The hypothalamic-pituitary axis (HP) is the main link between the central nervous system and endocrine system. The HP regulates hormone secretion during homeostasis, growth, reproduction, and several other physiological functions. Under control of the hypothalamus, the adenohypophysis (anterior pituitary lobe) produces and secretes hormones involved in these functions. This control occurs through the production of hypothalamic hypophysiotropic hormones that stimulate or inhibit synthesis and release of adenohypophysial hormones. The hypothalamus also secretes hormones such as vasotocin and oxytocin (in mammals), and isotocin and vasopressin (in fish) through its neuronal projections into the neurohypophysis (Bentley, 1998
; Unger and Glasgow, 2003
Several genes regulate induction, specification, survival, patterning, and cell differentiation in the adenohypophysis (reviewed in (Dasen and Rosenfeld, 2001
; Pogoda and Hammerschmidt, 2007
; Zhu et al., 2005
). However, little is known about the genetic regulation of later hypothalamic and neurohypophysial development or even when the HP matures and becomes physiologically functional.
To identify developmental markers of the HP, we constructed subtractive cDNA libraries between adult hypothalamus and pituitary and examined expression of these genes in embryos and larvae. We report the construction of these subtractive libraries and the identification of genes represented in them. We also present the expression patterns of some of these genes in the HP during the first 5 days of development. Some of these genes are previously known expressed sequence tags (ESTs), with undocumented or incompletely documented HP expression patterns. Other genes were previously unreported in zebrafish, some of which are novel and some of which are homologous to genes in other species.
1.1. Library construction and clone identification
To identify genes expressed in the HP, we constructed subtractive cDNA libraries between adult zebrafish hypothalamus and pituitary. The hypothalamic cDNA library was obtained by subtracting pituitary cDNAs from hypothalamic cDNAs: 448 clones were isolated (). Conversely, the hypothalamic cDNAs were subtracted from pituitary cDNAs to construct the pituitary cDNA library: 558 clones were isolated (). We eliminated clones that contained the most redundant sequences by membrane hybridization. We then characterized the remaining clones by sequencing and comparison with NCBI nucleotide and protein databases and the Sanger Institute Danio rerio genomic database.
Hypothalamus and pituitary library construction.
The subtraction process enriched cDNAs that are differentially expressed in hypothalamus or adenohypophysis. Our initial analysis suggested that the proportion of house keeping genes, such as mitochondrial and ribosomal genes, is low (). With the exception of the spermatogenesis associated protein 2 gene that we found in both libraries (1 sequence in the pituitary library, 2 similar sequences in the hypothalamic library), no other overlap was observed between the libraries, including mitochondrial and ribosomal sequences (not shown).
The sequences we identified consisted of previously published genes (known genes) and ESTs or novel genes (genes not cloned or identified in zebrafish yet or predicted only by genomic database analysis). Based on studies in other species and analysis of conserved domains, these genes can be classified into functional categories that include structural and physiological functions, cell metabolism, and protein synthesis.
We identified only one transcription factor in the pituitary library: the POU domain class1 transcription factor 1
) that plays a role in somatotrope, lactotrope, and thyrotrope development (; Gage et al., 1996
; Nica et al., 2004
). Because transcription factors are expressed at a lower frequency compared to many other mRNAs, the PCR amplification and subtraction method we used might not have been sensitive enough to identify genes expressed at these low levels. Alternatively, it is possible that many transcription factors are expressed in both tissues and were subtracted during the construction of the libraries. Also, some tissue specific transcription factors are probably no longer expressed in differentiated adult tissues.
List of genes expressed in the HP during the first 5d of development.
We also identified genomic DNA sequences that do not apparently correspond to genes. Some might represent non-coding RNA. Others that correspond to intronic sequences might have resulted from unspliced mRNA variants, although we did not find corresponding exon sequences of these genes. We can also not entirely exclude the possibility that some of these sequences might derive from genomic DNA contamination. Because our goal was to isolate genes expressed in the HP, we did not analyze these sequences further.
