Retinal growth hormone in perinatal and adult rats

Corneal parameters, ocular biometers, and retinal and choroidal thickness in acromegaly patients

BACKGROUND

To compare ocular findings of acromegaly patients with healthy individuals and investigate the relation of serum levels of insulin-like growth factor (IGF-1) along with growth hormone (GH) and pituitary tumor (adenoma) dimensions (TD) with specific ocular parameters.

METHODS

The ocular parameters of acromegaly patients (n = 38) were compared with those of healthy subjects (n = 36). These parameters were intraocular pressure, keratometric (K1-K2) values, central corneal thickness (CCT), total axial length along with anterior chamber-lens-vitreous length, retinal nerve fiber layer (RNFL) thickness, central foveal thickness (CFT), choroidal thickness (CT), ganglion cell layer thickness (GCLT), and inner plexiform layer thickness (IPLT). Also investigated was whether there was a correlation between disease duration, TD, GH, IGF-I, CCT, RNFL, CFT, GCLT, IPLT, and CT.

RESULTS

The lens length of the acromegaly group was increased (p = 0.014). GH and IGF-1 levels were positively correlated with CT and CCT, respectively (p = 0.041, r = 0.343) (p = 0.03, r = 0.347). Analysis of TD also found a highly negative correlation with the mean RNFL thickness of the acromegaly patients (p < 0.01, r = -0.603). The mean value of the inner parts of GCLT and IPLT was negatively correlated with TD (p = 0.041, r = -0.343 and p = 0.025, r = -0.379, respectively).

CONCLUSION

Serum IGF-1 and GH levels might be determinant factors in CCT and CT, respectively. The pituitary adenoma size increasing may be prone to lead RNFL, ganglion cell layer, inner plexiform layer thinning. Increased lens thickness was found in the acromegaly group.

Growth hormone (GH) and synaptogenesis

Growth hormone (GH) is known to exert several roles during development and function of the nervous system. Initially, GH was exclusively considered a pituitary hormone that regulates body growth and metabolism, but now its alternative extrapituitary production and pleiotropic functions are widely accepted. Through excess and deficit models, the critical role of GH in nervous system development and adult brain function has been extensively demonstrated. Moreover, neurotrophic actions of GH in neural tissues include pro-survival effects, neuroprotection, axonal growth, synaptogenesis, neurogenesis and neuroregeneration. The positive effects of GH upon memory, behavior, mood, sensorimotor function and quality of life, clearly implicate a beneficial action in synaptic physiology. Experimental and clinical evidence about GH actions in synaptic function modulation, protection and restoration are revised in this chapter.

Expression of growth hormone and growth hormone receptor genes in human eye tissues

In humans, the polygenic growth hormone (GH) locus is located on chromosome 17 and contributes with three types of proteins: pituitary GH which consists of at least two isoforms one of 22 kDa and the other of 20 kDa, placental GH, which also exhibits isoforms, and chorionic somatomammotropin hormone (CSH). While pituitary GH results from the expression of the GH-1 (GH-N) gene, placental GH is produced by the expression of the GH-2 (GH-V) gene and CSH is contributed by expression of the CSH-1 and CSH-2 genes. The location where GH-1 is expressed is the anterior pituitary and the rest of the genes in the locus are expressed in placenta. On the other hand, expression and synthesis of GH in extra-pituitary tissues, including the eye, has been recently described. However, the physiological role of GH in the eye has not yet been elucidated, although a possible neuroprotective role has been hypothesized. Thus, we analyzed GH-1, GH-2, CSH1/2, Pit-1, GHR, GHRH, GHRHR, SST, SSTR1, SSTR2, SSTR3, SSTR4, and SSTR5 to elucidate the expression and regulation of the GH locus in the human eye. Through qPCR analysis, we only found evidence of GH-1 expression in retina, choroid and trabecular meshwork; its transcript turned out to be the same as pituitary GH mRNA found in major species, and no splicing variants were detected. PIT1 was absent in all the ocular tissues implying an independent GH-1 expression mechanism. We found evidence of GHR in the cornea, choroid coat and retina. These results suggest autocrine and/or paracrine regulation, possibly exerted by GHRH and SSTs (since their mRNAs and receptors were found predominantly in retinal, choroidal and corneal tissues) since expression of both molecules was detected in different ocular tissues, as well as in the same tissues where GH-1 expression was confirmed. Our results add solid evidence about the existence of a regulatory local system for GH expression and release in the human eye.

