Neurotropic and modulatory effects of insulin-like growth factor II in Aplysia

Insulin-like growth factor 2 (IGF-2) rescues social deficits in NLG3-/y mouse model of ASDs

Autism spectrum disorders (ASDs) comprise developmental disabilities characterized by impairments of social interaction and repetitive behavior, often associated with cognitive deficits. There is no current treatment that can ameliorate most of the ASDs symptomatology; thus, identifying novel therapies is urgently needed. Here, we used the Neuroligin 3 knockout mouse (NLG3-/y), a model that recapitulates the social deficits reported in ASDs patients, to test the effects of systemic administration of IGF-2, a polypeptide that crosses the blood-brain barrier and acts as a cognitive enhancer. We show that systemic IGF-2 treatment reverses the typical defects in social interaction and social novelty discrimination reflective of ASDs-like phenotypes. This effect was not accompanied by any change in spontaneous glutamatergic synaptic transmission in CA2 hippocampal region, a mechanism found to be crucial for social novelty discrimination. However, in both NLG3+/y and NLG3-/y mice IGF-2 increased cell excitability. Although further investigation is needed to clarify the cellular and molecular mechanisms underpinning IGF-2 effect on social behavior, our findings highlight IGF-2 as a potential pharmacological tool for the treatment of social dysfunctions associated with ASDs.

Pattern detection in the TGFβ cascade controls the induction of long-term synaptic plasticity

Transforming growth factor β (TGFβ) is required for long-term memory (LTM) for sensitization in Aplysia. When LTM is induced using a two-trial training protocol, TGFβ inhibition only blocks LTM when administrated at the second, not the first trial. Here, we show that TGFβ acts as a "repetition detector" during the induction of two-trial LTM. Secretion of the biologically inert TGFβ proligand must coincide with its proteolytic activation by the Bone morphogenetic protein-1 (BMP-1/Tolloid) metalloprotease, which occurs specifically during trial two of our two-trial training paradigm. This paradigm establishes long-term synaptic facilitation (LTF), the cellular correlate of LTM. BMP-1 application paired with a single serotonin (5HT) pulse induced LTF, whereas neither a single 5HT pulse nor BMP-1 alone effectively did so. On the other hand, inhibition of endogenous BMP-1 activity blocked the induction of two-trial LTF. These results suggest a unique role for TGFβ in the interaction of repeated trials: during learning, repeated stimuli engage separate steps of the TGFβ cascade that together are necessary for the induction of long-lasting memories.

IGF2 in memory, neurodevelopmental disorders, and neurodegenerative diseases

Insulin-like growth factor 2 (IGF2) emerged as a critical mechanism of synaptic plasticity and learning and memory. Deficits in IGF2 in the brain, serum, or cerebrospinal fluid (CSF) are associated with brain diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). Increasing IGF2 levels enhances memory in healthy animals and reverses numerous symptoms in laboratory models of aging, neurodevelopmental disorders, and neurodegenerative diseases. These effects occur via the IGF2 receptor (IGF2R) - a receptor that is highly expressed in neurons and regulates protein trafficking, synthesis, and degradation. Here, I summarize the current knowledge regarding IGF2 expression and functions in the brain, particularly in memory, and propose a novel conceptual model for IGF2/IGF2R mechanisms of action in brain health and diseases.

Cell Cycle Re-entry in the Nervous System: From Polyploidy to Neurodegeneration

Terminally differentiated cells of the nervous system have long been considered to be in a stable non-cycling state and are often considered to be permanently in G0. Exit from the cell cycle during development is often coincident with the differentiation of neurons, and is critical for neuronal function. But what happens in long lived postmitotic tissues that accumulate cell damage or suffer cell loss during aging? In other contexts, cells that are normally non-dividing or postmitotic can or re-enter the cell cycle and begin replicating their DNA to facilitate cellular growth in response to cell loss. This leads to a state called polyploidy, where cells contain multiple copies of the genome. A growing body of literature from several vertebrate and invertebrate model organisms has shown that polyploidy in the nervous system may be more common than previously appreciated and occurs under normal physiological conditions. Moreover, it has been found that neuronal polyploidization can play a protective role when cells are challenged with DNA damage or oxidative stress. By contrast, work over the last two and a half decades has discovered a link between cell-cycle reentry in neurons and several neurodegenerative conditions. In this context, neuronal cell cycle re-entry is widely considered to be aberrant and deleterious to neuronal health. In this review, we highlight historical and emerging reports of polyploidy in the nervous systems of various vertebrate and invertebrate organisms. We discuss the potential functions of polyploidization in the nervous system, particularly in the context of long-lived cells and age-associated polyploidization. Finally, we attempt to reconcile the seemingly disparate associations of neuronal polyploidy with both neurodegeneration and neuroprotection.

Effect of photoperiod and light intensity on learning ability and memory formation of the pond snail Lymnaea stagnalis

Natural light is regarded as a key regulator of biological systems and typically serves as a Zeitgeber for biological rhythms. As a natural abiotic factor, it is recognized to regulate multiple behavioral and physiological processes in animals. Disruption of the natural light regime due to light pollution may result in significant effects on animal learning and memory development. Here, we investigated whether sensitivity to various photoperiods or light intensities had an impact on intermediate-term memory (ITM) and long-term memory (LTM) formation in the pond snail Lymnaea stagnalis. We also investigated the change in the gene expression level of molluscan insulin-related peptide II (MIP II) is response to the given light treatments. The results show that the best light condition for proper LTM formation is exposure to a short day (8 h light) and low light intensity (1 and 10 lx). Moreover, the more extreme light conditions (16 h and 24 h light) prevent the formation of both ITM and LTM. We found no change in MIP II expression in any of the light treatments, which may indicate that MIP II is not directly involved in the operant conditioning used here, even though it is known to be involved in learning. The finding that snails did not learn in complete darkness indicates that light is a necessary factor for proper learning and memory formation. Furthermore, dim light enhances both ITM and LTM formation, which suggests that there is an optimum since both no light and too bright light prevented learning and memory. Our findings suggest that the upsurge of artificial day length and/or night light intensity may also negatively impact memory consolidation in the wild.

Postsynaptic effects of Aplysia cysteine-rich neurotrophic factor in the induction of activity-dependent long-term facilitation in Aplysia californica

The spatial and temporal coordination of growth factor signaling is critical for both presynaptic and postsynaptic plasticity underlying long-term memory formation. We investigated the spatiotemporal dynamics of Aplysia cysteine-rich neurotrophic factor (ApCRNF) signaling during the induction of activity-dependent long-term facilitation (AD-LTF) at sensory-to-motor neuron synapses that mediate defensive reflexes in Aplysia We found that ApCRNF signaling is required for the induction of AD-LTF, and for training-induced early protein kinase activation and late forms of gene expression, exclusively in postsynaptic neurons. These results support the view that ApCRNF is critically involved in AD-LTF at least in part through postsynaptic mechanisms.