The pituitary library (supplemental Table 1
) contained genes encoding most of the adenohypophysial hormones including: follicle stimulating hormone beta-subunit
), glycoprotein hormones alpha polypeptide
), growth hormone
), luteinizing hormone beta-subunit
), somatolactin beta
) and thyroid stimulating hormone beta subunit
). One exception was somatolactin alpha
) that we did not detect. Together, these represent 68.5% of all identified sequences.
is expressed in both the pituitary and hypothalamus and should therefore have been subtracted, pomc
was represented abundantly (36.4%) in the pituitary library, and was absent from the hypothalamic library. Presumably, this results from very high expression levels of pomc
in the pituitary compared to the hypothalamus at the time of tissue dissection. In spite of our precautions to limit stress by MS-222 anesthesia and chilling on ice, pomc
encoded ACTH biosynthesis might have been elevated in response to stress (Engelmann et al., 2004
In the hypothalamic library (supplemental Table 2
), ependymin (epd)
was the most frequently represented gene (36.7%). This gene encodes a specific CNS glycoprotein, an adhesion molecule involved in memory consolidation (Schmidt, 1995
; Schmidt et al., 1995
; Shashoua, 1991
). We found only one gene that has been reported to regulate pituitary hormone production and/or secretion: adenylate cyclase activating polypeptide 1b
, 1.4%). This gene, also known as pituitary adenylate cyclase activating peptide
), functions in various species as a hormone releasing factor (Gracia-Navarro et al., 2002
; Zhou et al., 2005
). We also found genes corresponding to neuropeptides, such as proenkephalin-like
(1.7%) and prodynorphin
(0.3%) that belong to the opioid gene family (Dores et al., 2002
), and pro-melanin concentrating hormone-like
encoding the hypothalamic hormone, Melanin concentrating hormone (Mch; 0.6%; manuscript in preparation).
1.2. Gene expression patterns during the first 5d of development
The genes represented in these libraries were expressed in a tissue-specific manner in the mature, functional HP of adults. Therefore, the onset of expression during development of the hypothalamus, adenohypophysis, and neurohypophysis would make these genes interesting potential markers for the functional maturation of the HP.
To test whether the genes isolated in the adult pituitary and hypothalamic libraries are expressed in embryos and larvae, we analyzed their expression patterns between Prim-5 (24h) stage and day 5 (5d) of development. We analyzed whole-mount embryos by mRNA in situ
hybridization, if the length of the isolated sequence permitted such analysis (45 genes out of 116 total; 39% genes), or consulted the ZFIN gene expression database (http://zfin.org
, 57 genes out of 116 total; 49% genes).
Because we subtracted cDNAs exclusively between hypothalamic and pituitary tissues and not from the entire fish, several genes were also expressed in other tissues, for example in the CNS, or throughout the animal (Suppl. Fig. 1–21
). In addition, because we constructed the libraries from adult fish, some genes were not expressed between Prim-5 (24h) stage and 5d. Probably, these are expressed later during development and adulthood (see section 1.3).
Our analysis indicates that, among the genes with expression pattern data, 23.5% (24 out of 102 total) are expressed in the HP during the first 5 days of development (). 63.7% (65 out of 102 total) are expressed outside of the HP and 12.7% (13 out of 102 total) are not expressed during the first 5d of development. Because we analyzed gene expression by whole mount in situ
hybridization, it is possible that the sensitivity of this assay was insufficient to detect low expression levels. For instance, some gene expression not detected by in situ
hybridization was successfully detected by PCR (see section 1.3; Suppl. Fig. 22
1.2.1. Genes expressed in the adenohypophysis
All the genes expressed in the adenohypophysis during the first 5d of development were isolated from the pituitary library.
) belongs to the granin family (chromogranin, secretogranin), a group of acid secretory proteins found in the secretory granules of endocrine cells. Even though their precise role is unclear, proteins of this family are thought to be involved in secretion (Holthuis et al., 1996
; Holthuis and Martens, 1996
). Consistent with this postulated role, scg3
is found in zebrafish in endocrine organs such as adenohypophysis (), pancreas, and epiphysis (not shown) at Prim-5 (24h) stage. Additional weak expression is observed in the telencephalon and midbrain. Expression in these domains continues through protruding mouth (72h) stage. scg3
expression also appears in the retina and epiphysis by 5d (ZFIN, http://zfin.org
). In the adenohypophysis, cells in the most anterior part of the gland, called the rostral pars distalis (RPD; ), express scg3
Genes expressed in the adenohypophysis
hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase
) plays a role in steroid biosynthesis. It was identified in zebrafish using microarrays to analyze global hematopoietic and vascular endothelial gene expression (Covassin et al., 2006
). In addition, hsd3b7
is expressed in the adenohypophysis () and in the telencephalon (Suppl. Fig. 3
) from Prim-5 (24h) stage to protruding mouth (72h) stage. We find that all adenohypophysial cells express the gene. Additional expression of hsd3b7
outside of the HP is shown in Suppl. Fig. 3
solute carrier family 16 (monocarboxylic acid transporters) member 9a
) belongs to the monocarboxylic acid transporter family. slc16a9a
is expressed from Prim-5 (24h) stage to 5d in the RPD (), which contains lactotrope, corticotrope, and melanotrope cells. Additional expression outside of the HP is visible from 72h and later (Suppl. Fig. 6
adrenomedullin 2 (adm2)
is a predicted gene that belongs to the calcitonin/calcitonin related gene
family. In rat, it is also known as intermedin
) and is expressed in the intermediate lobe of the pituitary and in specific adenocorticotrope cells in the anterior lobe (Lin Chang et al., 2005
). Lin Chang et al. showed that PRL release increases in a dose dependent manner in response to Intermedin treatment. In female rats, these results suggested a role of Intermedin in the regulation of PRL release during reproduction.
