LECT2 functions as a hepatokine that links obesity to skeletal muscle insulin resistance.

Recent articles have reported an association between fatty liver disease and systemic insulin resistance in humans, but the causal relationship remains unclear. The liver may contribute to muscle insulin resistance by releasing secretory proteins called hepatokines. Here we demonstrate that leukocyte cell-derived chemotaxin 2 (LECT2), an energy-sensing hepatokine, is a link between obesity and skeletal muscle insulin resistance. Circulating LECT2 positively correlated with the severity of both obesity and insulin resistance in humans. LECT2 expression was negatively regulated by starvation-sensing kinase adenosine monophosphate-activated protein kinase in H4IIEC hepatocytes. Genetic deletion of LECT2 in mice increased insulin sensitivity in the skeletal muscle. Treatment with recombinant LECT2 protein impaired insulin signaling via phosphorylation of Jun NH2-terminal kinase in C2C12 myocytes. These results demonstrate the involvement of LECT2 in glucose metabolism and suggest that LECT2 may be a therapeutic target for obesity-associated insulin resistance.

Eyeblink conditioning contingent on hippocampal theta enhances hippocampal and medial prefrontal responses.

Trace eyeblink classical conditioning (tEBCC) can be accelerated by making training trials contingent on the naturally generated hippocampal 3- to 7-Hz theta rhythm. However, it is not well-understood how the presence (or absence) of theta affects stimulus-driven changes within the hippocampus and how it correlates with patterns of neural activity in other essential trace conditioning structures, such as the medial prefrontal cortex (mPFC). In the present study, a brain-computer interface delivered paired or unpaired conditioning trials to rabbits during the explicit presence (T(+)) or absence (T(-)) of theta, yielding significantly faster behavioral learning in the T(+)-paired group. The stimulus-elicited hippocampal unit responses were larger and more rhythmic in the T(+)-paired group. This facilitation of unit responses was complemented by differences in the hippocampal local field potentials (LFP), with the T(+)-paired group demonstrating more coherent stimulus-evoked theta than T(-)-paired animals and both unpaired groups. mPFC unit responses in the rapid learning T(+)-paired group displayed a clear inhibitory/excitatory sequential pattern of response to the tone that was not seen in any other group. Furthermore, sustained mPFC unit excitation continued through the trace interval in T(+) animals but not in T(-) animals. Thus theta-contingent training is accompanied by 1) acceleration in behavioral learning, 2) enhancement of the hippocampal unit and LFP responses, and 3) enhancement of mPFC unit responses. Together, these data provide evidence that pretrial hippocampal state is related to enhanced neural activity in critical structures of the distributed network supporting the acquisition of tEBCC.

Impaired motor functions in mice lacking the RNA-binding protein Hzf.

Local protein synthesis in dendrites plays an important role in some aspects of neuronal development and synaptic plasticity. Neuronal RNA-binding proteins regulate the transport and/or translation of the localized mRNAs. Previously, we reported that hematopoietic zinc finger (Hzf) is one of the neuronal RNA-binding proteins that regulate these processes. The Hzf protein is highly expressed in neuronal cells including hippocampal pyramidal neurons and cerebellar Purkinje cells, and plays essential roles in the dendritic mRNA localization and translation. In the present study we demonstrated that mice lacking Hzf (Hzf(-/-) mice) exhibited severe impairments of motor coordination and cerebellum-dependent motor learning. These findings raise the possibility that the post-transcriptional regulation by Hzf may contribute to some aspects of synaptic plasticity and motor learning in the cerebellum.

Characterization of hippocampal theta rhythm in wild-type mice and glutamate receptor subunit delta2 mutant mice during eyeblink conditioning with a short trace interval.

We have shown that glutamate receptor subunit delta2 (GluRdelta2) null mutant mice, which have serious morphological and functional deficiencies in the cerebellar cortex, are severely impaired in delay eyeblink conditioning but not in trace eyeblink conditioning, even with a 0-trace interval. Such 0-trace conditioning does not depend critically on the hippocampus in wild-type mice, but it does in GluRdelta2 mutant mice. Here we examined the hippocampal electroencephalogram (EEG) during 0-trace conditioning in GluRdelta2 mutant and wild-type mice. During the apparatus habituation sessions, the total hippocampal theta activity (4-12 Hz) of GluRdelta2 mutant mice was less than that of wild-type mice. Activity in the higher frequency band (8-12 Hz, type 1) in GluRdelta2 mutant mice was significantly less than it was in wild-type mice, but activity in the lower frequency band (4-8 Hz, type 2) was not. As learning proceeded during the acquisition sessions, the total theta activity decreased in many of the wild-type mice, while this phenomenon was less prominent in GluRdelta2 mutant mice. Further analysis showed that the type 1 activity in wild-type mice increased in the early sessions and then decreased, while that in GluRdelta2 mutant mice did not change. Type 2 activity tended to decrease in both types of mice as the conditioning proceeded. These results indicate that the distribution of hippocampal EEG frequency and its properties during conditioning are different between wild-type and GluRdelta2 mutant mice, suggesting that the cerebellar cortical dysfunction may cause an alteration in the electrophysiological characteristics of the hippocampus.

