Expression of GDNF and its receptors in the periodontal mechanoreceptor

Twins with hereditary sensory and autonomic neuropathy type IV with preserved periodontal sensation

Turkish twin brothers affected with hereditary sensory and autonomic neuropathy type IV (HSAN IV) are reported. Their clinical findings were generally typical for HSAN IV. Interestingly they both had preserved periodontal sensation. Mutation analysis of the NTRK1 gene showed a homozygous c.2001C>T substitution in exon 15 in both twins. This base substitution is predicted to change a polar, positively charged amino acid arginine to the highly active amino acid cystein at position 654 (p.Arg654Cys). The parents were heterozygous for the mutation. This mutation has been reported previously in one Japanese and one Arab patients. The preserved periodontal sensation has not previously been reported in patients affected with HSAN IV. This preserved sensation in our patients might have been through Ruffini endings, the periodontal mechanoreceptors which have been reported to be present in TrkA knockout mice. Here we report the first twins affected with HSAN IV and the observation that periodontal sensation is not affected by mutation in NTRK1.

Immunolocalization of aquaporin-1 in the mechanoreceptive Ruffini endings in the periodontal ligament

Previous ultrastructural studies have suggested an axon-Schwann cell interaction in the periodontal Ruffini ending, a primary mechanoreceptor. However, no information is available on the transport mechanism between them. The present study examined the immunolocalization of aquaporin-1 (AQP1) and -4 (AQP4), a member of the water-selective channel, in the periodontal Ruffini endings of the rat incisors and trigeminal ganglion. In addition, the expression of mRNA for AQP1 and 4 was detected in the trigeminal ganglion by a RT-PCR technique. A single PCR product of the sizes anticipated for AQP1 and 4 was detectable in a reverse transcripted cDNA sample from the trigeminal ganglion, whose neurons innervate the periodontal Ruffini endings. An AQP1 immunoreaction was recognizable in the axon terminals of the periodontal Ruffini endings as well as their associated terminal Schwann cells, as confirmed with a double staining with AQP1 and either PGP9.5 or S-100 protein. However, no immunoreaction for AQP4 was found in periodontal Ruffini endings. Although the AQP4 immunoreaction was localized in some satellite cells - but never in neurons - of the trigeminal ganglion, 16.1% trigeminal neurons showed the AQP1 immunoreaction. Furthermore, the AQP1 immunoreaction was found in certain satellite cells which surrounded AQP1-positive or -negative neurons. An analysis of a cross-sectional area of these positive neurons demonstrated that approximately 66.9% of the positive neurons were 400-1000 microm2 (671.4+/-172.4 microm2), indicating that they could be categorized as medium-sized neurons which mediate mechanotransduction. These findings suggest that AQP1 controls water transport in the periodontal Ruffini endings.

Involvement of GDNF and its receptors in the maturation of the periodontal Ruffini endings

Our recent study revealed an intense immunoreaction for GDNF and its receptors in the Ruffini endings, primary mechanoreceptors in the periodontal ligament, of young rats. However, no information is available for the expression of GDNF and its receptors during their development. The present study aimed to reveal postnatal changes in the immuno-expression of GDNF, GFRalpha1 and RET in the periodontal Ruffini endings of the rat incisors by double immunofluorescent staining. At postnatal day 3 (PO 3d), no structure with GDNF-, GFRalpha1-, or RET-immunoreaction existed in the periodontal ligament. The PGP 9.5-positive nerve fibers without GDNF- and RET-immunoreaction displayed a dendritic fashion at PO 1w, with a GFRalpha1-reaction found around these nerves. At PO 2w, GDNF-positive terminal Schwann cells occurred near the thick and dendritic axons, a part of which showed a RET-reaction, with no reactive cells near the thin nerves. The terminal Schwann cells became positive for GFRalpha1, but lacked RET-immunoreaction. At PO 3w, when the formation of the periodontal Ruffini endings had proceeded, GDNF-positive terminal Schwann cells began to increase in number. This stage-specific immuno-expression pattern suggests that GDNF is a key molecule for the maturation and maintenance of the periodontal Ruffini endings.

Involvement of neurotrophin-4/5 in regeneration of the periodontal Ruffini endings at the early stage

Little is known about the role of neurotrophin-4/5 (NT-4/5) in the regeneration of mechanoreceptors. Therefore, the present study examined the regeneration process of Ruffini endings in the periodontal ligament in nt-4/5-deficient and wildtype mice following transection of the inferior alveolar nerve by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by computer-assisted quantitative image analysis. Furthermore, rescue experiments by a continuous administration of recombinant NT-4/5 were performed and analyzed quantitatively. At postoperative day 3 (PO 3d), almost all PGP 9.5-positive neural elements had disappeared; they began to appear in both types of animals at PO 7d. At PO 10d, almost all nerve fibers showed a beaded appearance, with fewer ramifications in both types of mice. Although the regeneration proceeded in the wildtype, a major population of the periodontal Ruffini endings continued to display smooth outlines at PO 28d in the nt-4/5 homozygous mice. The reduction ratio of neural density reached a maximum at PO 3d, decreased at PO 10d, and later showed a plateau. In a rescue experiment, an administration of NT-4/5 showed an acceleration of nerve regeneration in the homozygous mice. These findings indicate that the nt-4/5-depletion causes a delay in the regeneration of the periodontal Ruffini endings, but the delay is shortened by an exogenous administration of NT-4/5. Combined with our previous findings of bdnf-deficient mice (Harada et al. [2003] Arch Histol Cytol 66:183-194), these morphological and numerical data suggest that multiple neurotrophins such as NT-4/5 and brain-derived neurotrophic factor (BDNF) play roles in their regeneration in a stage-specific manner.