The Kottmann Laboratory studies the regulation of structural plasticity underlying motor adaption and procedural -learning and -memory. While we conduct basic, molecular biology based research using genetically altered mice, our work is of high clinical relevance for neurological and neuropsychiatric disorders involving the basal ganglia such as Parkinson's disease and addictive states in which behavioral patterns are disturbed, and for spinal cord injury and diseases involving spinal motor neurons and skeletal muscle. Our approaches and our genetic and viral research tools might have utility beyond our immediate research interests and we invite collaborations. The Kottmann laboratory has three main areas of focus:
(1) Regulation of structural plasticity in basal ganglia circuits:
Many parts of the CNS in higher vertebrates are constantly remodeled during adult life allowing memory formation and learning. It is widely assumed that plasticity processes underlying memory and learning must be closely regulated by homeostatic feedback in order to preserve overall stability and function of the mature CNS. Computations in the basal ganglia are critical for procedural -learning and -memory and the acquisition of skilled motor behaviors and motor habits. The basal ganglia also represent the anatomic substrate for numerous neurodegenerative and psychiatric disorders including Parkinson’s disease (PD), Huntington disease (HD), addiction and schizophrenia. It is hypothesized that corrupted regulation of structural plasticity could contribute to the manifestation of structural and functional pathology of diseases impacting the adult basal ganglia. The mechanisms maintaining cellular and neurochemical homeostasis in the mature basal ganglia in the healthy brain are not fully elucidated but signaling by neurotrophic factors has emerged as a likely process.
Using a candidate gene approach we discovered that all mesencephalic dopamine (DA) neurons express the cell signaling molecule Sonic Hedgehog (Shh) throughout life. To test the physiological relevance of Shh expression by DA neurons we produced a mouse line in which Shh expression is genetically ablated in mature DA neurons. These studies demonstrated that non cell autonomous Shh signaling originating from DA neurons is absolutely required for the long term maintenance of striatal cholinergic (ACh) and fast spiking (FS) interneurons as well as DA neurons.
Further experimentation revealed that DA neurons of the mesencephalon and their projection targets ACh and FS neurons are linked together in a reciprocal trophic support loop: DA neurons produce Shh which acts on ACh and FS neurons, while ACh and FS neurons produce the dopaminotrophic factor GDNF (glia derived neurotrophic factor). Using mice with conditional ablation of GDNF or of the receptor for GDNF, Ret, in conjunction with acute manipulations of Shh signaling, we demonstrated that Shh signaling inhibits the transcription of GDNF in ACh and FS neurons and, conversely, that GDNF signaling inhibits the transcription of Shh in DA neurons. Our data reveals the existence of a reciprocal trophic support loop with rheostat properties: the attenuation of Shh expression in DA neurons by physiological cell stress, results in the relief of repression of GDNF in synaptically connected ACh and FS neurons and, in return, in enhanced expression of GDNF leading to greater trophic support of physiologically compromised DA neurons. Similarly, the attenuation of GDNF expression in ACh or FS neurons will result in increased Shh expression in DA neurons providing elevated trophic support to stressed ACh or FS neurons in return.
Supporting the notion that compromised neurotrophic factor signaling could cause basal ganglia diseases, we observe in animals that cannot express Shh in DA neurons adult onset and progressive locomotion and gait disturbances reminiscent of PD that can be alleviated by treatment with L-Dopa, the standard pharmacological strategy for the management of PD.
Interestingly, both GDNF and Shh also have neuromodulatory activity. We therefore tested the impact of Shh signaling on ACh neuron activity. These experiments revealed that Shh regulates the setpoint of the level of extra cellular acetylcholine (tone) by regulating muscarinic autoreceptor M2 transcription.
Guided by these experiments we currently investigate whether Shh expression by DA neurons influences the rate of reinforcement learning, the balance between goal oriented and habitual motor behaviors, and the kinetics by which motor habits are formed.
Using conditional gene ablation approaches, we also have begun an investigation whether there are other sources in the adult brain besides DA neurons that could provide Shh to the basal ganglia.
Publication: Gonzalez Reyes et al.,(2012) “Sonic Hedgehog Maintains Cellular and Neurochemical Homeostasis in the Adutl Nigrostriatal Circuit.” Neuron 75, 306-319.