In zebrafish, adm2 is weakly expressed in the adenohypophysis at 24h. adm2 expression intensifies by long-pec (48h) stage and is reduced at 72h (). We observed no expression at 4d or 5d. adm2 expressing cells in the adenohypophysis coexpress pomc in the RPD and the posterior part of the adenohypophysis called pars intermedia (PI). Lactotrope, corticotrope, and melanotrope cells are located in these subdomains of the adenohypophysis.
uo:ion002 is a novel gene. A peptide domain of the acyltransferase family was recognized as a putative conserved domain in the protein encoded by this gene, indicating a potential role of this gene in lipid metabolism. Expression of uo:ion002 starts at 48h and is maintained until 5d. It is exclusively expressed in the PI which contains melanotrope cells ().
1.2.2. Genes expressed in the hypothalamus
Genes expressed in the hypothalamus during the first 5d of development were found in the hypothalamic library except for serpin peptidase inhibitor, clade I (neuroserpin), member 1 which was found in the pituitary library.
cytochrome P450 family 19 subfamily A polypeptide 1b
(Lassiter and Linney, 2007
) and pro-melanin concentrating hormone like
) are expressed in the hypothalamus during the first 5 days of development.
adenylate cyclase activating polypeptide 1b
), also known as pituitary adenylate cyclase-activating peptide (pacap)
was originally discovered in hypothalamic extracts and was shown to stimulate cAMP accumulation in pituitary cells (Miyata et al., 1989
; Miyata et al., 1990
). Among several postulated roles, pacap
is known as a releasing factor for hormones, including adenohypophysial hormones (Rawlings and Hezareh, 1996
was shown to increase the secretion of Growth Hormone (Gracia-Navarro et al., 2002
; Wong et al., 2000
; Zhou et al., 2005
), Gonadotropin (Petersen et al., 1996
; Wong et al., 2000
), Thyrotropin (TSH; (Okada et al., 2006
), and Prolactin (Rawlings and Hezareh, 1996
In zebrafish, adcyap1b is expressed in the telencephalon at Prim-5 (24h) stage (). At 48h () and 72h (), the expression pattern expands throughout the anterior CNS, including the telencephalon, hypothalamus, ventral mesencephalon, and cerebellum. Double in situ hybridization with pomc mRNA probe indicates that the hypothalamic expression is located close to the adenohypophysis (). This proximity is consistent with a role in pituitary hormone regulation. We did not observe adcyap1b expression in the neurohypophysis.
Genes expressed in the hypothalamus
potassium channel tetramerization domain containing 4 (kctd4) is an EST. kctd4 is expressed in the telencephalon and hypothalamus from Prim-5 (24h) stage through 5d. The hypothalamic expression at 24h is located close to the adenohypophysis (). Later, this expression domain extends throughout the anterior hypothalamus, close to the adenohypophysis although not in the neurohypophysis (). Weak hindbrain expression starts at 48h (). kctd4 is an interesting marker because its expression pattern resembles that of the hypophysiotropic gene adcyap1b.
calcium/calmodulin-dependent protein kinase 1D
) was known as an EST. The calcium/calmodulin-dependent protein kinase best documented in the literature is camkII
. It is a multifunctional serine/threonine kinase acting by phosphorylating proteins which functions are Ca2+
dependent. CamkII is involved in a variety of functions, such as cell division, differentiation, synaptic plasticity and transmission, dendritic morphology plasticity, and cardiac contraction (Colbran and Brown, 2004
; Griffith, 2004
; Hudmon and Schulman, 2002
In zebrafish, camk1d expression starts at 48h in the hypothalamus, telencephalon and midbrain (). This expression persists at 72h (). No expression was found in the neurohypophysis ().
