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Endocrinology Vol. 140, No. 12 5698-5704
Copyright © 1999 by The Endocrine Society

ARTICLES

Alterations in the Growth Hormone/Insulin-Like Growth Factor I Pathways in Feline GM1 Gangliosidosis1

Nancy R. Cox, Nancy E. Morrison, James L. Sartin, Frances C. Buonomo, Barbara Steele and Henry J. Baker

Scott-Ritchey Research Center (N.R.C., N.E.M., H.J.B.) and the Department of Anatomy, Physiology and Pharmacology (J.L.S., B.S.), Auburn University College of Veterinary Medicine, Auburn, Alabama 36849; and Protiva-A Unit, Monsanto Co., St. Louis, Missouri 63017

Address all correspondence and requests for reprints to: Nancy R. Cox, D.V.M., Ph.D., Scott-Ritchey Research Center, Auburn University College of Veterinary Medicine, Auburn, Alabama 36849. E-mail address: coxnanc{at}vetmed.auburn.edu


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Cats affected with feline GM1 gangliosidosis, an autosomal, recessively inherited, lysosomal enzymopathy, have progressive neurological dysfunction, premature thymic involution, stunted growth, and premature death. Although increased membrane GM1 gangliosides can result in increased apoptosis of thymocytes, there is not a direct correlation between thymocyte surface GM1 and thymic apoptosis in vivo, suggesting that other factors may be important to the pathogenesis of thymic involution in affected cats. Because GH and insulin-like growth factor I (IGF-I) are important hormonal peptides supporting thymic function and affecting growth throughout the body, particularly in the prepubescent period, several components of the GH/IGF-I pathway were compared in GM1 mutant and normal age-matched cats. GM1 mutant cat serum IGF-I concentrations were reduced significantly compared with those in normal cats by 150 days of age, and GM1 mutant cats had no peripuberal increase in serum IGF-I. Additionally, IGF-binding protein-3 was reduced, and IGF-binding protein-2 was elevated significantly in GM1 mutant cats more than 200 days of age. Liver IGF-I messenger RNA and pituitary GH messenger RNA both were reduced significantly in GM1 mutant cats. After stimulation by exogenous recombinant canine GH, serum IGF-I levels increased significantly in GM1 mutant cats, indicating that GH/IGF-I signaling pathways within the liver remain intact and suggesting that alterations are external to the liver.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
GM1 GANGLIOSIDOSIS is an autosomal, recessively inherited defect of the lysosomal hydrolase ß-galactosidase, resulting in blocked catabolism of GM1 gangliosides as well as other glycolipids and glycoproteins with the same terminal sugar moiety. Individuals affected with this metabolic disease have progressive neurological dysfunction and die prematurely (1). Major clinical signs are associated with the nervous system dysfunction, but the cellular pathogenesis of the disease, even within the nervous system, remains unclear. All cells in affected individuals carry the genetic defect, and individuals are affected clinically to various degrees (1).

Feline GM1 gangliosidosis was first described in 1971 as a model for human type 2 GM1 gangliosidosis (2) and has been used to study morphological, pharmacological, genetic, and biochemical alterations in the nervous system and to evaluate possible therapies for this progressive and fatal disease (1, 3, 4, 5, 6). In addition to neurological abnormalities, GM1 mutant cats have been shown to have significantly reduced body mass and abrupt premature thymic involution by 7 months of age when neurological signs become clinically severe (7). Testes of pubescent male GM1 mutant cats have normal appearing spermatogonia but few spermatozoa (1, 4). Further study of the thymic involution indicated that there was a reduction of all thymocyte subpopulations (most notably in the CD4+CD8+ subpopulations). Although in vitro addition of exogenous GM1 gangliosides increases apoptotic cell death in thymocytes from very young cats (8), there is not a direct correlation between the amount of thymocyte surface GM1 gangliosides and the occurrence of thymic apoptosis in vivo (7), suggesting that the process of thymic involution in these cats is more complex than originally hypothesized and that other factors that could contribute to thymic involution should be considered.

