Molecular signatures and cellular diversity during mouse habenula development

Single-Cell Transcriptomic Analysis of the Potential Mechanisms of Follicular Development in Stra8-Deficient Mice

Follicle development is a critical process in mammalian reproduction, with significant implications for ovarian reserve and fertility. Stra8 is a known key factor regulating the initiation of meiosis; however, oocyte-like cells still appear in Stra8-deficient mice. Nevertheless, the underlying mechanism remains unclear and requires further investigation. Therefore, we used single-cell RNA sequencing to construct a comprehensive transcriptional atlas of ovarian cells from both wild-type and Stra8-deficient mice at embryonic stages E14.5 and E16.5. With stringent quality control, we obtained a total of 14,755 single cells of six major cell types. A further fine-scale analysis of the germ cell clusters revealed notable heterogeneity between wild-type and Stra8-deficient mice. Compared to the wild-type mice, the deficiency in Stra8 led to the downregulation of meiosis-related genes (e.g., Pigp, Tex12, and Sycp3), and the upregulation of apoptosis-related genes (e.g., Fos, Jun, and Actb), thereby hindering the meiotic process. Notably, we observed that, following Stra8 deficiency, the expression levels of Sub1 and Stk31 remained elevated at this stage. Furthermore, an RNA interference analysis confirmed the potential role of these genes as regulatory factors in the formation of primordial follicle-like cells. Additionally, Stra8 deficiency disrupted the signaling between germ cells and pregranulosa cells that is mediated by Mdk-Sdc1, leading to the abnormal expression of the PI3K/AKT signaling pathway. Together, these results shed light on the molecular processes governing germ cell differentiation and folliculogenesis, emphasizing the complex role of Stra8 in ovarian function.

The disappointment centre of the brain gets exciting: a systematic review of habenula dysfunction in depression

BACKGROUND

The habenula is an epithalamic brain structure that acts as a neuroanatomical hub connecting the limbic forebrain to the major monoamine centres. Abnormal habenula activity is increasingly implicated in depression, with a surge in publications on this topic in the last 5 years. Direct activation of the habenula is sufficient to induce a depressive phenotype in rodents, suggesting a causative role in depression. However, the molecular basis of habenula dysfunction in depression remains elusive and it is unclear how the preclinical advancements translate to the clinical field.

METHODS

A systematic literature search was conducted following the PRISMA guidelines. The two search terms depress* and habenula* were applied across Scopus, Web of Science and PubMed databases. Studies eligible for inclusion must have examined the habenula in clinical cases of depression or preclinical models of depression and compared their measures to an appropriate control.

RESULTS

Preclinical studies (n = 63) measured markers of habenula activity (n = 16) and neuronal firing (n = 22), largely implicating habenula hyperactivity in depression. Neurotransmission was briefly explored (n = 15), suggesting imbalances within excitatory and inhibitory habenula signalling. Additional preclinical studies reported neuroconnectivity (n = 1), inflammatory (n = 3), genomic (n = 3) and circadian rhythm (n = 3) abnormalities. Seven preclinical studies (11%) included both males and females. From these, 5 studies (71%) reported a significant difference between the sexes in at least one habenula measure taken. Clinical studies (n = 24) reported abnormalities in habenula connectivity (n = 15), volume (n = 6) and molecular markers (n = 3). Clinical studies generally included male and female subjects (n = 16), however, few of these studies examined sex as a biological variable (n = 6).

CONCLUSIONS

Both preclinical and clinical evidence suggest the habenula is disrupted in depression. However, there are opportunities for sex-specific analyses across both areas. Preclinical evidence consistently suggests habenula hyperactivity as a primary driver for the development of depressive symptoms. Clinical studies support gross habenula abnormalities such as altered activation, connectivity, and volume, with emerging evidence of blood brain barrier dysfunction, however, progress is limited by a lack of detailed molecular analyses and limited imaging resolution.

Analysis of a shark reveals ancient, Wnt-dependent, habenular asymmetries in vertebrates

The mode of evolution of left-right asymmetries in the vertebrate habenulae remains largely unknown. Using a transcriptomic approach, we show that in a cartilaginous fish, the catshark Scyliorhinus canicula, habenulae exhibit marked asymmetries, in both their medial and lateral components. Comparisons across vertebrates suggest that those identified in lateral habenulae reflect an ancestral gnathostome trait, partially conserved in lampreys, and independently lost in tetrapods and neopterygians. Asymmetry formation involves distinct mechanisms in the catshark lateral and medial habenulae. Medial habenulae are submitted to a marked, asymmetric temporal regulation of neurogenesis, undetectable in their lateral counterparts. Conversely, asymmetry formation in lateral habenulae results from asymmetric choices of neuronal identity in post-mitotic progenitors, a regulation dependent on the repression of Wnt signaling by Nodal on the left. Based on comparisons with the mouse and the zebrafish, we propose that habenular asymmetry formation involves a recurrent developmental logic across vertebrates, which relies on conserved, temporally regulated genetic programs sequentially shaping choices of neuronal identity on both sides and asymmetrically modified by Wnt activity.