Expression of growth hormone gene in the baboon eye

The human growth hormone (GH) locus is comprised by two GH (GH1 and GH2) genes and three chorionic somatomammotropin (CSH1, CSH2 and CSH-L) genes. While GH1 is expressed in the pituitary gland, the rest are expressed in the placenta. However, GH1 is also expressed in several extrapituitary tissues, including the eye. So to understand the role of this hormone in the eye we used the baboon (Papio hamadryas), that like humans has a multigenic GH locus; we set up to investigate the expression and regulation of GH locus in adult and fetal baboon ocular tissues. We searched in baboon ocular tissues the expression of GH1, GH2, CSH1/2, Pit1 (pituitary transcription factor 1), GHR (growth hormone receptor), GHRH (growth hormone releasing hormone), GHRHR (growth hormone releasing hormone receptor), SST (somatostatin), SSTR1 (somatostatin receptor 1), SSTR2 (somatostatin receptor 2), SSTR3 (somatostatin receptor 3), SSTR4 (somatostatin receptor 4), and SSTR5 (somatostatin receptor 5) mRNA transcripts and derived proteins, by qPCR and immunofluorescence assays, respectively. The transcripts found were characterized by cDNA cloning and sequencing, having found only the one belonging to GH1 gene, mainly in the retina/choroid tissues. Through immunofluorescence assays the presence of GH1 and GHR proteins was confirmed in several retinal cell layers. Among the possible neuroendocrine regulators that may control local GH1 expression are GHRH and SST, since their mRNAs and proteins were found mainly in the retina/choroid tissues, as well as their corresponding receptors (GHRH and SSTR1-SSTR5). None of the ocular tissues express Pit1, so gene expression of GH1 in baboon eye could be independent of Pit1. We conclude that to understand the regulation of GH in the human eye, the baboon offers a very good experimental model.

Growth Hormone (GH) and Gonadotropin-Releasing Hormone (GnRH) in the Central Nervous System: A Potential Neurological Combinatory Therapy?

This brief review of the neurological effects of growth hormone (GH) and gonadotropin-releasing hormone (GnRH) in the brain, particularly in the cerebral cortex, hypothalamus, hippocampus, cerebellum, spinal cord, neural retina, and brain tumors, summarizes recent information about their therapeutic potential as treatments for different neuropathologies and neurodegenerative processes. The effect of GH and GnRH (by independent administration) has been associated with beneficial impacts in patients with brain trauma and spinal cord injuries. Both GH and GnRH have demonstrated potent neurotrophic, neuroprotective, and neuroregenerative action. Positive behavioral and cognitive effects are also associated with GH and GnRH administration. Increasing evidence suggests the possibility of a multifactorial therapy that includes both GH and GnRH.

Growth hormone reverses excitotoxic damage induced by kainic acid in the green iguana neuroretina