Rapid thymic growth followed by progressive involution and general body growth with muscle, bone, and reproductive tract development are all controlled by complex pathways of the hypothalamic-pituitary-endocrine-cellular axes and involve a variety of hormones (9, 10, 11, 12, 13, 14, 15, 16). The studies reported here focused on insulin-like growth factor I (IGF-I) and GH because these interrelated peptide hormones play critical roles in the growth and development of multiple systems, particularly in early postnatal to pubescent stages (12, 17, 18, 19, 20, 21, 22, 23, 24). Additionally, recent intriguing studies show that IGF-I modulates pathways involved in neuronal plasticity and neurodegeneration and suggest that IGF-I may be an important neuroprotective agent in a variety of pathological conditions (reviewed in Refs. 25, 26). A hypothalamic/anterior pituitary/liver pathway has been well established as the primary positive and negative feedback circuit controlling the production of GH and IGF-I (27, 28, 29, 30). Others have shown that alterations to hypothalamic neurons due to infection or other diseases can result in growth abnormalities (31, 32). As initial steps to understanding the relationship of GM1 gangliosidosis to the neuroendocine/immune axis, these studies focused on the alterations to several components of the GH/IGF-I pathways.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Experimental animals
All cats were derived from established research colonies of domestic cats with feline GM1 gangliosidosis due to deficiency in activity of lysosomal ß-galactosidase (3). This closed breeding colony has been in existence for over 20 yr, and cats are periodically screened and shown to be free of antibody titers for virulent cat viral pathogens, including feline immunodeficiency virus and feline leukemia virus, which might alter immune function. Artificial light is maintained on a 14-h light, 10-h dark cycle throughout the year to facilitate estrous cycling. Although there is some individual variation, cats affected with GM1 gangliosidosis generally begin showing mild clinical signs of the disease (discrete head or limb tremors and slight dysmetria) at 3–4 months of age, are moderately affected (moderate dysmetric ataxia with loss of balance and spasticity) by 5–6 months, and are severely affected (severe spastic tetraparesis to tetraparalysis, often with blindness and onset of seizures) after 7 months. Cats are seldom maintained after 11 months of age for humane reasons. Because GM1 gangliosidosis is an autosomal, recessively inherited disease, very limited numbers of affected cats are available for study at any one point in time; therefore, studies of each GM1 mutant cat are compared with those of an age-matched (usually a littermate) control cat. Where possible, control cats of the same sex as the GM1 mutant cats were used. Animal housing and experimental protocols were approved by the institutional animal care and use committee, Auburn University.

Serum and tissue collection
Blood samples were collected from GM1 mutant and age-matched normal cats tranquilized with ketamine HCl (Ft. Dodge Animal Health, Ft. Dodge, IA). Sera were collected and stored frozen at -80 C. Liver and pituitary tissue samples were collected at necropsy, flash-frozen in liquid nitrogen, and stored until use at -80 C.

Sample preparation and IGF-I determinations
Ten microliters of sample serum were mixed with 400 µl 1 M glycine buffer (pH 3.2) in a polypropylene tube, and 500 µl assay buffer (35 mM sodium phosphate monobasic, 13 mM EDTA, 0.2 g/liter protamine sulfate, and 0.5 ml Tween-20, pH 7.5) were added (final pH 3.5). The samples were capped, vortexed, and incubated at 37 C for 48 h. After incubation, 90 µl 0.5 N NaOH were added. Tubes were recapped, vortexed, and frozen at -80 C. Samples were coded randomly and submitted to Dr. F. Buonomo (Monsanto Co., St. Louis, MO) for analysis of serum IGF-I by a heterologous double antibody RIA using recombinant bovine IGF-I as the standard (33). Serial dilutions of extracted normal feline serum were observed to be parallel to the bovine IGF-I standard curve, as determined by linear regression analysis. Addition of bovine IGF-I standard to extracted feline sera resulted in the recovery of 89.7% of the standard. Intraassay variation was 5.4%; interassay variation was 13%. The sensitivity of the assay was 0.25 ng/ml.