Molecular diversity and migration of GABAergic neurons in the developing ventral midbrain

Dopaminergic neurons in the ventral midbrain (mDA) are surrounded by GABAergic neurons. The full extent of GABAergic neuron subtypes occupying this region and the mechanisms that underlie their development and function are largely unknown. Therefore, we performed single-cell RNA sequencing (scRNA-seq) of fluorescence-activated cell sorting (FACS)-isolated GABAergic neurons in the developing mouse ventral midbrain. Several distinct GABAergic neuron subtypes were identified based on transcriptomic profiles and spatially assigned to the ventral midbrain using in situ hybridization and immunohistochemistry for specific markers. A subset of GABAergic clusters that co-expressed mDA markers was studied in more detail and showed distinctive molecular, functional, and wiring properties. Finally, migration of different GABAergic neuron subtypes required netrin-1 from different cellular sources acting via distinct receptor mechanisms. Overall, our work provides insight into the heterogeneity and spatial organization of GABAergic neurons in the developing ventral midbrain and begins to dissect the mechanisms that underlie their development.

The behavioral relevance of a modular organization in the lateral habenula

Behavioral strategies for survival rely on the updates the brain continuously makes based on the surrounding environment. External stimuli-neutral, positive, and negative-relay core information to the brain, where a complex anatomical network rapidly organizes actions, including approach or escape, and regulates emotions. Human neuroimaging and physiology in nonhuman primates, rodents, and teleosts suggest a pivotal role of the lateral habenula in translating external information into survival behaviors. Here, we review the literature describing how discrete habenular modules-reflecting the molecular signatures, anatomical connectivity, and functional components-are recruited by environmental stimuli and cooperate to prompt specific behavioral outcomes. We argue that integration of these findings in the context of valence processing for reinforcing or discouraging behaviors is necessary, offering a compelling model to guide future work.

Transcriptomic analysis of the human habenula in schizophrenia

Pathophysiology of many neuropsychiatric disorders, including schizophrenia (SCZD), is linked to habenula (Hb) function. While pharmacotherapies and deep brain stimulation targeting the Hb are emerging as promising therapeutic treatments, little is known about the cell type-specific transcriptomic organization of the human Hb or how it is altered in SCZD. Here we define the molecular neuroanatomy of the human Hb and identify transcriptomic changes in individuals with SCZD compared to neurotypical controls. Utilizing Hb-enriched postmortem human brain tissue, we performed single nucleus RNA-sequencing (snRNA-seq; n=7 neurotypical donors) and identified 17 molecularly defined Hb cell types across 16,437 nuclei, including 3 medial and 7 lateral Hb populations, several of which were conserved between rodents and humans. Single molecule fluorescent in situ hybridization (smFISH; n=3 neurotypical donors) validated snRNA-seq Hb cell types and mapped their spatial locations. Bulk RNA-sequencing and cell type deconvolution in Hb-enriched tissue from 35 individuals with SCZD and 33 neurotypical controls yielded 45 SCZD-associated differentially expressed genes (DEGs, FDR < 0.05), with 32 (71%) unique to Hb-enriched tissue. eQTL analysis identified 717 independent SNP-gene pairs (FDR < 0.05), where either the SNP is a SCZD risk variant (16 pairs) or the gene is a SCZD DEG (7 pairs). eQTL and SCZD risk colocalization analysis identified 16 colocalized genes. These results identify topographically organized cell types with distinct molecular signatures in the human Hb and demonstrate unique genetic changes associated with SCZD, thereby providing novel molecular insights into the role of Hb in neuropsychiatric disorders.

One Sentence Summary

Transcriptomic analysis of the human habenula and identification of molecular changes associated with schizophrenia risk and illness state.

The many enigmas of nicotine

This review discusses the diverse effects of nicotine on the various nicotinic acetylcholine receptors of the central and peripheral nervous system and how those effects may promote the usage and addiction to tobacco products.

Understanding the habenula: A major node in circuits regulating emotion and motivation

Over the last decade, the understanding of the habenula has rapidly advanced from being an understudied brain area with the Latin name 'habena" meaning "little rein", to being considered a "major rein" in the control of key monoaminergic brain centers. This ancient brain structure is a strategic node in the information flow from fronto-limbic brain areas to brainstem nuclei. As such, it plays a crucial role in regulating emotional, motivational, and cognitive behaviors and has been implicated in several neuropsychiatric disorders, including depression and addiction. This review will summarize recent findings on the medial (MHb) and lateral (LHb) habenula, their topographical projections, cell types, and functions. Additionally, we will discuss contemporary efforts that have uncovered novel molecular pathways and synaptic mechanisms with a focus on MHb-Interpeduncular nucleus (IPN) synapses. Finally, we will explore the potential interplay between the habenula's cholinergic and non-cholinergic components in coordinating related emotional and motivational behaviors, raising the possibility that these two pathways work together to provide balanced roles in reward prediction and aversion, rather than functioning independently.

Development, wiring and function of dopamine neuron subtypes

The midbrain dopamine (mDA) system is composed of molecularly and functionally distinct neuron subtypes that mediate specific behaviours and are linked to various brain diseases. Considerable progress has been made in identifying mDA neuron subtypes, and recent work has begun to unveil how these neuronal subtypes develop and organize into functional brain structures. This progress is important for further understanding the disparate physiological functions of mDA neurons and their selective vulnerability in disease, and will ultimately accelerate therapy development. This Review discusses recent advances in our understanding of molecularly defined mDA neuron subtypes and their circuits, ranging from early developmental events, such as neuron migration and axon guidance, to their wiring and function, and future implications for therapeutic strategies.