(2) Regulation of neurogenesis in the adult forebrain:
In translational stem cell research, particular interest has been devoted to neural precursor/stem cells resident in regions that display neurogenesis in adult mammals. This is due to the promise that neuronal stem cells resident in the adult brain could be coaxed into replenishing brain tissue with functional neurons and glia that are lost in neurodegenerative disease or aging. In principal support of this notion it has been found that many neurodegenerative diseases lead to changes in the cytoarchitecture and qualitative outcome of neurogenesis in the subventricular zone (SVZ) of the forebrain, pointing to pathological as well as adaptive and possibly corrective functional alterations in the SVZ dependent on the specific disease. A physiological adaptation of neurogenic outcome to current physiological needs of the adult CNS requires at least two functions:
a) The generation of a cell type specific signal for functional and/or structural deterioration
b) A mechanism by which this signal is translated into appropriate alterations in cell fate determination in the neurogenic niche.
We demonstrate that the morphogen Sonic Hedgehog (Shh), expressed outside of the germinal niche by adult dopaminergic (DA) neurons of the mesencephalon, is a key regulator of adult neurogenesis. We show that the genetic ablation of Shh from DA neurons causes an overall reduction of neurogenic activity, but an increase in the numbers of dopaminergic, periglomerular neurons of the olfactory bulb, and olfactory dysfunction. In particular, we produced evidence that Shh expression by DA neurons is up-regulated dynamically in correlation with the severity of cell physiological stress and neuronal dysfunction in connected neurons (Gonzalez et al., 2012). Thus our data suggests that Shh expressed by DA neurons could be both, a cell type specific sentinel for neuronal dysfunction and a morphogen whose expression at different levels could skew the qualitative outcome of SVZ neurogenesis towards cell identities of physiological need.
Current work aims to establish an allelic series of Shh expression levels by DA neurons and producing conditional overexpression alleles for Shh in DA neurons that can be switched on and off in mice. We also establish biochemical and functional assays to quantify the amount of biological active Shh produced by DA neurons and delivered to the SVZ. Determination of the qualitative outcome of SVZ neurogenesis as a function of different amounts of Shh delivered by DA neurons will allow us to determine the Shh dose dependent production of particular neuronal cell fates. We will then use conditional cell fate tracing to determine the destination and potential function of neurons born in response to Shh signaling from DA neurons.
Publication: manuscript in preparation.
(3) Function of trophic factor and morphogen production and signaling by spinal motor neurons:
Within the spinal motor neuron (MN) unit several tissues are interconnected that display drastically different capacities for plasticity and repair: MNs are thought not to be replenished and to retain their muscle subtype specific phenotype throughout life once born. In contrast, within the spinal cord and along their peripheral axonal projections, MNs are supported by glia, which demonstrate remarkable phenotypic transitions, expansion and turnover during disease and upon physical injury. The sole targets of MNs, skeletal muscle fibers, react to changes in bio mechanical demands and injury by the de novo production of task appropriate contractile elements and entire fibers. The plasticity and regenerative capacity of glia and muscle tissues results from germinal niches, that are formed by (1) neural stem cells resident in the ependymal cell layer surrounding the central canal in the mature spinal cord, (2) satellite (muscle stem-) cells present in each muscle fiber, and (3) the production of mitotically active repair cells, “Bungner cells”, by de-differentiation of mature Schwann cells along MN axons. The plasticity and repair processes are selective for the muscle and nerve involved. How these processes are induced, and whether and how they are coordinated is not well understood.
We and others found that mature MNs can express the morphogen Sonic Hedgehog (Shh) throughout life. Shh is essential for the ordered differentiation and congruent growth of many tissues and organs during embryogenesis where it becomes secreted from Spemann type organizer tissues like notochord and floorplate, forming gradients of morphogenic activity that results in the tissue patterning of abutting fields of naïve precursor cells. We observe that in mature MNs Shh expression is induced by muscle denervation. We also reveal that Shh production by MNs is essential for the maintenance of slow twitch muscle fibers suggesting that MN produced Shh is transported through and released from MN axons. Based on our data we hypothesize that motor units involved in functional or structural repair or plasticity, orchestrate motor unit wide processes through the graded up-regulation of Shh in effected MNs. Using a combination of genetic gain and loss of function paradigms, previously established by my lab (Gonzalez-Reyes 2012) in combination with disease- and injury- models in mice, we investigate whether MNs in the adult spinal cord act as spatially restricted signaling centers by secreting graded levels of Shh to regulate the activity and differentiation stem cells within the mature spinal cord and, transported by motor neuron axons, orchestrate muscle plasticity and repair, and coordinate Schwann cell responses to injury.
Publication: manuscript in preparation.