stathmin 1/ oncoprotein 18 b (stmn1b)
was known as an EST. It plays a role in microtubule destabilization. In vertebrates, it is expressed in neurons and in cells with a potential to proliferate. The expression is reduced in terminally differentiated cells (Cassimeris, 2002
In zebrafish, stmn1b is expressed in the hypothalamus, telencephalon and neural crest at 24h (). stmn1b is expressed in the telencephalon, hypothalamus and mesencephalon at 48h () and 72h (). Expression in the hypothalamus is near the adenohypophysis. Some, but not all neurohypophysial cells express stmn1b ().
pleckstrin and Sec7 domain containing 3 like (psd3l) is a novel gene. According to conserved domains, it is a guanine nucleotide exchange factor for small GTPases. The Sec7 domain is the central domain.
psd3l is expressed in the hypothalamus and telencephalon at 24h (). This expression is maintained through 48h () and 72h (). We found no expression in the neurohypophysis ().
serpin peptidase inhibitor, clade I (neuroserpin), member 1 (serpini1)
is a novel gene. It is a member of the serine proteinase inhibitor (serpin) gene family. It was shown in other species that it reacts preferentially with plasminogen activator in the brain. It is released from neurons and is involved in synaptic plasticity (Yepes and Lawrence, 2004
serpini1 was found in the pituitary library, however we detected no expression in the adenohypophysis during the first 5d of development. Expression in the hypothalamus is visible from 24h (). At 48h () and 72h (), we observed additional expression in telencephalon and midbrain (). Expression in the hypothalamus is close to the adenohypophysis but not in the neurohypophysis ().
1.2.3. uo:ion003 is expressed in both adenohypophysis and hypothalamus
Genes represented in the subtractive libraries were expected to be expressed exclusively in the hypothalamus or in the pituitary. However, one gene, uo:ion003 (from the pituitary library), was expressed both in adenohypophysis and hypothalamus during the first 5d of development and in adult brain ( and ). uo:ion003 is a novel gene. No putative conserved domains were detected. At 24h, uo:ion003 is expressed in both hypothalamus and adenohypophysis (). This expression persists through 48h () and 72h ( and ). In addition, uo:ion003 is expressed in the telencephalon, midbrain and hindbrain at all stages. In the adenohypophysis, uo:ion003 is expressed in the posterior pars distalis ( and ), but not in the area that corresponds to the neurohypophysis ().
Genes expressed in hypothalamus and adenohypophysis and in neurohypophysis
Genes expressed in adult hypothalamic-pituitary axis
1.2.4. crabp1a is expressed in the neurohypophysis
In teleosts, neurohypophysis and adenohypophysis do not form morphologically distinct lobes. Hypothalamic neurons project and interdigitate with the adenohypophysis. This interdigitation can be observed morphologically and is defined as the neurohypophysis (Chapman et al., 2005
). During dissection, it was not possible to separate the neurohypophysis from the hypothalamus or the adenohypophysis. Consequently, neurohypophysial cDNAs contributed to hypothalamic and pituitary cDNA pools and might have been subtracted. However, depending on the location and expression level of particular mRNAs within the neurons contributing to the neurohypophysis (in the cell body, axon, or dendrites), the corresponding cDNAs might have been abundantly represented in either of the cDNA pools.
We identified cellular retinoic acid binding protein 1a
) in the pituitary library. Crabp1a belongs to the intracellular lipid-binding protein family that binds fatty acids, retinoids, and steroids. In mammals, Crabp1 function is not fully understood. Based on its expression in sites of ongoing plasticity that respond to retinoic acid, Crabp1 has been proposed to play a role in retinoic acid targeting, metabolism, and presumably also in neurogenesis (Thompson Haskell et al., 2002
; Zetterstrom et al., 1999
; Zetterstrom et al., 1994
; Zhou and Wei, 2001
). Two orthologs of crabp1
exist in the zebrafish genome: crabp1a
that we identified in the present study and crabp1b
(Liu et al., 2005
; Sharma et al., 2003
crabp1a is expressed at 24h in hypothalamic cells, near the adenohypophysis (), and in the neurohypophysis at 48h () and 72h (). Saggital sections of the adenohypophysis confirmed that crabp1a is expressed in the neurohypophysis, dorsal to the adenohypophysis (visualized by pomc expression; ). In addition, crabp1a is also expressed in the telencephalon, in the tegmentum, and in the anterior spinal chord from 48h until 4d ().
