Coordinated upregulation of α1‐ and β11‐tubulin mRNAs during collateral axonal sprouting of central peptidergic neurons

Abstract

An in situ hybridization study was performed to determine the relationship between levels of mRNAs for the axonal growth-associated alpha 1-tubulin and beta II-tubulin isotypes and the process of collateral axonal sprouting by identified central nervous system (CNS) neurons. A unilateral hypothalamic knife-cut was used to hemisect the hypothalamoneurohypophysial tract, which results in a robust collateral sprouting response by the uninjured neurons of the contralateral supraoptic nucleus (SON) (Watt and Paden: Exp Neurol 111:9-24, 1991). At 10 and 30-35 days after the lesion, cryosections of the SON were obtained and hybridized with 35S-labeled cDNA probes specific to alpha 1- and beta II-tubulin mRNAs. Quantitative evaluation of the resulting autoradiographs revealed that alpha 1-tubulin mRNA levels were significantly increased by 10 days in SON neurons that were undergoing collateral sprouting compared to controls and that this increase was sustained at 30-35 days post-lesion. Less marked increases in hybridization intensity of the beta II-tubulin probe were also apparent in sprouting neurons at both 10 and 30-35 days after the lesion, but were statistically significant only at 10 days. The measured increases in intensity of hybridization of alpha 1- and beta II-tubulin probes are likely to be conservative estimates of the underlying increase in alpha 1- and beta II-tubulin mRNA levels because sprouting SON neurons undergo significant hypertrophy. High levels of both alpha 1- and beta II-tubulin mRNAs were also seen in surviving axotomized SON neurons ipsilateral to the hypothalamic lesion. We conclude that the pattern of regulation of alpha 1- and beta II-tubulin mRNAs in CNS neurons which are capable of supporting new axonal growth includes three elements: maintenance of significant basal alpha 1- and beta II-tubulin mRNA pools in mature neurons, rapid increases in the pool size of the mRNAs following stimulation of collateral sprouting, and sustained elevation of mRNA levels during the period of axonal sprouting.

Distribution of growth‐associated class I α‐tubulin and class II β‐tubulin mrnas in adult rat brain

“…Sprouting by sympathetic neurons following transection of contralateral axons projecting to a shared terminal field is accompanied by increased expression of al-tubulin mRNA (Mathew and Miller, 1990). Similarly, we have recently demonstrated increased levels of al-tubulin and PII-tubulin mRNAs in neurosecretory neurons of the supraoptic nucleus of the hypothalamus that are undergoing collateral sprouting in response to contralateral hemisection of the hypothalamo-neurohypophysial tract (Paden et al, 1995;Watt et al, 1992). Partial deafferentation of the hippocampus by lesions of the entorhinal cortex also results in upregulation of aI-tubulin mRNA in polymorphic hilar neurons and in pyramidal neurons of the CA1 and CA3 regions known to respond with collateral sprouting (Geddes et al, 1990).…”

CNTF receptor alpha is expressed by magnocellular neurons and expression is upregulated in the rat supraoptic nucleus during axonal sprouting

“…The later observation is particularly striking given our finding that uninjured magnocellular neurons upregulate levels of CNTFRα mRNA simultaneously with increased expression of CNTF by SON astrocytes (Watt et al, 2006). These coordinated events coincide with the onset of axonal sprouting (Watt and Paden, 1991; Paden et al, 1995). A similar coordinated upregulation of CNTF and CNTFRα has also been observed in the deafferented outer molecular layer of the hippocampus between 3 and 10 days following entorhinal cortex ablation (Lee et al, 1997), a period during which prolific terminal sprouting is known to occur within the deafferented outer molecular layer (Steward et al, 1988).…”

“…The unilateral lesion severs the contralateral hypothalamo-neurohypophysial tract resulting in a 42% reduction in terminal density in the NL (Watt and Paden, 1991). This robust sprouting response restores the axon density of the NL to normal values within 30 days post-denervation and is dependent upon a sustained increase in neurosecretory and metabolic activity by the sprouting neurons (Paden et al, 1995; Watt et al, 1999). …”

Perivascular cells increase expression of ciliary neurotrophic factor following partial denervation of the rat neurohypophysis

“…Complete unilateral transection of the hypothalamo-neurohypophysial tract by the knife cut was verified histologically in every animal included in the studies described below. The effectiveness of the lesion preparation has been previously described in detail (Watt and Paden, 1991; Paden et al, 1995; Watt et al, 1999). …”