It is known that growth hormone (GH) is expressed in extrapituitary tissues, including the nervous system and ocular tissues, where it is involved in autocrine/paracrine actions related to cell survival and anti-apoptosis in several vertebrates. Little is known, however, in reptiles, so we analyzed the expression and distribution of GH in the eye of green iguana and its potential neuroprotective role in retinas that were damaged by the intraocular administration of kainic acid (KA). It was found, by Western blotting, that GH-immunoreactivity (GH-IR) was expressed as two isoforms (15 and 26kDa, under reducing conditions) in cornea, vitreous, retina, crystalline, iris and sclera, in varying proportions. Also, two bands for the growth hormone receptor (GHR)-IR were observed (70 and 44kDa, respectively) in the same tissues. By immunofluorescence, GH-IR was found in neurons present in several layers of the neuroretina (inner nuclear [INL], outer nuclear [ONL] and ganglion cell [GCL] layers) as determined by its co-existence with NeuN, but not in glial cells. In addition, GH and GHR co-expression was found in the same cells, suggesting paracrine/autocrine interactions. KA administration induced retinal excitotoxic damage, as determined by a significant reduction of the cell density and an increase in the appearance of apoptotic cells in the INL and GCL. In response to KA injury, both endogenous GH and Insulin-like Growth Factor I (IGF-I) expression were increased by 70±1.8% and 33.3±16%, respectively. The addition of exogenous GH significantly prevented the retinal damage produced by the loss of cytoarchitecture and cell density in the GCL (from 4.9±0.79 in the control, to 1.45±0.2 with KA, to 6.35±0.49cell/mm(2) with KA+GH) and in the INL (19.12±1.6, 10.05±1.9, 21.0±0.8cell/mm(2), respectively) generated by the long-term effect of 1mM KA intraocular administration. The co-incubation with a specific anti-GH antibody, however, blocked the protective effect of GH in GCL (1.4±0.23cell/mm(2)) and INL (11.35±1.06), respectively. Furthermore, added GH induced an increase of 90±14% in the retinal IGF-I concentration and the anti-GH antibody also blocked this effect. These results indicate that GH and GHR are expressed in the iguana eye and may be able to exert, either directly of mediated by IGF-I, a protective mechanism in neuroretinas that suffered damage by the administration of kainic acid.

Growth hormone in the eye: A comparative update

Comparative studies have previously established that the eye is an extrapituitary site of growth hormone (GH) production and action in fish, amphibia, birds and mammals. In this review more recent literature and original data in this field are considered.

Comparing acromegalic patients to healthy controls with respect to intraocular pressure, central corneal thickness, and optic disc topography findings

Aims:

The aim was to compare the intraocular pressure (IOP), central corneal thickness (CCT), and optic disc topography findings of biochemically controlled acromegalic patients and the control group and to evaluate the effect of the duration of acromegaly and serum growth hormone and insulin-like growth factor-1 (IGF-1) levels on these ocular parameters.

Materials and Methods:

IOP measurement with Goldmann applanation tonometry, CCT measurement with ultrasonic pachymetry, and topographic analysis with Heidelberg retinal tomograph III were performed on 35 biochemically controlled acromegalic patients and 36 age- and gender-matched controls.

Results:

Mean IOP and CCT were 14.7 ± 2.9 mmHg and 559.5 ± 44.9 μm in the acromegaly patients and 13.0 ± 1.6 mmHg and 547.1 ± 26.7 μm in controls (P = 0.006 and P = 0.15, respectively). A significant moderate correlation was found between the duration of acromegaly and CCT (r = 0.391) and IOP (r = 0.367). Mean retinal nerve fiber layer (RNFL) thickness was significantly lower in the acromegalic patients (0.25 ± 0.05 mm) as compared to controls (0.31 ± 0.09 mm) (P = 0.01). A significant moderate correlation was detected between IGF-1 level and disc area (r = 0.362), cup area (r = 0.389) and cup volume (r = 0.491).

Conclusion:

Biochemically controlled acromegalic patients showed significantly higher CCT and IOP levels and lower RNFL thickness compared to healthy controls and the duration of disease was correlated with CCT and IOP levels.

Corneal thickness in children with growth hormone deficiency: the effect of GH treatment

OBJECTIVE

The eye represents a target site for GH action, although few data are available in patients with GH deficiency (GHD). Our aim was to evaluate central corneal thickness (CCT) and intraocular pressure (IOP) values in GHD children to assess the role played by GHD or GH treatment on these parameters.

DESIGN

In 74 prepubertal GHD children (51M, 23F, aged 10.4±2.4years) we measured CCT and IOP before and after 12months of treatment. A baseline evaluation was also made in 50 healthy children matched for age, gender and body mass index. The study outcome considered CCT and IOP during treatment and their correlations with biochemical and auxological data.

RESULTS

No difference in CCT and IOP between GHD children at baseline and controls was found (all p>0.005). GHD children after 12months of therapy showed greater CCT (564.7±13.1μm) than both baseline values (535.7±17μm; p<0.001) and control subjects (536.2±12.5μm; p<0.001), with a concomitantly higher corrected mean IOP (15.6±0.7mmHg; p<0.001) than both baseline (12.5±0.8mmHg; p<0.001) and controls (12.3±0.5mmHg; p<0.001), without correlation with auxological and biochemical parameters.

