Cephalochordates, urochordates, and vertebrates evolved from a common ancestor over 520 million years ago. To improve our understanding of chordate evolution and the origin of vertebrates, we intensively searched for particular genes, gene families, and conserved noncoding elements in the sequenced genome of the cephalochordate Branchiostoma floridae, commonly called amphioxus or lancelets. Special attention was given to homeobox genes, opsin genes, genes involved in neural crest development, nuclear receptor genes, genes encoding components of the endocrine and immune systems, and conserved cis-regulatory enhancers. The amphioxus genome contains a basic set of chordate genes involved in development and cell signaling, including a fifteenth Hox gene. This set includes many genes that were co-opted in vertebrates for new roles in neural crest development and adaptive immunity. However, where amphioxus has a single gene, vertebrates often have two, three, or four paralogs derived from two whole-genome duplication events. In addition, several transcriptional enhancers are conserved between amphioxus and vertebrates-a very wide phylogenetic distance. In contrast, urochordate genomes have lost many genes, including a diversity of homeobox families and genes involved in steroid hormone function. The amphioxus genome also exhibits derived features, including duplications of opsins and genes proposed to function in innate immunity and endocrine systems. Our results indicate that the amphioxus genome is elemental to an understanding of the biology and evolution of nonchordate deuterostomes, invertebrate chordates, and vertebrates.

We investigated whether mutations in the gene encoding gonadotropin-releasing hormone 1 (GNRH1) might be responsible for idiopathic hypogonadotropic hypogonadism (IHH) in humans. We identified a homozygous GNRH1 frameshift mutation, an insertion of an adenine at nucleotide position 18 (c.18-19insA), in the sequence encoding the N-terminal region of the signal peptide-containing protein precursor of gonadotropin-releasing hormone (prepro-GnRH) in a teenage brother and sister, who had normosmic IHH. Their unaffected parents and a sibling who was tested were heterozygous. This mutation results in an aberrant peptide lacking the conserved GnRH decapeptide sequence, as shown by the absence of immunoreactive GnRH when expressed in vitro. This isolated autosomal recessive GnRH deficiency, reversed by pulsatile GnRH administration, shows the pivotal role of GnRH in human reproduction.

SUM M A R YGonadotropin-releasing hormone (GnRH) is the central regulator of gonadotropins, which stimulate gonadal function. Hypothalamic neurons that produce kisspeptin and neurokinin B stimulate GnRH release. Inactivating mutations in the genes encoding the human kisspeptin receptor (KISS1R, formerly called GPR54), neurokinin B (TAC3), and the neurokinin B receptor (TACR3) result in pubertal failure. However, human kisspeptin loss-of-function mutations have not been described, and contradictory findings have been reported in Kiss1-knockout mice. We describe an inactivating mutation in KISS1 in a large consanguineous family that results in failure of pubertal progression, indicating that functional kisspeptin is important for puberty and reproduction in humans. I t is still unknown how puberty in humans, occurring during the early years of the second decade of life, is initiated. 1 The hallmark of puberty is increased secretion of the gonadotropins, luteinizing hormone (LH) and folliclestimulating hormone (FSH), which act in concert to stimulate the gonads to drive sex-hormone secretion and gametogenesis. The production of gonadotropins from pituitary gonadotropic cells is controlled by the pulsatile delivery of GnRH. Inactivating mutations in the genes encoding GNRH1 2 or the GNRH receptor (GNRHR) 3 give rise to normosmic idiopathic hypogonadotropic hypogonadism in humans. 4 However, GnRH neurons lack sex-steroid receptors. This suggests the existence of GnRH-regulating neurons, which would mediate this effect.A major breakthrough in identifying such candidate neurons was the finding that inactivating mutations in genes encoding the human kisspeptin receptor (KISS1R, formerly called GPR54), the cognate receptor for a hypothalamic peptide, kisspeptin, resulted in pubertal failure. 4,5 More recently, mutations in TAC3 or TACR3 (encoding neurokinin B and its receptor, respectively) were shown to result in the same phenotype. 6 Kisspeptin and neurokinin B are coexpressed, along with dynorphin, in sex-hormone-responsive neurons in the arcuate nucleus (infundibular nucleus in primates), and their coordinated activity appears to regulate GnRH secretion. 7 Gene defects associated with normosmic idiopathic hypogonadotropic hypogonadism have been described in all the neuropeptides and receptors identified as stimulators of GnRH except for the kisspeptin gene (KISS1).Although Kiss1-and Kiss1r-knockout mouse models largely produce phenocopies (i.e., affected noncarriers) of human normosmic idiopathic hypogonadotropic hypogonadism resulting from inactivating mutations of KISS1R, there is evidence of remarkable residual activity of the hypothalamic-pituitary-gonadal axis.

