Target-dependent expression of the netrin-1 receptor, UNC5C, in projection neurons of the ventral tegmental area

UNC5C: Novel Gene Associated with Psychiatric Disorders Impacts Dysregulation of Axon Guidance Pathways

The UNC-5 family of netrin receptor genes, predominantly expressed in brain tissues, plays a pivotal role in various neuronal processes. Mutations in genes involved in axon development contribute to a wide spectrum of human diseases, including developmental, neuropsychiatric, and neurodegenerative disorders. The NTN1/DCC signaling pathway, interacting with UNC5C, plays a crucial role in central nervous system axon guidance and has been associated with psychiatric disorders during adolescence in humans. Whole-exome sequencing analysis unveiled two compound heterozygous causative mutations within the UNC5C gene in a patient diagnosed with psychiatric disorders. In silico analysis demonstrated that neither of the observed variants affected the allosteric linkage between UNC5C and NTN1. In fact, these mutations are located within crucial cytoplasmic domains, specifically ZU5 and the region required for the netrin-mediated axon repulsion of neuronal growth cones. These domains play a critical role in forming the supramodular protein structure and directly interact with microtubules, thereby ensuring the functionality of the axon repulsion process. We emphasize that these mutations disrupt the aforementioned processes, thereby associating the UNC5C gene with psychiatric disorders for the first time and expanding the number of genes related to psychiatric disorders. Further research is required to validate the correlation of the UNC5C gene with psychiatric disorders, but we suggest including it in the genetic analysis of patients with psychiatric disorders.

DCC Receptors Drive Prefrontal Cortex Maturation by Determining Dopamine Axon Targeting in Adolescence

BACKGROUND

Dopaminergic input to the prefrontal cortex (PFC) increases throughout adolescence and, by establishing precisely localized synapses, calibrates cognitive function. However, why and how mesocortical dopamine axon density increases across adolescence remains unknown.

METHODS

We used a developmental application of axon-initiated recombination to label and track the growth of dopamine axons across adolescence in mice. We then paired this recombination with cell-specific knockdown of the netrin-1 receptor DCC to determine its role in adolescent dopamine axon growth. We then assessed how altering adolescent PFC dopamine axon growth changes the structural and functional development of the PFC by quantifying pyramidal neuron morphology and cognitive performance.

RESULTS

We show, for the first time, that dopamine axons continue to grow from the striatum to the PFC during adolescence. Importantly, we discover that DCC, a guidance cue receptor, controls the extent of this protracted growth by determining where and when dopamine axons recognize their final target. When DCC-dependent adolescent targeting events are disrupted, dopamine axons continue to grow ectopically from the nucleus accumbens to the PFC and profoundly change PFC structural and functional development. This leads to alterations in cognitive processes known to be impaired across psychiatric conditions.

CONCLUSIONS

The prolonged growth of dopamine axons represents an extraordinary period for experience to influence their adolescent trajectory and predispose to or protect against psychopathology. DCC receptor signaling in dopamine neurons is a molecular link where genetic and environmental factors may interact in adolescence to influence the development and function of the prefrontal cortex.

Dopamine Development in the Mouse Orbital Prefrontal Cortex Is Protracted and Sensitive to Amphetamine in Adolescence

The prefrontal cortex (PFC) is divided into subregions, including the medial and orbital prefrontal cortices. Dopamine connectivity in the medial PFC (mPFC) continues to be established throughout adolescence as the result of the continuous growth of axons that innervated the nucleus accumbens (NAcc) prior to adolescence. During this period, dopamine axons remain vulnerable to environmental influences, such as drugs used recreationally by humans. The developmental trajectory of the orbital prefrontal dopamine innervation remains almost completely unstudied. Nonetheless, the orbital PFC (oPFC) is critical for some of the most complex functions of the PFC and is disrupted by drugs of abuse, both in adolescent humans and rodents. Here, we use quantitative neuroanatomy, axon-initiated viral-vector recombination, and pharmacology in mice to determine the spatiotemporal development of the dopamine innervation to the oPFC and its vulnerability to amphetamine in adolescence. We find that dopamine innervation to the oPFC also continues to increase during adolescence and that this increase is due to the growth of new dopamine axons to this region. Furthermore, amphetamine in adolescence dramatically reduces the number of presynaptic sites on oPFC dopamine axons. In contrast, dopamine innervation to the piriform cortex is not protracted across adolescence and is not impacted by amphetamine exposure during adolescence, indicating that dopamine development during adolescence is a uniquely prefrontal phenomenon. This renders these fibers, and the PFC in general, particularly vulnerable to environmental risk factors during adolescence, such as recreational drug use.