Characterization of IGF-binding proteins (IGFBPs)
Western ligand blot procedures were used to compare IGFBPs in GM1 mutant and normal cat sera after PAGE under nonreducing conditions (34). Recombinant bovine IGF-I was used as the iodinated ligand. Bands that bound [125I]IGF-I (Amersham Pharmacia Biotech, Piscataway, NJ) were compared with predetermined mol wt standards run on the same gel to establish mol wt. Visualized bands in cat sera had mol wt similar to those previously described in other species (35). Identification of IGFBP-2 was confirmed by Western blotting with antibovine IGFBP-2 antibody (Upstate Biotechnology, Inc., Lake Placid, NY). Comparisons of IGFBPs from GM1 mutant and normal cats were determined by densitometry.

RNA extraction and Northern blot preparation
Total RNA was extracted from the pituitary glands and livers from both GM1 mutant cats and normal age-matched cats using a guanidium thiocyanate-phenol-chloroform method (RNAzol, Tel-Test, Inc., Friendswood, TX) (36, 37). Livers were extracted three times and enriched for polyadenylated messenger RNA (mRNA; PolyATtract, Promega Corp., Madison, WI). Liver polyadenylated mRNA (10 µg) and pituitary RNA (4 µg) samples were separated by size using electrophoresis on 1% agarose-formaldehyde gels and transferred overnight to nylon membranes (Nytran, Schleicher & Schuell, Inc., Keene, NH). RNAs were fixed to the membrane by UV irradiation. Prehybridization with denatured salmon sperm DNA (50 µg/ml) was performed overnight at 42 C with gentle shaking in a buffer containing 50% formamide, 5 x Denhardt’s solution (1 x Denhardt’s solution is 0.2% BSA fraction V, 0.2% Ficoll 400, and 0.2% polyvinylpyrrolidone), 5 x SSPE (1 x SSPE is 0.15 M NaCl, 0.01 M NaH2PO4, and 1 mM EDTA), 50 µg/ml polyadenylic acid, and 0.1% SDS. Liver blots were probed for IGF-I mRNA using porcine complementary DNA (cDNA) probes donated by Dr. F. Simmen (University of Florida, Gainesville, FL). Pituitary blots were probed with bovine GH cDNA probe provided by Dr. P. Rotwein (Washington University, St. Louis, MO). Probes were labeled with 32P to at least 5 x 108 cpm/mg DNA using a random primer kit (Prima-A-Gene Kit, Promega Corp.). Northern blots were hybridized for 72 h using approximately 3 x 106 cpm labeled probe/blot, then washed three times in 2 x SSC, 0.1% SDS at room temperature for 15 min each, blotted to remove excess moisture, and placed on x-ray film for 7–10 days for IGF-I message or 3–6 days for GH message. The intensity of the resolved bands was estimated using scanning laser densitometry. For GH blots, the bands at approximately 0.8–1 kb were used for analysis, whereas for IGF-I blots, all transcripts were scanned. For the purpose of normalization, each blot was stripped of respective probe and reprobed using radiolabeled ribosomal 28S cDNA (for GH mRNA blots) or radiolabeled ß-actin cDNA (for IGF-I mRNA blots; donated by R. C. Bird, Auburn University) to control for loading differences (38).