CONCLUSIONS

12months of GH treatment in children with GHD, regardless of auxological and biochemical data, affect CCT and IOP. Our findings suggest careful ocular evaluation in these patients to prevent undesirable side effects during the follow-up.

Release of retinal growth hormone in the chick embryo: local regulation?

The neural retina is an extrapituitary site of growth hormone (GH) production and an autocrine or paracrine site of retinal GH action. Retinal GH is released from retinal tissue and may be secreted into the vitreous. Ontogenetic changes in the abundance of retinal GH during embryogenesis indicate that the amount of GH released may be regulated. The presence of pituitary GH secretagogues (GH-releasing hormone, GHRH; thyrotropin-releasing hormone, TRH; and ghrelin) and pituitary GH inhibitors (somatostatin, SRIF and insulin-like growth factor, IGF-1) within the neural retina may indicate the involvement of these factors in retinal GH release. This possibility is supported by the finding that GHRH is colocalized with GH in chick retinal ganglion cells (RGCs) and in immortalized cells (QNRD) derived from quail neuroretinal cells and by the induction of GH mRNA in incubated QNRD cells. In summary, these results provide evidence for the autocrine or paracrine regulation of retinal GH release in the ganglion cells of the embryonic chick retina.

Selective inner retinal dysfunction in growth hormone transgenic mice

OBJECTIVE

The discovery of locally produced growth hormone (GH) and its receptor in the retina of rodents raises the possibility that GH might modulate retinal function. To test this hypothesis, we determined the retinal electroretinogram (ERG) of bovine GH (bGH) transgenic mice.

DESIGN

ERGs were recorded from 11 wild type (WT) and 9 bGH mice, at 2 months of age in response to a series of light flashes at increasing intensity. Three ERG components were assessed for their amplitude and timing: a-wave, b-wave and oscillatory potentials (OPs). OPs were isolated with a 75-300 Hz digital filter. Retina layer sizes, nuclei number and vascularization were assessed by respectively staining cross sections with DAPI and Bandeiraea simplicifolia.

RESULTS

OPs were selectively affected in the bGH mouse compared to WT. When OP amplitude values were normalized to the a-wave amplitude (to account for inter-animal variability in WT and bGH groups), OP2, OP3, and OP4 showed amplitude reductions (of 65%, 72%, and 68%, respectively) in the bGH mouse compared to the WT. This was accompanied by a prolongation of the implicit time for the peak of OP3 (28.1 vs 31.1 ms, WT vs bGH) and OP4 (37.8 vs 41.6 ms), while the implicit time of a- and b-waves were unaffected. Fast Fourier transform analysis revealed that the OPs' dominant frequency was significantly reduced (P<0.05) in the bGH mice (100 Hz) compared to WT (108Hz). There was no significant change in retinal histology except for a significant increase in the axial length of the eye in bGH mice.

CONCLUSIONS

Mice expressing bGH display a selective inner retinal defect as demonstrated using ERG recordings. The specific OP defect observed in these mice is similar to the ERG results obtained in patients with diabetic retinopathy and in related animal models.

Growth hormone promotes the survival of retinal cells in vivo

Growth hormone (GH) is synthesized and present in the developing chick retina, where it may have local actions in retinal cell differentiation similar to those of conventional growth factors. We have previously shown that retinal GH has neuroprotective effects in retinal ganglion cells. In this paper, we extend our earlier functional studies by examining the in vivo effects of a GH siRNA (NR-cGH-1) after microinjection into the eye cup of the developing chick embryo in ovo. We show that intra-vitreous cGH siRNA lowers both GH mRNA and insulin-like growth factor-1 (IGF-1) mRNA levels in the retina in vivo, and concomitantly elevates the numbers of apoptotic cells in the retina. These effects are apparent 6h after treatment, and persist for at least 24h. The apoptotic cells induced by GH withdrawal were primarily located close to the optic fissure of the developing eye, and were distributed in clusters, suggesting that there are sub-populations of retinal cells that are particularly susceptible to apoptotic stimuli. These results support our view that a GH/IGF-1 axis in retinal cells regulates retinal cell survival in vivo.

