Insulin-like growth factor-I improves cerebellar dysfunction but does not prevent cerebellar neurodegeneration in the calcium channel mutant mouse, leaner

Insulin-like growth factor 1 signaling in motor neuron and polyglutamine diseases: From molecular pathogenesis to therapeutic perspectives

The pleiotropic peptide insulin-like growth factor 1 (IGF-I) regulates human body homeostasis and cell growth. IGF-I activates two major signaling pathways, namely phosphoinositide-3-kinase (PI3K)/protein kinase B (PKB/Akt) and Ras/extracellular signal-regulated kinase (ERK), which contribute to brain development, metabolism and function as well as to neuronal maintenance and survival. In this review, we discuss the general and tissue-specific effects of the IGF-I pathways. In addition, we present a comprehensive overview examining the role of IGF-I in neurodegenerative diseases, such as spinal and muscular atrophy, amyotrophic lateral sclerosis, and polyglutamine diseases. In each disease, we analyze the disturbances of the IGF-I pathway, the modification of the disease protein by IGF-I signaling, and the therapeutic strategies based on the use of IGF-I developed to date. Lastly, we highlight present and future considerations in the use of IGF-I for the treatment of these disorders.

IGF-1 in autosomal dominant cerebellar ataxia - open-label trial

Background

The objective of this clinical open-label trial was to test the safety, tolerability and efficacy of IGF-1 therapy for autosomal dominant cerebellar ataxia (ADCA) patients.

Results

A total of 19 molecularly confirmed patients with SCA3, 1 patient with SCA6 and 6 patients with SCA7 completed our study. They were 8 females and 18 males, 28 to 74 years of age (average ± SD: 49.3 ± 14.1). Patients were treated with IGF-1 therapy with a dosage of 50 μg/kg twice a day for 12 months. The efficacy of this therapy was assessed by change from baseline on the scale for the assessment and rating of ataxia (SARA). Ten patients, consecutively selected, continued their assigned dosages in a second year open-label extension trial. A statistically significant improvement in SARA scores was observed for patients with SCA3, patients with SCA7 and all patients grouped together after the first year of IGF-1 therapy, while a stabilization of the disease was confirmed during the second year (extension study). The single patient with SCA6 showed 3 improvement points in SARA score after 3 four-month periods of IGF-1 therapy when compared with baseline measurements. Our data indicate that IGF-1 is safe and well tolerated in general.

Conclusions

Our data, in comparison with results from previous cohorts, indicate a trend for IGF-1 treatment to stabilize the disease progression for patients with SCA, indicating that IGF-1 therapy is able to decrease the progressivity of ADCA.

Frataxin deficiency unveils cell-context dependent actions of insulin-like growth factor I on neurons

BACKGROUND

Friedreich's ataxia (FRDA) is a neurodegenerative disease caused by deficiency of the mitochondrial iron chaperone frataxin (Fxn). FRDA has no cure, but disease-modifying strategies to increase frataxin are under study. Because insulin-like growth factor I (IGF-I) has therapeutic effects in various types of cerebellar ataxia and exerts protective actions on mitochondrial function, we explored the potential Fxn-stimulating activity of this growth factor on brain cells.

RESULTS

IGF-I normalized frataxin levels in frataxin-deficient neurons and astrocytes through its canonical Akt/mTOR signaling pathway. IGF-I also stimulated frataxin in normal astrocytes but not in normal neurons, whereas IGF-I stimulated the Akt/mTOR pathway in both types of cells. This cell context-dependent action of IGF-I on neurons suggested that the intrinsic regulation of Fxn in neurons is different than in astrocytes. Indeed, neurons express much higher levels of frataxin and are much more sensitive to Fxn deficiency than astrocytes; i.e.: only neurons die in the absence of frataxin. In addition, the half-life of frataxin is shorter in neurons than in astrocytes, while after blockade of the proteasome only neurons responded to IGF-I with an increase in frataxin levels. We also explore a potential therapeutic utility of IGF-I in FRDA-like transgenic mice (YG8R mice) and found that treatment with IGF-I normalized motor coordination in these moderately ataxic mice.

CONCLUSION

Exposure to IGF-I unveiled a cell-specific regulation of frataxin in neurons as compared to astrocytes. Collectively, these results indicate that IGF-I exerts cell-context neuroprotection in frataxin deficiency that maybe therapeutically effective.

