We have identified a critical role for the AS- and autism-linked ubiquitin ligase Ube3a in experience-dependent neocortical development. We have found that visual experience fails to strengthen functional connectivity of excitatory neurons in the visual cortex of Ube3am−/p+ mice. The lack of experience-dependent development of excitatory circuits is likely caused by deficits in synaptic plasticity, because both LTP and LTD are severely attenuated in Ube3am−/p+ mice. Paradoxically, the plasticity deficit is itself driven by sensory experiences, as sensory deprivation prevents the loss of normal synaptic plasticity, while brief sensory experiences can reinstate the plasticity deficit. Therefore, Ube3a is necessary to maintain normal synaptic plasticity during ongoing activity-dependent remodeling. Finally, we have shown that Ube3am−/p+ mice lack the cortical plasticity that is normally induced by monocular deprivation in vivo, demonstrating a role for Ube3a in naturally occurring experience-dependent modifications of neuronal circuits. The neocortical plasticity deficits caused by the absence of Ube3a may thus underlie the learning deficits observed in Angelman syndrome.
Two possibilities could explain the plasticity deficits in Ube3am−/p+
mice. First, cooperative excitatory drive may be attenuated in Ube3a-deficient mice, thus increasing the induction parameters for LTD and LTP. This could arise from a decrease in the number of excitatory synapses or their release probability, both of which are consistent with the observed reduction in spine density and mEPSC frequency of layer 2/3 pyramidal neurons. The possibility that Ube3am−/p+
mice exhibit abnormal neurotransmitter release might be particularly relevant for the LTD deficit in layers 2/3, as recent evidence suggests that LTD induced at this synapse requires endocannabinoid-mediated reductions in presynaptic neurotransmitter release33
. Future studies are needed to determine whether presynaptic plasticity and/or release mechanisms are disrupted by the absence of Ube3a, and whether this might contribute to the observed plasticity deficits.
Alternatively, the machinery for plasticity induction or expression may itself be compromised in Ube3a-deficient mice, including changes in calcium entry, receptor trafficking, or pre/postsynaptic signaling molecules. Indeed, the absence of Ube3a impairs the function of calcium/calmodulin-dependent protein kinase II (CaMKII)8
, an enzyme which normally facilitates LTP induction34
both in the hippocampus35
and in the visual cortex27,36
. Thus, although not yet examined in the neocortex, reductions in CaMKII activity in Ube3am−/p+
mice might contribute to the observed deficits in neocortical LTP. Intriguingly, the cortical LTP deficit observed in CaMKII knockout mice becomes progressively more severe with development27
, raising the possibility that the age-dependent increase in the LTP deficits observed in Ube3a-deficient mice may be a consequence of an increasing requirement of CaMKII for LTP induction37
. Like LTP, impairments in LTD could also arise from changes in signaling molecules. One such candidate molecule is PP1, as its enzymatic activity is implicated in LTD induction38,39
and is attenuated in Ube3am−/p+
. If the plasticity induction mechanisms for LTD and LTP are indeed compromised in the neocortex in the absence of Ube3a, then this could feasibly contribute to the observed reductions in spine density.
The ubiquitin-proteasome system is necessary for both LTP40
, and our data demonstrate that Ube3a is one critical component of this system. However, the protein substrate(s) of Ube3a, whose degradation is required to maintain normal synaptic plasticity, remain unknown. Extensive GeneChip analyses of the murine visual cortex have revealed that visual experience affects the expression of many genes42,43
. If one of these visual experience-induced proteins is a substrate for Ube3a, this protein could be abnormally overexpressed in Ube3am−/p+
mice given visual experience. Furthermore, if the protein has an ability to suppress synaptic plasticity, experience-dependent accumulation of such a protein would attenuate LTD and LTP in the absence of Ube3a. An ideal substrate of Ube3a that could account directly for the experience-induced plasticity deficits, would thus be a protein that is upregulated by activity but, at high levels, suppresses subsequent synaptic remodeling.
We suggest that the plasticity deficits observed in vitro
may contribute to or underlie synaptic learning deficits associated with AS. In support of the idea that Ube3a is required for naturally occurring plasticity during a critical period of development, we have found that ocular dominance plasticity is absent in Ube3am−/p+
mice (). Because LTD and monocular deprivation-induced depression of the deprived eye response share a common pathway44
, visual experience-induced loss of LTD could be the direct cause of the lack of ocular dominance plasticity. Consistent with this idea, Ube3a-deficient mice lacked the loss of responsiveness from the deprived eye that is normally observed following brief monocular deprivation.
abnormalities in the visual system of human AS patients are consistent with the observed aberrant development of visual cortical circuits in Ube3am−/p+
mice. If the deficits in experience-dependent encoding in the visual cortex are generalizable to other areas of the brain, then these same changes in synaptic physiology may explain the observed deficiencies in learning and cognition in human AS patients. It is notable that LOVD restores plasticity in Ube3am−/p+
mice ( and Supplementary Fig. 4e, f
). The demonstration that the physiological substrates of synaptic plasticity remain intact raises the possibility that behavioral or pharmacological manipulations could improve brain function in patients with AS. Moreover, because it has been speculated that sensory experience-dependent brain development is abnormal in other neurodevelopmental disorders such as Rett syndrome, Fragile X syndrome, and autism7
, our experimental approach may help to elucidate roles of experience in these disorders.