Exogenous GH injections
Recombinant canine GH (Ala l-canine GH; lot NBP5784686) was diluted in excipient (lot NBP5787226; a gift from Monsanto Co.) to give a final concentration of 2 mg/ml. After determining that their serum IGF-I levels were significantly reduced in each of five GM1 mutant cats (at ~200 days of age) compared with those in their normal age-matched controls, 0.5 mg GH was injected sc into these cats daily for 3 days. Each age-matched control cat also was injected with GH to establish biological activity of the hormone for each mutant-normal pair experiment. Blood was collected before the first injection and then at 24-h intervals; each sampling was immediately before subsequent injections of GH. Serum was collected and frozen at -80 C until IGF-I analysis.

Statistics
Paired data (GM1 mutant and age-matched control cats or GM1 mutant cats pre- and post-GH injections) were compared using paired Student’s t test. For data that could not be paired (for example, when data from additional normal age-matched cats were included), comparisons were made using Student’s t test.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
IGF-I in serum from GM1 mutant cats
IGF-I concentrations were determined from serum samples collected over time or at necropsy from GM1 mutant cats and age-matched normal cats. Our studies focused on young cats (<=330 days of age) because GM1 mutant cats are seldom maintained for more than 10–11 months for humane reasons. Cat pairs were grouped into three age groups, which corresponded well to the progression of clinical disease in the GM1 mutant cats. Mean age, stage of disease, phenotype, and IGF-I levels for all three groups are shown in Table 1Go. Although there was substantial cat to cat variation, IGF-I levels tended to increase as normal cats approached puberty. The average age at onset of puberty for domestic cats is 8.5–10 months of age (39); in this colony, female cats have their first estrus at 8–12 months of age, whereas male cats first sire kittens at 12–18 months of age (Baker, H. J., personal communication). Mean serum IGF-I levels in group 3 normal cats (near puberty, sexes combined) were elevated compared with those in normal prepuberal cats in groups 1 and 2 (P < 0.001 and P < 0.01, respectively; see Table 1Go). In contrast, mean serum IGF-I levels were similar in all GM1 mutant cats regardless of age. Mean levels of serum IGF-I in both group 2 and 3 GM1 mutant cats were significantly lower than mean levels in age-matched control cats (P < 0.001; see Table 1Go). The relationship of serum IGF-I levels to body weight (IGF-I per kg BW) indicated that reductions in serum IGF-I levels were not directly associated with body weight (see Table 2Go).


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  		Table 1. Mean serum IGF-I concentrations in normal and GM1 mutant cats

 

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  		Table 2. Body weight-matched analysis of serum IGF-I in GM1 mutant and normal cats

 
Comparison of IGFBPs in GM1 mutant and normal age-matched cats
Densitometric analysis of serum ligand blots demonstrated that bands corresponding to IGFBP-3 were reduced in sera from GM1 mutant cats compared with those in normal age-matched cats (P < 0.01), whereas bands corresponding to IGFBP-2 were increased (P < 0.001; see Table 3Go).


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  		Table 3. Comparison of IGF binding proteins in sera from GM1 mutant (n = 4) and normal cats (n = 6) more than 200 days of age

 
Evaluation of IGF-I mRNA in GM1 mutant cat liver
Feline GM1 gangliosidosis results in hepatocellular lysosomal hypertrophy due to the massive accumulation of large mol wt glycoproteins as well as asialo gangliosides and other glycolipids with nonreducing galactose in ß-D linkage (40). Because the liver is a major site for production of circulating IGF-I (41), we used Northern blot methodology to determine whether livers from cats severely affected with GM1 gangliosidosis (>200 days of age) contained IGF-I mRNA in amounts comparable to age-matched normal cat livers (see Fig. 1Go). Results demonstrate that liver IGF-I mRNA levels in the GM1 mutant cats were reduced by approximately 50%.



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  		Figure 1. A, IGF-I mRNA extracted from livers of normal and GM1 mutant cats more than 200 days of age were compared by Northern blot analyses as described in Materials and Methods. The IGF-I mRNA concentration is expressed as the mean ± SE ratio of IGF-I mRNA to ß-actin mRNA to correct for gel loading. n = 6 for each group. *, P < 0.01. B, Representative Northern blot for IGF-I mRNA and ß-actin for a GM1 mutant cat (M) and an age-matched normal cat (N).

