CNTNAP2 is a direct FoxP2 target in vitro and in vivo in zebra finches: complex regulation by age and activity

Mutations of FOXP2 are associated with altered brain structure, including the striatal part of the basal ganglia, and cause a severe speech and language disorder. Songbirds serve as a tractable neurobiological model for speech and language research. Experimental downregulation of FoxP2 in zebra finch Area X, a nucleus of the striatal song control circuitry, affects synaptic transmission and spine densities. It also renders song learning and production inaccurate and imprecise, similar to the speech impairment of patients carrying FOXP2 mutations. Here we show that experimental downregulation of FoxP2 in Area X using lentiviral vectors leads to reduced expression of CNTNAP2, a FOXP2 target gene in humans. In addition, natural downregulation of FoxP2 by age or by singing also downregulated CNTNAP2 expression. Furthermore, we report that FoxP2 binds to and activates the avian CNTNAP2 promoter in vitro. Taken together these data establish CNTNAP2 as a direct FoxP2 target gene in songbirds, likely affecting synaptic function relevant for song learning and song maintenance.     

An evolutionary perspective on FoxP2: strictly for the birds?

FoxP2 mutations in humans are associated with a disorder that affects both the comprehension of language and its production, speech. This discovery provided the first opportunity to analyze the genetics of language with molecular and neurobiological tools. The amino acid sequence and the neural expression pattern of FoxP2 are extremely conserved, from reptile to man. This suggests an important role for FoxP2 in vertebrate brains, regardless of whether they support imitative vocal learning or not. Its expression pattern pinpoints neural circuits that might have been crucial for the evolution of speech and language, including the basal ganglia and the cerebellum. Recent studies in songbirds show that during times of song plasticity FoxP2 is upregulated in a striatal region essential for song learning. This suggests that FoxP2 plays important roles both in the development of neural circuits and in the postnatal behaviors they mediate.     

Molecular microcircuitry underlies functional specification in a basal ganglia circuit dedicated to vocal learning

Similarities between speech and birdsong make songbirds advantageous for investigating the neurogenetics of learned vocal communication--a complex phenotype probably supported by ensembles of interacting genes in cortico-basal ganglia pathways of both species. To date, only FoxP2 has been identified as critical to both speech and birdsong. We performed weighted gene coexpression network analysis on microarray data from singing zebra finches to discover gene ensembles regulated during vocal behavior. We found ～2,000 singing-regulated genes comprising three coexpression groups unique to area X, the basal ganglia subregion dedicated to learned vocalizations. These contained known targets of human FOXP2 and potential avian targets. We validated biological pathways not previously implicated in vocalization. Higher-order gene coexpression patterns, rather than expression levels, molecularly distinguish area X from the ventral striato-pallidum during singing. The previously unknown structure of singing-driven networks enables prioritization of molecular interactors that probably bear on human motor disorders, especially those affecting speech.     

Song Practice Promotes Acute Vocal Variability at a Key Stage of Sensorimotor Learning

Background

Trial by trial variability during motor learning is a feature encoded by the basal ganglia of both humans and songbirds, and is important for reinforcement of optimal motor patterns, including those that produce speech and birdsong. Given the many parallels between these behaviors, songbirds provide a useful model to investigate neural mechanisms underlying vocal learning. In juvenile and adult male zebra finches, endogenous levels of FoxP2, a molecule critical for language, decrease two hours after morning song onset within area X, part of the basal ganglia-forebrain pathway dedicated to song. In juveniles, experimental ‘knockdown’ of area X FoxP2 results in abnormally variable song in adulthood. These findings motivated our hypothesis that low FoxP2 levels increase vocal variability, enabling vocal motor exploration in normal birds.

Methodology/Principal Findings

After two hours in either singing or non-singing conditions (previously shown to produce differential area X FoxP2 levels), phonological and sequential features of the subsequent songs were compared across conditions in the same bird. In line with our prediction, analysis of songs sung by 75 day (75d) birds revealed that syllable structure was more variable and sequence stereotypy was reduced following two hours of continuous practice compared to these features following two hours of non-singing. Similar trends in song were observed in these birds at 65d, despite higher overall within-condition variability at this age.

Conclusions/Significance

Together with previous work, these findings point to the importance of behaviorally-driven acute periods during song learning that allow for both refinement and reinforcement of motor patterns. Future work is aimed at testing the observation that not only does vocal practice influence expression of molecular networks, but that these networks then influence subsequent variability in these skills.     