Cornea in acromegalic patients as a possible target of growth hormone action

BACKGROUND

GH exerts its effects on many organs and the eye also seems to be a target site, although few authors have investigated the corneal thickness in patients with acromegaly.

AIM

To perform a detailed ophthalmological evaluation in acromegalic patients, in relation to disease activity.

MATERIAL AND METHODS

Twenty-eight acromegalic patients (11 males, 17 females) and 22 voluntary healthy subjects underwent complete metabolic and ophthalmological evaluation, including retinal thickness (RT), central corneal thickness (CCT), and intraocular pressure values (IOP).

RESULTS

Significantly greater CCT values were found in all acromegalic patients in comparison with controls (567 vs 528.5 μm; p<0.001), without concomitant greater corrected IOP. No difference was found for RT. Analyzing these data according to disease activity, uncontrolled patients showed greater CCT values (573.5 vs 559 μm; p=0.002) and corrected IOP (17.4 vs 16 mmHg; p=0.001) than the controlled ones. CCT also correlated with basal and nadir GH after oral glucose load levels, IGF-I levels, and duration of active disease.

CONCLUSIONS

Acromegaly is characterized by greater CCT values, supporting the hypothesis that GH excess may have stimulatory effects on the cornea as well as on other target organs. Higher GH levels, disease control status and duration of active disease seem to be the main causes of increased corneal thickness. We suggest a careful and detailed corneal evaluation in acromegalic patients to prevent the potential risk of increased IOP, in addition to the already-known complications.

Extrapituitary growth hormone

Pituitary somatotrophs secrete growth hormone (GH) into the bloodstream, to act as a hormone at receptor sites in most, if not all, tissues. These endocrine actions of circulating GH are abolished after pituitary ablation or hypophysectomy, indicating its pituitary source. GH gene expression is, however, not confined to the pituitary gland, as it occurs in neural, immune, reproductive, alimentary, and respiratory tissues and in the integumentary, muscular, skeletal, and cardiovascular systems, in which GH may act locally rather than as an endocrine. These actions are likely to be involved in the proliferation and differentiation of cells and tissues prior to the ontogeny of the pituitary gland. They are also likely to complement the endocrine actions of GH and are likely to maintain them after pituitary senescence and the somatopause. Autocrine or paracrine actions of GH are, however, sometimes mediated through different signaling mechanisms to those mediating its endocrine actions and these may promote oncogenesis. Extrapituitary GH may thus be of physiological and pathophysiological significance.

Growth hormone expression and neuroprotective activity in a quail neural retina cell line

We have previously shown that growth hormone (GH) is produced within cells of the chick embryo retina where it appears to act as an autocrine/paracrine anti-apoptotic factor in the regulation of programmed cell death during retinal development. These investigations were carried out on cultured chick embryo retinal ganglion cells (RGCs) as well as on the chick embryo retina in ovo, using GH protein knock-down by immunoneutralization. We have now investigated the putative neuroprotective actions of GH using a quail embryo neural retina cell line (QNR/D) treated with GH siRNA to silence the local synthesis of GH. We now show that knock-down of GH by gene silencing in cells of this cultured embryonic neural retina cell line, using NR-cGH-1 siRNA, correlates with the increased appearance in the cultures of cells with apoptotic nuclear morphology. This result is consistent with our previous results using protein knock-down by immunoneutralization. We thus validate, using different technology and a different culture system, our contention that GH, produced locally by cells of the neural retina acts in an autocrine or paracrine manner to regulate cell survival in the retina.

