Localized production of TGF-beta mRNA in tumour promoter-stimulated mouse epidermis

Tumour promoters induce a wide spectrum of morphological and biochemical alterations when applied to mouse epidermis in vivo. These include the induction of RNA, DNA and protein synthesis during discrete phases of proliferation and differentiation. This constitutes an ideal model for studying molecular events underlying the disruption of epidermal homeostasis by TPA, and its subsequent re-establishment. Transforming growth factor-beta (TGF-beta) can induce either growth stimulation, inhibition, or differentiation, depending on the target cell. A function has been proposed for TGF-beta in wound healing and in tumour promotion, but the main source of TGF-beta is generally thought to be platelets, macrophages or lymphocytes, and a direct role for this growth factor in regulating tissue homeostasis in vivo has not been demonstrated. We show here that when the tumour promoter 12-tetradecanoyl-phorbol-13-acetate (TPA) is applied to the skin of mice, very high levels of TGF-beta messenger RNA are induced in the epidermal cells. In situ hybridization techniques show that the main site of TGF-beta synthesis is in the suprabasal differentiating epidermal cells. These results suggest that TGF-beta may be a natural regulator of epidermal homeostasis which is important in tumour promotion.     

Mutations in voltage-gated potassium channel KCNC3 cause degenerative and developmental central nervous system phenotypes

Potassium channel mutations have been described in episodic neurological diseases. We report that K+ channel mutations cause disease phenotypes with neurodevelopmental and neurodegenerative features. In a Filipino adult-onset ataxia pedigree, the causative gene maps to 19q13, overlapping the SCA13 disease locus described in a French pedigree with childhood-onset ataxia and cognitive delay. This region contains KCNC3 (also known as Kv3.3), encoding a voltage-gated Shaw channel with enriched cerebellar expression. Sequencing revealed two missense mutations, both of which alter KCNC3 function in Xenopus laevis expression systems. KCNC3(R420H), located in the voltage-sensing domain, had no channel activity when expressed alone and had a dominant-negative effect when co-expressed with the wild-type channel. KCNC3(F448L) shifted the activation curve in the negative direction and slowed channel closing. Thus, KCNC3(R420H) and KCNC3(F448L) are expected to change the output characteristics of fast-spiking cerebellar neurons, in which KCNC channels confer capacity for high-frequency firing. Our results establish a role for KCNC3 in phenotypes ranging from developmental disorders to adult-onset neurodegeneration and suggest voltage-gated K+ channels as candidates for additional neurodegenerative diseases.     

The role of nonlinear dynamics of the syrinx in the vocalizations of a songbird

Birdsong is characterized by the modulation of sound properties over a wide image of timescales. Understanding the mechanisms by which the brain organizes this complex temporal behaviour is a central motivation in the study of the song control and learning system. Here we present evidence that, in addition to central neural control, a further level of temporal organization is provided by nonlinear oscillatory dynamics that are intrinsic to the avian vocal organ. A detailed temporal and spectral examination of song of the zebra finch (Taeniopygia guttata) reveals a class of rapid song modulations that are consistent with transitions in the dynamical state of the syrinx. Furthermore, in vitro experiments show that the syrinx can produce a sequence of oscillatory states that are both spectrally and temporally complex in response to the slow variation of respiratory or syringeal parameters. As a consequence, simple variations in a small number of neural signals can result in a complex acoustic sequence.     

Support for a synaptic chain model of neuronal sequence generation

In songbirds, the remarkable temporal precision of song is generated by a sparse sequence of bursts in the premotor nucleus HVC. To distinguish between two possible classes of models of neural sequence generation, we carried out intracellular recordings of HVC neurons in singing zebra finches (Taeniopygia guttata). We found that the subthreshold membrane potential is characterized by a large, rapid depolarization 5-10 ms before burst onset, consistent with a synaptically connected chain of neurons in HVC. We found no evidence for the slow membrane potential modulation predicted by models in which burst timing is controlled by subthreshold dynamics. Furthermore, bursts ride on an underlying depolarization of ～10-ms duration, probably the result of a regenerative calcium spike within HVC neurons that could facilitate the propagation of activity through a chain network with high temporal precision. Our results provide insight into the fundamental mechanisms by which neural circuits can generate complex sequential behaviours.     

An ultra-sparse code underlies the generation of neural sequences in a songbird

Sequences of motor activity are encoded in many vertebrate brains by complex spatio-temporal patterns of neural activity; however, the neural circuit mechanisms underlying the generation of these pre-motor patterns are poorly understood. In songbirds, one prominent site of pre-motor activity is the forebrain robust nucleus of the archistriatum (RA), which generates stereotyped sequences of spike bursts during song and recapitulates these sequences during sleep. We show that the stereotyped sequences in RA are driven from nucleus HVC (high vocal centre), the principal pre-motor input to RA. Recordings of identified HVC neurons in sleeping and singing birds show that individual HVC neurons projecting onto RA neurons produce bursts sparsely, at a single, precise time during the RA sequence. These HVC neurons burst sequentially with respect to one another. We suggest that at each time in the RA sequence, the ensemble of active RA neurons is driven by a subpopulation of RA-projecting HVC neurons that is active only at that time. As a population, these HVC neurons may form an explicit representation of time in the sequence. Such a sparse representation, a temporal analogue of the 'grandmother cell' concept for object recognition, eliminates the problem of temporal interference during sequence generation and learning attributed to more distributed representations.