Synaptic deficits are rescued in the p25/Cdk5 model of neurodegeneration by the reduction of β-secretase (BACE1)

Alzheimer's disease (AD) is the most common cause of dementia, and is characterized by memory loss and cognitive decline, as well as amyloid β (Aβ) accumulation, and progressive neurodegeneration. Cdk5 is a proline-directed serine/threonine kinase whose activation by the p25 protein has been implicated in a number of neurodegenerative disorders. The CK-p25 inducible mouse model exhibits progressive neuronal death, elevated Aβ, reduced synaptic plasticity, and impaired learning following p25 overexpression in forebrain neurons. Levels of Aβ, as well as the APP processing enzyme, β-secretase (BACE1), are also increased in CK-p25 mice. It is unknown what role increased Aβ plays in the cognitive and neurodegenerative phenotype of the CK-p25 mouse. In the current work, we restored Aβ levels in the CK-p25 mouse to those of wild-type mice via the partial genetic deletion of BACE1, allowing us to examine the Aβ-independent phenotype of this mouse model. We show that, in the CK-p25 mouse, normalization of Aβ levels led to a rescue of synaptic and cognitive deficits. Conversely, neuronal loss was not ameliorated. Our findings indicate that increases in p25/Cdk5 activity may mediate cognitive and synaptic impairment via an Aβ-dependent pathway in the CK-p25 mouse. These findings explore the impact of targeting Aβ production in a mouse model of neurodegeneration and cognitive impairment, and how this may translate into therapeutic approaches for sporadic AD.

Safety aspects of longitudinal administration of IGF-I/IGFBP-3 complex in neonatal mice

OBJECTIVE

Very preterm birth is associated with a high risk of morbidity. Infants born very preterm have low serum levels of insulin-like growth factor I (IGF-I), that further decrease after birth. IGF-I is essential for brain development and low serum levels have been associated with retinopathy of prematurity. The present study aimed to investigate the effects of prolonged administration of a low dose of rhIGF-I/rhIGFBP-3 on glucose levels and total body weight, as well as liver, spleen and brain weights, and gray and subcortical white matter in newborn mice.

DESIGN

The study was performed as three different trials. In all experiments C57BL/6N mice were injected with a rhIGF-I/rhIGFBP-3 complex or saline. In the first experimental trial, blood glucose levels were assessed 30 min, 1 h, 1.5 h, 3 h, 6 h, 24 h and 48 h after the rhIGF-I/rhIGFBP-3 or saline injection on postnatal day (PND) 6. In the second trial, mice were injected daily from PND 3 to 11 and sacrificed on PND 12 for analysis of IGF-I serum levels. In the third trial, body and organ weights and effects on gray and white matter were assessed on PND 18 after PND 3-11 treatments as above. Effects on gray and white matter were measured using immunoreactivity for microtubule-associated protein-2 (MAP-2), myelin basic protein (MBP), 2',3'-cyclic nucleotide 3' phosphodiesterase (CNPase), neurofilament and oligodendrocyte lineage transcription factor 2 (Olig2).

RESULTS

Blood glucose levels were unchanged in the rhIGF-I/rhIGFBP-3-treated group compared to baseline. In the control group glucose levels increased 30 min after the second saline injection; levels were not elevated at the subsequent time point. Three hours after the rhIGF-I/rhIGFBP-3 or saline, glucose levels were lower in rhIGF-I/rhIGFBP-3-treated animals than in saline treated (p=0.026). At PND 18, total body weight was higher in rhIGF-I/rhIGFBP-3-treated mice compared with controls (p<0.05), but there were no differences between groups in brain, liver or spleen weights. No differences in gray matter area were found between groups. Analyses of white matter markers showed an increased number of Olig2-positive cells in rhIGF-I/rhIGFBP-3-treated mice compared with controls (p<0.001). There were no differences between groups in terms of MBP, CNPase or neurofilament immunoreactivity.

CONCLUSIONS

Prolonged administration of rhIGF-I/rhIGFBP-3 did not have a negative impact on blood glucose levels and was beneficial for total body growth.

