Loss of forebrain cholinergic neurons and impairment in spatial learning and memory in LHX7‐deficient mice

Abstract

The identification of the genetic determinants specifying neuronal networks in the mammalian brain is crucial for the understanding of the molecular and cellular mechanisms that ultimately control cognitive functions. Here we have generated a targeted allele of the LIM-homeodomain-encoding gene Lhx7 by replacing exons 3-5 with a LacZ reporter. In heterozygous animals, which are healthy, fertile and have no apparent cellular deficit in the forebrain, b-galactosidase activity reproduces the pattern of expression of the wild-type Lhx7 locus. However, homozygous mutant mice show severe deficits in forebrain cholinergic neurons (FCNs), while other classes of forebrain neurons appear unaffected. Using the LacZ reporter as a marker, we show that in LHX7-deficient mice FCN progenitors survive but fail to generate cholinergic interneurons in the striatum and cholinergic projection neurons in the basal forebrain. Analysis of behaviour in a series of spatial and non-spatial learning and memory tasks revealed that FCN ablation in Lhx7 mutants is associated with severe deficits in spatial but only mild impairment of non-spatial learning and memory. In addition, we found no deficit in long-term potentiation in mutant animals, suggesting that FCNs modulate hippocampal function independently of its capacity to store information. Overall our experiments demonstrate that Lhx7 expression is required for the specification or differentiation of cholinergic forebrain neurons involved in the processing of spatial information.

Transcription factor LIM homeobox 7 (Lhx7) maintains subtype identity of cholinergic interneurons in the mammalian striatum

“…S1 and Materials and Methods). The fate of Lhx7-depleted cells was followed by combining Lhx7 fl with Lhx7 LacZ , a null allele generated previously by inserting the LacZ reporter into the Lhx7 locus (12). To confirm that Lhx7 fl is converted into a null allele upon Cre-mediated recombination, we analyzed brain sections from Lhx7 +/LacZ , Lhx7 fl/LacZ , and β-actin::Cre;Lhx7 fl/LacZ animals (15).…”

“…"Terminal selector genes" encode transcription factors that coordinately regulate the expression of diverse molecular, morphological and functional characteristics that constitute the terminal phenotype of neurons (23). Lhx7 and Lhx6 belong to this class of genes because, respectively, they regulate multiple aspects of cholinergic and GABAergic interneuron differentiation in the mammalian forebrain (8,10,(12)(13)(14). In vertebrates terminal selector genes have been studied mostly in the context of lineage restriction and forward neuronal differentiation, but their subsequent role in maintaining neuronal subtype identity is largely unknown.…”

“…S1D) and CSIs were dramatically reduced in the striatum (Fig. S1 E-G) (8,(12)(13)(14). Interestingly, reduction of CSIs was associated with increased number of Lhx6-expressing striatal interneurons (Fig.…”

“…Interestingly, reduction of CSIs was associated with increased number of Lhx6-expressing striatal interneurons (Fig. S1F), a characteristic feature of Lhx7-null mice (8,(12)(13)(14). Therefore, upon Cre-mediated recombination, Lhx7 fl is converted into a null allele.…”

“…In contrast, Lhx6 is undetectable in CSIs, which invariably coexpress Isl1 and Lhx7. Genetic studies have established the requirement of Lhx7 and Isl1 for the specification of forebrain cholinergic neurons (11)(12)(13)(14), but the potential role of these factors in maintaining the identity of CSIs following their specification and differentiation is currently unknown.…”