Common features of neural activity during singing and sleep periods in a basal ganglia nucleus critical for vocal learning in a juvenile songbird

Reactivations of waking experiences during sleep have been considered fundamental neural processes for memory consolidation. In songbirds, evidence suggests the importance of sleep-related neuronal activity in song system motor pathway nuclei for both juvenile vocal learning and maintenance of adult song. Like those in singing motor nuclei, neurons in the basal ganglia nucleus Area X, part of the basal ganglia-thalamocortical circuit essential for vocal plasticity, exhibit singing-related activity. It is unclear, however, whether Area X neurons show any distinctive spiking activity during sleep similar to that during singing. Here we demonstrate that, during sleep, Area X pallidal neurons exhibit phasic spiking activity, which shares some firing properties with activity during singing. Shorter interspike intervals that almost exclusively occurred during singing in awake periods were also observed during sleep. The level of firing variability was consistently higher during singing and sleep than during awake non-singing states. Moreover, deceleration of firing rate, which is considered to be an important firing property for transmitting signals from Area X to the thalamic nucleus DLM, was observed mainly during sleep as well as during singing. These results suggest that songbird basal ganglia circuitry may be involved in the off-line processing potentially critical for vocal learning during sensorimotor learning phase.     

鸣禽前脑X区结构功能的研究进展

鸣禽鸣唱与人类说话一样,都是在教习和听觉反馈下形成的感知运动学习过程。鸣禽鸣唱的发育和成熟巩固依赖于发声通路和前端脑通路组成的鸣唱系统的完整。前端脑通路中的X区在鸣唱学习记忆中扮演着重要角色。本文就X区的形态组织结构、在鸣唱发育与成熟巩固中的作用、突触可塑性的研究进展进行了综述,并且将X区与哺乳动物基底神经节的学习记忆功能做了比较。     

Twitter evolution: converging mechanisms in birdsong and human speech

Vocal imitation in human infants and in some orders of birds relies on auditory-guided motor learning during a sensitive period of development. It proceeds from 'babbling' (in humans) and 'subsong' (in birds) through distinct phases towards the full-fledged communication system. Language development and birdsong learning have parallels at the behavioural, neural and genetic levels. Different orders of birds have evolved networks of brain regions for song learning and production that have a surprisingly similar gross anatomy, with analogies to human cortical regions and basal ganglia. Comparisons between different songbird species and humans point towards both general and species-specific principles of vocal learning and have identified common neural and molecular substrates, including the forkhead box P2 (FOXP2) gene.     

鸣禽发声器官在鸣啭过程中的功能

鸣禽的鸣啭是一种习得行为,与人类的学习过程较为相似.因此鸣禽作为一种动物模型在研究人类学习记忆方面得到广泛的应用.鸣管和鸣肌是鸣禽鸣啭的主要器官,对鸣啭过程起着复杂的调节作用.此外,不同的鸣禽在鸣啭时,其发声器官具有不同的侧别优势.对近年在鸣禽发声器官功能方面的研究进行综述.     

FoxP2 in song-learning birds and vocal-learning mammals

FoxP2 is the first identified gene that is specifically involved in speech and language development in humans. Population genetic studies of FoxP2 revealed a selective sweep in recent human history associated with two amino acid substitutions in exon 7. Avian song learning and human language acquisition share many behavioral and neurological similarities. To determine whether FoxP2 plays a similar role in song-learning birds, we sequenced exon 7 of FoxP2 in multiple song-learning and nonlearning birds. We show extreme conservation of FoxP2 sequences in birds, including unusually low rates of synonymous substitutions. However, no amino acid substitutions are shared between the song-learning birds and humans. Furthermore, sequences from vocal-learning whales, dolphins, and bats do not share the human-unique substitutions. While FoxP2 appears to be under strong functional constraints in mammals and birds, we find no evidence for its role during the evolution of vocal learning in nonhuman animals as in humans.     

Timing and prediction the code from basal ganglia to thalamus

When is an inhibitory synapse not inhibitory? In this issue of Neuron, Person and Perkel demonstrate that thalamic neurons can translate extrinsic GABAergic input from the basal ganglia into highly precise patterns of sustained spiking in a circuit that is essential for vocal learning in songbirds. Postinhibitory rebound serves as a mechanism that preserves precise spike timing information, enabling reliable propagation of activity throughout this pathway. The results have broad implications for basic mechanisms of functional processing in both thalamus and basal ganglia and serve to increase our understanding of how acoustic units of vocal sounds are transformed into motor gestures during the sensitive period for song learning.     