1.3. Gene expression in adults
Because we subtracted cDNAs from adult hypothalamus and pituitary tissues, genes we identified that were not expressed in the HP during the first 5d of development might presumably be expressed later during development and adulthood. To analyze expression in the adult, we performed PCR on adult hypothalamus and pituitary cDNAs and in situ hybridized mRNA probes for a subset of genes to adult brain.
We performed PCR on adult hypothalamus and adult pituitary cDNAs using primers for 20 genes represented in each library. Primers were designed to amplify a fragment between 350 and 500 bp (not the full length cDNA). In each library, 80–85% of the genes (17 genes from the pituitary library, 16 genes from the hypothalamic library) were amplified from both adult pituitary and hypothalamic cDNA. 15% of the genes (3 genes) were amplified only from the adult pituitary cDNA (genes from the pituitary library) or from the adult hypothalamic cDNA (genes from the hypothalamic library). Only one gene (from the hypothalamic library) was not amplified from either cDNA set. However, we cannot rule out a failure of the PCR or primers used (Suppl. Fig. 22
). Apparently, these results are independent of the number of clones represented in the library for each gene. For example, genes that are represented in a large number of clones in the libraries (for example epd
in the hypothalamic library or pomc
in the pituitary library) were amplified from both sets of cDNAs, whereas genes represented by 1–3 clones were amplified from only one set of cDNA (Suppl. Fig. 22
At first sight, these results appear inconsistent with the theoretically predicted outcome of the subtraction method, however, they are probably due to a higher sensitivity to gene expression differences than anticipated.. When most of the genes analyzed were detected by PCR from both hypothalamic and pituitary cDNAs, there was almost no identical sequence represented in both libraries (including mitochondrial and ribosomal). Only spermatogenesis associated protein 2 was represented in both hypothalamus and pituitary library, corresponding to 1 sequence in the pituitary library, 2 identical sequences in the hypothalamic library, out of 840 total sequences.
It is possible that hypothalamus and pituitary express different isoforms of the same gene. If this is true, the sequences represented in each library would be specific for the particular isoform. However, no identical genes were represented in both hypothalamic and pituitary library by different sequences. Interestingly, for some of the genes analyzed, we amplified a fragment of different size from hypothalamic cDNAs and from pituitary cDNAs. In some cases, two fragments of different sizes were amplified from one set of cDNAs, when a single fragment was amplified from the other set of cDNAs (Suppl. Fig. 22
The fact that most of the analyzed genes were amplified by PCR from both sets of cDNAs can also be explained by a difference in the level of expression in the hypothalamus compared to the pituitary. Accordingly, genes represented in each library are specifically expressed or expressed at higher level in one tissue compared to the other. For example, pomc was represented abundantly only in the pituitary library, even though it is expressed in both pituitary and hypothalamus.
Even though the intensity of the amplified fragment obtained by PCR is obvious in some cases, we based our analyses on the presence or absence of amplification, without taking the intensity of the amplified product into account. Quantitative PCR would be the best way to address a difference in gene expression levels between hypothalamus and pituitary. However, because the goal of this study was to identify useful markers for the HP, we instead analyzed the expression of some genes in adult brain by in situ hybridization.
Consistent with a differential expression level, some cDNAs (6 out of 13) amplified from both tissues were detected by in situ hybridization in only one of the tissues. kctd4, psd3l, natriuretic peptide precursor C - like and uo:ion 006 (from the hypothalamic library) expression was found only in hypothalamus and uo:ion002 and neuromedin s (from the pituitary library) was found only in the adenohypophysis (). Three genes (out of 13) were detected by in situ hybridization in both hypothalamus and adenohypophysis (proline-rich nuclear receptor coactivator 2, uo:ion 001 (not shown) and uo:ion 003, from the pituitary library; ). The other 4 genes analyzed (interleukin 1 receptor accessory protein-like 2 from hypothalamic library; adrenomedullin 2, serpini1 and uo:ion005 from the pituitary library) had no expression detectable by in situ hybridization.
Our subtractive library analysis from adult hypothalamus and pituitary identified genes expressed in the embryonic, larval, and adult HP and serves as the basis for a more extended analysis of HP development. Specifically, we will use these genes to analyze development, maturation, and function of the HP.