 
Stimulation of IGF-I production by exogenous GH
To determine whether hepatocytes from severely affected GM1 mutant cats were capable of responding to GH with the production and release of IGF-I into the circulation, exogenous canine recombinant GH was injected, and serum levels of IGF-I were determined. GM1 mutant cats with serum IGF-I concentrations that were below those of their normal age-matched controls were used for this study (GM1 mutant mean ± SEM, 103.7 ± 10.2 ng/ml; normal mean ± SEM, 188.1 ± 25.1 ng/ml; P < 0.01; n = 5 GM1 mutant/normal cat pairs). Sera from cats were collected before and 24 h after sc injection of GH (0.5 mg/cat). Because of concern that a single injection might not be adequate to provoke the production of IGF-I in the GM1 mutant cats, the cats were injected twice more with GH (24 and 48 h), sampling before each additional injection and at 72 h after the initial injection. Bioactivity of the canine recombinant GH was verified by measurement of serum IGF-I levels in normal control cats after GH injections in parallel with GM1 mutant cat injections (data not shown). A single injection of GH significantly increased serum IGF-I in GM1 mutant cats (P < 0.05), raising levels to values very similar to normal cat baseline levels (see Fig. 2Go). Levels of serum IGF-I in all GM1 mutant cats increased with multiple GH treatments, although the time to reach peak levels varied from cat to cat (individual data points not shown).



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  		Figure 2. Elevations in serum IGF-I in GM1 mutant cats more than 200 days of age after the administration of recombinant canine GH. Each cat received three sc injections of 0.5 mg/cat at 24-h intervals. Serum samples from GM1 mutant cats were collected before each GH administration (pre-GH, 24 h, and 48 h). Additional serum samples were taken 24 h after the third GH treatment (72 h). Data are expressed as the mean serum IGF-I ± SE (nanograms per ml) and are based on samples from five GM1 mutant cats; serum IGF-I baseline data were from five normal age-matched cats.

 
Evaluation of GH mRNA in GM1 mutant and normal cat pituitaries
The pituitary is the major site of production of endogenous GH (30, 42). Northern blot methodology was used to compare GH mRNA in GM1 mutant cat pituitaries to that found in pituitaries from normal age-matched cats. Results are shown in Fig. 3Go and demonstrate that GH mRNA is reduced in pituitaries from GM1 mutant cats more than 200 days of age [mean ± SEM change from normal, -58.45 ± 17.19% (n = 4 GM1 mutant cats); mean normal set at 0 ± 9.19 SEM (n = 4 normal cats); P < 0.05].



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  		Figure 3. A, GH mRNA extracted from pituitaries of normal and GM1 mutant cats more than 200 days of age were compared by Northern blot analyses as described in Materials and Methods. The GH mRNA concentration is expressed as the mean ± SE ratio of GH mRNA to 28S ribosomal RNA to correct for gel loading. n = 4 for each group. *, P < 0.05. B, Representative Northern blot for GH mRNA and 28S ribosomal RNA for a GM1 mutant cat (M) and an age-matched normal cat (N).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Consistent with patterns seen in human patients as well as other animals in which levels of serum IGF-I are low at birth and progressively increase through the prepubescent period to puberty, serum IGF-I levels in the normal cats progressively increased with age. Sera from GM1 mutant cats more than 150 days of age had IGF-I levels that were significantly lower than those of age-matched control cats, with the greatest differences seen from sera taken from cats more than 200 days of age. Indeed, there were no significant differences in the levels of IGF-I among GM1 mutant cats of the three age groups, indicating that these cats do not show the increases in serum IGF-I normally seen in prepubescence and at puberty. The onset of puberty in the domestic cat is dependent upon body weight (39). Because GM1 mutant cats are smaller than control cats of the same age, puberty may be delayed in these cats; however, body weight (i.e. growth) is controlled primarily via the GH/IGF-I axis and the availability of nutritional building blocks. The measurement of serum IGF-I frequently is used not only to determine the availability of IGF-I to cells but also as a reliable, fairly stable indicator of GH levels in blood over time. The low levels of serum IGF-I in these cats could reflect either lowered circulating GH resulting in reduced IGF-I production or increased circulating GH, which is unable to bind and/or appropriately activate GH receptors. Alternatively, IGF-I may be short-lived in circulation, possibly due to altered IGFBP concentrations. Nutritional factors are also important modulators of the GH/IGF-I axis, and malnutrition can result in a state of GH resistance with normal to elevated levels of serum GH but low levels of IGF-I.

