Current insight into the transient X-zone in the adrenal gland cortex.

Mouse models have been widely used in the study of adrenal gland development and diseases. The X-zone is a unique structure of the mouse adrenal gland and lineage-tracing studies show that the X-zone is a remnant of the fetal adrenal cortex. Although the X-zone is considered analogous to the fetal zone in the human adrenal cortex, the functional significance of the X-zone has remained comparatively more obscure. The X-zone forms during the early postnatal stages of adrenal development and regresses later in a remarkable sexually dimorphic fashion. The formation and regression of the X-zone can be different in mice with different genetic backgrounds. Mouse models with gene mutations, hormone/chemical treatments, and/or gonadectomy can also display an aberrant development of the X-zone or alternatively a dysregulated X-zone regression. These models have shed light on the molecular mechanisms regulating the development and regression of these unique adrenocortical cells. This review paper briefly describes the development of the adrenal gland including the formation and regression processes of the X-zone. It also summarizes and lists mouse models that demonstrate different X-zone phenotypes.

DHCR24, a Key Enzyme of Cholesterol Synthesis, Serves as a Marker Gene of the Mouse Adrenal Gland Inner Cortex.

Steroid hormones are synthesized through enzymatic reactions using cholesterol as the substrate. In steroidogenic cells, the required cholesterol for steroidogenesis can be obtained from blood circulation or synthesized de novo from acetate. One of the key enzymes that control cholesterol synthesis is 24-dehydrocholesterol reductase (encoded by DHCR24). In humans and rats, DHCR24 is highly expressed in the adrenal gland, especially in the zona fasciculata. We recently reported that DHCR24 was expressed in the mouse adrenal gland's inner cortex and also found that thyroid hormone treatment significantly upregulated the expression of Dhcr24 in the mouse adrenal gland. In the present study, we showed the cellular expression of DHCR24 in mouse adrenal glands in early postnatal stages. We found that the expression pattern of DHCR24 was similar to the X-zone marker gene 20αHSD in most developmental stages. This finding indicates that most steroidogenic adrenocortical cells in the mouse adrenal gland do not synthesize cholesterol locally. Unlike the 20αHSD-positive X-zone regresses during pregnancy, some DHCR24-positive cells remain present in parous females. Conditional knockout mice showed that the removal of Dhcr24 in steroidogenic cells did not affect the overall development of the adrenal gland or the secretion of corticosterone under acute stress. Whether DHCR24 plays a role in conditions where a continuous high amount of corticosterone production is needed requires further investigation.

RNA-Seq Reveals Sub-Zones in Mouse Adrenal Zona Fasciculata and the Sexually Dimorphic Responses to Thyroid Hormone.

The sex-specific prevalence of adrenal diseases has been known for a long time. However, the reason for the high prevalence of these diseases in females is not completely understood. Mouse studies have shown that the adult adrenal gland is sexually dimorphic at different levels such as transcriptome, histology, and cell renewal. Here we used RNA-seq to show that in prepubertal mice, male and female adrenal glands were not only sexually dimorphic but also responded differently to the same external stimulus. We previously reported that thyroid hormone receptor ß1 (TRß1) in the adrenal gland is mainly expressed in the inner cortex and the fate of this TRß1-expressing cell population can be changed by thyroid hormone (triiodothyronine; T3) treatment. In the present study, we found that adrenal glands in prepubertal mice were sexually dimorphic at the level of the transcriptome. Under T3 treatment, prepubertal females had 1162 genes differentially expressed between the saline and T3 groups, whereas in males of the same age, only 512 genes were T3-responsive. Immunostaining demonstrated that several top sexually dimorphic T3-responsive genes, including Cyp2f2 and Dhcr24, were specifically expressed in the adrenal inner cortex, precisely in an area partially overlapping with the X-zone. Under T3 treatment, a unique cortical layer that surrounds the adrenal X-zone expanded significantly, forming a distinct layer peculiar to females. Our findings identified novel marker genes for the inner adrenal cortex, indicating there are different sub-zones in the zona fasciculata. The results also highlight the sex-specific response to thyroid hormone in the mouse adrenal gland.

Microwaving and Fluorophore-Tyramide for Multiplex Immunostaining on Mouse Adrenals - Using Unconjugated Primary Antibodies from the Same Host Species.

Immunostaining is widely used in biomedical research to show the cellular expression pattern of a given protein. Multiplex immunostaining allows labeling using multiple primary antibodies. To minimize antibody cross-reactivity, multiplex immunostaining using indirect staining requires unlabeled primary antibodies from different host species. However, the appropriate combination of different species antibodies is not always available. Here, we describe a method of using unlabeled primary antibodies from the same host species (e.g., in this case both antibodies are from rabbit) for multiplex immunofluorescence on formalin-fixed paraffin-embedded (FFPE) mouse adrenal sections. This method uses the same procedure and reagents used in the antigen retrieval step to strip the activity of the previously stained primary antibody complex. Slides were stained with the first primary antibody using a general immunostaining protocol followed by a binding step with a biotinylated secondary antibody. Then, an avidin-biotin-peroxidase signal development method was used with fluorophore-tyramide as the substrate. The immunoactivity of the first primary antibody complex was stripped through immersion in a microwaved boiling sodium citrate solution for 8 min. The insoluble fluorophore-tyramide deposition remained on the sample, which allowed the slide to be stained with other primary antibodies. Although this method eliminates most false positive signals, some background from antibody cross-reactivity may remain. If the samples are enriched with endogenous biotin, a peroxidase-conjugated secondary antibody may be used to replace the biotinylated secondary antibody to avoid the false positive from recovered endogenous biotin.