Mitochondrial transcription factor A induces the declined mitochondrial biogenesis correlative with depigmentation of brown eggshell in aged laying hens

Eggshell color is an important characteristic for poultry eggs. Eggs from aged hens usually have poor shell color that is unacceptable for the table egg market. The objective of this study was to examine effects of pigment synthesis and mitochondrial biogenesis on brown eggshell color of aged laying hens. In this trial, 8 hens laying eggs with darker shell color and 8 hens laying eggs with lighter shell color were selected from 300 62-week-old Hy-Line brown-egg laying hens. Results showed that egg weight (P < 0.05), eggshell weight (P < 0.01), protoporphyrin IX (Pp IX) content of the eggshell and the shell gland (P < 0.001), and biliverdin content of the shell gland (P < 0.001) were significantly declined in the light-shell group compared with the dark-shell group. Relative mRNA expression of δ-aminolevulinic acid synthase1 (ALAS1) (P < 0.05), coproporphyrinogen oxidase (P < 0.01), ATP-binding cassette transporter ABCG2 (P < 0.01), and mitochondrial transcription factor A (P < 0.05) was reduced in hens laying lighter brown eggshell. Moreover relative mRNA expression of mitochondrial DNA copy number (P < 0.01), mitochondrial NADH dehydrogenase subunit 4 (P < 0.05), mitochondrial ATP synthase F0 subunit 8 (P < 0.05), and mitochondrial cytochrome c oxidase 1 (P < 0.01) was significantly decreased in the shell gland of the light-shell group. In addition, NAD(+) contents of the shell gland were increased in the dark-shell group (P < 0.01). Brown eggshell depigmentation is a result of decreased Pp IX content in the eggshell and the shell gland. Decreased mitochondrial biogenesis may contribute to the depigmentation of brown eggshell by targeting ALAS1 and ALAS1-mediated Pp IX biosynthesis.