The majority of serum IGFs are bound to IGFBPs, which protect IGF-I from enzymatic degradation and modulate their bioavailability (41). Although there are six known IGFBPs, data from other species including human indicate that the predominant forms identified by serum ligand blot are IGFBP-3, IGFBP-2, IGFBP-1, and IGFBP-4 (35). IGFBP-3 and IGFBP-2 are primarily responsible for binding the majority of IGF-I in postnatal serum. IGFBP-3 is the primary binding protein and is regulated by GH and IGF-I (43) and by extreme variations in nutrient uptake (44). IGFBP-2 is the second most abundant IGFBP in postnatal serum and is inversely related to GH status and nutrient availability (41, 45). Comparison of band densities of IGFBPs in sera from normal and GM1 mutant cats indicated that GM1 mutant cats have reduced IGFBP-3 (45% of mean normal) and increased IGFBP-2 (827% of mean normal) densities. These findings suggested that in GM1 mutant cats, GH-driven signaling pathways for the production of IGFBP-3 and IGFBP-2 were altered. Such alterations could be due to inappropriate GH stimulation, altered postreceptor pathway abnormalities, or severe malnutrition. The finding that the mean mRNA for IGF-I was reduced in livers from GM1 mutant cats more than 200 days of age compared with that in normal cats further suggested that GH/IGF-I metabolism in the liver was affected in these cats.

The major source of circulating IGFs as well as IGFBPs is the liver (41). To determine whether livers of GM1 mutant cats were able to respond to GH by release of IGF-I, exogenous recombinant canine GH was given to GM1 mutant cats and normal cats, and serum IGF-I levels were determined over time. This in vivo study directly established that GM1 mutant cats had functional GH receptors on cells that were capable of producing significant amounts of circulating IGF-I and IGFBPs despite the presence of increased lysosomal storage products. The data further indicated that pathways within the liver were still intact and suggest that uptake of nutrients were adequate for assimilation into proteins. Because exogenous GH was capable of binding to GH receptors, resulting in increases in circulating IGF-I, the data also suggest that either serum GH levels were inadequate or that abnormal GH was produced in GM1 mutant cats.

GH is produced primarily by the somatotropic cells in the anterior pituitary for release into the circulation. In these studies, GM1 mutant cats more than 200 days of age had reduced pituitary GH mRNA levels compared with age-matched controls, suggesting that lack of production of GH mRNA is a component of the altered GH/IGF-I pathway in this disease. Retrospective analyses of serum samples taken at necropsy from GM1 mutant and normal cats suggested that GM1 mutant cats also may have reduced circulating GH, particularly in cats around 5 months of age [mean GH ± SEM: GM1 mutant ~150 days of age, 2.79 ± 0.13; normal ~150 days of age, 10.80 ± 3.09 (n = 6 pairs); GM1 mutant >200 days of age, 3.44 ± 0.30; normal >200 days of age, 5.70 ± 1.10 (n = 6 pairs)]. These differences were not statistically significant due in large part to the wide range of GH levels seen in the normal cats. Because these were single serum samples, pulsatility of GH release could not be established, and indeed, the variability seen could be due to pulsatile release of GH. It is interesting, however, that GH concentrations in all GM1 mutant cats regardless of age were consistently below 5 ng/ml with little variability. Reduced serum GH would explain the reduced serum levels of IGF-I and IGFBP-3 seen in these cats, as GH is required to stimulate pathways for their production. Pathways involving the production and release of GH into serum will be the focus of further study as additional GM1 mutant cats become available.

