Differential expression of thymosins β₄ and β₁₀ during rat cerebellum postnatal development

Abstract

The β-thymosins are a family of actin monomer-sequestering proteins widely distributed among vertebrate classes. The most abundant β-thymosins in mammalian species are thymosin β₄ (Tβ₄) and thymosin β₁₀ (Tβ₁₀), two small peptides (43 amino acids) sharing a high degree of sequence homology. In the present work, we have analyzed the distribution of Tβ₄ and Tβ₁₀ in the developing and adult rat cerebellum using in situ hybridization and immunohistochemistry techniques. Our results show that the temporal and cellular patterns of expression of both β-thymosins are different. In the young (7 and 18 postnatal days) and adult (1 and 4 months old) rat cerebellum, Tβ₄ was mainly expressed in the glia (microglia, Golgi epithelial cells and oligodendrocytes), neurons (granule cells and Purkinje cells), and in the capillaries. In 14-month-old rats, the Tβ₄ immunoreactivity was only detected in some microglia cells. In young and adult animals, most of the Tβ₁₀ immunoreactivity was localized in several types of neuronal cells including granule cells, Golgi neurons and Purkinje cells. In old animals, a faint Tβ₁₀ signal could be detected in a few Purkinje cells. Our results suggest that each β-thymosin could play a different function in the control of actin dynamics.

Introduction

The β-thymosins are a family of highly conserved small actin-binding peptides [30] widely distributed throughout the vertebrate phylum [14], being also present in some marine invertebrates [28]. Thymosin β₄ (Tβ₄) and thymosin β₁₀ (Tβ₁₀), the most abundant β-thymosins in mammalian species [37], coexist in some tissues at varying ratios [37] but, in general, Tβ₁₀ is less abundant than Tβ₄. Although both peptides share a high degree of sequence homology, they show distinct patterns of expression in several tissues [22], [23], [30].

The presence of both β-thymosins in the mammalian nervous system, during pre- and postnatal development, is now well established. Analysis with specific RNA and protein probes demonstrated that Tβ₄ and Tβ₁₀ are expressed in neural tissues from early stages of mammalian embryogenesis [5], [6], [10], [17], [20], [22], [24], [32]. After birth, the levels of Tβ₁₀ mRNA show a more pronounced descent than those of Tβ₄ mRNA [19], [22]. In the adult rat forebrain, Tβ₄ gene is highly expressed in several areas of the brain including the cerebral cortex, the hippocampal–entorhinal region, the infundibular region, the substantia nigra pars compacta the supraoptic, medial amygdaloid and dorsal premammillary nuclei [7], [16], and in the cerebellum [3]. Moreover, Tβ₄ mRNA is induced in some neurons and glial cells following focal brain ischemia [33] and treatment with kainic acid [7]. Tβ₁₀ is also localized in several areas of adult forebrain including hippocampus, neocortex and several brain nuclei [8] and it appears to be absent in the adult cerebellum [3]. Furthermore, treatment of adult rats with kainic acid led to an activation of the Tβ₁₀ gene [8]. Altogether, these results provide strong evidence that β-thymosins play an important role in the control of actin polymerization in both neurons and glial cells. Consequently, these peptides could be involved in processes such as motility, synaptogenesis, axon growth or plastic changes in dendritic spines as well as in the restoration of neuronal circuits and glial activation that occur after brain damage.

The cerebellum is a good model for studying the participation of β-thymosins in processes of neuronal and glial migration and differentiation [1]. The aim of the present study is to increase our knowledge about the spatial and temporal distribution of β-thymosins in the developing and mature rat cerebellum, within an age interval ranging from 7 days after birth to 14 months old, using in situ hybridization and immunohistochemical techniques. Our results show that the temporal and cellular distribution of both β-thymosins are clearly different during postnatal cerebellum development.