Pulsatile gonadotropin-releasing hormone (GnRH) is crucial to normal reproductive function and abnormalities in pulse frequency give rise to reproductive dysfunction. Kisspeptin and neurokinin B (NKB), neuropeptides secreted by the same neuronal population in the ventral hypothalamus, have emerged recently as critical central regulators of GnRH and thus gonadotropin secretion. Patients with mutations resulting in loss of signaling by either of these neuroendocrine peptides fail to advance through puberty but the mechanisms mediating this remain unresolved. We report here that continuous kisspeptin infusion restores gonadotropin pulsatility in patients with loss-of-function mutations in NKB (TAC3) or its receptor (TAC3R), indicating that kisspeptin on its own is sufficient to stimulate pulsatile GnRH secretion. Moreover, our findings suggest that NKB action is proximal to kisspeptin in the reproductive neuroendocrine cascade regulating GnRH secretion, and may act as an autocrine modulator of kisspeptin secretion. The ability of continuous kisspeptin infusion to induce pulsatile gonadotropin secretion further indicates that GnRH neurons are able to set up pulsatile secretion in the absence of pulsatile exogenous kisspeptin.

GnRH is the key regulator of the reproductive axis in vertebrates, but little is known about GnRH before the origin of vertebrates. We have identified two genes encoding GnRH in a protochordate, Ciona intestinalis, thought to be related to the ancestral animal that gave rise to vertebrates. Each gene, Ci-gnrh1 and Ci-gnrh2, encodes in tandem three GnRH peptides, each of which is unique compared with known forms. Ci-gnrh1 encodes three peptides and contains no introns, whereas Ci-gnrh2 encodes three more peptides but has two introns. This is the first report in which more than one GnRH peptide is encoded on a single gene. The Ciona genes reveal consensus promoter elements that are conserved compared with human GNRH1. Both tunicate genes are expressed as mRNA early and throughout development, measured at the stages of four-cell, gastrulation, tail release, and tail resorption. In a closely related tunicate species, Ciona savignyi, we used in silico analysis to identify two similar genes encoding six peptides, only one of which is unique compared with C. intestinalis. Immunohistochemistry showed that at least one GnRH peptide was in the nerve net that surrounds the dorsal strand. Synthetic forms of the seven novel tunicate peptides induced release of gametes in adult tunicates. In contrast, the peptides did not activate the human GnRH-I receptor or cause release of LH in a rat pituitary cell assay. These data provide insight into the structural evolution of the GnRH peptides and their genes and show a functional role for GnRH in tunicate spawning.

SUM M A R YGonadotropin-releasing hormone (GnRH) is the central regulator of gonadotropins, which stimulate gonadal function. Hypothalamic neurons that produce kisspeptin and neurokinin B stimulate GnRH release. Inactivating mutations in the genes encoding the human kisspeptin receptor (KISS1R, formerly called GPR54), neurokinin B (TAC3), and the neurokinin B receptor (TACR3) result in pubertal failure. However, human kisspeptin loss-of-function mutations have not been described, and contradictory findings have been reported in Kiss1-knockout mice. We describe an inactivating mutation in KISS1 in a large consanguineous family that results in failure of pubertal progression, indicating that functional kisspeptin is important for puberty and reproduction in humans. I t is still unknown how puberty in humans, occurring during the early years of the second decade of life, is initiated. 1 The hallmark of puberty is increased secretion of the gonadotropins, luteinizing hormone (LH) and folliclestimulating hormone (FSH), which act in concert to stimulate the gonads to drive sex-hormone secretion and gametogenesis. The production of gonadotropins from pituitary gonadotropic cells is controlled by the pulsatile delivery of GnRH. Inactivating mutations in the genes encoding GNRH1 2 or the GNRH receptor (GNRHR) 3 give rise to normosmic idiopathic hypogonadotropic hypogonadism in humans. 4 However, GnRH neurons lack sex-steroid receptors. This suggests the existence of GnRH-regulating neurons, which would mediate this effect.A major breakthrough in identifying such candidate neurons was the finding that inactivating mutations in genes encoding the human kisspeptin receptor (KISS1R, formerly called GPR54), the cognate receptor for a hypothalamic peptide, kisspeptin, resulted in pubertal failure. 4,5 More recently, mutations in TAC3 or TACR3 (encoding neurokinin B and its receptor, respectively) were shown to result in the same phenotype. 6 Kisspeptin and neurokinin B are coexpressed, along with dynorphin, in sex-hormone-responsive neurons in the arcuate nucleus (infundibular nucleus in primates), and their coordinated activity appears to regulate GnRH secretion. 7 Gene defects associated with normosmic idiopathic hypogonadotropic hypogonadism have been described in all the neuropeptides and receptors identified as stimulators of GnRH except for the kisspeptin gene (KISS1).Although Kiss1-and Kiss1r-knockout mouse models largely produce phenocopies (i.e., affected noncarriers) of human normosmic idiopathic hypogonadotropic hypogonadism resulting from inactivating mutations of KISS1R, there is evidence of remarkable residual activity of the hypothalamic-pituitary-gonadal axis.

