Expression And Function Of The Pan-ErbB Ligand HB-EGF In The Central Nervous System

ErbB signaling pathway plays an important role in the development and maintenance of many types of cells in the central nervous system(CNS).The previous study in our laboratory has shown that activating or inhibiting the ErbB signaling pathway disrupts the development of CNS myelin in mice during adolescence.Heparin-binding epidermal growth factor-like growth factor(HB-EGF)is a member of the epidermal growth factor(EGF)family,which is a pan-ligand for ErbB receptors.HB-EGF is expressed in many organs and plays an important role in many biological processes.At present,it is not clear about its expression profile and functions in the CNS.To explore the specificity and function of HB-EGF expression in the CNS,we obtained hbegf-LoxP mice.The mice have the endogenous hbegf gene replaced by intact hbegf cDNA that is fused with a Loxp-flanked lacZ gene.In the presence of Cre recombinase,the hbegfcDNA sequence is deleted and the endogenous hbegf promoter initiates the expression of the lacZ reporter.Therefore,the expression of ?-gal,the protein product of lacZ gene,not only represents the knockout of hbegf,but also reliably reflects the expression of HB-EGF during ontogeny.We crossed nestin-cre mice that target all neural cells in the CNS with hbegf-LoxP mice.In the offspring of hbegf conditional knockout mice,we used X-gal staining to label ?-gal and study the expression pattern of HB-EGF in the CNS,and further explore its role in the CNS development.X-gal staining results showed that HB-EGF was selectively expressed in specific brain regions in the CNS.At P0,HB-EGF was only expressed in the deep layer of the cortex;At P10 and P20,the expression of HB-EGF began to be observed in the hippocampus and cerebellum.After that,there was no significant change in the expression pattern of HB-EGF with age.Interestingly,HB-EGF in the cerebral cortex is mainly expressed in the deep layered excitatory neurons of sensory cortical areas,including the somatosensory cortex,visual cortex and auditory cortex.Through immunofluorescence staining analysis,it was found that in HB-EGF knockout mice at P35,the corpus callosum corresponding to the sensory cortex was hypomyelinated and myelin protein decreased.The numbers of oligodendrocytes(OLs)at each differentiation stage decreased,and so did the proliferative oligodendrocyte precursor cells(OPCs).In the corpus callosum at the front part of the brain,myelin proteins and the numbers of OLs at each differentiation stage were not affected.Therefore,HB-EGF specifically regulates the development of myelin in the corpus callosum corresponding to the sensory cortex.For a long time,it is generally believed that the development of OLs in the brain has the same molecular basis.It is only known that the development of OLs in the cerebrum and spinal cord is regulated in a slightly different manner.Until recently,through the single-cell RNA sequencing of OL lineage cells from different brain regions of mice at different ages,it is revealed that OLs are spatially and temporally heterogeneous.Our findings provide new insights into the regional regulation of OLs' diversity and the CNS myelin development.