AF4 is a critical regulator of the IGF-1 signaling pathway during Purkinje cell development

Deregulation of the insulin-like growth factor 1 (IGF-1) signaling pathway is a recurrent finding in mouse models and human patients with cerebellar ataxia and thus represents a common pathological cascade in neuronal cell death that may be targeted for therapy. We have previously identified a point mutation in AF4, a transcription cofactor of RNA polymerase II elongation and chromatin remodeling, that causes progressive and highly specific Purkinje cell (PC) death in the ataxic mouse mutant robotic, leading to the accumulation of AF4 in PCs. Here we confirm that the spatiotemporal pattern of PC degeneration in the robotic cerebellum correlates with the specific profile of AF4 upregulation. To identify the underlying molecular pathways, we performed microarray gene expression analysis of PCs obtained by laser capture microdissection (LCM) at the onset of degeneration. Igf-1 was significantly downregulated in robotic PCs compared with wild-type controls before and throughout the degenerative process. Consistently, we observed a decrease in the activation of downstream signaling molecules including type 1 IGF receptor (IGF-1R) and the extracellular signal-regulated kinase (ERK) 1 and ERK2. Chromatin immunoprecipitation confirmed that Igf-1 is a direct and the first validated target of the AF4 transcriptional regulatory complex, and treatment of presymptomatic robotic mice with IGF-1 indeed markedly delayed the progression of PC death. This study demonstrates that small changes in the levels of a single transcriptional cofactor can deleteriously affect normal cerebellum function and opens new avenues of research for the manipulation of the IGF-1 pathway in the treatment of cerebellar ataxia in humans.

Insulin-like growth factor I treatment for cerebellar ataxia: Addressing a common pathway in the pathological cascade?

In the present work we review evidence supporting the use of insulin-like growth factor I (IGF-I) for treatment of cerebellar ataxia, a heterogeneous group of neurodegenerative diseases of low incidence but high societal impact. Most types of ataxia display not only motor discoordination, but also additional neurological problems including peripheral nerve dysfunctions. Therefore, a feasible therapy should combine different strategies aimed to correct the various disturbances specific for each type of ataxia. For cerebellar deficits, and most probably also for other types of brain deficits, the use of a wide-spectrum neuroprotective factor such as IGF-I may prove beneficial. Intriguingly, both ataxic animals as well as human patients show altered serum IGF-I levels. While the pathogenic significance of IGF-I, if any, in this varied group of diseases is difficult to envisage, disrupted IGF-I neuroprotective signaling may constitute a common stage in the pathological cascade associated to neuronal death. Indeed, treatment with IGF-I has proven effective in animal models of ataxia. Based on this pre-clinical evidence we propose that IGF-I should be tested in clinical trials of cerebellar ataxia in those cases where either serum IGF-I deficiency (as in primary cerebellar atrophy) or loss of sensitivity to IGF-I (as in ataxia telangiectasia) has been reported. Taking advantage of the widely protective and anabolic actions of IGF-I on peripheral tissues, this neurotrophic factor may provide additional therapeutic advantages for many of the disturbances commonly associated to ataxia such as cardiopathy, muscle wasting, or immune dysfunction.

Differential expression of T-type calcium channels in P/Q-type calcium channel mutant mice with ataxia and absence epilepsy

Mutations in P/Q-type calcium channels generate common phenotypes in mice and humans, which are characterized by ataxia, paroxysmal dyskinesia, and absence seizures. Subsequent functional changes of T-type calcium channels in thalamus are observed in P/Q-type calcium channel mutant mice and these changes play important roles in generation of absence seizures. However, the changes in T-type calcium channel function and/or expression in the cerebellum, which may be related to movement disorders, are still unknown. The leaner mouse exhibits severe ataxia, paroxysmal dyskinesia, and absence epilepsy due to a P/Q-type calcium channel mutation. We investigated changes in T-type calcium channel expression in the leaner mouse thalamus and cerebellum using quantitative real-time polymerase chain reaction (qRT-PCR) and quantitative in situ hybridization histochemistry (ISHH). qRT-PCR analysis showed no change in T-type calcium channel alpha 1G subunit (Cav3.1) expression in the leaner thalamus, but a significant decrease in alpha 1G expression in the whole leaner mouse cerebellum. Interestingly, quantitative ISHH revealed differential changes in alpha 1G expression in the leaner cerebellum, where the granule cell layer showed decreased alpha 1G expression while Purkinje cells showed increased alpha 1G expression. To confirm these observations, the granule cell layer and the Purkinje cell layer were laser capture microdissected separately, then analyzed with qRT-PCR. Similar to the observation obtained by ISHH, the leaner granule cell layer showed decreased alpha 1G expression and the leaner Purkinje cell layer showed increased alpha 1G expression. These results suggest that differential expression of T-type calcium channels in the leaner cerebellum may be involved in the observed movement disorders.