The pathogenesis of the alterations in the GH/IGF-I pathways seen in the GM1 mutant cats remains unclear, but is likely to be multifactorial because of the complex interplay of hypothalamic neurotransmitters, hormones, and nutrition. Because of their surface membrane location, GM1 gangliosides are known to modulate numerous signaling mechanisms in many types of cells, but are most abundant in the nervous system (46). It is possible that progressively increased levels of GM1 gangliosides alter neuronal input into the hypothalamus and modulate the production of GH and, therefore IGF-I. Reduced GH and IGF-I levels would explain the lack of appropriate growth in body size and weight, premature thymic involution, and lack of male reproductive maturation seen in the GM1 mutant cats (9, 10, 11, 12, 13, 14, 15, 16). Others have shown that reduction of GH due to alterations in the hypothalamic/pituitary axis can result in the wasting syndromes seen in other diseases such as feline and human acquired immunodeficiency syndrome, rabies, cancer, and cerebral palsy (32, 47, 48, 49).

Because metabolic studies to evaluate the function of the intestinal mucosa of GM1 mutant cats have not been performed, malnutrition cannot be ruled out as a contributing factor in this disease. The presence in intestinal cells of excess membrane GM1 gangliosides or glycolipids and glycoproteins with the same undegradable terminal sugar moiety could affect nutrient absorption. However, malnutrition is unlikely to be the initial causative factor for low IGF-I levels for a number of reasons. Reduced serum IGF-I levels occur before the time when neurological disease could affect nutrient intake by GM1 mutant cats and do not appear to be directly related to body weights of affected cats. GM1 mutant cats continue to have good appetites when their neurological disease is severe enough to require assisted feeding. Their feces are normal in amount, appearance, and consistency. GM1 mutant cats have substantial abdominal and thoracic fat stores throughout their lifetimes. In children, malnutrition results in an increase in immature and a decrease in mature T lymphocytes (50), whereas GM1 mutant cats have severely reduced populations of immature CD4+CD8+ thymocytes, whereas mature CD8+ cells increase (7). In humans, starvation and malnutrition are associated with normal or elevated serum GH levels related to decreased hepatic GH binding or other GH resistance mechanisms (41). In GM1 mutant cats, reduced levels of GH mRNA were found. Additionally, functional GH receptors were present in affected cats and were capable of binding and responding to exogenous GH.

The finding that circulating levels of IGF-I are lower in GM1 mutant cats is intriguing. Circulating IGF-I enters the brain through brain capillaries and choroid plexuses of the ventricular system, and IGF-I receptors are expressed diffusely throughout the brain (reviewed in Ref. 26). Recent studies have shown IGF-I to be critical for growth, maintenance, rescue, and repair of cells within the nervous system, including neurons and oligodendroglial cells (51, 52). Progressive neurological abnormalities seen in feline GM1 gangliosidosis could reflect reduced trophic influence of IGF-I on neurons. Regardless of the initiating factor(s) for reduced serum IGF-I, our documentation that the circulating levels of this growth factor are reduced in feline GM1 gangliosidosis may prove to be important to understanding the pathogenesis of and developing therapeutic strategies for this currently incurable neurological disease.


   Acknowledgments
 
We thank Gertrude Baker, Stephanie Vaught, Atoska Gentry, Terry Whitehead, and Diana Groh for their technical assistance with this project.


   Footnotes
 
1 This work was supported in part by the Auburn University Biogrant Program and the Scott-Ritchey Research Center. Back

Received March 16, 1999.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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