Discussion
In the present study, we examined the function of ADAM11 by generating mice that lacked the Adam11 gene. It has been reported that the Adam11 gene is expressed in the brain, heart, liver and testis, as manifested in Northern blot analysis [19]. Interestingly, ADAM11-deficient mice appeared normal and survived more than one year without apparent histological abnormality, including the brain, heart, liver and testis. The expression patterns of the Adam11 gene in the brain have been studied using in situ hybridisation [25]. This study has shown high expression of ADAM11 mRNA in the hippocampus and cerebellum. We therefore focused on examining the hippocampus- and cerebellum-dependent behavioural tests to study the functions of ADAM11 in the nervous system.
It is widely accepted that the hippocampus is involved in the performance of the hidden platform version of the water maze test, since lesions of the hippocampal pathways lead to a severe impairment of learning in this task [29]. In the hidden platform task, ADAM11-deficient mice needed more time to reach the platform than wild-type mice. Spatial learning was also assessed using a probe trial, in which the platform was removed from the pool on the day following nine days of training. The wild-type mice spent significantly more time searching for the platform in the target quadrant than the other quadrants. However, ADAM11-deficient mice did not search selectively in the target quadrant. On the other hand, in the visible platform version, in which the motor ability and visual acuity of the mouse are tested, ADAM11-defieient mice were normal. These results indicated that ADAM11-deficient mice showed a spatial learning impairment.
The cerebellum is in charge of the smooth coordination of somatic motor activity, regulation of muscle tone, and mechanisms that influence and maintain equilibrium [30]. It is widely accepted that the cerebellum is involved in the performance of the rotating rod task, since mice with structural abnormalities in the cerebellum [31] or with disruptions in genes enriched in the cerebellum [32] exhibit performance deficits in this task. In the rotating rod test in this study, although the retention time of ADAM11-deficient mice on the rod was not different from wild-type mice at 0 rpm (stationary), ADAM11-deficient mice fell more quickly from the rod than wild-type mice at 5, 10 and 15 rpm. These results indicate that ADAM11-deficient mice have a deficit in motor coordination. On the other hand, in the visual platform version of the water maze task, motor ability for swimming in ADAM11-deficient mice was normal. These results suggested that there might be a different mechanism of motor function between swimming and the rotating rod task. It has been also reported that the cerebellum is involved in another important distinct function: learning associated with component movement (motor learning) [33]. The retention time of synaptotagmin (Syt) IV-deficient mice on the rod was significantly shorter than wild-type mice, and they improved it approximately to the same rates as did wild-type mice during rotating rod performance [34]. These data suggest the basis for the performance deficit on the rotating rod task in the Syt IV mutants is in motor coordination rather than in motor learning. The results in the present study showed that ADAM11-deficient mice slightly, though not significantly, improved motor learning ability during rotating rod performance. However, the deficit of motor learning in ADAM11-deficient mice was not clear in this study. Because they could not ride on the rotating rod, they might not learn this task. To examine motor learning ability in ADAM11-deficient mice in detail, we will need to use other cerebellar-dependent learning tasks, such as a classical eyelid conditioning test. There was no impairment in the grip strength and wire suspension test in ADAM11-deficient mice. Spontaneous motor activity and walking patterns of ADAM11-deficient mice were also found to be normal, suggesting that the dysfunction found in the rotating rod test was not derived from muscle weakness or other peripheral disturbances.
These results suggest that limited brain sites, mainly the hippocampus and cerebellum, might contribute to the dominant phenotype, spatial learning impairment and motor discoordination in ADAM11-deficient mice. In situ hybridisation analysis has detected Adam11 gene expression in the pyramidal cells of CA1–CA3 fields and granule cells of the dentate gyrus in the hippocampus and in granular cells in the cerebellum [25]. It is unlikely that morphological changes during development led to impairment of spatial learning and motor coordination, and morphological alterations in the cytoarchitecture of the hippocampus and cerebellum were not observed in ADAM11-deficient mice after histological investigation using HE staining. However, because it is thought that ADAM11 plays a role in neuron-neuron or neuron-glial cell interactions, a more precise morphological investigation will be needed. It has been reported that adhesion molecules, such as neural cell adhesion molecule (NCAM), are not only implicated in cell interactions during nervous system development, but are also recognised as important mediators of synaptic plasticity in the adult nervous system [35]. Changes in neuron excitability in the hippocampus, such as decreases in the post-burst after hyperpolarization (AHP), have been thought to affect long-term potentiation (LTP) [36], an experimental model of the synaptic changes thought to underlie learning and memory [37]. It has also been reported that mice lacking NMDA receptor for both NR2A and NR2C subunits showed motor dysfunction and complete loss of both spontaneous and evoked excitatory postsynaptic currents (EPSCs) in cerebellar granule cells [38]. Therefore, electrophyological studies to enable analysis of synaptic transmission and plasticity in the hippocampus and cerebellum of ADAM11-deficient mice will be needed. These studies might clarify whether ADAM11 plays an important signalling or structural role at the synaptic level and whether it participates in synaptic regulation.
ADAM11-deficient mice remained alive for more than one year without ataxia or tremor, but showed a deficit in spatial learning and motor dysfunction. On the other hand, mice lacking the Adam22 gene showed severe ataxia, exhibited marked hypomyelination of the peripheral nerves, and died before weaning [22]. Mice lacking the Adam23 gene showed severe ataxia and tremor and died before weaning [23]. These findings suggest that ADAM11, ADAM22 and ADAM23 have separate and important roles in the nervous system. A genome database search revealed orthologs of ADAM11, ADAM22 and ADAM23 genes to exist in vertebrates such as mammals, fish, and amphibians, but not in invertebrates. Although the precise molecular functions of these ADAMs are still unclear, further investigations will provide clues to understanding the nervous system of higher organisms.