Signaling mechanisms mediating local GH action in the neural retina of the chick embryo

Growth hormone (GH) is found in the retina and vitreous of the chick embryo, where it appears to act as a growth and differentiation factor, having neuroprotective effects on retinal ganglion cells (RGCs). Here, we review the molecular mechanisms of the anti-apoptotic effect of GH in chick RGCs. GH treatment of RGCs reduces Akt levels, while raising Akt-phos levels, consistent with a role for Akt signaling pathways in the GH neuroprotective action. The induction of apoptosis by immunoneutralization with GH antiserum is accompanied by an increase in caspase-3 and caspase-9 activation, and also PARP-1 cleavage. Calpain activation also appears to be a major caspase-independent pathway to PARP-1 cleavage and apoptosis in these cells, supporting the view that caspase and calpain inhibitors are major neuroprotective agents for RGCs, and that pathways that activate both caspases and calpains are important for the anti-apoptotic actions of GH in these cells. These pathways involve the activation of cytosolic tyrosine kinases (Trks) and extracellular-signal-related kinases (ERKs). Occupation of the GH receptor by GH involves downstream intracellular Trk pathways. The Akt and Trk pathways appear to converge on the activation of cAMP response element binding protein (CREB), which is able to initiate transcription of pro- or anti-apoptotic genes. These results indicate that the action of GH in the neuroprotection of embryonic RGCs involves pathways common to with other neurotrophins, and that GH can be considered to be a growth and differentiation factor in the development of the embryonic retina. We have also investigated the relationship between the overlapping anti-apoptotic effects of GH and insulin-like growth factor-1 (IGF-1), two functionally closely related factors. We find that simultaneous immunoneutralization of GH and IGF-1 does not increase the level of apoptosis in the cultures above that achieved by immunoneutralization of GH alone. We therefore conclude that the neuroprotective actions of GH in the developing retina are likely mediated in large part through the action of IGF-1.

Growth hormone promotes axon growth in the developing nervous system

Postnatally, endocrine GH is primarily produced by pituitary somatotrophs. GH is, however, also produced in extrapituitary sites, including tissues of the developing nervous system such as the neural retina. Whereas GH roles in the nervous system are starting to emerge, they are still largely unknown. We show here that GH in the neural retina is mainly present in the axons of retinal ganglion cells (RGCs) in embryonic day (E) 4-12 chick embryos, but it is no longer present at E14-18. This temporal window corresponds to the period of RGC axon growth. GH receptor mRNA was also detected within cells of the E7 RGC layer and GH receptor protein colocalized with GH in RGC axons. The possibility that GH promotes axon growth was thus investigated. Exogenous GH induced a significant increase in axon elongation at 10(-9) and 10(-6) M in E7 RGC culture purified by immunopanning. RNA interference-mediated gene silencing was used to examine whether endogenous GH similarly alters axon outgrowth. The ability of GH small-interfering RNA to knock down GH was first tested using HEK cells on a LacZ-cGH expression plasmid and found to reach 90%. Upon transfection of GH small-interfering RNA to immunopanned RGC culture, a 63% knockdown of endogenous GH was detected and RGC axon length was found to be reduced by 40%. Taken together, these data suggest that GH acts as an autocrine or paracrine signaling molecule to promote axon growth in a developing nervous tissue, the neural retina of chick embryos.

Peptide hormones as developmental growth and differentiation factors

Peptide hormones, usually considered to be endocrine factors responsible for communication between tissues remotely located from each other, are increasingly being found to be synthesized in developing tissues, where they act locally. Several hormones are now known to be produced in developing tissues that are unrelated to the endocrine gland of origin in the adult. These hormones are synthesized locally, and are active as differentiation and survival factors, before the developing adult endocrine tissue becomes functional. There is increasing evidence for paracrine and/or autocrine actions for these factors during development, thus, placing them among the conventional growth and differentiation factors. We review the evidence for the view that thyroid hormones, growth hormone, prolactin, insulin, and parathyroid hormone-related protein are developmental growth and differentiation factors.

Growth hormone in the visual system: comparative endocrinology

Growth hormone (GH) is rarely considered to be involved in ocular development or vision or to be present in the visual system. Basic and clinical studies nevertheless support roles for GH in the ocular function of most vertebrate groups and for its extrapituitary production in ocular tissues. The comparative endocrinology of endocrine, autocrine or paracrine GH in the visual system of vertebrates is the focus of this brief review.

