Altered White Matter and Layer VIb Neurons in Heterozygous Disc1 Mutant, a Mouse Model of Schizophrenia

Satb2 and Nr4a2 are required for the differentiation of cortical layer 6b

Cortical layer 6 is divided into two sublayers, and layer 6b is situated above the white matter with distinct architecture from layer 6a. Layer 6b arises from the subplate and contains the earliest born neurons in the development of cerebral cortex. Although great progress has been made in understanding the cortical morphogenesis, there is a dearth of knowledge regarding the molecular mechanisms governing the development of layer 6b neurons. Here we report that transcription factor special AT-rich binding protein 2 (Satb2) and nuclear receptor subfamily 4 group A member 2 (Nr4a2) are required for the normal differentiation layer 6b neurons. Upon conditional deletion of Satb2 in the cortex (Satb2Emx1 CKO) or selectively inactivation of Satb2 in layer 6b neurons only (Satb2Nr4a2CreER CKO), the expressions of layer 6b-specific genes (i.e., Ctgf, Cplx3, Trh and Tnmd) were significantly reduced, whereas that of Nr4a2 was dramatically increased, underscoring that Satb2 is involved in the differentiation of layer 6b neurons in a cell-autonomous manner. On the other hand, when Nr4a2 was deleted in the cortex, the expressions of Trh and Tnmd were upregulated with unchanged expression of Ctgf and Cplx3. Notably, the defective differentiation resulting from the deletion of Satb2 remained in Satb2/Nr4a2 double CKO mice. In summary, our findings indicated that both Satb2 and Nr4a2 are required for the differentiation of layer 6b neurons possibly via different pathways.

Hidden in the white matter: Current views on interstitial white matter neurons

The mammalian brain comprises two structurally and functionally distinct compartments: the gray matter (GM) and the white matter (WM). In humans, the WM constitutes approximately half of the brain volume, yet it remains significantly less investigated than the GM. The major cellular elements of the WM are neuronal axons and glial cells. However, the WM also contains cell bodies of the interstitial neurons, estimated to number 10 to 28 million in the adult bat brain, 67 million in Lar gibbon brain, and 450 to 670 million in the adult human brain, representing as much as 1.3%, 2.25%, and 3.5% of all neurons in the cerebral cortex, respectively. Many studies investigated the interstitial WM neurons (IWMNs) using immunohistochemistry, and some information is available regarding their electrophysiological properties. However, the functional role of IWMNs in physiologic and pathologic conditions largely remains unknown. This review aims to provide a concise update regarding the distribution and properties of interstitial WM neurons, highlight possible functions of these cells as debated in the literature, and speculate about other possible functions of the IWMNs and their interactions with glial cells. We hope that our review will inspire new research on IWMNs, which represent an intriguing cell population in the brain.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Advancing preclinical models of psychiatric disorders with human brain organoid cultures

Psychiatric disorders are often distinguished from neurological disorders in that the former do not have characteristic lesions or findings from cerebrospinal fluid, electroencephalograms (EEGs), or brain imaging, and furthermore do not have commonly recognized convergent mechanisms. Psychiatric disorders commonly involve clinical diagnosis of phenotypic behavioral disturbances of mood and psychosis, often with a poorly understood contribution of environmental factors. As such, psychiatric disease has been challenging to model preclinically for mechanistic understanding and pharmaceutical development. This review compares commonly used animal paradigms of preclinical testing with evolving techniques of induced pluripotent cell culture with a focus on emerging three-dimensional models. Advances in complexity of 3D cultures, recapitulating electrical activity in utero, and disease modeling of psychosis, mood, and environmentally induced disorders are reviewed. Insights from these rapidly expanding technologies are discussed as they pertain to the utility of human organoid and other models in finding novel research directions, validating pharmaceutical action, and recapitulating human disease.