**4.2 Neuroendocrine regulation**

220 Basic and Clinical Endocrinology Up-to-Date

IGF-1 on oligodendrocyte survival, development and proliferation (McMorris et al. 1986; McMorris and Dubois-Dalcq 1988; Mozell and McMorris 1991). In addition, formation of hippocampal granule cells and striatal parvalbumin-containing neurons was reduced in IGF-1-/- mice and therefore seems to require IGF-1 (Beck et al. 1995). Most dentate granule neurons as well as the myelination of axons are generated during postnatal development (Altman and Bayer 1990, 1990; Morell et al. 1972; Matthieu, Widmer, and Herschkowitz 1973) suggesting that IGF-1 plays a more important role during late embryonic and postnatal development. Intrauterine development was analysed in detail in single and combined homozygous knockout mice for IGF-1 and IGF-2 suggesting that before E13.5 the IGF-1R exclusively mediates IGF-2 actions and later, with increasing IGF-1 expression, the IGF-1R interacts with both IGFs (Baker et al. 1993). IGF-2 was found to be upregultaed in IGF-1-/- mice suggesting that IGF-2 may partially compensate for the loss of IGF-1

Homozygous IGF-1R knockout mice (IGF-1R-/-) die at birth and have smaller brains (Liu et al. 1993). A similar growth retardation of the CNS was observed in transgenic mice overexpressing human IGFBP-1 (hIGFBP-1) suggesting reduced IGF-1 action via IGF-1Rs (D'Ercole et al. 1994). Conversely, transgenic mice overexpressing human IGF-1 (hIGF-1) show increased brain growth and myelination (Carson et al. 1993; Ye, Carson, and D'Ercole 1995). In both mouse models, the transgenic hIGFBP-1 and hIGF-1 mouse, the cerebral cortex, hippocampus and diencephalon were the most affected brain regions (Ye, Carson, and D'Ercole 1995). Myelination was increased in hIGF-1 and reduced in hIGFBP-1 transgenic mice as well as the number of oligodendrocytes and the expression of myelinspecific proteins, respectively (Ye, Carson, and D'Ercole 1995). During development, the increase of myelin protein expression in the cerebral cortex of hIGF-1 transgenic mice correlates with the hIGF-1 transgene mRNA levels (Ye, Carson, and D'Ercole 1995). In conclusion, IGF-1 plays a crucial role in regulating neuronal growth and differentiation (Werther et al. 1998), but also enhances oligodendrocyte survival and myelination (Beck et

Several studies suggested that transcriptional programs control the development of axons or dendrites including their growth and branching (Jan and Jan 2003; Goldberg 2004; Polleux, Ince-Dunn, and Ghosh 2007). Recently, the role of FoxO-transcription factors, major downstream-targets of IIS (Insulin/IGF-1 signalling pathway), within the developing brain and their action of conducting IGF-1 signalling started to become clearer. FoxO proteins were found to be key-regulators of neuronal polarity in the mammalian brain and to trigger differentiation from immature neurons to post-mitotic neurons with specified axon- and dendrite-formations (de la Torre-Ubieta et al. 2010). In addition, protein kinase Pak1 was identified as direct target of FoxO-transcription factors linking FoxO-dependent transcription in the nucleus to an enzyme that promotes axonal polarity by controlling actin and microtubule dynamics (Edwards et al. 1999; Wittmann, Bokoch, and Waterman-Storer 2004) as well as dendritic spine morphogenesis and synapse differentiation (Hayashi et al. 2004; Hayashi et al. 2007; Nikolic 2008). Apart from Pak1, FoxO knockdown neurons demonstrated a number of downregulated polarity genes, such as Par6, R-Ras, APC and CRMP2 (de la Torre-Ubieta et al. 2010). Additionally, FoxO3a was shown to regulate homeostasis of neuronal stem cells (NSCs) both *in vitro* and *in vivo* by controlling a set of genes that determines cell cycle re-entry and optimal oxygen and glucose metabolism (Renault et al. 2009). However, the effect of FoxO3a deficiency in NSCs only became apparent in adult animals, as NSCs were shown to influence learning, memory and mood

expression and function (Ye et al. 2002).

al. 1995).

