FERM domain-containing protein 6 identifies a subpopulation of varicose nerve fibers in different vertebrate species.

FERM domain-containing protein 6 (FRMD6) is a member of the FERM protein superfamily, which is evolutionary highly conserved and has recently been identified as an upstream regulator of the conserved growth-promoting Hippo signaling pathway. In clinical studies, the FRMD6 gene is correlated with high significance to Alzheimer's disease and cognitive impairment implicating a wider role of this protein in the nervous system. Scare data are available on the localization of endogenous FRMD6 in neural tissues. Using a FRMD6-directed antiserum, we detected specific immunoreactivity in varicose nerve fibers in the rat central and peripheral nervous system. FRMD6-immunoreactive (-ir) neurons were found in the sensory ganglia of cranial nerves, which were marked by a pool of labeled cytoplasmic granules. Cross-species comparative studies detected a morphologically identical fiber population and a comparable fiber distribution in tissues from xenopus and human cranial nerves and ganglia. In the spinal cord, FRMD6-ir was detectable in the terminal endings of primary afferent neurons containing substance P (SP). In the rat diencephalon, FRMD6-ir was co-localized with either SP- or arginine vasopressin-positive fibers in Broca's diagonal band and the lateral septum. Dense fiber terminals containing both FRMD6-ir and growth hormone-releasing hormone were found in the median eminence. The intimate association of FRMD6 with secretory vesicles was investigated in vitro. Induction of exocytotic vesicles in cultured cells by ectopic expression of the SP precursor molecule preprotachykinin A led to a redistribution and co-localization of endogenous FRMD6 with secretory granules closely mimicking the observations in tissues.

Quantitative evaluation of neurons in the mucosal plexus of adult human intestines.

The consequence of presence versus absence of mucosal neurons is not consistently assessed. Here, we addressed two questions. First, based on resected gut specimens of 65 patients/body donors suffering from different diseases, counts of mucosal neurons per mm(2) were analysed with respect to age, gender and region. Second, we evaluated resected megacolonic specimens of four patients suffering from chronic Chagas' disease. Mucosal wholemounts were triple-stained for calretinin (CALR), peripherin (PER) and human neuronal protein Hu C/D (HU). Counts revealed no clear correlation between the presence of mucosal neurons and age, gender or region. Mucosal neurons were present in 30 of 36 specimens derived from males (83%) and in 20 of 29 from females (69%). The numbers per mm(2) increased from duodenum to ileum (1.7-10.8) and from ascending to sigmoid colon (3.2-9.9). Out of 149 small intestinal mucosal neurons, 47% were co-reactive for CALR, PER and HU (large intestine: 76% of 300 neurons) and 48% for PER and HU only (large intestine: 23%). In 12 megacolonic specimens (each 3 from 4 patients), all 23 mucosal neurons found (1.9 per mm(2)) displayed co-reactivity for CALR, PER and HU. We suggest that the presence or the absence of mucosal neurons is variable, ongoing studies will address our assumption that they correspond in their morphochemical characteristics to submucosal neurons. Furthermore, both the architecture and neuron number of the megacolonic mucosal plexus displayed no dramatic changes indicating that mucosal nerves might be less involved in chagasic/megacolonic neurodegeneration as known from the myenteric plexus.

Nucleocytoplasmic transfer of the NF2 tumor suppressor protein merlin is regulated by exon 2 and a CRM1-dependent nuclear export signal in exon 15.

The neurofibromatosis 2 protein merlin is a classical tumor suppressor protein. Germline mutations predispose to the development of schwannomas, meningiomas and ependymomas. Merlin has been implicated in cellular migration and adhesion. This function is reflected in its subcellular localization at the plasma membrane and known interacting partners. Merlin has been regarded as an exception in not exerting a functional role within the nucleus as other tumor suppressors do. Here, we show that detection of wild-type protein in the nucleus is a rare event. However, splicing out of exon 2 leads to unrestricted entry into the nucleus. Skipping of adjacent exon 3 has no comparable effect ruling out an unspecific effect due to misfolding of the 4.1/JEF domain. Exon 2 functions as a cytoplasmic retention factor as it is able to confer sole cytoplasmic localization to a GFP fusion protein. Nuclear entry of merlin is thus regulated by alternative splicing within the 4.1/JEF domain and analogous to band 4.1 protein. Merlin's ability to enter the nucleus is complemented by a full nuclear-cytoplasmic shuttle protein with a functional Rev-type nuclear export sequence (NES) within exon 15 that facilitates export via the CRM1/exportin pathway. Deletion of this NES or treatment with the CRM1-specific inhibitor leptomycin B leads to overall nuclear accumulation of merlin isoforms missing exon 2. A cellular function different to the wild-type protein is implied for naturally occurring splice variants lacking exon 2. A putative effect of merlin as a transcriptional regulator and identification of nuclear binding partners remains to be elucidated.