Role of hippocampal NMDA receptors in trace eyeblink conditioning.

We examined the effects of acute injections of competitive N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) into the dorsal hippocampus on contextual fear conditioning and classical eyeblink conditioning in C57BL/6 mice. When injected 10 to 40 min before training, APV severely impaired contextual fear conditioning. Thus, APV injection under these conditions was sufficient to suppress hippocampal NMDA receptors. To investigate the role of hippocampal NMDA receptors on eyeblink conditioning, we carried out daily training of mice during 10-40 min after injection of APV. In the delay eyeblink conditioning, in which the unconditioned stimulus (US) is delayed and terminates simultaneously with the conditioned stimulus (CS), APV-injected mice acquired the conditioned responses (CRs) as well as artificial cerebrospinal fluid (aCSF)-injected control mice did. However, in the trace eyeblink conditioning, in which the CS and US were separated by a stimulus-free trace interval of 500 ms, APV-injected mice showed severe impairment in acquisition of the CR. There was no significant difference in pseudo-conditioning between APV- and aCSF-injected mice. These results provide evidence that the NMDA receptor in the dorsal hippocampus is critically involved in acquisition of the CR in long trace eyeblink conditioning.

The hippocampus plays an important role in eyeblink conditioning with a short trace interval in glutamate receptor subunit delta 2 mutant mice.

Mutant mice lacking the glutamate receptor subunit delta2 exhibit changes in the structure and function of the cerebellar cortex. The most prominent functional feature is a deficiency in the long-term depression (LTD) at parallel fiber-Purkinje cell synapses. These mutant mice exhibit severe impairment during delay eyeblink conditioning but learn normally during trace eyeblink conditioning without the cerebellar LTD, even with a 0 trace interval. We investigated the hippocampal contribution to this cerebellar LTD-independent "0 trace interval" learning. The mutant mice whose dorsal hippocampi were aspirated exhibited severe impairment in learning, whereas those that received post-training hippocampal lesions retained the memory. The wild-type mice showed no impairment in either case. These results suggest that the hippocampal component of the eyeblink conditioning task becomes dominant when cerebellar LTD is impaired.

Time-limited role of the hippocampus in the memory for trace eyeblink conditioning in mice.

We examined the role of the hippocampus in memory retention after trace eyeblink conditioning in mice. After establishing the conditioned response (CR) in the trace paradigm, mice received a bilateral aspiration of the dorsal hippocampus and its overlying neocortex on the next day (1-day group) or after 4 weeks (4-week group). Control mice received a neocortical aspiration on the same schedule as the hippocampal-lesion group. After 2 weeks of recovery, these groups received additional conditioning for 3 days. Frequency of the CR of the 1-day group was as low as spontaneous values on the first day in the post-lesion session and never reached pre-surgical level during the post-lesion sessions, while that of the control group did reach pre-surgical level during the post-lesion sessions although there was a transient decline just after lesion. In contrast to the 1-day group, the 4-week-hippocampal lesion group retained the CR and showed a further increase, without significant difference from the control group. The temporal pattern of the CR also was unchanged by the hippocampal lesion 4 weeks after learning. These results suggest a time-limited role for the hippocampus in memory retention after trace conditioning in mice: the CR acquired recently requires an intact hippocampus for its retention, but the CR acquired remotely does not. This is similar to the result reported in rabbits. Therefore, the mechanism and time course of memory consolidation after trace eyeblink conditioning may be similar in mice and rabbits.

Scopolamine impairs eyeblink conditioning in cerebellar LTD-deficient mice.

We investigated the effect of the muscarinic acetylcholine receptor (mAChR) antagonist scopolamine on eyeblink conditioning in glutamate receptor subunit 62 null mice, which have severe impairments in cerebellar long-term depression (LTD). Mice were injected intraperitoneally with scopolamine (0.5 mg/kg) or saline, and conditioned using a delay paradigm with tone and periorbital shock but with no overlap between them. The saline-injected mutant mice learned this paradigm normally, as predicted from our previous study. When scopolamine was injected, learning was impaired more severely in the mutant mice than in the wild type mice. Basic sensory and motor performances were not affected. These results suggest that eyeblink conditioning in cerebellar LTD deficient mice depends largely on neural functions susceptible to blocking of mAChRs.