Embryonic brain IGF-1R and therefore the action of the IGF-1 pathway in the developing brain was found to play a crucial role in determining somatotrophic plasticity and hence postnatal GH and IGF-1 signalling. These observations were made in a brain specific IGF-1R knockout mouse model (bIGF-1RKO-/-; bIGF-1RKO+/-) by the group of Holzenberger (Kappeler et al. 2008). Whereas homozygous mutants (bIGF-1RKO-/-) showed severe growth retardation and were infertile, heterozygous mutants (bIGF-1RKO+/-) were healthy and had an increased mean lifespan compared to controls (Kappeler et al. 2008). In bIGF-1RKO+/ mutants, serum IGF-1 and GH were lowered and pituitaries, like most other organs, were smaller compared to controls (Kappeler et al. 2008). As IGF-1Rs are only diminished in the brain but not in the anterior pituitary in this mouse model, the observed phenotype was proposed to result from alterations in GH-regulatory neurons of the hypothalamus. Hypothalamic GHRH expression was found to be significantly lower, as well as the Pit-1 mRNA levels. bIGF-1RKO+/- mice had preserved gonado- and thyrotropic functions but were growth retarded with a body length 5 % shorter than controls and a body weight of about 90% of controls at the age of 90 days (Kappeler et al. 2008). Interestingly, weight gain with age was slightly higher in adult bIGF-1RKO+/- mice than in controls, and female mutants finally reached the same body weight than controls (Kappeler et al. 2008). The gain of weight was most probably due to an enlargement of subcutaneous adipose tissue (AT), as there was less or no increase in visceral AT (Kappeler et al. 2008). In line with GH-deficient mouse models (Berryman et al. 2004; Berryman et al. 2006), the bIGF-1RKO+/- mice showed impaired glucose homeostasis and fat metabolism. Heterozygous inactivation of IGF-1R had no detectable effects on behaviour or other brain functions apart from the somatotrophic deficit. Compared to control littermates, bIGF-1RKO+/- mice had a significantly longer mean lifespan. However maximum lifespan was unchanged probably due to increased late-life mortality caused by hyperglycemia and dyslipidemia (Kappeler et al. 2008). Early dietary restriction showed a similar neuroendocrine response as the brain specific IGF-1R knockout (Kappeler et al. 2008), indicating a connection between nutrition, somatotrophic hormones, growth and their determination of lifespan.

Similar observations, in which morbidity in later life was determined during prenatal development, were made in humans. Long-term studies have been conducted on the Dutch famine birth cohort as well as on the Chinese famine cohort. Babies exposed in utero to calory restriction in late or mid gestation were growth retarded and showed impaired glucose tolerance in later life (Ravelli et al. 1998; Painter, Roseboom, and Bleker 2005). However, babies exposed in early gestation were not smaller than controls but demonstrated the most striking consequences of in utero undernutrition, namely a threefold increase in coronary heart disease, hyper-/dyslipidemia and more frequent obesity (Painter, Roseboom, and Bleker 2005; Ravelli et al. 1999; Roseboom, van der Meulen, Osmond, Barker, Ravelli, Schroeder-Tanka et al. 2000; Roseboom, van der Meulen, Osmond, Barker, Ravelli, and Bleker 2000). Interestingly, mortality rates at older (50+) ages are significantly higher in the exposed group, and residual life expectancy at age of 50 was reduced by approximately 3 years (Lindeboom, Portrait, and van den Berg 2010), which is in line with the increased late-life mortality observed in the bIGF-1R+/- mice. In the cohort of

Role of Central Insulin-Like Growth Factor-1

Millward, Zolnierowicz, and Hemmings 1999).

apoptosis and axonal transport.

processing and development of AD (Haass and Selkoe 2007).