Hyperprolactinemia is the most common cause of hypogonadotropic anovulation and is one of the leading causes of infertility in women aged 25-34. Hyperprolactinemia has been proposed to block ovulation through inhibition of GnRH release. Kisspeptin neurons, which express prolactin receptors, were recently identified as major regulators of GnRH neurons. To mimic the human pathology of anovulation, we continuously infused female mice with prolactin. Our studies demonstrated that hyperprolactinemia in mice induced anovulation, reduced GnRH and gonadotropin secretion, and diminished kisspeptin expression. Kisspeptin administration restored gonadotropin secretion and ovarian cyclicity, suggesting that kisspeptin neurons play a major role in hyperprolactinemic anovulation. Our studies indicate that administration of kisspeptin may serve as an alternative therapeutic approach to restore the fertility of hyperprolactinemic women who are resistant or intolerant to dopamine agonists. IntroductionHyperprolactinemia is the most common cause of hypogonadotropic anovulation (WHO Group I) and represents a major etiology of infertility, with highest incidence in women aged 25-34 years (1). In men, hyperprolactinemia is also frequently associated with hypogonadotropic hypogonadism. This gonadotropic deficiency has been proposed to result from direct suppression of prolactin (PRL) on gonadotrophin-releasing hormone (GnRH) release, but evidence supporting this mechanism has never been provided. PRL is synthesized and secreted by the lactotrope cells of the pituitary, and high levels of circulating PRL are mainly caused by lactotroph adenomas, which account for approximately 40% of all pituitary tumors. Pulsatile GnRH replacement can reverse hypogonadotropic hypogonadism and infertility induced by hyperprolactinemia in women as well as men (2, 3), suggesting that PRL excess in humans affects hypothalamic release of GnRH rather than directly affecting pituitary or gonad function. However, very few GnRH neurons in mice express PRL receptors (PRLRs) (4), suggesting that PRL exerts its actions on upstream neurons regulating the GnRH neuron. Because GnRH neurons are stimulated by kisspeptin (Kp) neurons (5, 6), which unequivocally express PRLR (7), we hypothesized that GnRH deficiency resulting from hyperprolactinemia is caused by reduced Kp input, which is now considered to be a primary gatekeeper governing reproduction (8,9). Here, we show that hyperprolactinemia in mice induces hypogonadotropic anovulation and diminished Kp expression and that peripheral Kp administration restores GnRH and gonadotropin secretion and ovarian cyclicity. Therefore, we suggest that hyperprolactinemic women resistant or intolerant to dopamine agonists could take advantage of this therapeutic approach as a treatment for their infertility.

In vertebrates, GnRH binds to its receptor and stimulates predominantly G(q/11)-mediated signal transduction in gonadotropes. However, little is known about the GnRH receptor and its signaling pathway in tunicates, a group that arose before the vertebrates. Although tunicates have had duplications of a few genes in the last 600 million years, the early vertebrates had duplications of the full genome. Also unknown is the nature of GnRH signaling in the tunicate, which lacks both a pituitary gland and sex steroids. However, we know that tunicates have GnRH peptides because we previously reported six GnRH peptides encoded within the tunicate genome of Ciona intestinalis. Here we clone and sequence cDNAs for four putative GnRH receptors from C. intestinalis. These are the only invertebrate GnRH receptors found to date. Each Ciona GnRH receptor was expressed in COS-7 cells, incubated with each of the six C. intestinalis GnRHs and assayed for a signaling response. GnRH receptors 1, 2, and 3 responded to Ciona GnRH peptides to stimulate intracellular cAMP accumulation. In contrast, only GnRH receptor 1 activated inositol phosphate turnover in response to one of the Ciona GnRHs. The green monkey type II GnRH receptor cDNA was tested as a comparison and a positive control. In conclusion, the four GnRH receptors encoded within the C. intestinalis genome were all transcribed into messenger RNA, but only three of the Ciona GnRH receptors were biologically active in our assays. The Ciona GnRH receptors almost exclusively activated the cAMP pathway.