Representation of Early Sensory Experience in the Adult Auditory Midbrain: Implications for Vocal Learning

Vocal learning in songbirds and humans occurs by imitation of adult vocalizations. In both groups, vocal learning includes a perceptual phase during which juveniles birds and infants memorize adult vocalizations. Despite intensive research, the neural mechanisms supporting this auditory memory are still poorly understood. The present functional MRI study demonstrates that in adult zebra finches, the right auditory midbrain nucleus responds selectively to the copied vocalizations. The selective signal is distinct from selectivity for the bird''s own song and does not simply reflect acoustic differences between the stimuli. Furthermore, the amplitude of the selective signal is positively correlated with the strength of vocal learning, measured by the amount of song that experimental birds copied from the adult model. These results indicate that early sensory experience can generate a long-lasting memory trace in the auditory midbrain of songbirds that may support song learning.     

Auditory feedback in learning and maintenance of vocal behaviour

Songbirds are one of the best-studied examples of vocal learners. Learning of both human speech and birdsong depends on hearing. Once learned, adult song in many species remains unchanging, suggesting a reduced influence of sensory experience. Recent studies have revealed, however, that adult song is not always stable, extending our understanding of the mechanisms involved in song maintenance, and their similarity to those active during song learning. Here we review some of the processes that contribute to song learning and production, with an emphasis on the role of auditory feedback. We then consider some of the possible neural substrates involved in these processes, particularly basal ganglia circuitry. Although a thorough treatment of human speech is beyond the scope of this article, we point out similarities between speech and song learning, and ways in which studies of these disparate behaviours complement each other in developing an understanding of general principles that contribute to learning and maintenance of vocal behaviour.     

Variable Food Begging Calls Are Harbingers of Vocal Learning

Vocal learning has evolved in only a few groups of mammals and birds. The developmental and evolutionary origins of vocal learning remain unclear. The imitation of a memorized sound is a clear example of vocal learning, but is that when vocal learning starts? Here we use an ontogenetic approach to examine how vocal learning emerges in a songbird, the chipping sparrow. The first vocalizations of songbirds, food begging calls, were thought to be innate, and vocal learning emerges later during subsong, a behavior reminiscent of infant babbling. Here we report that the food begging calls of male sparrows show several characteristics associated with learned song: male begging calls are highly variable between individuals and are altered by deafening; the production of food begging calls induces c-fos expression in a forebrain motor nucleus, RA, that is involved with the production of learned song. Electrolytic lesions of RA significantly reduce the variability of male calls. The male begging calls are subsequently incorporated into subsong, which in turn transitions into recognizable attempts at vocal imitation. Females do not sing and their begging calls are not affected by deafening or RA lesion. Our results suggest that, in chipping sparrows, intact hearing can influence the quality of male begging calls, auditory-sensitive vocal variability during food begging calls is the first step in a modification of vocal output that eventually culminates with vocal imitation.     

Song tutoring triggers CaMKII phosphorylation within a specialized portion of the avian basal ganglia

In several songbird species, a specialized anterior forebrain pathway (AFP) that includes part of the avian basal ganglia has been implicated specifically in song learning. To further elucidate cellular mechanisms and circuitry involved in vocal learning, we used quantitative immunoblot analysis to determine if early song tutoring promotes within the AFP phosphorylation of calcium/calmodulin-dependent kinase II (CaMKII), a multifunctional kinase whose phosphorylation at threonine 286 is critical for many forms of neural plasticity and behavioral learning. We report that in young male zebra finches likely to have begun the process of song acquisition, brief tutoring by a familiar conspecific adult promotes a dramatic increase in levels of phosphorylated CaMKII (pCaMKII) in Area X, the striatal/pallidal component of the AFP. In contrast, pCaMKII levels in this region were not elevated if 1) the tutor did not sing, 2) the tutor sang but was visually isolated from the pupil, or 3) the tutor was an unfamiliar adult. In young males that had not previously heard any conspecific song, first exposure to a song tutor produced a more modest, but significant rise in pCaMKII levels. Young females (who do not develop song behavior) did not exhibit any effect of tutoring on pCaMKII levels in that portion of the basal ganglia that corresponds to Area X in males. These data are consistent with the hypothesis that Area X participates in encoding and/or attaching reward value to a representation of tutor song that is accessed later to guide motor learning.