Growth hormone and its receptor in projection neurons of the chick visual system: retinofugal and tectobulbar tracts

Recent studies have shown the presence of growth hormone (GH) in the retinal ganglion cells (RGCs) of the neural retina in chick embryos at the end of the first trimester [embryonic day (E) 7] of the 21 day incubation period. In this study the presence of GH in fascicles of the optic fiber layer (OFL), formed by axons derived from the underlying RGCs, is shown. Immunoreactivity for GH is also traced through the optic nerve head, at the back of the eye, into the optic nerve, through the optic chiasm, into the optic tract and into the stratum opticum and the retinorecipient layer of the optic tectum, where the RGC axons synapse. The presence of GH immunoreactivity in the tectum occurs prior to synaptogenesis with RGC axons and thus reflects the local expression of the GH gene, especially as GH mRNA is also distributed within this tissue. The distribution of GH-immunoreactivity in the visual system of the E7 embryo is consistent with the distribution of the GH receptor (GHR), which is also expressed in the neural retina and tectum. The presence of a GH-responsive gene (GHRG-1) in these tissues also suggests that the visual system is not just a site of GH production but a site of GH action. These results support the possibility that GH acts as a local growth factor during early embryonic development of the visual system.

Expression, translation, and localization of a novel, small growth hormone variant

A novel transcript of the GH gene has been identified in ocular tissues of chick embryos. It is, however, unknown whether this transcript (small chicken GH, scGH) is translated. This possibility was therefore assessed. The expression of scGH mRNA was confirmed by RT-PCR, using primers that amplified a 426-bp cDNA of its coding sequence. This cDNA was inserted into an expression plasmid to transfect HEK 293 cells, and its translation was shown by specific scGH immunoreactivity in extracts of these cells. This immunoreactivity was directed against the unique N terminus of scGH and was associated with a protein of 16 kDa, comparable with its predicted size. Most of the immunoreactivity detected was, however, associated with a 31-kDa moiety, suggesting scGH is normally dimerized. Neither protein was, however, present in media of the transfected HEK cells, consistent with scGH's lack of a signal sequence. Similar moieties of 16 and 31 kDa were also found in proteins extracted from ocular tissues (neural retina, pigmented epithelium, lens, cornea, choroid) of embryos, although they were not consistently present in vitreous humor. Specific scGH immunoreactivity was also detected in these tissues by immunocytochemistry but not in axons in the optic fiber layer or the optic nerve head, which were immunoreactive for full-length GH. In summary, we have established that scGH expression and translation occurs in ocular tissues of chick embryos, in which its localization in the neural retina and the optic nerve head is distinct from that of the full-length protein.

Retinal ganglion cell survival in development: mechanisms of retinal growth hormone action

Several variants of growth hormone (GH) are found in the retina and vitreous of the chick embryo, where they appear to act as cell survival factors, having neuroprotective effects on retinal ganglion cells (RGCs). Here, we investigate the molecular mechanisms of the anti-apoptotic effect of GH in cultured RGCs. GH treatment increased Akt phosphorylation in these cells, which is an anti-apoptotic event. Whereas unphosphorylated Akt was detected in both nucleus and cytoplasm of RGCs by immunocytochemistry, the phosphorylated form of Akt (Akt-phos) was located primarily in the cytoplasm of both normal and apoptotic cells, although levels were markedly lower in the latter. It was found that GH treatment of RGCs reduced Akt levels, while concomitantly raising Akt-phos levels, consistent with a role for Akt signaling pathways in GH neuroprotective action. This was substantiated using Wortmannin, which, like GH antiserum, inhibited Akt phosphorylation and initiated apoptosis. The addition of Wortmannin to RGC cultures simultaneously with GH significantly reduced the anti-apoptotic effect of GH. The induction of apoptosis by GH antiserum was clearly accompanied by an increase in caspase-3 activation and PARP-1 cleavage, both of which were significantly reduced in the presence of the broad spectrum caspase inhibitor, Q-VD-OPh, which itself had a dramatic neuroprotective effect on cultured RGCs. Calpain activation appeared to be a major caspase-independent pathway to PARP-1 cleavage and apoptosis in these cells. Calpain inhibitor III (MDL 28170) was able to reduce PARP-1 cleavage and abrogate the apoptogenic effect of GH antiserum. The results support the view that caspase and calpain inhibitors are major neuroprotective agents for RGCs, and that pathways that activate both caspases and calpains are important for the anti-apoptotic actions of GH in these cells.