Mesocortical Dopamine Phenotypes in Mice Lacking the Sonic Hedgehog Receptor Cdon

Motivated behaviors and many psychopathologies typically involve changes in dopamine release from the projections of the ventral tegmental area (VTA) and/or the substantia nigra pars compacta (SNc). The morphogen Sonic Hedgehog (Shh) specifies fates of midbrain dopamine neurons, but VTA-specific effects of Shh signaling are also being uncovered. In this study, we assessed the role of the Shh receptor Cdon in the development of VTA and SNc dopamine neurons. We find that Cdon is expressed in the proliferating progenitor zone of the embryonic ventral midbrain and that the number of proliferating cells in this region is increased in mouse Cdon(-/-) embryos. Consistent with a role of Shh in the regulation of neuronal proliferation in this region, we find that the number of tyrosine hydroxylase (TH)-positive neurons is increased in the VTA of Cdon(-/-) mice at birth and that this effect endures into adulthood. In contrast, the number of TH-positive neurons in the SNc is not altered in Cdon(-/-) mice at either age. Moreover, adult Cdon(-/-) mice have a greater number of medial prefrontal cortex (mPFC) dopamine presynaptic sites, and increased baseline concentrations of dopamine and dopamine metabolites selectively in this region. Finally, consistent with increased dopamine function in the mPFC, we find that adult Cdon(-/-) mice fail to exhibit behavioral plasticity upon repeated amphetamine treatment. Based on these data, we suggest that Cdon plays an important role encoding the diversity of dopamine neurons in the midbrain, influencing both the development of the mesocortical dopamine pathway and behavioral outputs that involve this neural circuitry.

Resilience to amphetamine in mouse models of netrin-1 haploinsufficiency: role of mesocortical dopamine

RATIONALE

Signaling through the netrin-1 receptor, deleted in colorectal cancer (DCC), in dopamine neurons controls the extent of their innervation to the medial prefrontal cortex (mPFC) during adolescence. In mice, dcc haploinsufficiency results in increased mPFC dopamine innervation and concentrations in adulthood. In turn, dcc haploinsufficiency leads to resilience to the effects of stimulant drugs of abuse on dopamine release in the nucleus accumbens and behavior.

OBJECTIVES

First, we set out to determine whether increased mPFC dopamine innervation causes blunted behavioral responses to amphetamine in adult dcc haploinsufficient mice. Second, we investigated whether unc5c, another netrin-1 receptor expressed by dopamine neurons, is involved in these effects. Third, we assessed whether haploinsufficiency of netrin-1 itself leads to blunted behavioral responding to amphetamine, whether this phenotype emerges before or after adolescence and whether increased mPFC dopamine input is the underlying mechanism.

RESULTS

Adult, but not adolescent, dcc, unc5c and netrin-1 haploinsufficient mice exhibit blunted behavioral responses to amphetamine. Furthermore, adult dcc, unc5c, and netrin-1 haploinsufficient mice have exaggerated mPFC dopamine concentrations in comparison to their wild-type littermates. Importantly, resilience to amphetamine-induced behavioral activation in all the three mouse models is abolished by selective dopamine depletion in the medial prefrontal cortex.

CONCLUSIONS

dcc, unc5c, or netrin-1 haploinsufficiency leads to increased dopamine content in the mPFC and to resilience against amphetamine-induced behavioral activation. Our findings raise the hypothesis that DCC, UNC5C, and netrin-1 act in concert to organize the adolescent development of mesocortical dopamine innervation and, in turn, determine behavioral responses to drugs of abuse.

The substantia nigra conveys target-dependent excitatory and inhibitory outputs from the basal ganglia to the thalamus

The basal ganglia (BG), which influence cortical activity via the thalamus, play a major role in motor activity, learning and memory, sensory processing, and many aspects of behavior. The substantia nigra (SN) consists of GABAergic neurons of the pars reticulata that inhibit thalamic neurons and provide the primary output of the BG, and dopaminergic neurons of the pars compacta that modulate thalamic excitability. Little is known about the functional properties of the SN→thalamus synapses, and anatomical characterization has been controversial. Here we use a combination of anatomical, electrophysiological, genetic, and optogenetic approaches to re-examine these synaptic connections in mice. We find that neurons in the SN inhibit neurons in the ventroposterolateral nucleus of the thalamus via GABAergic synapses, excite neurons in the thalamic nucleus reticularis, and both excite and inhibit neurons within the posterior nucleus group. Glutamatergic SN neurons express the vesicular glutamate receptor transporter vGluT2 and receive inhibitory synapses from striatal neurons, and many also express tyrosine hydroxylase, a marker of dopaminergic neurons. Thus, in addition to providing inhibitory outputs, which is consistent with the canonical circuit, the SN provides glutamatergic outputs that differentially target thalamic nuclei. This suggests that an increase in the activity of glutamatergic neurons in the SN allows the BG to directly excite neurons in specific thalamic nuclei. Elucidating an excitatory connection between the BG and the thalamus provides new insights into how the BG regulate thalamic activity, and has important implications for understanding BG function in health and disease.