Developmental Patterning and Neurogenetic Gradients of Nurr1 Positive Neurons in the Rat Claustrum and Lateral Cortex

Abstract

The claustrum is an enigmatic brain structure thought to be important for conscious sensations. Recent studies have focused on gene expression patterns, connectivity, and function of the claustrum, but relatively little is known about its development. Interestingly, claustrum-enriched genes, including the previously identified marker Nurr1, are not only expressed in the classical claustrum complex, but also embedded within lateral neocortical regions in rodents. Recent studies suggest that Nurr1 positive neurons in the lateral cortex share a highly conserved genetic expression pattern with claustrum neurons. Thus, we focus on the developmental progression and birth dating pattern of the claustrum and Nurr1 positive neurons in the lateral cortex. We comprehensively investigate the expression of Nurr1 at various stages of development in the rat and find that Nurr1 expression first appears as an elongated line along the anterior-posterior axis on embryonic day 13.5 (E13.5) and then gradually differentiates into multiple sub-regions during prenatal development. Previous birth dating studies of the claustrum have led to conflicting results, therefore, we combine 5-ethynyl-2′-deoxyuridine (EdU) labeling with in situ hybridization for Nurr1 to study birth dating patterns. We find that most dorsal endopiriform (DEn) neurons are born on E13.5 to E14.5. Ventral claustrum (vCL) and dorsal claustrum (dCL) are mainly born on E14.5 to E15.5. Nurr1 positive cortical deep layer neurons (dLn) and superficial layer neurons (sLn) are mainly born on E14.5 to E15.5 and E15.5 to E17.5, respectively. Finally, we identify ventral to dorsal and posterior to anterior neurogenetic gradients within vCL and DEn. Thus, our findings suggest that claustrum and Nurr1 positive neurons in the lateral cortex are born sequentially over several days of embryonic development and contribute toward charting the complex developmental pattern of the claustrum in rodents.

Current Status of the Hypothesis of a Claustro-Insular Homolog in Sauropsids

“…In our developmental analysis of the chick claustroinsular complex we interpreted the objective process of MD/MV division, the differential histogenetic and molecular development of which we actually first described step by step [Puelles et al, 2016a], as a secondary dorsoventral subdivision of the claustro-insular complex (LPall). This is topologically comparable to the secondary differentiation of mammalian disgranular and agranular insular subareas, both of which have distinct deep claustral components [Puelles, 2014;Fang et al, 2021).…”

“…Rather than a separate migration pathway for claustral cells [Bayer and Altman, 1991], there apparently existed just a standard radial pathway for the claustro-insular complex. I checked the corresponding published cell birthday data in rodents (amply commented in Puelles [2014]: see also the recent analysis by Fang et al [2021] in the rat) and verified that claustrum neurons (as well as subplate cells and the dorsal endopiriform nucleus) were long known to be born before insular ones (that is, before any cortical plate pyramidal cells). This heterochrony of finally inside-out stratified cell populations of the claustro-insular complex suggested a relationship of both claustrum and insula cells with a particular packet of radial glial processes passing successively through the claustrum and the insula (and not through the claustrum and any part of the olfactory cortex, contrary to what Puelles et al [2000] assumed).…”

A multifaceted architectural framework of the mouse claustrum complex

“…As a result, we present a multifaceted definition for the borders of the CC, 2017). Still, although Nurr1 is clearly enriched in the CC it also expresses in the insular cortex (Fang et al, 2021). Therefore, the use of genetic markers to differentiate between cortex and CL warrants some caution.…”

Temporal origin of mouse claustrum and development of its cortical projections

“…Many of the claustrum molecular markers are not detectable in young mouse brains, or appear insufficiently selective to the claustrum when their distribution is assessed from publicly available databases (Bruguier et al 2020). However some, such as the orphan nuclear receptor 4A2 ( Nr4a2 )/Nurr1, have been used to identify the claustrum in neonatal rat brains (Fang et al 2021). Retrograde tracing from retrosplenial cortex (RSP) in adult mice is increasingly used as a strategy to identify the claustrum (Zingg et al 2018; Marriott et al 2021; Shelton et al 2022).…”

“…Previous studies have attempted to establish whether claustrum is the lateral continuation of the cortical subplate, which contains some of the earliest born neurons of the cerebral cortex, revealing a complex picture of both similarities and differences in gene expression between these two structures (Bruguier et al 2020). In rats, several groups have investigated when claustrum neurons are generated, resulting in a range of likely birth dates from embryonic day (E)13.5 to E15 (Bayer and Altman 1991; Fang et al 2021), but to the best of our knowledge the equivalent birth dates for mouse claustrum neurons have not been determined. The range of birth dates established for rat claustrum includes the range suggested for subplate, but is equally consistent with a birth-date more similar to cortical layers 5 and 6a.…”