**5.2 Amyloid-β**

Receptor Signalling in Ageing and Endocrine Regulation 223

terminal kinase (JNK) and ERK1/2 (Robertson et al. 1993; Hanger et al. 1992; Flaherty et al. 2000; Cho and Johnson 2004; Stoothoff and Johnson 2005). Abnormal high phosphorylation is called "hyperphosphorylation". Hyperphosphorylated tau proteins form so called paired helical filaments, which are characteristic for AD. The degradation of tau is inhibited by phosphorylation at the caspase cleavage sites. It has been shown that the mutation of Ser422, which causes a stable phosphorylation at this site, prevents caspase cleavage (Guillozet-Bongaarts et al. 2006). GSK-3β is one of the major tau kinases and is inactivated upon phosphorylation of Akt at Ser9 connecting insulin and IGF-1 signalling to tau phosphorylation. The major tau phosphatase in human brain is PP2A (Sontag et al. 1996), which is as well regulated via the IR/IGF-1R pathway suggesting that IR/IGF-1R signalling maintains an equilibrium of phosphorylation and dephosphorylation of tau (Liu et al. 2008;

Aβ is generated by proteolytic cleavage of the amyloid precursor protein (APP), a type-1 integral membrane protein. APP was first described and cloned in 1987 (Kang et al. 1987; Tanzi et al. 1987; Goldgaber et al. 1987; Robakis et al. 1987). The APP gene is located on chromosome 21. Hence, patients with trisomy 21 show a higher risk to develop Alzheimer´s disease, because of the additional APP allele. Accordingly, the duplication of the isolated APP gene causes cerebral amyloid angiopathy and amyloidosis suggesting that increased APP expression itself is sufficient to cause Alzheimer-like pathology (Rovelet-Lecrux et al. 2006; Sleegers et al. 2006). Another risk factor for AD are mutations of the APP gene (Vassar 2004; Bertram and Tanzi 2005). APP contains a N-terminal extracellular domain and a shorter C-terminal cytoplasmic domain. Alternative splicing of the APP gene results in different isoforms of APP which are distinguishable by length. APP with 751 and 770 (APP751 and APP770) amino acids mainly occur in non-neuronal tissue. APP695 is mainly localised in neurons (Kang and Muller-Hill 1990). The function of APP and the APP-like proteins (APLP) is not clear yet. These proteins are possibly involved in cell adhesion,

The β-secretase BACE1 (β-site APP-cleaving enzyme) plays an essential role in the production of Aβ. It cleaves APP at Asp+1 at the N-terminus. The resulting fragments are called APPsβ and the C-terminal fragment C99. Upon cleavage of C99 by the γ-secretase, a complex formed by presenilin, nicastrin, Aph-1 and Pen-2, Aβ peptides (4 kDa) and the APP intracellular domain (AICD) with a size of 6 kDa are generated. Aβ-peptides mainly occur in two variants: Aβ40 which ends at residue 40 and Aβ42 ending at residue 42 after cleavage. Predominantly, the Aβ42 is prone to aggregate and forms toxic oligomers. Furthermore, APP is cleaved by the α-secretases ADAM10 (a disintegrin and metalloproteinase-like 10) or TACE (tumour necrosis factor-alpha convertase). This results in the C-terminal fragment C83 and APPsα. The cleavage of APP by α- or β-secretase is dependent on the competition between both enzymes. In case the β-secretase cleavage of APP increases, α-secretase processing decreases and vice versa (Vassar et al. 1999; Skovronsky et al. 2000) (Figure 3). In a healthy brain, there is more production of Aβ40 (~90 %) than there is of Aβ42 (~5-10 %) (Walsh and Selkoe 2007). The accumulation of Aβ42 is an important step in the formation of amyloid plaques (Iwatsubo et al. 1994). The Aβ42:Aβ40 ratio is a diagnostic tool for APP

In addition to age-associated Aβ42 accumulation, mutations in presenilin 1, presenilin 2 and the APP gene lead to familiar early-onset AD (Tabaton and Tamagno 2007; Sherrington et al.

the Chinese famine, fetal and infant exposure to undernutrition also resulted in increased risk for metabolic syndrome (Li et al. 2010; Li et al. 2011; Yang et al. 2008; Luo et al. 2006). In summary, IGF-1 signalling during early development is defined by the number of brain IGF-1Rs or their sensitivity and determines endocrine as well as metabolic function in later life possibly playing a key-role in pathogenesis of age-associated diseases.


Table 1. Function of IGF-1 in the brain
