stimulation of PtdCho synthesis is associated with cell cycle progression in other cell types (16,17), and in IIC9 cells it is accompanied by relocation of the CCT␣ protein from the nucleus to the cytoplasm (18).…”

“…Three CCT isoforms are expressed differentially in tissues, but CCT␣ is the dominant isoform that is expressed ubiquitously (12,13). Several mechanisms that regulate PtdCho synthesis have been uncovered, and most of these center on CCT.…”

Comparative genomics and evolution of eukaryotic phospholipid biosynthesis

“…Disruption of CCT genes has distinct phenotypes: deletion of mouse CCTα results in early embryonic lethality [89] whereas mice with impaired CCTβ2 expression are viable but have gonadal dysfunction and reduced fertility [90]. Disruption of the D. melanogaster CCT1 gene results also in a reproductive phenotype [91] attributed to aberrant membrane trafficking of ovarian growth and differentiation factor receptors [92].…”

Phosphatidylcholine Biosynthesis via CTP:Phosphocholine Cytidylyltransferase β2 Facilitates Neurite Outgrowth and Branching

“…The most pronounced expression of CT␤2 mRNA in the mouse is in the brain (30). However, since ϳ90% of the cells in the brain are glial cells, not neurons (reviewed in Ref.…”